Cambridge International AS & A Level Biology 9700: Transport in plants – Study Guide

This study guide teaches topic 7, Transport in plants, from the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers sections 7.1 (Structure of transport tissues) and 7.2 (Transport mechanisms). This is AS Level content, examined on Paper 1 (Multiple Choice) and Paper 2 (AS Level Structured Questions); the syllabus states that AS knowledge will also be required on Paper 4. The drawing and measuring skills in 7.1 match the microscope skills listed for Paper 3 (Advanced Practical Skills).

Useful links: the Cambridge A Level Biology hub, the printable 9700 checklist, the condensed transport in plants revision notes and the transport in plants practice questions. For a whole-course check, try the AS Level diagnostic. Water potential and active transport come from topic 4; test yourself on them with the cell membranes and transport practice.

What this topic covers

Syllabus ref.	What you must be able to do	Stage
7.1.1	Draw plan diagrams of transverse sections (TS) of stems, roots and leaves of herbaceous dicotyledonous plants	AS
7.1.2	Describe the distribution of xylem and phloem in those sections	AS
7.1.3	Draw and label xylem vessel elements, sieve tube elements and companion cells	AS
7.1.4	Relate the structure of these cells to their functions	AS
7.2.1	State that some mineral ions and organic compounds are transported dissolved in water	AS
7.2.2	Describe the apoplast and symplast pathways from soil to xylem	AS
7.2.3–7.2.4	Explain transpiration and the cohesion-tension mechanism, including hydrogen bonding and adhesion	AS
7.2.5	Make annotated drawings of TS xerophyte leaves to explain adaptations	AS
7.2.6–7.2.8	Describe source-to-sink transport; explain phloem loading and mass flow	AS

7.1 Distribution of xylem and phloem

All three organs are from herbaceous dicotyledonous plants (non-woody plants with two seed leaves).

Organ (TS)	Where the vascular tissue is	Xylem and phloem arrangement
Stem	Vascular bundles in a ring, near the outside	In each bundle, xylem is towards the centre and phloem towards the outside
Root	One central vascular cylinder, surrounded by the endodermis	Xylem forms a central star or cross shape; phloem lies in groups between the arms of the xylem
Leaf	Main vascular bundle in the midrib, with smaller veins through the mesophyll	Xylem towards the upper surface, phloem towards the lower surface

Plan diagrams

A plan diagram shows the distribution of tissues, not cells. The syllabus practical section asks for:

• no individual cells drawn
• tissue layers in the correct proportions

Good drawing practice also means:

• clear, single, continuous lines with no shading
• label lines drawn with a ruler, ending exactly on the tissue

Before you draw, look at the whole section. Estimate what fraction of the diameter each layer takes up, then keep those proportions.

7.1 Structure related to function

Xylem vessel elements

Xylem vessel elements are joined end to end to form xylem vessels, which carry water and mineral ions up from the roots.

Feature	How it helps
Dead cells with no cytoplasm or organelles	Leaves an empty lumen, so water flows with little resistance
End walls lost or reduced	Forms a continuous tube for an unbroken column of water
Walls thickened with lignin	Strengthens the vessel so it does not collapse under tension; also waterproofs it
Pits (unlignified areas in the wall)	Let water move sideways into and out of neighbouring vessels and cells
Narrow lumen	Water adheres to the walls, which helps the column move

Phloem sieve tube elements and companion cells

Sieve tube elements are joined end to end to form sieve tubes, which carry assimilates such as sucrose and amino acids.

Cell	Feature	How it helps
Sieve tube element	Living, but no nucleus and few organelles	More space for mass flow of sap
Sieve tube element	Sieve plates (end walls with pores)	Let sap flow from one element to the next
Sieve tube element	Thin layer of cytoplasm round the edge	Keeps the lumen clear
Companion cell	Nucleus and dense cytoplasm	Carries out metabolism for both cells
Companion cell	Many mitochondria	Supply ATP for the proton pumps in loading
Companion cell	Many plasmodesmata into the sieve tube element	Pathway for assimilates to pass into the sieve tube

When you draw these cells from a micrograph, draw walls with two lines, include only what you can see, and keep relative sizes right: a companion cell is much narrower than its sieve tube element.

Worked example 1 – actual size from a photomicrograph

A xylem vessel measures 27 mm across on a photomicrograph with magnification ×600. Calculate its actual diameter in µm.

actual size = image size ÷ magnification
27 mm = 27 000 µm
27 000 ÷ 600 = 45 µm

Worked example 2 – eyepiece graticule

20 divisions of a stage micrometer (1 division = 10 µm) line up with 50 eyepiece units (epu). A vessel is 35 epu wide.

20 × 10 = 200 µm = 50 epu
1 epu = 200 ÷ 50 = 4 µm
vessel = 35 × 4 = 140 µm

Recalibrate at every objective lens. The value of 1 epu changes with magnification.

7.2 Water: soil to xylem

Some mineral ions and organic compounds are carried through the plant dissolved in water.

