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Plants, animals and microorganisms need to respire to produce ATP, which is required for:

  • Active transport - much of an organisms energy is used for this
  • Secretion - large mlecules made in some cells are released by exocytosis
  • Endocytosis - bulk movement of larger molecules into the cell
  • Metabolic reactions - synthesis of large molecules from smaller ones, proteins from amino acids, steroids from cholesterol, cellulose from β-glucose. These are all anabolic
  • Replication of DNA and synthesis of organelles before a cell divides
  • Movement - bacterial flagella, eukaryotic cilia and undulipodia, muscle contractions
  • Activation of chemicals e.g. phosphorylation of glucose

Adenosine triphosphate (ATP) is comprised of an adenine group attached to a ribose sugar and three phosphate molecules. ATP provides the immediate source of energy for biological processes. ATP:

  • Transfers energy
  • Is the universal energy molecule and an immediate source of energy
  • The phosphates can be removed by hydrolysis to release 30kJ (per mole) energy
  • Releases energy for metabolism such as - muscle contraction, active transport, phosphorylation, glycolysis, during movement binding to proteins to change their shape
  • ADP can attach a phosphate (forming ATP) during respiration/photosynthesis
  • Energy is released in small 'packets' to prevent cell damage

Coenzymes aid in oxidation and reduction during reactions.
NAD and FAD accept hydrogen and become reduced. Reduced NAD and reduced FAD carry electrons to the electron transport chain for oxidative phosphorylation, and hydrogen ions for chemiosmosis. Coenzyme A carries acetate to the Krebs cycle.

Glycolysis takes place in the cytoplasm. 
1. An ATP molecule is hydrolysed and the phosphate attaches to the glucose molecule at C6.
2. Glucose-6-phosphate is turned into fructose-6-phosphate.
3. Another ATP is hydrolysed, and the phosphate attaches to C1.
4. The hexose sugar is activated by the energy released from the hydrolysed ATP moelcules. It now cannot leave the cell and is known as hexose-1,6-bisphosphate.
5. It is split into two molecules of triose phosphate (3C).
6. Two hydrogen atoms are removed from each triose phosphate, which involves dehydrogenase enzymes.
7. NAD combines with the hydrogen atoms to form reduced NAD.
8. Two molecules of ATP are formed by substrate level phosphorylation.
9. Four enzyme-catalysed reactions convert each triose phosphate into a molecule of pyruvate.
10. Two more moelcules of ATP are formed, so there is a net gain of two ATP.

During aerobic respiration in animals, pyruvate is actively transported into mitochondria. 

The Mitochondrial Matrix:

  • Enzymes that catalyse the stages of aerobic respiration (highly-concentrated mixture of hundreds of enzymes)
  • Molecules of coenzyme NAD
  • Oxaloacetate - the 4 carbon compound that accepts acetate from the link reaction
  • Mitochondrial DNA, some of which codes for mitochondrial enzymes and other proteins
  • Mitochondrial ribosomes where the proteins are assembled

The Inner Membrane:

  • Different lipid composition than the outer layer (impermeable to most small ions, including protons, or else aerobic respiration would stop if damaged)
  • Is folded into many cristae to give a large surface area
  • Has embedded on it many electron carriers and ATP synthase enzymes
  • High protein-to-phospholipid ratio

The Outer Membrane:

  • It contains


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