B4 - Bioenergetics

• Photosynthesis: carbon dioxide + water --->(light) glucose + oxygen
• Photosynthesis is a process in which energy is transferred from the environment to a plant cell's chloroplast by light. It is an endothermic reaction.
• The glucose produced in photosynthesis may be:

  • used for respiration

  • converted into insoluble starch for storage

  • used to produce fat or oil for storage

  • used to produce cellulose, which strengthens the cell wall

  • used to produce amino acids for protein synthesis.
• To produce proteins, plants also use nitrate ions that are absorbed from the soil.
• The factors which affect the rate of photosynthesis are:

  • Light intensity
  • Carbon dioxide concentration
  • Temperature

  (Amount of chlorophyll also affects it)
• The ways to measure the rate of photosynthesis are:

  • Rate of oxygen output
  • Rate of carbon dioxide uptake
  • Rate of carbohydrate production
• Without enough light, a plant cannot photosynthesise very quickly - even if there is plenty of water and carbon dioxide and a suitable temperature.
• Increasing the light intensity increases the rate of photosynthesis, until some other factor – a limiting factor – becomes in short supply.
• Carbon dioxide – with water – is one of the reactants in photosynthesis.

  If the concentration of carbon dioxide is increased, the rate of photosynthesis will therefore increase.

  Again, at some point, a factor may become limiting.
• The chemical reactions that combine carbon dioxide and water to produce glucose are controlled by enzymes. As with any other enzyme-controlled reaction, the rate of photosynthesis is affected by temperature.
• At low temperatures, the rate of photosynthesis is limited by the number of molecular collisions between enzymes and substrates. At high temperatures, enzymes are denatured.
• Chlorophyll absorbs the light energy required to convert carbon dioxide and water into glucose.

  Chlorophyll is green - so absorbs the red and blue parts of the electromagnetic spectrum and reflects the green part of the spectrum.
• Leaves with more chlorophyll are better able to absorb the light required for photosynthesis.
• With the law of limiting factors, each factor does not work in isolation. Several factors may interact, and it may be any one of them that is limiting photosynthesis.
• Cellular respiration is an endothermic reaction process which is continually occurring in living cells.
• Respiration in cells can take place aerobically (using oxygen) or anaerobically (without oxygen), to transfer energy.
• Organisms need energy for:

  • chemical reactions to build larger molecules

  • movement

  • keeping warm.
• Aerobic Respiration: glucose + oxygen --> carbon dioxide + water
• Anaerobic Respiration: glucose --> lactic acid
• As the oxidation of glucose is incomplete in anaerobic respiration much less energy is transferred than in aerobic respiration.
• Anaerobic respiration in plant and yeast cells is represented by the equation:

  glucose --> ethanol + carbon dioxide
• Anaerobic respiration in yeast cells is called fermentation and has economic importance in the manufacture of bread and alcoholic drinks.
• During exercise the human body reacts to the increased demand for energy. The heart rate, breathing rate and breath volume increase during exercise to supply the muscles with more oxygenated blood.
• If insufficient oxygen is supplied anaerobic respiration takes place in muscles. The incomplete oxidation of glucose causes a build up of lactic acid and creates an oxygen debt.
• During long periods of vigorous activity muscles become fatigued and stop contracting efficiently.
• Blood flowing through the muscles transports the lactic acid to the liver where it is converted back into glucose.
• Oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells.
• Metabolism is the sum of all the reactions in a cell or the body.
• The energy transferred by respiration in cells is used by the organism for the continual enzyme controlled processes of metabolism that synthesise new molecules.
• Metabolism includes:

  • conversion of glucose to starch, glycogen and cellulose

  • the formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids

  • the use of glucose and nitrate ions to form amino acids which in turn are used to synthesise proteins

  • respiration

  • breakdown of excess proteins to form urea for excretion.