I85PREP
⌘ K
LGCSE Biology (0180) study materials
LGCSE Biology (0180) study resources

LGCSE Biology (0180) practice questions

One question at a time. Build your LGCSE Biology (0180) understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Cells and Cell Processes

Which structure controls the entry and exit of substances in a cell and is described as partially permeable?

  1. The cell surface membrane Correct answer.
  2. The nucleus
  3. The mitochondrion
  4. The cell wall

Explanation

The cell surface membrane is a thin partially permeable layer made of lipids and proteins. It allows small molecules such as oxygen and water to pass while restricting many larger or charged substances, so it regulates exchange with the environment.

Question 2Cells and Cell Processes

A student compares a plant cell and an animal cell. Which feature is present in the plant cell but absent in the animal cell?

  1. A cellulose cell wall Correct answer.
  2. Cytoplasm
  3. A nucleus
  4. Ribosomes

Explanation

Plant cells are surrounded by a rigid cellulose cell wall that provides support and shape. Animal cells lack this wall and are bounded only by a flexible cell surface membrane, though both cell types share ribosomes, nuclei and cytoplasm.

Question 3Cells and Cell Processes

Which statement best describes diffusion?

  1. Movement of particles against a concentration gradient using energy from ATP
  2. Movement of water across a partially permeable membrane
  3. Net movement of particles from a region of higher to lower concentration Correct answer.
  4. Uptake of large particles by the folding of the cell surface membrane

Explanation

Diffusion is the passive net movement of particles down a concentration gradient, from where they are more concentrated to where they are less concentrated. It requires no energy input from respiration and continues until the concentrations are even.

Question 4Cells and Cell Processes

Which process requires energy released by respiration in a cell?

  1. Active transport of mineral ions into root hair cells Correct answer.
  2. Diffusion of carbon dioxide out of a respiring cell
  3. Osmosis of water into a root hair cell
  4. Diffusion of oxygen into a leaf

Explanation

Active transport moves ions against their concentration gradient and must be powered by energy from respiration. Diffusion and osmosis are passive and rely only on the random movement of particles down a gradient.

Question 5Cells and Cell Processes

What is the main function of the mitochondria in a cell?

  1. To store starch grains
  2. To control the activities of the cell
  3. To release energy by aerobic respiration Correct answer.
  4. To synthesise proteins from amino acids

Explanation

Mitochondria are the site of the reactions of aerobic respiration, which release energy from glucose for the cell's activities. Cells with high energy demands, such as muscle cells, contain many mitochondria.

Question 6Cells and Cell Processes

A plant cell is placed in pure water. Which change is most likely to occur?

  1. The cell shrinks and crenates
  2. The cell becomes turgid Correct answer.
  3. The cell becomes plasmolysed
  4. The cell bursts immediately

Explanation

Pure water has a higher water potential than the cell contents, so water enters by osmosis and presses the cytoplasm against the cell wall. The rigid cellulose wall resists further expansion, making the cell turgid rather than bursting.

Question 7Cells and Cell Processes

Which molecule carries genetic information in the nucleus of a cell?

  1. Cellulose
  2. DNA Correct answer.
  3. Glucose
  4. Haemoglobin

Explanation

DNA is the molecule that stores genetic information in the form of a base sequence and is found mainly in the nucleus. Sections of DNA called genes carry the instructions for making proteins.

Question 8Cells and Cell Processes

Which feature of a red blood cell adapts it for carrying oxygen efficiently?

  1. It contains many chloroplasts
  2. It has a thick cell wall
  3. It has a large nucleus
  4. It has a biconcave shape with a large surface area Correct answer.

Explanation

The biconcave disc shape gives a large surface area relative to volume for absorbing and releasing oxygen, and the lack of a nucleus leaves more room for haemoglobin. Together these adaptations maximise oxygen transport.

Question 9Cells and Cell Processes

A potato strip is placed in a concentrated sugar solution. After 30 minutes the strip is shorter and flaccid. Which explanation is correct?

  1. The sugar solution dissolved the cell walls
  2. Sugar moved into the potato cells by osmosis
  3. Water moved out of the potato cells by osmosis Correct answer.
  4. Water moved into the potato cells by osmosis

Explanation

The sugar solution has a lower water potential than the potato cell contents, so water leaves the cells through the partially permeable membranes by osmosis. Loss of water reduces turgor and shortens the tissue.

