Cells and Energy
• All food consumed is ultimately broken down into glucose units before it
can be utilised by the body.
• The chemical energy in glucose and other organic compounds is not used
directly by cells.
• Cells carry out a series of reactions that release chemical energy from
glucose and transfer it to ATP. The energy is then available for use by cells.
• The series of energy releasing reactions that break down organic
compounds of food, releasing chemical energy and transferring it to ATP, is
known as cellular respiration (or sometimes, just respiration). Not to be
confused with respiration as in breathing.
• Cellular respiration occurs all the time in the cells of all living things.
Energy from glucose
• Process of energy transfer from glucose to ATP is not 100 per cent
efficient.
• About 40 per cent of the chemical energy present in glucose is
transferred to ATP and the remaining 60 per cent appears as heat energy.
• The heat energy produced by living cells cannot be used to drive energyrequiring activities, such as muscle contraction or transport against a
concentration gradient.
• Instead heat energy is used to maintain the core body temperature of
animals such mammals and birds within a narrow range. Insulating layers
of fat, fur or feathers traps the heat energy released from cellular
respiration.
Cellular Respiration
1. A series of biochemical pathways
2. Involves many chemical reactions and enzymes
3. Process that gradually breaks down food (organic
substances) to release small packets of energy
4. Small packets of energy used to convert ADP into ATP
5. ATP is the chemical energy currency used by cells
6. 3 main stages are
(i) Glycolysis
(ii) Krebs Cycle (Citric acid cycle)
(iii) Electron Transport Chain (ETC) and Chemiosmosis
Three stages of respiration
• Glycolysis
– Occurs in cytosol
• Citric acid cycle/Kreb’s Cycle
– Occurs in matrix of mitochondria
– Also known as Kreb’s Cycle
• Electron transport
– Occurs in cristae of mitochondria
Reaction for Cellular Respiration
• Strictly speaking, ‘cellular respiration’ refers to the aerobic
breakdown of glucose to drive the production of ATP; that is,
the pathways that evolved when oxygen is available to
mitochondria in eukaryotic cells.
• The general simplified formula for the complete aerobic
breakdown of glucose is:
What happens when there is no
oxygen?
• If oxygen is not available, glycolysis is followed by
fermentation and no more energy in the glucose molecule
will be harvested—no further ATP is produced.
• This process is referred to as anaerobic respiration.
• Pyruvate is converted via an anaerobic pathway to either
lactic acid (in most animals) or alcohol and carbon dioxide (in
most plants, and in microorganisms such as yeast and
bacteria).
• Fermentation is necessary as it prevents the accumulation of
pyruvate and thus allows glycolysis to continue.
Anaerobic respiration in mammals
• In the absence of oxygen, an enzyme present in human muscle
tissue converts pyruvate to lactate (lactic acid) molecules.
• The total energy yield for anaerobic respiration is two ATP per
glucose molecule.
• If strenuous exercise continues, lactate builds up in the muscles,
the pH falls and pain and muscle fatigue occur.
• When strenuous exercise stops, the oxygen supply to the
muscles is adequate for normal needs and anaerobic respiration
stops.
• Accumulated lactate in muscle tissue is converted back to
pyruvate and enters the Krebs cycle.
Summary of pathway
Glucose (6C)
Click on a section for
more information
ATP
ATP
ATP
O2
Glycolysis
Krebs
Cycle
ETC &
Chemiosmosis
H2 O
CO2
Fermentation
(without O2)
Alcohol and
Lactic acid
Conclusion and summary in diagram form and in table form
Glycolysis
Splits a glucose
molecule into
2 - 3 Carbon
molecules called
PYRUVATE.
products: ATP, NADH and pyruvate
Preparation for the Citric Acid Cycle
C
The pyruvate loses a
carbon leaving the 2
carbon molecule
C
Acetyl CoA
CO2
products: CO2,
Acetyl CoA and NADH
Glycolysis
Inputs
•
•
•
•
Glucose
NAD
(2 ATP)
4 ADP
Outputs
•
•
•
•
2 Pyruvate
NADH
(2 ADP)
4 ATP
The Citric Acid Cycle
products:
CO2, ATP, NADH, FADH
Krebs cycle
• Before each pyruvate
enters the Krebs cycle
it loses one carbon
dioxide molecule
• The NAD carrier picks
up the hydrogen
• The remaining 2
carbons bond to
Coenzyme A to enter
the Krebs cycle
•
Write as formula
Krebs cycle (cont.)
