(a) (i) How do the three stages in fatty acid oxidation converge to conserve energy as ATP ? 5 (ii) What is β-oxidation ?…

(a)(i) How the three stages of fatty acid oxidation converge to conserve energy as ATP

The three stages are linked through shared electron carriers. In stage 1, β-oxidation of a fatty acyl-CoA releases acetyl-CoA and produces FADH₂ and NADH. In stage 2, the acetyl-CoA is oxidised to CO₂ in the citric acid cycle, producing NADH, FADH₂ and GTP. In stage 3, the NADH and FADH₂ from both stages donate electrons to the mitochondrial respiratory chain; the proton gradient that results drives ATP synthase.

For palmitate (C16), seven rounds give 7 FADH₂ (about 10.5 ATP) and 7 NADH (about 17.5 ATP), and eight acetyl-CoA give about 80 ATP. Subtracting 2 ATP equivalents for activation gives a net of about 106 ATP.

(a)(ii) β-oxidation and its steps for saturated fatty acids

β-oxidation is the repeated removal of two-carbon units as acetyl-CoA from the carboxyl end of a fatty acid, by oxidation at the β-carbon. Fatty acids are first activated to acyl-CoA in the cytosol, carried into the mitochondrial matrix by the carnitine shuttle (CPT-I, translocase, CPT-II), and oxidised there (in plants, mainly in peroxisomes and glyoxysomes). Each round has four steps:

1. Oxidation: acyl-CoA dehydrogenase forms trans-Δ²-enoyl-CoA, reducing FAD to FADH₂.
2. Hydration: enoyl-CoA hydratase adds water, giving L-β-hydroxyacyl-CoA.
3. Oxidation: β-hydroxyacyl-CoA dehydrogenase forms β-ketoacyl-CoA, reducing NAD⁺ to NADH.
4. Thiolysis: thiolase cleaves it with CoA to release acetyl-CoA and an acyl-CoA two carbons shorter.

The shorter acyl-CoA re-enters the cycle. Palmitoyl-CoA needs seven rounds and gives eight acetyl-CoA. Acetyl-CoA enters the citric acid cycle (or, in germinating oilseeds, the glyoxylate cycle).

(b) Transfer of energy between trophic levels

Energy flow in an ecosystem is unidirectional: solar energy fixed by producers passes to herbivores (primary consumers), then to carnivores (secondary and tertiary), and to decomposers.

Only a small part is transferred at each step. According to Lindeman's 10% law, about 10% of the energy of one trophic level is stored in the next; the rest is lost as respiratory heat, unassimilated material (faeces) and excreta that go to decomposers. So the number of trophic levels is limited to four or five.

Energy is shown in a pyramid of energy, which is always upright; the pyramids of biomass and numbers may be inverted, e.g. in a pond with phytoplankton and zooplankton, or a tree supporting many insects. In the grazing food chain, the energy source is living producers, and in the detritus food chain it is dead organic matter; in many ecosystems the latter carries more energy. Food webs link the chains.

(c) Gaseous pollutants, sources and health effects

Major gaseous pollutants and sources:

• Sulphur dioxide (SO₂): burning coal and oil in power plants and industry, and smelters.
• Oxides of nitrogen (NOₓ): vehicles, power plants, industrial combustion and fertilisers.
• Carbon monoxide (CO): incomplete combustion in vehicles, biomass burning and cooking fires.
• Ozone (O₃): a secondary pollutant from NOₓ and volatile organic compounds in sunlight.
• Others: hydrocarbons and VOCs, ammonia, hydrogen sulphide and CO₂ (a greenhouse gas).

Health effects:

• SO₂ and NOₓ: irritate the eyes and respiratory tract, causing bronchitis, asthma and reduced lung function; they form acid rain and fine particles.
• CO: combines with haemoglobin to form carboxyhaemoglobin (about 200 times the affinity of oxygen), cutting oxygen delivery and causing headache, dizziness and death in high doses.
• O₃: an oxidant that damages lung tissue and aggravates asthma.
• VOCs, e.g. benzene: carcinogenic.

India's National Ambient Air Quality Standards (2009), the Air (Prevention and Control of Pollution) Act, 1981 and the National Clean Air Programme deal with these pollutants.