What you'll learn
- How root hair cells absorb water and mineral ions.
- How xylem and phloem are adapted for transport.
- How stomata control transpiration and water movement.
- How environmental factors affect water uptake, and how to calculate rates.
The big picture: why plants need transport
Transport means moving substances from one part of an organism to another. Plants need transport because they are multicellular: a leaf cell making sugar is far away from a root cell absorbing water.
Photosynthesis is the process where plants use light energy to make glucose from carbon dioxide and water. Some glucose is converted into sucrose, a soluble sugar that can be moved around the plant.
Two transport jobs
Xylem carries water and mineral ions from roots to leaves, while phloem carries sucrose from leaves to places where it is needed or stored.
This overview shows the main routes through the plant.
Root hair cells: absorbing water and mineral ions
A root hair cell is a specialised cell on the root surface. The long “hair” shape gives it a large surface area for absorbing substances from the soil. An adaptation is a feature that helps a structure do its job.
Mineral ions are charged particles dissolved in soil water. Examples include nitrate ions, needed to make amino acids and proteins, and magnesium ions, needed to make chlorophyll.
Osmosis and active transport
- Osmosis is the net movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane, which allows some substances through but not others.
- Active transport is movement of substances from a lower concentration to a higher concentration, against a concentration gradient, using energy released in respiration. A concentration gradient is a difference in concentration between two areas.
Root hair cells are adapted by having:
- a long extension, giving a large surface area for absorption
- a thin cell wall, giving a short distance for water to move
- a large vacuole with concentrated cell sap, helping water enter by osmosis
- mitochondria, which release energy for active transport of mineral ions
Explaining mineral ion uptake
- Compare the concentrations: if nitrate ion concentration is lower in the soil than inside the root hair cell, nitrates will not diffuse into the cell overall.
- The required movement is from lower concentration to higher concentration, so it is against the concentration gradient.
- Therefore the root hair cell uses active transport, requiring energy from respiration; its large surface area helps more ions enter per second.
Xylem and phloem: plant transport tissues
A tissue is a group of similar cells working together. Plants have transport tissues called xylem and phloem.
Xylem
Xylem is tissue that transports water and mineral ions from the roots up through the stem to the leaves. Xylem vessels are made from dead cells joined end to end, forming hollow tubes.
The walls are strengthened with lignin, a waterproof strengthening material. Lignified xylem walls help stop the vessel collapsing as water is pulled upwards.
Phloem
Phloem is living tissue that transports sucrose around the plant. This movement is called translocation.
Translocation
Translocation is the movement of sucrose in phloem from sources, such as leaves where sucrose is made, to sinks, such as growing shoots, roots, fruits or storage organs where sucrose is used or stored.
Phloem cells are living and use energy to move sucrose. Unlike xylem flow, translocation can happen up or down the plant, depending on where sucrose is needed.
Mixing up xylem and phloem
Do not say xylem carries sucrose. Xylem carries water and mineral ions in dead, lignified vessels; phloem carries sucrose in living cells and uses energy.
Identifying xylem from structure
- A tube with no end walls forms a continuous hollow pathway, which suits moving a column of water through the plant.
- Thick lignified walls are a xylem feature because lignin strengthens the vessel and helps prevent collapse.
- So the tissue is xylem, and its function is transporting water and mineral ions from roots towards leaves.
Transpiration and stomata
Transpiration is the loss of water vapour from the leaves, mainly through tiny pores called stomata. One pore is a stoma. Water vapour is water in gas form.
Here is the sequence:
- Water moves from the xylem into leaf cells.
- Water evaporates from moist cell surfaces inside the leaf.
- Water vapour diffuses out through the stomata. Diffusion is the net movement of particles from a higher concentration to a lower concentration.
- This creates a pull that draws more water up the xylem from the roots.
The movement of water and mineral ions up the plant in xylem is called the transpiration stream.
Guard cells are pairs of cells around each stoma. They open stomata to allow gas exchange, but this also increases water loss. When stomata close, less water vapour escapes.
Transpiration pull
Water loss from the leaf creates a pull on the water column in xylem, so water and dissolved mineral ions are drawn upwards from the roots.
Leaf adaptations for photosynthesis and gas exchange
If you are taking Separate Biology, this is assessed directly in spec point 6.11B. If you are doing Combined Science, it is still useful context for transpiration.
A leaf is adapted for photosynthesis and gas exchange. Gas exchange means swapping gases with the surroundings: carbon dioxide enters, while oxygen and water vapour leave.
