All Free Biology MCQs with Answers

191. In a kidney dialysis machine, waste is removed from the patient's blood because

• A. the dialysis fluid contains enzymes that destroy urea
• B. the blood is heated so that urea evaporates
• C. urea is actively pumped across the membrane
• D. the dialysis fluid contains no urea, so urea diffuses out across a partially permeable membrane

Explanation: Dialysis fluid is made with the same concentrations of glucose and mineral ions as normal blood plasma but with no urea at all, so only the wastes follow a concentration gradient out while useful substances have no gradient and stay. The fluid flows in the opposite direction to the blood, keeping the gradient steep along the whole membrane. Nothing is pumped, since the whole process is diffusion.

192. Glucose appears in the urine of an untreated diabetic because

• A. the blood glucose level exceeds the capacity of the tubule carriers to reabsorb it
• B. glucose is too large to be reabsorbed
• C. the glomerulus stops filtering glucose
• D. insulin is secreted into the filtrate

Explanation: Reabsorption of glucose depends on a fixed number of carrier proteins in the proximal tubule, and once the blood concentration passes the renal threshold of roughly 180 mg per 100 cm3 the carriers are saturated and the surplus stays in the filtrate. The unabsorbed glucose then holds water osmotically, which explains the large urine volume and the constant thirst of an untreated diabetic.

193. The thermoregulatory centre of the human body is located in the

• A. medulla oblongata
• B. hypothalamus
• C. cerebellum
• D. skin itself

Explanation: The hypothalamus contains receptors that monitor the temperature of the blood flowing through it and also receives information from temperature receptors in the skin, then coordinates sweating, shivering and changes in blood flow. The medulla controls heart rate and breathing, and the cerebellum handles balance and coordination of movement. Skin receptors detect the change but cannot organise the response.

194. When the body becomes too hot, the arterioles supplying the skin capillaries

• A. constrict, so less blood reaches the surface
• B. are unaffected, since only sweating matters
• C. dilate, so more blood flows near the surface and more heat is lost
• D. carry blood away from the skin into deeper shunt vessels

Explanation: Vasodilation of the arterioles lets more warm blood flow through the surface capillaries, so heat is lost by radiation and conduction and the skin looks flushed. In the cold these arterioles constrict and blood is diverted through deeper shunt vessels, keeping the skin pale and conserving heat. The capillaries themselves have no muscle, so they cannot dilate or constrict on their own.

195. Sweating cools the body because

• A. sweat is colder than the body
• B. sweat carries away dissolved urea
• C. the sweat glands use up energy in secreting it
• D. evaporation of the water in sweat takes latent heat from the skin

Explanation: Changing liquid water into vapour requires a large amount of latent heat, and that heat is drawn from the skin surface, so the body cools only when the sweat actually evaporates. This is why sweating is far less effective on a humid day, when evaporation is slow and the sweat simply drips off. Sweat leaves the body at body temperature, so it is not cold when secreted.

196. Shivering helps to raise body temperature because

• A. rapid involuntary muscle contractions release heat from respiration
• B. it increases the blood flow to the skin
• C. it reduces the surface area of the body
• D. it stops sweat being produced

Explanation: The muscles contract and relax rapidly without doing any useful external work, so almost all the energy released by increased respiration in the muscle fibres appears as heat. The metabolic rate can rise several fold during vigorous shivering. It is a short term emergency response, since it is tiring and depends on the glycogen stored in muscle.

197. The goose pimples seen on human skin in the cold are produced by

• A. dilation of the surface capillaries
• B. contraction of the erector pili muscles, which pull the hairs upright
• C. the emptying of the sweat glands
• D. the thickening of the subcutaneous fat layer

Explanation: Each hair has a small erector muscle that pulls it erect, and in a furry mammal this traps a thicker layer of still air as insulation. Human body hair is far too sparse for this to help, so the raised bumps are essentially a vestigial response inherited from hairier ancestors. Subcutaneous fat does insulate, but its thickness cannot change from minute to minute.

198. Mammals and birds are described as endotherms because they

• A. always live in warm climates
• B. have a body temperature that follows the temperature of the surroundings
• C. generate most of their body heat from their own metabolism and hold it nearly constant
• D. are unable to lose heat through the skin

Explanation: An endotherm keeps its core temperature steady using heat from respiration, which lets enzymes work at their optimum whatever the weather and allows activity at night and in cold regions. The cost is a high food requirement, since much of that energy is spent simply on staying warm. An ectotherm such as a lizard depends on external heat and warms up by basking instead.

199. Which of the following is an example of excretion rather than egestion?

• A. The passing of undigested cellulose in faeces
• B. The loss of dead cells from the surface of the skin
• C. The removal of hair when it is cut
• D. The removal of carbon dioxide by the lungs

Explanation: Excretion is the removal of the waste products of the body's own metabolism, and carbon dioxide is made by respiration inside every cell, so the lungs are excretory organs. Faeces consist largely of material that was never absorbed into the cells at all, so removing them is egestion. This distinction is a standard exam point and turns on whether the substance was ever part of the body's metabolism.

200. Which of the following belongs to the first line of defence against infection?

• A. Production of antibodies by plasma cells
• B. Intact skin and the mucous membranes of the body openings
• C. Activation of cytotoxic T lymphocytes
• D. Formation of memory cells after an infection

Explanation: The first line is a physical and chemical barrier that stops pathogens entering at all: unbroken skin, mucus that traps microbes, cilia that sweep them out and acidic secretions. It is non specific, acting the same way against every organism, and it needs no previous exposure. Antibodies, T cells and memory cells all belong to the third line, the specific immune response.