Cambridge International AS & A Level Biology 9700: Transport in plants – Revision Notes

Condensed for the final weeks. For full explanations and worked examples, use the transport in plants study guide.

These notes cover topic 7 of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027: sections 7.1 (Structure of transport tissues) and 7.2 (Transport mechanisms). It is AS Level content, examined on Paper 1 and Paper 2; the syllabus says AS knowledge is also required on Paper 4. Other links: the Cambridge A Level Biology hub, the printable 9700 checklist, the practice questions and the AS Level diagnostic.

Key definitions

• Transpiration: evaporation of water from the internal surfaces of leaves, followed by diffusion of water vapour to the atmosphere.
• Apoplast pathway: water moves through cellulose cell walls and intercellular spaces.
• Symplast pathway: water moves through cytoplasm, from cell to cell via plasmodesmata.
• Casparian strip: a band of waterproof suberin in endodermal cell walls that blocks the apoplast.
• Cohesion: hydrogen bonding between water molecules.
• Adhesion: hydrogen bonding between water molecules and cellulose in cell walls.
• Assimilates: substances made by the plant, such as sucrose and amino acids, carried in phloem.
• Source: where assimilates are loaded into phloem. Sink: where they are used or stored.
• Xerophyte: a plant adapted to reduce water loss by transpiration.

7.1 Distribution in herbaceous dicots

TS of	Xylem	Phloem
Stem	Inner side of each vascular bundle; bundles in a ring near the outside	Outer side of each bundle
Root	Central star or cross shape	Groups between the xylem arms
Leaf (midrib)	Towards the upper surface	Towards the lower surface

Plan diagram checklist: no cells drawn; correct proportions of layers; clean single lines; no shading; ruled label lines touching the tissue.

7.1 Structure → function

Cell	Structure	Function it serves
Xylem vessel element	Dead, empty lumen	Low resistance to water flow
	No end walls	Continuous water column
	Lignified walls	Resists collapse under tension; waterproof
	Pits	Sideways movement of water
Sieve tube element	No nucleus, few organelles, peripheral cytoplasm	Clear lumen for mass flow
	Sieve plates	Sap flows between elements
Companion cell	Many mitochondria	ATP for proton pumps
	Plasmodesmata to sieve tube	Route for assimilates into the sieve tube
	Nucleus, dense cytoplasm	Metabolism for the sieve tube element

Drawing cells from micrographs (7.1.3)

The practical section of the syllabus sets out what a cell drawing should show:

• the correct shapes of the cells
• cell walls drawn with two lines, or three lines where two cells touch
• the correct relative sizes and proportions
• only the cell contents you can actually see

For xylem, draw the thickened wall and any pits you can see, with an empty lumen. For phloem, show the sieve plate across the end of the sieve tube element and a narrower companion cell alongside it. Label with ruled lines.

Small worked reminders

Actual size. A vessel is 18 mm wide on a ×400 photomicrograph. 18 mm = 18 000 µm; 18 000 ÷ 400 = 45 µm.

Calibration. 10 stage micrometer divisions of 10 µm match 40 eyepiece units: 100 ÷ 40 = 2.5 µm per eyepiece unit.

Rate. A label moves 540 mm in 45 min: 540 ÷ 45 = 12 mm min⁻¹. Multiply by 60 and divide by 1000 for m h⁻¹: 0.72 m h⁻¹.

Microscope calculations

You need	Formula
Actual size	image size ÷ magnification
Magnification	image size ÷ actual size
1 eyepiece unit	(stage micrometer divisions × value of each) ÷ eyepiece units they span

Convert to the same unit first: 1 mm = 1000 µm.

Method in steps: soil to xylem

1. Water enters root hair cells by osmosis.
2. It crosses the cortex by the apoplast (cellulose walls) or symplast (cytoplasm and plasmodesmata).
3. At the endodermis, the Casparian strip (suberin) blocks the apoplast.
4. Water must enter the symplast of endodermal cells, crossing a membrane.
5. Water then moves into the xylem, entering through pits in the lignified walls.

Method in steps: xylem to air

1. Water evaporates from mesophyll cell walls into air spaces.
2. Water vapour diffuses out through stomata to the atmosphere.
3. Water leaves leaf xylem to replace it; the column is under tension.
4. Cohesion (hydrogen bonds between water molecules) pulls the whole column up.
5. Adhesion to cellulose in the walls supports the column.
6. Lignin stops the vessels collapsing.

