Xylem vs phloem: what and which way
Plants have two transport tissues. Xylem is dead, hollow, lignified vessels that carry water and mineral ions one way only — from the roots upward to the leaves. Phloem is living tissue that carries sucrose and amino acids in both directions (up to growing shoots, down to the roots); this movement is called translocation. A rule of thumb: xylem = water + minerals = up only; phloem = sugars + amino acids = any direction.
Transpiration: definition and rate factors
Transpiration is the loss of water vapour from a plant by evaporation and diffusion through the stomata. Water travels roots → xylem → mesophyll, evaporates, then diffuses out; the transpiration stream pulls more water up. The rate rises with more light (stomata open), higher temperature (faster evaporation) and air movement (steeper gradient); it falls with high humidity. A potometer measures water uptake as a transpiration proxy.
Double circulation and cardiac output
The heart drives a double circulation — blood passes through it twice per circuit: the pulmonary circuit (right side → lungs → left side) and the systemic circuit (left side → body → right side). The left ventricle wall is thicker, pumping at higher pressure to the whole body; valves stop backflow. Cardiac output = heart rate × stroke volume. A trained athlete has a larger stroke volume, so reaches the same output at a lower heart rate.
Drawn from real examiner reports.
Lumen size is not an artery wall feature
Arteries carry blood at high pressure; veins at low pressure — a minority wrongly say veins are higher. Asked about the artery wall, give thick muscular and elastic tissue linked to high pressure; "thicker wall" alone may not score. Lumen size is not a wall feature — never cite it when the question says "wall". Veins have valves; arteries do not.
June 2024 Paper 1BR Q7(a)(i): candidates lost marks citing lumen size as a wall feature; a minority said veins carry higher pressure than arteries.
Water loss alone fails as a definition
The mark-scheme definition needs two parts: loss of water vapour (not liquid) by evaporation and diffusion through the stomata. "Plants losing water" omits the mechanism and scores zero. Asked to explain the transpiration stream, add that evaporation lowers water potential, so tension pulls water up the xylem — don't stop at water leaving the stomata.
Cardiac output: read the right table row
Cardiac output = heart rate × stroke volume (cm³/min). It is a one-step calculation, so marks are usually lost by reading the wrong volunteer's row (untrained instead of trained) or by omitting the unit cm³/min. Label which value is heart rate (beats/min) and which is stroke volume (cm³/beat) before multiplying — do not swap them.
June 2023 Paper 1BR Q5(b)(i): the common error was reading data from the wrong (untrained) volunteer's row; units were also omitted.
Bad diet is too vague for CHD marks
Coronary heart disease questions want specific risk factors: high-fat diet, high cholesterol, obesity, lack of exercise, stress and smoking. Vague answers such as "bad diet", "unhealthy diet" or just "diet" are rejected. Name the precise factor — e.g. "a diet high in saturated fat", not "diet".
June 2024 Paper 1BR Q7(b)(i): "bad diet" or "diet" alone were rejected; specific factors (high-fat diet, high cholesterol, obesity, stress, lack of exercise) were required.
Carbon monoxide forms carboxyhaemoglobin
Explaining how carbon monoxide harms oxygen transport, you must name the product: CO binds to haemoglobin in red blood cells to form carboxyhaemoglobin, which cannot carry oxygen, so the blood's oxygen-carrying capacity falls. Calling CO merely "a poisonous gas" scores nothing — the mark is for the carboxyhaemoglobin mechanism.
Nov 2024 Paper 2B Q6(b): CO forms carboxyhaemoglobin, reducing oxygen carriage; weak answers just called CO "a poisonous gas".
Vessel questions: composition vs pressure
When a question asks about the blood composition in a vessel (oxygen and carbon dioxide levels), answer with oxygenated/deoxygenated content — not blood pressure. Many candidates discussed pressure when the question was about composition, and vice versa. Read whether the question wants composition or pressure, and answer only that.
June 2023 Paper 1BR Q5(a)(ii): the question asked about blood composition (oxygen/CO2 levels) but many wrongly discussed blood pressure; blood in vessel X is deoxygenated.
Do not reverse xylem and phloem loads
A frequent slip is giving sucrose for xylem or water for phloem. Xylem carries water and mineral ions (upward, dead lignified vessels); phloem carries sucrose and amino acids (both directions, living cells). Keep the terms apart too — transpiration is water vapour leaving via stomata; translocation is sugars and amino acids moving in phloem.
Chain: feature to adaptation to function
Answer transport questions as a chain: feature → adaptation → function — e.g. "capillary wall one cell thick → short diffusion distance → fast gas exchange". Never stop at the structural feature; always state the functional consequence it produces.
