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Essential idea: Cell respiration
supplies energy for the
functions of life.
Topic 2: Molecular Biology
2.8 Respiration
Nature of Science
Assessing the ethics of scientific
research—the use of invertebrates in
respirometer experiments has ethical
implications. (4.5)
Understandings
2..8.U1 Cell respiration is the controlled release of
energy from organic compounds to produce ATP.
2.8.U2 ATP from cell respiration is immediately
available as a source of energy in the cell.
2.8.U3 Anaerobic cell respiration gives a small yield
of ATP from glucose.
2.8.U4Aerobic cell respiration requires oxygen and
gives a large yield of ATP from glucose.
Applications and Skills
2.8.A1 Application: Use of anaerobic cell
respiration in yeasts to produce ethanol and
carbon dioxide in baking.
2.8.A2 Application: Lactate production in humans
when anaerobic respiration is used to maximize the
power of muscle contractions.
2.8.S1 Skill: Analysis of results from experiments
involving measurement of respiration rates in
germinating seeds or invertebrates using a
respirometer.
Key Terms
Summary of Energy Reactions:
Energy Reactions
Photosynthesis
Light Energy --> Chemical energy
sunlight
6CO2 + 6H2O ----> C6H12O6 + 6O2
chlorophyll
Respiration
Chemical Energy --> Chemical energy
C6H12O6 + 6O2 ----> 6CO2 + 6H2O + Heat
ADP + P
Occurs in green plant cells
ATP
Occurs in all living cells
Energy for life
Every living cell in your body needs energy. The average
adult has about 50 million million cells – that’s a lot of
energy!
energy
Where does all this energy come
from?
In body cells, the energy needed for life comes from
the chemical energy stored in glucose.
What is needed for releasing energy?
Burning is a chemical reaction in which energy is
released in the form of heat.
What other substance is needed for energy to be
released from this fuel?
fuel
oxygen
?
Respiration
In respiration, carbohydrates like
glucose (fuel) are broken down to
provide energy for the body.
This must occur in a controlled manner
in order for the body to capture the
energy in a usable form.
What is breathing?
DON’T GET THESE MIXED UP
What is aerobic respiration?
Respiration is the process used by the body’s cells
to release the chemical energy stored in glucose.
When oxygen is involved in this energy-releasing
process, it is called aerobic respiration.
What do you think aerobic means?
aerobic = ‘with oxygen’
Aerobic respiration is an efficient process that
generates enough energy to supply the whole body.
Our bodies perform aerobic respiration most of the
time, as long as the supply of oxygen remains high
enough.
Respiration is the process by which organisms extract the
energy stored in complex molecules and use it to generate
adenosine triphosphate (ATP).
What is this ‘Useable Form of Energy?
In this way they obtain energy to fuel their metabolic pathways.
ATP provides the immediate
source of energy for biological
processes such as active
transport, movement and
metabolism.
ATP
Adenosine triphosphate
adenine
ATP contains a sugar (ribose), a
base (adenine) and three
phosphate groups.
ribose
phosphates
When ATP is hydrolysed to form ADP and inorganic phosphate, 30.5 kJ of
energy are released.
+
ATP
+

H 2O
ADP
+ 30.5 kJ
inorganic
phosphat
e
Biological systems transfer the energy in glucose to ATP
because unlike glucose…
Why ATP?
glucose
ATP

ATP releases its energy instantly in a single reaction.

The hydrolysis of ATP releases a small amount of
energy, ideal for fuelling reactions in the body.
Types of Respiration
During aerobic respiration, a respiratory substrate, e.g.
glucose, is split in the presence of oxygen to release
carbon dioxide and water. A large number of ATP
molecules are produced, releasing the energy from the
glucose.
C6H12O6 + 6 O2  6 CO2 + 6 H2O + 36 ATP
In anaerobic respiration, glucose is converted (in the
absence of oxygen) to either lactate or ethanol. The ATP
yield is low.
C6H12O6  2 C2H5OH + 2 CO2 + 2 ATP
ethanol
C6H12O6  2 C3H6O3 + 2 ATP
lactate
Where Does Respiration Occur?
Respiration occurs in all living cells. In eukaryotes the early stages
of respiration occur in the cytoplasm. The later stages of
respiration are restricted to the mitochondria.

