Cell Respiration (HL IB Biology)

Cell respiration as a system for producing ATP

• Cell respiration is the controlled release of energy from organic compounds to produce ATP
• Respiration is a series of chemical reactions that happens in every cell
• Its purpose is to release energy in usable forms from chemical energy stored in food e.g. glucose
• Respiration is a catabolic process
• Glucose is the main respiratory fuel used in cells
  • Lipids and proteins can also be used but they must undergo several changes before they can enter the respiratory pathway
  • Glucose can enter glycolysis directly which makes it easier to oxidise than lipids and proteins
  • Since proteins are primarily structural molecules, they will only be used as a respiratory fuel in conditions where glucose and lipids are not available
• Organic food substances contain a lot of chemical energy
• This energy cannot be released in one, uncontrolled step in cells, which would cause cell damage and tissue death
• Enzymes control the release of energy through a series of chemical reactions called a pathway
• This ends in the production of ATP (adenosine triphosphate)
  • To make ATP, a phosphate group is linked to adenosine diphosphate (ADP)
  • This process requires energy which comes from the breakdown of organic molecules
• The energy that is released is used for
  • Fuelling anabolic processes
  • Muscle contraction
  • Fuelling active transport
  • Moving molecules around the cell
  • Generating heat to maintain body temperature in warm-blooded animals

Differences between anaerobic and aerobic respiration in humans

• Respiration involves the transfer of chemical potential energy from nutrient molecules (such as carbohydrates, fats and proteins) into a usable energy form (through the synthesis of ATP) that can be used for work within an organism
• It is a vital process that takes place in the cells of all living organisms
• There are two forms of respiration depending on the oxygen availability of the cell:
  • Aerobic respiration
  • Anaerobic respiration
• Aerobic respiration is the process of breaking down a respiratory substrate in order to produce ATP using oxygen
  • The substrate is completely oxidised, thereby releasing a large amount of energy
• Anaerobic respiration takes place in the absence of oxygen and also breaks down a respiratory substrate but produces less ATP for the cell
• The main respiratory substrate involved in respiration is glucose

Aerobic respiration

• Aerobic cell respiration requires oxygen and gives a large yield of ATP from glucose
• The presence of oxygen allows glucose to be broken down fully into carbon dioxide and water
• This yields far more energy (approx. 36 ATP molecules) than anaerobic respiration (2 ATP molecules) per molecule of glucose
• CO₂ is a waste product and has to be excreted
  • Except in plants where it is used for photosynthesis
• Water is a by-product and contributes to the organism's water needs
  • Some animals that live in deserts drink very little but survive on this water
• Most of the reactions of aerobic respiration, in eukaryotes, take place in the mitochondria

Aerobic respiration releases energy during the reaction between glucose and oxygen

Anaerobic respiration

• In cells, there is a much lower energy yield from respiration in anaerobic conditions than in aerobic conditions
• The reactions of anaerobic respiration will occur in the cytoplasm of cells and does not involve the mitochondria
• There can be different ways in which oxygen becomes unavailable
  • When oxygen supply can't keep up with demand in heavily respiring cells
    • But a short supply of ATP is still required e.g. vigorous exercise requiring a lot of muscle contraction
  • In conditions where oxygen cannot reach the organisms e.g. in waterlogged soil
• In anaerobic respiration, glucose is only partially oxidised meaning only a small part of its chemical energy is released and transferred to ATP
  • The only ATP-producing reaction that continues is the first stage of respiration (around 2 ATP molecules per molecule of glucose)
• As there is no oxygen, none of the remaining reactions (of aerobic respiration) can take place
  • This means that around 36 ATP molecules are not produced anaerobically that would otherwise have been produced in the presence of oxygen
  • 2 ATP molecules are better than zero ATP molecules, so anaerobic respiration can give a short discharge of energy when oxygen runs out
• Different types of organisms produce different products when respiring anaerobically
  • Plants and yeasts produce ethanol and CO₂
  • Animals produce lactate

Anaerobic respiration in humans (and other animal cells) will partially oxidise glucose to form lactic acid

Comparison of Aerobic & Anaerobic Respiration in Humans Table

	Aerobic respiration	Anaerobic respiration
Oxidation of glucose	Complete	Incomplete
Oxygen required	Yes	No
Relative ATP yield	High (∼36 molecules)	Low (2 molecules)
Products	CO2 and H2O	Lactate
Location of reactions	Cytoplasm and mitochondria	Cytoplasm