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TOPIC: CELL RESPIRATION
Key Knowledge:
- General structure of the biochemical pathways in cell respiration from initial reactant to final product
- The main inputs, outputs and locations of glycolysis, Krebs Cycle and electron transport chain including ATP yield (details of biochemical pathway mechanisms are not required)
- The location, inputs and the difference in outputs of anaerobic fermentation in animals and yeasts
- Uses and applications of anaerobic fermentation of biomass for biofuel production
- Factors affecting the rate of cell respiration (temperature, glucose availability and O 2 concentration)
CELL RESPIRATION
Cellular respiration is the controlled release of energy from the breakdown of organic compounds. These compounds are produced by autotrophs (via photosynthesis) or can be synthesised from other pre-existing
molecules within the cell (e.g. excess glucose can be converted to fats). Usable carbon compounds include:
- Carbohydrates: The main organic molecule used in cell respiration is the monomer glucose (C 6 H 12 O 6 )
- Triglycerides: Fats produce more energy per gram than sugars, but are harder to transport and digest
- Proteins: Not a primary source as produces nitrogenous by-products (which are toxic if not excreted)
ENERGY CONVERSIONS
Organic molecules store energy in their chemical bonds – but this energy is not easily accessible for use by
the cell. Cell respiration transfers this stored energy into coenzymes. Two types of coenzymes are used:
- ATP: Immediately available energy source (energy is released for use when ATP is hydrolysed to ADP)
- Hydrogen carriers: Transitional energy source (carries high energy electrons and protons for transfer)
ATP can be produced directly from organic molecules via substrate level phosphorylation ( pink arrow ) or it
can be indirectly synthesised by hydrogen carriers (needs O 2 ) via oxidative phosphorylation ( yellow arrow ).
TYPES OF CELL RESPIRATION
Cellular respiration can involve one of two reaction pathways: anaerobic respiration or aerobic respiration
ANAEROBIC RESPIRATION AEROBIC RESPIRATION
Partial breakdown of glucose Oxygen is not required for a small ATP yield Occurs entirely in the cytosol Involves glycolysis and fermentation Products: Lactic acid / Ethanol + CO 2
Complete breakdown of glucose Oxygen is required for a large ATP yield Occurs in the mitochondria Involves glycolysis, Krebs cycle and ETC Products: Carbon dioxide and water
Glucose = Piggy Bank (stored chemical energy)
Hydrogen Carrier = Wallet (transitional energy source)
ATP = Cash (usable energy)
ANAEROBIC RESPIRATION
Anaerobic respiration involves the partial breakdown of carbohydrates (glucose) in the absence of oxygen.
It occurs in the cytosol and results in a low yield of ATP (net production = 2 ATP). This ATP is produced via
substrate level phosphorylation. The process of anaerobic respiration involves glycolysis and fermentation.
GLYCOLYSIS
Both anaerobic and aerobic respiration begins with the breakdown of glucose in the cytosol via glycolysis.
Glycolysis splits glucose into two molecules of pyruvate in a process that consumes two molecules of ATP.
However, four molecules of ATP are produced via substrate level phosphorylation, resulting in a net gain of
two ATP molecules. Additionally, the coenzyme NAD is loaded with hydrogen to form molecules of NADH.
FERMENTATION
In the presence of oxygen, the hydrogen carriers produced by glycolysis may be used by the mitochondria
to produce large amounts of ATP (via oxidative phosphorylation). However, in the absence of oxygen the
hydrogen carriers must be unloaded to allow for glycolysis to continue (NADH must be unloaded to NAD). Fermentation involves the conversion of pyruvate via a reaction that unloads hydrogen carriers to restore
stocks of NAD. In plants and yeasts, pyruvate is irreversibly converted into ethanol and carbon dioxide. In
animals, pyruvate is converted into lactic acid (however, this reaction can be reversed if oxygen is present).
AEROBIC RESPIRATION
Aerobic respiration completes the breakdown of glucose begun by glycolysis. This process requires oxygen and occurs within the mitochondrion. Aerobic respiration occurs via two distinct reactions:
- Krebs Cycle: Pyruvate is broken down to make carbon dioxide and large amounts of hydrogen carriers
- Electron Transport Chain: Hydrogen carriers are unloaded to produce ATP (oxidative phosphorylation)
6 CO 2
Carbon dioxide
6 H 2 O
Water
C 6 H 12 O 6
Glucose
6 O 2
Oxygen
ATP
C 6 H 12 O 6
Glucose
2 × C 3 H 4 O 3
Pyruvate
+ ATP
NADH
Hydrogen carrier
+ ATP
ENERGY INVESTMENT ENERGY PAYOFF (COENZYME LOADING)
NAD+
glucose
NADH
pyruvate
FERMENTATION
lactate (muscles)
ATP
GLYCOLYSIS Fermentation is reversible in animals
(but is irreversible in plants or yeast)
This means that lactic acid can be converted back into pyruvate when exercise is over and the pyruvate can then be digested aerobically to make ATP (via oxidative phosphorylation)
OVERVIEW OF AEROBIC RESPIRATION
ANAEROBIC VERSUS AEROBIC
Cell respiration involves the partial (anaerobic) or complete (aerobic) digestion of glucose to produce ATP
for use by the cell. Both pathways begin with the initial breakdown of glucose (by glycolysis) to form two
molecules of pyruvate. In anaerobic respiration, this pyruvate is converted within the cytosol into either
lactic acid (animals) or ethanol and carbon dioxide (plants and yeast). In aerobic respiration, the pyruvate
is converted into carbon dioxide and water within the mitochondrion. Aerobic respiration requires oxygen to proceed and produces a larger yield of ATP (oxidative phosphorylation utilises the hydrogen carriers).
