ocr as biology unit 1 revision notes

8/12/2019 OCR AS Biology Unit 1 Revision Notes

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AS Biology

Unit 1- Cells, Exchangeand Transport (F211)


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Microscopes

Magnification- the number of times bigger an image is than the

object.

Resolution- the ability to distinguish to close together points asdistinct from each other

Sample staining- any process that helps to reveal or distinguish

different features. In light microscopy, stains may be colours of

fluorescent dyes. In electron microscopy, they are metal particles or

metal salts.

Sectioning- specimens are embedded in wax. Thin sections are then

cut without distorting the structure of the specimen. This is

particularly useful when making sections of soft tissue, such as brain.

Magnification=Image Size (m)

Actual Size (m)

To convert from mm to m times by 1000.

Light Microscope Transmission

Electron

Microscope

Scanning

Electron

Microscope

Magnification X1,500 X500,000 X100,000

Resolution 200nm 0.1nm 0.1nm

Advantages Inexpensive Goodmagnification

and resolution

Goodmagnification

and resolution,

3D images

produced

Disadvantages Low magnification

and resolution

Samples have to be dead, samples

have to be in a vacuum, extremely

expensive and require high degrees

of skill and training.


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Organelles

Nucleus- Contains the genetic information of the cell

Nucleolus- Makes RNA and ribosomesNuclear envelope- Contains holes called nuclear pores,

which allow relatively large molecules to pass through

Rough endoplasmic reticulum- Transport proteins that

were made on the attached ribosomes.

Smooth endoplasmic reticulum- Makes lipids and

steroids.

Golgi apparatus- Modifies proteins and packages them into

vesicles. They can then be transported to the surface forexocytosis.

Ribosomes- Site of protein synthesis

Mitochondria- Where ATP is made

Lysosomes- Contain digestive enzymes that break down

waste material in the cell

Chloroplasts- Site of photosynthesis in plant cells

Centrioles- Form spindle fibres during cell division

Flagella and cilia- Cellular extensions, which move in awave like manner. Flagella are long and few in number and

cilia are short and numerous.

Cell surface membrane- Controls the entry and exit of

substances into and out of the cell.


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Production and Secretion of Proteins

1. The instructions to make proteins are in the nucleus of the cell2. The gene containing the instructions for the production of the

hormones is copied onto a piece of mRNA3. The mRNA leaves the nucleus through the nuclear pores and

attaches to a ribosome.

4. The ribosome uses the codes to assembled the protein5. The assembled proteins inside the rough ER is pinched off in a

vesicle and transported to the Golgi apparatus.

6. Golgi apparatus processes and packages the molecules, readyfor release.

7. The molecules are pinched off in vesicles from the Golgiapparatus and moves towards the cell surface membrane.

8. Vesicles fuse with the cell surface membrane and themembrane opens to release the molecules outside- this is

exocytosis.

Cytoskeleton

The cytoskeleton is made up of

1. Microfilaments2. Microtubules3. Intermediate filaments

Its function is to

1. Keep the cells shape and strength and stability2. Whole cell movement3. Movement of organelles

Microtubules do not move, but they provide an anchor for protein to

move along e.g. kinesin attach one end to an organelle and the otherend to a microtubule. Using ATP it swivels, pushing the organelle

along. The head then reattaches itself to the microtubule and the

process is repeated.

Flagella and cilia are each made from one cylinder containing 9

microtubules. Flagella move with the aid of the protein, Dynein.

When a molecule of dynein swivels it pulls one microtubule past the

next, causing the cilium to bend.

Cilia move out of time with each other to create a wave.


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Prokaryotes and Eukaryotes

Prokaryotes EukaryotesNo nucleus (DNA

suspended freely)

Nucleus (contains DNA)

No membrane bound

organelles

Membrane bound

organelles (mitochondria,

chloroplast etc.)

Peptidoglycan cell wall Cellulose cell wall

Spiral flagella Waved flagellaSmaller ribosomes Larger ribosomes

Single-loop chromosomes Linear chromosomes

Single-celled One or more cell

Contains plasmids Do not contain plasmids


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Cell Membrane

Cholesterol- Gives the membrane stability by sitting between the

fatty acid tails and therefore making the barrier more complete,

preventing molecules like water and ions passing through the

membrane.

Glycolipids- Phospholipid molecules that have a carbohydrate partattached. They are used for cell signaling, cell surface antigens and

cell adhesion.

