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Botswana BGCSE Science Double Award (0569) study materials
Botswana BGCSE Science Double Award (0569) study resources

Botswana BGCSE Science Double Award (0569) practice questions

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Question 1Biology: Cells, Organisms and Life Processes

Which structure controls the activities of a cell and contains its genetic material?

  1. The cytoplasm
  2. The cell wall
  3. The vacuole
  4. The nucleus Correct answer.

Explanation

The nucleus holds the chromosomes, which are made of DNA, and it directs the chemical reactions and division of the cell. Genetic instructions for making proteins are stored there and copied out to the cytoplasm.

Question 2Biology: Cells, Organisms and Life Processes

Which feature is present in a plant cell but absent from an animal cell?

  1. Mitochondria
  2. A cellulose cell wall Correct answer.
  3. Ribosomes
  4. A cell membrane

Explanation

Plant cells are surrounded by a tough wall made mainly of cellulose, which animal cells never have. Both cell types possess a membrane, ribosomes and mitochondria because both carry out protein synthesis and respiration.

Question 3Biology: Cells, Organisms and Life Processes

A student views onion cells using a microscope with a 10x eyepiece and a 40x objective lens. What is the total magnification?

  1. 50x
  2. 400x Correct answer.
  3. 4000x
  4. 30x

Explanation

Total magnification is found by multiplying the eyepiece magnification by the objective magnification. Ten multiplied by forty gives four hundred, so the image appears four hundred times larger than the specimen.

Question 4Biology: Cells, Organisms and Life Processes

Which part of a flowering plant absorbs most of the water needed for photosynthesis?

  1. The leaves
  2. The stem
  3. The flowers
  4. The root hair cells Correct answer.

Explanation

Root hair cells are long, thin extensions of root cells that greatly increase the surface area for absorbing water and mineral salts from the soil. Water then travels up the stem to the leaves in the xylem.

Question 5Biology: Cells, Organisms and Life Processes

Which gas is released by a green plant during photosynthesis?

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

Explanation

During photosynthesis, carbon dioxide and water are combined using light energy to form glucose, and oxygen is released as a by-product. The oxygen leaves the leaf mainly through the stomata.

Question 6Biology: Cells, Organisms and Life Processes

Which word equation summarises aerobic respiration?

  1. Glucose + oxygen → carbon dioxide + water Correct answer.
  2. Glucose → alcohol + carbon dioxide
  3. Glucose → lactic acid
  4. Carbon dioxide + water → glucose + oxygen

Explanation

Aerobic respiration uses oxygen to break down glucose completely, releasing energy together with carbon dioxide and water. The other equations describe photosynthesis or anaerobic respiration.

Question 7Biology: Cells, Organisms and Life Processes

Which type of cell has no nucleus when it is mature, so that it can carry more oxygen?

  1. A red blood cell Correct answer.
  2. A white blood cell
  3. A nerve cell
  4. An egg cell

Explanation

A mature red blood cell loses its nucleus and much of its internal structure, leaving more room for haemoglobin to carry oxygen. Its flexible biconcave shape also gives a large surface area for gas exchange.

Question 8Biology: Cells, Organisms and Life Processes

Which process allows water molecules to move from a dilute solution into a more concentrated solution through a partially permeable membrane?

  1. Active transport
  2. Diffusion of gases
  3. Osmosis Correct answer.
  4. Digestion

Explanation

Osmosis is the movement of water through a partially permeable membrane from a region of higher water concentration to a region of lower water concentration. It requires no energy from the cell and continues until concentrations balance.

Question 9Biology: Cells, Organisms and Life Processes

An enzyme found in the human stomach works best at about pH 2. Which statement explains this?

  1. The enzyme is destroyed by acid and replaced continuously.
  2. The enzyme's active site has a shape that fits its substrate best at that pH. Correct answer.
  3. The enzyme is denatured by acid and so acts faster.
  4. All enzymes work fastest in acidic conditions.

Explanation

Each enzyme has an active site whose shape is affected by pH. At the optimum pH the shape fits the substrate closely, so many enzyme-substrate complexes form and the reaction is fast. Away from the optimum, the active site changes shape and the enzyme is denatured.

Question 10Biology: Cells, Organisms and Life Processes

A potato strip is placed in pure water. What is most likely to happen to the strip?