Water enters root hair cells by osmosis, down a water potential gradient. It then crosses the cortex by two pathways.

Apoplast pathway

• Water moves through the cellulose cell walls and the spaces between cells, without crossing a membrane.
• Cellulose fibres have spaces between them, and water is held to the cellulose by hydrogen bonds, so water is drawn along as water ahead of it moves on.
• Xylem walls are thickened with lignin; water enters and leaves the vessels through pits.

Symplast pathway

• Water moves through the cytoplasm of cells, passing from cell to cell through plasmodesmata.

The endodermis

• The endodermis is the layer of cells surrounding the vascular tissue in the root.
• Its cell walls contain the Casparian strip, a band of suberin. Suberin is waterproof.
• The Casparian strip blocks the apoplast pathway. Water must pass into the cytoplasm of the endodermal cells (the symplast) to reach the xylem.
• Because water and ions must cross a cell surface membrane here, the plant can control which mineral ions enter the xylem.

7.2 Transpiration and the transpiration pull

Transpiration is the evaporation of water from the internal surfaces of leaves (the walls of mesophyll cells), followed by diffusion of water vapour through the stomata to the atmosphere.

Cohesion-tension

1. Water evaporates from mesophyll cell walls into the air spaces. Water vapour diffuses out through the stomata.
2. Water moves from the xylem in the leaf to replace it. This lowers the pressure at the top of the xylem, putting the column under tension.
3. Water molecules form hydrogen bonds with each other. This cohesion keeps the column continuous as it is pulled up: the transpiration pull.
4. Water molecules also form hydrogen bonds with the cellulose in the walls. This adhesion helps water move up and stops the column pulling away from the walls.
5. Lignin stops the vessels collapsing under the tension.

Xerophytes

Xerophytes are adapted to reduce water loss by transpiration. You must be able to make annotated drawings of TS xerophyte leaves: labels name the feature, and annotations explain how it reduces water loss. Marram grass is a common example.

Adaptation	How it reduces water loss
Leaf rolled, with stomata on the inner surface	Traps humid air, reducing the water vapour gradient
Stomata sunken in pits or grooves	Humid air collects above the stoma, so less diffusion
Hairs near the stomata	Trap still, humid air
Thick waxy cuticle	Reduces evaporation through the epidermis
Fewer stomata / smaller leaf surface area	Less surface for water vapour to diffuse out of

Annotations must name the gradient: “reduces the water vapour potential gradient between the leaf and the air”.

7.2 Translocation in phloem

Assimilates dissolved in water, such as sucrose and amino acids, move in phloem sieve tubes from sources to sinks. A source releases assimilates (for example, a photosynthesising leaf). A sink uses or stores them (for example, roots, growing tips, fruits). A storage organ can be a sink in one season and a source in another.

Loading: companion cells

1. Proton pumps in the companion cell membrane use ATP to pump hydrogen ions (H⁺) out of the cell.
2. This builds up a high concentration of H⁺ outside the cell.
3. H⁺ diffuse back in through cotransporter proteins, carrying sucrose with them into the companion cell.
4. Sucrose passes from the companion cell into the sieve tube element through plasmodesmata.

Mass flow

1. Sucrose loaded at the source lowers the water potential of the sieve tube sap.
2. Water enters by osmosis (from the nearby xylem), raising the hydrostatic pressure at the source.
3. At the sink, sucrose is removed and used or stored. Water leaves by osmosis, so pressure is lower.
4. The sap moves by mass flow down this hydrostatic pressure gradient, from source to sink.

Worked example 3 – speed of translocation

A radioactive label fed to a leaf is detected 540 mm down the stem after 45 minutes.

rate = 540 ÷ 45 = 12 mm min⁻¹
12 × 60 = 720 mm h⁻¹ = 0.72 m h⁻¹

Common errors

• Drawing cells in a plan diagram.
• Placing phloem inside xylem in a stem bundle, or putting the root’s xylem in a ring.
• Saying xylem vessels are “living” or sieve tube elements “have a nucleus”.
• Saying lignin makes water stick to the walls – the syllabus links adhesion to cellulose.
• Saying water “evaporates out of the stomata”. It evaporates inside the leaf; vapour diffuses out.
• Explaining cohesion without naming hydrogen bonds.
• Saying the Casparian strip is made of lignin. It is suberin.
• Saying sucrose is actively pumped. The protons are pumped; sucrose enters by cotransport.
• Explaining mass flow with “concentration gradient” alone instead of hydrostatic pressure.

Where next

• Tables, steps and a quick self-test: transport in plants revision notes
• Original exam-style questions with marked answers: transport in plants practice
• Microscopy and magnification from topic 1: cell structure study guide

Official syllabus

Cambridge International, Cambridge International AS & A Level Biology 9700 syllabus for 2025, 2026 and 2027 (Version 1, published September 2022), Cambridge University Press & Assessment. Topic 7, Transport in plants: sections 7.1 and 7.2.