Mitochondria contain highly folded inner membranes
that hold key respiratory proteins (including the
enzyme that makes ATP) over a large surface area.

Mitochondria provide an isolated
environment to maintain optimum
conditions for respiration.

Mitochondria have their own DNA
and ribosomes, so can manufacture
their own respiratory enzymes.
The structure of the mitochondria
An overview of respiration
Respiration in detail…………
Cell respiration occurs in 3 main stages:
1. Glycolysis
2. Krebs Cycle
Transport
3. Electron
 Glycolysis means “splitting of sugar”
 A six-carbon sugar (glucose) is broken into two
three-carbon molecules of pyruvate, which still
hold most of the energy of glucose.
 This occurs in the cytoplasm and does
not require oxygen.
 There is a net yield of two ATP molecules.
The first stage of respiration:
glycolysis
The stages of glycolysis
The fate of pyruvate
Krebs cycle
Counting carbons
For each molecule of glucose, glycolysis produces:
Keeping track of the products

2×

2×

2×
For each molecule of glucose, the link reaction produces:

2×

2×

2×
For each molecule of glucose, Krebs cycle generates:
Keeping track of the products

4×

6×
produced by redox reactions

2×
produced by redox reactions

2×
produced by substrate-level phosphorylation
produced by decarboxylation
The NADH and FADH2 contain the potential energy
originally locked in glucose. This energy is now transferred
to ATP by oxidative phosphorylation in the electron
transport chain.
The electron transport chain
Understanding the ETC
How much ATP is produced?
Process
ATP in
ATP produced
glycolysis
2
4
2
link reaction
0
0
0
Krebs cycle
0
2 (per glucose)
Net ATP out
2 (per glucose)
Via the electron transport chain and chemiosmosis, each NADH can
yield 2.5 ATP and each FADH2 1.5 ATP.
From one molecule of glucose, glycolysis yields 2 NADH, the link
reaction yields 2 NADH and the Krebs cycle yields 6 NADH and 2 FADH2.
10 × 2.5 = 25 ATP from NADH
2 × 1.5 = 3 ATP from FADH2
total = 2 + 2 + 25 + 3
= 32 ATP overall
The theoretical yield of 32 ATPs for each glucose molecule is rarely
achieved. In fact respiration is only about 32% efficient.
Efficiency of aerobic respiration

Some protons leak across the mitochondrial membrane,
so not all are available to generate ATP via chemiosmosis.

Some ATP is used up moving
pyruvate into the mitochondria
by active transport.

Some ATP is used up moving hydrogen from reduced NAD
made during glycolysis into the mitochondria.

Some energy is lost as heat. This heat helps to maintain a
suitable body temperature for enzyme-controlled reactions.
Adaptations of mitochondria
Evidence for chemiosmosis
The theory of chemiosmosis
states that the energy in a
chemical gradient established
by electron movement is used
to generate ATP.
matrix
Evidence includes:

The proton gradient across the inner membrane can be
measured as it corresponds to a pH gradient.

Isolated ATP synthase enzymes can produce ATP using a
proton gradient even if no electron transport is occurring.

