# Transport

> Edexcel International GCSE Biology · 4BI1
> Source: https://www.owlsprep.com/study/edexcel-igcse-biology-s2-transport/

This guide covers all transport content for Edexcel IGCSE Biology (4BI1) 2.51–2.69, including transport needs in unicellular/multicellular organisms, plant transport systems, human blood, heart function and circulatory pathways.

**Prerequisites:** [Understanding of diffusion and osmosis (Edexcel IGCSE Biology S2_T03)](https://www.owlsprep.com/study/edexcel-igcse-biology-s2-movement-substances/); [Basic cell structure and organisation (Edexcel IGCSE Biology S2_T01)](https://www.owlsprep.com/study/edexcel-igcse-biology-s2-cell-structure/)

## Learning objectives

- Explain why unicellular organisms rely on diffusion and multicellular organisms need specialised transport systems
- Describe the functions of xylem and phloem in plants, and explain transpiration and related practicals (Biology-only)
- Identify components of human blood and their roles, including immune response and vaccination (Biology-only)
- Describe heart structure, circulatory pathways, blood vessel adaptations and coronary heart disease risk factors

## Why Organisms Need Transport Systems

All organisms need to move essential substances (oxygen, glucose, mineral ions) into cells and waste products (carbon dioxide, urea) out of cells. The method used depends on the organism's size and surface area to volume (SA:V) ratio.

**Surface Area to Volume Ratio (SA:V)** — A measure of how much surface area an organism has relative to its total volume; smaller organisms have significantly larger SA:V ratios than larger organisms.

Unicellular organisms (e.g. amoeba) have a very large SA:V ratio and a maximum diffusion distance of less than 1mm, so all substances can move in and out of the cell quickly enough via diffusion alone, with no need for a transport system. Multicellular organisms have a much smaller SA:V ratio, and cells deep inside the body have a diffusion distance of several centimetres, which is too slow to meet metabolic demands, so specialised transport systems are required.

**Worked example:** Explain why a 1cm single-celled organism can rely on diffusion for transport, but a 10cm multicellular organism cannot.

1. Step 1: Calculate SA:V for the 1cm organism: surface area = 6 × 1² = 6cm², volume = 1cm³, so SA:V = 6:1. The maximum diffusion distance to the centre of the cell is 0.5cm, so substances reach all parts of the cell quickly.
2. Step 2: Calculate SA:V for the 10cm organism: surface area = 6 × 10² = 600cm², volume = 1000cm³, so SA:V = 0.6:1, 10x smaller than the unicellular organism.
3. Step 3: The maximum diffusion distance to inner cells of the multicellular organism is 5cm, which is far too slow to deliver enough oxygen and glucose to meet energy demands, so a dedicated transport system is needed.

> **Exam tip:** Exam questions often ask you to link SA:V ratio directly to the need for transport systems; always include both the small SA:V of multicellular organisms and long diffusion distance in your answer for full marks.

## Plant Transport Systems

Flowering plants have two specialised transport tissues: xylem and phloem, arranged in vascular bundles throughout the roots, stems and leaves.

- **Xylem**: Transports water and mineral ions absorbed from the soil *upwards* from the roots to all other parts of the plant. Made of dead, hollow lignified cells, transport only occurs in one direction.
- **Phloem**: Transports sucrose (made from glucose during photosynthesis) and amino acids around the plant in a process called translocation, which works in both directions from sources (areas that produce glucose, e.g. leaves) to sinks (areas that use or store glucose, e.g. roots, fruits, developing seeds).

**Worked example:** Compare the role of xylem and phloem in supplying resources to developing strawberry fruits on a plant.

1. Step 1: Xylem carries water and mineral ions from the roots up to the strawberry fruits, providing water for cell expansion and mineral ions for healthy growth.
2. Step 2: Phloem carries sucrose and amino acids made in the photosynthesising leaves (source) to the developing fruits (sink) via translocation, to provide energy and building materials for fruit growth.

> **Biology-only Content (Paper 2 Only)**
>
> The following content (water uptake, transpiration, potometer practical) is only assessed in Edexcel IGCSE Biology (4BI1), not Double Award Science.

Water is absorbed by root hair cells, which have a large surface area to maximise osmosis: water moves from the dilute soil solution into the more concentrated cytoplasm of root hair cells. Transpiration is the evaporation of water from the surface of plant leaves through open stomata, which pulls a continuous column of water up the xylem from the roots (the transpiration stream).

The rate of transpiration is affected by four key environmental factors:

- **Temperature**: Higher temperatures increase the rate of water evaporation, so transpiration is faster.
- **Wind speed**: Higher wind removes water vapour from around leaves, increasing the concentration gradient for diffusion, so transpiration is faster.
- **Humidity**: Higher humidity reduces the concentration gradient between the leaf and air, so transpiration is slower.
- **Light intensity**: Higher light opens stomata for photosynthesis, so more water evaporates, making transpiration faster.

