Cambridge IGCSE Biology Revision

It is an animal and a bird vertebrate: feathers and a beak are the stated identifying features, while the backbone establishes vertebrate status. Excretion removes metabolic wastes or excess substances; egestion removes undigested food as faeces.

A and B are more closely related because their compared DNA base sequences are more similar, supporting a more recent common ancestor. A virus has genetic material and a protein coat.

Cell wall supports and strengthens; chloroplasts carry out photosynthesis (in photosynthesising cells). A root hair cell absorbs water and mineral ions; its syllabus-listed specialised function is absorption.

Actual size = 24 mm ÷ 6 = 4 mm.

The gallery teaches diagram labelling for plant, animal and bacterial cells. The separate requirement to identify those cell types from actual images is still uncovered.

0.085 × 1000 = 85 μm.

Osmosis: water crossed the partially permeable tubing from the more dilute surrounding water toward the more concentrated solution. The gain is evidence of net water entry; sucrose should not be described as the particle responsible for that mass gain.

The cell is plasmolysed. The surrounding solution has lower water potential, so water leaves by osmosis; the cell loses turgor and the protoplast pulls away from the wall.

Protein and fat/oil, respectively. The lilac Biuret result indicates protein; the cloudy white ethanol emulsion indicates fat or oil.

T G A C: pair A with T and C with G at every position.

At 60°C the enzyme may be denatured: its active-site shape changes, so starch fits less well and fewer enzyme-substrate interactions produce product. The example suggests a difference but does not establish a universal optimum.

Below the optimum, greater kinetic energy increases effective collision frequency. Above the optimum, denaturation changes the active-site shape and reduces substrate fit, so rate falls.

Starch formed in the illuminated region but not in the covered region, supporting the need for light for photosynthesis under these conditions. The iodine result detects starch, a product/storage form of carbohydrate.

CO₂ was limiting over the rising section; when the curve levels off, another condition such as light intensity or temperature may limit further increase.

Chewing is physical digestion because it changes particle size without chemically changing food molecules. Amylase performs chemical digestion of starch. Smaller pieces provide greater surface area for enzyme action.

Alkaline bile neutralises the acidic mixture to provide a suitable pH for enzyme action. Emulsification divides fat into smaller droplets, increasing the surface area for lipase; neither action makes bile an enzyme.

The stain shows water moved into and along the xylem. A leaf section with stain in the vein xylem, followed by stained mesophyll near the vein, would support the onward route from xylem to leaf tissues.

Transpiration is expected to increase: warmth speeds evaporation, while moving dry air maintains a steep water-vapour gradient at the leaf surface. (The prediction assumes other factors, such as light and stomatal state, are unchanged.)

It is a vein. Blood in veins is at relatively low pressure, so valves prevent backflow and help maintain return toward the heart.

The two passages separate pulmonary and systemic circuits and restore pressure after the lungs before blood travels around the body. The left ventricle needs a thicker muscular wall to generate the higher systemic pressure; the right pumps only to the nearby lungs.

Provide a clean treated water supply so the pathogen is not ingested, and safely dispose of waste/sewage so contamination does not reach the well. Each removes a link between the pathogen source and the next host.

Vaccination has produced memory cells; on re-exposure these cells enable a faster antibody response. Breast milk transfers antibodies made by another individual, giving temporary passive protection without producing the infant's memory cells.

Those controls make the comparison fair: a greater change in cloudiness can then be attributed to a difference in carbon dioxide content rather than unequal limewater amount or exposure time.

Thoracic volume increases and pressure inside falls below atmospheric pressure. Air therefore moves into the lungs down the pressure gradient.

Glucose → alcohol + carbon dioxide. It occurs without oxygen and releases much less energy per glucose molecule than aerobic respiration.

Heart rate remains high to carry lactic acid in blood from muscles to the liver, while deeper and faster breathing supplies oxygen; the liver aerobically respires the lactic acid.

It is a ureter. Urine passes to the bladder, where it is stored, and leaves through the urethra.

All filtered glucose is reabsorbed back into the blood by the nephron. Urine contains urea, excess water and excess ions.

Receptor → sensory neurone → relay neurone in the CNS → motor neurone → effector. The hand muscle is the effector that contracts to withdraw the hand.

Circular iris muscles contract, decreasing pupil diameter. Auxin becomes more concentrated on the shaded side of the shoot, where it stimulates more cell elongation; that side grows longer and the shoot bends toward the light.

Influenza is caused by a virus, so an antibiotic will not treat it. Stopping early may leave bacteria that were less susceptible alive; they can reproduce and pass on resistance, increasing the chance that a resistant population develops.

The antibiotic kills susceptible bacteria more readily; the 2 resistant bacteria are more likely to survive, reproduce and pass resistance on. If unnecessary courses repeatedly impose this selection, resistant descendants make up a larger proportion, even if the antibiotic did not cause those bacteria to choose resistance.

Water availability is the independent variable. Keep seed species/age, number, temperature, light and observation period the same; use replicates if possible. Count the number of seeds that germinate in each group at the same time intervals, rather than judging growth from unequal samples.

It is cross-pollination. The pollen grain germinates on the stigma, a pollen tube grows down to and enters an ovule, and the pollen nucleus fuses with a nucleus in that ovule.

The affected child must be aa. Each unaffected parent must have supplied a recessive a allele, so both parents are heterozygous carriers Aa. The unaffected sibling could be AA or Aa; the diagram alone does not distinguish those genotypes.

Female gametes are Xᴺ and Xⁿ; male gametes are Xᴺ and Y. The combinations are XᴺXᴺ, XᴺXⁿ, XᴺY and XⁿY, each 1/4. One of the two possible sons is XⁿY, so 1/2 of sons are expected to be colour-blind.

Height is continuous because it spans a range and is affected by both genes and environment. Flower colour here is discontinuous because it falls into separate categories; it is usually caused by genes only.

The thick waxy cuticle reduces water loss by evaporation. In a dry habitat this helps the plant maintain water, survive and reproduce; if the feature is inherited, its allele can be passed to offspring and become more common over generations.

Efficiency = (1 800 ÷ 12 000) × 100 = 15%. Some energy is used in respiration and lost as heat; other energy is in uneaten material or waste, so less is transferred to the next trophic level.