Part 1 · The structure of a leaf

Read the passage and look at the diagram.

Plants are organisms made of organs including the stem, roots and leaves, which carry out the tasks a plant needs to survive. Plant organs are made up of groups of tissues.

The leaf is an organ made of several tissues: a waxy cuticle (reduces water loss); the upper epidermis (transparent, lets light through); the palisade mesophyll (tightly packed cells with many chloroplasts near the top, for the most light); the spongy mesophyll (air gaps for gas exchange); the lower epidermis (has stomata, controlled by guard cells); and veins (xylem and phloem) that transport substances.

Part 2 · Xylem and phloem

Read the passage and look at the diagram.

Plants have two transport systems made of two tissues. Xylem transports water and dissolved mineral ions from the roots up to the leaves (transpiration). Xylem is made of dead cells joined end to end with no end walls and strengthened with lignin; water moves up in one direction.

Phloem transports dissolved sugars (sap) from the leaves to the rest of the plant (translocation). Phloem is made of columns of living cells with small pores in the end walls, and sugars move in both directions. Glucose is needed all over the plant for respiration, to build cellulose and other molecules, or to be stored as starch.

Part 3 · Observing stomata

Read the method and the calculation.

Scientists count stomata to investigate their distribution, recording the density per square millimetre (mm²). The microscope's field of view is known.

If the field of view has a diameter of 0.40 mm, the radius is 0.20 mm, so the area of the field of view is:

If 12 stomata are counted in that field of view, the density is:

Method

1. Paint the surface of the leaf with clear nail varnish.
2. Allow to dry.
3. Peel off the nail varnish with forceps / sellotape.
4. Place on a dry microscope slide and count the stomata.
5. Repeat and calculate a mean.
6. Calculate the area of the field of view.
7. Calculate the stomatal density.

Part 1 · The photosynthesis reaction

Read the passage and look at the diagram.

Plants and algae are producers - they make their own food (glucose) by photosynthesis. Photosynthesis uses light energy to convert carbon dioxide and water into glucose and oxygen. Water is absorbed through the roots and carbon dioxide diffuses into the leaves through the stomata; oxygen is a by-product that diffuses out.

Photosynthesis takes place in chloroplasts, which contain the green pigment chlorophyll. Chlorophyll absorbs the light energy needed.

Photosynthesis is an endothermic reaction because it absorbs energy (light) from the surroundings.

Part 2 · Leaf adaptations

Read the table of leaf adaptations.

Leaves are adapted for photosynthesis. The table shows how the plant is adapted:

Part 3 · Root hair cells

Read the passage and look at the diagram.

Roots absorb water (a reactant in photosynthesis) and dissolved mineral ions. Root hair cells, found near the tips of growing roots, are adapted to take up water and mineral ions efficiently.

Root hair cells have three main adaptations

1. They greatly increase the surface area available for water and ions to move into the cell.
2. They have a large permanent vacuole that speeds up the movement of water into the cell by osmosis.
3. They have many mitochondria that provide the energy needed for the active transport of mineral ions into the cell.

Root hair cells have no chloroplasts because they are underground and get no light for photosynthesis.

Part 1 · Transpiration and the transpiration stream

Read the passage and look at the diagram.

For a plant to photosynthesise it needs carbon dioxide, which enters through the stomata - so the stomata must be open. But when they are open, plants lose water vapour through them as it evaporates and leaves by diffusion. This loss of water vapour is called transpiration.

The transpiration stream is the constant movement of water through the xylem, from the roots up the stem to the leaves, where it evaporates through the stomata. Increasing the rate of photosynthesis increases the rate of transpiration.

Part 2 · Desert plant adaptations

Read about how desert plants are adapted, and look at the diagram.

Plants such as cacti that live in deserts (xerophytes) have special adaptations to reduce water loss by transpiration:

• Thick, waxy skin to reduce evaporation of water and to reflect heat.
• Large, fleshy stems to store water.
• Thorns or thin, spiky leaves to reduce the surface area for water loss.
• Fewer stomata to reduce evaporation of water.
• Stomata that open at night, when it is cooler, to reduce evaporation of water.
• Deep roots to reach water far underground, or long shallow roots to cover a large area and absorb more water.

Part 1 · Factors affecting transpiration rate

Read the sentence, then study the table.

Transpiration is the evaporation of water through the stomata, and several environmental factors affect its rate:

Part 2 · Measuring transpiration rate

Read about how transpiration rate is measured.

There are many ways to investigate the effect of different factors on the rate of transpiration in plants:

Two common methods

1. Weighing the mass of the plant before and after, in different conditions.
2. Using a potometer, which measures the water taken up by the shoot.

To measure the effect of the different factors, you can create different conditions around the leaves of the shoot:

You can only change one factor at a time; all the others must be controlled (kept the same) so that you know only the factor you changed had an effect. You should also do a control trial, where no factors are changed, so you can compare your results and see the effect of the factor on the rate.