Transport in Plants and Animals

What you'll learn

This revision guide covers how organisms transport essential substances throughout their bodies. You'll explore the human circulatory system, blood components, and how plants move water and nutrients. Understanding these transport systems is crucial for WJEC GCSE Biology Paper 1 and forms the foundation for understanding how organisms maintain life processes.

Key terms and definitions

Double circulatory system — a circulatory system where blood passes through the heart twice in one complete circuit of the body (once to the lungs, once to the rest of the body)

Plasma — the liquid component of blood that transports dissolved substances including glucose, amino acids, carbon dioxide, urea and hormones

Haemoglobin — the red oxygen-carrying protein found in red blood cells that reversibly binds to oxygen

Xylem — non-living plant tissue that transports water and mineral ions from roots to leaves through the transpiration stream

Phloem — living plant tissue that transports dissolved sugars (sucrose) from sources (leaves) to sinks (growing regions and storage organs) through translocation

Transpiration — the loss of water vapour from plant leaves through stomata, which creates the pull that moves water up the plant

Artery — a blood vessel with thick muscular walls that carries blood away from the heart at high pressure

Capillary — a tiny blood vessel with walls one cell thick that allows exchange of substances between blood and tissues

Core concepts

The human circulatory system

The human circulatory system is a double circulatory system consisting of two separate circuits:

Pulmonary circulation:

• Right ventricle pumps deoxygenated blood to the lungs
• Gas exchange occurs in lung capillaries
• Oxygenated blood returns to the left atrium

Systemic circulation:

• Left ventricle pumps oxygenated blood to all body organs
• Oxygen and nutrients delivered to tissues
• Deoxygenated blood returns to the right atrium

The double system is efficient because:

• Blood is pumped at high pressure to the body after being re-pressurized at the heart
• Oxygenated and deoxygenated blood remain completely separated
• Different pressures can be maintained in each circuit

Heart structure and function

The human heart has four chambers:

Upper chambers (atria):

• Thin-walled as they only pump blood to the ventricles
• Right atrium receives deoxygenated blood from the vena cava
• Left atrium receives oxygenated blood from the pulmonary vein

Lower chambers (ventricles):

• Thick muscular walls to pump blood out of the heart
• Right ventricle pumps blood to the lungs via the pulmonary artery
• Left ventricle has the thickest wall as it pumps blood around the entire body via the aorta

Valves prevent backflow of blood:

• Atrioventricular valves between atria and ventricles
• Semilunar valves in the aorta and pulmonary artery

The cardiac cycle:

1. Atria contract, pushing blood into ventricles
2. Ventricles contract, forcing blood out through arteries
3. Heart muscle relaxes, allowing chambers to refill
4. Cycle repeats (approximately 70 times per minute at rest)

The heart muscle receives its own blood supply through coronary arteries. Blockage of these arteries causes a heart attack.

Blood vessels

Three types of blood vessels transport blood:

Arteries:

• Carry blood away from the heart
• Thick walls containing smooth muscle and elastic tissue
• Small lumen (internal space) relative to wall thickness
• Withstand high pressure from heart contractions
• Pulse can be felt where arteries pass over bones
• No valves needed (except at heart exit)

Veins:

• Carry blood towards the heart
• Thinner walls with less muscle and elastic tissue
• Large lumen relative to wall thickness
• Transport blood at low pressure
• Contain valves to prevent backflow
• Blood flow assisted by skeletal muscle contraction

Capillaries:

• Microscopic vessels linking arteries and veins
• Walls only one cell thick (single layer of endothelial cells)
• Permeable to allow exchange of substances
• Very narrow lumen (often single blood cell width)
• Extensive branching creates large surface area
• Found in close contact with all body cells

Blood composition and function

Blood consists of plasma and cells:

Plasma (55% of blood volume):

• Straw-colored liquid
• 90% water
• Transports: glucose, amino acids, proteins, hormones, antibodies, urea, carbon dioxide
• Distributes heat around the body

Red blood cells (erythrocytes):

• Biconcave disc shape increases surface area for oxygen absorption
• No nucleus to maximize space for haemoglobin
• Contain haemoglobin which binds reversibly with oxygen
• In lungs: oxygen + haemoglobin → oxyhaemoglobin
• In tissues: oxyhaemoglobin → oxygen + haemoglobin
• Produced in bone marrow
• Lifespan approximately 120 days

White blood cells (leucocytes):

• Part of the immune system
• Have a nucleus
• Two main types:
  • Phagocytes: engulf and digest pathogens
  • Lymphocytes: produce antibodies specific to antigens on pathogens
• Can change shape to squeeze through capillary walls

Platelets:

• Small cell fragments without a nucleus
• Essential for blood clotting
• Prevent blood loss and pathogen entry at wound sites
• Release chemicals triggering conversion of fibrinogen to fibrin
• Fibrin forms a mesh trapping blood cells to form a clot

Plant transport systems

Plants have two separate transport systems:

Xylem tissue:

• Transports water and dissolved mineral ions upwards
• Made of dead cells forming continuous hollow tubes
• Cell walls strengthened with lignin
• No end walls between cells
• Found towards the inside of vascular bundles
• Movement is one-way only (roots to leaves)
• Process driven by transpiration pull

Phloem tissue:

• Transports dissolved sugars (mainly sucrose)
• Made of living cells called sieve tube elements
• Cells have perforated end walls (sieve plates)
• Companion cells provide energy for active transport
• Found towards the outside of vascular bundles
• Movement can be up or down the plant (multi-directional)
• Process called translocation requires energy

