Cell Structure

Plant Cell

Animal Cell

Fungal Cell(yeast)

ribosomes

cell membrane

cell wall

capsule

plasmids

cytoplasm

DNABacterial Cell

Nucleus stores genetic information / controls cell activities

Cytoplasm site of chemical reactions

Cell membrane controls entry and exit of materials

Cell wall gives shape and support to the cell and stops it bursting due to osmosis

Chloroplast site of photosynthesis

Vacuole stores water and minerals help support the cell, and when full it pushes outwards against the wall to help provide support

Mitochondrion site of aerobic respiration

Ribosome site of protein synthesis

Plasmid a small circular piece of DNA that controls characteristics which help bacteria adapt to their environment eg. antibiotic resistance. Bacteria swap characteristics with each other by swapping plasmids

Structure Function

( )

The cell wall of a plant cell is made of cellulose, the cell walls of fungal cells and bacterialcells are not.

Calculating cell size

• Cells are too small to be described in terms of millimetres (mm).

• They are measured in micrometres ( µm ) ( also called microns)

To calculate the size of a cell seen under the microscope;

1. If the diameter of the field of view is given in millimetres, the first thing to do is change it to micrometres

2. Count how many cells you see going from one side to the other

3. Divide the diameter by the number of cells

1mm = 1000 (µm)

So, 1 µm = 1/1000mm

For example,

The diagram shows some cells as observed under a microscope at a magnification of 100X

The diameter of the field of view is 1 millimetre.

What is the average length of each cell inmicrometres?

1 millimetre

Answer

1mm = 1000 (µm)So field of view = 1000 µm4 cells stretch across the diameterAverage length = 1000 4

Average length = 250µm

Field of view

Transport across Cell Membranes

The cell membrane consists of lipids and proteins.

The diagrams show the fluid mosaic model of the cell membrane.

A double layer of constantly moving or fluid lipid molecules, with a patchy

mosaic of protein molecules.

Some protein molecules form channels or pores through the membrane.

The membrane can be described as porous.

The cell membrane is selectively permeable, that is it selects or controls what

substances can enter or leave the cell.

Passive transport is when a substance moves across the cell membrane down a

concentration gradient. That is, from a high concentration of that substance to

a low concentration of it.

Passive transport does not require any energy.

High concentration

Low

concentration

Diffusion and osmosis are examples of passive transport.

Diffusion is the movement of molecules from an area of high concentration to

an area of low concentration down a concentration gradient.

In respiration, living cells gain glucose and oxygen by diffusion and release

carbon dioxide by diffusion.

In photosynthesis, living plant cells take in carbon dioxide by diffusion and

release oxygen by diffusion.

Osmosis is a special case of diffusion involving water.

Osmosis is the movement of water from a high water concentration to a low

water concentration across a selectively permeable membrane.

The water will move from right (high water concentration) to left (low water

concentration)

However, the solute cannot move as it is too large to fit through the pores in

the selectively permeable membrane.

Active transport is when a substance moves across a cell membrane against the

concentration gradient, from a low concentration to a high concentration.

This requires energy from respiration.

Protein carriers in the cell membrane move molecules into or out of the cell

against the concentration gradient.

Water molecule

Solute molecule

Chromosomes, Genes and DNA

nucleus

Cell chromosomes DNA

• Chromosomes are located in the nucleus of the cell and each chromosome is made up of

a molecule of a chemical called DNA which is unique to every individual.

• A short piece of DNA in a chromosome makes a gene and genes control the different

characteristics of living things by producing particular proteins.

• DNA is passed down from parent to offspring.DNA

Chromosomes are made up of sections called genes and genes are made up of the nucleic acid

DNA (deoxyribonucleic acid). DNA carries the complete genetic information of an organism

in the form of a code. This code determines the sequence of amino acids in a protein.

From the Cell to DNA

Structure of DNA DNA consists of two strands made up of a backbone and bases. The strands are held

together by weak hydrogen bonds between the bases forming a structure called a double

helix.

