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Biology Year 11 Module 1

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Slide 2 Inspection Copy for school evaluationonly. Copying NOT permitted.

Cells asthe Basis

of Life1. Different

Types of Cells

c) BiochemicalControl... Enzymes

What is this topic about?To keep it as simple as possible, (K.I.S.S.

Principle) this topic covers:

1. DIFFERENT TYPES of CELLSEukaryotic & prokaryotic cells. How we know...light microscopes, electron microscopes, x-ray

crystallography, isotopic “tracers”.

2. CELL STRUCTURESMain features of plant & animal cells.

Organelles... the nucleus, mitochondria, E.R.,ribosomes, golgi body, lysosomes, chloroplasts.

Structure of membranes.

3. CELL FUNCTIONSa) STUFF GETS IN & OUT

Diffusion & osmosis. Active v. passive transport.Endocytosis & exocytosis.

Importance of the SA/Vol. ratio.

b) FOOD & ENERGY for CELLSPhotosynthesis & cellular respiration.

What cells need, and need to get rid of.

c) BIOCHEMICAL CONTROL... ENZYMESProperties & importance of enzymes.

Effects of temperature & pH on enzyme activity.

Topic Outline

2. CellStructures

3. CellFunctions

b) Food &Energy for

Cells

a) Stuff GetsIn & Out

Eukaryotic & Prokaryotic

Plant v. animal

Diffusion & osmosis

Photosynthesis

Properties of enzymes

Effects of temperature & pHon enzyme activity

Cellular respiration

What cells need

Active v. passivetransport

Endocytosis& Exocytosis

Importance ofSA/Vol. ratio

Major organelles visible with light &electron microscopes.

Membrane structure

Technologies to understand cells





Introduction Comparison: Light & Electron ‘Scopes

Light (Optical) ElectronMicroscope Microscope

How the beam of light beam of electronsimage focused by focused by magneticis formed glass lenses fields

Magnification generally about up to 10,000,000 X500 X. (5000 times moreMaximum powerful)about 2,000 X

Resolution about 0.2 μμm about 0.0002 μμm(ability to see max. (1,000 times betterfine details) detail)

micrometres (μμm) 1 μμm = 0.000001(10-6)metre.

1 micrometre is 1/1000 of a millimetre.

The Cell TheoryThe “Cell Theory” is one of the fundamental concepts in Biology. It simply states:

• All living organisms are composed of cells or are the product of cells. (e.g. viruses)

• All cells are produced from pre-existing cells.

The evidence supporting the Cell Theory has come mainly from the useof microscopes to examine living things.

Our knowledge of cell structure and function has developed as thetechnology of microscopes advanced over the last 300 years or so.

Initially only light (optical) microscopes were available, but since the1930’s, electron microscopes have revealed more detail of cell structureand function.

How Big Are Cells Anyway?Typical Plant Cell

20-100 μμm

Bacterial Cells0.1 - 5 μμm

Typical Animal Cell5 - 20 μμm

SCALE: 100 μμm(0.1 mm)


Pathologist using a “Light” (or “Optical”) Microscope to

view blood cells.

University students using a

“Scanning ElectronMicroscope” (SEM).

Photo byDaniel Schwen

(used under Creative Commons

Attribution-Share Alike2.5 Generic Licence)





keep it simple science® 1. Different Types of Cells

The first scientific observation of cells was madewith a newly-invented (and very primitive)microscope by Robert Hooke (English, 1665). He sawa lot of boxy, identical cells, but soon it was realisedthat cells occur in many sizes, shapes and types.

However, the full details of the different cell typeshad to wait about 300 years until moderntechnologies could unravel all the scientific facts.

Some of these technologies will be describedshortly, but meanwhile, what are the different celltypes?

Eukaryotic CellsFamiliar living things, including allplants & animals, are composed of cellsdescribed as “eukaryotic”. (“eu” = true, “karyo” =“kernel” (Greek). Here karyotic refers to the nucleus of acell)

All eukaryotic cells have a distinct cell nucleuscontaining thread-like structures calledchromosomes. The chromosomes hold thegenetic information in the form of DNA. Essentially, thenucleus is the “control-centre” of the cell.

As well as the nucleus, every eukaryotic cell also contains a variety ofother structures built from, or surrounded by, membranes. Collectively,these are called “organelles”. In this topic you will study some detailsof the important organelles.

Any living thing composed of eukaryotic cells may be described as a“eukaryote”. This includes all plants & animals, the fungi and a varietyof single-celled creatures such as protozoa & diatoms.

Prokaryotic Cells (“pro” = before)In contrast to a plantor animal cell, abacterial cell is verydifferent.

There is NO nucleuswith chromosomes.Certain structuresare present withinthe cell, but none aremembrane-based.

Prokaryotic cells are generally much smallerthan any eukaryotic cell for reasons that will becovered later.

In an evolutionary sense, the prokaryotes arethe more ancient & primitive, while eukaryotesare more advanced & more recent.

Archaea & EubacteriaA relatively recent discovery has complicatedthe simple division between prokaryotes &eukaryotes: it is now known that there are 2distinct types of prokaryotes.

The “Eubacteria” (true-bacteria) have beenknown for 150 years and were thought to bethe full story of prokaryotic cells. In the 1980’snew technology revealed another totallydifferent type: the “Archaea” (means “ancient”)

Archaean cells are prokaryotic, but verydifferent chemically to “normal” bacteria. Theirlineage dates back perhaps 3.5 billion years!

Hooke’smicroscope

EUKARYOTICAnimal Cellwith lots of

membrane-basedorganelles.

PROCARYOTICBacterial Cell

has no organelles boundby membranes.





keep it simple science® Technologies to Understand Cells

The Light MicroscopeOur understanding of cells and their structure & function wasinitially due entirely to the optical microscope. Here is a briefhistory:

Robert Hooke 1665Hooke is credited with being thefirst person to see cells and namethem.

Using a primitive microscope, helooked at a piece of cork (deadtree bark) and saw tiny “boxes”like the rooms and compartmentsof a gaol or monastery. (hence“cells”)

Anton vanLeeuwenhoekIn 1676, van Leeuwenhoek used a very simple microscope, but itwas equipped with an excellent lens, through which he saw livingmicro-organisms swimming around in a drop of water.

WhatHooke

saw

These are Hooke’s drawings ofwhat he saw in the cork.

Van Leeuwenhoek’ssketches of the“animalcules”

(microscopic livingthings) which he

discovered.

Over the next 150 years, microscopes improved,and it was suspected that cells were present in allliving things.

Robert Brown, 1827Brown was the first to discover structures insidecells. He discovered and described the nucleusinside plant cells.

By about 1840, the“Cell Theory” wasbecoming acceptedby most biologists,because cells wereobserved in everyorganism studied.Louis Pasteur’sdiscoveries showedthat infectiousdiseases were causedby “germs”, whichwere microscopic,cellular organisms.

Rudolf Virchow, 1859 and Walther Flemming, 1879Between them, these two German scientistsclarified the process of cell division, by which cellsproduce more cells. This established the principlethat all cells come from pre-existing cells.

Portrait of Louis Pasteur

in his laboratory





keep it simple science® Technologies to Understand Cells (cont.)

The Electron MicroscopeThe electron microscope was invented between about 1926to 1933. A number of scientists, engineers & companieswere involved. For full details you should search a reliablewebsite such as Wikipedia.

The first commercial equipment became available about1938, but because of WWII this technology did not havemuch scientific impact until the 1950’s.

Electron microscopes use beamsof electrons, focused by electric &magnetic fields, to form images atmagnifications & resolutions farsuperior to a light microscope. (see slide 3) Objects cannot beviewed by eye, but are displayed onscreens, as photos, or captured asdigital images in computers.

In the sections which follow, youwill see examples of images ofcells seen by both light microscopeand by electron ‘scope.

The electron microscope revealedcellular details which hugely increased our understandingof the structure & function of living cells.

