Topic 5 - A: Photosynthesis and respiration

• Why is energy important?

  Plant and animal cells need energy for biological processes to occur

  1. plants need energy for things like photosynthesis, active transport (e.g. to take in minerals via their roots), DNA replication, cell division and photosynthesis
  2. animals need energy for things like muscle contraction, maintenance of body temperature, active transport, DNA replication, cell division and protein synthesis
• Photosynthesis and energy: photosynthesis is the process where energy from light is used to make glucose from water (H2O) and carbon dioxide (CO2). The light energy is converted to chemical energy in the form of glucose - C6H12O6
• The overall equation of photosynthesis:

  6CO2 + 6H20 + Energy = C6H12O6 + 6O2
• Energy is stored in the glucose until the plants (or other photosynthesising organisms e.g. algae) release it by respiration.
• Animals obtain glucose by eating plants and other animals, which have eaten plants, then respire the glucose to release energy
• Photosynthesis is an example of a metabolic pathway - the process occurs in a series of small reactions controlled by enzymes
• Plant and animal cells release energy from glucose - this process is called respiration - the energy is used to power all the biological processes in a cell
• There are two types of respiration: aerobic and anaerobic
• Aerobic respiration - respiration using oxygen
• Anaerobic respiration - respiration without oxygen
• Aerobic respiration produces carbon dioxide and water, and releases energy:

  C6H12O6 + 6O2 = 6CO2 + 6H2O +Energy
• Anaerobic respiration in plants and yeast produces ethanol and carbon dioxide and releases energy.
• In humans, anaerobic respiration produces lactate and releases energy
• Aerobic and anaerobic respiration are examples of metabolic pathways
• ATP: adenine triphosphate is the immediate source of energy in a cell
• A cell cannot get its energy directly from glucose, so in respiration the energy released from glucose is used to make ATP
• ATP is made from the nucleotide base adenine, combined with a ribose sugar and three phosphate groups. It carries energy around the cell to where it is needed
• ATP is synthesised via a condensation reaction between ADP (adenine diphosphate) and inorganic phosphate (Pi) using energy from an energy-releasing reaction e.g. the breakdown of glucose in respiration
• The energy is stored as chemical energy in the phosphate bond - the enzyme ATP synthase catalyses this reaction
• Adding phosphate to a molecule is known as phosphorylation
• ADP is phosphorylated to ATP
• ATP then diffuses to the part of the cell that needs energy. Here it is broken down back into ADP and inorganic phosphate (Pi). Chemical energy is released from the phosphate bond and used by the cell. ATP hydrolase catalyses this reaction
• This process is known as hydrolysis - the splitting (lysis) of a molecule using water (hydro). The ADP and inorganic phosphate are recycled and the process starts again
• Properties of ATP: specific properties that make it a good energy source:

  1. ATP stores or releases only a small, manageable amount of energy at a time, so no energy is wasted as heat
  2. It's a small soluble molecule so it can be easily transported around the cell
  3. It's easily broken down, so energy can easily be released instantaneously
  4. It can be quickly remade
  5. It can make other molecules more reactive by transferring one of its phosphate groups to them (phosphorylation)
  6. ATP can't pass out of the cell, so the cell always has an immediate supply of energy
• Plants can carry out both photosynthesis and respiration. Both processes can occur at the same time, and at different rates. The rate at which photosynthesis takes place is partly dependent on the light intensity of the environment the plant is in
• There is a particular level of light intensity at which the rate of photosynthesis exactly matches the rate of respiration - the compensation point for light intensity
• One way to work out the compensation point for a plant is to measure the rate at which oxygen is produced and used by a plant at different light intensities.
• Because photosynthesis produces oxygen and respiration uses it, in this case, the compensation point is the light intensity at which oxygen is being used as quickly as it is produced
• The rate of CO2 production could also be measured - photosynthesis used CO2 and respiration produces it