2.8 Cell Respiration

Essential Idea:

Cell respiration supplies energy for the functions of life.

--> Cellular respiration is the gradual and controlled release of energy by breaking down organic compounds to produce ATP

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

==> Adenosine Triphosphate:

• energy is held in the bonds between atoms, in particular the high energy bond that joins the second and third phosphates
• ATP = currency of the cell
• Efficiency of respiration is measured by the yield of ATP
• It cannot be transferred between cells

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

What do we need energy for?

• Brain
• Muscles
• Cell reparation
• Digestion
• Membrane Transport
• etc. 

Parts of respiration:

1) Glycolysis

2) Krebs cycle

3) Electron transport chain & oxidative phosphorylation

Cellular respiration involves enzymes that control the process to ensure that energy is produced when it is required.

Two main ways of respiration:

Cell respiration can take place with or without the presence of oxygen. This leads to these two types:

1) Anaerobic respiration = no oxygen

2) Aerobic respiration = oxygen

Anaerobic cell respiration gives a small yield of ATP from glucose.

• Occurs only in the cell cytoplasm as it involves only glycolysis
  --> In the absence of O₂, glycolysis couples with fermentation or anaerobic respiration to produce ATP
• Two main types of anaerobic cell respiration:
  • Alcoholic fermentation
  • Lactic fermentation
• Fermentation consists of glycolysis plus reactions that regenerate NAD⁺, which can be reused by glycolysis
• Humans use the ability for anaerobic cell respiration just as yeast and certain bacteria to produce different types of food

==> Yeast respires anaerobically to produce ethanol and CO₂ when fermenting raw ingredients
==> CO₂ produced by yeast is used to make bread rise, while the ethanol is evaporated away during baking
==> Also used to produce Bioethanol 

• Obligate anaerobes = can use anaerobic cell respiration OR fermentation,
  facultative anaerobes can do both (because of pyruvate)
• Lactic acid fermentation (Animals/Humans):
  Glucose → lactate + 2 ATP molecules
• Alcoholic fermentation (Plants/Yeast):
  Glucose → ethanol + carbon dioxide + 2 ATP molecules
  

  Both ethanol and lactate are toxic in excess à must be removed or be produced in limited amounts)

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

• Can yield far more ATP than under anaerobic conditions à we are breathing to get this oxygen
• Most of the energy contained in a glucose molecule can be harvested and converted into ATP with this
• All steps of cellular respiration are completed instead of only one as in anaerobic respiration
• 6-carbon glucose is systematically and gradually broken down to 6 CO₂ molecules
  à Each step yields potential energy, which can be converted into ATP in the electron transport chain at the end

Glucose + Oxygen --> Carbon Dioxide + Water +Energy 

Respiration in sports:

• certain human activities require anaerobic respiration such as weightlifting and sprinting
  à aerobic respiration is producing a greater yield of ATP, but anaerobic respiration can supply ATP very quickly as oxygen is not required
• This rapid generation enables us to maximise the power of muscle contractions
• Anaerobic cell respiration produces lactate
  à There is a limit to the concentration that the body can tolerate & this limits how much or how long anaerobic respiration can be done for.
• Afterwards lactate must be broken down. This involves the use of oxygen. It can take several minutes for enough oxygen to be absorbed for all lactate to be broken down. The demand for oxygen that builds up during a period of anaerobic respiration is called the oxygen debt.

Respirometer:

A respirometer is a simple apparatus that can measure the rate of respiration. Cellular respiration uses oxygen and produces carbon dioxide and water. We can measure the consumption of oxygen as an indication of the respiration rate.

In Tube A, the organism to be tested (or germinating seed) is positioned, and the tap is closed. The organism starts respiring, consuming O2 and producing CO2 and H2O. The alkaline solution at the bottom of Tube A will absorb the CO2. Tube B is the control where no O2 is used or CO2 produced because no living organism is present. The capillary connecting the two tubes is a manometer.

The reduction in oxygen in Tube A will reduce the pressure in Tube A and will move the coloured liquid in the manometer in the direction of Tube A, providing an indirect measurement of the oxygen consumed, allowing the rate (amount of oxygen consumed per time unit) to be calculated.

Examples of investigations with respirometer: Compare the respiration rate of different organisms; The effect of temperature on respiration rate; Respiration rates could be compared in active and inactive organisms…

Continuation in 8.2

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