Leaf structure and factors affecting photosynthesisIB MYP Sciences: Revision notes

A leaf is adapted to carry out photosynthesis. From the top to the bottom, its layers are:

• Waxy cuticle: a waterproof layer that reduces water loss by evaporation
• Upper epidermis: a thin, transparent layer of cells that lets light through
• Palisade layer: tightly packed, tall cells with many chloroplasts; the main site of photosynthesis, near the top where there is the most light
• Spongy mesophyll: loosely packed cells with air spaces, so gases can diffuse through the leaf
• Lower epidermis with stomata (pores) that let gases in and out

The leaf is thin and flat, giving a large surface area to absorb light and a short distance for gases to diffuse.

Stomata (one is a stoma) are tiny pores, mostly on the lower surface of the leaf. Each stoma is surrounded by two guard cells.

• Carbon dioxide diffuses in through the stomata for photosynthesis
• Oxygen and water vapour diffuse out

When the guard cells take in water they become swollen (turgid) and curve, so the stoma opens. When they lose water they become limp and the stoma closes, which reduces water loss, for example in a hot, dry place.

Guard cells also contain chloroplasts, unlike the other epidermal cells.

The veins in a leaf contain two transport tissues:

• Xylem: carries water and minerals up from the roots to the leaf. Water is needed for photosynthesis and to keep cells firm.
• Phloem: carries sugars (made in photosynthesis) away from the leaf to other parts of the plant, such as roots, growing tips and fruits. This is called translocation.

Veins also support the leaf and keep it spread out so it can absorb light.

A limiting factor is the factor in shortest supply, which restricts the rate of photosynthesis. The three main ones are:

• Light intensity: less light means less energy absorbed by chlorophyll
• Carbon dioxide concentration: less carbon dioxide means fewer molecules to react
• Temperature: low temperatures slow the enzymes; above about 40 °C the enzymes are denatured and the rate falls quickly

If you increase the limiting factor, the rate goes up, until a different factor becomes limiting.

Farmers use this in glasshouses by adding lamps, extra carbon dioxide and heating.

On a graph of rate against a factor (for example light intensity):

1. The line rises steeply: the factor on the x-axis is the limiting factor.
2. The line levels off (a plateau): the factor on the x-axis is no longer limiting. Something else is in shortest supply, for example carbon dioxide or temperature.
3. For temperature, the line rises, reaches a peak, then falls as the enzymes are denatured.

To describe a graph, quote data: 'the rate increased from 12 to 40 units as light intensity increased from 0 to 50 units, then stayed constant'.

A common investigation uses pondweed in water. Oxygen bubbles are released from the cut stem.

• Independent variable: for example the distance of a lamp (light intensity)
• Dependent variable: the number of bubbles per minute (the rate)
• Control variables: temperature, concentration of sodium hydrogencarbonate (a source of carbon dioxide), same pondweed, same time

Evaluation: counting bubbles is inaccurate, as they vary in size. Collecting the gas in a gas syringe or measuring tube is more accurate. The lamp may heat the water, so use a water bath or an LED lamp. Repeat and calculate a mean.