TYPE OF CELL
RESPIRATION
ATP YIELD
Glycolysis Krebs Cycle Electron Transport Chain
Aerobic Cell Respiration (^2) × ATP 2 × ATP 26 × ATP
Anaerobic Fermentation 2 × ATP
BIOFUELS
Biofuels are an energy source produced from the anaerobic fermentation of biomass (i.e. organic material
from plants or animals). Biofuels are a renewable resource and are typically associated with a lower carbon
footprint (because biomass is typically produced via photosynthesis, which uses CO 2 as an input). Biomass
has historically been produced from agricultural feedstocks (edible crops), which requires large amounts of arable land and drives up local food prices (as less crops are being used as a food source). Biomass can also
be produced from non-edible plant components and certain municipal wastes; however, these sources are
associated with higher costs of production. More recently, algae has been used as a source of biomass. The
algae can photosynthesise at low costs and does not require large quantities of land (can be maintained in
a photobioreactor). Bioethanol is a common biofuel that can be used to supplement or replace traditional
fossil fuels (i.e. petrol) in fuel tanks. Drawbacks of bioethanol include the fact that it has a lower energy
output than fossil fuels, is harder to vaporise (more difficult to use in colder temperatures) and is more likely to corrode materials (such as car engines) upon extended exposure (i.e. higher maintenance costs).
4 ATP
2 ATP
ATP
ATP
ATP
ATP
Total net ATP yield: 30
ATP Glycolysis
Glucose
2
Pyruvate
Acetyl-CoA
Krebs Cycle
CYTOSOL
Substrate level Oxidative
NADH
NADH
NADH
FADH 2
Aerobic Stages:
Glycolysis Glucose → Pyruvate Substrate Level: 2 ATP
Krebs Cycle Pyruvate → CO 2 Substrate Level: 2 ATP
Electron Transport Oxygen → Water Oxidative: 26 ATP
Intermediates:
NADH = NAD + H+^ + e–
FACTORS AFFECTING RESPIRATION RATE
The rate of respiration can be measured by either the consumption of inputs (glucose and oxygen) or the
formation of product (carbon dioxide). However, these conditions may be affected by the pathway used:
- Anaerobic respiration does not use oxygen and carbon dioxide is only produced as a by-product of
yeast or plant fermentation (animal cells convert pyruvate into lactic acid via a reversible reaction)
Factors that affect aerobic respiration include: temperature, glucose concentration and oxygen availability
TEMPERATURE
Cell respiration is catalysed by a variety of enzymes and
is therefore impacted by ambient temperatures. If the
temperature is too low, the activation energy threshold
cannot be reached. As temperatures increase, reaction
rate will also increase as more kinetic energy results in
more frequent enzyme-substrate collisions. At optimal
temperatures, activity will peak, as higher temperatures will denature the enzymes involved in cell respiration.
GLUCOSE CONCENTRATION
Glucose is the initial substrate for both pathways of respiration (anaerobic and aerobic). Higher glucose
levels will result in increased frequency of collisions
with glycolytic enzymes over a given period of time.
Above a certain glucose level, the rate of respiration
will plateau. This is because the environment is now
saturated with glucose and some other condition has become the limiting factor that determines the rate.
OXYGEN AVAILABILITY
Increasing oxygen levels will result in higher rates of aerobic respiration. This is because oxygen is needed
to maintain the functioning of the electron transport
chain. Higher oxygen concentrations will increase the
rate of respiration up to a certain point, above which
the respiration rate will plateau as the environment
is now saturated with oxygen and some other factor has become the rate-limiting factor for respiration.
EXERCISE INTENSITY
One condition that will increase the rate of cellular respiration is exercise (muscles require ATP in order to
contract). Strenuous physical exertion will utilise anaerobic respiration, as ATP requirements exceed levels of oxygen intake. The resultant accumulation of lactic acid within the muscles will cause fatigue, rendering
high levels of exercise unsustainable. Aerobic respiration is then used as the level of activity diminishes.
Temperature (°C)
Rate of
Respiration
Rate of Re
spiration
Oxygen Concentration
Rate of Respiration
Glucose Concentration