  1. It will dissolve completely.
  2. It will become shorter and floppy.
  3. It will become firmer and slightly longer. Correct answer.
  4. It will stay exactly the same length.

Explanation

Pure water has a higher water concentration than the potato cells, so water enters the cells by osmosis. The cells become turgid, making the strip firm and slightly longer.

Question 11Biology: Cells, Organisms and Life Processes

Which statement best explains why leaves are thin and flat?

  1. They make the plant heavier so it stays upright.
  2. They provide a large surface area for absorbing light and allow gases to diffuse quickly. Correct answer.
  3. They allow the plant to store more food in a small space.
  4. They reduce water loss by keeping the leaf cool.

Explanation

A broad, thin leaf exposes a large area to sunlight and gives gases a short distance to travel to and from the cells. This increases the rate of photosynthesis and gas exchange.

Question 12Biology: Cells, Organisms and Life Processes

Which structure in the human breathing system is the site of gas exchange?

  1. The bronchi
  2. The diaphragm
  3. The trachea
  4. The alveoli Correct answer.

Explanation

Alveoli are tiny air sacs at the ends of the bronchioles, and their thin, moist walls are surrounded by capillaries. Oxygen diffuses into the blood while carbon dioxide diffuses out, which is the exchange of gases.

Question 13Biology: Cells, Organisms and Life Processes

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

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

Explanation

The aorta is the large artery leaving the left ventricle, and it carries oxygenated blood to the body. Arteries have thick, elastic walls to withstand the high pressure produced by the heart.

Question 14Biology: Cells, Organisms and Life Processes

Which component of blood transports oxygen around the body?

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

Explanation

Red blood cells contain haemoglobin, which combines with oxygen in the lungs and releases it in the tissues. Their biconcave shape gives a large surface area for this uptake.

Question 15Biology: Cells, Organisms and Life Processes

Which sequence correctly describes the path of food through the human digestive system?

  1. Mouth → oesophagus → stomach → small intestine → large intestine Correct answer.
  2. Mouth → small intestine → stomach → oesophagus → large intestine
  3. Mouth → oesophagus → small intestine → stomach → large intestine
  4. Mouth → stomach → oesophagus → small intestine → large intestine

Explanation

Food is chewed in the mouth, passes down the oesophagus to the stomach, and then enters the small intestine where most absorption occurs. Undigested material finally passes into the large intestine.

Question 16Biology: Cells, Organisms and Life Processes

Which substance speeds up the breakdown of starch into simpler sugars in the mouth and small intestine?

  1. Lipase
  2. Amylase Correct answer.
  3. Protease
  4. Hydrochloric acid

Explanation

Amylase is a carbohydrase enzyme that catalyses the breakdown of starch into maltose and other sugars. Protease digests proteins and lipase digests fats, so neither acts on starch.

Question 17Biology: Cells, Organisms and Life Processes

Which feature of a fish gill helps it to absorb oxygen efficiently?

  1. A single large tube through which water flows slowly
  2. A hard outer shell that protects the gills
  3. A thick, dry surface that prevents water loss
  4. Many thin filaments with a large surface area and a good blood supply Correct answer.

Explanation

Gills are made of many thin filaments and lamellae, giving a very large surface area for diffusion. Each is richly supplied with blood and kept moist by flowing water, so oxygen diffuses rapidly into the blood.

Question 18Biology: Cells, Organisms and Life Processes

A plant is left in the dark for two days. What is most likely to happen to the starch in its leaves?

  1. It will be converted into protein for growth.
  2. It will remain unchanged because starch is stable.
  3. It will decrease because starch is converted to sugar and used or transported away. Correct answer.
  4. It will increase because photosynthesis continues.

Explanation

Without light, photosynthesis stops and no new glucose is made, so the plant uses stored starch for respiration. The starch is converted back to soluble sugar and carried away, which is why leaves lose starch in darkness.

Question 19Biology: Cells, Organisms and Life Processes

Which statement correctly compares anaerobic respiration in yeast with aerobic respiration?

  1. Anaerobic respiration in yeast releases less energy and produces alcohol and carbon dioxide. Correct answer.
  2. Aerobic respiration produces alcohol and carbon dioxide.
  3. Anaerobic respiration requires oxygen but aerobic respiration does not.
  4. Both release the same amount of energy from each glucose molecule.