There are four types of base: adenine (A), thymine (T), guanine (G) and cytosine (C). Base

pairing is said to be complimentary since only certain bases can pair:

Adenine pairs with thymine (A – T)

Guanine pairs with cytosine (G – C)

Unit 1.3 – Cell division and its role in growth and repair

New cells are produced from existing cells by cell division. Cell division allows organisms to

increase the number of cells and these can then be used for growth or repair of damaged

body parts (e.g. cuts or broken bones).

Cell division is controlled by the nucleus of a cell.

During cell division, a parent cell will divide to form two new cells. These two new cells will be

identical to each other and to the parent cell.

The nucleus of a cell contains thread-like structures called chromosomes.

Each new cell produced by cell division will contain the same number of chromosomes as the

parent cell.

Cell division is a controlled cycle; each cell which is made by cell division will become a parent

cell and divide itself. If cell division isn’t controlled then large numbers of cells can be

produced, this is the cause of cancers.

Nucleus

A cell

parent cell

cell division

2 identical cells

cell

nucleus

chromosome

Unit 1.3 - Producing new cells A diploid cell is a cell that contains 2 matching sets of chromosomes. These cells divide in a

process known as mitosis

The Stages of Mitosis

• Just before a cell divides, each chromosome doubles up to form two identical

chromatids joined by a centromere.

• The chromosomes shorten and thicken and become visible.

• They move to the equator of the cell and attach to spindle fibres by the centromere.

• The chromatids are pulled to the opposite ends of the cell.

• Nuclear membranes form around the new chromosomes and the cytoplasm divides.

• There are now two new cells each with the same number of chromosomes as the

original cell.

Mitosis The stages of mitosis are shown below:

Production of Proteins

To make proteins the nucleic acid mRNA (messenger RNA) is needed. mRNA is a single

stranded molecule which carries a copy of the code from DNA.

Proteins are assembled at the ribosome so mRNA carries the code from the nucleus to the

ribosome. Proteins are made by joining amino acids together. Remember the sequence of

bases on DNA determines the sequence of amino acids in a protein.

1.5 Protein and Enzymes

Part A There are many different proteins. Proteins are made of units called AMINO ACIDS. There

are twenty different amino acids (aa). These are often depicted by using different shapes.

Each amino acid can join with any other amino acid e.g

aa 1 aa2 aa3 aa4 aa 5

The structure of a protein depends on the SEQUENCE in which the amino acids are joined

together. Proteins, therefore, have DIFFERENT SHAPES e.g. STRUCTURAL PROTEIN and

GLOBULAR PROTEIN. These shapes determine protein FUNCTION .

Part B

COLLAGEN has a super-coiled helical shape that is long, stringy and strong and resembles a

rope. This structure provides support. Collagen is an example of a STRUCTURAL PROTEIN.

Collagen gives STRENGTH to BONES, TEETH, CARTILAGE, TENDONS and SKIN. It also

gives skin ELASTICITY and is used in COSMETIC / BURNS SURGERY

HAEMOGLOBIN is a protein, found in blood. It is folded and compact. Its spherical shape

allows it to move through blood vessels. Haemoglobin is an example of a GLOBULAR

PROTEIN. Haemoglobin carries oxygen in the blood.

ENZYMES, HORMONES and ANTIBODIES are all globular proteins. Each has a different

function.

ENZYMES speed up chemical reactions, HORMONES act as messengers in the body, helping

to coordinate body activities and ANTIBODIES defend the body from foreign invaders

Part C Enzymes are made by all living cells. Their function in a cell is to work as a BIOLOGICAL

CATALYST. A catalyst is a substance which SPEEDS UP A CHEMICAL REACTION and

REMAINS UNCHANGED at the end of the reaction.

Without enzymes, the reactions which go on inside ALL LIVING CELLS would be so slow that

life would simply grind to a halt.

Enzymes act with a particular substance known as a SUBSTRATE.

