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Albert Li Jia Wei

Science notes

PSLE preparations

Albert Li9/16/2009

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Body systemThe body system is made up of several different systems, including the digestive system,

the respiratory system vice versa. They are all interdependent and our body would not

be able to work properly without any one of them.

The digestive systemThe process of digestion is rather complicated; therefore, I could be splitting them into

smaller parts to explain. I would first be talking about the uses of each part in the

system, then, I would move on to explain how to answer some commonly seen questions

in the test paper.

Processes

There are four processes that will take place in our gut. Glands are also connected to

parts of our guts to release enzymes. The four main processes are ingestion, digestion,

absorption and egestion. Ingestion is the process whereby food is put into the mouth.

Digestion is the process whereby food is broken down into smaller, soluble substances.

Absorption is the process whereby digested food molecules move through the walls the

walls of the small intestine into the bloodstream where they are transported to different

parts of the body. Egestion is the process whereby undigested food is passed out of the

body through the anus.

The Mouth

When food is placed in my mouth, we will chew the food into smaller pieces with our

teeth. The teeth will cut and grind the food into smaller pieces and speed up the process

of digestion and aid in the swallowing process. The teeth are also made up of different

types. The front ones, known as incisors, will cut and break the food into smaller pieces.

Then, the grinding teeth at the back of our mouth cavity, known as molars, will mash the

food up into pieces. The process does not involve any chemical change of food

substances; hence, we can say that the teeth play a part in the mechanical digestion of

the food we eat. The salivary glands will secrete saliva which serves to digest the cooked

starch into maltose as saliva contains salivary amylase. However, the food usually does

not stay in the mouth long enough for all the starch to be digested into maltose. Saliva

will also soften the food and make it easier for us to swallow the food. The tongue then

rolls the food into a ball and sends it to the back of the mouth cavity, so that we can

swallow. The food then enters the oesophagus through the process of swallowing.

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The oesophagus The oesophagus or the gullet joins the mouth to the stomach. Peristalsis then occurs in

the gullet as the oesophagus move the down the alimentary canal. Peristalsis refers to

the wave-like muscle contraction of the gut, to propel food forward. The gullet contains

muscles. The contraction and relaxation of the muscles help to push the food substances

through the systems.

Hence, the alternate

contraction and

relaxation of the

muscles along the wall

of the oesophagus

forces the food ball

down towards the

stomach. Although no digestion occurs on the oesophagus, digestion of the starch by the

saliva amylase may still continue.

The stomachThe stomach is a muscular bag that contracts and relaxes. Peristalsis of the stomach will

break the food down into smaller pieces to speed up the rate of digestion, and to mix the

food substances with the gastric juices. The gastric juices in the stomach are secreted by

the gastric glands found on the wall of the stomach. The gastric juice contains protease

that will digest protein into polypeptides. Dilute hydrochloric acid which provides acidic

medium for the proteases to function optimally. The acid also kills any harmful bacteria

present in food. The stomach also contains water. The walls of the stomach will produce

mucus which functions to lubricate the food and protect the walls of the stomach from

being corroded by the acid produced. This is also why if we do not eat our meals

regularly, the digestive juices and acids will be secreted on an empty stomach, leading to

gastric ulcers on the walls of the stomach as the acid digests the stomach wall. The food

will stay in the stomach for about two to six hours and will appear as a black liquid after

some time.

The small intestine

The small intestine is the main organ in the digestive system that is involved in the

process of digestion. The small intestine is a long coiled tube of about six meters long. It

is also connected to two other important glands, the liver and the pancreas, which

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secretes digestive juices into the small intestine. In the small intestine, food is mixed

with intestinal juices secreted by glands found on the intestinal walls. Amylase, protease

and lipase, from the pancreas can be found in the intestinal juices. Bile from the liver can

also be found. Absorption of food through the walls of the alimentary canal into the

bloodstream will occur in the small intestine. Digestion will be complete in the small

intestine. However, not all the food that we eat can be digested in the small intestine.

*AbsorptionThe absorption of food takes place in the small intestine. Digested

food, which exists as small soluble molecules, will pass through

the walls of the small intestine into the bloodstream during

absorption. Hence, the movement of food though the walls of the

alimentary canal is called absorption. Only small, soluble and

diffusible substances such as glucose and amino acids can pass

through the walls of the small intestine. Larger particles like

starch or protein cannot pass through and be absorbed. In the small intestine, finger-like

projections, known as Villi are present. The diagram above shows a cross section of a

single villus in the small intestine. There are thousands of Villi present n the small

intestine. The Villi are highly convoluted or coiled up, and contain blood capillaries that

will transport the absorbed nutrients to all parts of the body. The Villi bring blood closer

to aid in absorption. It also increases the surface area to speed up the rate of absorption.

The liverThe liver is a large reddish gland that is responsible for the secretion of bile into the small

intestine. Bile is a yellowish-green liquid that is produced by the liver and stored in the

gall bladder before being secreted into the small

intestine via a bile duct. While bile does not

contain any digestive enzymes, it plays an

important role of breaking the fats down into

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smaller oil droplets. This will increase the area for lipase to act on. Hence, the rate of

digestion will increase in the presence of bile.

The pancreasThe pancreas is a gland that produces alkaline digestive juices. The pancreatic juice

contains an amylase, a protease and a lipase. The alkaline nature of the juice helps to

neutralize the acid in the food from the stomach. It is also necessary to have an alkaline

medium for the enzymes secreted into the small intestine to function optimally. Hence,

some of the processes that will take

place in the presence of the

pancreatic juice includes, the

complete digestion of maltose by

pancreatic juices, the digestion of

maltose by maltase, the digestion of

polypeptides to amino acids by

protease and the digestion of fats to

fatty acids and glycerol by lipases.

Large intestine

The large intestine is where undigested food substances like fibers, will enter the body. It

is important to include fiber in our diet since it provides the bulk of the food masses

passing through the alimentary canal. The fiber also absorbs the poisonous wastes, it can

help too to satisfy our hunger and can help to control and maintain our weight. Fiber can

be obtained from whole meal bread, wheat, fruits and vegetables. The large intestine is

about 1.5 meters long and consists of colon, the rectum and the anus. In the large

intestine, water is absorbed and mineral salts such as zinc, iron and calcium, which are

necessary for the development of the body, are also absorbed. The absorption of water

and minerals occur at the colon, leaving behind an almost solid waste known as feaces.

The feaces is then stored temporarily in the rectum and is excreted from our body

through the anus.

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Frequent questions***

Define digestion: Digestion is the process whereby large, insoluble food

substances are broken down into smaller, diffusible molecules so that

they can be absorbed by the body.

How do the Villi help in the absorption of food: The Villi

finger-like projections in the small intestine and are highly convoluted or

coiled up, they contain blood capillaries that will transport the absorbed

nutrients to all parts of the body. The Villi bring blood closer to aid in

absorption. It also increases the surface area to speed up the rate of

absorption.

