Life Processes — Important Questions

Answer: (b) Nephron. Each kidney contains a large number of nephrons; blood is filtered at the glomerulus inside Bowman's capsule, and the tubule then reabsorbs useful substances. (A neuron is a nerve cell, an alveolus is an air sac in the lung, and a villus increases absorptive area in the intestine.)

Answer: (c) Small intestine. It receives bile, pancreatic juice and intestinal juice that complete digestion of carbohydrates, proteins and fats, and its inner lining has finger-like villi that provide a large surface area for the absorption of digested food into the blood.

Answer: (b) Lactic acid. When muscles do not get enough oxygen during heavy exercise, anaerobic respiration converts pyruvate into lactic acid. Its accumulation in the muscles causes cramps. (Yeast, in contrast, forms ethanol and $CO_2$; complete breakdown to $CO_2 + H_2O$ needs oxygen.)

Answer: (a) The left ventricle receives oxygenated blood from the lungs and pumps it to the body. Oxygenated blood returns from the lungs to the left atrium, passes to the left ventricle, which then pumps it through the aorta to the whole body. Its thick muscular wall generates the high pressure needed for systemic circulation.

Answer: (a) Both A and R are true and R is the correct explanation of A. The left ventricle must generate high pressure to push blood through the long systemic circuit to the whole body, so it needs a thicker, more muscular wall; the right ventricle only sends blood the short distance to the lungs at lower pressure.

The trachea is supported by C-shaped rings of cartilage to keep the air passage permanently open and prevent it from collapsing when the air pressure inside falls during breathing.

If these rings were absent, the trachea would collapse and close when we breathe out (or when pressure drops), cutting off the flow of air to the lungs and making normal breathing impossible.

(i) The alveoli are extremely numerous and balloon-like, providing a very large surface area for the exchange of gases.

(ii) Their walls are very thin and richly supplied with a dense network of blood capillaries, so oxygen and carbon dioxide can diffuse rapidly between the air and the blood. (Their moist surface further aids diffusion.)

(a) $6CO_2 + 6H_2O \xrightarrow{light,\ chlorophyll} C_6H_{12}O_6 + 6O_2$

(b) Raw materials:

• Carbon dioxide — taken from the air through the stomata.
• Water — absorbed from the soil by the roots and carried up by the xylem.
• Sunlight — trapped by chlorophyll in the leaves.

(c) Role of stomata: Stomata are tiny pores (mainly on the leaf surface) through which $CO_2$ enters and $O_2$ leaves, allowing the gaseous exchange needed for photosynthesis; they also allow water vapour to escape during transpiration. Their opening and closing is controlled by guard cells.

(a) The two conducting tissues are:

• Xylem — transports water and dissolved minerals from the roots to the leaves and other parts.
• Phloem — transports food (dissolved sugars) made in the leaves to all other parts (translocation).

(b) Upward transport of water: Root hairs absorb water from the soil, creating a difference in ion concentration that draws water into the root and builds root pressure, pushing water up. More importantly, water evaporating from the leaf surface through the stomata (transpiration) creates a suction/pull called the transpiration pull. Because water molecules stick together (cohesion) in continuous columns inside the xylem vessels, this pull draws water upward all the way from the roots to the top of even very tall trees, especially during the day.

(a) Glucose is first broken in the cytoplasm into pyruvate (3-carbon), which is then broken down in three ways:

• Lack of oxygen (in our muscles): pyruvate is converted to lactic acid + a little energy.
• Absence of oxygen (in yeast — fermentation): pyruvate is converted to ethanol + carbon dioxide + a little energy.
• Presence of oxygen (aerobic, in mitochondria): pyruvate is broken to carbon dioxide + water + a large amount of energy.

(b) The aerobic pathway releases the most energy because glucose is completely oxidised to $CO_2$ and water, whereas anaerobic breakdown leaves energy locked in lactic acid/ethanol (incomplete oxidation).

(c) During strenuous exercise the muscles run short of oxygen and build up lactic acid. We breathe faster to take in more oxygen so that this lactic acid can be oxidised and to meet the increased energy demand.

Mouth (buccal cavity): Food is chewed by teeth and mixed with saliva. Salivary glands secrete salivary amylase (ptyalin), which begins the digestion of starch into sugar. The tongue helps mix food and roll it into a bolus.

Oesophagus: The bolus is pushed down to the stomach by rhythmic muscular movements called peristalsis.

Stomach: The stomach wall secretes gastric juice containing:

• Hydrochloric acid (HCl) — makes the medium acidic (activating enzymes) and kills bacteria.
• Pepsin — a protein-digesting enzyme.
• Mucus — protects the stomach lining from the acid.

Small intestine: This is the site of complete digestion. It receives:

• Bile (from the liver, stored in the gall bladder) — makes the medium alkaline and emulsifies fats into small droplets.
• Pancreatic juice (from the pancreas) — contains trypsin (proteins), pancreatic amylase (carbohydrates) and lipase (fats).
• Intestinal juice — completes digestion into glucose, amino acids and fatty acids/glycerol.

The digested food is then absorbed through the finger-like villi, which provide a large surface area and are rich in blood vessels. The undigested waste passes to the large intestine, where water is absorbed, and is finally egested through the anus.

(a) Double circulation: In human beings, blood passes through the heart twice during one complete circulation of the body. It consists of two circuits:

• Pulmonary circulation: deoxygenated blood from the right side of the heart goes to the lungs (gets oxygenated) and returns to the left side of the heart.
• Systemic circulation: oxygenated blood from the left side of the heart goes to the body tissues and returns (deoxygenated) to the right side of the heart.

(b) Why it is necessary: The four-chambered heart keeps oxygenated and deoxygenated blood from mixing. This ensures a highly efficient supply of oxygen to the body, which is essential for warm-blooded animals that need a lot of energy to maintain a constant body temperature.

(c) Difference (any one): Arteries carry blood away from the heart, have thick, elastic walls and no valves (blood flows under high pressure); veins carry blood towards the heart, have thinner walls and possess valves to prevent backflow.

(a) The cup-shaped structure is the Bowman's capsule (which encloses the glomerulus, a cluster of capillaries where filtration occurs).

(b) Substances selectively reabsorbed (any two): glucose, amino acids, most of the water, and useful salts.

(c) Urine is mainly composed of water, urea and dissolved salts (the wastes not needed by the body).

(d) The artificial process is dialysis (haemodialysis). It works on the principle of diffusion of wastes across a selectively permeable membrane from the blood into a dialysing fluid, removing urea and excess substances that the failed kidneys can no longer filter.