You need to be aware that there are 2 main types ofelectron microscope. Each has its own advantages &disadvantages.

Transmission Elect. Micro. (TEM)To form a biological image with aTEM the sample has to be dried &fixed into a special resin, thensliced extremely thinly.

The electrons pass through thesample so the image is flat and2-D and shows the fine details ofthe structures within.

TEM images can achieveextremely high magifications & resolution, but thepreparation of the specimens is difficult & highly technical.

Scanning Elect. Micro. (SEM)A Scanning Electron Microscopeimage often appears 3-D and canshow amazing surface details. Thisis because the specimen has beencoated with a layer of heavy metal(eg gold) only one or 2 atoms thick.

The electron beam does not passthrough it, but is scattered from it.Computer analysis of the scatteringeffects generate an image of thesurface topography.

Any colours are artificial andcomputer-generated.

Modern Electron MicroscopePhoto by David J Morgan(used under Creative Commons

Attribution-Share Alike 2.0 Licence)

TEM imageof a singlebacterial

cell.Photo by

PeterHighton

(used under Creative

CommonsAttribution-Share Alike1.0 Generic

Licence)

SEM image of bacterial cellsbeing attacked by a humanimmune cell. Photo: NIAID(used under Creative Commons

Attribution-Share Alike 2.0 Licence)





Technologies to Understand Cells (cont.)Microscopes, both optical & electron, have allowed detailed images of cells and cell parts.

However, understanding exactly what is going inside a cell is largely a matter of molecular structures and chemical reactions.The technologies described here will give you a simple over-view of how we have discovered the functioning of cells.


X-Ray CrystallographyWe now know that all of the thousands of chemical reactions in aliving cell are dependant on, or controlled by, huge biologicalmolecules, especially the proteins & the nucleic acids (of whichDNA is the most famous). Furthermore, we know that it is theprecise 3-dimentional shape of these “macro-molecules” whichis critical to their functioning. (More on this later in this topic.)

How can we study the shape of a molecule?

Just over 100 years ago, x-rays were discovered and immediatelyscientists began using x-rays for all sorts of reasons, includingmedical imaging of broken bones, etc.

An Australianfather & son team,William &Lawrence Bragg,used x-rays toprobe thestructure ofmatter. Theybeamed x-raysthrough purecrystals &captured on filmthe patterns of the

scattered rays. They figured out how the diffraction patternsrelated to the arrangement of atoms within the crystal. They wereawarded the Nobel Prize for Physics in 1915. At the time, no-onecould predict how important this would be for Biology!

Crystalx-raybeam

X-rays diffracted by the crystallattice & form Interference

patterns which are captured onthe film.

Photographic filmsensitive to x-rays

Unravelling of DNABy the 1950’s it was known that asubstance known as DNA was thebasis of heredity. Its chemicalcomposition was known, but no-one could figure out how it couldfunction as a gene.

James Watson (USA) & FrancisCrick (UK) (and others) used x-rayscattering patterns fromcrystallised DNA to discover thenow famous double-helix shape.

Armed with the chemical analysisAND the shape, Watson & Crickwere able to develop a theory forthe functioning of DNA.

This led to understanding the“genetic code” and later to the“Human Genome Project”. Theknowledge gained is now acornerstone for modern Biology& Medicine.

Meanwhile, X-Ray Crystallographycontinues to quietly contributemore & more knowledge of theshapes of biological molecules,helping us understand how it allworks.

Part of an x-raydiffraction image of a

large protein. Mathematical analysis

of this pattern bycomputer can determine

the 3-D shape of themolecule.

Photo: Jeff Dahl(used under Creative

Commons Attribution-ShareAlike 3.0 Licence)

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Isotopic TracersWithin each microscopic living cell, thousands of chemicalreactions are constantly occurring. Many processesinvolve a sequence or chain of reactions which need tooccur in strict order, each one controlled by huge macro-molecules with a precise shape to “grab” chemicals andeither ram them together, or tear them apart, then “handthem on” to the next step.

How have we been able to unravel such complexityoccurring within a pin-point-sized bag of life? Traditional,test-tube chemical analysis does NOT get you very far.

Isotopes You should already be aware that all chemical elementsoccur in 2 or more variant forms called isotopes. Thedifference is the number of neutrons in the nucleus of eachatom. Some isotopes are unstable & may spontaneouslyemit various radiations... they are “radioactive”.

One of the best known examples concerns 2 of theisotopes of carbon:“Carbon-12” “Carbon-14”

Example of the “Tracer” MethodYou should be familiar with the overall chemistry ofphotosynthesis in plants:

carbon + water glucose + oxygendioxide

CO2 + H2O glucose + O2Now, here is a simple question about this process:

Where does the oxygen (O2) come from? Is it the oxygenoriginally in the CO2 or is it from the H2O?

If a plant is exposed to CO2 containing some atoms of adifferent isotope of oxygen, that isotope will be laterdetected entirely in the glucose.

However, if a plant is exposed to H2O containing someatoms of the different isotope of oxygen, the isotope willbe later detected entirely in the oxygen gas released fromthe plant.

Therefore, all the oxygen gas in our atmosphere (whichhas been released from photosynthesising plants) wasoriginally in water molecules. This experiment has “traced”the pathway of oxygen atoms through the process.

This is an extremely simple example of how the “tracermethod” can be used to study chemical pathways in livingcells.

More Technologies to Understand Cellskeep it simple science

®

6p+6n0 6p+

8n0

C126 C14

6

These atoms have thesame number ofelectronss so they arechemically identical andreact the same way.However, carbon-14 isradio-active and can beidentified by the radiationit emits.

This is just a “taste” of some important technologies. If interested, you might research the “Ultra-Centrifuge” (allowsparts of cells to be separated), Gas Chromatography (allows separation & identification of the dozens of chemicals in a

chain of reactions) and Automatic Sequencing equipment to study DNA and/or proteins.

Discusssion / Activity 1The following activity might be for class discussion, or there may be paper copies for you to complete.

If studying independently, please use these questions to check your comprehension before moving on.

Cell Types & Technologies Student Name .................................1. a) Outline the major differences between eukaryotic & prokaryotic cells.

b) For each named living thing, identify it as either eukaryote or prokaryote. (E or P)

palm tree ........... mouse ........... anthrax bacteria ............ mushroom .........

2. What accident of history led to our use of the word “cell” for these “units of life”?

3. a) In general terms, (no numbers required) how does the magnification & resolution of an electronmicroscope compare to that of an optical (light) microscope?

b) What is the meaning of the 2 words underlined in part (a)?

c) There are 2 types of electron microscope. Name them, and outline the differences in terms of thepathway of electrons and how you would recognise an image formed from each type.

4. a) Outline what x-ray crystallography is, and what it can tell us about cell structure or function.

b) Outline what “isotopic tracing” is, and what it can tell us about cell structure or function.


Inspection Copy for school evaluationonly. Copying NOT permitted.







Examining CellsThe syllabus requires that you examine a variety of cells; prokaryotic and eukaryotic. Hopefully, you will do some prac.work using a microscope toexamine fresh, living cells as well as prepared slides of eukaryotic cells. With a typical school light microscope you might not be able to examineprokaryotes at all. You will probably study TEM & SEM images to become familiar with prokaryotic cells.


You will probably learn how to use a microscope, look at some cellsthrough it and sketch them. You probably will NOT view bacteria (too

small), but might see the following examples.Try to identify all the visible cell parts that you see and label them.

cytoplasmnucleus

cellmembrane

Paramecium (unicellular organism)

magnified 100X

cytoplasmnucleus

cellmembrane

Human Cheek Cellsmagnified 400X

cytoplasmnucleuscell wall

Onion Skinmagnified 100X

Human Bloodmagnified 400X

Learn to sketch inside a circle which represents the “field ofview” of the microscope.

Sketch only a few of the cells, to scale.

Always label your sketches

Even at maximummagnification you willprobably not see any

detail

Sketching Cells Through the Microscope Photos Taken Through a Microscope

Cross-Section of part of a Plant Stem.Colours are due to staining.

