(a) State four modes of nutrition in plants. (b) Outline the process involved in the light and dark reactions of photosynthesis. (c) Describe briefly the mo...

Answer Details

(a) Four modes of nutrition in plants

• Autotrophic (holophytic) nutrition: green plants manufacture their own food by photosynthesis, e.g. most green plants.
• Saprophytic nutrition: feeding on dead organic matter, e.g. Mucor, Rhizopus, mushroom.
• Parasitic nutrition: feeding on a living host, e.g. Cuscuta (dodder), mistletoe.
• Carnivorous (insectivorous) nutrition: trapping and digesting insects, e.g. sundew, pitcher plant, bladderwort.

(Symbiotic nutrition, e.g. lichen, is also acceptable.)

(b) Light and dark reactions of photosynthesis

Light reaction (light-dependent stage):

• Takes place in the grana of the chloroplast and requires light.
• Chlorophyll absorbs (traps) light energy.
• The trapped energy splits water molecules into hydrogen and oxygen (photolysis of water); the oxygen is released as a by-product.
• The light energy is used to form energy-rich ATP, and hydrogen is picked up by a carrier (NADP) to form reduced NADP (NADPH₂).

Dark reaction (light-independent stage / Calvin cycle):

• Takes place in the stroma of the chloroplast and does not need light directly.
• Carbon dioxide is fixed (combined) using the hydrogen (from reduced NADP) and the energy (from ATP) produced in the light stage.
• This reduces carbon dioxide to form simple sugar (glucose), which may then be built up into starch.

Overall: \[ 6CO_2 + 6H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6O_2 \]

(c) Modifications and mechanisms of feeding habits

• (i) Filter feeding in mosquito larva: the larva lives in water and hangs just below the surface. It has a pair of brushes of fine hairs (mouth brushes) around the mouth. These hairs beat rapidly and set up water currents that sweep small floating food particles (plankton, tiny organisms and organic matter) in the water towards the mouth, where they are filtered out and swallowed. In this way the larva strains (filters) food from the water.
• (ii) Parasitic feeding in tapeworm: the tapeworm lives in the small intestine of its host (man), where digested food is abundant. It has a head (scolex) bearing hooks and suckers for firm attachment to the intestinal wall, so that it is not swept away. It has no mouth and no gut; instead it has a thin, flat, ribbon-like body with a very large surface area covered by a resistant cuticle that protects it from the host's digestive enzymes. It simply absorbs already-digested, soluble food from the host through its whole body surface (osmosis and diffusion). Its long, segmented body also allows it to produce large numbers of eggs for reproduction.