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The substance on which an enzyme acts is called its SUBSTRATE. Normally an enzyme will

only act upon ONE SUBSTRATE.

Each enzyme has a characteristic shape on its surface called an active site. A substrate

molecule will fit into this active site. A reaction then takes place while the enzyme and the

substrate are joined together. The products of the reaction leave the active site, freeing it

for another molecule of the substrate to join.

Each enzyme has a different shaped active site which is SPECIFIC to its substrate. The

shape of the active site can be described as COMPLEMENTARY to that of its substrate.

Product released

from enzyme.

Substrate moves

towards active site

Active

site

enzyme

molecule

Reaction occurs

On the enzyme

molecule

PPaarrtt DD

EEffffeecctt ooff TTeemmppeerraattuurree oonn EEnnzzyymmee AAccttiivviittyy The graph below shows the effect of temperature on the activity of an enzyme:

The rate of activity of an enzyme is LOW at LOW TEMPERATURES, increasing to a

maximum at the OPTIMUM TEMPERATURE and then further DECREASING RAPIDLY as the

temperature is increased further.

Most enzymes will STOP WORKING if the temperature rises above 45 oC. This is because

enzymes are PROTEINS and are therefore DENATURED by heat (i.e. the shape of the

enzyme molecule is altered). This CANNOT BE REVERSED by cooling the enzyme down again.

HHEEAATT

pH

Rate of

Enzyme

Activity

pepsin

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Most enzymes work best within a NARROW pH RANGE.

The graph below shows how the activity of the enzymes PEPSIN and CATALASE is affected

by pH.

PEPSIN (found in acidic conditions of the stomach) works BEST in the range around pH1 -

pH4 with an OPTIMUM pH about pH2.

CATALASE has a working range of around pH5.5 - pH9 with an OPTIMUM pH value

around pH7.

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ALL enzymes have the following properties:

They are always PROTEINS

They are UNCHANGED by the reaction in which they take part and, therefore, can be

used over again.

They are destroyed (DENATURED) by HEAT

They are sensitive to pH

They are SPECIFIC in their action.

They work best in OPTIMUM CONDITIONS.

catalase

Genetic engineering and therapeutic uses of cells

A gene is a small section of a chromosome that codes for a particular protein

e.g. insulin.

Insulin is a hormone that regulates the blood sugar levels.

Insulin is required by people who suffer from diabetes, as their bodies do not

produce enough.

Bacterial cells

Bacterial cells contain a large circular chromosome which controls the cells

activities and other smaller circular DNA called Plasmids.

The plasmids can be extracted and used in genetic engineering.

Allow bacteria to swap

characteristics

Genetic engineering

Genetic engineering is the transfer of genes from one organism to another e.g.

human to bacteria.

Genetic engineering can be used to produce large quantities of insulin and other

proteins.

Genetic engineering involves transferring the desired gene (insulin gene) into a

bacterial cell. Once inside, the bacterial cell will reproduce rapidly producing

many bacterial cells with the insulin gene. The bacterial cells will produce the

insulin which can then be extracted and purified.

Steps involved in genetic engineering to produce insulin.

1.Chromosome extracted

and insulin gene identified

2. Gene cut out

3. Plasmid extracted

4.Plasmid cut open5. Gene inserted into plasmid

6. Plasmid inserted into bacterial host cell

7. Bacterium grows and multiplies

8. Insulin mass produced

by duplicates of plasmid

How does this work?

1. The required gene is identified in the correct organism.

2. This gene is cut out and removed from its chromosome using and enzyme.

3. A plasmid is extracted from a bacterial cell.

4. The same enzyme as used in step 2 is used to cut open the plasmid.

5. The required gene is inserted into the plasmid using a different enzyme to the

one used in steps 2 and 4 to seal it in.

6. The plasmid containing the new gene is inserted back into the bacterial cell it

was removed from.

