How do plants transport water, minerals and food, and control water loss?
The structure and function of xylem and phloem, the transpiration stream and the factors affecting its rate, and the role of stomata, guard cells and root hair cells in water movement and exchange.
An SQA National 5 Biology answer on transport systems in plants, covering the structure and function of xylem and phloem, the transpiration stream and the factors affecting its rate, and the role of stomata, guard cells and root hair cells.
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
The SQA wants you to describe the xylem and phloem transport tissues (their structure and what each carries), explain the transpiration stream and the factors that change its rate, and describe how stomata, guard cells and root hair cells are involved in water movement and exchange.
Xylem and phloem
The lignin in xylem also gives the plant support, which is one reason woody plants are strong.
The transpiration stream
The stream also carries the dissolved minerals up from the roots, so transpiration is how the plant moves water and minerals to the leaves.
Factors affecting the rate
Each of these makes water leave the leaf surface more quickly, speeding up the whole stream.
Stomata, guard cells and root hairs
The stomata are tiny pores, mostly on the underside of the leaf. Each is opened and closed by two guard cells, which lets the plant control gas exchange (for photosynthesis) and water loss. When guard cells close the stomata, less water is lost, which helps in dry conditions.
At the other end of the plant, root hair cells are long, thin extensions that greatly increase the surface area for absorbing water and mineral salts from the soil.
Examples in context
Example 1. Wilting on a hot day. On a hot, windy day a plant transpires fast. If the roots cannot take up water quickly enough, the cells lose turgor and the plant wilts. The guard cells may then close the stomata to cut water loss, which also slows photosynthesis.
Example 2. Celery in dye. Standing a celery stalk in coloured water shows the xylem at work: after a few hours the dye has risen up the stalk through the xylem tubes, marking the path of the transpiration stream from the cut base up towards the leaves.
Try this
Q1. State what the xylem transports and in which direction. [1 mark]
- Cue. Water and dissolved minerals, upwards from roots to leaves.
Q2. Name the cells that open and close the stomata. [1 mark]
- Cue. Guard cells.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksCompare the structure and function of xylem and phloem in a plant.Show worked answer →
A 4-mark compare question needs structure and function for both tissues.
Xylem is made of dead, hollow tubes strengthened by rings of lignin. It carries water and dissolved minerals upwards from the roots to the leaves.
Phloem is made of living cells joined end to end with sieve plates between them, with companion cells alongside. It carries dissolved food (mainly sugar) in both directions through the plant.
Markers reward (1) xylem dead and lignified, (2) xylem carrying water and minerals up, (3) phloem living with sieve plates, and (4) phloem carrying sugar both ways.
SQA N5 style3 marksName three factors that increase the rate of transpiration and explain why one of them does so.Show worked answer →
Three factors are needed, with an explanation for one.
Increased wind speed, increased temperature, and decreased humidity (drier air) all increase the rate of transpiration. A larger leaf surface area also increases it.
For example, increasing the temperature gives the water molecules more energy, so water evaporates faster from the leaf surfaces and diffuses out through the stomata more quickly.
Markers reward three correct factors and a valid explanation of how one of them speeds up water loss.
Related dot points
- The four-chambered heart and the pathway of blood, the structure and function of arteries, veins and capillaries, and the components of blood including red and white cells, platelets and plasma.
An SQA National 5 Biology answer on transport systems in animals, covering the four-chambered heart and the pathway of blood, the structure and function of arteries, veins and capillaries, and the components of blood including red and white cells, platelets and plasma.
- The need for efficient exchange surfaces with a large surface area, and the adaptations of the alveoli for gas exchange in the lungs and of the villi for the absorption of nutrients in the small intestine.
An SQA National 5 Biology answer on the absorption of materials, covering the need for efficient exchange surfaces with a large surface area, and the adaptations of the alveoli for gas exchange in the lungs and of the villi for the absorption of nutrients in the small intestine.
- The word equation for photosynthesis, the two stages of the light reactions and carbon fixation, the uses of the sugar made, and the limiting factors that control the rate of photosynthesis.
An SQA National 5 Biology answer on photosynthesis, covering the word equation, the two stages of the light reactions and carbon fixation, the uses of the sugar made, and the limiting factors that control the rate of photosynthesis.
- Passive transport by diffusion and osmosis down concentration gradients, the effects of osmosis on animal and plant cells in hypertonic, hypotonic and isotonic solutions, and active transport against the gradient using energy from respiration.
An SQA National 5 Biology answer on transport across cell membranes, covering the selectively permeable membrane, passive transport by diffusion and osmosis, the effects of osmosis on animal and plant cells in different solutions, and active transport against the concentration gradient using energy from respiration.
- The stages of mitosis and its role in growth, repair and maintaining the diploid chromosome number, stem cells and meristems as sources of unspecialised cells, and the specialisation of cells into tissues, organs and systems.
An SQA National 5 Biology answer on producing new cells, covering the stages of mitosis and its role in growth and repair, the diploid chromosome number, stem cells and meristems as sources of unspecialised cells, and the specialisation of cells into tissues, organs and systems.
Sources & how we know this
- SQA National 5 Biology Course Specification — SQA (2019)