SEM photo. Colours arecomputer enhanced.

The red cells are bacteriainfecting human tissue.

These roundcells are in human

blood. The rod-shaped cells arebacteria which cause a disease

called Anthrax.Colours are caused by using a dye

to stain cells for easier viewing.

A simple water plant which growsin hair-like filaments.

Low magnification, natural colour.

Lots more imagesin the following

section

2. Cell Structures School Inspection only.Copying NOT permitted.




keep it simple science® Cell Organelles Visible with a Light Microscope

Generalised ANIMAL CELL

SmallVacuoles

(if any at all)

GeneralisedPLANT CELL

There are probably no actual cellswhich looks just like these. Real shapes vary greatly.

CellMembrane

Nucleus

Cytoplasm

Cell Wall on the outside of the

cell membrane

Chloroplasts which absorb lightand make food for

the plant

LargeVACUOLE

Differences Between Plant & Animal CellsPlant cells have a tough CELL WALL on the outside of their cell

membrane. Animal cells never have a cell wall.

Many plant cells contain a large VACUOLE. Animal cells rarely havevacuoles, and if present they are small.

Many plant cells contain CHLOROPLASTS. These are green in colourbecause they contain the pigment chlorophyll. Chloroplasts are the sites

of PHOTOSYNTHESIS, where plants make food.

Note: not all plant cells have chloroplasts... for example, cells in the underground roots cannot photosynthesise,

so do not contain any chloroplasts.The Electron Microscope reveals much more

detail than this... next slide.

Photo (through a microscope) of amass of plant cells. The dark blobs

are vacuoles of stored food. What else can you identify?





What the Electron Microscope RevealsThe superior magnifying power and resolution of the electron microscope has given us a much more

detailed knowledge of the cell and its organelles. The diagram below left shows a plant cell with the added details that the electron microscope has revealed.

The extra organelles (labelled in blue) shown are generally NOT visible with a light microscope.

Chloroplast internal structure

Stacks of flatmembranes (grana)

contain the chlorophyll.

Mitochondrion.Site of cellular

respiration.

Lysosome

Golgi apparatus

Vacuole Cell Wall Cell Membrane

NucleusExtra detailrevealed.

The tiny Ribosomes

are oftenattached

to the E.R.

Endoplasmic Reticulum(E.R.)

A network of membranestructures connected to the

nucleus & extendingthroughout the cytoplasm.

Nucleus

Nucleolus

Cellmembrane

Golgi

Mitochondria

Lysosomes

Electron Microscope (TEM)view of an animal cell

2 μμm

Photo by Itayba(used under Creative Commons Attribution-Share Alike3.0 Unported Licence)






keep it simple science® Cell Organelles... Structure & Function

Note that the organelles detailed in the following slides are typically found ONLY in eukaryotic cells. In fact, the presence of these structures defines a eukaryote.

Nuclear material“chromatin”.

(Chromosomes unwoundand spread out)

The Nucleus This is the control centre of the cell.

Inside the nucleus are the chromosomes containing DNA,the genetic material. There is often a nucleolus present.This is the site for production of RNA, a “messenger”chemical which leaves the nucleus carrying instructions toother organelles. The nuclear membrane has holes or“pores” to allow RNA to exit.

This structure helps the organelle do its job more efficiently.

Mitochondria (singular: mitochondrion)

This is where cellular respiration occurs

Glucose + Oxygen Carbon + Water + ATP(sugar) Dioxide

The ATP produced by respiration carries chemical energyall over the cell to power all the processes of life. Themitochondria are therefore, the “power stations” of the cell,converting the energy of food into the readily usable formof ATP.

Inside a mitochondrion is a folded membrane with manyprojections (“cristae”). This structure provides a greatersurface area, where the enzymes (control chemicals) forrespiration are attached in correct sequence for the stepsof the process.

Inner membranefolded into “cristae”

with respirationenzymes attached.

Sketch of aMitochondrion

Image of actualMITOCHONDRIA using an

Electron Microscope (TEM)

Nuclear membranewith pores, for RNA exit

Nucleolusfor RNA

manufacture





keep it simple science® More Cell Organelles

The Golgi Apparatus is a semi-circulararrangement of membranes which are concerned withpackaging chemicals into small membrane sacs(“vesicles”) for storage or secretion.

One type of “vesicle” producedby a Golgi Body is theLysosome. These membranesacs contain digestive enzymeswhich can destroy any foreignproteins which enter the cell.

Lysosome enzymes alsorapidly digest the contents of acell which has died, so thatyour body can clean up the remains and replace the deadcell.

GOLGI BODY Curvedmembrane sacs

Vesicles pinch-off forstorage or secretion

Lysosomes formthis way

Endoplasmic Reticulum (E.R.)E.R. is a network of membranes which form channels andcompartments throughout the cytoplasm of the cell. Its function can becompared to the internal walls of an office building which divide thebuilding into “rooms” where different operations can be kept separateso that each does not interfere with others.

The E.R. structure provides channels for chemicals and“messengers” to travel accurately to the correct locations, andfor chemical production to occur in isolation from otheroperations. This structure helps cells function

Often found attached to the E.R. are the tiny Ribosomes.These are the sites of production of proteins, the mainstructural and functional chemicals of living cells. RNA“messengers” from the nucleus attach to a ribosome to makethe specific proteins that the cell needs.

ENDOPLASMICRETICULUM

RIBOSOMESattached to membranes

Membranes

Membranes enclosechannels and “rooms”

Nucleus

Mitochondrion

E.R.membranescoated withribosomes






Chloroplasts Chloroplasts are found only in

photosynthetic plant cells. The electron microscope has

revealed that the chloroplast is notjust a bag of chlorophyll, but has an

organised internal structure which makes its functioning

more efficient.

CHLOROPLAST

Doublemembraneenvelope

Membrane stacks(“grana”)

containing chlorophyll

“Stroma”zone

Yet More Cell Organelles

The “grana” are stacked membrane sacs containing chlorophyll, which absorbs the light energy for photosynthesis. Thislight-capturing step is kept separate from the “stroma” zone, where the chemical reactions to make food are completed.

The Importance of MembranesExcept for the tiny ribosomes, all the cell organelles are built from, and surrounded by, membranes.

The membranes provide:-

• the infrastructure of the cell.• channels for chemicals to move through.• packaging for chemicals which need to be stored.• points of attachment for chemicals (“enzymes”).• control over what moves in or out of each organelle, and in or out of the entire cell.

The “membrane-bound” organelles help the cell’s variousfunctions to be carried out with greater efficiency.

Having these membrane-based organelles is the definingcharacteristic of the “Eukaryotic” group of organisms, whichincludes all plants & animals.

Prokaryotic cells (such as bacteria) do have lots of tinystructures inside, but do NOT have any membrane-typeorganelles, and can only operate efficiently by being very small.

TEM Photo by and3k & caper437(used under Creative Commons Attribution-Share

Alike3.0 Unported Licence)

1 μμm

Grana

Stroma






Chemicals in CellsINORGANIC CHEMICALS

These include small simple molecules like water (H2O)and carbon dioxide (CO2), as well as mineral ions such

as calcium, nitrate, phosphate, chloride, etc.

Although these are often considered of lesserimportance, you should remember that all living things

are 75%- 95% water.

The Chemicals That Cells Are Made From

ORGANIC CHEMICALS“Organic” chemicals are based on the element carbon,

which can form chains, rings and networks and so buildthe very complex molecules needed to make a living cell.

Many are “polymers” made by joining together manysmaller molecules to form huge “macro-molecules”.

There are four main categories to know about... Next slide.

Carbohydrates

Proteins

Nucleic Acids

Lipids

Although not specified by the syllabus, it will help greatly if you have some basic knowledge about...






