How are plant tissues organised, and how do plants transport water and food?
Plant tissues and organs, the structure and function of the leaf, the roles of xylem and phloem, transpiration and translocation, and the effect of environmental factors on the rate of transpiration.
A focused answer to AQA GCSE Biology 4.2.3, covering plant tissues and organs, leaf structure, xylem and phloem, transpiration and translocation, and the factors that affect transpiration rate.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
AQA wants you to describe plant tissues and organs, explain how the leaf is adapted for photosynthesis and gas exchange, distinguish the roles of xylem and phloem, and explain transpiration and translocation, including the factors that change the transpiration rate.
Plant tissues and the leaf
The leaf is a plant organ made of several tissues, each adapted to a job:
- Epidermal tissue: covers and protects the leaf; the upper surface has a waxy cuticle to reduce water loss and the lower epidermis contains the stomata.
- Palisade mesophyll: near the top of the leaf, packed with chloroplasts; the main site of photosynthesis because it gets the most light.
- Spongy mesophyll: has air spaces that allow gases (carbon dioxide and oxygen) to diffuse to and from the cells.
- Xylem and phloem: form the transport tissue running through the veins.
- Guard cells: surround each stoma (pore) and change shape to open or close it, controlling gas exchange and water loss.
The root also has adapted tissue: root hair cells have a large surface area for absorbing water (by osmosis) and mineral ions (by active transport).
Xylem and phloem
Transpiration and translocation
Transpiration is the loss of water vapour from the surfaces of the leaves, mostly through the stomata. As water evaporates from the leaf, more water is pulled up the xylem from the roots, forming the transpiration stream, which also carries mineral ions up the plant. Transpiration provides water for photosynthesis and for keeping cells turgid, and helps cool the plant.
The guard cells control water loss: in bright light they take in water, become turgid and open the stomata for photosynthesis; in dry or dark conditions they lose water, become flaccid and close the stomata to conserve water. This is a balance between letting carbon dioxide in for photosynthesis and not losing too much water.
Try this
Q1. State one difference between the substances carried by xylem and phloem. [1 mark]
- Cue. Xylem carries water and mineral ions; phloem carries dissolved sugars.
Q2. Explain why transpiration is faster on a windy day. [2 marks]
- Cue. Wind removes water vapour from around the leaf, keeping a steep concentration gradient so water diffuses out faster.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksCompare the structure and function of xylem and phloem in a plant.Show worked answer →
A 4-mark compare question rewards linked comparative points.
Xylem is made of dead, hollow cells strengthened with lignin, whereas phloem is made of living cells. Xylem transports water and dissolved mineral ions, whereas phloem transports dissolved sugars made in the leaves (translocation). Xylem moves substances in one direction only, upwards from the roots, whereas phloem can move substances in both directions, to wherever they are needed for growth or storage.
Markers reward at least three comparisons (living versus dead cells, what each carries, and direction of movement) using comparative language.
AQA 20214 marksA student measured the rate of transpiration of a plant in still air and in moving air from a fan. Predict and explain how the rate would differ, and describe how a potometer could be used to measure the rate.Show worked answer →
A 4-mark question rewards a prediction with reasoning plus the method.
The rate of transpiration would be faster in the moving air. Moving air removes the water vapour from around the leaf, so the concentration of water vapour just outside the stomata stays low, keeping a steep concentration gradient, so water diffuses out of the leaf faster.
A potometer measures water uptake (which is close to the rate of transpiration) by recording how far a bubble of air moves along a capillary tube in a set time; the faster the bubble moves, the faster the transpiration. The student should control other factors such as temperature and light.
Markers reward the prediction (faster in moving air), the steep gradient reasoning, and the potometer measuring the rate by the movement of a bubble.
Related dot points
- The levels of organisation from cells to tissues, organs and organ systems, the meaning of each level, and how the digestive system is an example of an organ system, including the action of enzymes.
A focused answer to AQA GCSE Biology 4.2.1, covering the levels of organisation from cells to tissues, organs and organ systems, with the digestive system as the worked example.
- The structure of the heart and the double circulatory system, the roles of the arteries, veins and capillaries, how the heart rate is controlled, and the structure and function of the lungs in gas exchange.
A focused answer to AQA GCSE Biology 4.2.2, covering the structure of the heart, the double circulatory system, the three types of blood vessel, control of heart rate, and the structure of the lungs for gas exchange.
- Diffusion of gases and solutes, osmosis as the movement of water across a partially permeable membrane, active transport against a concentration gradient, the factors affecting these, and the importance of surface area to volume ratio and exchange surfaces.
A focused answer to AQA GCSE Biology 4.1.3, covering diffusion, osmosis and active transport, the factors that affect them, surface area to volume ratio, and the required practical on osmosis.
- Photosynthesis as an endothermic reaction, the word and symbol equations, the site of photosynthesis, and the uses of the glucose produced by the plant.
A focused answer to AQA GCSE Biology 4.4.1.1, covering photosynthesis as an endothermic reaction, the word and symbol equations, where it happens, and how plants use the glucose they make.
- The definition of health, the difference between communicable and non-communicable disease, risk factors for non-communicable diseases, the effect of lifestyle, the development of cancer, and the interaction of different diseases.
A focused answer to AQA GCSE Biology 4.2.2, covering the definition of health, communicable and non-communicable disease, risk factors and lifestyle, the development of cancer, and how diseases interact.
Sources & how we know this
- AQA GCSE Biology (8461) specification — AQA (2016)