36 Leaf Structure and Photosynthesis

Key Idea: Leaves are the main site for photosynthesis in plants. Leaf structure aids the photosynthetic process.

` The main function of leaves is as photosynthetic organs in which light energy is captured for use in photosynthesis.

Leaves are green because they reflect the green wavelengths of light not involved in photosynthesis. ` The structure of leaves maximises the capture of sunlight energy and facilitates the diffusion of gases used and produced by photosynthesis into and out of the leaf tissue. Gases enter and exit the leaf through stomata (pores) in the leaf. Guard cells each side a stoma lose or gain water to control the size of the pore and regulate the movement of gases. Inside the leaf, large air spaces and the loose arrangement of the spongy mesophyll provides a large surface area for gas exchange. ` https://bit.ly/3TTDvgr

Dicot leaf structure

O2

CO2

CO2

O2

Entry and exit of gases through the stomata

CO2

O2

Net gas exchanges in a photosynthesising dicot leaf

Cuticle forms a barrier to the diffusion of gases Upper epidermis (lacks chloroplasts) Palisade mesophyll cell with chloroplasts Spongy mesophyll cell with chloroplasts Leaf vein (xylem and phloem) Substomatal air space Lower epidermis

Guard cell Stoma (pore)

3D

E

Ms Mp

SEM of black walnut leaf showing epidermis (E), palisade mesophyll (Mp), and spongy mesophyll (Ms) with its numerous air spaces.

` Respiring plant cells use oxygen (O2) and produce carbon dioxide (CO2). These gases move in and out of the plant and through the air spaces by diffusion. Flowering plants have many air spaces between the cells of the stems, leaves, and roots. These air spaces are continuous and gases are able to move freely through them and into the plant’s cells via the stomata (sing. stoma). ` The situation is more complex when the plant is photosynthesising. Overall there is a net use of CO2 and a net production of oxygen. CO2 uptake (in photosynthesis) maintains a gradient in CO2 concentration between the inside of the leaf and the atmosphere. Oxygen is produced in excess of respiratory needs and diffuses out of the leaf. These exchanges are indicated by the arrows on the diagram.

1. (a) How do gases enter and leave the leaf tissue?

(b) How is this movement regulated?

2. Identify the region of a dicot leaf where most of the chloroplasts are found:

3. What is the purpose of the air spaces in the leaf tissue?

4. The theoretical output of O2 from photosynthesis is the same as the theoretical input of O2 to respiration. Explain why there is a net output of O2 from plants.

The amount of light falling on a plant affects its ability to photosynthesise. A greater intensity of light enables a higher rate of photosynthesis, but increases the risk of the leaf drying out or burning. The leaves of plants are adapted to their environment (e.g. sun or shade) to maximise photosynthesis and minimise water loss and sun damage.

Sun plant Sun plant

A sun leaf, when exposed to high light intensities, can A sun leaf, when exposed to high light intensities, can absorb much of the light available to the cells. absorb much of the light available to the cells.

Palisade mesophyll layer Palisade mesophyll layer often 2-3 cells thick often 2-3 cells thick

Intense light Intense light Thick leaves Thick leaves

Shade plant Shade plant

A shade leaf can absorb the light available at lower A shade leaf can absorb the light available at lower light intensities. If exposed to high light, most would light intensities. If exposed to high light, most would pass through. pass through.

Low lightLow light intensityintensity Palisade mesophyll Palisade mesophyll layer, only 1 cell thick layer, only 1 cell thick

Thin leaves Thin leaves

Chloroplasts are mostly Chloroplasts are mostly restricted to palisade restricted to palisade mesophyll cells (few in mesophyll cells (few in spongy mesophyll). spongy mesophyll). Chloroplasts occur Chloroplasts occur throughout the mesophyll throughout the mesophyll (as many in the spongy as (as many in the spongy as in the palisade mesophyll). in the palisade mesophyll).

Sun leaves

Plants adapted for full sunlight have high levels of respiration. Sun plants include many weed species found on open ground. They expend much more energy on the construction and maintenance of thicker leaves than do shade plants. The benefit of this investment is that they can absorb the higher light intensities available and grow more quickly.

Shade leaves

Shade plants typically grow in forested areas, partly shaded by the canopy of larger trees. They have lower rates of respiration than sun plants, mainly because they build thinner leaves. Less energy is needed to produce and maintain a smaller number of cells.

5. Explain why plants have most of their leaves on the outer edges of the plant (rather than hidden within the branches):

6. (a) Compare the respiration rates of sun and shade plants:

(b) Compare the rate of photosynthesis between sun and shade plants and explain the consequences of this:

7. The diagram on the right shows the effects of the light environment on leaf size:

(a) What happens to leaf size as we move from wet and shady to sunny and dry?

(b) Why would plants produce small leaves in sunny, dry areas?

Sunny

Shady

Wet Dry