Explanation

Anaerobic respiration in yeast breaks glucose down only partly, releasing less energy and forming ethanol and carbon dioxide. Aerobic respiration breaks glucose down completely, releasing much more energy with only carbon dioxide and water.

Question 20Biology: Cells, Organisms and Life Processes

Which structural feature of a sperm cell is directly related to its function of reaching and fertilising an egg?

  1. A large store of food for the embryo
  2. A tail that enables it to swim Correct answer.
  3. Many chloroplasts for making food
  4. A thick cellulose wall for protection

Explanation

A sperm cell has a flagellum-like tail that beats to propel it through the fluid of the reproductive tract towards the egg. It also has many mitochondria to supply the energy needed for swimming.

Question 21Biology: Cells, Organisms and Life Processes

A student places a plant cell in a concentrated sugar solution. Which change is expected?

  1. The cell becomes turgid because water enters the vacuole.
  2. The cytoplasm shrinks away from the cell wall because water leaves the cell. Correct answer.
  3. Nothing happens because plant cells cannot lose water.
  4. The cell wall dissolves and the cell bursts.

Explanation

The sugar solution has a lower water concentration than the cell sap, so water leaves the vacuole and cytoplasm by osmosis. The protoplast shrinks away from the cell wall, a condition called plasmolysis, and the cell becomes flaccid.

Question 22Biology: Cells, Organisms and Life Processes

Which explanation best accounts for the increased rate of breathing during vigorous exercise?

  1. Carbon dioxide levels fall, which triggers the breathing centre.
  2. Muscles need more oxygen and produce more carbon dioxide, which stimulates faster breathing. Correct answer.
  3. The body needs to cool the blood by taking in cold air.
  4. Less oxygen is required because muscles respire anaerobically.

Explanation

Working muscles respire faster, using more oxygen and releasing more carbon dioxide into the blood. The raised carbon dioxide concentration is detected by the brain, which increases the rate and depth of breathing to remove it and supply more oxygen.

Question 23Biology: Cells, Organisms and Life Processes

A leaf is tested with iodine solution and turns blue-black only in the areas that were exposed to light. Which conclusion is best supported?

  1. Iodine is produced only in the dark.
  2. Light is needed for the leaf to make starch. Correct answer.
  3. The leaf respires only in the light.
  4. Chlorophyll is present only in the green areas.

Explanation

Iodine turns blue-black where starch is present, so starch was formed only in the illuminated regions. This shows that light is required for photosynthesis and therefore for starch production.

Question 24Biology: Cells, Organisms and Life Processes

Which combination of features best adapts a villus in the small intestine for absorption?

  1. A single layer of surface cells, a rich blood supply and a large surface area Correct answer.
  2. A thick muscular wall and a small surface area
  3. A dry, keratinised surface and few blood vessels
  4. A long, narrow tube with a thick lining

Explanation

A villus is covered by a thin single layer of cells so dissolved food has a short diffusion distance, and it contains a dense network of capillaries to carry absorbed nutrients away. Its finger-like shape, together with many villi, provides a very large surface area.

Question 25Biology: Cells, Organisms and Life Processes

A plant stem is ringed so that its phloem is removed. Which observation is most likely after several days?

  1. Sugars accumulate above the ring and the roots are starved of food. Correct answer.
  2. Water uptake by the roots stops immediately.
  3. The xylem stops transporting water to the leaves.
  4. The leaves stop losing water through transpiration.

Explanation

Phloem carries dissolved sugars made in the leaves to other parts of the plant, including the roots. When the phloem is ringed, sugars collect above the cut and cannot reach the roots, which are then starved.

Question 26Biology: Cells, Organisms and Life Processes

Which organelle is the main site of aerobic respiration in a cell?

  1. Chloroplast
  2. Vacuole
  3. Ribosome
  4. Mitochondrion Correct answer.

Explanation

Aerobic respiration releases energy from glucose using oxygen, and most of the reactions of this process take place on the folded inner membranes of the mitochondrion. Cells with high energy demands, such as muscle cells, contain many mitochondria for this reason.

