Transport systems — needed as SA:V falls; xylem and phloem
A small unicellular organism has a large surface-area-to-volume ratio (SA:V) and short distances, so diffusion alone supplies it fast enough. As an organism grows, its SA:V falls and diffusion distances rise, so diffusion is too slow and a transport system is needed. In flowering plants: xylem carries water and dissolved mineral ions one way, from roots up to leaves; phloem carries sucrose and amino acids (food) to where they are needed, in either direction — translocation.
Blood — plasma carrying red cells, white cells and platelets
Plasma transports carbon dioxide, digested food (glucose, amino acids), urea, hormones and heat. Red blood cells carry oxygen: biconcave, no nucleus, packed with haemoglobin, forming oxyhaemoglobin where oxygen is plentiful and releasing it where low. White blood cells defend the body — phagocytes engulf pathogens (phagocytosis); lymphocytes make antibodies. Platelets help the blood clot. Carbon monoxide binds haemoglobin as carboxyhaemoglobin, so less oxygen is carried.
Heart — double circulation; cardiac output = rate × stroke volume
Humans have a double circulation: blood passes through the heart twice — pulmonary to the lungs, systemic to the body. The right side handles deoxygenated blood, the left oxygenated; the left ventricle wall is thicker as it pumps at high pressure round the body. Arteries carry blood from the heart at high pressure (thick, elastic wall); veins return it at low pressure (thinner wall, valves); capillaries are one cell thick. Cardiac output = heart rate × stroke volume.
Drawn from real examiner reports.
Describe the WALL, not the lumen
When asked how an artery differs from a vein, credit WALL features linked to pressure: an artery has a thick muscular, elastic wall to withstand high pressure; a vein has a thinner wall and valves to stop low-pressure blood flowing back. Citing lumen size is not a wall feature and scores nothing. Remember arteries carry blood at HIGHER pressure than veins, not the reverse.
Link blood flow → oxygen → respiration → energy
In coronary-heart-disease questions, a partly blocked coronary artery delivers less blood → less oxygen (and glucose) → less aerobic respiration → less energy released for heart muscle. Weak answers just say "the artery is narrower" without this oxygen-to-respiration link, which is where the marks are. Whenever oxygen is mentioned, connect it to respiration and energy release.
Composition ≠ pressure
When a question asks about the COMPOSITION of blood in a vessel — its oxygen or carbon dioxide levels — do not answer in terms of blood PRESSURE. They are different properties. Read whether the question wants how much oxygen or carbon dioxide the blood carries, or how high the pressure is, and answer that exact property.
Red blood cell: name feature AND benefit
For red blood cell adaptations, give the feature AND its benefit: biconcave shape → large surface area for absorbing oxygen; no nucleus → more room for haemoglobin; full of haemoglobin → binds oxygen. Saying only "large surface area", or "carries oxygen" without naming haemoglobin, is too vague to score both marks.
Athlete: bigger stroke volume, not less oxygen
For the trained-athlete question, the reasoning is: a fitter heart has a larger stroke volume, so it pumps more blood per beat and can beat more slowly at rest (lower pulse) yet still deliver enough oxygen. Do NOT say the athlete "needs less oxygen" — the same cardiac output comes from a bigger stroke volume and a lower heart rate.
Left = oxygenated, right = deoxygenated
Keep the heart sides straight: the RIGHT side pumps deoxygenated blood to the lungs; the LEFT side pumps oxygenated blood to the body. The left ventricle wall is thicker because it pushes blood at high pressure all round the body, not just to the nearby lungs. On a diagram, left and right are labelled from the body's view, so the left side appears on the right of the page.
Xylem vs phloem — don't swap
Do not swap the plant transport tissues. Xylem carries water and dissolved mineral ions one way, from roots up to leaves. Phloem carries sucrose and amino acids (dissolved food) to where they are needed, in either direction — this is translocation. A memory hook: pHloem carries pHood.
Cardiac output = heart rate × stroke volume
Cardiac output = heart rate × stroke volume; rearrange as stroke volume = cardiac output ÷ heart rate when needed. Read the data for the CORRECT person/row (a common slip is using the wrong volunteer's figures), and carry the unit (cm³/min) or lose the unit mark.
Answer structure as feature → benefit
For any structure question (heart wall, artery/vein, red blood cell, capillary), answer as a feature → benefit chain: name the structural feature, then say what it lets the vessel or cell DO. A feature stated without its function rarely scores the second mark.
Exercise chain: respire faster → heart rate up
For "why does heart rate rise in exercise", build the chain: exercise → muscles respire faster → need more oxygen and remove more carbon dioxide → heart beats faster and harder (adrenaline raises it too) → more blood delivered per minute to the muscles.
Why transport systems are needed. A single-celled organism has a large surface-area-to-volume ratio (SA:V) and short distances, so diffusion alone supplies its needs fast enough. A large multicellular organism has a small SA:V and long diffusion distances, so diffusion is too slow — it needs a transport system to move substances quickly.
Plant transport tissues:
| Tissue | Transports | Direction |
|---|---|---|
| Xylem | Water and dissolved mineral ions (from roots to leaves) | Upwards only |
| Phloem | Sucrose and amino acids (dissolved food) — translocation | Either direction (to where needed) |
Blood composition (spec-precise):
| Component | Job |
|---|---|
| Plasma | Liquid that transports carbon dioxide, digested food (glucose, amino acids), urea, hormones and heat |
| Red blood cell (RBC) | Transports oxygen using haemoglobin |
| White blood cell (WBC) | Defends against disease (phagocytosis + antibodies) |
| Platelets | Help the blood to clot and seal wounds |
Key definitions (two-element, mark-scheme form):
Red blood cell adaptations (feature → benefit):
White blood cells and immunity. Phagocytes engulf and digest pathogens (phagocytosis). Lymphocytes produce antibodies — proteins that lock onto a specific pathogen and destroy it or mark it for destruction.
Define a vein.
At rest, a person's heart beats 72 times per minute and pumps 75 cm³ of blood with each beat.
Calculate the cardiac output. Give the unit. (2 marks)