CARBOHYDRATESinclude the sugars, starch and others.

monosaccharides (mono = one, saccharide = sugar)are simple sugars such as glucose C6H12O6

disaccharides (di = two)are sugars made from TWO monosaccharides joined together,such as “table sugar” (sucrose).

polysaccharides (poly = many)are huge molecules made from thousands of sugar moleculesjoined in chains or networks.Examples are:Starch... made by plants, to store excess sugar.Glycogen... made by animals, to store sugar.Cellulose... made by plants as a structural chemical.

The CELL WALL of a plant cell is made from cellulose.

Uses of CarbohydratesSugars are energy chemicals. Glucose is made by plants inphotosynthesis, and is the “fuel” for cellular respiration to make ATPto power all cells.

Starch & Glycogen are polymer molecules used to store sugars as afood reserve. Starch is the main nutrient chemical in the plant foodswe eat.

Cellulose & Lignin are polymers of sugar used by plants structurally.Cellulose makes the tough cell wall of all plant cells. Lignin is a strongmaterial used to reinforce the walls of “veins” in plants.

Polysaccharide.Small part of aStarch molecule

Monosaccharidesugar

molecules

Disaccharidesugar

PROTEINSare the main structural chemicals of organelles, cells,

bone, skin & hair. Life is built from protein.

Proteins are polymers, made from amino acid moleculesjoined in chains.

Amino acidmolecules

Part of a protein molecule...a chain of amino acids

LIPIDS are the fats and oils.All cell membranes are built from lipid & protein.

Lipids are used as a way to store excess energy.Carbohydrates can be converted to fat for storage.

NUCLEIC ACIDS (DNA & RNA)are the most complex of all. DNA is the genetic information ofevery cell. RNA is the “messenger”sent out from the nucleus to control allcell activities.

DNA is a huge polymer of sugars,phosphate and “bases” coiled in adouble helix shape.

ORGANIC CHEMICALS





keep it simple science® The Structure of the Cell Membrane

The electron microscope and other modern analysis methods have revealed the structure of themembranes which surround a cell and form most of the cell organelles.

The membrane is extremely thin; just two molecules thick. The basicchemical unit is a “phospholipid” molecule; a lipid (fat) with phosphategroups attached. Each molecule has two distinct ends; one which isattracted to water molecules (“hydrophilic”) and the other is repelledby water (“hydrophobic”).“Hydro” = water. “philic” = to like. “phobic” = hate / fear.

Two layers of phospholipids form each membrane. The molecules clingto each other, and line up with their hydrophilic ends outwards. Thewater-loving ends are attracted to the watery environment both insideand outside the cell.

The hydrophobic ends are repelled from the watery surroundings, andcling together inside the membrane itself. A membrane is like a thinlayer of oil floating on water. It is fluid and flexible, but clings togetherforming an unbroken “skin” on the surface of a cell.

The membrane is NOT solid: it is in fact a liquid or “fluid” structure. Itis held together by the mutual attractions of the phospholipidmolecules. At the microscopic level, these attractive forces are strongenough for the fluid layer to form a barrier between the inside &outside of the cell.

Other molecules are embedded in the phospholipid bilayer. They aremostly proteins, many with carbohydrates attached.

These other molecules have various functions:

• “receptors” for messenger chemicals.• identification markers, so your body knows its own cells from any

foreign invaders.• to help chemicals get through the membrane.

This concept of the membrane is called the “Fluid-Mosaic Model”:this refers to a liquid structure and the different molecules embeddedwithin it are like the different shapes & colours in a “mosaic” tilepattern.

Membrane proteins

Onephospholipid

-philic

-phobic

MEMBRANE STRUCTUREOutside of cell

Inside of cellDouble layer of

phospholipid molecules

Nucleus

Golgi

Electron Microscope(TEM)

view of part of a cell.

The double-layerstructure of the cellmembrane is clearly

visible.





Modelling the Cell MembraneA simple model may help you understand the cell membrane a little better.

You might make a more sophisticated model in class. If not, you can easily do this at home.You need about 100 “cotton buds”, plastic drinking straw, sticky tape, rubber band.


Step 1Line up about a dozen cotton buds in aneat row. Use sticky tape to hold themtogether in a flat “panel”.

Repeat, until you have at least 5-6“panels” of cotton buds.

How the Model Relates to a Cell Membrane1. Chemical StructureEach cotton bud represents two phospholipid molecules joined tail-to-tail. In the top photo a texta line has been drawn to emphasise this.

The cotton wool represents the hydrophilic “head” of the molecule. Theshaft represents the two hydrophobic tails clinging together.

2. FlexibilityIf you gently manipulate your model, you can see that the entirestructure is flexible. Real membranes are thought to be even moreflexible and in fact are a liquid structure: the phospholipid moleculescling together, but the wall of molecules can warp & bend withoutrupturing.

3. Acts as a Barrier, but Some Things Can Get ThroughStand your model upright on the bench. Now gently sprinkle a fewgrains of rice (or similar) onto the cotton wool heads. Notice that therice sits on top & cannot penetrate your “membrane”.

Now sprinkle a few grains above the drinking straws.Your membrane can also let things through!(Be aware that this is a very simplistic model of what really happens!)

Step 2Stack your panels neatly on top of eachother so the model becomes more & more3-dimensional. Between two of the middlepanels, place one or two cut pieces ofplastic drinking straw.

Step 3Gently wrap arubber band loosely around your stack ofpanels. This holds the entire model togetherso it can be placed upright & gentlymanipulated. Be gentle & careful or else thecotton buds will tend to begin pointing in alldirections & lose their nice parallel pattern.

Outsideof cell

Intsideof cell

Schematicdiagram of a cell

membrane,according to the

“Fluid MosaicModel”.

Image byLady of Hats

(Mariana Ruiz)

Mem

bran

e




Cell Structure Student Name .................................

1. Name the parts labelled a,b,c,etc. in this plant cell.All are visible with a light ‘scope.

2. Match the lists. (connect matching items with an arrow)

Cell Organelle Function or DescriptionMitochondria Makes proteinsEndoplasmic Reticulum Makes food using light energyNucleus Control centre of cellGolgi Apparatus Cellular respiration siteRibosome Packaging of chemicalsChloroplast Network of membranes, internal compartments

3. Underline any organelle in Q2 which is usually only visible using an electron microscope.

4. a) The basic chemical unit in a membrane is a “phospholipid”. What is this?

b) In what important way are the 2 ends of each molecule different? (In your answer use “hydrophilic” & “hydrophobic”, and define these words)

c) The structure of the cell membrane is described by the “Fluid-Mosaic Model”.In what way is it “fluid”?

In what way is it “mosaic”?





a

b

c

de

f





3. Cell Functionsa) How Stuff Gets In & Out

How Chemicals Pass Through MembranesThe cell membrane as the boundary of a cell is a bit like growing a plant hedge as the boundary of a field. It stops the cows and horses getting out, but a mouse, or a lizard, can easily crawl through it.

Similarly, a membrane is “semi-permeable”; it prevents most (especially large) molecules getting through, butallows others to pass through easily. Small molecules like water (H2O), oxygen (O2) and carbon dioxide(CO2) pass freely through the membrane like a lizard through a hedge.

To understand how this happens, you must learn about the processes of DIFFUSION & OSMOSIS.


DiffusionDiffusion occurs in every liquid or gas because the atomsand molecules are constantly moving. The particles“jiggle” about at random in what is called “Brownianmotion”. (Named for its discoverer Robert Brown, the sameman who discovered the cell nucleus.)

Imagine a water solution containing a dissolved chemical,but it is NOT evenly distributed... it is more concentrated inone place than elsewhere. As the molecules jiggle about atrandom, they will automatically spread out to make theconcentration even out. This process is called DIFFUSION.

To startwith, thedissolvedmaterial isnot evenlydistributed. Diffusion

causes thedissolvedsolute to

spread outuniformly.

Highconcentration

Lowerconcentration

Equal concentrationthroughout

Later

In a living cell, there is often a “concentration gradient”from the outside to the inside of the cell.

