2h Transport¶
Part of 2 Structure and Functions in Living Organisms.
Transport systems solve the problem that large multicellular organisms cannot rely on diffusion alone. In this topic the course compares plant transport in xylem and phloem with animal transport in blood and the circulation.
Learning Objectives¶
| ID | Official specification wording | Main teaching sections |
|---|---|---|
2h-lo-1 |
2.51 understand why simple, unicellular organisms can rely on diffusion for movement of substances in and out of the cell 2.52 understand the need for a transport system in multicellular organisms Flowering plants 2.53 describe the role of phloem in transporting sucrose and amino acids between the leaves and other parts of the plant 2.54 describe the role of xylem in transporting water and mineral ions from the roots to other parts of the plant 2.55B understand how water is absorbed by root hair cells 2.56B understand that transpiration is the evaporation of water from the surface of a plant 2.57B understand how the rate of transpiration is affected by changes in humidity, wind speed, temperature and light intensity 2.58B practical: investigate the role of environmental factors in determining the rate of transpiration from a leafy shoot Humans |
Why Transport Systems Are Needed, Transport in Plants |
2h-lo-2 |
2.59 describe the composition of the blood: red blood cells, white blood cells, platelets and plasma 2.60 understand the role of plasma in the transport of carbon dioxide, digested food, urea, hormones and heat energy 2.61 understand 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 |
Blood Components |
2h-lo-3 |
2.62 understand how the immune system responds to disease using white blood cells, illustrated by phagocytes ingesting pathogens and lymphocytes releasing antibodies specific to the pathogen 2.63B understand 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 2.64B understand how platelets are involved in blood clotting, which prevents blood loss and the entry of micro-organisms |
Defence and Clotting |
2h-lo-4 |
2.65 describe the structure of the heart and how it functions 2.66 explain how the heart rate changes during exercise and under the influence of adrenaline 2.67 understand how factors may increase the risk of developing coronary heart disease 2.68 understand how the structure of arteries, veins and capillaries relate to their function 2.69 understand the general structure of the circulation system, including the blood vessels to and from the heart and lungs, liver and kidneys |
The Heart and Circulation |
Why Transport Systems Are Needed¶
Unicellular organisms have a large surface area to volume ratio, so diffusion across the cell surface is sufficient to meet their needs. Multicellular organisms have a much smaller surface area to volume ratio. Cells deep inside the body are too far from the surface for diffusion to supply oxygen and nutrients quickly enough or to remove waste efficiently. Transport systems connect exchange surfaces to the rest of the organism.
Transport in Plants¶
Xylem transports water and mineral ions from the roots upwards to the shoots and leaves. Xylem is adapted in several ways: - Lignin deposited in the cell walls causes the cells to die, becoming hollow tubes that join end-to-end to form a continuous column. - Water molecules are attracted to one another by hydrogen bonds, forming a continuous stream through the xylem — the transpiration stream. - Lignin also strengthens the xylem to withstand the pressure of the water column and contains bordered pits to allow water to move sideways into nearby cells.
Phloem transports sucrose and amino acids (the products of photosynthesis) from the leaves to all parts of the plant. This is called translocation and can occur in both directions — carrying food away from leaves for use, or transporting stored food from storage organs to growing areas. Phloem cells have sieve plates (perforated end walls) and many organelles removed to allow flow. Companion cells alongside the phloem have many mitochondria to supply the phloem with energy.
Transpiration is the evaporation of water from the surface of a plant, mainly through stomata in the leaves. Water lost by transpiration is replaced by uptake through root hair cells. Factors affecting transpiration rate:
| Factor | Effect on transpiration |
|---|---|
| Increased light intensity | More stomata open for gas exchange → faster transpiration |
| Increased temperature | Water molecules gain energy and evaporate faster; also more stomata open |
| Increased wind (air movement) | Removes water vapour from around the leaf, steepening the concentration gradient → faster transpiration |
| Increased humidity | Reduces the concentration gradient for water vapour between inside and outside the leaf → slower transpiration |
Practical investigation of transpiration: A potometer measures water uptake as a proxy for transpiration. A plant shoot is set up underwater (to prevent air entering the xylem), then connected to a capillary tube containing an air bubble. The distance the bubble moves per unit time indicates the rate of water uptake. Environmental conditions such as temperature, wind and light intensity can be altered to observe their effects.
Blood Components¶
Plasma is the straw-coloured liquid that makes up most of the blood volume. It transports: - Digested food substances (glucose, amino acids, fatty acids) - Carbon dioxide (from respiring cells to the lungs) - Urea (from the liver to the kidneys) - Hormones (throughout the body) - Heat energy
Red blood cells carry oxygen from the lungs to all cells in the body: - Contain haemoglobin, a red protein that combines with oxygen to form oxyhaemoglobin. - No nucleus — this frees up more space for haemoglobin. - Biconcave disc shape — maximises surface area for oxygen diffusion. - Flexible — allows them to pass through narrow capillaries.
White blood cells form part of the immune system, defending against pathogens in several ways: 1. Phagocytes engulf and destroy pathogens by phagocytosis — this is a non-specific response effective against any pathogen. 2. Lymphocytes produce specific antibodies that bind to antigens on the pathogen's surface, causing pathogens to clump together for easier destruction. 3. White blood cells can also produce antitoxins that neutralise the toxins released by bacteria.