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Concept map (abstract)

Digestion in animals

Human digestive system

Large intestineConsists of colon, rectum and anus

Absorption of water and mineral salt occursFeaces from the body will be removed

Small intestineEnzymes in the intestinal juices and pancreatic

juices will complete the process of digestionContains Villi that help in the absorption of food

PancreasSecretes alkaline pancreatic juices

containing amylase, protease and lipase

LiverSecretes bile that is

stored in the gall bladderBile emulsifies fat

StomachGlands secrete gastric juices into the stomach

Gastric glands juices include protease and dilute hydrochloric acid

Digestion of protease occur

OesophagusPassage that joins the stomach to the mouthPeristalsis moves food down into the stomach

No digestion occurs

MouthTeeth breaks up food into smaller pieces

Saliva moistens the food and salivary amylase starts digestion of starch into maltose

Peristalsis

Processes

Breaks down food into

small, soluble substances

Absorption

Function

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Circulatory system

The Heart

The heart is a ball of muscles that pumps blood to all parts of the body. It has two atria,

the chambers which receive blood returning to

the heart and two ventricle, the chambers that

pump blood out of the heart. Oxygen rich

blood from the lungs enters the left atrium of

the heart and into the left ventricles of the

heart before exiting to the rest of the body,

while deoxygenated blood enters from the

right atrium of the heart and into the right

ventricle before exiting to the lungs. Valves are

located in the heart to prevent blood from

flowing backwards.

Heart beat and pulse

Each cycle of the contraction and relaxation of the heart is called a heartbeat. With each

heart beat, blood is pumped through the blood vessels, causing them to expand and

relax. After each heartbeat, a pressure wave passes along the blood vessel, pushing the

blood through them. This is known as the pulse. We can take out pulse rate by pressing

our finger along the blood vessels in our wrist or neck, and count the number of

heartbeat per minute. The normal pulse rate of a healthy young male is 60 to 70 pulses

per minute. This is measured when a person is at rest. Our heart beat rate varies with

our age and the type of activities that we carry out. The faster the heart beat, the faster

the pulse rate. The beats faster when we exercise as our bodies need more food and

oxygen to release enough energy from the food for use. More carbon dioxide is thus

produced. The heart pumps faster so that the blood can supply more food and oxygen to

the cells rapidly remove carbon dioxide produced.

The blood vessels

There are three different types of blood vessels in our

body, artery, vein and the capillaries. Each vein has a

different use. The vein is used to carry the de-

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oxygenated blood back to the heart; the artery is to

transport the oxygenated blood to all parts of the

body. Lastly, the capillaries are for transporting both

types of blood as it connects two main vessels, they

are also the ones that spread to cover all parts of our

body. The arteries have thick walls so that it can

sustain the pressure at which the blood is pumped

from the heart. The veins have slightly thinner walls than arteries. The capillaries have a

very thin, one cell thick, permeable walls to allow exchange of materials between the

blood and cells.

Blood

Blood is the circulating tissue composed of fluid plasma and cells. The main components

of the blood are the red blood cells, the white blood cells, plasma and the platelets. The

blood actually acts as a medium of transportation of substances around the body.

Through the circulatory system, blood is pumped to all parts of our body. By transporting

blood throughout the body, it connects the aqueous environment of all the body cells

together.

Blood cells

There are all together three types of blood cells, the red blood cells, the white blood cells

and the platelets.

The red blood cells

Red blood cells are the most numerous blood cells. Its function is to transport

diffused oxygen from the lungs to all the cells in our body for cellular respiration.

Red blood cells are characterised by their biconcave disk shape and their bright

red colour. They contain haemoglobin which allows oxygen from the lungs to

bind loosely to them. The oxygen will eventually be given up to other cells. What

makes the red blood cell different is that they do not have a nucleus, thus they

cannot self divide and, red blood cells are actually produces by the bone marrow.

Platelets

Platelets are small fragments of cells. They help to clot the blood and stop bleeding.

When we bleed, the platelets with gather at the point of the bleeding and form

something like a plug and plug up the cut and clot up the blood, preventing excess blood

loss

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White blood cells

The main function of the white blood cells is to fight off infection. They do so by

producing antibodies. They play an important role is our body, as without them, we

would be prone to diseases and would not be able to fight them off.

Plasma

Majority of the blood volume consists of liquid plasma. The blood and other blood cells

are suspended in the plasma along with many other substances. Carbon dioxide,

dissolved nutrients, waste product together with antibodies, blood protein, hormones and

ions can be found in plasma blood. It is now known as tissue fluid. The tissue fluid acts as

a medium between cells and blood. The exchange of waste substances for useful

substances occurs between cells and the tissue fluids by osmosis, diffusion or active

transport.

Respiratory system

The respiratory system is very important to our body. It helps us to respire, as respiration

is a very important process, without it we will die.

LungThe lungs of mammals have a spongy texture

and are honeycombed with epithelium, having a

much larger surface area in total than the outer

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surface area of the lung itself. The lungs of humans are a typical example of this type of

lung.

Human lungs are located in two cavities on either side of the heart. Though similar in

appearance, the two are not identical. Both are separated into lobes by fissures, with

three lobes on the right and two on the left. The lobes are further divided into segments

and then into lobules, hexagonal divisions of the lungs that are the smallest subdivision

visible to the naked eye. The connective tissue that divides lobules is often blackened in

smokers and city dwellers. The medial border of the right lung is nearly vertical, while the

left lung contains a cardiac notch. The cardiac notch is a concave impression moulded to

accommodate the shape of the heart. Lungs are to a certain extent 'overbuilt' and have a

tremendous reserve volume as compared to the oxygen exchange requirements when at

rest. Such excess capacity is one of the reasons that individuals can smoke for years

without having a noticeable decrease in lung function while still or moving slowly; in

situations like these only a small portion of the lungs are actually perfuse with blood for

gas exchange. As oxy gen requirements increase due to exercise, a greater volume of

the lungs is perfusing, allowing the body to match its carbon dioxide or oxygen exchange

requirements. Additionally, due to the excess capacity, it is possible for humans to live

with only one lung, with the other compensating for its loss.

The environment of the lung is very moist, which makes it hospitable for bacteria. Many

respiratory illnesses are the result of bacterial or viral infection of the lungs.

Inflammation of the lungs is known as pneumonia; inflammation of the pleura

surrounding the lungs is known as pleurisy. Vital capacity is the maximum volume of air

that a person can exhale after maximum inhalation; it can be measured with a

spirometer. In combination with other physiological measurements, the vital capacity can

help make a diagnosis of underlying lung disease.

Processes of breathing

Breathing is largely driven by the muscular diaphragm at the bottom of the thorax.