7. The altered plasmid duplicates inside the cell and the cell multiplies.

8. The bacterial cell mass produces insulin, which is then taken for purification.

Other proteins and substances can be produced in the same way such as;

Medical

Genetically Modified Plants

• These are organisms which have had genes inserted from another

organism.

• These organisms are said to be genetically modified and are important in

the development of new varieties of animals, plants and micro-organisms,

GM organism Modification Benefit

Soya bean Resistance to weed

killer Increased yield

Tomato Prevent softening Stays ripe longer

Oilseed rape Produce oils Used in plastics

Brewer’s yeast

Higher alcohol

content and lower

carbohydrate

content

Light beers

Yeast Produce rennin Vegetarian cheese

Product Need

Insulin Regulates blood sugar

levels

Factor VIII Required for normal

blood clotting

Growth Factor Required for normal

cell growth

Crops can also be modified, to develop

– Drought tolerance

– Disease resistance

– Pest resistance

– Easier harvesting

– Richer in vitamins

Genetic Engineering

Advantages Disadvantages

Completely different species can be combined

Inserting genes into animals and plants has proved to be very difficult

Only desired characteristics are produced

Some complex proteins can only be made by plants or animals.

Only takes one generation to get desired results

Only a few plants have been found that will accept the plasmids carrying the foreign gene.

Large quantities of protein produced

Research and implementation are very expensive.

Easier to purifyCan go wrong, e.g. in production of factor VIII too much may be produced at a time.

Less contamination There is a question of ethics.

Therapeutic Uses of Cells

Fighting Disease

• Genetic engineering can produce proteins which act as vaccines, or even

the antibodies themselves.

• These proteins can then be adapted so that they can be taken orally,

resist the human digestive system and then be absorbed into the

bloodstream.

Gene Therapy

• Gene therapy is the replacement of a defective gene with a fully

functional gene.

• Cystic fibrosis is one of the most common genetic disorders

• A defective gene is unable to produce a protein required for normal cell

function and so the linings of the airways and pancreas become blocked

with very thick mucus.

• This defective gene has been identified and isolated.

• Scientists are currently investigating ways of inserting the correct gene

into the body cells.

• If this can be achieved, cystic fibrosis could be cured by gene therapy.

• Gene therapy is also being investigated for cancer.

• Special genes called ‘suicide genes’ are placed into the cancer cells.

These genes are only active in cancer cells – not normal cells.

Stem Cell Therapy

Stem cell therapy is a set of techniques that aim to replace cells damaged or

destroyed by d......................... with healthy functioning ones.

If successful, the healthy replacement stem cells will integrate into the body

and give rise to more cells that can take on the necessary functions for a

specific tissue.

What are Stem Cells?

Stem cells have several unique properties that separate them from other cells:

They are unspecialised cells.

They can self-renew, which means they are capable of replenishing

themselves for long periods of time by dividing.

They can differentiate into specialised cells such as a nerve or heart cell.

Sources of Stem Cells

Stem cells may be derived from several sources:

1. Embryonic stem cells: they are extracted from e........................ .

2. Adult stem cells: these are present in adult tissues such as the

b................. m......................, brain and blood.

3. Cord blood stem cells: this source of stem cells is derived from

u.................. cord blood.

Benefits of Stem Cells

Stem cells are currently used to treat cancers such as l......................... . You may

be familiar with the concept of bone marrow transplants, which have been used

for decades now to provide a healthy source of cells in the body. Other

diseases that stem cells may help include:

Parkinson's disease

Stroke

Spinal cord injuries

Retinal diseases

Alzheimer's disease

Type I diabetes

Ethical Debate

The use of stem cells from an embryo has prompted massive debate amongst

the public, politicians, scientists and religious groups. Because an embryo is

destroyed after stem cells are extracted, opponents argue that this is the

equivalent of killing a potential life. Fortunately, newer techniques are

currently being investigated which will allow for embryonic stem cell

extraction without either destroying an embryo or creating one.

Artificial Organs

Stem cells have been touted as the treatment of the future for many

diseases.