Question 10Cells and Cell Processes

In a typical enzyme-controlled reaction, what happens to the rate of reaction as temperature rises from 10 °C to 40 °C and then beyond the optimum?

  1. The rate falls continuously from 10 °C onwards
  2. The rate rises steadily and remains high at all higher temperatures
  3. The rate rises to a maximum at the optimum and then falls sharply Correct answer.
  4. The rate stays constant at all temperatures

Explanation

Warmer temperatures give molecules more kinetic energy, so successful collisions between enzyme and substrate increase and the rate rises to an optimum. Beyond the optimum the enzyme's active site is denatured and loses its shape, so the rate falls sharply.

Question 11Cells and Cell Processes

Which statement explains why an enzyme is described as specific?

  1. It is used up during the reaction it catalyses
  2. It can catalyse any reaction in the cell
  3. It works only at a particular temperature
  4. It catalyses only reactions involving a substrate that fits its active site Correct answer.

Explanation

The active site of an enzyme has a shape complementary to its substrate, so only a substrate that fits can form an enzyme–substrate complex. This lock-and-key relationship means each enzyme catalyses a limited set of reactions.

Question 12Cells and Cell Processes

A cell is observed to have a cell wall, cytoplasm, ribosomes and a circular loop of DNA but no membrane-bound nucleus. Which type of organism does this cell belong to?

  1. A flowering plant
  2. A fungus
  3. A bacterium Correct answer.
  4. A mammal

Explanation

Bacteria are prokaryotes: they lack a membrane-bound nucleus and have circular DNA in the cytoplasm. They also possess a cell wall and ribosomes but no mitochondria or other membrane-bound organelles.

Question 13Cells and Cell Processes

Which combination correctly describes the effects of a lack of oxygen on a respiring muscle cell?

  1. Anaerobic respiration produces lactic acid and releases less energy per glucose molecule Correct answer.
  2. Respiration stops completely because oxygen is essential for all respiration
  3. Anaerobic respiration produces ethanol and carbon dioxide
  4. Aerobic respiration increases and more carbon dioxide is released

Explanation

When oxygen is limited, muscle cells respire glucose anaerobically to lactic acid, releasing much less energy per glucose molecule than aerobic respiration. Lactic acid can build up and contribute to muscle fatigue.

Question 14Cells and Cell Processes

Which statement correctly describes the role of chlorophyll in photosynthesis?

  1. It converts glucose into starch for storage
  2. It releases oxygen from water without using light
  3. It absorbs carbon dioxide from the atmosphere
  4. It absorbs light energy and transfers it to drive the synthesis of carbohydrates Correct answer.

Explanation

Chlorophyll in chloroplasts absorbs mainly red and blue light, and the absorbed energy drives the reactions that build carbohydrates from carbon dioxide and water. Oxygen is released as a by-product of splitting water.

Question 15Cells and Cell Processes

A leaf is tested with iodine solution after being destarched and then exposed to light for several hours. Why is the leaf boiled in ethanol before adding iodine?

  1. To add carbon dioxide to the leaf
  2. To remove chlorophyll so the colour change is visible Correct answer.
  3. To kill the leaf and stop respiration
  4. To dissolve the starch so iodine can enter

Explanation

Boiling the leaf in ethanol decolourises it by removing chlorophyll, which would otherwise mask the blue-black colour produced when iodine reacts with starch. The leaf is first killed by boiling in water to stop further chemical change.

Question 16Cells and Cell Processes

Which adaptation of a root hair cell increases the rate of absorption of water and mineral ions?

  1. A long thin extension that increases surface area Correct answer.
  2. A thick waxy cuticle
  3. A reduced number of mitochondria
  4. A large number of chloroplasts

Explanation

The elongated hair-like extension greatly increases the surface area of the cell in contact with soil water, speeding absorption. Many mitochondria also supply energy for active transport of mineral ions.

Question 17Cells and Cell Processes

Which statement best describes the behaviour of an animal cell placed in distilled water?

  1. It becomes turgid and supported by a cell wall
  2. It loses water and shrinks
  3. It remains unchanged because it is impermeable
  4. It takes in water, swells and may burst Correct answer.

