(h) Transport
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
2.51Diffusion in unicellular organismsUnderstand why simple, unicellular organisms can rely on diffusion for movement of substances in and out of the cell.2.52Transport systemsUnderstand the need for a transport system in multicellular organisms.2.53Phloem transportDescribe the role of phloem in transporting sucrose and amino acids between the leaves and other parts of the plant.2.54Xylem transportDescribe the role of xylem in transporting water and mineral ions from the roots to other parts of the plant.255B Water absorption by root hair cellsUnderstand how water is absorbed by root hair cells.256B TranspirationUnderstand that transpiration is the evaporation of water from the surface of a plant.257B Factors affecting transpiration rateUnderstand how the rate of transpiration is affected by changes in humidity, wind speed, temperature and light intensity.258B Transpiration practicalPractical: investigate the role of environmental factors in determining the rate of transpiration from a leafy shoot.2.59Blood compositionDescribe the composition of the blood: red blood cells, white blood cells, platelets and plasma.2.60Transport by blood plasmaUnderstand the role of plasma in the transport of carbon dioxide, digested food, urea, hormones and heat energy.2.61Red blood cell adaptationsUnderstand how adaptations of red blood cells make them suitable for the transport of oxygen, including shape, the absence of a nucleus and the presence of haemoglobin.2.62Immune response to diseaseUnderstand how the immune system responds to disease using white blood cells, illustrated by phagocytes ingesting pathogens and lymphocytes releasing antibodies specific to the pathogen.263B Vaccination and memory cellsUnderstand how vaccination results in the manufacture of memory cells, which enable future antibody production to the pathogen to occur sooner, faster and in greater quantity.264B Blood clottingUnderstand how platelets are involved in blood clotting, which prevents blood loss and the entry of micro-organisms.2.65The heart and how it functionsDescribe the structure of the heart and how it functions.2.66Heart rate changesExplain how the heart rate changes during exercise and under the influence of adrenaline.2.67Coronary heart disease risk factorsUnderstand how factors may increase the risk of developing coronary heart disease.2.68Arteries, veins and capillariesUnderstand how the structure of arteries, veins and capillaries relate to their function.2.69Circulatory system structureUnderstand the general structure of the circulation system, including the blood vessels to and from the heart and lungs, liver and kidneys.
Rely on diffusion when every cell is close
A unicellular organism exchanges substances directly with its surroundings by diffusion because its single cell is always next to the environment.
Its small size gives a large surface-area-to-volume ratio and a short diffusion distance. Oxygen and dissolved food can enter, while carbon dioxide and other wastes can leave, fast enough for the cell's relatively low demand.
A unicellular organism does not need a specialised transport system. Diffusion is sufficient because distances and demand are small—not because diffusion becomes faster inside a single cell.
Explain why large organisms need transport systems
Large multicellular organisms need specialised transport systems because diffusion across the outer surface alone cannot supply every cell fast enough.
| Feature of a large organism | Consequence |
|---|---|
| small surface-area-to-volume ratio | too little exchange surface for the volume of living tissue |
| many internal cells | some cells are far from the external environment |
| long diffusion distances | substances take too long to reach cells or leave them |
| high metabolic demand | oxygen and nutrients are used, and wastes produced, faster |
A transport system moves materials rapidly between exchange surfaces and cells. Continuous delivery and removal also maintain steep concentration gradients for diffusion at both ends.
Transport does not replace diffusion: bulk flow covers long distances, while diffusion still moves substances across exchange surfaces and cell membranes.
Move sugars and amino acids through phloem
Phloem translocates dissolved sucrose and amino acids between leaves and other parts of a plant.
| Source or sink | Typical role |
|---|---|
| photosynthesising leaf | source: loads sucrose made from photosynthetic products |
| growing root, bud, flower or fruit | sink: uses sucrose and amino acids for respiration, growth and synthesis |
| storage organ | can be a sink while storing and a source when reserves are mobilised |
Translocation can occur up or down the plant because the source–sink relationship changes. Phloem is living tissue; movement is not restricted to the root-to-leaf direction.