They have even made it possible to rebuild areas of the body that have

suffered from tissue destruction eg. the growth of a windpipe for a lady

whose windpipe had been destroyed by tuberculosis.

Growing Artificial Organs

Yeast

Yeast are single-celled fungi which need an organic material to grow on.

Their preferred food source would be sugar, for example sucrose. They can

respire aerobically (in the presence of oxygen) or anaerobically (in the absence

of oxygen).

In industry we use yeast in anaerobic conditions as they give the most useful

products.

sugar carbon dioxide + ethanol

raw material products

Baking

In the baking industry, yeast is added to dough with some sugar to make the

dough rise.

It does this by making carbon dioxide in the dough. This collects as small

bubbles, making the dough bigger in volume and the bread lighter in texture.

Bubbles of carbon dioxide

At start After 2 hours

Like all living things, yeast has a range of temperatures that it is active in.

If the temperature is too low the yeast won’t respire and make carbon dioxide.

However, if the temperature is too high, the yeast will die and will no longer be

able to respire.

This is important for industry to know, especially in brewing.

Brewing

Brewing is where the yeast is grown on sugar to produce alcohol (ethanol).

malting

Hops

added for

flavour

Fermentation

Filtration

Bacteria

Bacteria such as E. coli can complete a life cycle and make another E. coli

within 20 minutes in a laboratory. This quick growth means you don’t need

to wait long for a small number to become a very large number. The more

bacteria you have, the more products you can make.

In industry, this means you spend less money growing the bacteria.

There are several types of bacteria which give us different products.

Lactic acid bacteria

This group of bacteria produce lactic acid as a product when grown on a

carbohydrate such as a basic sugar. The sugar is called lactose (some people are

lactose intolerant).

The lactic acid can be used to turn milk into yoghurt as it causes the milk

proteins to clump together making the milk thicken.

The steps of making yoghurt are designed to make sure only the bacteria we

want to grow in the milk can.

1. Use fresh pasteurised milk to ensure there is as few bacteria there as

possible.

2. Heat the milk up to 73oC to kill any remaining bacteria.

3. Cool the milk to 44oC and add lactic acid bacteria. This could be from a

packet or using live yoghurt in natural yoghurt from the supermarket.

4. Leave at 44oC for four hours to let the bacteria grow on the lactose to

make lactic acid

5. Store at 4oC to slow growth of the bacteria.

Lactic acid bacteria are also used in the first stage of making cheese.

Again the lactic acid starts clumping the proteins together before a special

enzyme called rennet is added. This makes the solids collect together, becoming

curds. This leaves a liquid behind called whey.

Biofuels

A biofuel is produced by living cells and will burn to give energy. These are

produced so successfully some cars are designed to be run on them.

They are important to society as they can replace the fossil fuels, coal, oil and

gas which are running out as they are finite.

Biofuels can be produced from many waste products from industry so they also

help find a use for material that would otherwise be rubbish.

The waste material could be sugar cane, which is a rich source of sugar for

yeast to grow on, producing ethanol.

This gets mixed with normal petrol and used in cars.

The waste can also include oils produced from sunflower, rapeseed and soya.

This undergoes a

process where it is

made suitable to go into

a diesel engine for a car

to use. This called

biodiesel. Filtered

vegetable oil can also be

used to make biodiesel,

instead of being put out

with food waste.

Biogas

Biogas is another type of biofuel, but is produced by micro-organisms breaking

down organic waste in anaerobic conditions (no oxygen). The gas produced is

methane which burns well and can be used for heating, cooking etc.

Sewage Treatment

Sewage needs to be treated before it can be released into rivers and streams.

This prevents disease-causing bacteria growing on it in our water ways, carrying

disease throughout the local community.

A wide range of specific bacteria are used for this as they use the organic

waste in sewage as a food source and none are the type to cause disease. They

all need oxygen to break down sewage into harmless products.

The wide range of bacteria allows all the different substances to be broken

down at the same time.