Name the variable for a fit athlete
Explaining a trained athlete's lower resting heart rate, name the variable: a larger stroke volume gives the same cardiac output with fewer beats. "Fitter" or "needs less oxygen" scores nothing; do not confuse aerobic with anaerobic respiration.
Link oxygen to respiration to energy
For a blocked coronary artery, link oxygen to respiration to energy: less blood flow → less oxygen → less aerobic respiration → less energy for heart-muscle contraction. Describing the diagram without this chain scores nothing.
Spec statements 2.51–2.69. This topic covers transport in plants (xylem, phloem, transpiration) and in humans (blood, heart, blood vessels, double circulation, cardiac output).
Simple, unicellular organisms (e.g. Amoeba) are small enough that diffusion can supply every part of the cell with oxygen and nutrients and remove waste products quickly. As organisms get larger, the surface-area-to-volume ratio (SA:V) decreases, and the diffusion distance from the surface to the centre becomes too great for diffusion alone. Multicellular organisms therefore need a dedicated transport system to carry substances to and from cells quickly.
| Feature | Xylem | Phloem |
|---|---|---|
| What it transports | Water and mineral ions | Sucrose and amino acids |
| Direction | Upward only (roots to leaves) | Both directions (leaves to roots OR to growing regions) |
| Cell state | Dead, hollow (no end walls) | Living |
| Wall | Thickened with lignin (waterproof and strong) | Thin |
Transpiration stream: water enters root hair cells by osmosis (high water potential in soil; low water potential in root), passes through the root into xylem, travels up the stem, reaches the leaf mesophyll cells, evaporates from their surfaces, and diffuses out through the stomata as water vapour.
Definition — mark-scheme form: Transpiration is the loss of water vapour from the surface of a plant by evaporation and diffusion through the stomata.
Two elements required: (1) loss of water vapour (not liquid water), (2) by evaporation and diffusion through the stomata. Saying "water loss" scores zero.
| Factor | Effect on rate | Reason |
|---|---|---|
| Increased light intensity | Increases | Stomata open wider; more water vapour escapes |
| Increased temperature | Increases | Molecules have more kinetic energy; evaporation and diffusion faster |
| Increased air movement (wind) | Increases | Removes water vapour from leaf surface; steeper concentration gradient |
| Increased humidity | Decreases | Less steep concentration gradient for water vapour |
A potometer measures water uptake by a cut leafy shoot — this is a valid proxy for transpiration rate. As the shoot transpires, water is drawn up and an air bubble in the capillary tube moves. Rate = distance moved by bubble ÷ time (with unit, e.g. mm/min). Change one variable at a time (light, humidity, wind speed, temperature) and keep all others constant as controls. Repeats improve reliability.
| Component | Role |
|---|---|
| Red blood cells (RBCs) | Carry oxygen as oxyhaemoglobin; biconcave disc shape, no nucleus, contain haemoglobin |
| White blood cells (WBCs) | Immune defence — phagocytes engulf pathogens; lymphocytes produce antibodies specific to pathogen antigens |
| Platelets | Cell fragments that initiate blood clotting at wound sites, preventing blood loss and entry of micro-organisms |
| Plasma | Liquid; transports carbon dioxide (), digested food (glucose, amino acids, fatty acids), urea, hormones and heat energy |
Carbon monoxide () binds permanently to haemoglobin to form carboxyhaemoglobin, reducing the blood's oxygen-carrying capacity — a key reason smoking increases cardiovascular disease risk.
The human circulatory system is a double circulation: blood passes through the heart twice in one complete circuit.
Units: cardiac output in ; heart rate in beats/min; stroke volume in /beat.
A trained athlete has a larger stroke volume than an untrained person; at rest they achieve the same cardiac output with a lower heart rate (more efficient heart). During exercise, both heart rate and stroke volume increase.
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Wall thickness | Thick (muscle + elastic tissue) | Thin | One cell thick |
| Lumen | Narrow | Wide | Very narrow |
| Valves | No | Yes (prevent backflow) | No |
| Blood pressure | High | Low | Low |
| Function | Carries blood AWAY from heart | Returns blood TO heart | Exchange with tissues |
Risk factors include: high-fat diet, high cholesterol, lack of exercise, obesity, stress, and smoking (carbon monoxide reduces oxygen delivery; nicotine raises heart rate; fatty deposits block coronary arteries). "Bad diet" alone is not specific enough — state the type (high fat, high cholesterol).
State the formula for cardiac output and give the correct units for each quantity.
State the substances transported by (a) xylem and (b) phloem in a flowering plant. (2 marks)