Glycoproteins- Protein molecules with a carbohydrate attached:

Act as antigens Enable the identification of cells as self or non-self Used in cell signaling Act as receptors or binding sites for hormones. They have a

specific shape that is complementary to the shape of the

communicating molecule which binds to the receptor Act as receptors on transport proteins to trigger movement Allow cell adhesion to hold cells together in a tissue Attach the water molecules to stabilize the membraneChannel proteins-Allow the movement of some substances, such as

the large molecule sugar, into and out of the cell as they cant travel

directly through the cell surface membrane.

Carrier proteins-Actively move substances across the cell surface

membrane.


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Function of membranes:

Separate cell contents from outside environment Cell recognition and signaling Holding the components of some metabolic pathways in place Regulating the transports of materials into or out of the cell Allow compartmentalisation Isolate harmful substances (e.g. lysosomes) Provide a surface (attachment of ribosomes)Temperature and permeability:

A high temperature boosts the kinetic energy of the componentmolecules of the membrane and the transported substance. Themembrane becomes more permeable.

Very high temperatures will denature the protein molecules,changing their shape and making the membrane permeable.

Eventually the membrane will be destroyed.


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Cell communication and signaling

Most messenger molecules are unable to directly crossthe membrane and must bind to the membrane boundreceptors in order to communicate with a cell.

Some integral proteins are receptors for hormones andneurotransmitters.

Different cells have specific receptors depending on therole in our body.

Via receptors and complementary shaped molecules onthe target cell.

Insulin

Pancreasliver and muscle cells

Purpose- regulate glucose

Transport- via the blood

Chemical message- insulin (protein)

Serotonin

NeuronsPurpose- nervous system

Transport- via the blood

Chemical messenger- serotonin

Drugs that bind to receptors and mimic the bodys normal

messengers are called agonists(e.g. HIV Virus)

Drugs that bind and block the bodys normal messengers arecalled antagonists(e.g. beta blockers)


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Diffusion

The movement of molecules from an area of high concentration of that

molecule, to an area of low concentration, down a concentration gradient.

Factors that affect diffusion

Temperature Concentration gradient Stirring/ mixing Surface area Distance/ thickness Size of moleculePassiveProcesses

DiffusionDown a concentration gradient. Smallmolecules/ lipid soluble.

Facilitated Diffusion Down a concentration gradient.

Charged/ hydrophilic molecules.

Through channel or carrier proteins.

Osmosis Down water potential gradient through

bilayer or protein pores.

Active

Processes

Active Transport Against concentration gradient via

carrier proteins that use ATP to change

shape.

Endo/exocytosis Bulk transport via vesicles that can fuse

or break from cell surface membrane.

Osmosis

Osmosis is the passage of water molecules through a partially permeable

membrane, from a region of high water potential, to a region of lower

water potential (down a water potential gradient.)

Water potential is denoted by the symbol and is measured in kilopascals

(kPa). Pure water has a value of 0, the more solutes that are dissolved the

more negative the water potential gets.

Animal cell

When water osmosis into an animal cell it can burst- this is called

haemolysis.

When water leaves the cell it shrinks- it becomes crenated.

Plant Cell

When water enters the cell, the cell wall prevents it from bursting- the cell

becomes turgid.

When water leaves the cell it pulls away from the cell wall- this is calledplasmolysis.


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Mitosis

Interphase

1. G1 Phase- Cells increase in size and ensure everything is ready forDNA synthesis.

2. S Phase- DNA in the cell is duplicated3. G2 Phase- Cell continues to grow and duplicated DNA is checked.

Mitosis

1. Prophase- DNA shortens and condenses by coiling to formchromosomes

2. Metaphase- the spindle fibres attach themselves to the centromeresof the chromosomes and align the chromosomes along the middle.

3. Anaphase- the spindle fibres shorten and the centromere splits.Sister chromatids are pulled apart.

4. Telophase- the nuclear envelope reforms before the chromosomesuncoil. The spindle fibres disintegrate.

Cytokinesis

Daughter cells split apart. A furrow forms and the cell is pinched in

two.

Homologous pair of chromosomes-The chromosomes that have the

same gene sequence pair up during the cell cycle. This pairing happens

between chromosomes that are homologous i.e. Chromosomes having the

same genes at the same loci but possibly different alleles.

Why mitosis is so important-

Asexual reproduction Growth- multicellular organisms grow by producing new extra cells. Repair- damaged cells need to be replaces by new ones. Replacement- red blood cells and skin cells are replaced by new

ones.

Budding in Yeast

The nucleus divides by mitosis. The cell swells on one side and bulges. Thenucleus, cytoplasm and organelles move into the bus and it pinches off as

the cell wall forms so the bud becomes a separate cell.

