Exchange in plants Text p.228

Homework White book p103,104 gas and solute exchange, 108 water flow through plants

Objectives Stomata and gas exchange in a leaf Root hairs and function Transpiration Factors affecting transpiration

There are small openings called stomata in the leaves of a plant. AQA Science © Nelson Thornes Ltd 2006  4

Stomata These open to allow gases in and out for photosynthesis and respiration. But at the same time water is lost by evaporation. AQA Science © Nelson Thornes Ltd 2006  5

Stomata Label a diagram of a stoma Time allowed… Could show stomata video in lesson folder

Lower epidermis cell Guard cell Pore for gas diffusion Uneven celulose cell wall nucleus chloroplast vacuole

How do guard cells work? When the cell is turgid the uneven cellulose cell wall causes it to become curved so it opens the pore When the cell is flaccid the cell is not curved and the pore closes

Stomata are the pores in leaves Function Open during the day to allow CO2 to enter for photosynthesis This is not ideal as the plant loses water vapour so they close at night. There is enough oxygen in the air spaces for the plant to respire through the night Other ways a plant has to minimise water loss are: waxy cuticle on upper surface of leaves Wilting (less contact with solar heat so less evaporation)

Gas exchange in a leaf The flattened shape of a leaf, and internal air spaces increase the surface area of the leaves. Large area for diffusion of carbon dioxide in and oxygen out For photosynthesis

Why are stomata on the underside of leaves? Less solar radiation Less wind Therefore less water loss Not clogged by dust Less chance of infection by airborne microbes

that most of the water and mineral ions are absorbed by root hair cells

Root hairs Label the diagram Ans q2 p229 Cellulose cell wall nucleus Cell membrane Label the diagram Ans q2 p229 cytoplasm vacuole Root hair extension to increase surface area

Q2 p229 thin, large surface area, root hairs, short diffusion distance, mitochondria and protein pumps for active transport in their membranes Large surface area, moist, small distances for diffusion, plants are not as effective in maintaining their concentration gradients, but have plenty of active transport systems to help them out

Summary Extension – what is transpiration? Why can it be described as a necessary evil?

Plants take in water through their roots in the soil… Transpiration Plants take in water through their roots in the soil…

…it moves up through the plant… Transpiration …it moves up through the plant… AQA Science © Nelson Thornes Ltd 2006  17

…..and is lost through the leaves in the transpiration stream. Corel 178(NT) …..and is lost through the leaves in the transpiration stream. AQA Science © Nelson Thornes Ltd 2006  18

Transpiration AQA Science © Nelson Thornes Ltd 2006  19

Rate of transpiration When it is hot and dry, photosynthesis and respiration take place quickly. As a result, plants also very quickly lose water. AQA Science © Nelson Thornes Ltd 2006  20

Graph of bubble movement against time. Transpiration rate Graph of bubble movement against time. AQA Science © Nelson Thornes Ltd 2006  21

The steeper the graph, the faster the transpiration rate! AQA Science © Nelson Thornes Ltd 2006  22

Which potometer experiment was run in drier air? Transpiration rate Which potometer experiment was run in drier air? AQA Science © Nelson Thornes Ltd 2006  23

The drier the air, the faster the transpiration rate! AQA Science © Nelson Thornes Ltd 2006  24

Factors affecting transpiration stream write how and why each affects it Light Humidity Wind speed Temperature

Transpiration (sounds like…) Perspiration (=sweating) The evaporation of water from the leaves As water evaporates more is pulled up through the xylem vessels A plant has to open its stomata to gain carbon dioxide for photosynthesis, but also loses water through transpiration through them Stomata close at night to prevent water loss by transpiration When it is hot and dry, photosynthesis and respiration take place quickly. As a result, plants also very quickly lose water.

Extension - Adaptations for plants growing in a dry environment Curled leaves. Stomata on the curled side only. Moist air trapped inside the curl. Thick waxy cuticle. AQA Science © Nelson Thornes Ltd 2006  27

Adaptations for plants growing in a dry environment Corel 46(NT) Marram grass leaves will even uncurl slightly in the wet and curl up more in the dry. AQA Science © Nelson Thornes Ltd 2006  28