Fats and Proteins
Animals and humans besides glucose also consume fats and proteins to harvest energy. Fats are broken down into glycerol and three fatty acids. First, the glycerol is phosphorylated and then enters the glycolytic pathway at the level of glyceraldehyde 3- Phosphate (PGAL) while fatty acids (2-C), enter the mitochondrion where their carbons are removed.
They form acetyl CoA (2-C) which is the entry point for the Krebs cycle (an 18-carbon fatty acid results in nine acetyl CoA molecules). One gram of fat provides about 2.5 times more. energy than carbohydrates and proteins.
Animals digest amino acids, excessive quantity body is starved then amino acids can be used as fuel. The size of the R-group determines whether the carbon chain is oxidizing in glycolysis (Pyruvate) or in the Krebs cycle or cetyl CoA.
Amino acids are degraded, and the amine group is removed to yield ammonia this process is called the deamination reaction.
4.3 Photorespiration
This process occurs only in photosynthesizing cells of the plants. It is opposite to photosynthesis, because in it oxygen is used instead of CO, and instead of oxygen, carbon dioxide is released (like respiration).
It differs from ordinary respiration of cells which occurs in mitochondria at night and in non-green tissues of plants while photorespiration takes place in the presence of light and only in photosynthetic cells. The oxygen is absorbed but unlike respiration does not produce energy (ATP).
4.3.1 How does RuBP react with oxygen in photorespiration?
Photorespiration is related to the functioning of the enzyme ribulose biphosphate (RuBP) carboxylase which also acts as oxygenase (combines with O, instead of CO2). Ass carboxylase The RuBP carboxylase is also known as Rubisco. When rubisco adds CO, to RuBP (an acceptor
molecule) it introduces two molecules of PGA while during oxygenase, it adds oxygen to RuBP and produces one molecule of PGA and one phosphoglycolate. The phosphoglycolate loses its phosphate to become glycolate. Some algae can excrete glycolate but higher plants cannot excrete it.
Therefore, plants must convert it back to intermediate in the Calvin cycle. The conversion of glycolate into glycine amino acid takes place by a series of reactions in mitochondria, chloroplast, and other cellular parts especially in peroxisomes. Glycolate glycine amino acid Glycine diffuses into mitochondria where every two glycine molecules are converted
into serine amino acid and CO2. 2 glycine→ Serine+CO2
This entire pathway is called photorespiration in which RuBP is converted into serine and CO, which uses ATP and NADPH, produced during light reactions like the Calvin cycle.
4.3.2 Disadvantages of photorespiration
It is the reverse of the Calvin cycle (here CO, is released instead of being fixed into carbohydrates). Photorespiration reduces the amount of carbon fixation into carbohydrates by 25%. The role of photorespiration in plants is not thoroughly understood. It is presumed that photorespiration may be necessary for the assimilation of nitrates from the soil.
4.3.3 Photosynthesis in C4 plants
In the normal process of photosynthesis, a 3-C compound called PGA is formed as the first detectable product of photosynthesis. Therefore, these plants are called C, plants. However, some plants are growing in dry and hot conditions which produce a four-carbon compound (C4) called oxaloacetate as the first product of photosynthesis in dark react RS-T
These plants are called C4 plants and this type of photosynthesis is called C4 photosynthesis.
C3 plants use rubisco to react CO, with RuBP, on the other hand, C4 plants use a different enzyme called phosphoenol pyruvate carboxylase (PEPCO) to fix CO, to a compound known as phosphoenol pyruvate (PEP). The PEP is reduced into another molecule called malate. The malate carries CO2 to a special type of cells called bundle sheath cells where the Calvin cycle proceeds.
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