Contraction of the diaphragm pulls the bottom of the cavity in which the lung is enclosed

downward, increasing volume and thus decreasing pressure, causing air to flow into the

airways. Air enters through the oral and nasal cavities; it flows through the larynx and

into the trachea, which branches out into the main bronchi and then subsequent

divisions. During normal breathing, expiration is passive and no muscles are contracted

(the diaphragm relaxes). The rib cage itself is also able to expand and contract to some

degree, through the action of other respiratory and accessory respiratory muscles. As a

result, air is sucked into or expelled out of the lungs. This type of lung is known as bellow

lungs as it resembles a blacksmith's bellows.When you inhale, the diaphragm and inter

costal muscles (those are the muscles between your ribs) contract and expand the chest

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cavity. This expansion lowers the pressure in the chest cavity below the outside air

pressure. Air then flows in through the airways (from high pressure to low pressure) and

inflates the lungs. When you exhale, the diaphragm and intercostal muscles relax and

the chest cavity gets smaller. The decrease in volume of the cavity increases the

pressure in the chest cavity above the outside air pressure. Air from the lungs (high

pressure) then flows out of the airways to the outside air (low pressure). The cycle then

repeats with each breath. In humans, the trachea divides into the two main bronchi that

enter the roots of the lungs. The bronchi continue to divide within the lung, and after

multiple divisions, bronchioles are given rise. The bronchial tree continues branching

until it reaches the level of terminal bronchioles, which lead to alveolar sacs. Alveolar

sacs are made up of clusters of alveoli, like individual grapes within a bunch. The

individual alveoli are tightly wrapped in blood vessels and it is here that gas exchange

actually occurs. Deoxygenated blood from the heart is pumped through the pulmonary

artery to the lungs, where oxygen diffuses into blood and is exchanged for carbon

dioxide in the haemoglobin of the erythrocytes. The oxygen-rich blood returns to the

heart via the pulmonary veins to be pumped back into systemic circulation. Within each

air sac, the oxygen concentration is high, so oxygen passes or diffuses across the

alveolar membrane into the pulmonary capillary. At the beginning of the pulmonary

capillary, the hemoglobin in the red blood cells has carbon dioxide bound to it and very

little oxygen. The oxygen binds to hemoglobin and the carbon dioxide is released.

Carbon dioxide is also released from sodium bicarbonate dissolved in the blood of the

pulmonary capillary. The concentration of carbon dioxide is high in the pulmonary

capillary, so carbon dioxide leaves the blood and passes across the alveolar membrane

into the air sac. This exchange of gases occurs rapidly (fractions of a second). The carbon

dioxide then leaves the alveolus when you exhale and the oxygen-enriched blood returns

to the heart. Thus, the purpose of breathing is to keep the oxygen concentration high

and the carbon dioxide concentration low in the alveoli so this gas exchange can occur!

You don't have to think about breathing because your body's autonomic nervous system

controls it, as it does many other functions in your body. If you try to hold your breath,

your body will override your action and force you to let out that breath and start

breathing again. The respiratory centers that control your rate of breathing are in the

brainstem or medulla. The nerve cells that live within these centers automatically send

signals to the diaphragm and intercostal muscles to contract and relax at regular

intervals. However, the activity of the respiratory centers can be influenced by several

factors. Oxygen can affect the activities; specialized nerve cells within the aorta and

carotid arteries called peripheral chemoreceptor monitor the oxygen concentration of the

blood and feedback on the respiratory centers. If the oxygen concentration in the blood

decreases, they tell the respiratory centers to increase the rate and depth of breathing.

Carbon dioxide too, peripheral chemoreceptor also monitor the carbon dioxide

concentration in the blood. In addition, a central chemoreceptor in the medulla monitors

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the carbon dioxide concentration in the cerebrospinal fluid (CSF) that surrounds the brain

and spinal cord; carbon dioxide diffuses easily into the CSF from the blood. If the carbon

dioxide concentration gets too high, then both types of chemoreceptor signal the

respiratory centers to increase the rate and depth of breathing. The increased rate of

breathing returns the carbon dioxide concentration to normal and the breathing rate

then slows down. Stretch receptors in the lungs and chest wall monitor the amount of

stretch in these organs. If the lungs become over-inflated (stretch too much), they signal

the respiratory centers to exhale and inhibit inspiration. This mechanism prevents

damage to the lungs that would be caused by over-inflation. Signals from higher brain

centers, nerve cells in the hypothalamus and cortex also influence the activity of the

respiratory centers. During pain or strong emotions, the hypothalamus will tell the

respiratory centers to speed up. Nerve centers in the cortex can voluntarily tell the

respiratory center to speed up, slow down or even stop (holding your breath). Their

influence, however, can be overridden by chemical factors (oxygen, carbon dioxide, and

pH). Nerve cells in the airways sense the presence of unwanted substances in the

airways such as pollen, dust, noxious fumes, water, or cigarette smoke. These cells then

signal the respiratory centers to contract the respiratory muscles, causing you to sneeze

or cough. Coughing and sneezing cause air to be rapidly and violently exhaled from the

lungs and airways, removing the offending substance. Of these factors, the strongest

influence is the carbon dioxide concentration in your blood and CSF followed by the

oxygen concentration.

Sometimes the respiratory centers go temporarily awry and send extra impulses to the

diaphragm. These impulses cause unwanted contractions (hiccups). The same thing

happens in unborn children; many pregnant women often feel their babies hiccup. This

happens because the respiratory centers of the developing child's brain are working just

like those of an adult even though they are not yet breathing air.

Respiration (Human, Fishes & plants)Respiration is the process in which oxygen is taken in and together with the glucose from

the food that we eat. The mixture is then, used to release the energy in the food and

carbon dioxide, together with water are by products. They are then released from the

body through the trachea. The energy meanwhile will be used by the cell at which

cellular respiration took place.

Fishes do not have lungs to respire, instead, they have gills. Their gills function such that

it has many fine blood vessels on each gill to take in the dissolved oxygen from the water

as it drinks the water and the water pass through. After the exchange of gases at the gill,

the water is then released through the two flaps on the gills as they open and the

process would repeat itself again.

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As for plants, it is different. The plants respire by releasing air through the openings on

the underside of the leaves called stomata. Oxygen will be given out together will excess

water in its gaseous state. The stomata have two guard cells and help to control the size

of the pore to regulate the amount of water lost to the surrounding through respiration.

This is why when the sun is very hot, the stomata will become smaller. It seemingly acts

like our nose and allow air in and out.

Magnet NotesA magnet is a special piece of metal that can attract other objects. But it can only attract

objects made of iron, steel, nickel or cobalt. These materials are called magnetic

materials. A magnet can be natural or man-made. Natural magnets are made of

lodestones. Man-made magnets are made from materials such as iron or steel. Magnets

can also come in many different shapes and sizes. The most common shapes are button,

ring, bar, strip, rod, U-shaped and horseshoe shape. The strongest parts of a magnet are

its two poles, the North and the South. The centre of the magnet is usually very weak. A

freely suspended magnet will always hang in a North- south position. Like poles of a

magnet (North-North) repels each other while the unlike poles (South- North) attract each

other. Magnetic force can pass through non-magnetic substances like glass, wood,

plastic. But it cannot pass through magnetic substances. Magnetic strength of a magnet

can be reduced by hitting it many times or by

heating it continuously. Electromagnet can be

formed using two methods. Either by stroking a

magnetic substance with a magnet or by

twirling wires over it and connect it to batteries.

The second method is usually used in factories

to retrieve large pieces of metal from the pile of

rubbish as it can be turned

N S

Stroking Method

N S

Electrical method

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on and off and is stronger than the stroking method. So when we slam refrigerator doors

often, we are reducing the strength of the magnet strip on the door.

Solar SystemThe sun, the moon, Earth and the other planets make up the solar system. The names of

the names of the planets from the nearest to the furthest are, Mercury, Venus, Earth,

Mars, Jupiter, Saturn, Uranus and Neptune. The planets travel on their own orbit, which is

a fixed path that an object takes around something else. The planets can be seen

because they reflect the light from the sun to our eyes, so we are able to see them.