A potometer is used to investigate transpiration rate by measuring how far an air bubble moves along a capillary tube over time, as the plant takes up water to replace water lost via transpiration.

**Worked example:** A student uses a potometer to measure transpiration rate of a leafy shoot. With a fan blowing air at the shoot, the air bubble moves 15mm in 5 minutes, compared to 3mm in 5 minutes without the fan. Explain this result.

1. Step 1: The fan increases wind speed around the leaves of the shoot.
2. Step 2: Wind removes water vapour that has evaporated from the leaf surface immediately, maintaining a steep concentration gradient between the high water vapour concentration inside the leaf and low concentration outside the leaf.
3. Step 3: This increases the rate of evaporation of water from the leaf, so transpiration rate increases. The plant takes up more water from the potometer to replace lost water, so the air bubble moves faster.

> **Exam tip:** Never mix up xylem and phloem function: remember *X*ylem = *W*ater (X and W are adjacent in the alphabet) to avoid losing easy marks.

*Calculator:* allowed

## Human Blood Components and Immune Function

Human blood is made of four key components suspended in plasma: red blood cells, white blood cells, and platelets.

- **Plasma**: Pale yellow liquid making up ~55% of blood volume. Transports carbon dioxide from respiring cells to lungs, digested food from the small intestine to cells, urea from the liver to kidneys, hormones around the body, and heat energy to regulate body temperature.
- **Red blood cells**: Carry oxygen from the lungs to respiring cells. Three key adaptations: biconcave disc shape (large surface area for oxygen diffusion), no nucleus (more space for haemoglobin), contains haemoglobin (red protein that binds oxygen in the lungs to form oxyhaemoglobin, which releases oxygen at respiring cells).
- **White blood cells**: Part of the immune system. Two main types: phagocytes, which engulf and ingest pathogens via phagocytosis; lymphocytes, which produce antibodies specific to antigens on the surface of a pathogen, clumping pathogens together for destruction by phagocytes.

> **Biology-only Content (Paper 2 Only)**
>
> The following content (vaccination, platelet function) is only assessed in Edexcel IGCSE Biology (4BI1), not Double Award Science.

Vaccination provides long-term immunity by introducing a weakened or dead version of a pathogen into the body, triggering an immune response without causing disease. Lymphocytes produce antibodies against the pathogen, and long-lived memory cells are formed. If the person is later exposed to the live pathogen, memory cells produce antibodies much faster and in greater quantity, destroying the pathogen before illness develops. Platelets are small cell fragments that trigger blood clotting at wound sites, preventing excess blood loss and stopping microorganisms entering the body.

**Worked example:** Explain two adaptations of red blood cells that make them efficient at carrying oxygen around the body.

1. Step 1: Red blood cells have a biconcave disc shape, which gives them a large surface area relative to their volume, maximising the rate of oxygen diffusion into and out of the cell.
2. Step 2: They have no nucleus, which frees up extra space inside the cell to store more haemoglobin, the protein that binds to oxygen, so each red blood cell can carry more oxygen per trip.

> **Exam tip:** When answering questions about white blood cells, always specify that antibodies produced by lymphocytes are *specific* to one pathogen; this is a common marking point many students miss.

## Heart and Circulatory System

Humans have a double circulatory system: blood passes through the heart twice per full circuit of the body, once via the pulmonary circuit (to the lungs to pick up oxygen) and once via the systemic circuit (to the rest of the body to deliver oxygen). The heart is a muscular organ with four chambers:

- Right atrium: Receives deoxygenated blood from the body via the vena cava
- Right ventricle: Pumps deoxygenated blood to the lungs via the pulmonary artery
- Left atrium: Receives oxygenated blood from the lungs via the pulmonary vein
- Left ventricle: Pumps oxygenated blood to the rest of the body via the aorta; has a thicker muscular wall than the right ventricle to generate higher pressure for long-distance transport around the body.

Valves (atrioventricular valves between atria and ventricles, semilunar valves at the exit of the ventricles) prevent backflow of blood, ensuring it only flows in one direction. There are three main types of blood vessel:

- **Arteries**: Carry blood away from the heart at high pressure; thick muscular/elastic walls, narrow lumen, no valves (except semilunar valves at the aorta and pulmonary artery).
- **Veins**: Carry blood back to the heart at low pressure; thin walls, wide lumen, valves to prevent backflow.
- **Capillaries**: Tiny vessels that carry blood through organs for substance exchange; one-cell-thick walls to reduce diffusion distance, permeable to small molecules like oxygen, glucose and carbon dioxide.