Chemicals that block the ETC inhibit the formation of a
proton gradient and prevent ATP synthesis.
Respiratory rate
The respiratory rate is the rate at
which an organism converts glucose
to CO2 and water. It can be
calculated by measuring an
organism’s rate of oxygen
consumption.
Studies on simple animals often
use a respirometer.
Respirometers measure the change in gas volume in a closed system.
Any change is due to the respiratory activity of the study organisms.
Potassium hydroxide or soda lime is used to absorb the carbon dioxide
produced, meaning any changes in volume are due to oxygen
consumption.
The respirometer
Respirometer experiments
Other substances as well as glucose can be respired. Different
respiratory substrates release different amounts of energy.
Respiratory substrates
Respiratory substrate
carbohydrate
Mean energy value (kJ g-1)
15.8
lipid
39.4
protein
17.0
The difference in the relative energy values of these respiratory
substrates is due to the amount of hydrogen atoms present in
each one. If more hydrogen atoms are available to reduce
coenzymes, more energy can subsequently be generated in
the electron transport chain.
Respiratory quotient (RQ) is the ratio of the volume of carbon dioxide
produced to the volume of oxygen used in the same period of time.
Respiratory quotient
RQ =
volume of CO2 given out
volume of O2 taken in
RQ gives an indication of the respiratory substrate being respired and
whether respiration is aerobic or anaerobic.
Type of respiration
Substrate
RQ
anaerobic
glucose
>1
aerobic
carbohydrate
1.0
protein
approx. 0.9
lipid
approx. 0.7
Respiration calculations
Anaerobic Respiration
Anaerobic respiration:
 Without oxygen
 Partial breakdown of glucose molecule
 Minimal release of energy: 1 glucose  2ATP
 Occurs in 2 stages:
1. Glycolysis (occurs in the cytoplasm)
2. Fermentation (occurs in the cytoplasm)
 Glycolysis: Glucose  Pyruvate + 2ATP
 During fermentation:
1. In animals: Pyruvate  Lactic acid
2. In plants and yeast: Pyruvate  Ethanol + CO2
Anaerobic respiration is useful to the body when
energy is needed in a hurry.
The problems with anaerobic respiration
glucose
lactic
acid
energy
There are two problems with anaerobic respiration:
 Anaerobic respiration releases much less energy
from
glucose compared to aerobic respiration.
 Lactic acid is a poisonous waste product.
Why is anaerobic respiration not the best way to get
energy from glucose?
Lactic acid is the product of anaerobic respiration
and is harmful because it can stop muscles from
doing their job.
Why is lactic acid so harmful?
If lactic acid builds up in muscle cells, it stops
muscles from contracting and relaxing and they
become fatigued.
The muscles ache and the body experiences cramp,
which forces the body to stop what it is doing and
rest.
Why can anaerobic respiration only be carried out
for short periods of time?
After anaerobic respiration, the body
is in recovery and must get rid lactic
acid.
Recovery and getting rid of lactic acid
glucose
lactic
acid
energy
The body is now at rest but the breathing rate
and heart rate remain high. Why does this
happen?
Oxygen is needed to get rid of lactic acid by
turning it into carbon dioxide and water.
lactic
acid
oxygen
carbon
dioxide
water
Why do the breathing and heart rates return to
normal after a few minutes of recovery?
During aerobic respiration, muscles get energy
Oxygen debt – build up
from glucose by ‘paying’ for it with oxygen.
glucose oxygen
carbon
dioxide
water
energy
During anaerobic respiration, muscles get energy
from glucose but do not ‘pay’ for it with oxygen.
glucose
lactic
acid
energy
This means that an oxygen debt is created.
When and how is this oxygen debt ‘paid
off’?
The oxygen debt caused by
Oxygen debt – pay off
anaerobic respiration is paid off
during the recovery period after
exercise.
It is the oxygen needed to get rid of lactic
acid that pays back the oxygen debt.
lactic
acid
oxygen
carbon
dioxide
water
oxygen
to pay back
‘oxygen debt’
When is the oxygen debt completely paid off?
Anaerobic respiration: word equation
activity
Revision Questions:
1. Write a chemical equation for the process of
aerobic respiration.
2. Draw a diagram of the mitochondria and
label the parts.
3. Explain why it is possible that the products
of anaerobic respiration in animal and plant
cells results in different products even
though they both start with Pyruvate.
4. Define cell respiration.
5. List the differences between aerobic and
anaerobic respiration.
6. Define autotroph and heterotroph in terms
of energy and respiration (will need to look
this up).
Anaerobic and Muscles
Aerobic Vs Anaerobic
Uses of Anaerobic Respiration
Summary
Glossary
What’s the keyword?
Multiple-choice quiz