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Higher Human Biology
Unit 1
Human Cells
KEY AREA 7: Cellular Respiration
Human Cells Learning Intentions
KEY AREA 7 – Cellular Respiration
1. Role of ATP
2. Metabolic Pathways of Cellular Respiration
3. Regulation of Cellular Respiration Pathways
7a) Role of ATP
Cellular respiration is a series of metabolic pathways
that releases energy from food and generates a high
energy compound called adenosine triphosphate (ATP)
ATP is composed of adenosine and three inorganic
phosphate (Pi) groups
Energy is released from ATP when the bond attached
to the terminal phosphate is broken by enzyme action
ATP releases energy and breaks down to form
Adenosine diphosphate (ADP) and inorganic phosphate
(Pi)
ADP + Pi  ATP (this reaction is called Phosphorylation)
7b) Role of ATP
ATP is very important as it acts as the link between catabolic energy releasing reactions (e.g.
respiration) and anabolic energy-consuming reactions (e.g. protein synthesis)
There is a rapid turnover of ATP in a cell
ATP is continuously made at the same time as it is being used up, so there is no need for humans to
have a vast store of ATP
Phosphorylation is an enzyme controlled process by which a phosphate group is added to a molecule
Phosphorylation also occurs when the phosphate and energy are transferred from ATP to the
molecules of a reactant in a metabolic pathway, making them more reactive
7c) Metabolic Pathways of Cellular Respiration
Cellular respiration is a series of metabolic pathways that releases energy from food and
generates a high energy compound called adenosine triphosphate (ATP)
There are 3 stages involved in Cellular Respiration:Stage 1 – Glycolysis (occurs in the cytoplasm of a cell)
Stage 2 – Citric Acid Cycle (occurs in the central matrix of mitochondria)
Stage 3 – Electron Transport Chain (occurs in the inner membrane of mitochondria)
7d) Glycolysis
1.
1st Stage of Respiration
2.
Occurs in the cytoplasm of a cell
3.
Glucose  Pyruvate (enzyme controlled process)
4.
Energy investment phase (2 ATP are needed for Glycolysis)
5.
1st Phosphorylation produces an intermediate which can continue to other metabolic pathways
6.
2nd Phosphorylation is catalysed by phosphofructokinase and is an irreversible reaction which
can only lead to Glycolysis pathway
7.
Energy payoff phase (4 ATP are produced)
8.
Net gain of 2 ATP per molecule of glucose
9.
Hydrogen ions are released from glucose by dehydrogenase enzyme
10. H+ ions are passed onto NAD forming NADH
11. No oxygen required for Glycolysis
7e) Glycolysis
7f) Citric Acid Cycle
Citric Acid Cycle
2nd Stage of Respiration
Occurs in the central matrix of mitochondia
a) Formation of Citrate
1. Oxygen is required (aerobic respiration)
2. Pyruvate is broken down to an Acetyl Group that combines with Coenzyme A
(Acetyl Coenzyme A)
3. H+ ions are released and bind to NAD forming NADH
4. Carbon Dioxide is released
5. Acetyl Coenzyme A + Oxaloacetate  Citrate
b) The Citric Acid Cycle
6. Occurs in the central matrix of the mitochondria
7. Citric Acid Cycle involves many enzyme controlled steps which results in:-
- Regeneration of Oxaloacetate
- Release of Carbon Dioxide
- Generation of ATP
8. Dehydrogenase enzymes remove H+ ions and high energy electrons and pass them
to NAD to make NADH, and FAD to make FADH2
7g) Citric Acid Cycle
7h) Electron Transport Chain
Electron Transport Chain
3rd Stage of Respiration
Occurs in the inner membrane of mitochondia
1. The electron transport chain consists of protein molecules found attached to the
inner membrane of the mitochondria
2. NADH and FADH2 from Glycolysis and the Citric Acid pathways release high-energy
electrons to the electron transport chain on the mitochondrial membrane and this
results in the synthesis of the bulk of ATP
ATP synthesis
3. High energy electrons are used to pump hydrogen ions across a membrane and flow
these ions back through the membrane synthesising ATP using the membrane
protein ATP synthase
4. Final electron acceptor is oxygen
5. Oxygen combines with hydrogen ions and electrons to form water
7i) Electron Transport Chain
Electron Transport Chain
7j) Substrates for Respiration
Substrates
Fate
Starch & Glycogen
Broken down into Glucose and feed into the GlycolyticPathway
Other Sugars
(e.g. maltose, sucrose) – converted to glucose and feed into
the Glycolytic Pathway
Fats
broken down into Fatty Acids & Glycerol. Glycerol feeds into
the Glycolytic Pathway, and the Fatty Acids enter the pathway
as Acetyl Coenzymes A for use in the Citric Acid Cycle
Proteins
Amino Acids are de-aminated forming intermediates that can
enter the respiratory pathway as pyruvate, acetyl coA, or
intermediates of the Citric Acid Cycle
7k) Regulation of Cellular Respiration Pathways

The cell conserves its resources by only producing ATP when required

As the rate of glycolysis and the citric acid cycle increases, ATP supply
increases

As the rate of glycolysis and the citric acid cycle decreases, ATP supply
decreases

If a cell produces more ATP than it needs, the high concentration of ATP
inhibits phosphofructokinase, and slows down the rate of glycolysis

When the concentration of ATP decreases, the enzyme is no longer inhibited
and glycolysis speeds up

The rate of glycolysis and the rate of the citric acid cycle are synchronised
by the inhibition of phosphofructokinase by citrate
-
If citrate accumulates, glycolysis slows down
-
If citrate consumption increases, glycolysis increases the supply of acetyl
groups to the citric acid cycle
7l) Regulation of Cellular Respiration Pathways
Human Cells Questions
KEY AREA 7 – Cellular Respiration
1. Testing Your Knowledge 1
2. Quick Quiz
Page 106
Q’s 1-3