AQA A2 Biology

My revision notes, updated as I go!

For Unit 4 of AQA A2 Biology


Photosynthesis - Chloroplasts

Photosynthesis takes place in the chloroplast of the plant cell. They consist of a double membrane, thylakoids which are stacked up into grana, lamellae which link grana molecules, photosynthetic pigments, stroma and starch grains.

Photosynthetic pigments (e.g chlorophyll and carotene) are coloured substances that absorb light energy for photosynthesis, they are found in the thylakoid membrane and are attached to proteins. The protein and pigment are called a photosystem.

Plants use two types of photosystems to capture light energy, 1 which absorbs light at 700nm and number 2 which absorbs light at 680nm.

Stroma is the gel liquid which fills the rest of the chloroplast and surrounds the thykaloids. It contains some sugars, enzymes and organic acids.

Carbs produced by photosynthesis and not used up straight away are stored as starch grains (round blobs) in the stroma.

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Photosynthesis - Light Dependant

Photosynthesis takes place in two stages - light dependant and light independant.

The light dependant stage takes place in the thykaloid membranes of the chloroplast.

The light energy is used to create ATP (adding another phosphate to ADP), and to reduce NADP to form Reduced NADP.

The products of the light dependant stage (ATP, Reduced NADP) are used in the light independant stage. The ATP is used as energy and the reduced NADP transfers a hydrogen.

During the light dependant stage, photolysis of water occurs (use of light to break water). Water is split into proton (H+), electron (e-) and oxygen, which is released into the atmosphere. The e- is used to replace the excited electron emitted from chlorophyll, the H+ goes back to the e- (after it has passed through electron carriers) to reduce NADP.

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Photosynthesis - Equations

The equation for Photosynthesis is -

6 CO2 + 6 H2O ------------> C6H12O6 + 6 02

The equation for Photolysis is -

2 H2O --------------> 4 H+ + 4 e + O2

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Photosynthesis - Light Independant

The light independent reaction takes place in the stroma of the chloroplast. This reaction is also known as The Calvin Cycle. Carbon fixation takes place in the Calvin Cycle, this is where carbon is 'fixed' into an organic molecule.

The products of the light independent stage are triose phosphate which can be used to make glucose and other useful organic substances.

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Photosynthesis - Limiting Factors

There are optimum conditions for photosynthesis.

High light intensity of a certain wavelength, temperature around 25 degrees, and C02 at 0.4%. All of these factors can limit photosynthesis, if one or any of the factors are too high or too low it will slow down photosynthesis.

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Aerobic Respiration - requires oxygen and produces carbon dioxide, water and lots of ATP.

Anaerobic Respiration (fermentation) - takes place without oxygen and produces lactate (animals) or ethanol and carbon dioxide (plants). It only produces a small amount of ATP.

Aerobic Respiration can be summarised into four stages -

1. Glycolysis - splitting of a 6 carbon molecule into two 3 carbon pyruvates

2. Link Reaction - conversion of pyruvate into C02 and 2 carbon molecule Acetyl coA.

3. Krebs Cycle - intro of Acetyl coA into a cycle of oxidation-reduction reactions that creates some ATP and a large number of electrons.

4. Electron Transport Chain - electrons from the Krebs cycle are used to synthesise ATP with water produced as a by product.

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Respiration - Glycolysis

Glycolysis is the first stage in both aerobic and anaerobic respiration. It occurs in the cytoplasm of all living cells. 2 pyruvates, 2 reduced NAD and 2 ATP are gained.

1 - Glucose Activation -this provides energy to activate glucose

Glucose is made more reactive by the adding two phosphate molecules (phosphorylation), which come from the hydrolysis of two ATP molecules to ADP.

2 - Splitting Glucose

Phosphorylated glucose is split into two 3 carbon molecules, triose phosphate.

3 - Triose phosphate oxidation.

H is removed from triose phosphates and transferred to NAD to form reduced NAD.

4 - ATP production

The triose phosphate is converted into 3 carbon pyruvate by enzyme controlled reactions. Two molecules of ATP are produced from ADP + Pi.

