WAEC SSCE - Biology - 2016 (Essay)

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(a)(i) Three forms in which living cells exist

1. As single, free-living (unicellular) organisms.
2. As a colony of cells.
3. As part of a many-celled (multicellular) organism (in tissues).

(a)(ii) One example each

• Unicellular (single cell): Amoeba (or Euglena, Paramecium).
• Colony: Volvox (or Spirogyra as a filament).
• Multicellular: a human being (or a maize plant).

(b) A diagram 6 to 10 cm long of a typical plant cell is required, labelling: cell wall, cell membrane, cytoplasm, nucleus, large central vacuole, chloroplast and (if shown) mitochondria.

(c)(i) Three differences between a plant cell and an animal cell

(c)(ii) Three similarities between a plant cell and an animal cell

1. Both have a cell membrane.
2. Both have cytoplasm.
3. Both have a nucleus (and mitochondria).

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(a) Organs of the alimentary canal of humans, in sequence

1. Mouth (buccal cavity)
2. Pharynx
3. Oesophagus (gullet)
4. Stomach
5. Small intestine (duodenum, then ileum)
6. Large intestine (colon)
7. Rectum
8. Anus

(b) Bird versus human alimentary canal

(i) Three structural differences

(ii) Three structural similarities

1. Both have a mouth for taking in food.
2. Both have an oesophagus leading to the stomach.
3. Both have a small and large intestine for digestion and absorption.

(c) How the dentition of herbivores is adapted for feeding

Herbivores have wide, chisel-like incisors that bite and crop grass and leaves. The canines are reduced or absent, leaving a gap (diastema) in which the tongue turns and rolls food. The premolars and molars are broad with ridged (interlocking) grinding surfaces and grow continuously to replace the wear caused by chewing tough, fibrous plant material. The lower jaw also moves sideways so the ridged cheek teeth grind the food thoroughly.

(d) Two roles of the pancreas in digestion

• It secretes pancreatic juice containing digestive enzymes (amylase, trypsin and lipase) that act on carbohydrates, proteins and fats.
• It secretes sodium hydrogen carbonate (an alkali) that neutralises the acidic food arriving from the stomach, giving the enzymes the right pH.

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(a) Two differences between tactic and nastic movements

(b) Two examples each

• (i) Tactic movement: Euglena and Chlamydomonas (also Amoeba, Paramecium).
• (ii) Nastic movement: Mimosa pudica (sensitive plant) and the flower of Portulaca (also the Venus fly-trap).

(c)(i) Three ways of caring for the mammalian skin

1. Bathe (wash) the skin regularly with soap and clean water.
2. Wear clean, well-aired clothing.
3. Eat a balanced diet rich in vitamins and treat cuts or skin infections promptly.

(c)(ii) Three stimuli to which the mammalian skin is sensitive: touch (pressure), temperature (heat and cold) and pain.

(d) Five effects of high temperature on a terrestrial habitat

1. It increases the rate of evaporation and transpiration, drying out the soil and plants.
2. It lowers the humidity of the air.
3. It causes wilting and can kill plants and seedlings.
4. It forces some animals to burrow, become nocturnal or migrate (aestivate) to avoid the heat.
5. It speeds up decay of organic matter and reduces the water available to organisms.

(e)(i) What is courtship behaviour?

Courtship behaviour is the special pattern of actions carried out by animals (usually the male) to attract a mate of the opposite sex and prepare for mating.

(e)(ii) Three courtship behaviours: display of bright colours or plumage (as in the peacock); special calls, songs or sounds; special dances or movements (and the offering of food or nest material).

(f) Three organisms that carry out holozoic nutrition: human being, dog and Amoeba.

(g) How fingerprinting is used to detect crime

Every person has a unique pattern of ridges on the fingertips that never changes through life. At a crime scene the finger marks (prints) left on surfaces are dusted, lifted and compared with the fingerprints of suspects. If a suspect's fingerprint pattern matches the print found at the scene, it is strong evidence that the person was present, so the criminal can be identified.

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(a)(i) Agglutination

Agglutination is the clumping together of red blood cells that occurs when incompatible bloods are mixed during transfusion. It is caused by a reaction between the antigens on the donor's red cells and the corresponding antibodies in the recipient's plasma.

(a)(ii) Blood transfusion compatibility

A tick means the transfusion is safe (no agglutination); a cross means agglutination occurs.

Group O is the universal donor (can give blood to all groups) and group AB is the universal recipient (can receive from all groups).

(b) Adaptations for obtaining food

(i) Mosquito larva: it has a breathing siphon so it can hang at the water surface, and tufts of mouth brushes that beat to create currents which sweep tiny food particles and micro-organisms in the water into its mouth (filter feeding).

(ii) Dodder plant: it is a parasite with no chlorophyll; it twines around a host plant and develops haustoria (suckers) that penetrate the host's tissues to absorb ready-made food and water.

(iii) Grasshopper: it has strong biting and chewing mouthparts (hard mandibles and maxillae) that cut and grind plant material, suiting its herbivorous diet.

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(a)(i) What is a habitat?

A habitat is the natural place or environment where an organism (plant or animal) lives, feeds and reproduces, for example a pond, a forest or the soil.

(a)(ii) Three roles of a decomposer in an ecosystem

• It breaks down dead plants and animals and other organic waste, so it clears the environment of dead matter.
• It releases (recycles) mineral nutrients such as nitrogen and carbon back into the soil, water and air for reuse by plants.
• It keeps the nutrient cycles going, so producers can continue to obtain the raw materials they need.

(b) Energy flow in a freshwater habitat

Energy from the Sun is trapped by producers (green water plants and algae/phytoplankton) during photosynthesis and stored as chemical energy in their food. This energy passes to primary consumers (herbivores such as water fleas and small fish) when they eat the plants, and then to secondary and tertiary consumers (larger fish and birds) along the food chain. At each transfer (trophic level) much of the energy is lost as heat during respiration and in undigested waste, so less energy is available at each higher level. Decomposers finally break down all dead organisms and release the remaining energy as heat. Energy therefore flows in one direction only, from the Sun through the organisms and out as heat, and is not recycled.

(c)(i) Three harmful effects of microorganisms to plants

1. They cause plant diseases such as leaf spot, blight and wilt.
2. They cause rotting and spoilage of fruits, tubers and stored produce.
3. They cause galls or decay that reduce crop yield.

(c)(ii) Six beneficial effects of microorganisms to humans

1. They decay dead matter and recycle nutrients, keeping the environment clean.
2. They fix atmospheric nitrogen, enriching the soil for crops.
3. They are used in making foods such as bread, yoghurt, cheese and local drinks (fermentation).
4. They are used to produce antibiotics such as penicillin.
5. Bacteria in the gut help in digestion and make some vitamins.
6. They are used in sewage treatment and in producing alcohol and other industrial products.