Key adaptations include:
- a broad, flat lamina or leaf blade, giving a large surface area for light absorption
- a thin shape, giving a short diffusion distance for gases
- a transparent upper epidermis, allowing light to reach photosynthetic cells
- palisade mesophyll cells near the top with many chloroplasts; chloroplasts contain chlorophyll for absorbing light
- spongy mesophyll with air spaces, allowing gases to diffuse through the leaf
- stomata, often mainly on the lower surface, controlled by guard cells
- veins containing xylem and phloem for transport
This cross-section shows how these structures fit together.
Linking air spaces to gas exchange
- Carbon dioxide must move from the outside air to photosynthesising cells inside the leaf.
- Air spaces in the spongy mesophyll create pathways through the leaf and increase the internal surface area for diffusion.
- This helps carbon dioxide reach leaf cells quickly, while oxygen and water vapour can diffuse out.
Environmental factors affecting water uptake
A rate means how much something changes per unit time. A higher transpiration rate usually means a higher rate of water uptake by the roots.
Light intensity
In brighter light, stomata usually open so carbon dioxide can enter for photosynthesis. Open stomata allow more water vapour to diffuse out, so transpiration increases.
Air movement
Moving air removes humid air from around the leaf surface. Humid air contains a lot of water vapour. Removing it keeps a steep concentration gradient, so water vapour diffuses out faster.
Temperature
At higher temperatures, water evaporates faster from leaf cell surfaces and particles diffuse faster. This increases transpiration. Very high temperatures may cause stomata to close to reduce water loss and prevent wilting, which is drooping because cells lose water and become less firm.
Wind does not suck water out
Wind increases transpiration mainly by removing the humid layer of air around the leaf, maintaining a steep diffusion gradient for water vapour.
Explaining faster water uptake in moving air
- Moving air removes water vapour from around the stomata, so the air outside the leaf becomes less humid.
- The concentration gradient for water vapour between the inside and outside of the leaf becomes steeper.
- Water vapour diffuses out faster, increasing transpiration pull, so more water is taken up by the roots per minute.
Measuring and calculating rate of water uptake
A potometer is apparatus used to estimate transpiration rate by measuring water uptake. It follows the movement of an air bubble in a capillary tube, which is a very narrow tube.
A good method is:
- Cut the shoot under water to prevent air entering the xylem.
- Fill the apparatus with water and make an airtight seal around the shoot.
- Introduce one air bubble and record its starting position.
- Measure the distance moved in a set time.
- Change one environmental factor at a time, while controlling the others, and repeat.
Potometers estimate transpiration
A potometer measures water uptake, not water loss directly. It is a good estimate because most water taken up replaces water lost by transpiration, but a small amount is used in photosynthesis and keeping cells firm.
For rate calculations:
rate=distance movedtime taken\text{rate}=\frac{\text{distance moved}}{\text{time taken}}rate=time takendistance movedIf the question gives the cross-sectional area of the capillary tube, you can calculate volume first:
volume=cross-sectional area×distance moved\text{volume}=\text{cross-sectional area}\times\text{distance moved}volume=cross-sectional area×distance moved rate=volumetime taken\text{rate}=\frac{\text{volume}}{\text{time taken}}rate=time takenvolumeCalculating rate of water uptake
A bubble moves 4.8 cm in 6.0 minutes. The capillary tube has a cross-sectional area of 0.030 cm². Calculate the volume of water taken up per minute.
- Calculate the volume of water taken up using the tube area and bubble distance: V=0.030 cm2×4.8 cm=0.144 cm3V=0.030\ \text{cm}^2 \times 4.8\ \text{cm}=0.144\ \text{cm}^3V=0.030 cm2×4.8 cm=0.144 cm3.
- Substitute into the rate equation: rate=0.144 cm36.0 min\text{rate}=\frac{0.144\ \text{cm}^3}{6.0\ \text{min}}rate=6.0 min0.144 cm3.
- Divide by the time: rate=0.024 cm3 min−1\text{rate}=0.024\ \text{cm}^3\ \text{min}^{-1}rate=0.024 cm3 min−1.
Units sanity check
If your time is in minutes, your rate should be “per minute”; if your time is in seconds, your rate should be “per second”.
In the exam
- For adaptation questions, always link structure → function → benefit.
- Keep xylem and phloem separate: xylem is dead and lignified; phloem is living and transports sucrose using energy.
- In transpiration questions, explain the diffusion gradient for water vapour rather than just saying “it goes faster”.
- For rate calculations, identify whether you are using distance moved or volume taken up, then divide by time.
Check yourself
- Why do root hair cells need energy to absorb some mineral ions?
- How does closing stomata affect transpiration pull in the xylem?
- Why does moving air increase the rate of water uptake by a plant?