Meiosis

Meiosis produces 4 genetically un-identical cells. This is because the genes

from the father and mother wrap around each other and exchange

chromosomes.


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Stem Cells

A stem cellis an undifferentiated cell that is capable of becoming differentiated

to a number of possible cell types.

Cells becoming specialised to carry out a particular function is known as

differentiation.

Cells can differentiate with changes to:

The number of a particular organelle The shape of the cell Some of the contents of the cellErythrocytes and Neutrophils

Erythrocytes (red blood cells) and neutrophils (white blood cells) both begin

with the same set of chromosomes, produced from undifferentiated stem cells inbone marrow.

Erythrocytes loose their nucleus, golgi apparatus, rough endoplasmic

reticulum and mitochondria in order to make room for haemoglobin. Their

shape changes to become biconcave to increase the surface are for picking up

oxygen.

Neutrophils keep their nucleus. Their cytoplasm contains lots of lysosomes to

digest microorganisms. They are flexible for phagocytosis.

Xylem and Phloem

Both come from dividing meristem cells such as cambium meristem cells.

Undergo differentiation to form the different kinds of cells in the transporttissues,

Xylem walls become waterproofed and reinforced (lignin.) This kills the cells

contents. The xylem therefore becomes a long, dead, hollow tube.

Sieve plates are formed between cells, companion cells on the side of the

phloem with lots of mitochondria.

Sperm Cell

Mitochondria- Energy needed for the

movement of undulipodium

Acrosome- a lysosome that releases enzymesonto the outside of the egg so that the sperms

nucleus can penetrate the egg in order to fertilise it.

Undulipodium- helps to propel the cell towards the egg.

Shape- Long, thin to help ease movement.

Tissues- A collection of cell that are similar and preform a common

function. Examples- Xylem/ phloem.

Organs- A collection of tissues working together to preform a particular

function. Example- Plant leaves.

Organ System- Organs working together to preform an overall life

function. Example- Reproductive system.


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Squamous and Ciliated Epithelium Tissue

Squamous- made up of cells that are flattened, so they are very

thin. The cells together form thin, smooth, flat surfaces. This

makes them ideal for the lining inside of tubes such as bloodvessels/ walls of the alveoli. It provides a short diffuse ion

pathway for the exchange of oxygen and carbon dioxide.

Ciliated- made up of column-shaped cells. This type of tissue is

often found on the inner surface of tubes, for example, in the

trachea, bronchi and bronchioles.


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Special Surfaces for Exchange

Three factors affect the need for a transport system:

1. Size- as an animal has several layers of cells, any oxygen or nutrientsdiffusing from the outside are used by outer layers of cells.

2. Surface area to volume ratio- in larger animals the surface area tovolume ratio is not large enough to supply all the oxygen and nutrients

needed by the internal cells. (As the organism gets larger the SA: V ratio

gets smaller.)

3. Level of activity- an active animal needs a good supply of nutrients andoxygen to supply energy for movement.

Good exchange surfaces

Have a large surface area to provide more space for molecules to passthrough. (Alveoli increase surface area in the lungs.)

Short diffusion pathway to reduce the diffusion distance. (Squamousepithelium in the alveoli only one cell thick.)

Fresh supply of molecules to maintain concentration gradient.Tissues in the Lungs

Cartilage- Supports the trachea and bronchi, holding them open. This

prevents collapse when the air pressure inside is low during inhalation.

Ciliated Epithelium- These cells have cilia (tiny hairs) that waft mucus up

the airway to the back of the throat. The mucus is swallowed and the acidin the stomach kills any bacteria.

Goblet Cells- Secretes mucus to trap tiny particles from the air (including

pollen and bacteria) to reduce the risk of infection.

Smooth Muscle- When it contracts it constricts the airway. This makes the

lumen narrower and restricts the flow of air to and from the alveoli. This

may be important if there are harmful substances in the air.

Elastic Fibres- When the smooth muscle contracts it can not reverse the

effect of the narrowing the lumen. When it relaxes the elastic fibres recoil

to their original size and shape. This helps to dilate (widen) the airway.


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Inspiration

1.Diaphragm contracts and flattens2.

External intercostal muscles contract to raise the ribs3.Volume of the chest cavity increases

4.Pressure decreases5.Air moves down the pressure gradient into the lungs.

Spirometers

Tidal Volume- The volume of air moved in and out during

the breathing when you are at rest.

Vital capacity- The largest amount of air that can be moved

in and out of the lungs in one breath.

