Respiration

Respiration1. Cellular (internal or tissue) respiration the metabolic processes within cells which release the energy from glucose.

2. Gaseous exchange (external respiration) the process involved in obtaining the oxygen for respiration and the removal of gaseous wastes.

Cellular respiration can be divided into three stages:

1. Glycolysis

2. Krebs (tricarboxylic acid) cycle

3. Electron (hydrogen) transport chain

Adenosine triphosphate (ATP)Adenosine triphosphate (ATP) is the short-term energy store of all cells. It is easily transported and is therefore the universal energy carrier. ATP is formed from the nucleotide adenosine monophosphate (AMP) by the addition of two further phosphate molecules/groups. The hydrolysis of ATP to ADP is catalyzed by the enzyme ATPase and the removal of the terminal phosphate yields 30.6 kJmol-1 of free energy.

ADP may be reconverted to ATP by the addition of phosphate molecules/groups in a process called phosphorylation (requires 30.6 kJ of energy), of which there are two main forms:1. Photosynthetic phosphorylation (photophosphorylation) occurs during photosynthesis in chlorophyll-containing cells.

2. Oxidative phosphorylation occurs during cellular respiration in all aerobic cells.

Uses of ATP

1. Anabolic processes It provides the energy needed to build up macromolecules from their components units, e.g.

polysaccharides synthesis from monosaccharides

proteins synthesis from amino acids

DNA replication

2. Movement It provides the energy for many forms of cellular movement including: muscle contraction, ciliary action, spindle action in cell division.3.Active transport It provides the energy necessary to move materials against a concentration gradient, e.g. ion pumps.

4.Secretion It is needed to form the vesicles necessary in the secretion of cell products.

5. Activation of chemicals It makes chemicals more reactive, enabling them to react more readily, e.g. the phosphorylation of glucose at the start of glycolysis.

Glycolysis

Glycolysis, the series of reactions in which six-carbon sugar is broken down to two molecules of the three-carbon pyruvate ion. Glycolysis occurs in the cytosol.

Glucose is first phosphorylated by reaction with ATP, which activates it. Conversion to fructose phosphate follows, and a further phosphate group is added, forming fructose bisphosphate (diphosphate). Note that two molecules of ATP are consumed per molecule of glucose, at this stage. Next the phosphorylated 6C-sugar is split (lysis) into two 3C-sugar phosphates, called triose phosphate (glyceraldehyde 3-phosphate or phosphoglyceraldehyde and dihyroxyacetone phosphate). Finally, the triose phosphate molecules are oxidised to pyruvate by the removal of hydrogen, producing reduced NAD (NADH + H+). Two molecules of ATP are formed for each triose phosphate oxidised. This means that there are four ATPs formed during oxidation, and a net gain of two ATPs in glycolysis, per molecule of glucose.

The reaction of glycolysis are common to both aerobic and anaerobic respiration, but the fate of the reduced NAD that is formed differs. In aerobic respiration it is oxidised in mitochondria with the formation ATP.

Glucose + 2NAD+ (2 pyruvate + 2ATP + 2NADH + 2H+

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