Plant Tissues and Transpiration

A tissue is a group of similar cells working together; an organ is several tissues working together to carry out a function. The leaf is a plant organ whose main job is photosynthesis, and it is built from a small set of named tissues.

For AQA, the leaf tissues you need are: the epidermis (upper and lower), the palisade mesophyll, the spongy mesophyll, the xylem and phloem (in the veins), and the guard cells surrounding the stomata.

A tissue is similar cells with a shared function. An organ is several tissues grouped to perform a function. The leaf is an organ; xylem and phloem are tissues inside it.

Every tissue's structure matches its function. The palisade cells sit near the top to catch light first; the spongy layer is full of air spaces so gases can diffuse; the epidermis is transparent to let light through to the cells below.

Tissue	Location in the leaf	Function and key adaptation
Upper epidermis	Top surface	Transparent, no chloroplasts — lets light pass through to the mesophyll; waxy cuticle on top reduces water loss
Palisade mesophyll	Just below upper epidermis	Tall column-shaped cells packed with chloroplasts — most photosynthesis happens here
Spongy mesophyll	Middle/lower part of leaf	Loosely packed with large air spaces — lets CO₂ and O₂ diffuse to and from cells
Xylem	In the veins	Carries water and mineral ions into the leaf
Phloem	In the veins	Carries dissolved sugars away from the leaf
Lower epidermis	Bottom surface	Contains the stomata and guard cells that control gas exchange
Guard cells	Surround each stoma	Open and close the stomata to control gas exchange and water loss

Meristem tissue is different: it is found at the growing tips of shoots and roots and contains cells that keep dividing to make new cells, so the plant can grow. It is not part of the finished leaf structure but is on the spec as a plant tissue.

The roots, stem and leaves together form a plant organ system for transporting substances around the plant. Two transport tissues do the carrying, and they work in opposite directions.

Xylem is made of dead, hollow cells joined end to end into continuous tubes. The end walls break down, the cells lose their contents, and the walls are strengthened with a tough waterproof material called lignin. This makes xylem a strong, open pipe — ideal for moving water upwards.

Phloem is made of living cells arranged into tubes of elongated cells. Cell sap can move from one phloem cell to the next through pores in the end walls, which lets it carry dissolved food in either direction.

Feature	Xylem	Phloem
What it transports	Water and mineral ions	Dissolved sugars (food)
Direction	Roots → stem → leaves (upward only)	Leaves → rest of plant (both ways)
Process name	Transpiration stream	Translocation
Cells	Dead, hollow	Living, elongated
Strengthening	Walls thickened with lignin	No lignin; pores in end walls

Water goes UP the xylem; sugars move BOTH WAYS in the phloem. Xylem is dead and hollow; phloem is living.

(Extra context — the detailed structure of phloem and the mechanism that drives translocation are not required by AQA 8461.)

Water enters the plant through the roots. The outer cells of the root grow long thin extensions called root hairs. These give each cell a huge surface area in contact with the soil water, so absorption is fast.

Two different processes bring substances in:

• Water enters by osmosis — it moves from the soil (high water concentration) into the cell (lower water concentration) down the gradient, using no energy.
• Mineral ions (such as nitrate) are usually in low concentration in the soil, so they must be taken in against their gradient by active transport, which requires energy from respiration.

Adaptations of a root hair cell:

Adaptation	Why it helps
Long, thin projection	Large surface area for absorbing water and ions
Thin cell wall	Short distance for water to cross
Many mitochondria	Release energy for active transport of mineral ions

From the root, water passes into the xylem and begins its journey up the plant in the transpiration stream.

Transpiration is the loss of water vapour from a plant. Water evaporates from the surfaces of the spongy mesophyll cells inside the leaf and then diffuses out through the open stomata as water vapour. As water leaves the top of the plant, more is pulled up the xylem to replace it. This continuous flow of water from roots to leaves is the transpiration stream.

The transpiration stream also delivers the mineral ions the plant absorbed at the roots up to the leaves.

Transpiration = evaporation of water from the leaf cells followed by diffusion of water vapour out through the stomata. It pulls the whole transpiration stream up the xylem.