Phloem transports sucrose and amino acids, not starch molecules. Do not confuse it with xylem, which carries water and mineral ions mainly upward.
Carry water and mineral ions through xylem
Xylem carries water and dissolved mineral ions from roots through stems to leaves and the rest of the plant.
| Feature | Transport advantage |
|---|---|
| dead cells joined end to end | forms a continuous hollow pathway |
| no end walls or cytoplasm | reduces resistance to water flow |
| lignified walls | resist collapse under tension and support the plant |
Water loss from leaves creates transpiration pull, drawing a continuous column of water upward through xylem. Mineral ions dissolved in that water travel with the transpiration stream.
Xylem flow is mainly one-way from roots upward. It is not the tissue that translocates sucrose and amino acids between sources and sinks.
Absorb water with root hair cells
Water enters root hair cells by osmosis, from soil with higher water potential to cell contents with lower water potential through a partially permeable cell membrane.
| Adaptation | Effect |
|---|---|
| long hair-like extension | large contact area with water between soil particles |
| thin wall and membrane | short pathway into the cell |
| concentrated cell sap | helps maintain a lower water potential than the soil solution |
Water then moves across the root into xylem. Mineral ions may be absorbed by active transport and can help lower cell water potential, but the water itself enters by osmosis.
Osmosis describes water movement only. Do not say water is actively transported into the root hair cell or that mineral ions enter by osmosis.
Build the transpiration stream
Transpiration is the loss of water vapour from a plant's surface, mainly through stomata in the leaves.
| Stage | Event |
|---|---|
| 1 | water evaporates from moist mesophyll cell surfaces into leaf air spaces |
| 2 | water vapour diffuses through open stomata down its concentration gradient |
| 3 | water leaving mesophyll is replaced from leaf xylem |
| 4 | tension pulls a continuous water column upward through xylem from the roots |
The transpiration stream supplies water for photosynthesis and turgor, carries mineral ions and can cool leaves by evaporation.
Evaporation occurs inside the leaf before water vapour diffuses out. Transpiration is the overall water loss, not a pump located in the stomata.
Predict environmental effects on transpiration
Environmental conditions alter transpiration by changing evaporation, the water-vapour gradient or stomatal opening.
| Change | Effect on rate | Mechanism |
|---|---|---|
| lower humidity | increases | steepens the water-vapour gradient from leaf to air |
| greater wind speed | increases | removes the moist boundary layer around the leaf |
| higher temperature | increases | gives water molecules more kinetic energy, increasing evaporation and diffusion |
| brighter light | usually increases | opens stomata for carbon dioxide entry, reducing resistance to water loss |
The converse changes usually reduce the rate. Once stomata are fully open or another factor becomes limiting, further change may have little additional effect.
State the mechanism, not only the direction. Light acts mainly through stomata; wind does not heat the leaf in the standard explanation, and high humidity slows loss by reducing the gradient.
Measure water uptake with a leafy shoot
A bubble potometer measures water uptake by a cut leafy shoot. Water uptake is used as a proxy for transpiration because most absorbed water is lost from leaves.
| Step | Action |
|---|---|
| 1 | cut the shoot under water and connect it to water-filled apparatus |
| 2 | seal every joint, dry the leaves and check that the apparatus is airtight |
| 3 | introduce one air bubble and record its starting position |
| 4 | expose the shoot to one measured condition for a fixed time |
| 5 | measure bubble distance; calculate volume as capillary cross-sectional area × distance, then divide by time |
| 6 | reset the bubble, repeat and calculate a mean |
Change one factor such as wind speed, light intensity, humidity or temperature. Control the other environmental factors, shoot species, leaf area and time; allow the shoot to acclimatise before readings.
A potometer does not measure transpiration directly: some absorbed water is used in photosynthesis, growth or turgor. Avoid air entering the xylem by cutting under water, and keep water away from electrical equipment.