Explanation

Distilled water has a higher water potential than the cytoplasm, so water enters the animal cell by osmosis. With no cell wall to resist expansion, the cell swells and may eventually burst, a process called lysis.

Question 18Cells and Cell Processes

A student measures the rate of an enzyme reaction at different substrate concentrations. At high substrate concentration the rate levels off. What is the best explanation?

  1. The temperature has fallen during the experiment
  2. The substrate has been denatured by the high concentration
  3. The enzyme has been used up in the reaction
  4. All the active sites are occupied, so enzyme concentration limits the rate Correct answer.

Explanation

At high substrate concentrations the enzyme molecules are working at maximum capacity because all active sites are occupied at any moment. Further substrate cannot increase the rate, so enzyme concentration becomes the limiting factor.

Question 19Cells and Cell Processes

Which statement correctly compares the energy released by aerobic and anaerobic respiration in muscle cells?

  1. Anaerobic respiration releases no energy at all
  2. Anaerobic respiration releases more energy per glucose molecule than aerobic respiration
  3. Aerobic respiration releases more energy per glucose molecule than anaerobic respiration Correct answer.
  4. Aerobic and anaerobic respiration release equal energy per glucose molecule

Explanation

Aerobic respiration completely breaks down glucose to carbon dioxide and water, releasing far more energy per molecule than the partial breakdown in anaerobic respiration. Anaerobic respiration to lactic acid releases only a small fraction of that energy.

Question 20Cells and Cell Processes

Which structure in a leaf cell is the main site of photosynthesis?

  1. The vacuole
  2. The ribosome
  3. The cell wall
  4. The chloroplast Correct answer.

Explanation

Chloroplasts contain chlorophyll and the enzymes needed for the light-dependent and light-independent reactions of photosynthesis. They are numerous in palisade mesophyll cells near the upper surface of the leaf.

Question 21Cells and Cell Processes

A cell is treated with a substance that prevents ATP production. Which process would be affected most directly?

  1. Active transport of ions across the membrane Correct answer.
  2. Movement of carbon dioxide out of the cell
  3. Osmosis of water into the cell
  4. Diffusion of oxygen across the cell surface membrane

Explanation

Active transport depends on energy from ATP to move ions against their concentration gradient, so blocking ATP production stops it. Diffusion and osmosis are passive and continue as long as a gradient exists.

Question 22Cells and Cell Processes

Two enzyme samples act on the same substrate, but sample X reaches its maximum rate at a much lower substrate concentration than sample Y. Both samples contain the same total mass of enzyme protein. Which conclusion is best supported?

  1. Sample X has a higher affinity for the substrate than sample Y Correct answer.
  2. Sample Y has been denatured before the experiment
  3. Sample Y contains no active sites
  4. Sample X releases more energy per reaction than sample Y

Explanation

If sample X reaches saturation at a lower substrate concentration, its active sites become fully occupied more readily, indicating a greater affinity for the substrate. Both samples can reach a similar maximum rate if the total amount of active enzyme is comparable.

Question 23Cells and Cell Processes

A plant is kept in bright light but in air containing very little carbon dioxide. Which immediate effect on photosynthesis is expected?

  1. The rate rises because light intensity is high
  2. The rate falls because chlorophyll is destroyed instantly
  3. The rate falls because carbon dioxide is a limiting factor Correct answer.
  4. The rate is unchanged because light is the only requirement

Explanation

Photosynthesis needs carbon dioxide as a raw material for making carbohydrates, so a shortage limits the rate even in bright light. The light-dependent stage may still run briefly, but overall carbohydrate synthesis slows.

Question 24Cells and Cell Processes

A eukaryotic cell is placed in a solution whose water potential is identical to that of its cytoplasm. Which observation is expected?

  1. Water continues to enter until the cell bursts
  2. Solutes move out by osmosis until the cell is empty
  3. Water leaves rapidly and the cell shrinks
  4. Net movement of water stops because there is no water potential gradient Correct answer.

Explanation

Osmosis depends on a difference in water potential between the two sides of the membrane. When the external solution and cytoplasm have the same water potential, there is no net movement of water, though individual molecules still cross in both directions.

Question 25Cells and Cell Processes

A microscopic section shows cells with many mitochondria, numerous folds of the cell surface membrane and many vesicles. Which function is most consistent with these features?

