Gas exchange in Plants Leaf’s structure Stomata Mechanisms of opening of the stomata

Cross-section of a leaf upper epidermis protect internal tissues from mechanical damage and bacterial and fungal invasion

Cross-section of a leaf Cuticle a waxy layer prevent water loss from the leaf surface

Cross-section of a leaf mesophyll

Cross-section of a leaf palisade mesophyll contains many chloroplasts columnar cells closely packed together to absorb light more efficiently

Cross-section of a leaf Irregular cells loosely packed together to leave numerous large air spaces to allow rapid diffusion of gases throughout the leaf Cross-section of a leaf spongy mesophyll (moist)

Cross-section of a leaf same as upper epidermis except the cuticle is thinner lower epidermis

Cross-section of a leaf Stoma (pl. Stomata) opening which allows gases to pass through it to go into or out of the leaf

Cross-section of a leaf Stomata also let water out during respiration. This process is called transpiration

Cross-section of a leaf guard cells control the size of stoma

Cross-section of a leaf vascular bundle (vein)

Cross-section of a leaf xylem to transport water and mineral salts towards the leaf

Cross-section of a leaf phloem to transport organic substances away from the leaf

Xerophytes Have Special Adaptations to Reduce the Rate of Transpiration Xerophytes are plants which live in hot, dry environments. These cacti have reduced leaf area as the leaves are reduced to spines Fleshy stems to hold water Silver surface to reflect sun

Adaptations to Reduce Water Loss in Xerophytes Thick waxy cuticle to reduce evaporation Reduced leaf area e.g. needles, spines Hairy leaves:- the hairs trap a layer of saturated air Sunken stomata:- the pits above the stomata become saturated Rolled leaves:- this reduces the area exposed to the air and keeps the stomata on the inside so increasing the water vapour inside the roll Increasing the water vapour around the stomata reduces the water potential gradient so slows water loss

Number Name of part Job of part 1 Upper epidermis Protection 2 Cuticle Waterproof layer 3 Stomata Movement of gases 4 Mesophyll Photosynthesis 5 Palisade cells 6 Spongy cells Allows movement of gases 7 Phloem Transport of food 8 Xylem Transport of water and minerals

The guard cells control the opening and closing of the stomata Guard cells flaccid (lack of water) Guard cells turgid (plenty of water) Thin outer wall Thick inelastic inner wall Stoma closed Stoma open

We think that light, water availability and carbon dioxide concentration control the opening of the stomata. Guard cells flaccid (lack of water) Guard cells turgid (plenty of water) Thin outer wall Thick inelastic inner wall. It’s fundamental for the movement Stoma closed Stoma open

Graph to show stomatal opening over 24 hours Some plants close stomata during hottest time-saving water 100 Increased light intensity causes more stomata to open stomatal opening/% An adaptation to hot dry environments Stomata close as the sun sets Dawn-stomata begin to open 12 2 4 6 8 10 12 2 4 6 8 10 12

Conclusions: Stomata are closed when there isn’t much water in the guard cells. Water gets inside the guard cells via osmosis These become turgid and because of their inelastic inner wall they curve and open the stoma. This process is influenced by water availability, carbon dioxide concentration and light intensity. The question is.... How do plants regulate the stomatal movement?

Regulating Stomatal Opening: Potassium ion pump - hypothesis Guard cells flaccid K+ K+ ions have the same concentration in guard cells and epidermal cells K+ K+ K+ K+ K+ K+ K+ Light activates K+ pumps which actively transport K+ from the epidermal cells into the guard cells K+ K+ K+ K+ K+ pumps use ATP produced during photosynthesis. It’s not a FREE process for the plant! Stoma closed

Regulating Stomatal Opening: Potassium ion pump - hypothesis H2O Increased concentration of K+ in guard cells Lowers the  (water potential) in the guard cells K+ K+ Water moves in by osmosis, down  (water potential) gradient K+ K+ K+ K+ K+ Stoma open K+ H2O K+ K+ K+ K+ K+ Stoma closed

To make sure that the guard cell don’t gain too many K+ and become too positive (therefore too turgid), some negative ions (anions) must be produced. Starch which is found in the cells is then converted into malate which is an anion. This process re-equilibrate the conditions in the guard cells.