2.8 - Cell respiration

Question 1

Cell respiration is controlled by release of energy from organic compounds to produce ATP

Answer 1

• All cells perform respiration
• Example: energy is released in muscle fibres by breaking down glucose into carbon dioxide and water. The energy can then be used for muscle contraction
• The source of organic compounds in humans is food
• Carbohydrates and lipids are often used but amino acids from proteins may be used if we eat more protein than needed
• Plants use carbohydrates or lipids previously made by photosynthesis
• Cell respiration is carried out using enzymes in way so that as much as possible of the energy released is retained in the form of ATP
• To make ATP a phosphate group is linked to adenosine diphosphate, or ADP. Energy is required to carry out this action, this energy comes from the breakdown of organic compounds
• ATP is not transferred from cell to cell and all cells require a continuous supply
• Cell respiration is an essential function of life in all cells

Question 2

ATP from cell respiration is immediately available as a source of energy in the cell

Answer 2

• Cells require energy for three main types of activity
  • Synthesizing large molecules like DNA, RNA and proteins
  • Pumping molecules or ions across membranes by active transport
  • Moving things around inside the cell, such as chromosomes, vesicles, or in muscle cells the protein fibres that cause muscle contraction
• ATP is the energy source for all of these processes
• Energy from ATP is immediately available, and it is released by splitting ATP into ADP and phosphate, the ADP and phosphate can then be reconverted to ATP be cell respiration.
• Whenever energy from ATP is used in cells it is ultimately converted into heat.
• Heat cannot be reused for cell activities and is eventually lost to the environment, this is why cells require a continual source of ATP for cell activities

Question 3

Anaerobic cell respiration gives a small yield of ATP from glucose

Answer 3

• Glucose is broken down in anaerobic cell respiration without the use of oxygen
• The yield of ATP is relatively small but the ATP can be produced quickly
• Anaerobic cell respiration is useful in three situations
  • when a short but rapid burst of ATP production is needed
  • when oxygen supplies run out in respiring cells
  • in environments that are deficient in oxygen, for example waterlogged soils
• the products of anaerobic respiration are not the same in all organisms
• in humans, glucose is converted to lactic acid, a.k.a lactate
• in yeast and plants glucose is converted to ethanol and carbon dioxide
• both ethanol and lactate are toxic in excess so must be removed from the cells that produce them or produced in strictly limited quantities

Question 4

Aerobic cell respiration requires oxygen and gives a large yield of ATP from glucose

Answer 4

• if oxygen is available to a cell, glucose can be more fully broken down, to release a greater quantity of energy than in anaerobic respiration
• the yield of ATP in anaerobic respiration is 2 molecules per glucose
• the yield of ATP in aerobic respiration is 30+ molecules per glucose
• aerobic respiration involves a series of chemical reactions
• carbon dioxide and water are produced, in most organisms, carbon dioxide is a waste product that has to be excreted, but the water is often more useful
• in eukaryotic cells most of the reactions of aerobic cell respiration including all of the reactions that produce carbon dioxide happen inside the mitochondria

Question 5

Application: use of anaerobic cell respiration in yeasts to produce ethanol and carbon dioxide in baking

Answer 5

• yeast can respire anaerobically or aerobically
• after kneading the dough, it is kept warm, any oxygen in the dough is soon used up so the yeast carries out anaerobic respiration
• the carbon dioxide produced by anaerobic respiration cannot escape from the dough and forms bubbles, the swelling of the dough due to the production of bubbles of carbon dioxide is called rising
• ethanol is also produced by anaerobic cell respiration but it evaporates during baking
• yeast converts sugars into ethanol in large fermenters by anaerobic respiration
• starch and cellulose must first be broken down into sugars using enzymes
• the ethanol produced by the yeasts is purified by distillation and various methods are then used to remove water from it to improve its combustion

Question 6

Lactate production in humans when anaerobic respiration is used to maximise the power of muscle contractions

Answer 6

• anaerobic cell respiration involves the production of lactate, so when it is being used to supply ATP the concentration of lactate in a muscle increases
• there is a limit to the concentration that the body can tolerate and this limits how much anaerobic respiration can be done