Platelets are tiny cell fragments without a nucleus. When a blood vessel is damaged: 1. Platelets arrive at the wound site. 2. A clotting cascade of reactions occurs in the blood plasma. 3. Platelets release chemicals that convert fibrinogen (a soluble plasma protein) into insoluble fibrin. 4. Fibrin forms a mesh that traps red blood cells, forming a clot. 5. The clot hardens into a scab, preventing bacteria from entering.
Defence and Clotting¶
Vaccination protects against pathogens by introducing a dead or inactivated form of the pathogen. The immune system responds by producing specific antibodies and forming memory cells that persist in the body. When the actual pathogen is later encountered, these memory cells enable antibodies to be produced much faster and in greater quantities, preventing symptoms from developing.
Benefits and limitations of vaccination:
| Benefits | Limitations |
|---|---|
| Can eradicate diseases (e.g. smallpox) and reduce incidence of many others | Not always 100% effective at providing immunity |
| Herd immunity — widespread vaccination reduces spread even to unvaccinated individuals | Rare side effects such as fever can occur |
The Heart and Circulation¶
The heart is a muscular organ at the centre of a double circulatory system: - Circuit 1: deoxygenated blood flows from the body into the right atrium, then the right ventricle pumps it to the lungs where it collects oxygen. - Circuit 2: oxygenated blood returns from the lungs to the left atrium, then the left ventricle pumps it to all parts of the body.
The left ventricle wall is thicker than the right because it must pump blood around the entire body rather than just to the nearby lungs.
Key vessel names: deoxygenated blood returns to the heart via the vena cava; oxygenated blood leaves for the body via the aorta; blood travels to and from the lungs via the pulmonary artery and pulmonary vein. The coronary arteries supply the heart muscle itself with oxygenated blood.
Structure of blood vessels:
| Vessel type | Structure and function |
|---|---|
| Arteries | Thick, muscular walls and elastic fibres to withstand high pressure from the heart; carry blood away from the heart |
| Veins | Wide lumen (reduces resistance) and valves to prevent backflow; carry blood at low pressure back to the heart |
| Capillaries | One cell thick wall and permeable to allow exchange of substances between blood and tissues; bring blood close to every cell |
Heart rate changes: - During exercise, muscles need more energy. Respiration rate rises, and heart rate increases to deliver more oxygen and remove more CO₂ from working muscles. Stroke volume (amount of blood per beat) also increases. - During intense anaerobic exercise, lactic acid accumulates and an oxygen debt builds up. Elevated heart and breathing rates continue after exercise to repay this oxygen debt. - Adrenaline, produced by the adrenal glands above the kidneys, prepares the body for action ('fight or flight'). It increases heart rate, increases breathing rate, diverts blood from the digestive system to working muscles, and causes pupils to dilate.
Coronary heart disease (CHD): The coronary arteries supply the heart muscle with blood. In CHD, fatty deposits (atherosclerosis) build up inside these arteries, narrowing them and reducing blood flow. This can cause ischaemia (insufficient oxygen delivery to heart muscle), potentially leading to a heart attack.
Risk factors: - Poor diet: high saturated fat increases cholesterol and the likelihood of plaque formation; high salt intake raises blood pressure, damaging vessel walls. - Smoking: nicotine narrows blood vessels and raises blood pressure; smoking also damages the vessel lining. - Stress: stress hormones increase blood pressure, contributing to vessel damage.
Key Terms¶
- Xylem: plant tissue that transports water and mineral ions upwards from the roots.
- Phloem: plant tissue that transports sucrose and amino acids throughout the plant (translocation).
- Translocation: the movement of dissolved sugars and amino acids through the phloem.
- Transpiration: the evaporation of water from a plant's surface, mainly through stomata.
- Transpiration stream: the continuous movement of water from roots through xylem to leaves, driven by transpiration.
- Plasma: the liquid component of blood that transports dissolved substances and heat.
- Haemoglobin: the red protein in red blood cells that carries oxygen.
- Phagocyte: a white blood cell that engulfs and destroys pathogens.
- Lymphocyte: a white blood cell that produces specific antibodies.
- Antibody: a specific protein produced by lymphocytes in response to an antigen.
- Antigen: a molecule on the surface of a pathogen that triggers an immune response.
- Memory cell: a long-lived immune cell that enables rapid antibody production on re-exposure to a pathogen.
- Vaccination: the introduction of dead or inactive pathogen material to stimulate immunity without causing disease.
- Platelet: a cell fragment involved in blood clotting.
- Fibrin: the insoluble protein that forms the mesh of a blood clot.
- Capillary: a tiny blood vessel one cell thick, where exchange between blood and tissues occurs.
- Aorta: the main artery leaving the left ventricle, carrying oxygenated blood to the body.
- Vena cava: the main vein returning deoxygenated blood from the body to the right atrium.
- Double circulatory system: a circulatory system with two separate circuits — one to the lungs, one to the body.
- Atherosclerosis: the build-up of fatty plaques in artery walls, narrowing the lumen.
- Coronary heart disease (CHD): disease caused by narrowing of the coronary arteries, reducing blood flow to the heart muscle.