Question 27Biology: Cells, Organisms and Life Processes

Which structure is present in a plant cell but absent from an animal cell?

  1. Nucleus
  2. Cellulose cell wall Correct answer.
  3. Cytoplasm
  4. Cell surface membrane

Explanation

Plant cells are surrounded by a rigid wall made mainly of cellulose, which gives shape and support and prevents the cell bursting when it takes in water. Animal cells have only a thin surface membrane and no cellulose wall.

Question 28Biology: Cells, Organisms and Life Processes

Enzymes are described as biological catalysts. Which statement best describes what this means?

  1. They supply the energy needed for a reaction to start.
  2. They raise the temperature of the reaction mixture.
  3. They are used up in the reaction they control.
  4. They speed up reactions without being changed themselves. Correct answer.

Explanation

A catalyst increases the rate of a reaction and is recovered unchanged at the end, so a small amount of enzyme can act on many substrate molecules. Enzymes achieve this by lowering the activation energy needed, not by supplying energy or being consumed.

Question 29Biology: Cells, Organisms and Life Processes

Which statement correctly describes the products of photosynthesis?

  1. Glucose and oxygen Correct answer.
  2. Oxygen and water
  3. Glucose and carbon dioxide
  4. Carbon dioxide and water

Explanation

In photosynthesis, green plants use light energy to combine carbon dioxide and water, producing glucose and releasing oxygen as a by-product. The glucose is used for respiration, stored as starch, or converted into other substances.

Question 30Biology: Cells, Organisms and Life Processes

A red blood cell is placed in a beaker of distilled water. Which change is expected to occur?

  1. It keeps the same shape because water moves equally in both directions.
  2. It swells and may burst as water enters. Correct answer.
  3. It becomes rigid because a cell wall prevents water entry.
  4. It shrinks as water leaves the cell.

Explanation

Distilled water has a higher water potential than the cytoplasm, so water enters the cell by osmosis across the partially permeable membrane. Lacking a cell wall, the red blood cell cannot resist the pressure and may swell until it bursts.

Question 31Biology: Cells, Organisms and Life Processes

A student observes cells under a microscope and sees many small green structures inside each cell. Which conclusion is best supported?

  1. The cells are dead and contain no organelles.
  2. The cells are bacterial cells without a nucleus.
  3. The cells are animal cells carrying out respiration.
  4. The cells are plant cells carrying out photosynthesis. Correct answer.

Explanation

The small green structures are chloroplasts, which contain the pigment chlorophyll and are the site of photosynthesis. Their presence indicates the cells are photosynthetic plant cells, such as those from a leaf.

Question 32Biology: Cells, Organisms and Life Processes

Bile is produced by the liver and stored in the gall bladder. Which function does bile perform in digestion?

  1. It breaks down fats into fatty acids and glycerol.
  2. It absorbs glucose through the wall of the small intestine.
  3. It digests proteins into amino acids in the stomach.
  4. It emulsifies fats into small droplets and neutralises stomach acid. Correct answer.

Explanation

Bile contains bile salts that break large fat droplets into many smaller ones, increasing the surface area for lipase to act on. It is also alkaline, so it neutralises the acidic chyme entering the small intestine, providing suitable conditions for intestinal enzymes.

Question 33Biology: Cells, Organisms and Life Processes

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

  1. Soft, brittle bones that fracture easily
  2. Weakness and tiredness due to anaemia Correct answer.
  3. Swelling of the neck caused by goitre
  4. Bleeding gums and slow wound healing

Explanation

Iron is needed to make haemoglobin, the red pigment in red blood cells that carries oxygen. A shortage of iron leads to fewer or less effective red blood cells, so less oxygen reaches the tissues, causing anaemia with tiredness and breathlessness.

Question 34Biology: Cells, Organisms and Life Processes

Water and mineral ions are absorbed by root hair cells. Which feature of a root hair cell makes absorption most efficient?

  1. Many chloroplasts that provide energy from light
  2. A thick, waterproof outer layer
  3. A large central vacuole that stores starch
  4. A long, thin extension that increases the surface area Correct answer.

Explanation

The root hair is a long, narrow extension of the cell that greatly increases the surface area in contact with soil water. A larger surface area allows more water to enter by osmosis and more mineral ions to be taken up by active transport.

