Cycling of Matter in Living Systems 3.3 The Leaf Tissue and Gas Exchange

Dermal Tissue The epidermis on the top and underside of the leaf is clear and very thin Guard cells form tiny openings or pores called stomata allow gas exchange to occur Stomata regulates the movement of gases and open into air chambers that connect with the cells of the ground tissue Carbon dioxide and oxygen can enter and leave the leaf by diffusion at any time. Most stomata are located on the underside of the leaf

Guard cells function to allow materials in and out when necessary Protect the leaves from losing too much water The process of water leaving the leaf through the stomata is called transpiration Without guard cells transpiration could dangerously dehydrate a plant The number of stomata vary depending on environmental conditions: hot, dry climate = few stomata, high humidity = many stomata

Ground Tissue Between the upper and lower epidermis are specialized ground tissues called mesophyll There are two different types of mesophyll tissues

Palisade Tissue Cell Found just below the upper epidermis Long, rigid, rectangular cells that are tightly packed together and arranged so that a large number of cells are exposed to the Sun’s rays. Responsible for photosynthesis Require carbon dioxide and produce oxygen

Spongy Mesophyll Tissue Located between the palisade tissue cells and the lower epidermis Loosely packed irregularly shaped cells Increased space allows for gas exchange by diffusion throughout the leaf Move oxygen towards the stomata or expulsion from the plant and will move carbon dioxide towards the palisade cells

Vascular Tissue Provides the leaf with the water needed for transpiration and photosynthesis and removes the sugars formed The “ribs” running through the leaf, called veins, contain the vascular tissue of the leaf. The xylem and phloem tissues are bunched together like a handful of straws in a vascular bundle

Xylem transports water necessary for photosynthesis and dissolved salts from the roots to the leaf Phloem transports the sugar manufactured in photosynthesis to the rest of the plant The vascular bundles are direct extensions of the vascular bundles of the stem They branch into finer veins within the spongy mesophyll

Gas Exchange in Plants All gas exchange occurs by diffusion Diffusion occurs through air spaces and then across cell membranes Homework: Check and Reflect p. 314 # 1 - 6