For example, because a cell keeps consuming oxygen forcellular respiration, the inside of the cell usually has a lowconcentration of O2 dissolved in the water of the cytoplasm.On the outside, there may be a lot of O2.

DIFFUSION DRIVES MOLECULES THROUGH THEMEMBRANES along the concentration gradient.

DIFFUSION of SMALL MOLECULES into a CELLIf the molecules can cross the membrane, diffusion will cause them to

move from higher to lower concentration.

Higherconcentration

outside cell

Lowerconcentration

inside

Many chemical substancesconstantly move in and out of

a living cell.





keep it simple science® Osmosis

Osmosis is a special case of diffusion which occurs when the concentration gradient involvesdissolved molecules or ions which CANNOT get through the membrane.

The opposite situation can also happen. A cell’s cytoplasmcontains many dissolved chemicals. If the outsideenvironment around the cell is more watery (lessconcentrated in dissolved substances) then osmosis willcause water to diffuse inwards.

This can cause cells to “pump up” with water and helpsmaintain their shape. It can also cause problems fororganisms living in fresh water environments.

For example, consider a cell which is surrounded by asolution containing a lot of dissolved sugar. The sugar cannotdiffuse through the membrane to equalise the concentrations.In such a situation, water (which can go through themembrane) will diffuse toward the high sugar concentration,as if attempting to equalise by diluting the sugar.

In this case, the cell will lose water and might shrink andshrivel up.

Loss of water by osmosis can be a problem for livingthings in water environments with high levels of dissolvedchemicals such as salt.

Sugar cannot getin through the

membrane

OSMOSISWater diffuses

OUT of cell

H2OH2O

H2O Dissolved chemicalscannot diffuse out...

...so water diffuses intothe cell.

This is how plantsabsorb water into theirroots, even when the

soil seems almost dry.

H2O

H2OH2O

Highconcentration

of sugaroutside cell





keep it simple science® Observing Diffusion & Osmosis

DiffusionYou might do one of these activities yourself, or see it demonstrated.

OsmosisYour teacher may have a more sophisticated experimentfor you to do. If not, this is a simple activity you could doin class or even at home.

Cut 2 pieces of fresh celery from the same stick. Trimthem to be exactly the same length.

If possible, pat dry with a tissue, then weigh each to thenearest 0.1g and record.

Place each into a small beaker of water or salt solution, asshown and leave overnight.Fluids (liquids and gases) seem to be able to mix

themselves together automatically... “Diffusion”.

The explanation is in the Moving-Particle Model ofmatter. In liquids and gases, the particles are moving

around. If 2 different gases or liquids are side-by-side,then the moving particles will automatically mix. Any dissolved molecules will spread out evenly.

Is diffusion faster in liquid or gas?What effect would temperature have?

Next day, pat each piece dry with a tissue and re-weigh.One piece of celery may have lost a small amount of mass.

Compare their lengths. They might not be exactly the same any more.

Bend or cut each piece and note the texture and“crispness”. One will be hard and crisp, the other softer

and “rubbery”.

Try to explain these results on the basis of movement ofwater (NOT salt!!) in/out of the living celery cells.

Gas Jar of air

glassseparator

When the separator is

removed, thetwo gases mix

themselvestogether.Gas Jar of

brown gas

The food colourspreads out

through the waterby itself.

Without anystirring, it auto-

mixes through thewater.

one dropof foodcolourdye

Water

Identical pieces ofcelery soaked indifferent liquids

for 24 hours.

One loses waterdue to osmosis.

PureWater

conc.Saltsoln.





keep it simple science® Other Ways Substances Get Through Membranes

Passive & Active TransportDiffusion and Osmosis are vitally important for manychemicals (especially water) to get in and out of cells.Diffusion and osmosis happen automatically and withoutthe cell having to use any energy. We say these are“passive transport” processes.

What about all the other important chemicals which cannotget through the membrane? Many proteins, carbohydratesand other molecules regularly move into or out of cells.How do they get in or out?

Cells have other ways to deliberately move substancesacross the membrane apart from diffusion and osmosis.The membrane contains special protein channels &mechanisms which can “carry” chemicals through themembrane.

These ways to transport materials across membranesrequire the cell to use energy (ATP from cellularrespiration) to move substances. We say these are “activetransport” processes.

Not only can “active transport” move substances whichcannot normally penetrate the membrane, but it can evendo so against the concentration gradient.

An analogy to this might help: a passive process, such asdiffusion, is like water running downhill in a pipe. Ithappens naturally without any energy expenditure.

However, active transport is like using a pump to forcewater uphill through the pipe. Energy will be required torun the pump to push water against gravity.

Some Active Transport MechanismsSodium-Potassium PumpOne notable example of an active transport mechanism is the “Na-K pump” whichis present in every animal cell.

Background info: Animal cells are the only cell type with NO cell wall. This means that ifthey swell up tightly with water (become “turgid”) they are in danger of bursting open. Incontrast, a plant cell has a tough, rigid cell wall. If a plant cell becomes turgid there is littledanger of bursting... plants habitually keep their cells turgid to support their leaves, etc.

The Na-K pump is like an air-lock system with 2 doors, but only one door can everbe open at any one time. By oscillating between these 2 “doors” the mechanismactively pumps sodium ions (Na+) OUT of the cell and potassium ions (K+) INTOthe cell. This allows the cell to maintain an “osmotic pressure” which prevents itabsorbing excess water and bursting.

Note how the “2-door system” requires ATP (the energy chemical made in themitrochondria) to power it... it is “active” transport.As well as maintaining osmotic pressure in every animal cell, the Na-K pump isessential for the sending of nerve signals, for kidney function & many other bodyprocesses.

Outside of Cell

Inside of Cell

Con

cent

ratio

nm

aint

aine

d

Schematic of the Na-K PumpImage by Lady of Hats (Mariana Ruiz)

As well as the Na-K pump, there are other “channel-based pumps” which move specific chemicals through the membrane.





keep it simple science® Endocytosis (“endo” = into / inside, “cyto” = cell, “-sis” implies a process)

Another, quite differentmechanism involves anumber of relatedprocesses collectivelyknown as “endocytosis”.

To keep it as simple aspossible (KISS Principle)this involves themembrane pinchingoutwards to surround thedesired substance andenvelop it.

The membrane thenrejoins to itself to sealthe cell, leaving the targetsubstance inside, sealedin a small vacuole or“vesicle”.

Phagocytosis(“Phago” = eating, “cyto” = cell)is a version of endocytosis which takessolid particles into a cell. The best knownoccurrence involves a type of white bloodcell called a “phagocyte” (literally an “eatingcell”) which absorbs infectious germs, deadcells & fragments. Once inside thephagocyte the encapsulated solids aredestroyed & digested by a cocktail ofenzymes.

This is also the mechanism which manysingle-cell organism eat food particles.

Image byLady of Hats

Pinocytosis (“Pino” = drink)is a version of endocytosis by which cells take in a small parcel of fluid, including anydissolved chemicals.

Receptor-Mediated Endocytosisis another variation which can target specific chemicals which the cell needs to absorb, such ashormones or specific types of protein. The “targetting” is achieved by receptor moleculesembedded on the outer surface of the membrane in a shallow “coated pit”. A receptor canrecognise (by shape) the target molecules & “lock-on” by forming a loose chemical bond. Oncethe receptors are “loaded”, the membrane is stimulated to encapsulate the pit into a vesicletaken inside the cell.

Later, the vesicle membrane is dissolved to release the absorbed chemical, possibly after beingtransported to the appropriate cell location which needs the target substance.





keep it simple science® Exocytosis (“exo” = outside)

As well as moving substances into the cell, there are many substances which need to be moved out of the cell.

“Secretion” refers to chemicals being released by a cell for some useful purpose. For example, the cells of your salivary glands secretesaliva into the food while you chew it. This moistens the food for easier swallowing, but also begins digesting the food with a digestive enzyme inthe saliva. Nerve cells secrete a “neurotransmitter” chemical across the nerve synapse to make the nerve signal carry on into the next neuron cell.

