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CSEC Biology practice questions

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Question 1Biology

Which organelle is the main site of ATP synthesis by oxidative phosphorylation in a eukaryotic cell?

  1. Ribosome
  2. Golgi apparatus
  3. Mitochondrion Correct answer.
  4. Lysosome

Explanation

The inner mitochondrial membrane carries the electron transport chain and ATP synthase, so most ATP from aerobic respiration is produced there. Ribosomes build proteins, the Golgi modifies and packages them, and lysosomes carry out hydrolysis reactions.

Question 2Biology

A student observes a cell with a rigid cellulose wall, a large central vacuole and chloroplasts. To which group does the cell most likely belong?

  1. A plant cell Correct answer.
  2. A bacterial cell
  3. An animal cell
  4. A fungal cell

Explanation

Cellulose walls, a large permanent central vacuole and chloroplasts together are diagnostic of plant cells. Animal cells lack walls and chloroplasts, bacteria lack cellulose and membrane-bound chloroplasts, and fungal walls are chitin.

Question 3Biology

Which process moves water across a selectively permeable membrane from a region of higher water potential to one of lower water potential?

  1. Diffusion of oxygen
  2. Active transport
  3. Phagocytosis
  4. Osmosis Correct answer.

Explanation

Osmosis is the net movement of water through a partially permeable membrane down a water potential gradient. Active transport uses energy to move solutes against a gradient, while phagocytosis is bulk engulfment of particles.

Question 4Biology

Which enzyme digests starch into maltose in the human digestive system?

  1. Amylase Correct answer.
  2. Lipase
  3. Pepsin
  4. Catalase

Explanation

Amylase hydrolyses the glycosidic bonds of starch to yield maltose, and it is secreted by salivary glands and the pancreas. Lipase acts on fats, pepsin on proteins and catalase on hydrogen peroxide.

Question 5Biology

In a monohybrid cross between two heterozygous tall pea plants (Tt x Tt), what proportion of the offspring is expected to show the recessive short phenotype?

  1. One half
  2. One quarter Correct answer.
  3. All of them
  4. Three quarters

Explanation

The Punnett square gives genotypes TT, Tt, Tt and tt in equal proportions, so only tt, one quarter of offspring, shows the recessive trait. The other three quarters carry at least one dominant T allele and are tall.

Question 6Biology

Which blood vessel carries oxygenated blood away from the heart to the body tissues?

  1. The aorta Correct answer.
  2. The pulmonary artery
  3. The vena cava
  4. The coronary vein

Explanation

The aorta is the large artery leaving the left ventricle, and it distributes oxygenated blood to the systemic circulation. The pulmonary artery carries deoxygenated blood to the lungs, and veins return blood to the heart.

Question 7Biology

Which gas is taken in by leaves and used as the carbon source in photosynthesis?

  1. Water vapour
  2. Nitrogen
  3. Oxygen
  4. Carbon dioxide Correct answer.

Explanation

Carbon dioxide diffuses into the leaf through stomata and is fixed in the Calvin cycle to build carbohydrate. Oxygen is a product of the light reactions, and nitrogen and water vapour are not carbon sources.

Question 8Biology

Which term describes the variety of different species present in an ecosystem?

  1. Genetic diversity
  2. Carrying capacity
  3. Species diversity Correct answer.
  4. Population density

Explanation

Species diversity refers to the number and relative abundance of different species in a habitat. Genetic diversity concerns variation of alleles within a species, population density counts individuals per area, and carrying capacity is the maximum population an environment can sustain.

Question 9Biology

A sample of pond water is found to contain many cells with a nucleus, cell wall and chloroplasts. Which conclusion is best supported?

  1. The sample contains animal cells
  2. The sample contains viruses
  3. The sample contains only bacteria
  4. The sample contains photosynthetic protists such as algae Correct answer.

Explanation

Presence of a nucleus rules out bacteria, and chloroplasts plus a cell wall point to photosynthetic eukaryotes such as algae. Animal cells lack walls and chloroplasts, and viruses are not cellular and have no nucleus or chloroplasts.