Method in steps: phloem loading and mass flow

1. Proton pumps in companion cells use ATP to pump H⁺ out.
2. H⁺ gradient forms (high outside).
3. H⁺ return through cotransporter proteins, carrying sucrose in.
4. Sucrose passes into the sieve tube through plasmodesmata.
5. Water potential in the sieve tube falls; water enters by osmosis.
6. Hydrostatic pressure rises at the source.
7. At the sink, sucrose is removed; water leaves; pressure falls.
8. Sap moves by mass flow down the hydrostatic pressure gradient.

Xerophyte features to annotate

Feature	Annotation
Rolled leaf	Traps humid air; lowers the water vapour potential gradient
Sunken stomata	Humid air held above stoma
Hairs	Trap still, moist air
Thick waxy cuticle	Less evaporation through the epidermis
Few stomata / small area	Less surface for diffusion of vapour

Links to water potential (topic 4)

• Water always moves by osmosis from higher (less negative) to lower (more negative) water potential.
• Root hair cells have a lower water potential than soil water, so water enters them.
• In phloem, loading sucrose lowers the water potential of the sap, so water enters from nearby xylem and raises hydrostatic pressure.
• At a sink, removing sucrose raises the water potential of the sap, so water leaves and pressure falls.

Phrases that carry the marks

• “Water evaporates from the internal surfaces of the leaf; water vapour diffuses out through the stomata.”
• “Hydrogen bonds between water molecules give cohesion.”
• “Hydrogen bonds between water and cellulose give adhesion.”
• “The column of water is under tension; lignin prevents the vessel collapsing.”
• “The Casparian strip, made of suberin, blocks the apoplast pathway.”
• “Proton pumps use ATP; H⁺ diffuse back through cotransporter proteins carrying sucrose.”
• “Mass flow down a hydrostatic pressure gradient from source to sink.”

Must-know distinctions

• Xylem vs phloem. Xylem: dead, lignified, water and ions, upwards only. Phloem: living, sieve plates, assimilates, source to sink in either direction.
• Apoplast vs symplast. Walls vs cytoplasm. Only the apoplast is blocked by the Casparian strip.
• Suberin vs lignin. Suberin: Casparian strip in the endodermis. Lignin: xylem vessel walls.
• Cohesion vs adhesion. Water–water vs water–cellulose.
• Evaporation vs diffusion. Evaporation inside the leaf; diffusion of vapour out through the stomata.
• Active vs passive in phloem. Proton pumping is active. Sucrose entry by cotransport depends on it. Mass flow itself is passive, down a pressure gradient.

Quick self-test

1. Where is phloem found in a TS of a herbaceous dicot root?
2. Give two features of a plan diagram.
3. What substance forms the Casparian strip?
4. Why must water enter the symplast at the endodermis?
5. What type of bond causes cohesion between water molecules?
6. Where exactly does water evaporate in a leaf?
7. A sieve tube element is 60 µm wide. Its image on a micrograph is 24 mm wide. Calculate the magnification.
8. One eyepiece unit equals 2.5 µm. A xylem vessel spans 18 eyepiece units. How wide is it?
9. Why do companion cells contain many mitochondria?
10. What is moved actively during phloem loading: H⁺ or sucrose?
11. Name two assimilates carried in phloem.
12. A label moves 1.2 m through phloem in 2 hours. Give the rate in mm min⁻¹.

Answers

1. In groups between the arms of the central xylem.
2. Any two: no cells drawn; correct proportions of tissue layers; single clear lines; no shading.
3. Suberin.
4. The Casparian strip blocks the apoplast, so water must cross a membrane into the cytoplasm.
5. Hydrogen bonds.
6. From the cell walls of mesophyll cells, into the air spaces.
7. 24 mm = 24 000 µm; 24 000 ÷ 60 = ×400.
8. 18 × 2.5 = 45 µm.
9. To make ATP for the proton pumps.
10. H⁺ (protons) are pumped actively; sucrose enters by cotransport.
11. Sucrose and amino acids.
12. 1200 mm ÷ 120 min = 10 mm min⁻¹.

Where marks are usually lost

• Drawing individual cells in a plan diagram, or getting the proportions of cortex to vascular tissue wrong.
• Swapping xylem and phloem positions in a stem bundle or leaf midrib.
• Calling the Casparian strip lignin.
• Writing “water evaporates through the stomata”.
• Missing “hydrogen bonds” when explaining cohesion or adhesion.
• Linking adhesion to lignin instead of cellulose.
• Stating that sucrose is actively transported without mentioning H⁺ and cotransporter proteins.
• Explaining mass flow without “hydrostatic pressure” and without saying where pressure is high and low.
• Annotating a xerophyte drawing with labels only, no explanation of how water loss is reduced.
• Forgetting to convert mm to µm before dividing.

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.