Bioremediation

Bioremediation is when bacteria are used to help us remove harmful substances

which are released into our environment. They can break them down

(biodegrade) into safer products.

Examples can include breaking down oil in oil spills, or in biodegrading things like

polystyrene which was through to be non-biodegradable.

Bacteria

Respiration Summary Notes National 4

Respiration is the chemical process where we release energy from our food

(glucose).

Every cell needs to release energy from glucose and there are two types of

respiration;

Aerobic – with oxygen

Anaerobic – without oxygen

The equation for aerobic respiration can be summarised as follows:

Glucose + oxygen Carbon dioxide + water + ENERGY

Raw materials Products

All animals and plants make the same products when there is oxygen present.

In anaerobic respiration, glucose is still required but no oxygen.

For this reason, less energy is produced.

In Animals and Bacteria, anaerobic respiration produces not Carbon Dioxide and

Water, but Lactic Acid.

Lactic acid is what makes your muscles sore when you are exercising. Once you

stop exercising and your breathing catches up to deliver enough oxygen, you can

return to aerobic respiration and the lactic acid is removed.

Glucose Lactic acid

(Remember – lactic acid was made by the bacteria in the milk to produce

yoghurt)

In plants and fungi, anaerobic respiration produces Carbon Dioxide and

Alcohol.

Glucose Carbon Dioxide + Alcohol + little

energy

Raw material Products

The carbon dioxide is released and cannot be regained.

(Remember – yeast (fungus) makes Carbon Dioxide and Alcohol for making

dough rise and in alcohol production)

Respiration Summary Notes National 5

Adenosine triphosphate (ATP) is a molecule made up from an adenosine molecule

bound to three inorganic phosphates (Pi).

Energy which is stored in ATP is released when the bond attaching the terminal

phosphate is broken by enzymes.

Once the terminal phosphate is broken off we are left with ADP + Pi (adenosine

di-phosphate and inorganic phosphate).

During respiration ATP is regenerated from ADP + Pi, in an enzyme controlled

process called phosphorylation.

this reaction can be summarised as:

ATP ADP + Pi

High Low

energy energy

state state

Adenosine Pi Pi

Pi

Terminal

phosphate

Adenosine

Pi

Pi

Pi

Energy

When an energy rich substances such as glucose are broken down they produce

energy which is used to produce ATP.

If glucose is burned in a dish it releases its energy quickly as heat and light.

However in a living cell respiration releases energy gradually through a series of

enzymes controlled steps.

There are many molecules of ATP in each living cell. As ATP is broken down to

produce ADP + Pi, the energy released is used to fuel biological processes such

as muscle contraction, transmission of nerve impulses, cell division and

protein synthesis.

In order to regenerate ATP, cells require glucose which is derived from

digested carbohydrates. Glucose is carried, dissolved in the blood plasma, to

every cell in the body where it is required. As the glucose is used up in the

cells the concentration levels in the cell remain low while the concentration in

the blood is higher allowing for diffusion of glucose from the blood into the

cells.

Respiration can happen with or without oxygen, however it is more efficient if

oxygen is available.

Aerobic respiration is the term used to describe respiration with oxygen.

Glucose + Oxygen Carbon dioxide + water

Oxygen is transported to the cells bound to a special protein called haemoglobin

in red blood cells; oxygen is used by the cells maintaining a low concentration

inside the cell.

As we breathe, more oxygen enters the blood, maintaining a high concentration

of oxygen in the blood reaching cells. This allows oxygen to move into the cells

by diffusion.

Stage 1 – Glycolysis

Respiration begins in the cytoplasm of a cell and involves the breakdown of one

glucose molecule into two molecules of pyruvate.

In order to do this the cell must use energy from 2 ATP molecules; at the

same time producing 4 ATP molecules giving a net gain of 2 ATP molecules.