Question 35Biology: Cells, Organisms and Life Processes

Which statement best explains why the human heart has valves?

  1. They increase the pressure of blood leaving the heart.
  2. They prevent blood from flowing backwards through the heart. Correct answer.
  3. They allow blood to pass in either direction between chambers.
  4. They filter waste products from the blood.

Explanation

Valves are flaps of tissue that open in one direction only, closing when blood tries to flow backwards. This ensures blood moves in a single direction from atria to ventricles and out into the arteries, maintaining efficient circulation.

Question 36Biology: Cells, Organisms and Life Processes

During exercise, a person's breathing rate and depth increase. Which combination of changes in the blood best explains this response?

  1. Oxygen concentration rises and carbon dioxide concentration falls.
  2. Glucose concentration falls while oxygen stays constant.
  3. Carbon dioxide concentration rises and oxygen concentration falls. Correct answer.
  4. Lactic acid is removed and oxygen concentration rises.

Explanation

Muscles respire faster during exercise, using up oxygen and releasing extra carbon dioxide into the blood. The brain detects the higher carbon dioxide concentration and signals the breathing muscles to work harder, increasing ventilation to remove carbon dioxide and take in more oxygen.

Question 37Biology: Cells, Organisms and Life Processes

A plant is watered with a solution containing a substance that blocks the xylem vessels in the stem. Which effect is most likely?

  1. The roots can no longer absorb water from the soil.
  2. Sugars cannot be transported from the leaves to the roots.
  3. Water and mineral ions cannot reach the leaves, so the plant wilts. Correct answer.
  4. The rate of photosynthesis in the leaves immediately increases.

Explanation

Xylem vessels carry water and dissolved mineral ions upward from the roots to the shoot and leaves. If they are blocked, water cannot replace that lost by transpiration, the cells lose turgor and the plant wilts.

Question 38Biology: Cells, Organisms and Life Processes

A student investigates the effect of temperature on an enzyme. The rate of reaction rises from 10 degrees Celsius to 40 degrees Celsius, then falls sharply at 60 degrees Celsius. Which explanation best accounts for the fall at 60 degrees Celsius?

  1. The substrate molecules have all been used up at that temperature.
  2. The enzyme is working more slowly because particles have less kinetic energy.
  3. The enzyme molecules have been denatured, so the active site no longer fits the substrate. Correct answer.
  4. The pH of the mixture has changed so the enzyme works faster.

Explanation

Above the optimum temperature, the increased vibration breaks the weak bonds that hold the enzyme's active site in shape. The active site is distorted so the substrate can no longer bind, and the enzyme is said to be denatured; this change is not reversible.

Question 39Biology: Cells, Organisms and Life Processes

A leaf from a destarched plant is partly covered with black paper and left in sunlight for several hours. The leaf is then tested with iodine solution. Which result and conclusion are correct?

  1. Only the exposed part turns blue-black, showing that light is needed for starch formation. Correct answer.
  2. The whole leaf turns blue-black, showing that light is not needed for starch formation.
  3. No part of the leaf changes colour, showing that the plant cannot make starch.
  4. Only the covered part turns blue-black, showing that light prevents photosynthesis.

Explanation

Starch is made only where photosynthesis has occurred, and photosynthesis requires light. Iodine turns blue-black where starch is present, so the exposed areas give a positive result while the covered areas do not, showing that light is needed.

Question 40Biology: Cells, Organisms and Life Processes

Which statement correctly describes the movement of substances between a leaf and the rest of the plant?

  1. Water leaves the leaf through the phloem during transpiration.
  2. Glucose is carried from the roots to the leaves in the xylem.
  3. Carbon dioxide enters the leaf through stomata and oxygen leaves through the same pores. Correct answer.
  4. Mineral ions are carried downward from the leaves to the roots in the phloem.

Explanation

Stomata are small pores in the leaf surface, mostly on the lower side, that open to allow carbon dioxide in for photosynthesis and let oxygen and water vapour out. Gas exchange in leaves depends on these pores and the air spaces inside the spongy mesophyll.

Question 41Biology: Human Physiology, Health and Reproduction

Which blood vessel carries oxygenated blood from the lungs to the heart?