Planets are objects that revolve around the sun and do not give off light on its own.

Man-made Satellite Man-made satellites are used in many different ways in space.

The Sun The sun is a huge star that gives off its own light. It is made up of a huge ball of gases. It

provides plans with light to perform photosynthesis as it cannot take place without light.

It also allows us to see things around us. Heat energy is also given off by the sun to warm

us up and prevent us from freezing. The sun rises in the east and sets in the west. Direct

•To study the Earth’s land, oceans, atmosphere an life.

Earth’s Observation

•To help ships and airplanes determine their exact location on Earth.

Navigation

•To observe the activities of enemy nations.

Defense

•To provide long distance communication links• To transmit data to various parts of the world.

Communications

•To observe the Earth’s upper atmosphere, stars, planets in the universe.

Scientific Research

•To study the weather pattern on the earth by obtaining the information of the clouds, winds and temperatures

•To find the amount and types of pollutant in the air.

Weather forecasting

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staring at sun can cause us temporary blindness. If too much ultra-violet ray reach the

Earth, our skin will become darker and we may have skin diseases.

The EarthIt is the fifth largest planet in the solar system. The earth is mostly made up of rock

metal and water. It is also surrounded by a layer of gas which traps heat for within the

earth and shield the earth from the sun’s harmful ultra-violet rays, and this layer of gas

form the atmosphere. The earth does not give off its own light, instead, it reflects the

light from the sun so, is visible when we look at it in space. The rotation of the earth on

its own axis which takes 24 hours each round, gives rise to the day and night cycle. It

takes one year to rotate on complete round around the sun and each year is 36514

days.

We are only able to live on earth as its position is just right from the sun and there is food

for us to eat and air for us to breathe, whereas water also exist in liquid state.

The MoonThere is no air or water on the moon so it does not support life. It does not give off its

own light as it merely reflect the sun’s light allowing us to see it at night. It is the earth’s

one and only natural satellite. The regular movement of the moon and the gravity of the

moon give rise to the high and low tides.

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Simple Machine notes

Force

A force can be push or a pull. A force can change the shape of an object, move a

stationary object, stop a moving object, slow down a moving object, and change the

speed of an object and change the direction an object moves in. we cannot see a force

but we can feel the effects of force after it interacts with another object or substance

(ECT. Gravitational force) Energy is needed to produce force. We get energy from the

food we eat. A force is a pull or a push. Many actions such as kicking a ball involve

forces. A force cannot be seen but we can see what a force can do. Some forces can act

at a distance, but others cannot.

Types of forces

Magnetic force: Magnetism is the force that electric currents exert on other electric

currents. Magnetism may be created by the motion of electrons in the atoms of certain

materials, which are called magnets. Magnetic force may also be produced by ordinary

electric current flowing through a coil of wire, called an electromagnet. The magnetic

force may cause attraction or repulsion--that is, it may pull magnets together or push

them apart. A magnet attracts iron, steel, nickel, and certain other materials. The

attracted materials then become magnets themselves in a process called magnetization.

A steel nail placed near a magnet, for example, becomes magnetized and can attract a

second nail. Magnetization occurs because the magnet causes spinning particles called

electrons in the atoms of the nail to align along the magnet's field lines. The atoms with

aligned electrons then act like tiny bar magnets.

Force of a magnet is strongest at the poles

Like poles repel, unlike poles attract

It can act at a distance

The strength of a magnet does not depend on its shape or size

The strength of a natural magnet, a lodestone, cannot be increased

Earth is the largest piece of magnet

The strength of a magnet varies

The strength of a magnet can be decreased by hammering, dropping or heating it

Temporary magnets can be created using the stroking and electromagnetism

methods

Elastic spring force: It is the force founding the spring when it is compressed or

stretched. When this force is exerted on a spring, the spring will in turn exert a force on

whatever is stretching or compressing it.

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The weight of the object stretches the elastic band or spring

The heavier the object, the greater the extension

Gravitational force: It is the force of attraction between the Earth and any object on Earth.

Gravity is a force that prevents things from falling out of the Earth

Gravity causes falling objects to fall downwards

Gravity causes a spring to stretch or extend

A very strong force is needed to resist the pull of gravity

There is zero gravity in space

Weight is the gravitational force put forth on an object by the planet on which the object

is located. The weight of any object depends on the distance from the object to the

centre of the planet and the mass of the object. An object's weight is largest if the object

is on the surface of the planet. The weight becomes smaller if the object is moved away

from the planet. The object has no weight in space where the gravitational force acting

on it is too weak to be measured. The weight of an object also depends on the mass of

the planet. If the mass of a planet is smaller than that of the earth, the gravitational

force there is smaller. For example, a man who weighs 91 kilograms on the earth would

weigh only 15 kilograms on the moon. He would weigh 34 kilograms on Mars, 82

kilograms on Venus, and 234 kilograms on Jupiter.

Friction is the property that objects have which makes them resist being moved across

one another. If two objects with flat surfaces are placed one on top of the other, the top

object can be lifted without any resistance except that of gravity. But if one object is

pushed or pulled along the surface of the other, there is a resistance caused by friction.

Friction has many important uses. It makes the wheels of a locomotive grip the rails of

the track. It allows a conveyor belt to turn on pulleys without slipping. You could not

walk without friction to keep your shoes from sliding on the pavement. This is why it is

hard to walk on ice. The smooth surface of the ice produces less friction than a

pavement and allows shoes to slip. Friction also has disadvantages. It produces heat

that may cause objects to wear. This is why oil and other lubricating liquids are used to

fill spaces between moving machinery parts. The liquid reduces friction and makes the

parts move more easily and produce less heat. Laws of friction: The basic law of friction

says that the force needed to overcome friction is proportional to the total normal, or

perpendicular, force pressing one surface against the other. That is, when the weight of

a box being pulled across a floor is doubled, the force necessary to pull it must be

doubled. When the box weighs four times as much, four times as much force must be

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used to pull it. The ratio between the weight being moved and the force pressing the

surfaces together is called the coefficient of friction (C.F.). The value of the C.F. depends

on the type of surfaces moving against each other. The coefficient of friction equals the

force needed to move an object divided by the force pressing the surface together. This

can be written C.F. = F/P.

The advantages of friction

It lets people walk without slipping

It enables car wheels to stop spinning and thus stop a moving vehicle

It starts movements without slipping

It enables us to hold on to things

It produces heat

The disadvantages of frictionIt slows down movement

It causes wear and tear

It forces us to use more energy

Friction can be reduced by using:

Rollers

Wheels

Ball bearings

Lubricants

Water

Simple MachinesMachines help us to do work more easily, quickly and efficiently. A force is needed to

operate these machines. Simple machines also help us to do work easily by reducing the

distance travelled by the load, distance the effort needs to travel, by reducing the effort

needed to lift the load. There are many types of simple machine. When, more than one

simple machine is used in one object it is complex machine.

LeverA lever enables us to use a smaller force to overcome a larger force. When we want to lift

a heavy load with the least effort, place the fulcrum as near to the load as possible, apply

20

effort as far from the fulcrum as possible. hen an effort moves a greater distance than

the load, less effort is used to lift the load.

