AQA A2 BIOLOGY UNIT 4: Respiration

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  • Created on: 18-04-14 14:17
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Respiration is the process by which chemical energy stored in glucose is converted to chemical energy
stored in ATP.
Glucose + Oxygen Carbon Dioxide + Water
C6H12O6 + O2 CO2 + H2O
The Mitochondria
Much of respiration takes place in the mitochondria.
Mitochondria have a double membrane: the outer
membrane contains many protein channels while the inner
membrane is selectively permeable to solutes.
The inner membrane is highly folder into projections called
cristae, giving a large surface area.
Stalked particles are enzyme complexes that synthesise
The space inside the inner membrane is the matrix which is
contains DNA, tRNA and ribosomes, and some genes are
replicated and expressed here.
There are two different forms of respiration depending upon whether oxygen is available or not:
Aerobic Respiration requires oxygen and produces carbon dioxide, water and much ATP
Anaerobic Respiration takes place in the absence of oxygen and produces lactate and carbon
dioxide but only a little ATP
Aerobic respiration can be divided into four stages:
Glycolysis The splitting of the 6-carbon glucose molecule into two 3-carbon pyruvate
Link Reaction The conversion of the 3-carbon pyruvate molecule into carbon dioxide and a
2-carbon molecule called acetylcoenzyme A
Krebs Cycle The introduction of acetylcoenzyme A into a cycle or oxidation-reduction
reactions that yield some ATP and a large number of electrons
Electron Transport Chain The use of the electrons produced in the Krebs cycle to synthesise ATP with
water produced as a by-product

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Glycolysis is the initial stage of both aerobic and anaerobic respiration.
It occurs in the cytoplasm of all living cells and is the process by which a hexose sugar, usually glucose,
is split into two molecules of the 3-carbon molecule, pyruvate.
There are four stages:
Activation of glucose by Before it can be split into two, glucose must first be made
phosphorylation more reactive by adding two phosphate molecules.
The phosphate molecules come from the hydrolysis of two
ATP molecules to ADP.…read more

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The pyruvate molecules produced during glycolysis possess potential energy that can only be
released using oxygen in a process called the Krebs cycle.
Before they enter the Krebs cycle, these pyruvate molecules must first be oxidised in a procedure
known as the link reaction take place exclusively inside mitochondria.
The Link Reaction
The pyruvate molecules produced in the cytoplasm during glycolysis are actively transported into the
matrix of mitochondria.
Here, pyruvate undergoes a series of reactions during which the following changes take place:
1.…read more

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The Krebs cycle involves a series of oxidation-reduction reactions that take place in the matrix of the
1. A 2 carbon acetylcoenzyme A from the link reaction combines with a 4 carbon molecule to produce
a 6 carbon molecule
2. This 6 carbon molecule loses carbon dioxide and hydrogen to give a 4 carbon molecule and a single
molecule of ATP produced as a result of substrate-level phosphorylation
3.…read more

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Hydrogen atoms and the electrons they possess are a valuable source of energy.
These hydrogen atoms are carried by coenzymes NAD and FAD into the next stage of the process: the
electron transport chain.
This is the mechanism by which the energy of the electrons within the hydrogen atoms is converted
into a form that cells can use ­ ATP.
The Electron Transport Chain and Mitochondria
Mitochondria are rod-shaped organelles that are found in eukaryotic cells.…read more

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If there is no oxygen (anaerobic conditions) then the final reaction to make water cannot take place.
Electrons can't leave the respiratory chain, and so NADH cannot unload any hydrogens to the
respiratory chain.
This means that there is no NAD in the cell; it's all in the form of NADH.
Without NAD as a coenzyme, some of the enzymes of the Krebs cycle and glycolysis cannot work, so the
whole of respiration stops.…read more

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Summary of respiration to see how much ATP is made from each glucose molecule.
ATP is made in two different ways:
· Some ATP molecules are made directly by the enzymes in glycolysis or the Krebs cycle. This is called
substrate level phosphorylation (since ADP is being phosphorylated to form ATP).
· Most of the ATP molecules are made by the ATP synthase enzyme in the respiratory chain. Since
this requires oxygen it is called oxidative phosphorylation.…read more


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