• During the Krebs cycle,
two more carbon
dioxide molecules are
given off
• A total of 10 hydrogen
molecules are picked
up by NAD and FAD
carriers
• Each pyruvate molecule
yields one ATP (meaning
2 per glucose molecule)
Krebs cycle
Inputs
•
•
•
•
•
2 Pyruvate
Coenzyme A
NAD
FAD
ADP
Outputs
•
•
•
•
•
Coenzyme A
CO2
NADH
FADH2
ATP
Electron Transport
H+ H+
NAD
H+
H+
outer membrane
H+
matrix
H+H+
H+
H+
inner membrane
or cristae
During electron transport, electrons from ‘loaded’ acceptors (NADH
and FADH2) are brought to the inner membranes of the mitochondria.
The electrons are passed back and forth across the membrane from
one cytochrome to another.
During this process their energy is gradually decreased and used
to transport H+ through the membrane.
Oxygen is the final electron acceptor and it joins with the H+ to
produce H2O.
If there is no oxygen, the electron chain cannot continue
because there is no way to release electrons .
products:
H2O, ATP
Electron transport chain
• The carriers NADH and FADH2 deliver the hydrogen ions
to the inner membrane
• Hydrogen ions pass into the inner membrane, passing
electrons along at the same time
• The hydrogen ions are used to generate 32 ATP through
ATP synthase
• Each oxygen molecule accepts hydrogen ions to create
water as a by-product
Electron Transport Chain
Inputs
•
•
•
•
FADH2
NADH
Oxygen
ADP
Outputs
•
•
•
•
FAD
NAD
Water
ATP
Outcome of the three stages
• In cells of your heart, liver and kidneys, two additional molecules of ATP
are generated to give a total of 38 ATP.
• This is because the NADH produced during glycolysis in those cells enters
the respiratory chain earlier than NADH produced in other kinds of cell.
TOTAL Cellular Respiration
Inputs
Outputs
• Glucose
• Oxygen
• Water
• Carbon dioxide
• Water
• 36 ATP
Remember: NAD, FAD and ADP are all carriers, they aren’t used
up by this reaction so you don’t include them in the equation
Alcoholic Fermentation
•
•
•
•
•
During fermentation by yeast, pyruvate is broken down to carbon dioxide and
ethanol (an alcohol).
The amounts of ethanol and carbon dioxide produced vary with different yeasts
and different environmental conditions.
In wine-making, grapes are crushed to release the juice which contains sugars.
Yeasts are added to this fluid, fermentation occurs which produces alcohol. When
the alcohol concentration reaches about 12 per cent (v/v), this kills the yeast cells
and fermentation stops.
Beer is made by fermenting sprouting barley grains using brewers’ yeast. Hops
are added to give colour, taste and aroma.
Spirits are produced by fermenting various products, such as fruit juice (brandy),
molasses (rum), barley grains (whisky). Spirits are distilled to increase the alcohol
content in the final product to about 40 per cent (v/v).
Comparison of anaerobic and aerobic
respiration
Other substrates for respiration
•
The products of digestion of fats (fatty acids
and glycerol) and the products of digestion of
proteins (amino acids) can also enter the
pathways of cellular respiration at various
points.
•
When starved of food for a long period, even
the proteins in muscles and other body tissues
will be broken down to provide the energy
necessary to survive.
•
During starvation in people, up to 97 per cent
of fat tissue, 31 per cent of skeletal muscle and
27 per cent of blood can be lost. The brain,
heart and diaphragm are not affected
•
Fats provide more energy per gram (39 kJ)
than either carbohydrates or proteins (about
17 kJ each).
Summary
Link between cellular respiration and
photosynthesis
•
Carbon dioxide and water are the waste products of
respiration.
•
These are the basic materials that a plant uses for
photosynthesis.
•
Photosynthesis is an endergonic (energy-requiring)
reaction.
•
Cellular respiration is an exergonic (energy-releasing)
reaction.
Respiration
C6H12O6 + 6O2 → 6CO2 + 6H2O + 36−38 ATP
Glucose + oxygen → carbon dioxide + water + energy
Photosynthesis
6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2
carbon dioxide + water + light → glucose + oxygen