PulleysA pulley has a wheel

that rotates freely. The

load is tied to a rope

that winds around the

wheel. There are two

types of pulley,

moveable and fixed

pulley.

Pulley SystemIt which consists of a fixed and a movable pulley is often used to raise heavy loads. It

changes the direction of a force and also reduces the effort needed to lift a load.

The load and effort move in opposite directions. It is commonly used in lifts and cranes.

Inclined plane

An inclined plane is a sloping surface. It helps us to do work by reducing effort to move

heavier load from a lower level to a higher level. The longer the slope, the lesser the

effort needed to travel up but the effort has to travel a longer distance.

Factors that affect the amount of effort needed:

The angle of inclination

The weight of the load

The steepness of the inclined plane

The roughness of the inclined plane

GearsGears are wheels with teeth that can interlock one another.

They help us do work by changing the direction of rotation or applied force or changing

the speed at which an object moves. Gears move in the opposite directions of the next

gear interlocked with it. Gears are usually placed together with other gears so that the

tooth of one gear fits into the notches of the other. Gears can be connected by a chain.

Examples are egg-beaters and bicycles.

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Wheel and AxleA wheel and axle is made up of a large wheel connected to a rod called axle. It makes us

to use a smaller force to overcome a bigger force at the axle.

Examples:

Screwdriver

Doorknob

Steering wheel

Pencil sharpener

Cross spanner

Faucets

A key

A simple machine is made of a small rod stuck to a big wheel. It is actually a lever;

the fulcrum is at the centre of the rod. If the wheel is bigger than the axle, less effort

is required. However, the distance moved by the effort is greater than the distance

moved by the load.

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Plants

Plants are living things; they play a very important role in maintaining the amount

of oxygen and controlling the amount of carbon dioxide in the atmosphere for us to

respire. A plant is generally considered as a living organism that manufactures its own

food (autotrophic). A plant is usually immobile and functions on the spot. However, they

are able to move and respond to changes. A good example would be plant left in a dark

box with only a small hole at the other end of the box. A few weeks later, you will find

that the plant is growing towards the hole. Doesn’t that show that plants are able to

move and respond to changes? A plant is basically made up of leaves, stem and roots. A

mushroom is definitely not a plant. It is a fungus as it does not have leaves, stem or

roots. Moreover, it cannot make its own food.

Leaves

The leaf of a plant can have many textures, smooth, hairy, waxy and rough. The

plant’s leaves have special ‘designs’. There are toothed edged, lobbed edges, and entire

edges. These are a few of the most common ones. The plants can also be classified by

the veins. Leaves help the plant to make food and are

also called, the plant’s ‘kitchen’. The leaf contains

chlorophyll which causes the leaf to appear green.

Some leaves may also appear yellow or orange, but

they still contain chlorophyll as it is hidden under the

color pigments of the leaf. The chlorophyll absorbs

sunlight and

together with

water from the

roots and the

carbon dioxide

that it gets, it

photosynthesizes and makes food and oxygen is a

byproduct of photosynthesis. It will be given out by

In the presence of light and chlorophyll

Carbon Dioxide

Water

Oxygen

Sugar/ Glucose

Main Vein

Leaf stalkBranch

VeinApex

Leaf Blade

23

the plant for us to respire and give out carbon dioxide for the plants. The plant has

veins on the leaves that will transport water from the roots to all parts of the leaf and

also collects the food at the stalk of the leaf and wait to be transported to other parts of

the plant. There are two different types of veins, the network vein and parallel vein. The

network vein is like a network just as its names states and branches to all parts of the

leaf. The parallel veins are positioned in a parallel position. The cactus has adapted to

have needle-like leaves to minimize water lose through transpiration.

The plant respires and gets carbon dioxide for photosynthesis from this small

opening found on the underside side of the leaf. It also releases excess water as water

vapor. It is mostly located at the underside of the leaf to prevent too much water loss as

the sun might dry the leaf up. It acts like our nose and allow air in and out.

Roots

The root may look plain and useless, but actually, it plays a vital role in ensuring

that the plant survives. When the plant photosynthesizes, the water that is needed

comes from the root. The root takes in water and the water is transported to the leaves.

It also anchors the plant firm in the ground and prevents it from flying out of the soil at

the slightest wind. The two most common types of root are the fibrous root and the tap

root

Other types of root

Storage Root It stores food in the roots

Breathing RootIt is able to take in air with its roots

Aerial RootIt is able to respire with its roots

Clasping Root It can clasp on other plants and ‘steal’ minerals and water from

them.

Buttress root It can keep a heavy trunk erected.

Tap root It has a long thick main root and has smaller roots growing from it. It is

good

at anchoring and can reach deep into the soil.

Fibrous rootIt is a cluster of roots growing, they are all about the same size and

it can

reach widely and reach further across the soil.

Prop rootspenetrate the soil deeply

Creeping roots It grows on the stem of other plants and this stem anchors them

24

The Stem

The stem is a very important part of the plant. Stems have a circular layer of cells

called the cortex under the epidermis, a thin, skin-like layer of cells lying in seeds, plants

and ferns which will protect he plant parts. Bundles of Xylem

tubes, that transport water and dissolved minerals absorbed by

the roots, and Phloem tubes, that transports food made by the leaf,

are arranged in a ring beneath the cortex. The xylem lies towards

the inside of the bundle, and the phloem tube is towards the

outside. Dicotyledonous stems also have a band of cells called

the cambium, which lies between the Xylem tube and the Phloem

tube within the bundle. It also helps to keep a plant upright.

Some plants have weak stems, so they cannot stand up on their

own; they cling on to supports or other plants for support.

Flower

The flower is a very interesting part of a plant. It grows when a plant matures. The flower

is made up of two parts, the male and female parts.

Not all types of plant have both the male and female

parts in one flower. The basic part of a flower is the

petal, stigma, ovary, style, ovule, filament, anther

and sepal. The anther, where the male reproductive

cells are, is supported by the filament. The stigma is

the female part of the plant where the pollen sacs will

land on. The style is the passage that leads to the

ovary and the ovules are female reproductive cells

that will fuse with the male reproductive cells to form

seeds. After the flower is fertilized, the petals will

drop off, the ovary will start to swell and a fruit is formed over time. The sepal is the part

that protects the plant when it is still a bud.

Pollination is the process whereby the pollen lands on the stigma. The pollen can

be transferred through several ways. Insects like bees and butterflies lands on flowers to

such the nectar and the pollen grain will stick to their legs and will drop off when they

land on the next plant. Another way is for the wind to blow the pollen and the pollen will

Phloem

Xylem

25

land on the flower, fertilizing it. But this method is only workable for flowers with large

droopy and feathery stigma to catch the pollen in the air. Flowers pollinated this way

does not need to have colorful petals as they do not need to attract insects.

Pollination can also be grouped into two types, self pollination and cross

pollination. Self pollination is the pollen of the same plant fertilizing the flowers of the

same plant, while cross pollination is the pollen of one plant fertilizing the flowers of

another plant. The advantage of self pollination is that the good points of one plant will

continue to be present in the young plant. The disadvantage is that if the adult plant has

a disease of is prone to a disease, the young plant would also be diseased or prone to

that disease. The advantages of cross pollination are that the plants may get good points

of both the plant that fertilized it and the plant that is fertilized. The disadvantage is that

the plant that fertilizes the plant may have some unknown disease and pass it on to the

young plant.