Chemicals destined for secretion are often packaged inside small vacuoles by the golgi body organelles. The actual secretion process occursrather like endocytosis running in reverse. (However, in full technical detail there are significant differences.)

“Excretion” refers to the removal of unwanted, possibly toxic, waste materials. These may be encapsulated in small vesicles to protect theinside of the cell from possibly dangerous substances.

The actual removal of these wastes follows the same pathway as for secretion... the process of exocytosis.

Both Endocytosis & Exocytosis, in all their variations, require the cell to use energy...they are ACTIVE TRANSPORT processes.

The energy is supplied in the form of ATP, manufactured in the mitochondria by cellular respiration.






Length ofone side= 1 unit

Length ofone side= 2 units

Length ofone side= 3 units

Importance of the Surface Area to Volume RatioWhy are cells so small? The answer requires a mathematical study...

Lengthof oneside

= 4 units

Consider this series of cubes of increasing size:

Surface Area:Six squares, each 1x1SA = 6 sq.units

Volume = 1x1x1 = 1 cu.unit

SA = 6vol



SA = 3vol



SA = 2vol



SA = 1.5vol

What’s this got to do with cells?The amount of food, oxygen or other substances a cell needs depends on its volume... the bigger the cell, the more it

needs according to its volume. But, all cells have to get whatever they need in through their cell membrane, and the size of the membrane is all about surface area.

As any cell gets bigger, it becomes more and more difficult for it to get enough food, water and oxygen because itsSA/Vol. ratio keeps shrinking. Getting rid of waste products also becomes more difficult.

Large cells are impossible... all single-celled organisms are microscopic, and all larger organisms are multi-cellular. The only way to be big is to have lots of small cells.

Notice that as the cubes get larger: • Surface Area increases, and... • Volume increases, but...• SA / Vol Ratio DECREASES, because the volume grows faster than the surface area.

This pattern is the same for any shape... as any object gets bigger, the ratio between its Surface Area and its Volume gets smaller.

Cells must feed their Volume, through their Surface Area

What is true for cells, is also true for membrane-based organelles. It is better to have many, small mitochondria rather than a few larger ones. A larger mitochondrion has a lower SA/Vol. ratio. It will be less efficient at absorbing glucose & oxygen & getting rid of wastes,

and exporting ATP to where it is needed. In a cell which uses a lot of energy (eg muscle cell) it is always found that there are a multitude of small mitochondria, never just a few very large ones.






Why are Prokaryotes so Small?Now we will try to answer a question which arose near the very beginning of this topic.Typically, a prokaryotic cell (eg bacterium) is only about 1/10 the size of an average animal cell, or even less inmany cases. In this photo we can only see a small part of the human cell... it is 100 times larger than the 2bacterial cells.

There are 2 things to consider to understand why prokaryotes are so small:

OrganellesThe presence of membrane-based organelles in eukaryotes makes all their functions much more efficient.Organelles allow the cell to carry out specialist functions in an enclosed space with all the control chemicals(enzymes) in place. The chemicals involved in a process are concentrated together where needed and othercellular processes cannot interfere with whatever the organelle is doing.

SA / Vol. RatioWithout any membrane-based organelles, a prokaryotic cell is inherently far less efficient. The only way it canthrive is to be as efficient as possible by having a high SA/Vol. ratio. This can only be achieved by being verysmall. Therefore, all prokaryotic cells are relatively small.

Get it?

SEM image of bacterial cells beingattacked by a human immune cell.

Photo: NIAID (used under Creative Commons Attribution-Share Alike 2.0 Licence)

How Stuff Gets In & Out... a SummaryWhen you think about substances movingthrough a membrane, there are 3 factors to beconsidered:

SA / Vol. RatioThis ratio basically determines whether a cell (ororganelle) is able to transport enough materials in& out across the membrane to meet its needs. Younow know that the smaller the cell is, the higherthe ratio, so the more likely it is to achievesufficient supply of nutrients and removal ofwastes.

Be aware that this is NOT entirely about size...shape matters as well. Elongated, irregular shapeswith lots of folds & projections have higher ratiosthan compact, regular shapes like a sphere.

Concentration GradientFor substances which can cross a membrane by passive transport (diffusion & osmosis ofwater) the difference in concentration of the substance inside the cell compared to itsconcentration on the outside is another important factor.

The bigger this difference, or “concentration gradient”, the faster will be the rate of diffusion.

The Nature of the SubstanceFinally, the nature of the chemical substance itself can have a big effect. For example, think about oxygen, a small molecule which can pass through the membraneeasily. If there is a large concentration gradient, its rate of diffusion through the membranecan be so fast that this can partly compensate for a poor SA/Vol ratio.

Conversely, dissolved ions or large proteins must rely on active transport to cross themembrane. In this case, not only is the SA/Vol ratio involved, but also the rate at which thecell can supply energy to drive the “pump”, or endocytosis cycle, or whatever activeprocess is involved.




Stuff Gets In & Out Student Name .................................1. Membranes are described as “semi-permeable”. What does this mean? Give examples.

2. Explain the difference between Diffusion & Osmosis using the phrase “concentration gradient”.

3.a) What is the difference between “active” & “passive” transport?

b) To which category do diffusion & osmosis belong?

c) In general terms, what are “endocytosis” & “exocytosis”? How are they different?

4. As any shape gets larger, what happens to its:a) surface area? b) volume? c) SA/Vol ratio?

b) How does this relate to living cells and why they are always microscopically small?









keep it simple science® 3. Cell Functions

b) Food & Energy for CellsAutotrophs & Heterotrophs (auto = “self”, hetero = “other, not self”, troph = “to eat, feeding”)An autotroph is an organism that makes its own food. All plants are autotrophic, making their own food by photosynthesis.

Any organism that cannot make its own food must be a heterotroph. All animals are heterotrophic, and so are the fungiand most bacteria. A heterotrophic animal eats plants or other animals which have eaten plants,

and so on according to the food chain involved.

Photosynthesis in PlantsAll plants make their own food from the simple, low-energyraw materials water (H2O) and carbon dioxide (CO2) usingthe energy of sunlight, to make the high-energy sugarglucose (C6H12O6), with oxygen gas (O2) as a by-product.

light

Phase 1In the grana,

chlorophyll absorbs light energy and uses

it to split water molecules intohydrogen and oxygen.

The oxygen is released.

PHOTOSYNTHESIS in the CHLOROPLAST

Phase 2In the stroma,

a cycle ofreactions

buildsglucose from

CO2 and thehydrogen from

the water.

Summarising photosynthesis with this brief equation isvery deceptive. Photosynthesis actually occurs as acomplex series of chemical steps inside the chloroplast.

There are 2 main stages, which take place in different partsof the chloroplast, as summarised below.

WATER + CARBON GLUCOSE + OXYGENDIOXIDE

chlorophyll

light energy

The energy of light isabsorbed by chlorophyll, the green pigment in the

leaves of plants.

high-energysugar (food)

fromsoil

fromair

releasedto air

Water & CO2are low-energy

chemicals

6H2O + 6CO2 C6H12O6 + 6O2

The energy of the lightis being stored as

chemical energy in theglucose molecules






How Organisms Use EnergyEverything that an organism does requires energy.

Organisms:-MoveGrow new cells & Repair body tissueReproduceSeek, Eat and Assimilate their foodRespond to happenings around themKeep their bodies warm

Cellular Respirationis the process which releases the energy stored in food. It takes place in every living cell on the planet and after photosynthesisis the next most important biological process on Earth.

Although the process can be written as a simple chemical reaction,this is very deceptive. Cellular respiration actually takes place as asequence of about 50 chemical steps... this equation is merely asummary of the overall process.

Don’t forget that the essential product of respiration is the energy-carrier “ATP”. The CO2 and H2O are merely waste products to berecycled in the ecosystem like all chemicals. Each 1 molecule ofglucose results in the production of up to 38 molecules of ATP.

A common misconception is that plants do PHOTOSYNTHESIS andmake food, while animals do RESPIRATION to use the food.

