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THE STOMATA Leaves The exchange of oxygen and carbon dioxide in the leaf occurs through pores called stomata (singular = stoma). Normally stomata open when the light strikes the leaf in the morning and close during the night. The immediate cause is a change in the turgor of the guard cells. The inner wall of each guard cell is thick and elastic. When turgor develops within the two guard cells flanking each stoma, the thin outer walls bulge out and force the inner walls into a crescent shape. This opens the stoma. When the guard cells lose turgor, the elastic inner walls regain their original shape and the stoma closes. The table shows the osmotic pressure measured at different times of day in typical guard cells. The osmotic pressure within the other cells of the lower epidermis remained constant at 150 lb/in2. When the osmotic pressure of the guard cells became greater than that of the surrounding cells, the stomata opened. In the evening, when The immediate cause is a change in the turgor of the guard cells. The inner wall of each guard cell is thick and elastic. When turgor develops within the two guard cells flanking each stoma, the thin outer walls bulge out and force the inner walls into a crescent shape. This opens the stoma. When the guard cells lose turgor, the elastic inner walls regain their original shape and the stoma closes. The table shows the osmotic pressure measured at different times of day in typical guard cells. The osmotic pressure within the other cells of the lower epidermis remained constant at 150 lb/in2. When the osmotic pressure of the guard cells became greater than that of the surrounding cells, the stomata opened. In the evening, when the osmotic pressure of the guard cells dropped to nearly that of the surrounding cells, the stomata closed. STOMATA ; Is a pore on the epidermis of the leaves and stems of plants which can open and close FUNCTIONS : 1. Enable gases exchange between leave cells and atmospheric 2. Regulate and control water loss from plants THE STRUCTURE OF STOMATA STOMATA The stomata, Is a tiny openings in the epidermis of a plant leaf, are surrounded by a pairs of guard cells. The guard cells open and close the stoma by changing shape THE STRUCTURE OF STOMATA THE STRUCTURE OF A STOMA TA: ( Stoma are found on epidermal layer of leaves, stems and part of flowers of flowering plant ) Stomata is bounded by a pair of guard cells which are kidney shape in dicot and dumbell-shaped in monocot leaves. Guard cells (gc) is a specialised epidermal cell which contain chloroplasts for photosynthesis. The cellulose cell walls of gc that surround the stoma (inner cell wall), are thicker and less elastic than those in contact with epidermis ( outer cell wall). The different elasticity of the cellulose cell wall is contributed by the microfibril cellulose that surround the wall of the cell. The two guard cells are fused at their end allowing the cell to bend as they expand. The diff. causes the outer wall elongates more than the inner wall when the cell is in turgor pressure. The presence of chloroplast enables the gc to photosynthesise, For gaseous exchange -During the day, CO2 diffuse into the leaves for photosinthesis and O2 produces from photusinthesis diffuses out. -During night, O2 diffuse into the leaves for respiration and CO2 diffuse out. Stomata regulate the rate of photosynthesis based on large number of internal and external signals. Allow transpiration to cooling leaves -During transpiration, water vapour diffuse out through stomata due to water vapour concentration gradient.(water vapour concentration higher in spaces of leaves and lower in atmosphere) -Diffusion of water vapour create a transpiration pull in the xylem so that water in transported up to the leaves for photosynthesis. Reduce exessive loss of water when pore close -Done physically as when water is lost from the guard cells, the cells become flaccid.The stoma is automatically closed There are two hypothesis 1. The starch-sugar hypothesis 2. The potassium ion accumulation hypothesis STARCH –SUGAR HYPOTHESIS At a day time, the guard cell undergoes photosynthesis due to the presence of chloroplast in the gc sugars are formed the accumulation of sugar, increase the osmotic concentration of gc, water potential in gc becomes lower than the water potential in neighbouring epidemal cells Difference in water potential causes the movement of water molecules from epidermal cells into guard cells by osmosis. The entrance of water, causes the gc to become swell ,and turgid Turgor pressure increases, the thinner outer cellulose cell walls elongate more than the inner thicker walls The gc bend outward and stoma opens. At night, Guard cells do not go through photosynthesis There is no sugar produced in guard cells , sugar converted to starch, reduces osmotic concentration in guard cells Water potential in guard cells become higher than the w.p in epidermis cells Water potential in guard cells become higher than the w.p in epidermis cells Water molecule diffuses out of gc into neighbouring epidermis cells Gc becomes flaccid because of no turgor pressure. Closing the stoma POTASSIUM IONS ( K+) ACCUMULATION HYPOTHESIS ( more recent hypothesis) Potassium ions from neighbouring epidermis cells actively transported to the guard cells Energy needed is supplied by ATP generates during phosphorylation of the light reaction of photosynthesis ATP ( from photophosphorylation) is use to drive the proton-pump in the cell membrane of gc, activate by the light ……..POTASSIUM IONS ( K+) ACCUMULATION HYPOTHESIS ( more recent hypothesis) As protons (H+) are pumped out of the gc , the inner gc becomes more negatively charged. This activate potassium channels to pumped in potassium ions, K+ into gc from epidermis cells. Cl– ions then enter to balance the charge The accumulation of K+/ ( malate ions in some plants ) causes the water potential of gc more negative than that in epidermis cells. Water from epidermis cells enters by osmosis into the guard cells The gc becomes turgid , Turgor pressure increased , thinner outer cellulose cell walls expand more than the inner wall Guard cells bend outward and the stoma opens Proton pump Potassium channel K H+ K+ channels activated, K+ ions pumped in H+ ion H+ pumped out of the guard cells, lowest the electrochemical gradient Stoma opens