Gas exchange in Plants

. 

2.2 Adaptations for Gas  exchange

angiosperm

ˈandʒɪə(ʊ)spəːm/

noun

BOTANY

1. a plant of a large group that comprises those that have flowers and produce seeds enclosed within a carpel, including herbaceous plants, shrubs, grasses, and most trees.

 The structure of the angiosperm leaf

The leaves of angiosperms must balance the conflicting needs of gas exchange and preventing desiccation. Angiosperm leaves come in a wide range of morphologies, suited to a wide range of habitats, but they share many adaptations. 

Leaf adaptation for light harvesting includes a large surface area and the ability to move by growth to the best position.

Plants rely entirely on diffusion for the exchange of gases. Leaves are therefore thin to shorten distances for diffusion and have a large surface area and are permeated by air spaces.

Epidermis covers the outside of the leaf, top and bottom, and secretes a waxy cuticle that reduces evaporation. (On many slides, the cuticle is difficult or impossible to see.) Leaves have a cuticle to prevent water loss which also reduces gaseous exchange.

Stomata (singular: stoma) are openings in the epidermis, allowing for gas exchange and transpiration. 

The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening. 

Air containing carbon dioxide and oxygen enters the plant through these openings and is used in photosynthesis in the mesophyll cells and respiration, respectively. 

Oxygen produced as a by-product of photosynthesis diffuses out to the atmosphere through these same openings. Also, water vapor is released into the atmosphere through these pores in a process called transpiration.

Guard cells can open or close the stomata, allowing Carbon dioxide in when needed but shutting to prevent water from leaving the plant in hot dry weather. 

How do Guard cells open and close?

The opening and closing of the stomata is controlled by the guard cells (stomatal rhythm)- 

In light, guard cells take up water by osmosis and become turgid. Because their inner walls are rigid they are pulled apart, opening the pore. In darkness water is lost and the inner walls move together closing the pore. 

The pores are OPEN during the day (allowing carbon dioxide in for photosynthesis and water vapour out for transpiration). 

Photosynthesis allows this opening by- 

• Photosynthesis produces ATP.
• ATP allows active transport of potassium ions into the guard cells (lowers the water potential).
• Starch within the plants is converted to soluble malate (lowers the water potential too)
• Water flows in via osmosis.

At night, the opposite is true, so the guard cells shut. 

Mesophyll cells fill the middle of the leaf and perform photosynthesis. They are packed with chloroplasts. "Mesophyll" means "middle of the leaf." 

• Palisade mesophyll cells are elongated elongated and densely arranged in a layer, or layers, and they contain many chloroplasts which arrange themselves according to the light intensity. The choloroplasts are efficiently exposed to sunlight using cytoplasmic streaming.  The chloroplasts are only exposed to the light for just enough time to get the energy required for photosynthesis. They then move out of the way in order for other chloroplasts to receive light. When they have used up the energy received they have moved up to the top of the cell again and can get more energy. 

The spongy mesophyll cells are surrounded by air spaces allowing diffusion of gases to occur. Light can pass through to the spongy mesophyll. The spaces between mesophyll cells allow carbon dioxide to diffuse to the cells and oxygen can diffuse away. The cells are moist so gases can dissolve.

Flowering plant leaves normally have multiple veins, or vascular bundles. The vascular bundles contain xylem and phloem.

Xylem transports water and inorganic nutrients from the roots up to the rest of the plant; xylem cells are thick-walled and reinforced with lignin to withstand the necessary pressure. Xylem cells are typically stained red in prepared microscope slides.

Phloem transports the sugars that are produced in photosynthesis from the leaves to the rest of the plant. Phloem cells are small and thin-walled; in many others, the phloem cells appear blue.

The role of leaf structures in allowing the plant to function and photosynthesise effectively.

29.
30..
31. Guard cells around the stomata can change shape to open and close the stomata so helping to control gas exchange and water loss.
32. Guard cells change shape because of changes in turgor; in the light, water flows in by osmosis so the cells expand.
33. The inner wall is inelastic so the pairs of cells curve away from each other and the pore opens.
34. Pores close due to the reverse process.

35. There are several theories about the mechanism and opening is affected by changing CO2 levels, but only the ‘malate’ theory is required. The movement of potassium ions from the epidermal cells into the guard cells creates a negative water potential in the guard cells. Water moves in by osmosis. 36. The movement of potassium ions is an active process requiring ATP.
37. Xerophytes may open stomata at night instead of during the day in order to conserve water, whilst other plants may close stomata during the day or night under drought conditions.

The leaf as an organ of gaseous exchange.

Stomatal opening and closing.




























Some of this material has been copied from Brian MacCauley's page at DeAnza College http://www.deanza.edu/faculty/mccauley/6a-labs-plants-04.htm