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Respiration - Link Reaction

Pyruvate molecules made in Glycolysis are actively transported into the matrix of the mitochondria, this is where the link reaction takes place.

Pyruvate is oxidised by removing H. This is accepted by NAD to form reduced NAD. 2 Carbon molecule acetyl combines with a molecule called coenzyme A to produce acetylcoenzyme A and a C02 molecule is formed from each pyruvate.

pyruvate + NAD + coA ----------> acetyl coA + reduced NAD + C02

Overall produced in the link reaction - (2x because 2 molecules of pyruvate are made in glycolysis from one glucose)

2x Reduced NAD

2x C02

2x acetyl coA

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Respiration - Krebs Cycle

The Krebs cycle involves a series of oxidation-reduction reactions that take place in the matrix of the mitochondria.

Acetylcoenzyme A (from the link reaction) enters the krebs cycle and joins with a 4 carbon compound to form a 6 carbon molecule.

The 6 carbon molecule then loses C02 and hydrogens H to give a 4 carbon compound and a single molecule of ATP.

The 4 carbon compound can then combine with a new molecule of Acetylcoenzyme A to begin the cycle again.

Also produced in the Krebs Cycle are reduced FAD from FAD and reduced NAD from NAD.

Overall one cycle of the Krebs cycle produces 1 coenzyme A, 2 C02, 1 ATP, 3 reduced NAD and 1 reduced FAD.

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Respiration - Electron Transport Chain

The electron transport chain is the mechanism by which the energy of the electrons in the H atoms is converted into a form which cells can use, ATP. It is found in the mitochondria, in the cristae of the mitochondria are the enzymes and proteins involved in the electron transport chain and ATP synthesis.

H atoms from the Krebs cycle are carried by the coenzymes NAD and FAD to the electron transport chain. The electrons are taken from the H atoms and are passed along a chain of electron transport carrier molecules in a series of oxidation-reduction reactions. The electrons lose energy as they pass down the chain and some of this is used to combine ADP+ Pi, to make ATP. At the end of the chain the electrons combine with the original H protons and oxygen to form water. Oxygen is the final acceptor of electrons in the electron transport chain.

Oxygen (the final acceptor) is important because without it the hydrogen ions and electrons would back up and the process of respiration would come to a halt.

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Respiration - Anaerobic Respiration

When oxygen is not available anaerobic respiration occurs. In anaerobic respiration glycolysis still takes place but the krebs cycle and the electron transport chain do not.

In plants and microorganisms like yeast, the pyruvate from glycolysis is converted to ethanol and C02.

pyruvate + reduced NAD ------> ethanol + C02 + NAD

In animals, the pyruvate is converted into lactate.

pyruvate + reduced NAD -------> lactate + NAD

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Populations - Ecosystems

An ecosystem is made up of all the interacting biotic (living) and abiotic (non-living) features in a specific area. Eg. freshwater pond, oak woodland.

Within each ecosystem are a number of species. Each species is made up of many groups of individuals that together make up a population.

A population is a group of interbreeding organisms of one species in a habitat. Populations of different species form a community.

A community is all the populations of different organisms living and interacting in a particular place at the same time.

A habitat is the place where a community of organisms lives. Within an ecosystem there are many habitats. Microhabitats are smaller habitats within a larger one. Eg. the mud at the bottom of a stream is a microhabitat for a bloodworm.

An ecological niche describes exactly how an organism fits into the environment. A niche refers to where an organism lives and what it does there, including all abiotic and biotic factors.

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Populations - Investigating Populations

To study a habitat it is necessary to count the number of individuals of a species in a given space. This is known as abundance.

Sampling techniques

random sampling using a frame or point quadrat.

- systematic sampling along a transect (belt or line)

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Alikem McRugged


Can you explain why radioactive GP level rises in the plant when light turned off; and why the level remains constant with light. I can never get my head round it and it seems pretty common in the exam

Former Member


Can you explain why radioactive GP level rises in the plant when light turned off; and why the level remains constant with light. I can never get my head round it and it seems pretty common in the exam

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