1. Section 7 – Cellular respiration
Unit 1 Living Cells

2. ATP Adenosine Tri-Phosphate High energy compound
- generated by respiration
Composed of adenosine (adenine & ribose)
- and three inorganic phosphates (Pi)
If terminal phosphate is broken off, energy is released
- forms ADP and Pi
To reform ATP (from ADP and Pi) requires energy input

3. ATP turnover ATP links energy-releasing (CATABOLIC) reactions
– e.g. respiration
with energy consuming (ANABOLIC) reactions
– e.g. protein synthesis
ATP turns over rapidly in the human body (400g/hr)
- however, has a very low fixed quantity (50g)

4. Phosphorylation Phosphorylation:
- the adding of phosphate to a molecule
- enzyme controlled
- e.g. ADP + Pi  ATP
Can also involve ATP donating a phosphate to something else
E.g. glucose-6-phosphate
This substance is now phosphorylated
- high energy state

5. Synthesis of ATP Respiration releases energy
- this energy is in the form of electrons
These help pump H+ ions across the inner membrane of the mitochondria
- against the concentration gradient
H+ ions flow back across the membrane through the enzyme ATP synthase
This rotates and catalyses the conversion of ADP + Pi  ATP

6. ATP synthase

7. Glycolysis (“glucose splitting”)
First stage of respiration
Occurs in the cytoplasm
Converts glucose  pyruvate
A series of enzyme controlled steps
Initially requires 2ATP
- energy investment phase
Later on there is a gain of 4ATP
- energy payoff phase
Overall net gain of 2ATP
H+ ions are released and collected by a coenzyme molecule (NAD)
- it gets converted into NADH
Glycolysis doesn’t require oxygen

8. Citric Acid Cycle Proceeds after glycolysis
- only if oxygen is present - (aerobic respiration)
- occurs in the central matrix of mitochondria
Initially:
- pyruvate  acetyl group + CO2
Acetyl group combines with coenzyme A
- forming acetyl coenzyme A
This combines with oxaloacetate  citrate
This enters the citric acid cycle
- a series of enzyme controlled stages

9. Citric Acid Cycle ctd. At four stages, H+ ions are released
- along with high energy electrons
At 3 stages, these are joined onto the coenzyme NAD
- to form NADH
At the fourth stage FAD acts instead of NAD
- to form FADH2
Cycle also produces 2 CO2
- & 1 ATP
FADH2
ATP

10. Electron Transport Chain
Involves a chain of proteins on the inner mitochondrial membrane
NADH and FADH2 release electrons and pass them into the chain
As electrons pass along the chain, they release energy
This pumps H+ ions across the matrix into the inter-membrane space
- maintains a high H+ concentration
H+ ions flow back via ATP synthase
- this drives the conversion of ADP + Pi  ATP
At the end, the electrons and H+ ions combine with oxygen
- to form water (H2O)
- In the absence of oxygen, electron transport chain doesn’t operate

11. Substrates for respiration
Carbohydrates:
Starch and glycogen are composed of glucose molecules
- they can be digested to release glucose
Other sugars can also be used
- some converted directly into glucose
- e.g. maltose
- others converted to intermediates
- e.g. fructose

12. Substrates for respiration
Fats:
For respiration, fats are broken down
- into glycerol and fatty acids
Glycerol is converted to an intermediate in glycolysis
Fatty acids are metabolised to enter the pathway as acetyl co-enzyme A

13. Substrates for respiration
Protein:
Dietary proteins are digested into amino acids
Excess amino acids undergo deamination
Forming urea/ammonia
- released in urine
And intermediates
- which enter the respiratory pathway

14. Substrates for respiration

15. Regulation of respiration
Stage 3 of glycolysis is catalysed by phosphofructokinase
An irreversible step
– commits to continuing glycolysis
- a key regulatory point
High concentrations of ATP inhibit phosphofructokinase
- slows down glycolysis
Enzyme also inhibited by citrate
This control is feedback inhibition
- prevents build up of an intermediate
- ATP only produced when needed
- conserves resources

16. Creatine phosphate system
ATP break down produces energy for muscular contraction
Muscle cells only contain enough ATP for a few contractions
For strenuous activity, creatine phosphate is broken down
- releasing phosphate
- helps converts ADP -> ATP
The ATP formed sustains contraction for a few seconds after the initial contraction
At rest, enzymes use up ATP to synthesise the reformation of creatine phosphate
- acts as a high energy reserve

17. Lactic Acid metabolism
If ATP and creatine phosphate reserves run out, cells will try to respire anaerobically
- due to a lack of oxygen for aerobic respiration
Therefore no citric acid cycle and electron transport system
- only glycolysis can proceed
As a result, only 2NADH and 2 ATP are produced
Pyruvic acid is converted to lactic acid
- aided by hydrogen from NADH
NAD is re-formed to aid glycolysis to continue
Build up of lactic acid causes fatigue
- only eased by exercise stopping
Lactic acid can be converted back to pyruvate
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