Separate the four blood components
Blood is a tissue made of red blood cells, white blood cells and platelets suspended in liquid plasma.
| Component | Main function |
|---|---|
| red blood cells | transport oxygen using haemoglobin |
| phagocytes and lymphocytes (white blood cells) | defend against pathogens |
| platelets | trigger clotting at damaged vessels |
| plasma | transports cells and dissolved substances, and distributes heat |
Platelets are cell fragments, not white blood cells. Plasma is the liquid carrier; it is not the same as the whole blood or the clear fluid inside every cell.
Route substances through plasma
Plasma is the liquid part of blood that carries dissolved substances between the organs that produce, absorb, use or remove them.
| Cargo | Main route |
|---|---|
| carbon dioxide | respiring tissues → lungs |
| digested food such as glucose and amino acids | small intestine → liver and body cells |
| urea | liver → kidneys |
| hormones | endocrine glands → target organs |
| heat | redistributed from active organs such as muscles and liver to the rest of the body |
Oxygen is transported mainly bound to haemoglobin inside red blood cells, whereas these listed substances travel in plasma. Always name both the substance and a biologically correct source or destination.
Adapt red blood cells for oxygen transport
A red blood cell is specialised to load oxygen in the lungs and release it to respiring tissues.
| Adaptation | Oxygen-transport advantage |
|---|---|
| biconcave disc | large surface-area-to-volume ratio and short diffusion path |
| no nucleus when mature | leaves more internal space for haemoglobin |
| packed with haemoglobin | haemoglobin binds oxygen reversibly |
| small and flexible | squeezes through narrow capillaries close to cells |
Red blood cells contain cytoplasm and haemoglobin but no nucleus. Iron is part of haemoglobin; the cell does not carry oxygen because it is hollow.
Compare phagocytes and lymphocytes
White blood cells defend the body in two distinct ways: phagocytes ingest pathogens, while lymphocytes make specific antibodies.
| Cell | Recognition and action |
|---|---|
| phagocyte | surrounds and engulfs a pathogen, then digests it with enzymes |
| lymphocyte | recognises a particular antigen and produces antibodies with complementary binding sites |
Antibodies bind specifically to their matching antigens and can clump pathogens or mark them for destruction. Some activated lymphocytes form memory cells.
Phagocytes do not produce antibodies, and lymphocytes do not normally engulf pathogens. Antibody specificity depends on complementary antigen-binding shape.
Create immune memory by vaccination
Vaccination exposes the immune system to harmless pathogen antigens so that protection develops without the full disease.
| Stage | Immune event |
|---|---|
| 1 | dead, weakened, inactive or antigen-containing material is introduced |
| 2 | specific lymphocytes recognise the antigen and produce antibodies |
| 3 | some lymphocytes remain as memory cells |
| 4 | later exposure to the same antigen triggers a secondary response |
| 5 | antibodies are produced sooner, faster and in greater quantity, often removing the pathogen before symptoms develop |
A vaccine does not kill every future pathogen directly and does not cause antibiotics to be made. Memory is antigen-specific, so protection against one pathogen may not protect against another.
Seal a wound by blood clotting
Blood clotting rapidly seals a damaged vessel, limiting blood loss and blocking microorganisms from entering the body.
| Stage | Event |
|---|---|
| 1 | a vessel is damaged and platelets collect at the site |
| 2 | clotting reactions produce strands of fibrin |
| 3 | the fibrin mesh traps blood cells and forms a clot |
| 4 | the clot dries into a scab while repair occurs underneath |
Platelets help start clotting but are not antibodies or phagocytes. A clot prevents both excessive bleeding and pathogen entry; these are separate benefits.