It’s true that plants do photosynthesis and make (virtually) all thefood on Earth, but respiration is carried out by all living things...animals AND plants.

Luckily for us animals, the plants carry out enough photosynthesisto feed themselves AND produce a surplus to feed us as well.

C6H12O6 + 6O2 6CO2 + 6H2O

energy transfer ATPADP+P

Cellular Respiration

More About ATPATP stands for “adenosine tri-phosphate”. The molecule can be represented by this simple diagram:

The bond holding the 3rd phosphategroup contains a lot of chemicalenergy.

ATP will readily transfer the 3rd phosphate group to otherchemicals (with help from an enzyme). When this occurs, energy is transferred which can force other reactions to go.

P P PAdenosine 3 phosphate groups

High-energy bond

The molecule now has only2 phosphate groups, so it iscalled “ADP”.

ATP is the “energy currency” of a cell. It can transferenergy to power any process. Then, the ADP goes backto a mitochondrion and is “re-charged” when energyfrom glucose (via cellular respiration) is used to joinanother phosphate group on to make ATP again.

P P PADP=adenosine di-phosphate

Energy transfer when P-group is detached

Glucose + Oxygen Carbon + Water (sugar) Dioxide

ATPThe process transfers energy to

Major energycompound in

foods

in air Waste products

Energy-carryingchemical used inall cells to power

life processes.






As you have learned previously, in all ecosystems there is a constantinput and flow of energy via the food chains, while the chemicalssuch as H2O, O2, and CO2 simply get re-cycled over and over.

The Most Important Process on EarthPhotosynthesis makes virtually all the food on Earth, for all living things.It also makes all the oxygen in the atmosphere for us animals to breathe.

For these two reasons, photosynthesis has to be considered themost important biological process on the planet.

Photosynthesis & Cellular Respiration

What is really happening is ENERGY FLOW through the food chains ofan ecosystem. Photosynthesis captures the energy of light and storesit in a high energy food compound like glucose. Cellular respirationreleases that stored energy in the form of ATP which can power allcellular and life activities... growing, moving, keeping warm etc.

Light energy

MITOCHONDRIA - site ofcellular respiration

GLUCOSE+

OXYGEN

ATP

CHLOROPLAST -site of

photosynthesis

CARBONDIOXIDE

+WATER

You will have noticed that these two vital processes, when written as summary equations, are exact opposites.This is really not true because the precise chemical pathway of one process is NOT the opposite of the other.

They both follow complex, multi-stage, quite different pathways.





keep it simple science® What Happens to Glucose in a Plant?

If photosynthesis only makes glucose, where do all the other biological chemicals in a plant come from?

Glucose is a monosaccharide sugar, a member of the carbohydrategroup. It is easy for a plant to convert glucose into other types ofcarbohydrate.

GLUCOSEmolecules

joined in pairs

joined in 1000’s

(polymerisation)

Other sugars, such as sucrose

CELLULOSEfor building new cell walls

STARCHfor storage of food

In fact, plants convert glucose to STARCH so rapidly that the cells ina plant leaf become packed with starch grains when it is

photosynthesising.

THIS IS THE BASIS OF EXPERIMENTS YOU MAY HAVE DONE (See next slide)

Glucose can also be converted chemically into lipids... fats and oils,since they contain exactly the same chemical elements (carbon,hydrogen & oxygen only - CHO).

GLUCOSE LIPIDS (oils)

Making proteins and nucleic acids is more difficult, since thesecontain additional chemical elements, especially nitrogen, phosphorusand sulfur.

This is where the “minerals” such as nitrate, phosphate and sulfatecome in. Soil minerals are often called “plant nutrients”, and agardener may say he/she is “feeding” the plants when applyingfertiliser, but these minerals are NOT food.

They are the essential ingredients needed so plants can make proteinsand DNA etc, from the real food... glucose.

GLUCOSE Polymerisation

Aminoacids PROTEIN

Aminoacids

chemicalconversion

Soil mineralsnitrate, sulfate, etc





Experiments with PhotosynthesisThe classic experiment you have probably done, is to partly cover a leaf with light-proof aluminium foil,

and then expose it to light for several days. The aim is to prove that light is necessary for photosynthesis.

Ligh

t

Iodine test showslots of starch here

No light,no starch

After several days, the leaf is decolourised (so the test canbe seen more easily) and then tested with IODINE solution.

Why Iodine? It detects STARCH, not glucose.

As explained before, the glucose produced byphotosynthesis is immediately converted to starch. Theiodine test is used because it is the test for starch.

Sure enough, you probably found that any part of the leafexposed to light turned black when soaked in iodine,while parts under the foil did not go black.

This shows that any part of a leaf allowed tophotosynthesise will build up a store of starch from theglucose it makes. The first product of photosynthesis isglucose, but it is rapidly converted to other things.

ExperimentalSet-up

ResultAluminium foil






keep it simple science® Summary: What Cells Need

Plant Cellseukaryotic autotrophic

Light for photosynthesis

Cell Requirements

ENERGY

SIMPLECHEMICALS

WASTE REMOVALNEEDS

Animal Cellseukaryotic heterotrophic

Complex, high energy carbohydrates(or lipids & proteins which can be

converted) made by other organisms

Bacterial Cellsprokaryotic

Varied. Some are autotrophs requiring light.Others are “chemotrophs” requiring

certain inorganic chemicals (eg SO2 ) asenergy sources.

Many are heterotrophs which feed onplant/animal wastes. Somes weirdos can

feed on chemicals such as petrol.

H2O & CO2 (photosynthesis)

O2 (cellular resp.)

A range of simple inorganic“minerals” (ions) including

nitrates, phosphates, sulfates,calcium, magnesium, etc.

In daylight, surplus O2 is“excreted” by simple diffusionfrom cells, then to the air via

stomates.

Plants such as mangrovesmay need to excrete excesssalt by active transport from

specialist cells.

Most animals cannot tolerate a build-up of CO2. Individual cells

excrete it by simple diffusion, butthen a specialist system involving

blood transport, lungs or gills, etc. isneeded to remove it from the body.Another critical toxic waste is urea

(from protein metabolism) excreted inurine via the kidneys, or similar.

Waste products can be CO2, methane,metal sulfides, lactic acid, etc.

depending on exactly how each speciesgets its energy.

However, since all prokaryotes are single-celled, excretion is carried out by simplediffusion, or by active transport across

the cell membrane.

All need H2O

Beyond that, the needs are highly varied.Many require O2, but others are poisoned

by it. Photosynthetic types need CO2,while some need SO2 or CO2 & H2 for

chemosynthesis.All have a need for simple ions like

calcium, potassium or iron,but precise details vary.

H2O

O2

A range of “minerals” & “vitamins”which are generally supplied in a“balanced diet”. (What this means

varies from one species to another)




Food & Energy for Cells Student Name .................................

1. a) In plants, photosynthesis occurs in 2 stages, in different parts of a chloroplast.Outline these 2 stages and precisely where each occurs.

b) How many molecules of water & CO2 are required to produce one molecule of glucose?

2. When you look at the summary chemical equations for photosynthesis & cellular respiration, theyseem to be exactly opposite processes. Comment on this statement.

3. Describe the ATP ADP cycle & explain why ATP can be considered as the “energy currency” of aliving cell.

4. Compare & contrast what exactly is needed to supply energy to an autotroph compared to aheterotroph (assume eukaryotic cells).










Metabolism is ChemistryEverything that happens inside a living cell is really a matter ofchemistry... “metabolism”. For example...

• For your body to grow, cells must divide and add more membranes,cytoplasm and organelles. This involves the chemical construction ofnew DNA molecules, new phospholipids for membranes and so on.

• All these chemical reactions require energy. Energy is delivered by theATP molecule, itself the product of a series of chemical reactions in themitochondria... cellular respiration.

All of these reactions are “metabolism”: the sum total of all thethousands of chemical reactions going on constantly in all the billions ofcells in your body.