Coronary heart disease (CHD) occurs when fatty deposits narrow the coronary arteries that supply oxygen to the heart muscle, reducing oxygen supply and causing chest pain or heart attacks. Modifiable risk factors include a diet high in saturated fat/salt, smoking, lack of exercise, obesity and high stress; non-modifiable risk factors include genetics and age.

Heart rate increases during exercise to deliver more oxygen and glucose to respiring muscle cells and remove excess carbon dioxide. The hormone adrenaline, released during stress or fear, also increases heart rate to prepare the body for 'fight or flight' action.

**Worked example:** Explain why the left ventricle has a thicker muscular wall than the right ventricle.

1. Step 1: The right ventricle pumps deoxygenated blood only to the lungs (pulmonary circulation), a short distance, so it only needs to generate a relatively low pressure — too high a pressure would damage the delicate lung capillaries.
2. Step 2: The left ventricle pumps oxygenated blood all around the body (systemic circulation), a much greater distance against greater resistance, so it must generate a much higher pressure.
3. Step 3: The thicker muscular wall of the left ventricle allows it to contract with much greater force to produce this higher pressure.

> **Exam tip:** A common exam trap is asking if all arteries carry oxygenated blood: remember the pulmonary artery carries deoxygenated blood to the lungs, so never generalise that all arteries carry oxygenated blood, or you will lose marks.

## Common pitfalls

- **Wrong:** Stating phloem only transports substances up the plant and xylem transports in both directions
  - Why it fails: This mixes up core function of the two plant transport tissues, a very common marking point
  - Correct: Remember xylem only transports water/mineral ions upwards from roots; phloem transports sucrose/amino acids in both directions via translocation
- **Wrong:** Claiming all arteries carry oxygenated blood and all veins carry deoxygenated blood
  - Why it fails: The pulmonary artery carries deoxygenated blood to the lungs, and the pulmonary vein carries oxygenated blood to the heart, so this generalisation is incorrect
  - Correct: Define arteries as vessels carrying blood away from the heart, veins as vessels carrying blood to the heart, regardless of oxygen content
- **Wrong:** Only listing 1 or 2 adaptations of red blood cells when asked for three
  - Why it fails: The mark scheme explicitly requires all three adaptations (biconcave shape, no nucleus, haemoglobin) for full marks
  - Correct: Learn all three adaptations and link each directly to its function for oxygen transport
- **Wrong:** Stating vaccination gives you ready-made antibodies to fight pathogens
  - Why it fails: Vaccines trigger your own immune system to make antibodies and memory cells, they do not contain pre-made antibodies for this topic
  - Correct: Explain vaccines contain weakened/dead pathogens that trigger an immune response, producing memory cells for long-term immunity
- **Wrong:** Forgetting the hepatic portal vein carries blood from the gut to the liver, not from the liver to the heart
  - Why it fails: The hepatic portal vein is a unique vessel not connected directly to the heart, and is frequently tested
  - Correct: Memorise the three liver vessels: hepatic artery (to liver), hepatic vein (away to heart), hepatic portal vein (from gut to liver)

## Cheatsheet

| Structure/Component | Key Function | Core Exam Facts |
| --- | --- | --- |
| Xylem | Transport water/mineral ions from roots to plant | One direction only (up), dead lignified cells |
| Phloem | Transport sucrose/amino acids around plant | Translocation, both directions (source → sink) |
| Red blood cell | Carry oxygen to respiring cells | 3 adaptations: biconcave shape, no nucleus, haemoglobin |
| Phagocyte | Immune response | Engulfs/ingests pathogens via phagocytosis |
| Lymphocyte | Immune response | Produces *specific* antibodies against pathogen antigens |
| Artery | Carry blood away from heart | Thick muscular/elastic wall, narrow lumen, high pressure |
| Vein | Carry blood to heart | Thin wall, wide lumen, valves prevent backflow, low pressure |
| Capillary | Substance exchange between blood and cells | One-cell-thick wall, short diffusion distance, permeable |
| Left ventricle | Pump blood to body | Thicker wall than right ventricle to generate higher pressure |
| CHD Risk Factors | Increase coronary artery narrowing risk | High saturated fat/salt diet, smoking, inactivity, obesity, genetics |

## What's next

Now that you have mastered transport systems in plants and humans, you are ready to move on to related Edexcel IGCSE Biology topics. Next, learn about excretion in humans, including how kidneys remove urea from the blood and regulate water balance, which builds on your knowledge of plasma function and renal blood vessels. Revise movement of substances into and out of cells to reinforce your understanding of diffusion, osmosis and active transport, which underpin all transport processes in living organisms. Practice transport topic past paper questions to test your knowledge, paying close attention to 'explain' command terms that require you to link structure to function for full marks. Memorise key definitions and labelled diagrams of the heart and plant vascular tissues to maximise your exam marks.

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