Soda lime is added to the spirometer to remove carbon

dioxide when it is breathed out.When air is breathed in the spirometer data logger moves

downwards.

Oxygen uptake can be calculated:

Change in volume

Change in time


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Circulatory Systems

Sing circulatory system- Blood flows through the heart

once during each circulation of the bodyDouble circulatory system-The blood flows through the

heart twice. Once to pick up oxygen (pulmonary circulation)

and then to carry oxygen to the body (systemic circulation.)

Open circulatory system- The blood is not always in

vessels (i.e. insects.)

Closed circulatory system- The blood is always contained

within vessels (i.e. fish)

Thickness of walls

The left ventricle has the thickest cardiac muscle as it has to

pump blood around the body. Next is the right ventricle

which has to pump the blood to the lungs. The atriums have

the least cardiac muscle as they only have to pump blood

into the ventricles.


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Cardiac cycle

When the atria are in diastole they fill with blood from the

vena cava/ pulmonary vein. They then contract (systole),

increasing the pressure. As the pressure is higher in the atria

then it is in the ventricle, the atroventricular valves

(bicuspid and tricuspid) open and blood flows into the

ventricles. The ventricle then contracts, increasing the

pressure and therefore causing the AV valve to close. This

also causes the semilunar valves to open and blood flows

into the pulmonary artery/ aorta.

Electrical Impulses

The sino-atrial node starts the excitation wave, which

spreads over the wall of the atria until it reaches the atrio-

ventricular node. The atria contract (atrial systole) and this

contraction is synchronised. There is a delay at the atrio-

ventricular node when the wave of excitation spreads down

septum and into the bundle of His and the Purkyne fibres.

This causes the ventricle to contract (ventricular systole)from the apex of the heart.


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Blood Vessels

Arteries

Carry blood at a high pressure, so in order towithstand the pressure the wall is thick, with

a thick layer of collagen to provide strength.

The endothelium is folded which prevents

damage as it can stretches under pressure

Must be able to maintain that high pressure.

There is a thick layer of elastic tissue to cause

recoil and a return to original size. There is a

thick layer of smooth muscle, which narrows

the lumen.

Veins

Carry blood at low pressure so do walls do

not need to be thick. Lumen is relatively

large to ease the flow of blood. The walls

have thinner layers of collagen, smooth

muscle and elastic tissue. They do no need to

stretch and recoil and are not activelyconstricted to reduce blood flow. Contain valves to prevent

blood flowing in the wrong direction. As the walls are thin, the

vein can be flattened by the action of the surrounding skeletal

muscles. Pressure is applied to the blood, forcing it to move

along in the direction dictated by the valves.

Capillaries

Walls consist of a single layer of flattened endothelial cells that

reduces the diffusion distance for the materials being exchanged.The lumen is the same diameter as the red blood cell (about

7m). This ensures that the red blood cells are squeezed as they

pass along the capillaries. The diffusion distance is shorter, so

they are more likely to give up their oxygen.


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Blood, Tissue Fluid and Lymph

Blood- The main transport fluid for substances to and from all regions of

the body.

Tissue Fluid- Leaks from the capillaries and passes around the cells.Oxygen and nutrients such as glucose can diffuse from the tissue fluid into

the cells and carbon dioxide and waste products such as urea diffuse out of

the cell.

Lymph- The fluid that drains from the tissues into lymph vessels and

eventually back into the blood.

Hydrostatic pressure is caused by the heart pumping blood. This

hydrostatic pressure pushes the blood fluid through the tiny gaps in the

capillaries walls. The fluid that leaves the blood consists of plasma with

dissolved nutrients and oxygen. All the red blood cells and platelets

remain in the blood. They are too large to fit through the gaps. The fluid

that leaves the capillaries is known as tissue fluid.

Feature Blood Tissue Fluid Lymph

Cells Erythrocytes,

leucocytes and

platelets

Some phagocytic

white blood cells

Lymphocytes

Proteins Hormones and

plasma proteins

Some hormones,

proteins secreted

by body cells

Some proteins

Fats Some transported

as lipoproteins

None More than in blood

Glucose 80-120mg per

100cm3

Less Less

Amino acids More Less Less

Oxygen More Less LessCarbon dioxide Little More More


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Haemoglobin

The ability of haemoglobin to take up and release oxygen depends on the

amount of oxygen in the surrounding tissue. The amount of oxygen is

measured by the relative pressure that it contributes to a mixture of gases.This is called partial pressure.