Question 10Biology

An enzyme-catalysed reaction is fastest at 37 degrees Celsius and slows sharply above 45 degrees Celsius. Which explanation is most consistent with this pattern?

  1. Above the optimum, the enzyme is converted into a substrate
  2. Above the optimum, the pH of the solution falls to zero
  3. Above the optimum, substrate molecules stop moving
  4. Above the optimum, hydrogen bonds break and the active site loses its shape Correct answer.

Explanation

Heating beyond the optimum disrupts bonds holding the tertiary structure, so the active site is no longer complementary to the substrate and the enzyme is denatured. Substrate molecules in fact move faster at higher temperature, so the slowdown is not due to reduced movement.

Question 11Biology

A red blood cell is placed in pure distilled water. Which change is expected?

  1. It keeps the same volume
  2. It swells and may burst by haemolysis Correct answer.
  3. It develops a cellulose wall
  4. It shrinks and becomes crenated

Explanation

Distilled water has a higher water potential than the cytoplasm, so water enters the cell by osmosis and the cell swells until the membrane may rupture. Shrinking occurs in a concentrated solution, where water leaves the cell.

Question 12Biology

Which feature of the alveoli most directly increases the rate of gas exchange?

  1. A very large total surface area with thin walls Correct answer.
  2. A small blood supply to each alveolus
  3. A thick layer of mucus on the inner surface
  4. A layer of cartilage in the alveolar wall

Explanation

Gas exchange rate depends on surface area and on the short diffusion distance, and the alveoli provide an enormous total area with walls one cell thick. A rich blood supply also helps, so reduced blood flow or added thickness would slow exchange.

Question 13Biology

Which sequence correctly traces the pathway of blood through the mammalian heart, starting from the right atrium?

  1. Right atrium, right ventricle, aorta, lungs, pulmonary vein, left atrium, left ventricle
  2. Right atrium, pulmonary vein, lungs, pulmonary artery, left atrium, right ventricle, aorta
  3. Right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta Correct answer.
  4. Right atrium, left atrium, right ventricle, left ventricle, aorta, lungs, pulmonary artery

Explanation

Deoxygenated blood passes from the right atrium to the right ventricle and is pumped to the lungs by the pulmonary artery. Oxygenated blood returns by the pulmonary vein to the left atrium, then the left ventricle, and leaves through the aorta.

Question 14Biology

A leaf is destarched overnight and then partly covered with black paper before being exposed to light for several hours. After decolourising and testing with iodine, which result is expected?

  1. Only the covered area turns blue-black, showing light prevents photosynthesis
  2. No part of the leaf turns blue-black
  3. Only the exposed areas turn blue-black, showing starch formed there Correct answer.
  4. The whole leaf turns blue-black, showing light is unnecessary

Explanation

Starch forms only where light reaches the leaf, so the exposed regions turn blue-black with iodine while the covered region stays brown. This demonstrates that light is required for photosynthesis.

Question 15Biology

Which statement best explains why carbon dioxide concentration often limits the rate of photosynthesis in a well-lit greenhouse?

  1. Carbon dioxide is the main product of the light-dependent reactions
  2. Carbon dioxide is needed to keep stomata closed
  3. Carbon dioxide is a raw material for the Calvin cycle, so its supply limits the rate at which carbohydrate is made Correct answer.
  4. Carbon dioxide absorbs light energy in place of chlorophyll

Explanation

Carbon dioxide is fixed in the Calvin cycle, so if its concentration is low the light-independent stage becomes the rate-limiting step. Adding carbon dioxide to a greenhouse can therefore increase the rate of photosynthesis.

Question 16Biology

A food chain shows grass, grasshopper, frog, snake. If the grass contains 10 000 kJ of energy, approximately how much energy is transferred to the snake?

  1. About 10 000 kJ
  2. About 1 000 kJ
  3. About 10 kJ Correct answer.
  4. About 5 000 kJ

Explanation

Roughly ten per cent of energy is transferred between trophic levels because most is lost in respiration, movement and heat. From grass to snake spans three transfers, so about 10 000 x 0.1 x 0.1 x 0.1, which is about 10 kJ.

