Chapter 15: Transport Systems in Living Things (Summary Sheet)

Part A · Why Living Things Need a Transport System

1 Why Multi-cellular Organisms Need a Transport System

The problem	Why it matters
Only the outer layer touches the surroundings	Yet the cells deep inside still need oxygen and food in, and carbon dioxide and waste out
The diffusion distance is too great	In a large body, substances would have to travel far too far to reach the inner cells
The surface area to volume ratio falls	Less exchange surface has to serve more living tissue
So a transport system is needed	Vessels carry a fluid that moves dissolved substances in bulk to where they are used

In humans: the heart is the pump, the blood vessels are the routes, and blood is the fluid carrying the cargo.
Answer shape: large body → inner cells far from the surface → diffusion alone too slow → a faster internal transport system is required.

2 What Diffusion Still Does

Particles spread from higher concentration to lower concentration, down a concentration gradient. No energy is needed.
Humans: oxygen and digested food diffuse from the blood in the capillaries into the body tissues; carbon dioxide and waste diffuse back into the blood.
Plants: gases and mineral salts diffuse into and out of plant cells.
The two work as a pair: the transport system covers the long distance, and diffusion finishes the job across the last short distance.

Part B · Transport in Humans

3 The Circulatory System — a Double Circulation

Blood is driven through the heart twice in one complete circuit.

Circuit	Route taken	What it achieves
Pulmonary	Right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium	Blood picks up oxygen and releases carbon dioxide
Systemic	Left ventricle → aorta → body tissues → vena cava → right atrium	Delivers oxygen and nutrients to cells; returns carbon dioxide and waste

Two circuits, not one: blood sent to the lungs goes only a short way, then returns for a fresh push to the body — so the tissues receive blood fast enough, at high enough pressure.

Exam tip: the internal structures of the heart and blood vessels are not required. What is required is the direction of flow and each vessel's job.

4 The Three Blood Vessels

Vessel	What it does	Why
Artery	Carries blood away from the heart	Receives blood while the heart's pressure is at its greatest
Vein	Carries blood towards the heart	Receives blood that has passed through the capillaries, so pressure is low
Capillary	The site of exchange of substances	Runs between the body cells, so materials pass between blood and cells

Trend, not a rule: the pulmonary artery carries deoxygenated blood to the lungs; the pulmonary vein brings oxygenated blood back.
Never name a vessel without its direction — “away from the heart” / “towards the heart” are the marks.

5 Each Vessel Is Built for Its Job

Feature	Artery	Vein	Capillary
Wall	Thick, elastic and muscular	Thin, only slightly elastic	One cell thick
Lumen	Small — pressure stays high	Large — blood returns easily	Extremely narrow — red blood cells in single file
Valves	None along the vessel	Present at intervals — stop backflow	None
Pressure	High	Low	Low, and falling along its length
Blood in it	Oxygenated (except the pulmonary artery)	Deoxygenated (except the pulmonary vein)	Oxygenated arriving, deoxygenated leaving

Answer in three steps: feature → what it does → advantage.
Elastic wall: stretches with each surge and springs back, so blood keeps moving between heartbeats.
Valves in veins: pressure is low and blood travels back up against gravity, so one-way valves stop it flowing backwards.

6 Exchange at the Capillaries

Fast exchange needs three things, and a capillary has all three: a large total surface area, a very short distance to cross (wall one cell thick), and blood that moves slowly (narrow lumen, cells in single file).
Out: oxygen and digested food diffuse into the body tissues.
In: carbon dioxide and other waste diffuse from the cells into the blood.
Arteries and veins only carry blood to and from the capillary bed — the transfer happens here and nowhere else.

7 Blood — a Liquid Tissue

Component	What it is	Its job
Plasma	Pale yellow liquid, a little over half the blood volume	Transports dissolved substances: digested food, urea, carbon dioxide, hormones, mineral salts, soluble proteins
Red blood cells	Contain haemoglobin; biconcave, no nucleus, no mitochondria	Carry oxygen — haemoglobin combines with oxygen in the lungs and releases it in the tissues
White blood cells	Have a nucleus, colourless, larger; numbers rise during infection	Defence: engulf micro-organisms (phagocytosis); lymphocytes produce antibodies
Platelets	Cell fragments, no nucleus	Start clotting, sealing the wound so less blood is lost and pathogens cannot enter

The red blood cell is shaped for its job: biconcave → large surface area, so oxygen diffuses in and out quickly; no nucleus → more room for haemoglobin; no mitochondria → it does not use the oxygen it carries.
Blood also spreads the heat released by respiration, helping keep body temperature steady.
Too few red blood cells → less haemoglobin → less oxygen reaches the muscles → less energy released → tiredness.

8 When the Transport System Goes Wrong

Drug abuse harms many body systems, the transport system among them: stimulants raise heart rate and blood pressure, straining the heart and vessels, and injected drugs carry pathogens straight into the blood.
A heart transplant raises ethical questions: the heart may be used only with consent, and because demand exceeds supply there must be a system of priority for allocation — medical need and likely benefit before wealth or status.