Fertilization occurs after pollination. The pollen rain that lands on the stigma

makes a pollen tube and travels to the ovary where its nucleus will fuse with the nucleus

of the female reproductive cell of the flower, the ovule. After the ovule is fertilized the

ovary becomes the fruit and the other parts of the flower outside the ovary wilts and

dies, leaving the fruit to grow. The fruit will contain and protect the seeds that are

needed to ensure the continuity of the species of plant.

Seed dispersal

Different plant can have different methods of seed dispersal. The seed is

dispersed so as to prevent the young plant from growing too near to the adult plant and

to prevent overcrowding and the competition for water and nutrients, promoting healthy

growth of the young plant.

A common type of dispersal is by wind. The seeds dispersed by wind are small

and light. They may also have special structures like umbrella structures or wing-like

structures that enable them to float for a longer distance away from the adult plant. The

wind can also carry away seeds that have a fluffy covering.

Fruits that disperse its seeds by water can float on water and have a fibrous husk

that can trap air. They also grow just beside rivers or seaside so that their seeds can be

carried away by water. Although fruit like coconut which is dispersed by water is heavy,

when their kernel, which contains water dries up, they simultaneously lose a lot of

weight.

Fruits which are dispersed by splitting must be dry and can open with explosive

force. When the seed is dispersed, they seed flies in all directions at high speed. There

26

are also other types which just split open, but with no explosive force as the wind will

carry the seeds away.

Some fruit disperse seeds with the help of animals. There are two types of animal

dispersal; the first type is the animal eats the whole fruit and the hard indigestible seeds

will pass out as feaces and the plant will grow. Plants dispersing seed in this way has nice

smelling and nice tasting fruit to lure animals to eat them. Another type is that the seeds

have stiff hooks and cling onto the fur of passing animals and drop off after a while as the

plant moves.

Plants can also reproduce in many other ways. One of them is by suckers. The

adult plant matures and a sucker will grow from its stem and that will be the young plant.

The young plant will get nutrients directly from the adult plant and as the young plant

grows larger, the adult plant dies. The advantage of this is that the adult plant will

provide it food and ensures that the young plant grows healthily. Another ways is by

using underground stem. The method is by digging the stem out, cutting the part of the

stem that has the bud and growing it. Some plants also can use their roots to reproduce

by cutting the ‘head’ of the root and planting it. As time passes, a young plant will grow

from it. But these only apply for special plants that have specially adapted roots and

underground stem. Plants also can reproduce from spores. But that only applies for

ferns. Ferns do not flower, thus they do not need pollination or flowers. They reproduce

by spores which are carried by the wind and when the spores land, the new fern grow.

Each fern has a lot of spore so as to ensure that some of the spore will develop into adult

ferns as it is not confirmed that the spores would land on a spot which is suitable for the

growth of the young fern.

Photosynthesis & Respiration

All green plants can photosynthesize. Plants get energy from the food the produce

through photosynthesis. Humans or other living things are unable to make their own

food. However green plants like algae and certain bacteria can trap sunlight to make

food. The food produced is glucose; glucose is a

carbohydrate that is the basic fuel and basic

building material for life. All organisms depend

on sugar as an energy source. Human and other

animals are unable to produce it on their own

and must rely ultimately on the sugar produced by

plants in the process called photosynthesis. The

leaves of plants that can make food are usually

green as they contain chlorophyll which aids in

trapping sunlight for sunlight for photosynthesis.

Without chlorophyll, the plant cannot make its

In the presence of light and chlorophyll

Carbon Dioxide

Water

Oxygen

Sugar/ Glucose

27

OxygenPlants

Respiration

food. The chlorophyll is embedded in the chloroplast. Coleus, croton and copper leaf

plants are plants with colorful leaves. They also have chlorophyll that is hidden under the

red and yellow pigments of the leaves.

Glucose and oxygen are products of photosynthesis.

Plants produce sugar in glucose for many uses. It can be

used to build leaves, flowers, fruits and seeds; it can

also be converted cellulose in building of their cell wall.

An extremely important product of photosynthesis is

oxygen. The oxygen that the plants give off is what we

breathe in for respiration and the carbon dioxide that we breathe out is taken in by the

plants for photosynthesis. Some plants not only carry out photosynthesis but also trap

insects to get more nutrients.

Food Source & Energy Source of Animals

All life on Earth depends, directly on directly, on photosynthesis as a source of food,

energy, and oxygen, making it one of the most important biochemical processes known.

Plants usually make more sugar than they need. The excess sugar is converted into

starch and stored in different parts of the plants. These stored reserves are used by

plants for extra energy or building materials. The storage part of the plant can be either

the roots, seeds or stem. Iodine is used to test for starch. In a starch test, there is iodine

in a substance if the iodine turns dark blue. As plants can make their own food, they are

called food producers. Animals and other organisms do not have the ability to make food

so they depend on other organisms and are called food consumers. There are several

types of food consumers:

Herbivores: Plant eaters Insectivore*: Insect eaters

Carnivores: Meat Eaters Frujitivore*: fruit eater

Omnivores: Plants and meat eater

Respiration

All living things respire. Unlike photosynthesis which only takes place in the presence of

light, respiration takes place all the time. During respiration, glucose and oxygen are

converted into energy for work, carbon dioxide and water.

Living things take in oxygen and give out carbon dioxide

during respiration. When they breathe in oxygen, glucose

is converted to energy used to do work. Limewater is

OxygenPlants

Photosynthesis

28

used to test the presence. It will turn chalky when comes into contact with carbon

dioxide. And remain clear if there is no presence of carbon dioxide.

GerminationGermination can only occur under certain conditions. These conditions include the

availability of air, the presence of water and warmth. Without any one of these, the seed

would not be able to germinate without any one of these conditions. However, at the

germination stage, the plant would not require light since it does not make its own food

and would get food from the seed leaves (cotyledons).

Conditions of habitat

DesertThe desert is very hot during the day and is very dry. Thus some animals have

special adaptations to survive in such conditions. Some animals only come out

during the night to avoid the high temperatures in the day. Others have

adaptations to reduce water loss like the camel which sweats very little and give

little urine. The fennec fox is another example as it has large ears to reduce heat

gained. The most common plant there is the cactus. The cactus has needle-like

29

leaves to reduce water loss through transpiration and it has a storage stem to

store the water in its stem. It is very airy there and the temperature fluctuates

from day to night.

Rotting log

The rotting log community always contains a rotting log as its name states and

the log has to be in the process of decomposing, thus it is called the rotting log.

The rotting log community must contain decomposers like the termite, fungus or

bacteria to decompose the log. The decomposing log provides accommodation

and food for the animals living in it. The rotting log is softened when it is

decomposing thus easier for the organisms in it to digest it. The organisms that

can be found there include the millipede, the centipede and spiders. The air

there is quite stale.

SeasideThe seaside community contains sandy soil and can retain little water thus the

plants there have to have roots that can absorb water fast enough for

photosynthesis. The animals there include crabs, starfishes. It is well ventilated

there and the temperature is very much the same there all the time.

Garden

The garden community is very common to most of as most of us have a garden.

