RespirationCells at work

Basic cell processesAll cells need to:– obtain nutrients – synthesise materials for growth – provide energy for movement and metabolism– eliminate wastes– produce a variety of biological molecules• assemble them into new organelles • use them for repair and maintenance.

Cell replication• Additional demands on the activities of cells: – doubling of the cytoplasm– replication of DNA– division of the nucleus– synthesis of new membranes– division of the cell with an equal distribution of

organelles to each new (daughter) cell.• Some cells can go through this cycle of doubling

followed by cell division in less than an hour.

Providing energy in cells

• ALL cells use chemical energy to carry out their energy-requiring activities.

• Chemical energy is stored in the bonds (connections) that join atoms together in molecules.

• If a molecule is broken apart, the energy of these bonds is released.

• Organic molecules such as glucose, fats and proteins have many energy-containing bonds that can be broken apart in cells to release energy.

ATP: chemical energy for cells

• Cells store immediately usable chemical energy in the form of ATP molecules. • ATP has a high-energy terminal phosphate bond that is easily broken.• This releases a small ‘packet’ of energy, which can be used to carry out all the

energy-dependent processes of cells (energy for work).• Once an ATP molecule has given up its energy, it becomes ADP• ADP can be ‘recharged’ and then used again.• The recharging process requires some energy, but much less than it would

take to make an entirely new ATP molecule.

ATP = adenosine triphosphate

ADP = adenosine diphosphate

Generating heat

• When cells convert energy from one form into another, some energy is always lost to the surroundings, usually as heat.

• For example, during exercise we use the chemical energy of ATP to make our muscles contract.

• The more work we do, the more energy we use and the more energy is lost as heat, which warms up our muscles.

Providing energy in cells• Cells get the energy to make ATP by breaking apart glucose molecules.

• The chemical energy from the glucose molecule is released in a series of small steps that involve many enzymes, so that the energy is released in many small ‘packets’, each of which is used to produce ATP from ADP.

• Almost 40% of the chemical energy released during the breakdown of glucose to carbon dioxide and water is converted to energy stored in the bonds of ATP molecules.

• The types of chemical reactions that occur and the amount of energy that can be harvested from glucose depend on whether oxygen is present or not.

Respiration equation

• Sometimes ‘cellular respiration’ is used to refer to both the aerobic and anaerobic energy-releasing pathways in cells.

• However, biochemists define cellular respiration as the aerobic pathways of energy release, which occur in mitochondria in eukaryote cells.

• Simplified formula for the complete aerobic breakdown of glucose is:

Glycolysis• Initial stage in the breakdown of glucose.• ‘Lysis’ or splitting of glucose into two pyruvate

molecules.• Occurs in the cytosol of the cell.• Anaerobic - does not require oxygen.

Glycolysis occurs in the cytosol

• For each molecule of glucose, glycolysis produces two ATP molecules very rapidly.

• This stage is common to both aerobic and anaerobic pathways (‘an’ meaning without).

• What happens next depends on whether or not there is an adequate supply of oxygen.

Aerobic respiration: in mitochondria• Requires oxygen.• Glucose is broken down further.• An additional 34 - 36 molecules of ATP are produced. • Occurs in mitochondria.• Inner membrane forms folds called cristae - provide a large

surface area where the chemical reactions can take place.• Aerobic respiration involves two linked pathways - the Krebs

cycle and the electron transport chain.

Energy from other moleculesOther organic molecules – eg lipids and proteins - can be broken down to generate ATP.

Fermentation• Anaerobic (without oxygen)• Occurs in the cytosol.• No further formation of ATP.• Pyruvate produced during glycolysis undergoes

fermentation into either lactic acid (Animals) or carbon dioxide and alcohol (plants, yeasts, bacteria).

• Fermentation is necessary to prevent the accumulation of pyruvate and thus allows glycolysis to continue.

• Accumulation of a product can slow a chemical reaction.

Oxygen debt - Lactic acid

Lactic acid and fatigue

• Latest scientific evidence suggests that accumulation of lactic acid in exercising muscle does not cause muscle fatigue.

• Cramping during strenuous exercise could be due to:– lactic acid build up– dehydration– electrolyte imbalance (Ca, K or Na ions)– factors linked to spinal cord reflex muscle control.

• Pain is due to an irritation of the sensory nerve endings. • Healthy bodies recover quickly - usually within an hour.• Delayed soreness is most likely caused by tiny tears in the

muscle.

Comparing pathways

Anaerobic respiration (glycolysis) Aerobic respiration (cellular respiration)

Oxygen not required Oxygen required

Rapid ATP production Slower rate of ATP production

Mammals sustain over short time only Sustain indefinitely

Less efficient energy transfer More efficient energy transfer

2 mole ATP produced per mole of glucose used 36 mole ATP produced per mole of glucose used (some tissues produce 38)

Various end products:Lactic acid (humans)Ethanol + carbon dioxide (yeast)Butyl alcohol (some bacteria)

End products are:Carbon dioxide + water

Value of both pathways

• For each molecule of glucose, aerobic respiration produces almost 20 times the number of ATP molecules produced by glycolysis.

• So it is not surprising that most cells of animals and plants normally carry out aerobic respiration.

• They rely on the generation of ATP by glycolysis only for very short periods (seconds) or when there is not enough oxygen available for aerobic respiration.