Question 17Biology

Which statement correctly describes the role of a motor neurone in a reflex arc?

  1. It carries impulses from a receptor to the central nervous system
  2. It carries impulses from the central nervous system to an effector Correct answer.
  3. It detects the stimulus and generates the impulse
  4. It releases hormones into the blood at the synapse

Explanation

A motor neurone transmits impulses away from the central nervous system to a muscle or gland, which acts as the effector. Sensory neurones carry impulses from receptors to the central nervous system, and receptors detect the stimulus.

Question 18Biology

Which adaptation of a xerophytic plant reduces water loss by transpiration?

  1. A thin, waxy cuticle
  2. Numerous stomata on the upper surface
  3. Broad, thin leaves
  4. Sunken stomata in pits Correct answer.

Explanation

Stomata sunk in pits trap a layer of humid air that reduces the water potential gradient and limits evaporation. Broad leaves, many stomata and a thin cuticle all increase water loss and are typical of mesophytes.

Question 19Biology

Which human activity contributes most directly to eutrophication of a lake?

  1. Clearance of land leading to soil erosion
  2. Burning of fossil fuels releasing sulfur dioxide
  3. Discharge of hot cooling water from a power station
  4. Runoff of nitrate fertiliser from farmland Correct answer.

Explanation

Nitrate and phosphate runoff raise nutrient levels, causing algal blooms whose decay depletes oxygen and kills aquatic animals, which is eutrophication. Sulfur dioxide causes acid rain, and hot water causes thermal pollution.

Question 20Biology

In a cross between a heterozygous tall plant and a short homozygous recessive plant, what phenotypic ratio is expected among the offspring?

  1. 3 tall to 1 short
  2. 1 tall to 3 short
  3. All tall
  4. 1 tall to 1 short Correct answer.

Explanation

A test cross of Tt with tt gives gametes T and t from the heterozygote and only t from the recessive parent, so half the offspring are Tt and tall and half are tt and short. The ratio is therefore one to one.

Question 21Biology

A mutation changes a DNA triplet so that a different amino acid is inserted into a polypeptide. Which level of protein structure is most directly affected at that point?

  1. Quaternary structure
  2. The hydrogen bonds of the double helix
  3. Primary structure Correct answer.
  4. The lipid bilayer

Explanation

The primary structure is the sequence of amino acids joined by peptide bonds, so substituting one amino acid alters it directly. This may then alter folding and hence secondary and tertiary structure, but the immediate change is in the sequence.

Question 22Biology

Which feature of the kidney tubule is most important for reabsorbing glucose from the filtrate?

  1. A low blood pressure in the glomerulus
  2. Microvilli and many mitochondria in the proximal tubule cells Correct answer.
  3. A thick, impermeable capsule around the glomerulus
  4. Absence of carrier proteins in the tubule membrane

Explanation

Glucose is reabsorbed by active transport and co-transport, which needs a large surface area for carriers and a good ATP supply. Microvilli enlarge the apical surface and numerous mitochondria supply the energy for the proximal tubule cells.

Question 23Biology

Two populations of a species become separated by a river for many generations. Which outcome best describes speciation by geographical isolation?

  1. Both populations lose all genetic variation and become extinct
  2. Different selection pressures cause the populations to diverge until they can no longer interbreed Correct answer.
  3. One population immediately changes its genes to suit the new habitat
  4. The populations interbreed more freely and become genetically identical

Explanation

Geographical isolation prevents gene flow, so mutation and natural selection act independently on each population. Over many generations the gene pools diverge and reproductive isolation may arise, giving separate species.

Question 24Biology

A person with type AB blood can receive red cells from which donors without an ABO incompatibility reaction?

  1. Only groups A and B
  2. Only group AB
  3. Only group O
  4. Groups A, B, AB and O Correct answer.

Explanation

Group AB plasma contains neither anti-A nor anti-B antibodies, so the recipient's antibodies will not agglutinate donor cells of A, B, AB or O. This makes AB the universal recipient for red cell transfusion within the ABO system.