The garden community has garden soil that contains many dead organisms to be

decomposed and be simplified into simple substances and return to the soil as

nutrients for the plant. The soil there is thus the most optimum for the growth of

the plants. The animals found there include butterflies, caterpillar and all types

of animals that we usually see along the road. The air there is also very airy and

the temperature is very much the same.

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Pond

The pond community unlike other habitats has a lot of water and fishes live

there. There are also many plants that live around or in the pond like the

arrowhead, hydrilla and many others. Basically, these plants can be classified

into three groups, floating, partially submerged and fully submerged. There are

also many insects living around there, like the dragonfly nymph, the water spider

and many others. The air there is very airy and the temperature maintains at

around the same range.

Leaf litter

The leaf litter community is rather damp and stuffy. There is bound to be many

trees around as the dead leaves would fall and create a heap, called the leaf

litter community. The leaves are decomposing and many insects live in them and

feed on them. The air there is stuffy and the area of the community is not very

large.

Field Community

PollutionsPollutions

Pollutions occur when substances which can make the environment dirty or unhealthy

are released into the environment.

31

Air

pollution

32

CFCs and the ozone layer

Ozone is a thin layer

of gas in the

atmosphere which

shields the earth

from most of the

harmful ultraviolet (UV) rays from the Sun. In the recent years, the ozone layer has been

depleted by humans as they release chlorofluorocarbon (CFCs) into the environment.

33

Water

Pollution

34

Acid rain Acid rain often occurs when rainwater dissolves harmful gases in the air to form a weak acid.

Negative impacts

Destruction of trees in the environment

Organisms that live in the rivers or lakes are harmed.

Our water supply is contaminated.

Acid rain can react with stones and metals, causing buildings to corrode

Land PollutionBiodegradable products are able to be broken down into simpler forms by decomposers

and can be recycled in nature. Non-biodegradable products cannot be broken down by

decomposers and will

not decay.

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Noise Pollution

Deforestation

Deforestation is the clearing of forest. This affects our environment a lot as the trees

which take in the carbon dioxide that we breathe out and turns them into oxygen, are cut

down. That causes the carbon dioxide and oxygen to be unbalanced. Then the carbon

dioxide that we breathe out would be accumulated and there will be global warming. The

water cycle is also affected by deforestation. Trees extract groundwater through their

roots and release it into the atmosphere. When part of a forest is removed, the trees no

longer evaporate away this water, resulting in a much drier climate. The cutting of tree

prevents the trees from getting a grip on the soil and the will also is soil erosion.

36

EnvironmentLiving organisms are also known as organisms. Organisms are often found in groups with

other organisms of its same species. A population consists of all the organisms of the

same kind that live and reproduce in a particular place. For example, all the duckweeds

plant in pond is considered as a population. The population size is the total number of

live organisms in a population. Adults and young must be when counting the population

size. The place where a population lives is its habitat. A habitat provides the living thing

with everything that it needs to stay alive. There are many different types of habitats,

like forest, field, garden and many others. All these habitats are for various organisms.

The living conditions are different habitats. Thus, the types of organisms found in

different from the types of organisms found in another habitat. For example, many of the

organisms that can live in the pond would not be able to live in the forest community.

Populations of several kinds are interdependent on each other. They depend on each

other for survival, or for competing with one another for food. The animals depend on the

plants for food, shelter and oxygen. While the plants on the other hand, are dependent

on animals for seed dispersal, pollination and nutrients. Populations living together in a

habitat form a community together.

Pond Community A pond community is aquatic. Aquatic means anything relate to water.

37

Leaf litter community

Leaf litter forms from fallen leaves, bits of bark and dead plant matter. It s usually found

wherever there are a lot of plants. A leaf litter habitat is made up of leaf litter and the soil

below the leaves is dark and warm, with little air movements. It may be damp. Many

populations of organisms can be in a leaf litter habitat. The leaf litter is food for bacteria,

fungi and some other animal population. Other animal population in the leaf litter

community feeds on each other.

38

Cells

Cells are the smallest building block of life and can be large or small relative to a regular

cell. Cells can also group together to form a larger organism, however, they can also live

as individual organisms.

Cells structure

Plasma Membranes

The plasma membrane is a thin layer of semi-permeable covering of the cell made of

protein and fats, that allows certain substances in and others out and it is very flexible

and appears in most cells. It increases the surface area for absorption of nutrients and

disposal of wastes, it also prevents larger molecules like starch and protein from leaving

the cell so that certain substances can be prevented from leaving the cell. The cell

membrane is also there to separate one cell from an adjacent cell.

Nucleus

The nucleus is the most prominent organelle in the cell and is the control center of the

cell where all the commands are given out. It is usually spherical in shape and found near

the center of the animal cell. It is bound by the nuclear membrane and isolates the

genetic materials of the cell from there rest of the cell. The nuclear membrane has many

perforations and it is scattered all over the nuclear membrane. In the cell, there is one or

more of a small, spherical structure called nucleolus which is not bound by a membrane

and is in charge or forming ribosome. There is also nucleoplasm in the cell that acts like

cytoplasm and fills up the nucleus; it contains chromatin which is made up of DNA. The

nucleus is involved in cell division and carries out instructions for protein synthesis. The

nuclear pores on the membrane allow certain molecules to enter or leave the nucleus.

Cytoplasm

The cytoplasm is what fills up the cell apart from the organelles and is a median for

cellular activities to occur. It is jelly-like and metabolism takes place.

Mitochondria

The mitochondria often appear in a rod or cylindrical shape. It is a permanent structure in

the cell. It can also be said to be the power house of the cell and it is where aerobic

respiration occurs to generate energy from the glucose molecules in the cell. It is

involved in chemical energy conversion during metabolic activities of the cell.

Vacuole

Vacuoles are sacs filled with sap, a watery solution of sugar, salts, and pigments. These

are the excess food stored in the cell after digestion. In plants, the vacuoles appear as

39

one large section of the cell in the middle, however, in animal cells, they appear as tiny

granules.

Ribosome

Ribosome is made up of two sub units that aids in protein synthesis and is the biological

catalysts and makes reactions of metabolisms. Many types of cell have vast numbers of

ribosome and they also construct a type of acid known as nucleic acid.

Endoplasmic Reticulum

The endoplasmic reticulum consists of a network of folded membranes, forming

interconnected sheets, tubes or sacs. The structure originates from the outer membrane

of the nucleus, to which they may stay attached to. The cytoplasm of metabolically

active cells is usually packed with endoplasmic reticulum. There are two distinct types of

endoplasmic reticulum, the rough ER (RER) and the smooth ER (SER).

Smooth Endoplasmic Reticulum has no ribosomes attached to it and is the site of

synthesis for substances needed by the cell. The SER is also responsible for the

manufacturing of lipids. In the cytoplasm of voluntary muscle fibers, a special form of

SER is at the site of storage of calcium ions which have an important role in the

contraction of muscle fibers.

Rough endoplasmic reticulum has ribosomes attached to it and vesicles are formed from

swelling at the margins that get pinched off. A vesicle is a small spherical organelle

bounded by a single membrane, which is used to store and transport substances around

the cell. For example, RER is the site of synthesis for protein such as digestive enzymes.

