Temperature and light intensity in photosynthesis, the structure and functions of capillaries, hepatitis, and biomass compared with biogas

Understanding the topic

Question 3 moves from plant physiology to human circulation, then to liver disease and renewable energy. Each part rewards a stated mechanism rather than a list, and the last one turns on a single distinction: biomass is the feedstock, biogas is one carrier obtained from it.

(a) Explain how temperature and light intensity affect the rate of photosynthesis in plants.

Photosynthesis converts carbon dioxide and water into carbohydrates using light energy. Its rate is controlled by limiting factors, especially light intensity, temperature and carbon-dioxide supply.

At low light intensity, light is the limiting factor. Increasing intensity supplies more photons to chlorophyll, so the light-dependent reactions generate ATP and NADPH faster and the photosynthetic rate rises almost proportionally. At the light-saturation point, the chloroplast machinery or another factor such as carbon dioxide becomes limiting, so further light gives little increase. Extremely intense light may damage photosystems and reduce the rate, a process called photoinhibition. Below the light-compensation point, respiration uses as much or more carbohydrate than photosynthesis produces.

Temperature mainly affects enzyme-controlled reactions. At low temperature, molecules move slowly and enzymes involved in carbon fixation work less rapidly. As temperature rises, the rate increases to an optimum that varies with plant type and adaptation. Beyond the optimum, enzymes lose efficiency, stomata may close to conserve water, carbon-dioxide entry falls and photorespiration may increase. At very high temperature, enzymes and membranes can be damaged, causing a sharp decline.

The two factors interact: raising temperature will not increase photosynthesis when light is insufficient, and stronger light has limited effect when temperature or carbon dioxide is already limiting.

(b) Describe the structure and functions of capillaries in the human blood circulatory system.

Capillaries are the smallest blood vessels and form dense networks between arterioles and venules. Their diameter is roughly that of a red blood cell, so cells often pass through one at a time. A capillary wall consists mainly of a single layer of flattened endothelial cells resting on a thin basement membrane. It contains no thick muscular or elastic coat. Tiny intercellular gaps and, in some organs, pores make the wall selectively permeable. Precapillary sphincters regulate how much blood enters particular capillary beds.

This structure is suited to exchange. The wall is only one cell thick, giving a short diffusion distance. The combined cross-sectional area of millions of capillaries slows blood flow, allowing time for transfer. Oxygen and nutrients diffuse from blood into tissue fluid and cells, while carbon dioxide and other wastes move toward the blood. Hydrostatic pressure near the arterial end filters water and small solutes outward; much fluid returns by osmotic forces near the venous end, while excess enters lymphatic vessels. Capillaries also exchange hormones, distribute heat and permit white blood cells to move into tissues during inflammation.

Their permeability varies by organ: brain capillaries form tight barriers, kidney capillaries are fenestrated for filtration, and liver sinusoids are wider and more permeable.

(c) What is hepatitis? Give its types and briefly explain its common symptoms along with preventions.

Hepatitis means inflammation of the liver. It may be caused by viruses, alcohol, medicines, toxins, metabolic disorders or autoimmune disease. The principal viral forms are A, B, C, D and E. Hepatitis A and E usually spread by contaminated food or water and commonly cause acute illness. Hepatitis B spreads through infected blood, sexual contact and mother-to-child transmission. Hepatitis C spreads mainly through infected blood, while hepatitis D occurs only in a person infected with hepatitis B. B, C and D may become chronic and lead to cirrhosis or liver cancer.

Some infected people have no early symptoms. Common features include fatigue, fever, poor appetite, nausea, vomiting, discomfort in the right upper abdomen, dark urine, pale stools, joint pain and jaundice, which makes the skin and eyes yellow. Severe cases may produce bleeding, confusion or liver failure.

Prevention depends on route. Safe water, sanitation, handwashing and properly cooked food reduce A and E. Effective vaccines are available for A and B; preventing B also prevents D. Screening blood, using sterile needles and medical instruments, avoiding shared razors, practising safer sex and testing pregnant women reduce B and C. There is no widely used vaccine for C, so early testing and antiviral treatment are important. Alcohol and unnecessary hepatotoxic medicines should be avoided during liver illness.

(d) Differentiate between biomass and biogas. Briefly explain various methods for obtaining energy from biomass.

Biomass is organic material of recent biological origin, including wood, crop residues, animal manure and biodegradable municipal waste. It is the solid or liquid feedstock itself. Biogas is a combustible gas produced when microorganisms break down wet biomass without oxygen. It usually contains methane and carbon dioxide, with small amounts of other gases. Biogas is therefore one energy carrier obtained from biomass, not an alternative to it.

Energy can be recovered from biomass by several methods:

1. Direct combustion: dry wood, bagasse or crop residue is burned to produce heat or steam for electricity. It is simple but smoky and inefficient in poor stoves.
2. Anaerobic digestion: manure, sewage or food waste decomposes in a sealed digester. Methane-rich biogas fuels cooking, heating, engines or generators; the digestate can be used as fertilizer.
3. Gasification: limited oxygen converts solid biomass into synthesis gas containing mainly carbon monoxide and hydrogen, which can fuel boilers or engines.
4. Pyrolysis: heating without oxygen produces bio-oil, combustible gas and biochar.
5. Fermentation: sugars and starches are converted into ethanol; oils can be processed into biodiesel.

These five are parallel routes from one feedstock, not stages of one process. Each begins with biomass and ends in a different carrier:

• Combustion gives heat and electricity.
• Anaerobic digestion gives biogas and a fertilizer digestate.
• Gasification gives synthesis gas.
• Pyrolysis gives bio-oil, gas and biochar.
• Fermentation gives ethanol or biodiesel.

Biomass can be renewable when regrowth and waste recovery match use. Unsustainable harvesting, however, causes deforestation, smoke and competition for land.