Do not confuse transpiration with translocation: transpiration moves water up the xylem; translocation moves sugars through the phloem.

Anything that makes water evaporate or diffuse out of the leaf faster speeds up transpiration. Four factors are on the spec. Work through each by asking: does this increase evaporation or steepen the diffusion gradient of water vapour leaving the leaf?

Factor	Change	Effect on rate	Reason
Temperature	Increase	Faster	Water molecules gain energy and evaporate more quickly; diffusion out of the leaf speeds up
Air movement (wind)	Increase	Faster	Wind blows away the humid air at the leaf surface, keeping the diffusion gradient steep
Humidity	Increase	Slower	Moist air outside reduces the gradient between leaf and air, so less water diffuses out
Light intensity	Increase	Faster	Brighter light makes the stomata open wider for photosynthesis, so more water escapes

Worked prediction — a plant is moved from a still, cool, humid greenhouse to a warm, breezy, sunny field. Predict the change and explain it.

• Temperature rises → faster evaporation.
• Wind increases → humid air swept away, gradient stays steep.
• Humidity falls → steeper gradient out of the leaf.
• Light increases → stomata open wider.

All four changes push the same way, so the rate of transpiration increases sharply. Each step is justified by a steeper gradient or faster evaporation, not just "because it is warmer".

A potometer measures water uptake, which is used as an estimate of transpiration rate. As the plant draws up water, an air bubble moves along a capillary tube; the distance it travels in a known time gives the rate.

Data: the bubble moves 60 mm in 5 minutes.

Rate of transpiration as distance per minute:

$\text{rate}=\frac{\text{distance moved}}{\text{time}}=\frac{60\text{mm}}{5\text{min}}=12\text{mm/min}$

To express this as a volume of water, multiply the distance by the cross-sectional area of the tube. For a tube of radius $0.5\text{mm}$:

$A=\pi r^2=\pi \times (0.5{)}^2\approx 0.785{\text{mm}}^2$

$\text{volume}=60\text{mm}\times 0.785{\text{mm}}^2\approx 47.1{\text{mm}}^3$

$\text{volume rate}=\frac{47.1{\text{mm}}^3}{5\text{min}}\approx 9.4{\text{mm}}^3/\text{min}$

So the plant takes up water at about 12 mm/min along the tube, equal to roughly 9.4 mm³/min by volume.

To compare two conditions fairly, change only one factor at a time and reset the bubble between readings. Faster bubble movement means a faster transpiration rate.

The plant faces a problem: stomata must open to let carbon dioxide in for photosynthesis, but every time they open they also let water vapour out. The guard cells manage this trade-off by opening and closing the stomata.

How it works:

• In the light, guard cells take in water and become turgid (swollen). Their uneven walls make them bend, opening the stoma for gas exchange.
• In the dark or when short of water, guard cells lose water and become flaccid, so the stoma closes, reducing water loss.

Most stomata are on the lower surface of the leaf, away from direct sun, which helps limit water loss.

The role of stomata and guard cells is to control gas exchange and water loss. Open in light for photosynthesis; close in the dark or drought to conserve water.

1. Saying xylem is alive

Xylem cells are dead and hollow, with walls strengthened by lignin. It is the phloem that is made of living cells. Mixing these up is one of the most common errors.

2. Getting the directions the wrong way round

Xylem carries water and mineral ions upward only, from roots to leaves. Phloem carries dissolved sugars both ways (translocation). Do not write that phloem only goes down.

3. Confusing transpiration with translocation

Transpiration = water moving up the xylem and evaporating from leaves. Translocation = sugars moving through the phloem. The similar-sounding names trap students.

4. Saying higher humidity speeds up transpiration

Higher humidity slows transpiration, because moist air outside the leaf reduces the diffusion gradient. Only temperature, wind and light increase the rate.

5. Explaining factors without mentioning the gradient

A factor "increases the rate" is not enough. Explain why — usually because evaporation is faster or the diffusion gradient of water vapour out of the leaf is steeper.

6. Forgetting root hair cells use two different processes

Water enters by osmosis (passive); mineral ions enter by active transport (needs energy). Do not say both enter the same way.