These are ‘packaged’ in the vesicles and are discharged from the cells.

Golgi apparatus

The Golgi apparatus consists of a stack-like structure that is a collection of flattened

membranous sacs. One side of the stack of membranes is formed by the fusion of

membranes of vesicles from ER. At the opposite side of the stack, Vesicles are formed

from swellings at the margins that become pinched off. The Golgi apparatus is present in

all cells, but is especially prominent in metabolically active cells such as secretary cells. It

is the site of synthesis for certain chemicals, including hormones and polysaccharide

macromolecules, which are then packages into vesicles. In animal cells these vesicles

may form lysosome, while those in plant cells my contain polysaccharide for cell wall

formation.

Lysosomes

Lysosomes are small spherical vesicles bound by a single membrane. They contain a

concentrated mixture of hydrolytic enzymes, which are produced by either the Golgi

apparatus or by the ER. Lysosomes are involved in the breakdown of contents of

40

imported food vacuoles. An example might be a harmful bacterium that has invaded the

body and been engulfed by one of the body’s defense cell. It is then broken down, and

the products of digestion escape into the liquid of the cytoplasm. Lysosomes may also

fuse with and digest any broken-down organelles in the cytoplasm. When an organism

dies, the hydrolytic enzymes in the lysosome of the cells escape into the cytoplasm and

cause self digestion.

Cell wall

The cell wall in plant cells is a firm and turgid layer of cellulose that gives the cell its

shape and it does not interfere with the cell membrane’s work of regulating entry and

exit of substances as the perforations in it is large enough for almost all substances to go

through.

Chloroplast

In plant cells, there is another unique cell part, the chloropast which is the part in the cell

that contains chlorophyll and trap sunlight to make food. Without the chloroplast, the

plant cells would not appear green and would not be able to photosynthesize.

Division of Labour

Division of labour is needed since each cell has specified jobs and not any other cell is

able to take over the job of a specified type of cell. For example, a cheek cell is definitely

not able to replace a brain cell thus, cell specialization in the division of labor is very

important. This allows multiple processes to go on concurrently.

Diffusion & Osmosis

Osmosis is generally the net movement of water molecules across a partially permeable

layer from a substance of higher water potential to a substance of lower water potential.

Water potential is the density of water molecules in a certain area. The water would

move across from the substrate of higher water potential until both sides have an equal

water potential. When a substrate has a higher water potential than the average water

potential in your cell, it is said that the substance is hypotonic and water moves into the

cell and a plant cell would become turgid while an animal cell would lyse. When the

substrate has an equal water potential as the average water potential in you cell, it is

called isotonic and there is no net movement of water in and out of the cell. When the

substance has a lower water potential than the average water potential in your cell, it is

said that the substance is hypertonic and water would move out of the cell, causing it to

be flaccid. Diffusion is quite the same thing as osmosis except that it involves a

substance that can move and travel across in natural, for example, the diffusion of gas in

41

the or air, or the diffusion of blue paint in a beaker of water. Diffusion is affected by many

different factors like the temperature of the substance since it affects the movement of

molecule, the thickness of the barrier separating the two substances and the size of the

molecule these factors all can affect the rate of diffusion and osmosis.

Digestive System

The process of digestion

Mouth

The mouth is the place where digestion starts. The teeth start helping by chewing and

tearing the food up into smaller pieces (mastication) so that digestion can occur over a

larger surface area for enzyme reaction and digestion can happen faster. The tongue

secretes enzyme amylase which helps to catalyse the digestion of starch into maltose in

the food. It is also to soften the food. The tongue then rolls the chewed up food into a ball

called bolus and it is rolled to the back of the tongue so that it can go down into the

throat. The saliva also contains mucus that helps to make food more slippery and allows

it to go down the esophagus more easily.

42

Gullet

The gullet or the esophagus is a passage way which connects the mouth to the stomach.

The food from the mouth is pushed down this tube by peristalsis; this is the action of the

muscles contracting and relaxing, pushing the ball of food down. This is an involuntary

action. No chemical reaction occurs here apart from the digestion of starch into maltose.

The gullet itself is just a long tube of muscles.

Stomach

The stomach is a muscular bag that contains hydrochloric acid and it is also where most

of the digestion takes place. The food is kept in the stomach for about 2-4 hours and

remains there until mostly digested. The food that has been churned by the stomach is

called chime. Alcohol and glucose are also absorbed here. The hydrochloric acid in the

stomach is highly acidic and thus the average pH of the stomach is about 2. This acidic

environment allows many of the processes in the stomach to go on and also kills the

bacteria in the food. The hydrochloric acid also helps to loosen the bonds in the meat, to

render easy absorption of calcium and iron at a later stage in the process of digestion.

Due to its acidic environment, the stomach has to secrete mucus in order to prevent the

hydrochloric acid from digesting the walls of the stomach. In the stomach, various

enzymes are secreted, pepsin, rennin and peptides. The pepsin aids in breaking down

the protein into peptides and the rennin converts protein into insoluble curds for

hydrolysis of pepsin. The pyloric sphincter controls the entry of food to the duodenum.

Liver

The liver is one of the crucial parts of the digestive system and it is where green bile that

is needed for emulsification of fats is formed and sent to the gall bladder to be stored

and transferred to the small intestine to emulsify the fats and allow a larger area for

reaction of lipase, making it easier for the digestion of fats and lipids in the food.

Pancreas

The pancreas is one of the main secretor of alkaline enzymes in the digestive system.

The pancreas is a feathery like structure that secretes enzymes like amylase, trypsin,

lipase and even water as it helps in hydrolysis of food. It also secretes insulin to convert

excess glucose to glycogen in the liver.

Small Intestine

The small intestine is where the final stages of digestion take place, it is also where bile

is secreted into the digestive system, and the bile helps to emulsify the fats so that the

digestion of fats into fatty acid and glycerol can occur at a faster rate. The pH here is

relatively normal as it is

pH7 since the pancreatic

43

juice is alkaline and thus the hydrochloric acid from the stomach may be neutralized so

that the acids in the pancreatic juice can be activated. In the duodenum, the last stages

of digestion take place and are completed and the food moves on to the duodenum. Just

to name a few of the reactions that occur, the peptides are digested into amino acids,

lactose is digested into glucose and galactose, sucrose is digested into glucose and

fructose and the maltose is digested into sucrose. In the ileum, the walls of the small

intestine are covered with tiny protrusions called micro-villi. The villi are present to aid in

the absorption of nutrients into the bloodstream as the surface area for absorption is

increased and is more efficient. On the villi, there are millions of micro-villi, each villus

has blood capillaries linked to the hepatic system and in the middle of the main villi, and

there is the lacteal which is linked to the lymphatic system of the body. The fats and

lipids are absorbed into the lacteal, into the lymphatic system to the liver. The other

nutrients however, are absorbed into the blood capillaries and transported to all parts of

the body.

Large Intestine

The large intestine is the alimentary canal and is mainly made up of the colon; the colon

is the part where no enzyme activity takes place, however, water is absorbed from the

food into the body The large intestine pushes the digested food through the large

intestine by peristalsis. Since there are no chemical to neutralize the enzymes from the

small intestine, the walls of the large intestine secretes mucus to prevent the enzymes

from harming the walls of the large intestine.