Trace one-way flow through the heart
The heart is a muscular double pump: its right side sends deoxygenated blood to the lungs, and its left side sends oxygenated blood around the body.
| Flow step | Structure |
|---|---|
| 1 | vena cava → right atrium |
| 2 | right atrium → valve → right ventricle |
| 3 | right ventricle → pulmonary artery → lungs |
| 4 | lungs → pulmonary vein → left atrium |
| 5 | left atrium → valve → left ventricle |
| 6 | left ventricle → aorta → body |
| Feature | Function |
|---|---|
| valves | prevent backflow |
| septum | prevents oxygenated and deoxygenated blood mixing |
| thick left-ventricle wall | generates high pressure for the whole body |
| thinner right-ventricle wall | pumps only to nearby lungs |
| coronary arteries | supply heart muscle with oxygen and glucose |
Arteries carry blood away from the heart and veins return it; oxygen content does not define the vessel. The pulmonary artery is deoxygenated and the pulmonary vein oxygenated.
Raise heart rate for exercise or adrenaline
Heart rate rises when muscles need faster delivery of oxygen and glucose and faster removal of carbon dioxide during exercise.
More muscular contraction increases respiration and ATP demand. Increasing heart rate raises blood flow, supporting aerobic respiration and helping transport heat and respiratory products away.
Adrenal glands release the hormone adrenaline into the blood during stress or excitement. Adrenaline acts on the heart's pacemaker to increase heart rate as part of the fight-or-flight response.
Adrenaline is a hormone carried in plasma; it is not produced by the heart. A higher heart rate supports increased respiration—it is not itself cellular respiration.
Connect CHD risk factors to coronary blockage
Coronary heart disease occurs when coronary arteries become narrowed or blocked, reducing blood and oxygen supply to heart muscle.
| Risk factor | How risk can increase |
|---|---|
| smoking | damages artery lining, raises clot risk and blood pressure |
| high saturated-fat or cholesterol diet | promotes fatty deposits that narrow coronary arteries |
| inactivity and obesity | increase strain on the circulatory system and often raise blood pressure |
| persistent high blood pressure or stress | damages vessels and increases heart workload |
| diabetes, inherited alleles and increasing age | can increase susceptibility independently or with lifestyle factors |
A narrowed lumen limits oxygen and glucose delivery. Complete blockage can stop aerobic respiration in part of the heart muscle, causing tissue death and a heart attack.
A risk factor changes probability; it does not prove that one individual will develop CHD. Correlation in population data also does not isolate cause unless other variables are controlled.
Match blood vessels to pressure and exchange
Arteries, veins and capillaries have structures matched to the pressure, direction and exchange role of the blood they carry.
| Vessel | Structure | Structure–function link |
|---|---|---|
| artery | thick muscular, elastic wall; relatively narrow lumen; no valves along most of its length | withstands high pressure; elastic recoil smooths flow away from heart |
| vein | thinner wall with less muscle and elastic tissue; wide lumen; valves | low-resistance return at low pressure; valves prevent backflow |
| capillary | wall one cell thick; extremely narrow lumen; branching network | short diffusion distance, slow close flow and large total exchange area |
Vessel type is defined by direction relative to the heart, not oxygen content. Capillaries are not simply tiny veins: their one-cell-thick walls are specialised for exchange.
Map pulmonary, hepatic and renal circulation
Humans have double circulation: blood passes through the heart once in the pulmonary circuit and again in the systemic circuit during one complete journey.
| Organ route | Named vessels and direction |
|---|---|
| lungs | heart → pulmonary artery → lungs → pulmonary vein → heart |
| body tissues | heart → aorta → organ arteries → capillaries → organ veins → vena cava → heart |
| liver | hepatic artery brings oxygenated blood; hepatic portal vein brings nutrient-rich blood from the gut; hepatic vein drains to vena cava |
| kidneys | renal arteries bring blood from aorta; renal veins return blood to vena cava |
The pulmonary circuit oxygenates blood. The systemic circuit distributes oxygen and nutrients and returns carbon dioxide and other wastes. Separate pumps allow high systemic pressure without exposing lung capillaries to the same pressure.
The hepatic portal vein is unusual because it connects gut capillaries to liver capillaries before blood returns to the heart. The renal vein carries blood away from the kidney, even though all veins carry blood toward the heart overall.