EnzymesEvery reaction requires a catalyst... a chemical which speeds the reactionup and makes it happen, without being changed in the process. In livingcells there is a catalyst for every different reaction.

Biological catalysts are called enzymes.• Enzymes are protein molecules.• Each has a particular 3-dimensional shape, which fits its “substrate”perfectly.• Enzymes are highly “specific”. This means that each enzyme will onlycatalyse one particular reaction, and no other.• Enzymes only work effectively in a relatively narrow range oftemperature and pH (acidity).

The Importance of ShapeMany of the properties of enzymes are related to their precise 3-dimensional shape.

The shape of the enzyme fits the“substrate” molecule(s) as closelyas a key fits a lock.

This is why enzymes are “substrate-specific”...only one particular enzyme can fit each substrate molecule. Eachchemical reaction requires a different enzyme.

Changes in temperature and pH (acidity) can cause the shape of theenzyme to change. If it changes its shape even slightly, it might not fitthe substrate properly any more, so the reaction cannot run asquickly and efficiently. This is why enzymes are found to work best atparticular “optimum” temperature and pH values.

Various DifferentSubstrateMolecules

Only thisone fits

Enzymemolecule

Enzyme shapeat optimum pH

andtemperature

Shape changesslightly at

different pH ortemp.

Substrate...

...no longerfits enzyme

3. Cell Functionsc) Biochemical Control... Enzymes






PolymerizationPolypeptidechain

Product releasedfrom enzyme

Substrate molecules are chemically

attracted to the enzyme’s active site

Protein, with precise 3-D shape...

Substrate molecules broughttogether and react with each other

Amino acidmolecules

Twists & folds...becomes an

ENZYME molecule

Enzyme’s “Active Site”

has a shape to fit thesubstrate(s) exactly

ENZYME ENZYME

ENZYME can react with more substrate

From Amino Acids to Enzyme to Metabolic Control

Precisely folded

protein is an enzyme

This schematic diagram outlines how an enzyme is made & how it can control a metabolic reaction in a cell.






Temperature

1/tim

e ta

ken

for

reac

tion

(rat

e)

You may have measured the rate of a chemical reaction being catalysed by anenzyme, such as:• the rate of milk clotting by junket tablets.

• the rate of digestion of some starch by amylase.

• the rate of decomposition of hydrogen peroxide by “catalase” enzyme.

Enzyme Activity GraphsYou may do experimental work to measure the “activity” of an enzyme under different conditions of temperature or pH.

A common way to measure the rate of a reaction is tomeasure the time taken for a reaction to reachcompletion... the shorter the time taken, the faster thereaction. This is why the reciprocal of time taken(1/time) is used as the measure of rate of reaction.

The Effect of TemperatureWhen enzyme activity is measured at different temperatures, the results produce a graph as below.

ExplanationsAs temperature rises the rate increases because the molecules move faster and aremore likely to collide and react. All chemical reactions show this response.

However, beyond a certain “peak” temperature, the enzyme’s 3-D shape begins tochange. The substrate no longer fits the active site so well, and the reaction slows. If thetemperature was lowered again, the enzyme shape, and reaction rate could be restored.

If the temperature reaches an extreme level, the distortion of the enzyme’s shape mayresult in total shut-down of the reaction. The enzyme may be permanently distorted outof shape, and its activity cannot be restored. We say the enzyme has been “denatured”.

ExperimentalPoints

Not all enzymes will “peak” at the same temperature, or have exactly the same shape graph. Inmammals, most enzymes will peak at around the animal’s normal body temperature, and often workonly within a narrow range of temperatures.

An enzyme from a plant may show a much broader graph, indicating that it will work, at least partly,at a wider range of temperatures.

An enzyme from a thermophilic bacteria from a hot volcanic spring will show a totally different“peak” temperature, indicating that its metabolism will perform most efficiently at temperatures thatwould kill other organisms. 0 20 40 60 80 100

Temperature (oC)R

eact

ion

Rat

e

MammalEnzyme

PlantEnzyme

Thermophilicbacteriaenzyme

Optimum Temperature of Enzymes






The pH ScaleThe acidity or alkalinity of any solution is measured on a numerical scale known as “pH”.

On the pH scale, anything which is neutral (neither acid nor alkaline) has a pH = 7.

The inside environment of a cell, and most parts of an organism’s body, is always very close to pH 7... i.e. neutral.An exception is in the stomach where conditions are strongly acidic. (approx. pH 2)

76 8543 119 10

Neutralincreasingacidity

increasingalkalinity

The shape of the pH graph is usually symmetrical on either side

of the “peak”.

The explanation for the shape is as follows:

At the optimum pH the enzyme’s 3-Dshape is ideal for the substrate, so

reaction rate is maximum.

At any pH higher or lower thanoptimum, the enzyme’s shape begins to

change. The substrate no longer fits, so activity is less.

At extremes of pH, the enzyme can bedenatured and shows no activity at all.2 3 4 5 6 7 8 9 10

pH

1/tim

e (r

ate)

Enz

yme

Act

ivity

Enzy

me

Act

ivity

1 2 3 4 5 6 7 8 9 10 11pH

Intra-cellular enzyme

Pepsin.(Stomachenzyme)

The Effect of pHWhen the temperature is kept constant and an enzyme tested at various pH levels, the results will produce a graph as shown.

Generally, all intra-cellularenzymes (i.e. those from within a

cell) will show peak activity atabout pH = 7, very close to

neutrality.

The digestive enzyme “pepsin”from the stomach shows an

optimum pH about 2 or 3, meaningthat it works best in the acidic

environment.





keep it simple science® The “Bottom Line” for a Cell to Thrive

Now that you know some basics about enzymes, we can end this topic by discussing just why animals (and people) candie in a “heat wave” (or a blizzard), why pouring vinegar on weeds kills them and why it is so important to excrete

the CO2 your cells are constantly producing while making ATP.

Temperature Impacts on CellsTo stay alive, a human’s body temperature must be close to 37oC. If it varies by more thanabout 4oC either side of this, it is life-threatening!

Now you can figure out why.

Impacts of Changing pHLikewise, the pH of your cellular & body fluids(eg blood) is also critical for your survival.

Same reason... if the pH goes up or down byjust 0.5 of a pH unit some critical enzymemolecules will change their 3-D shape & mightnot fit their substrate properly. This could slowdown, or stop, some vital biochemical pathway.

This is why it is (for example) very important toget rid of the CO2 you constantly produce inyour hundreds of billions of cells busily makingATP by cellular respiration.

The problem with CO2 is not that it is“poisonous” in some vague, mysterious way.Specifically, its danger is pH change!

When CO2 dissolves in your blood beyondcertain concentrations, it increases acidity. Thiscan quickly lead to “acidosis” in your bodyfluids which can kill you (by malfunction of vitalenzymes) within minutes.Temperature 32 37 42

Enzy

me

Act

ivity

Somewhere in your cells there are critical chemicalpathways controlled by enzymes with very narrowactivity curves, as shown by this graph.

These pathways might be in one of the many stepsin cellular respiration in all your mitochondria.Maybe it’s an enzyme involved with exocytosis of aneuro-transmitter which passes nerve signals fromone nerve cell in your brain to another.Whatever it is, it is vital to your survival.

Now look at the graph:If your body temperature drops to 32o, or goesabove, say, 40o, this enzyme will STOPFUNCTIONING. This could stop ATP production, orstop nerve signals in your brain. Either one couldstop your heart!

Body temperature & pH are critical to survival because vital enzymes can only perform efficiently in a narrow range of temperature and/or pH.




Enzymes Student Name .................................

1. What is meant by “metabolism”?

2.a) Enzymes are said to be “substrate specific”. What does this mean?

b) Explain how the shape of an enzyme molecule is linked to this specificity.

3. Sketch the shape of the graph of enzyme activity plotted against:

a) temperature. b) pH.

Act. Act.

temp pH

4. How is the shape of these graphs connected to enzyme shape?






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