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.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

On this page you'll learn about why transport systems are needed, transport in plants, and blood components. You'll also cover defence and clotting and the heart and circulation. The notes bring these ideas together into one clear overview of transport.

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. Lignin strengthens the xylem walls, and the xylem forms the route by which water travels upwards through the plant.

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.

Transpiration is the evaporation of water from the surface of a plant, mainly through stomata in the leaves. This water loss from the leaves is what pulls more water up through the xylem in the transpiration stream.

Water lost by transpiration is replaced by uptake through root hair cells. Water enters the root hair cells by osmosis, passes across the root cortex, and then joins the xylem.

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.

Teacher insight

A standard potometer does not measure transpiration directly. It measures water uptake, which is only an estimate of transpiration because some of the water taken up is used inside the plant. A mass potometer is closer to a true measure because it tracks loss of mass from the whole system.

When exam questions ask you to describe, state the change in rate, such as "higher light intensity increases transpiration". When they ask you to explain, give the reason, such as "more stomata are open, so more water vapour can leave the leaf".

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:

Phagocytes engulf and digest pathogens by phagocytosis — this is a non-specific response effective against any pathogen.
Lymphocytes produce specific antibodies that bind to antigens on the pathogen's surface, causing pathogens to clump together for easier destruction.
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:

Platelets arrive at the wound site.
A clotting cascade of reactions occurs in the blood plasma.
Platelets release chemicals that convert fibrinogen (a soluble plasma protein) into insoluble fibrin.
Fibrin forms a mesh that traps red blood cells, forming a clot.
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. This faster, bigger secondary response usually destroys the pathogen before symptoms develop.

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; they do not carry blood into the chambers of the heart.

Explore the heart as a schematic¶

Use this schematic to lock in the basic heart layout rather than one exact picture. Open full interactive.

Teacher insight

This model is deliberately simplified. Different heart diagrams can look quite different, so the important thing is to recognise the basic layout of the chambers and vessels and apply that understanding to any diagram, rather than memorising one exact picture.

Compare vessel structure across the circuit¶

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, which reduces resistance, and valves to prevent backflow; carry blood at low pressure back to the heart
Capillaries	One-cell-thick wall and permeable surface to allow exchange of substances between blood and tissues; bring blood close to every cell

Use this interactive to compare how vessel walls and lumens change as blood moves away from the heart and back again. The main revision focus here is the contrast between arteries, capillaries and veins; arterioles and venules are included mainly to show how the circuit narrows into capillary beds and widens again afterwards. Open full interactive.

For quick recall, keep the big pattern in mind: arteries have thick elastic walls for high pressure, capillaries have one-cell-thick walls for exchange, and veins have wide lumens with valves for low-pressure return.

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.

Common Confusions¶

Transpiration vs water uptake: A standard potometer measures water uptake, not transpiration directly.
Root pressure vs transpiration stream: The main pull moving water up the plant comes from evaporation at the leaves, not from water being pushed up from the roots.
Antibody vs antigen: An antigen is the marker on a pathogen. An antibody is the specific protein made by lymphocytes that binds to that antigen.
Coronary arteries vs major vessels: Coronary arteries supply the heart muscle itself. They are different from the major vessels carrying blood into and out of the heart chambers.

Key Terms¶

Xylem: plant tissue that transports water and mineral ions upwards from the roots.
Phloem: plant tissue that transports sucrose and amino acids.
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: introduction of harmless pathogen material to stimulate immunity.
Immunity: protection against a 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.