Gas Exchange In plants

All organisms need to ‘breathe.’ PLANTS ARE NO DIFFERENT but they ‘breathe’ or respire at night and photosynthesise during the day. Some of the oxygen produced in photosynthesis is used in cellular respiration, the rest is diffused out. Therefore in plants oxygen and carbon dioxide are both waste products AND nutrients

Where does exchange occur in plants…. Small plants have thin leaves - one cell thick so gas exchange can occur directly with the external environment by diffusion. More complex plants – vascular plants – they exchange gases through small openings in the epidermis, or outer layer of their leaves, stems and roots, called STOMATA. The stomata regulate the rate of gas exchange. If the stomata are closed the gas exchange virtually stops altogether. Plant cells are packed loosely – allows rapid diffusion through the extracellular spaces

More about stomata.. Tiny pores in the epidermis. Bordered by guard cells, and as the name implies they guard the ‘hole’ or are involved in opening and closing the pore. The size and number of stomata vary according to the plant species. Generally found on the underside of the leaf

Take a careful look on pg 145 Guard cells – these are joined in pairs at the ends. Have thicker walls Have inelastic fibres around the cell walls to prevent them from expanding and opening when they need to be closed. Water passes into the guard cells and they become turgid or “full and firm” buckle and open the stomata: ~ this means to open, there is lots of light, water and low internal carbon dioxide levels ~ At night then, they usually close as the need for such rapid and volumous gas exchange is reduced. Aquatic plants differ in that oxygen often diffuses from Arial plants of the plant- mangroves

TRANSPORT SYSTEM IN PLANTS Plants need water, light and carbon dioxide to photosynthesise. We have already said simple plants take all this in by simple diffusion. More complex plants have a transport system – both for water they need and the sugars they produce as a result of photosynthesis

Page 161 Xylem – transports water and nutrients Phloem – transports the sugars Both called vascular tissue Take own notes in a little more detail on pages 162/163 (simple, in dot points)

Roots In a plant these are essential for the survival for the following reasons: ~ they provide stability ~ Increase the amount of nutrients etc which can be uptaken by the increased surface area with many fine root hairs ~ can access ground water if levels are low externally

Transpiration The movement or loss of water from the leaves No energy required Evaporation – driven by the sun Rate is dependent on temperature, wind, available surface area and humidity – we look at this more in Arid zones and ecosystems

Goodluck for your exams Year 11 ~ Ms Hillebrand 