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

Explanation

The pulmonary vein is the only vein that carries oxygenated blood, returning it from the lungs to the left atrium of the heart. The pulmonary artery carries deoxygenated blood the opposite way, and the vena cava returns deoxygenated blood from the body.

Question 42Biology: Human Physiology, Health and Reproduction

Which component of blood is mainly responsible for transporting oxygen around the body?

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

Explanation

Red blood cells contain haemoglobin, which binds oxygen in the lungs and releases it in the tissues. Plasma carries dissolved substances but only a small fraction of the oxygen, so red cells are the main oxygen carriers.

Question 43Biology: Human Physiology, Health and Reproduction

A person breathes in. Which change occurs in the thorax to draw air into the lungs?

  1. The rib cage moves downwards and inwards.
  2. The diaphragm relaxes and moves upwards.
  3. The volume of the thorax decreases.
  4. The diaphragm contracts and moves downwards. Correct answer.

Explanation

During inhalation the diaphragm muscle contracts and flattens, moving downwards, while the intercostal muscles raise the ribs. These actions increase the volume of the thorax and lower the pressure inside, so air flows in.

Question 44Biology: Human Physiology, Health and Reproduction

Which structure in the digestive system absorbs most of the digested nutrients into the blood?

  1. Small intestine Correct answer.
  2. Large intestine
  3. Oesophagus
  4. Stomach

Explanation

The small intestine has a very long length and a lining covered with villi, giving a large surface area for absorbing digested nutrients into the bloodstream. The large intestine mainly absorbs water, and the stomach and oesophagus absorb very little.

Question 45Biology: Human Physiology, Health and Reproduction

Which organ produces urine by filtering the blood?

  1. Bladder
  2. Liver
  3. Kidney Correct answer.
  4. Lung

Explanation

The kidneys filter waste products such as urea from the blood and produce urine, which then passes to the bladder. The liver processes many substances but does not filter blood into urine.

Question 46Biology: Human Physiology, Health and Reproduction

Which part of the human nervous system consists of the brain and spinal cord?

  1. Peripheral nervous system
  2. Autonomic nervous system
  3. Somatic nervous system
  4. Central nervous system Correct answer.

Explanation

The central nervous system is made up of the brain and spinal cord, which coordinate and process information. The peripheral nervous system consists of the nerves connecting the central system to the rest of the body.

Question 47Biology: Human Physiology, Health and Reproduction

Which disease is caused by a deficiency of vitamin C in the diet?

  1. Rickets
  2. Kwashiorkor
  3. Anaemia
  4. Scurvy Correct answer.

Explanation

Vitamin C is needed to make healthy connective tissue, especially collagen, so a deficiency causes scurvy with bleeding gums and poor wound healing. Rickets is caused by lack of vitamin D, and anaemia by lack of iron.

Question 48Biology: Human Physiology, Health and Reproduction

Which process produces gametes with half the normal chromosome number?

  1. Mitosis
  2. Fertilisation
  3. Binary fission
  4. Meiosis Correct answer.

Explanation

Meiosis is a type of cell division that halves the chromosome number, producing gametes that are haploid. At fertilisation two haploid gametes fuse, restoring the normal diploid number in the zygote.

Question 49Biology: Human Physiology, Health and Reproduction

A student counts a pulse rate of 72 beats per minute at rest. After running for two minutes the rate rises to 140 beats per minute. What is the main reason for this increase?

  1. The blood becomes thicker during exercise.
  2. Muscles need more oxygen and faster removal of carbon dioxide. Correct answer.
  3. The heart produces more red blood cells during exercise.
  4. The lungs stop working during exercise.

Explanation

During exercise muscles respire faster, using more oxygen and producing more carbon dioxide. The heart beats faster to deliver oxygen and glucose more quickly and to carry carbon dioxide to the lungs for removal.

Question 50Biology: Human Physiology, Health and Reproduction

Which feature of the alveoli makes gas exchange more efficient?

  1. They have a large total surface area and thin walls. Correct answer.
  2. They contain many blood-clotting cells.
  3. They are filled with mucus at all times.
  4. They are lined with cartilage rings.

Explanation

Millions of alveoli give a very large surface area, and their walls are only one cell thick so gases diffuse over a short distance. They are also surrounded by capillaries, keeping a steep concentration gradient for diffusion.