  1. Support of the organism through a rigid wall
  2. Storage of starch grains for later use
  3. Absorption of substances requiring energy and a large surface area Correct answer.
  4. Rapid diffusion of gases only

Explanation

Many mitochondria supply ATP for active processes, membrane folds increase surface area for absorption, and vesicles suggest transport of materials into or within the cell. Together these features indicate an absorptive cell with high energy demands.

Question 26Cells and Cell Processes

Which part of a bacterial cell controls the movement of substances into and out of the cell?

  1. The circular loop of DNA
  2. The ribosomes
  3. The cell surface membrane Correct answer.
  4. The cell wall

Explanation

The cell surface membrane is a partially permeable phospholipid bilayer that regulates which substances enter or leave the cell. The cell wall provides shape and support but is fully permeable in bacteria, so it does not control entry.

Question 27Cells and Cell Processes

A student observes a cell under a microscope and sees a nucleus, mitochondria, chloroplasts and a cellulose cell wall. Which conclusion is best supported?

  1. The cell is a fungal cell
  2. The cell is an animal cell
  3. The cell is a plant cell Correct answer.
  4. The cell is a bacterial cell

Explanation

Chloroplasts and a cellulose cell wall together with a true nucleus and mitochondria are characteristic of a photosynthetic plant cell. Animal cells lack both chloroplasts and a cell wall, while bacterial cells have no nucleus or membrane-bound organelles.

Question 28Cells and Cell Processes

Which statement describes the main function of the mitochondria in a cell?

  1. They store water and dissolved salts
  2. They synthesise proteins from amino acids
  3. They release energy from glucose during aerobic respiration Correct answer.
  4. They package and modify proteins for secretion

Explanation

Mitochondria are the site of the later stages of aerobic respiration, where glucose is broken down and energy is released to make ATP. Their folded inner membrane provides a large surface area for the enzymes involved.

Question 29Cells and Cell Processes

A cell is placed in a solution with a lower water potential than its cytoplasm. Which prediction about the movement of water is correct?

  1. Water moves into the cell by osmosis, causing the cell to swell
  2. Water moves equally in both directions, so there is no net change
  3. Water does not cross the membrane because the membrane is impermeable
  4. Water moves out of the cell by osmosis, causing the cell to shrink Correct answer.

Explanation

Osmosis is the net movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential. Because the external solution has the lower water potential, water leaves the cell and the cell shrinks.

Question 30Cells and Cell Processes

A student investigates diffusion using cubes of agar containing a pH indicator. A small cube and a large cube are placed in acid, and the time taken for the acid to reach the centre of each cube is recorded. Which result and explanation is correct?

  1. The small cube is coloured first because it has a larger surface area to volume ratio Correct answer.
  2. The small cube is coloured first because it has a larger volume
  3. Both cubes are coloured at the same time because diffusion rate is independent of size
  4. The large cube is coloured first because it has a larger surface area to volume ratio

Explanation

The rate of diffusion depends on the surface area to volume ratio, which increases as an object becomes smaller. The small cube therefore has more surface area relative to its volume, so the acid diffuses to its centre in a shorter time.

Question 31Cells and Cell Processes

During the cell cycle, what happens during interphase before mitosis begins?

  1. The chromosomes are pulled to opposite poles
  2. The cytoplasm divides to form two cells
  3. The nuclear membrane breaks down
  4. The DNA is replicated and the cell grows Correct answer.

Explanation

Interphase is the period of growth and DNA replication that prepares the cell for division, so each chromosome is duplicated before mitosis. The other events listed occur during mitosis or cytokinesis, not interphase.

Question 32Cells and Cell Processes

An enzyme is added to a substrate solution and the reaction proceeds. A competitive inhibitor is then added in a low concentration. Which change is most likely to be observed?

  1. The rate of reaction decreases and cannot be restored by adding more substrate
  2. The rate of reaction decreases, but a higher substrate concentration can restore it Correct answer.
  3. The rate of reaction increases because more enzyme active sites become available
  4. The rate of reaction is unaffected because the inhibitor does not bind to the enzyme

Explanation

A competitive inhibitor binds to the active site and blocks substrate molecules, so the rate falls. Because it competes with the substrate, increasing the substrate concentration reduces the inhibitor's effect and can restore the rate.

