Cell Respiration (DP IB Biology) : Revision Note

Cell Respiration

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

Comparing Anaerobic & Aerobic Cell Respiration

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