Question 25Biology

A plant is kept in a sealed transparent chamber in the dark for several hours. Which change in the chamber air is expected?

  1. Carbon dioxide decreases and oxygen increases
  2. Both carbon dioxide and oxygen increase
  3. Carbon dioxide increases and oxygen decreases Correct answer.
  4. Both carbon dioxide and oxygen remain unchanged

Explanation

In darkness the plant carries out only respiration, which takes up oxygen and releases carbon dioxide. Photosynthesis, which would reverse these changes, requires light and cannot occur.

Question 26Biology

Which structure in a plant cell controls the movement of substances into and out of the cell?

  1. Cell wall
  2. Cell surface membrane Correct answer.
  3. Large permanent vacuole
  4. Chloroplast

Explanation

The cell surface membrane is a partially permeable phospholipid bilayer that regulates which substances enter or leave the cell. The cell wall is fully permeable and provides structural support rather than selective control.

Question 27Biology

Which carbohydrate is the main storage polysaccharide found in animal liver and muscle tissue?

  1. Glycogen Correct answer.
  2. Cellulose
  3. Sucrose
  4. Starch

Explanation

Glycogen is a branched polymer of glucose stored in liver and muscle cells and broken down to release glucose when needed. Starch is the equivalent storage carbohydrate in plants, while cellulose is structural.

Question 28Biology

Which process moves water molecules across a partially permeable membrane from a dilute solution to a more concentrated solution?

  1. Diffusion of ions
  2. Osmosis Correct answer.
  3. Active transport
  4. Phagocytosis

Explanation

Osmosis is the net movement of water through a partially permeable membrane down a water potential gradient, from a region of higher water potential to lower water potential. It requires no energy input from the cell.

Question 29Biology

Which enzyme breaks down starch into maltose during digestion?

  1. Protease
  2. Amylase Correct answer.
  3. Lipase
  4. Catalase

Explanation

Amylase catalyses the hydrolysis of starch into maltose and is produced by the salivary glands and pancreas. Each enzyme is specific to its substrate because of the shape of its active site.

Question 30Biology

Which component of the blood is mainly responsible for transporting oxygen from the lungs to the tissues?

  1. Red blood cells Correct answer.
  2. Platelets
  3. Plasma proteins
  4. White blood cells

Explanation

Red blood cells contain haemoglobin, which binds oxygen in the lungs and releases it in respiring tissues. Their biconcave shape and lack of a nucleus maximise the space available for haemoglobin.

Question 31Biology

Which type of tooth is adapted with a flat, broad surface for crushing and grinding food?

  1. Incisor
  2. Premolar Correct answer.
  3. Incisor of the lower jaw
  4. Canine

Explanation

Premolars have broad, flattened crowns with cusps that crush and grind food during chewing. Incisors are chisel-shaped for cutting, and canines are pointed for tearing.

Question 32Biology

Which structure in the human respiratory system contains the vocal cords and connects the pharynx to the trachea?

  1. Diaphragm
  2. Alveolus
  3. Larynx Correct answer.
  4. Bronchus

Explanation

The larynx, or voice box, sits at the top of the trachea and contains the vocal cords, which vibrate as air passes over them. It links the pharynx above to the trachea below.

Question 33Biology

Which term describes the permanent loss of fertile land to desert-like conditions because of drought and poor land management?

  1. Deforestation
  2. Succession
  3. Eutrophication
  4. Desertification Correct answer.

Explanation

Desertification is the degradation of fertile land into desert, driven by overgrazing, poor irrigation and prolonged drought. It reduces the land's ability to support crops and natural vegetation.

Question 34Biology

A student views a stained slide of human tissue and observes cells with no cell wall, a distinct nucleus and many mitochondria. Which tissue is most likely being observed?

