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class notes - cell division
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Respiration – BiologyCellular respiration
Releasing energy
Respiration releases energy – it is an exothermic process.
Photosynthesis is the process by which plants, and some bacteria, synthesise food
molecules – which they then use, in addition to other things, for respiration. The process of
photosynthesis requires energy – it is endothermic.
- Respiration happens in cells.
Why organisms need energy?
All organisms need energy to live.
This energy is used:
- to drive the chemical reactions needed to keep organisms alive – the reactions to
build complex carbohydrates, proteins and lipids from the products of
photosynthesis in plants, and the products of digestion in animals, require energy
- movement – in animals, energy is needed to make muscles contract, while in plants,
it is needed for transport of substances in the phloem.
Respiration is only around 40 per cent efficient. As animals respire, heat is also released. In
birds and mammals, this heat is distributed around the body by the blood. It keeps these
animals warm and helps to keep a constant internal temperature.
Energy is also used:
- for cell division
- to maintain constant conditions in cells and the body – homeostasis
- to move molecules against concentration gradients in active transport
- for the transmission of nerve impulses
Aerobic and anaerobic respiration-
Aerobic respiration
Respiration using oxygen to break down food molecules is called aerobic respiration.
Glucose is the molecule normally used for respiration – it is the main respiratory substrate.
Glucose is oxidised to release its energy.
The word equation for aerobic respiration is:
- glucose + oxygen → carbon dioxide + water + energy released
, Chemical symbols:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy released
Respiration is a series of reactions, but this summarises the overall process.
The first stages of respiration occur in the cytoplasm of cells, but most of the energy
released is in the mitochondria.
Anaerobic respiration-
Most organisms cannot respire without oxygen but some organisms and tissues can
continue to respire if the oxygen runs out. These organisms and tissues use the process of
anaerobic respiration.
Human muscle can respire anaerobically for short periods of time – even though the process
is relatively inefficient, it's better to continue respiring and be able to run away from danger
– or run a race.
The glucose in muscle is converted to lactic acid:
- glucose → lactic acid + energy released
Some plants, and some fungi such as yeast can respire anaerobically – it's preferable to
release less energy but remain alive.
Glucose in yeast cells is converted to carbon dioxide and ethanol, which we refer to simply
as 'alcohol':
- glucose → ethanol + carbon dioxide + energy released
Anaerobic respiration occurs only in the cytoplasm of cells.
Aerobic and anaerobic respiration compared:
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