Question 33Cells and Cell Processes

A student compares the rate of oxygen uptake of pondweed in light and in darkness at the same temperature. Which explanation correctly accounts for the difference?

  1. In light, respiration stops, so all oxygen produced by photosynthesis is released
  2. In darkness, respiration stops, so oxygen uptake falls to zero
  3. In darkness, photosynthesis is faster than respiration, so more oxygen is used
  4. In light, photosynthesis produces oxygen while respiration still uses it, so net oxygen release is greater Correct answer.

Explanation

In light, photosynthesis and respiration occur together; photosynthesis produces oxygen and respiration consumes it, giving a net release of oxygen. In darkness photosynthesis stops and respiration continues, so oxygen is taken up from the surroundings.

Question 34Cells and Cell Processes

A cell is treated with a drug that prevents the formation of spindle fibres. Which stage of mitosis would be most directly affected?

  1. Interphase, because the DNA could not replicate
  2. Telophase, because the nuclear membrane could not reform
  3. Prophase, because the chromosomes could not condense
  4. Anaphase, because chromosomes could not be separated to opposite poles Correct answer.

Explanation

Spindle fibres attach to chromosomes at their centromeres and pull them apart during anaphase. Without spindle formation, sister chromatids cannot be separated correctly, so anaphase is the stage most directly disrupted.

Question 35Organisation and Maintenance of the Organism

Which structure in a leaf is the main site of light absorption for photosynthesis?

  1. Palisade mesophyll cells Correct answer.
  2. Guard cells
  3. Xylem vessels
  4. Spongy mesophyll air spaces

Explanation

Palisade mesophyll cells are packed with chloroplasts and lie near the upper leaf surface, so they absorb most of the light that reaches the leaf. Their column shape and dense chloroplast content maximise light capture for photosynthesis.

Question 36Organisation and Maintenance of the Organism

A student observes that a plant wilts on a hot day. Which change in the plant best explains wilting?

  1. Thickening of the cuticle
  2. Increased lignin in vessel walls
  3. Loss of turgor pressure in cells Correct answer.
  4. Reduced chloroplast number

Explanation

Wilting occurs when cells lose water by transpiration faster than it is replaced, so vacuoles shrink and turgor pressure falls. Without turgor the soft tissues can no longer hold the stem and leaves upright.

Question 37Organisation and Maintenance of the Organism

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

  1. Vena cava
  2. Coronary vein
  3. Pulmonary artery
  4. Aorta Correct answer.

Explanation

The aorta is the large artery leaving the left ventricle, and it distributes oxygenated blood to the systemic circulation. Arteries carry blood away from the heart, and the aorta is the main systemic artery.

Question 38Organisation and Maintenance of the Organism

Which feature of an alveolus increases the rate of gas exchange?

  1. A narrow lumen with valves
  2. A large surface area with a thin wall Correct answer.
  3. A layer of cartilage in the wall
  4. A thick, moist lining

Explanation

Gas exchange is fast when the surface area is large and the diffusion distance is short, so the many alveoli with walls one cell thick maximise diffusion of oxygen and carbon dioxide. A moist surface also helps gases dissolve before diffusing.

Question 39Organisation and Maintenance of the Organism

Which process moves glucose from the small intestine into the blood against a concentration gradient?

  1. Active transport Correct answer.
  2. Filtration
  3. Diffusion
  4. Osmosis

Explanation

Active transport uses energy from respiration to move substances across a membrane against their concentration gradient, which is how glucose is absorbed when intestinal levels fall. Carrier proteins in the epithelial membrane perform this movement.

Question 40Organisation and Maintenance of the Organism

Which enzyme digests starch, and what are its products?

  1. Lipase, producing fatty acids and glycerol
  2. Protease, producing amino acids
  3. Catalase, producing water and oxygen
  4. Amylase, producing maltose Correct answer.

Explanation

Amylase hydrolyses starch into maltose, a disaccharide, which is later broken down further by maltase into glucose. Starch digestion begins in the mouth and continues in the small intestine.

Question 41Organisation and Maintenance of the Organism

Which structure prevents food from entering the trachea during swallowing?

  1. Diaphragm
  2. Epiglottis Correct answer.
  3. Uvula
  4. Sphincter muscle of the stomach

Explanation

During swallowing the epiglottis folds down over the opening of the trachea so that the bolus passes into the oesophagus. This reflex prevents choking and keeps the airway clear.