  1. Palisade mesophyll
  2. Epidermal tissue of a leaf
  3. Root hair tissue
  4. Muscle tissue Correct answer.

Explanation

Animal cells such as muscle cells lack a cell wall and have a nucleus and numerous mitochondria to supply ATP for contraction. The other options are plant tissues, which contain cells with cellulose walls.

Question 35Biology

A potato strip is placed in a sucrose solution and its mass decreases by 12 percent after one hour. Which conclusion is best supported?

  1. The potato cells gained water by osmosis.
  2. The sucrose solution had a lower water potential than the potato cell sap. Correct answer.
  3. The sucrose solution was more dilute than the potato cell sap.
  4. The potato cells were killed by the sucrose solution before any movement occurred.

Explanation

A loss in mass shows that water left the potato cells by osmosis, which happens when the surrounding solution has a lower water potential than the cell sap. The external solution was therefore more concentrated, not more dilute.

Question 36Biology

Which change would most increase the rate of an enzyme-controlled reaction when substrate is plentiful and the enzyme is not yet saturated?

  1. Adding more substrate without changing enzyme concentration
  2. Lowering the temperature well below the optimum
  3. Adding a heavy metal ion that binds the active site
  4. Increasing the enzyme concentration Correct answer.

Explanation

With substrate plentiful, enzyme concentration is the limiting factor, so adding more enzyme provides more active sites and raises the rate. Once all active sites are occupied, further substrate has little effect.

Question 37Biology

A person's diet is very low in iron. Which symptom is most directly expected?

  1. Scurvy caused by reduced collagen formation
  2. Constipation caused by reduced fibre intake
  3. Rickets caused by poor calcium absorption
  4. Anaemia caused by reduced haemoglobin synthesis Correct answer.

Explanation

Iron is a component of haemoglobin, so a deficiency reduces haemoglobin synthesis and causes anaemia, with tiredness and breathlessness. The other symptoms are linked to vitamins, calcium or fibre rather than iron.

Question 38Biology

Which feature of the small intestine most increases the surface area available for absorption of digested food?

  1. Presence of bile in the lumen
  2. Secretion of hydrochloric acid
  3. Presence of villi and microvilli on the inner surface Correct answer.
  4. Presence of a thick muscular wall

Explanation

Villi are finger-like projections of the intestinal lining, and microvilli are further folds on their surface cells. Together they greatly increase the surface area for absorption of nutrients.

Question 39Biology

During vigorous exercise, which change helps deliver more oxygen to muscle cells?

  1. Constriction of the coronary arteries
  2. Increased heart rate and increased stroke volume Correct answer.
  3. Reduced heart rate and reduced stroke volume
  4. Reduced ventilation rate

Explanation

Both a faster heart rate and a larger stroke volume raise cardiac output, increasing blood flow and oxygen delivery to respiring muscles. Ventilation also rises to support this.

Question 40Biology

Which statement best explains why a person's breathing rate increases during exercise?

  1. The diaphragm contracts less frequently during exercise.
  2. Oxygen concentration in the blood rises and stimulates breathing.
  3. Blood temperature falls and stimulates breathing.
  4. Carbon dioxide concentration in the blood rises and stimulates breathing. Correct answer.

Explanation

Increased respiration releases more carbon dioxide, which raises its concentration in the blood. Chemoreceptors detect this and stimulate the breathing centre, increasing the rate and depth of ventilation.

Question 41Biology

Which process in the carbon cycle returns carbon dioxide to the atmosphere from dead organic matter?

  1. Nitrogen fixation
  2. Transpiration
  3. Decomposition by microorganisms Correct answer.
  4. Photosynthesis

Explanation

Decomposers such as bacteria and fungi break down dead organic matter and release carbon dioxide through respiration. Photosynthesis removes carbon dioxide from the atmosphere instead.

Question 42Biology

Two heterozygous black guinea pigs are crossed, and black is dominant to white. What proportion of the offspring is expected to be white?

  1. Three quarters
  2. All offspring
  3. One half
  4. One quarter Correct answer.

Explanation

A cross of Bb by Bb gives genotypes BB, Bb, Bb and bb in equal expected proportions. Only the bb genotype shows the recessive white phenotype, which is one quarter of the offspring.

