a. State three features of the small intestine that increase the rate of absorption of digested food. [3 marks] b. Explain briefly what happens to the gluco...

(a) Three features of the small intestine that increase the rate of absorption

1. It is very long and coiled, providing a large surface area for absorption.
2. Its inner wall is folded and carries numerous finger-like villi (and microvilli) which greatly increase the absorbing surface.
3. The villi have thin (one-cell-thick) walls and a rich supply of blood capillaries and lacteals, so absorbed food passes quickly into the blood and lymph.

(b) Fate of the glucose formed during photosynthesis

Some of the glucose is used at once in respiration to release energy. The rest is converted to starch for temporary storage in the leaf, or changed to sucrose and translocated through the phloem to other parts of the plant, where it is used for growth or stored (as starch, oils or proteins) in fruits, seeds, roots and stems. Glucose is also used to build cellulose for cell walls and, with nitrogen, to make proteins.

(c) Mode of feeding

(i) Euglena: Euglena is mainly holophytic (autotrophic). It contains chloroplasts and manufactures its own food by photosynthesis in the presence of sunlight. In darkness it can feed saprophytically (saprozoically) by absorbing dissolved organic matter through its body surface. This double mode of nutrition makes it mixotrophic.

(ii) Spirogyra: Spirogyra is holophytic (autotrophic). It has a spiral chloroplast containing chlorophyll and manufactures its own food by photosynthesis, using water, carbon dioxide and sunlight.

(d) Three characteristics of an estuarine habitat

1. The water is brackish (a mixture of fresh and salt water), so salinity varies.
2. Salinity, water level and temperature change with the tides.
3. The water is often muddy/turbid and rich in nutrients and silt.

(e) Three differences between an aquatic habitat and a terrestrial habitat