MEMBUKA DAN MENUTUPNYA STOMATA

The opening and closing of stomates For photosynthesis to proceed efficiently, the leaf mesophyll(kata keterangan tempat) must receive(Kata kerja) a continuous supply(kata benda) of carbon dioxide from the atmosphere. The epidermis (Kata benda) with its cuticle presents(kata keterangan) a formidable barrier(kata kerja) to carbon dioxide diffusion, however. Consequently, vascular plants have evolved(kata kerja) stomatal pores on the surface of their leaves (keterangan tempat) (and photosynthetic stems) to permit rapid(kata benda) carbon dioxside entry and oxygen release. One disadvantage of these openings (keterangan benda), however, is that they allow water vapor(kata benda) in the air space of a leaf (or stem) to escape(kata kerja) into the atmosphere(keterangan tempat), rendering the leaf susceptible to dehydration, particularly during hot, dry conditions(keterangan sifat) For all vascular plants, this problem has been partly solved by the evolution of a mechanism to control(kata kerja) the opening and closing of the stomates but other “solutions” have evolved(kata kerja) as well (Essay 15-1). Under moderate conditions of soil moisture, humidity and temperature, the stomates(kata benda) are open during the day and closed at night. Thus, carbon dioxide can enter the leaf during periods of photosythesis(keterangan waktu), and the loss of water vapor is greatly curtailed(kata kerja) at night when there is no need for carbon dioxide. Let us now investigate(kata kerja) how this remarkable mechanism works(kata benda). Each stomate on the leaf surface is surrounded(kata kerja) by a pair of specialized epidermal cells(keterangan benda) called guard cells(kata benda) (Figure 15-3b). The guard cells separate from one another when they swell up with water. The separation(keterangan sifat) creastes a pore-the stomate- between them. When the guard cell lose water and deflate, they collapse on each other, closing the pore. (menerangkan dan diterangakan) How is the update and loss of water in the guard cell regulated? According to current theory, there are ion pumps(kata benda) located in the plasma membranes(keterangan tempat) of the guard cell that, with the expenditure of ATP energy, transport potassium ions inward. At day break (with stomates initially closed), light activated(kata kerja) these pumps and potassium ions begin to accumulate(keterangan kerja) in the guard cell (figure 15-4). Recall from chapter 4 that solutes decrease water potensial, as the potassium ion concentration increases(keterangan sifat) in the guard cell, their water potential drops to below that of the surrounding epidermal cell. This water potential gradient favors the entry of water into the guard cell, causing them to swell and separate, creating a pore in the process (Menerangkan Sebab akibat). At nightfall, the potassium pump is no longer activated. The potassium ions diffuse out(kata kerja) of the guard cells, water follows and the deftating cells collapse over the pore. The question remaining is, how does light activate the ion pump? Although we have no clear answer to this question, there are some clues. The guard cells(kata benda), unlike all other epidermal cells, have chloroplasts. Perhaps these chloroplasts.