Question 43Biology

Which type of variation shows a continuous range of values, such as human height?

  1. Variation caused only by mutation
  2. Continuous variation controlled by many genes and the environment Correct answer.
  3. Variation between separate species
  4. Discontinuous variation controlled by a single gene

Explanation

Continuous variation is controlled by many genes acting together and is also influenced by environmental factors, producing a smooth range of values. Discontinuous variation falls into distinct categories.

Question 44Biology

A bacterium is exposed to an antibiotic, and a few cells with a resistance mutation survive and reproduce. Which statement best explains the population that develops?

  1. The resistant bacteria were selected for and passed the resistance allele to their offspring. Correct answer.
  2. The antibiotic caused a new resistance gene to appear in every cell.
  3. The bacteria learned to resist the antibiotic during exposure.
  4. All bacteria survived and then gradually lost resistance.

Explanation

The antibiotic acts as a selective pressure: cells already carrying the resistance allele survive and reproduce, so the allele becomes more common in later generations. This is natural selection acting on existing variation.

Question 45Biology

Which human activity releases chlorofluorocarbons that break down ozone in the upper atmosphere?

  1. Burning fossil fuels in power stations
  2. Adding fertiliser to farmland
  3. Using CFCs as refrigerants and propellants Correct answer.
  4. Discharging untreated sewage into rivers

Explanation

Chlorofluorocarbons used in refrigeration and aerosols release chlorine atoms in the stratosphere, which catalyse the breakdown of ozone. This thins the ozone layer and increases ultraviolet radiation reaching the surface.

Question 46Biology

A woman is a carrier of a recessive sex-linked disorder. Which statement about her sons is correct?

  1. All sons will be carriers but unaffected.
  2. No son can inherit the disorder.
  3. Each son has a one in two chance of being affected. Correct answer.
  4. Only sons born after the first child can be affected.

Explanation

Sons inherit their single X chromosome from their mother. If she carries the recessive allele on one X, each son has a one in two chance of receiving that X and showing the disorder.

Question 47Biology

In a cross between two heterozygous plants showing incomplete dominance, the offspring have three phenotypes. What is the expected phenotypic ratio?

  1. Nine to three to three to one
  2. One to one
  3. Three to one
  4. One to two to one Correct answer.

Explanation

With incomplete dominance the heterozygote has its own intermediate phenotype, so a cross of two heterozygotes gives one homozygous dominant, two heterozygous and one homozygous recessive, a ratio of 1:2:1.

Question 48Biology

A cell has 24 chromosomes at the start of meiosis. How many chromosomes are present in each cell at the end of meiosis II?

  1. 24
  2. 6
  3. 48
  4. 12 Correct answer.

Explanation

Meiosis halves the chromosome number: the first division reduces the diploid number to haploid, and the second division separates chromatids without further halving. Each of the four resulting cells therefore has 12 chromosomes.

Question 49Biology

In a food web, a large amount of energy is lost between each trophic level. Which factor accounts for most of this loss?

  1. Energy converted into light by consumers
  2. Energy passed on as biomass to the next trophic level
  3. Energy lost as heat from respiration and in undigested material Correct answer.
  4. Energy stored permanently in the bodies of consumers

Explanation

At each level, organisms use energy for respiration and release much of it as heat, while some material is excreted undigested. Only about ten percent of the energy is transferred as biomass to the next trophic level.

Question 50Biology

A patient loses a large volume of blood, causing a fall in blood pressure. Which response is expected from the body's homeostatic mechanisms?

  1. Vasoconstriction of arterioles and an increase in heart rate Correct answer.
  2. A decrease in the release of adrenaline
  3. A reduction in the reabsorption of water by the kidneys
  4. Vasodilation of arterioles and a fall in heart rate

Explanation

A fall in blood pressure is detected by baroreceptors, which trigger vasoconstriction to raise peripheral resistance and a faster heart rate to maintain cardiac output. Adrenaline release also rises to support this response.