Transpiration and factors affecting it

Transpiration is the loss of water vapour from leaves through stomata:

The transpiration stream:

1. Water evaporates from mesophyll cells into air spaces
2. Water vapour diffuses out through stomata
3. Water is pulled up xylem vessels to replace lost water
4. Water enters roots from soil by osmosis
5. Continuous column of water moves from roots to leaves

Factors increasing transpiration rate:

Temperature:

• Higher temperature increases kinetic energy of water molecules
• Faster evaporation and diffusion
• Stomata open wider in warm conditions

Humidity:

• Lower humidity creates steeper concentration gradient
• Water vapour diffuses out faster
• High humidity slows transpiration

Wind speed:

• Wind removes water vapour from leaf surface
• Maintains concentration gradient
• Still air allows water vapour to accumulate

Light intensity:

• Light causes stomata to open for gas exchange
• More open stomata increase water loss
• Stomata close in darkness (reduced transpiration at night)

Adaptations to reduce water loss:

• Waxy cuticle on leaf surface
• Stomata mainly on lower leaf surface
• Stomata can close in hot, dry conditions
• Some plants have rolled leaves or hairs to trap moist air

Root hair cells and water uptake

Root hair cells are specialized for absorbing water and mineral ions:

Structural adaptations:

• Long extension (root hair) increases surface area
• Thin cell wall for short diffusion distance
• Large vacuole maintains water potential gradient
• Many mitochondria provide energy for active transport of minerals

Water uptake:

• Water enters by osmosis down a water potential gradient
• Soil water has higher water potential than root cell contents
• Water moves through root cortex to xylem

Mineral ion uptake:

• Absorbed by active transport (requires energy)
• Minerals often at lower concentration in soil than in root cells
• Movement is against concentration gradient
• Root hair cells have many mitochondria for ATP production

Worked examples

Example 1: Explain why the left ventricle has a thicker muscular wall than the right ventricle. [3 marks]

Mark scheme answer:

• The left ventricle pumps blood around the entire body / to all organs (1 mark)
• This is a greater distance than the right ventricle which only pumps to the lungs (1 mark)
• Therefore it needs to generate higher pressure / more force (1 mark)

Example 2: A student investigated the rate of transpiration using a potometer. They measured water uptake in different conditions. Explain why increased wind speed increases the rate of transpiration. [3 marks]

Mark scheme answer:

• Wind removes / blows away water vapour from the leaf surface (1 mark)
• This maintains a steep concentration gradient between the inside and outside of the leaf (1 mark)
• So water vapour diffuses out of the stomata faster (1 mark)

Example 3: Describe how the structure of an artery is related to its function. [4 marks]

Mark scheme answer:

• Thick walls to withstand high pressure (1 mark)
• Elastic tissue allows artery to stretch and recoil / maintains pressure (1 mark)
• Smooth muscle in walls can contract to control blood flow (1 mark)
• Small lumen maintains high pressure (1 mark)

Common mistakes and how to avoid them

• Confusing arteries and veins by color: Remember arteries carry blood AWAY from the heart (not "red blood"). The pulmonary artery carries deoxygenated blood. Learn by position relative to the heart, not oxygen content.

• Saying red blood cells carry carbon dioxide: Red blood cells primarily transport oxygen. Carbon dioxide is mainly carried dissolved in plasma (about 70%), with some forming hydrogencarbonate ions. Only a small amount binds to haemoglobin.

• Confusing transpiration with translocation: Transpiration is water loss through stomata. Translocation is the transport of dissolved sugars through phloem tissue. These are completely different processes.

• Stating that veins have no valves: Veins contain valves to prevent backflow of blood. Only arteries (except at the heart exits) lack valves because high pressure prevents backflow.

• Writing that xylem transports food or glucose: Xylem transports water and dissolved mineral ions only. Phloem transports dissolved sugars (mainly sucrose, not glucose) produced during photosynthesis.

• Claiming stomata close during the day: Stomata open during the day for gas exchange (CO₂ in for photosynthesis). They typically close at night to reduce water loss, unless conditions are very hot and dry.

Exam technique for "Transport in Plants and Animals"

• Structure-function questions: When asked to relate structure to function, make explicit links. Don't just describe structure—explain HOW that structure enables the function. Use "so that" or "which allows" to connect your points.

• Compare questions: When comparing blood vessels or transport tissues, make direct comparisons. Use comparative language: "thicker than," "whereas," "but" rather than describing each separately. A 3-mark compare question needs three comparative points.

• Explaining trends in data: For practical investigations (e.g., transpiration rate), identify the pattern, quote data values, and explain using scientific principles. A typical 4-mark question allocates: 1 mark for trend, 1 for data, 2 for explanation.

• Label diagrams carefully: Heart diagrams must use correct terminology (atrium not atria for singular, vena cava not just "vein"). Draw label lines with a ruler to specific points, not circles around areas.

Quick revision summary

The human double circulatory system consists of the heart pumping blood through arteries, capillaries, and veins. Blood contains plasma, red blood cells with haemoglobin for oxygen transport, white blood cells for immunity, and platelets for clotting. Plants use xylem to transport water upwards via transpiration and phloem to transport sugars via translocation. Transpiration rate increases with temperature, wind speed, light intensity, and decreases with humidity. Root hair cells absorb water by osmosis and minerals by active transport.