Question 42Organisation and Maintenance of the Organism

Which component of blood is mainly responsible for transporting oxygen?

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

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 increase the space available for haemoglobin.

Question 43Organisation and Maintenance of the Organism

A person's diet lacks iron. Which symptom is most likely to develop?

  1. Scurvy caused by reduced collagen
  2. Goitre caused by an enlarged thyroid
  3. Rickets caused by weak bones
  4. Anaemia caused by reduced haemoglobin Correct answer.

Explanation

Iron is a component of haemoglobin, so a deficiency reduces the amount of oxygen-carrying pigment in red blood cells. This produces anaemia, with tiredness and breathlessness on exertion.

Question 44Organisation and Maintenance of the Organism

Which adaptation of a root hair cell increases the uptake of mineral ions from soil?

  1. A long, thin extension that increases surface area Correct answer.
  2. A thick waxy cuticle covering the cell
  3. Numerous chloroplasts in the cytoplasm
  4. A large central vacuole filled with air

Explanation

The elongated projection of a root hair cell greatly increases the surface area over which ions and water can be absorbed. This raises the rate of uptake from the surrounding soil solution.

Question 45Organisation and Maintenance of the Organism

Which sequence correctly describes the path of blood through the heart after it returns from the lungs?

  1. Right ventricle, left atrium, aorta, lungs
  2. Right atrium, right ventricle, pulmonary artery, lungs
  3. Left atrium, left ventricle, aorta, body Correct answer.
  4. Left ventricle, left atrium, vena cava, body

Explanation

Oxygenated blood from the lungs enters the left atrium, passes into the left ventricle, and is pumped into the aorta for distribution to the body. The left ventricle has a thick muscular wall to generate the pressure required for the systemic circuit.

Question 46Organisation and Maintenance of the Organism

Which effect does the hormone adrenaline have on the body during exercise?

  1. It reduces blood flow to skeletal muscles.
  2. It lowers blood glucose by promoting glycogen formation.
  3. It slows the heart rate and reduces breathing depth.
  4. It increases heart rate and widens the airways. Correct answer.

Explanation

Adrenaline prepares the body for vigorous activity by raising heart rate, widening the bronchioles and increasing blood flow to muscles. It also promotes the release of glucose from glycogen stores to supply respiring cells.

Question 47Organisation and Maintenance of the Organism

Which statement best explains why a mammal loses heat rapidly in cold, wet conditions?

  1. Evaporation of water from the skin removes latent heat. Correct answer.
  2. Sweat glands stop producing sweat.
  3. Vasodilation of skin arterioles conserves heat.
  4. Shivering generates additional body heat.

Explanation

When the skin is wet, water evaporates and takes latent heat from the body surface, which increases heat loss. In cold conditions this can lower core temperature faster than the body can replace the heat.

Question 48Organisation and Maintenance of the Organism

A plant is kept in darkness for several days. Which observation is most likely?

  1. The plant produces more chlorophyll to compensate.
  2. The rate of photosynthesis falls and stored starch is used up. Correct answer.
  3. Transpiration stops completely.
  4. Starch stores in the leaves increase.

Explanation

Photosynthesis requires light, so in darkness the plant cannot make new glucose and must rely on stored carbohydrate. Stored starch in the leaves is broken down and used for respiration, so a starch test would become negative.

Question 49Organisation and Maintenance of the Organism

Which change occurs in the lungs during inhalation?

  1. The diaphragm contracts and flattens. Correct answer.
  2. The volume of the thorax decreases.
  3. The external intercostal muscles relax.
  4. The diaphragm relaxes and moves upward.

Explanation

During inhalation the diaphragm contracts and flattens while the external intercostal muscles raise the ribs. This increases the volume of the thorax, lowering the pressure inside the lungs so air flows in.

Question 50Organisation and Maintenance of the Organism

Which feature of a fish gill allows efficient gas exchange in water?

  1. A counter-current flow of water and blood Correct answer.
  2. Air-filled chambers within the filaments
  3. A thick, waterproof covering
  4. A single gill arch with no lamellae

Explanation

In fish gills water flows over the lamellae in the opposite direction to the blood, maintaining a concentration gradient along the whole length of the exchange surface. This counter-current arrangement allows a high proportion of the dissolved oxygen to be extracted.