Fetal haemoglobin

Fetal haemoglobin has a higher affinity for oxygen then the adult

haemoglobin. This means fetal haemoglobin can bind to oxygen in the

placenta at relatively low partial pressure of oxygen, where the mothers

haemoglobin is dissociating (releasing oxygen.) (The curve shifts to the

left.)

Carbon Dioxide

Carbon dioxide is transported:

5% directly dissolved in the plasma10% combined with haemoglobin to form carbominohaemoglobin.85% transported in the form of hydrogencarbonate ions (HCO3-)

The Bohr effect- the change in the shape of the oxyhaemoglobin curve

when the carbon dioxide is present- this causes the oxyhaemoglobin to

release oxygen more readily. (The curve shifts to the right.)


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Transpiration

Xylem

Long, thick walls that have been impregnated by

lignin. As the xylem develops, the lignin

waterproofs the walls of the cell,

consequently, the cells die and their end walls

and contents break down. This leaves a long

column of hollow, dead cells. The lignin

strengthens the walls and prevents the

vessel from collapsing- the vessels stay open

even when water is in short supply.

The thickening of the lignin forms patterns on

the cell walls. This prevents the vessel from

becoming too rigid and allows the stem or

branch to be flexible.

In some places the lignification is not complete.

Pits and bordered pits, like pores in the walls, are left which

allow water to leave the vessel to either join another vessel

or pass into cells.

TranspirationThe loss of water by evaporation from the aerial parts of a plant

The stomata needs to be open in order for gaseous exchange to take

place. Therefore, water can be evaporated through the leaves.

Water uptake and movement up the stem

Minerals are actively transported into the root hair cell (using ATP.)

This reduces the water potential in the root hair cell- therefore water

moves down the water potential gradient into the roots (by osmosis.)

The water moves across cortex by osmosis. It can move through oneof three pathways:

1. Apoplast- through the cell wall, prevented by the casparian stripso it has to eventually join the symplast pathway.

2. Symplast- through the cytoplasm3. Vacuolar- through vacuolesWhen going up the xylem vessel the water molecules are attracted to

each other due to intermolecular hydrogen bonds. This is called

cohesion. Adhesion is the attraction of water molecules to the walls

of the xylem. This means that when one water molecule evaporates,the others follow up the xylem.


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Factors that affect transpiration

Number of leaves Number, size and position of stomata Presence of cuticle Light (the stomata open to photosynthesis) Temperature Relative humidity Air movement/ wind Water availabilityPotometer

1. Select plant to be used in experiment2. Underwater, cut the stem at an angle of about 33oc3. Keep the cutting beneath water level, thus ensuring the column of

water in the xylem is not broken.

4. Fill the photometer with water, being sure to introduce an airbubble.

5. Carefully insert the top of the cutting into the top of thephotometer (still under water) and ensure and air tight seal.

6. The plant can now be exposed to different environmentalconditions. Leave to acclimatize and then water uptake can be

measured.7. Results can be graphed as followed; rate of water transpired

against time.

Xerophytes

A plant that is adapted to reduce water loss so that it can survive in

very dry conditions is called a xerophyte.

Adaptations of xerophytes

A waxy cuticle to reduce water loss Smaller leaves Closing the stomata when possible Hairs to hold water and therefore reduce water vapour gradient. Pits to trap water (reducing the water potential gradient) Rolled leaves to trap water vapour


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Translocation

Translocation-The transport of assimilates throughout the plant in

the phloem tissue.

Source-Releases sugars into the phloem.

Sink-Removes sugars from the phloem.

At the source H+ ions are pumped out of the companion cells and

come back with sucrose (using co-transporter proteins.) They are

actively transported out and then diffuse back in.

At the sink sucrose molecules move by diffusion or active transport

from the sieve tube element into the surrounding cells. This increases

the water potential in the sieve tube element causing water to diffuse

out.

Evidence for and against translocation

For Against

Radioactive labeled carbon-16 issupplied to the plant and it shows up

in the phloem

Aphids feed on the sugars in thephloem

Ringing a tree to remove the phloemresults in a build up of sugars

The companion cells have manymitochondria

Translocation can be stopped byusing a metabolic process, the process

inhibits the production of ATP

The rate of flow is so high it cant justbe diffusion alone

The PH of the companion cells arehigher than that of surrounding cells

(due to H+ ions)

The concentration of sucrose it higherin the source than in the sink

Not all the solutes in the phloem sapmove at the same rate

Sucrose is moved to all parts at thesame rate, despite concentration

The role of sieve plates is unclear

ocr as biology unit 1 revision notes

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