Topic 8 of IGCSE Biology (Cambridge 0610) is transport in plants. You need to state the function of xylem and phloem, describe how water moves from roots to leaves (including the role of transpiration), explain factors that affect transpiration rate (light, temperature, wind, humidity), and describe how root hair cells are adapted for absorption. This revision guide takes you through each of these with clear explanations and exam-style tips for full marks.

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Xylem: water and mineral ions

Xylem vessels are dead, hollow tubes (no cytoplasm) that run from the roots through the stem to the leaves. They carry water and mineral ions from the roots upwards to the leaves. The flow is one-way only (roots → leaves). The walls of xylem cells are strengthened with lignin, which makes them rigid and helps support the plant. Water is lost from the leaves by transpiration (evaporation through stomata), and this loss creates a pull (tension) that draws water up the xylem from the roots. You may be asked to state the function of xylem (water and mineral ions upward) and one structural feature (dead; hollow; no cytoplasm; lignified walls). Do not say xylem carries “sap” without specifying water and mineral ions.

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Phloem: sucrose and translocation

Phloem is made of living cells. It carries sucrose (and amino acids) from the leaves (where they are made in photosynthesis) to other parts of the plant: roots, growing tips, flowers, fruits, and storage organs. This movement of sucrose (and other dissolved food) is called translocation. Unlike xylem, phloem can transport in both directions (e.g. sucrose to roots or to growing shoots). Phloem carries sucrose (or “dissolved sugars”); do not just say “food” without specifying. You may be asked to state the function of phloem (sucrose and amino acids to sink tissues) and to compare xylem and phloem (direction: one-way vs two-way; contents: water and minerals vs sucrose; living vs dead).

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Transpiration and the transpiration stream

Transpiration is the loss of water vapour from the leaves of a plant, mainly through the stomata. When water evaporates from the surfaces of mesophyll cells and diffuses out through the stomata, it creates a pull (tension) on the column of water in the xylem. Because water molecules stick together (cohesion) and stick to the xylem walls (adhesion), the whole column is pulled up from the roots. This flow of water from roots to leaves is the transpiration stream. Transpiration is a consequence of the plant having stomata open for gas exchange (CO₂ in for photosynthesis); it also helps to cool the plant and to bring mineral ions up in the xylem. You may be asked to describe how water moves from roots to leaves (transpiration pull; cohesion; adhesion; one-way flow in xylem).

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Factors affecting transpiration rate

The rate of transpiration is affected by light, temperature, wind, and humidity. Light: stomata open in light (for photosynthesis), so more water is lost; transpiration rate increases. Temperature: higher temperature increases evaporation, so transpiration rate increases. Wind: wind removes water vapour from near the leaf, keeping a steep concentration gradient, so transpiration rate increases. Humidity: high humidity means the air already has a lot of water vapour, so the concentration gradient is shallow and transpiration rate decreases. You may be asked to explain how each factor affects transpiration (link to stomata opening or to evaporation and concentration gradient).

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Root hair cells

Root hair cells are found on the surface of plant roots. Each cell has a long, thin root hair that projects into the soil, which increases the surface area for absorption of water and mineral ions. Water enters by osmosis (from the soil solution into the cell, down a water concentration gradient). Mineral ions enter by active transport (against the concentration gradient; energy from respiration; carrier proteins), because their concentration in the soil is often lower than in the root. You may be asked to describe how root hair cells are adapted for absorption: large surface area (root hair); osmosis for water; active transport for mineral ions.

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Exam tips and command words

• State the function: Xylem: water and mineral ions upward; phloem: sucrose (and amino acids) to sink tissues.
• Describe: How water moves from roots to leaves (transpiration pull; cohesion; one-way in xylem).
• Explain: How light, temperature, wind, and humidity affect transpiration rate (link to stomata or evaporation).
• Compare: Xylem vs phloem (direction, contents, living/dead, one-way/two-way).

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Common mistakes to avoid

• Saying phloem carries “food” without specifying sucrose (or “dissolved sugars”).
• Confusing transpiration (water vapour loss from leaves) with translocation (movement of sucrose in phloem).
• Saying xylem carries “sap” (use water and mineral ions for xylem; sucrose for phloem).
• Not stating that transpiration creates the pull that draws water up the xylem.

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Revision checklist

• State the function of xylem and phloem and one structural difference (e.g. dead vs living).
• Describe the path of water from root to leaf and the role of transpiration.
• Explain how light, temperature, wind, and humidity affect transpiration rate.
• Describe how root hair cells are adapted for absorption (surface area; osmosis; active transport).

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Next steps

Book a free trial with an IGCSE Biology tutor to practise xylem, phloem, and transpiration questions, or explore Tutopiya’s learning portal for more revision resources.