Part C · Transport in Plants

9 Xylem and Phloem — Two Tissues, Two Cargoes

Tissue	Carries	From → to	Direction
Xylem	Water and mineral salts	Roots → stem → leaves and every other part	One way, upward
Phloem	Food, mainly sucrose, made in the leaves	Leaves → growing tips, fruits, roots, storage organs	Up or down, to wherever food is used or stored

Together they thread from root tips to leaves; side by side they form a vascular bundle, which also stiffens the shoot.
The xylem is a continuous hollow tube with no end walls, so water moves up with very little resistance.
Name the tissue and the form of the cargo: raw water and mineral salts in the xylem, manufactured sugar in the phloem. “Food and water together” loses the mark.

10 Root Hair Cells — Water Enters by Osmosis

Step	What happens
The root hair	A long, thin outgrowth from an outer root cell, pushing into the film of water between soil particles — thin and very numerous, so the root has an enormous surface area
The cell sap inside	Holds a high concentration of sugars and mineral salts, so it is far more concentrated than the dilute soil solution
Water moves in	By osmosis, across the partially permeable cell membrane, from the soil solution into the cell sap
Mineral salts	Mainly by active transport, using energy from respiration to move ions in even against the gradient

Diffusion, osmosis and active transport — compared

	Diffusion	Osmosis	Active transport
What moves	Any particles — a gas or a dissolved substance	Water only	Dissolved mineral salts (ions)
Direction	Higher concentration → lower concentration, down the gradient	Higher water potential → lower water potential, down the gradient	Lower concentration → higher concentration, against the gradient
Membrane needed?	No	Yes — a partially permeable membrane	Yes — the cell membrane
Energy from respiration?	No — passive	No — passive	Yes — it uses energy
Example here	Oxygen diffusing from the blood into the tissue cells	Water entering a root hair cell	Mineral salts taken in at the root hair cells

11 The Path of Water Through the Plant

In order: soil → root hair cell (by osmosis) → cell to cell across the root → xylem → up the stem as a continuous column → leaf cells (by osmosis) → evaporates into the air spaces → water vapour out through the stomata.
No pump drives water up a plant: water is pulled up as it is lost from the leaves, and each cell's sap is slightly more concentrated than the one before it, so water keeps moving on by osmosis.
Water moves up; mineral salts are taken in at the roots and carried with it.

12 Transpiration and the Stomata

Transpiration is the loss of water vapour from the plant, chiefly through the stomata — pores mostly on the lower leaf surface, shaded and out of moving air.
Two guard cells border each stoma: they swell with water to open it, and lose water to close it. Open by day for photosynthesis, closed at night.
It is not only a loss: it cools the leaf and keeps the water column moving.
Losing water faster than the roots replace it → cells lose firmness → the shoot droops → the plant wilts.

13 What Changes the Rate of Transpiration

Factor	Rate	Reason
Light intensity	Up	Light opens the stomata
Temperature	Up	Warmer molecules have more kinetic energy, so they evaporate and diffuse away faster
Wind speed	Up	Moving air sweeps away the damp layer around the leaf, keeping the gradient steep
Humidity	Down	Moist air gentles the gradient, so water vapour diffuses out more slowly
Number of stomata	Up	More pores, more routes out
Leaf surface area	Up	More surface for evaporation and diffusion
Thickness of cuticle	Down	The waxy layer resists water passing through

Say which condition makes the gradient steeper or flatter, then link it to diffusion out of the leaf.

14 Evidence from Investigations

Investigation	What it shows
Celery or a white flower standing in dyed water	The dye appears inside the xylem, tracing the tissue that carried the water up from the roots
A potted plant with its soil covered, weighed every half hour	Water can leave only through the plant, so the steady fall in mass is water lost — and the rate can be compared between conditions

Inference shape: observation → name the tissue the evidence points to → say why the other tissue cannot explain it.

Exam tip: in plant answers, give the tissue and the form of the cargo — water and mineral salts in the xylem, food in the phloem — and the direction it travels.

Part D · Say It Like This

15 Say It Like This

The question asks	The phrase that earns the mark
Why does a large organism need a transport system?	“The distance from the surface to the cells deep inside is too great, so diffusion alone would be too slow to supply oxygen and food and to remove waste.”
How does oxygen reach a muscle cell?	“Oxygen diffuses from the blood in the capillary, down a concentration gradient and through the one-cell-thick wall, into the muscle cell.”
Name the vessel and its direction.	“Arteries carry blood away from the heart, veins carry blood towards the heart, and capillaries are the site of exchange of substances.”
Why do veins have valves?	“Because the pressure in veins is low and blood must travel back up against gravity, so one-way valves stop it flowing backwards.”
How is water taken in at the root?	“Water enters the root hair cell by osmosis, through its partially permeable cell membrane, because the cell sap is more concentrated than the dilute solution in the soil.”
Why is transpiration faster on a hot, windy day?	“Heat gives the water molecules more energy to evaporate, and the wind blows away the humid air around the stomata, so the concentration gradient stays steep.”