Stage 2 – Kreb’s cycle (with oxygen only)

Pyruvate enters into the Kreb’s cycle a series of enzyme controlled reactions

which takes place in the mitochondria of the cell. During this part of the

process hydrogen is removed by high energy carrier molecules. Carbon dioxide

is also given off as a by-product.

Stage 3

The hydrogen is used in the hydrogen transfer system to produce ATP before

finally combining with oxygen to form water.

Including glycolysis, each molecule of glucose produces 38 ATP.

Carbon dioxide and water are breathed out as waste products.

Fermentation

There are occasions when oxygen is not available. Under these conditions

fermentation takes place.

2 x Pyruvate

Glucose

Pyruvate cannot enter the Kreb’s cycle in the absence of oxygen so only

glycolysis can take place and only 2 ATP can be produced overall. The pyruvate

therefore must enter an alternative pathway. This is different in animals and

plants.

In animals pyruvate is converted to lactic acid which causes fatigue and cramp

in muscle tissue. When oxygen becomes available lactic acid is converted back

to pyruvate which can then enter the Kreb's cycle. The reaction is said to be

reversible.

In plants pyruvate is broken down to carbon dioxide and ethanol (an alcohol).

This is a non-reversible reaction.

+

Fermentation is less efficient than aerobic respiration producing 20 times less

energy. Fermentation takes place in the cytoplasm and only involves energy

produced in glycolysis.

Lactic

Acid

Pyruvate

Pyruvate Carbon

dioxide

Ethanol

Photosynthesis

Photosynthesis is the process which allows green plants to make their own food.

It can be summarised with the word equation

Carbon Dioxide + Water Glucose + Oxygen

Raw materials Products

This takes place in the chloroplast of the plant cell.

Testing for Starch

The glucose the plant makes during photosynthesis can be stored in the leaves as starch. Testing

a leaf for starch can show if the plant has been photosynthesising.

Light

Chlorophyll

Chemistry of Photosynthesis

Light Reactions (Photolysis)

This is the first stage of photosynthesis and involves the splitting of water into oxygen and

hydrogen using energy captured from sunlight by chlorophyll. This reaction also produces

ATP.

Due to requirement of light energy to break apart water molecules this reaction is classified

as light dependent, meaning it will only occur in the presence of light.

Carbon Fixation

This is the second stage of photosynthesis. It involves a series of enzyme controlled

reactions joining together carbon dioxide and hydrogen to form glucose. This reaction

requires ATP and hydrogen, passed on from photolysis, and cannot occur without the

products of photolysis.

The glucose produced during photosynthesis can be used in various ways by the plant cell

Water

Light energy from

the sun captured by

chlorophyll

Oxygen

Released as a

by-product

Hydrogen

ADP + Pi

ATP

Light energy

Carbon Dioxide

Hydrogen

Glucose

Enzyme controlled

ATP

ADP + Pi

Used in the second stage

– Carbon Fixation.

1. Used immediately for energy to power cell process, such as mitosis or protein

synthesis.

2. It can be converted to starch for long term storage.

3. It can be converted to cellulose and used to create cell walls

1. Limiting Factors

A limiting factor is a factor that by its presence or absence controls the rate at which a

reaction happens.

For photosynthesis there are various factors that can affect the rate of photosynthesis

Light Intensity

Temperature

Carbon dioxide concentration

Increasing light intensity or carbon dioxide concentration should increase the rate of

photosynthesis, until the plant is receiving as much as it can utilise at which point they

cease being a limiting factor

At the point labelled Y the limiting factor is the light intensity as the rate of photosynthesis

increases as the light intensity increases.

At the points labelled X the rate of photosynthesis shows no further increase even if the

light intensity is increased, therefore another factor such as carbon dioxide concentration

must be limiting the rate of photosynthesis.

Temperature usually works in the same way. Remember, however, that above a certain

temperature the enzymes controlling photosynthesis will become denatured and the rate of

photosynthesis will actually start decreasing. This is shown on the following graph.

A - 0.01% CO2

B - 0.10% CO2

C - 0.50% CO2

X

Y