Plant Biology Module 10 Learning Objectives
Outline the physiological changes that accompany stomatal opening and closing. (Figures 8-8 and 8-9)
1) Blue light, which is a component of sunlight, triggers the activation of proton pumps located in the guard-cell plasma. 2) The proton pumps use ATP energy to actively transport protons out of the guard cells. As the proton pumps in the plasma membranes of guard cells transport protons out of the guard cells, a proton gradient with a charge and concentration difference forms on the two side of the guard-cell plasma membrane. 3) The resulting gradient of H+ drives the facilitated diffusion of large numbers of potassium ions into guard cells. This movement occurs through voltage-activated ion channels, which open when a certain difference in charge between the two sides of the guard-cell plasma membrane is attained. 4) Chloride ions are also taken into the guard cells through ion channels in the guard-cell plasma membrane. The negatively charge chloride ions help electrically balance the positively charged potassium ions. 5) Water enters the guard cells from the surrounding epidermal cells. by osmosis. The increased turgidity of the guard cells changes their shape, because the thickened inner cell walls do not expand as much as the outer walls, so the stoma open. (Blue light acitivates proton pumps ---> proton pump moves H+ out of guard cells ---> K+ and Cl- move into guard cells through voltage-activated ion channels ---> water diffuses by osmosis into guard cells ---> guard cells change shape, and stoma opens)
Define leaf abscission. Explain why it occurs and what physiological and anatomical changes precede it.
Abscission- The normal falling off of leaves or other plant parts, such as fruits or flowers. The leaves fall off as winter approaches in temperate climates, or at the beginning of the dry period in tropical climates with pronounced wet and dry seasons. Abscission is also needed because of a plant's water requirements, which become critical during physiological drought of winter. If there were leaves on the plant, transpiration would occur. But what would happen is that the lost water can't be replaced. As autumn approaches, sugars, amino acids, and many essential minerals are mobilized and transported from the leaves to other plant parts. Chlorophyll breaks down allowing some of the carotenoids to become evident. In addition, red water-soluble pigments called anthocyanins are synthesized and store din the vacuoles of epidermal leaf cells in some species. The various combinations of carotenoids and anthocyanins are responsible for the brilliant colors found in autumn landscapes in temperate climates.
Describe the major tissues of the leaf (epidermis, mesophyll, xylem, and phloem). Be able to recognize different leaf morphologies.
Epidermis- The upper epidermis covers the upper surface, and the lower epidermis covers the lower surface. A feature of leaf epidermal cells is that the cell wall facing toward the outside environment is somewhat thicker than the cell wall facing inward. This extra thickness may provide the plant with additional protection against injury or water loss. Epidermal cells secrete a waxy layer, the cuticle, that reduces water loss from their exterior walls. A leaf's exposed upper epidermis generally has a thicker cuticle tan the shaded lower epidermis. The epidermis contains minute openings, or stomata, for gas exchange. The epidermis of many leaves is covered with various hairlike structures called trichomes. Trichomes of some plants help reduce water loss from the leaf surface by retaining a layer of moist air next to the leaf and by reflecting excessive sunlight, thereby protecting the plant from overheating. Mesophyll- The photosynthetic ground tissue of the leaf. It is sandwiched between the upper epidermis and the lower epidermis. Mesophyll cells are loosely arranged with many air spaces between them that facilitate gas exchange. In many plants, the mesophyll is divided into two sublayers, the palisade mesophyll and the spongy mesophyll. The palisade mesophyll are the columnar cells that are closely together in a layer. It is the main side of photosynthesis in the leaf. The spongy mesophyll are the cells that are more loosely and more irregularly arranged. It's main function is to allow diffusion of gases, particularly CO2, throughout the leaf's interior. Xylem- Conducts water and dissolved minerals. It is usually located on the upper side of a vein, toward the upper epidermis. Phloem- Conducts dissolved sugars. It is usually confined to the lower side of a vein.
Contrast leaf structure in eudicots and monocots.
Eudicots- Composed of a broad, flattened blade and a petiole. Their leaves typically have netted venation. As a result, a cross section of a eudicot blade often shows veins in both cross-sectional and lengthwise views. Monocots- Often lack a petiole. They're narrow, and the base of the leaf often wraps around the stem, forming a sheath. In a cross section of a monocot leaf, the parallel venation pattern produces evenly spaced veins, all of which appear in cross section. Figure 8-6. I have a picture of this on my phone as well.
Describe the major tissues of the leaf (epidermis, mesophyll, xylem, and phloem). Label a leaf diagram.
Figure 8-3. I also took a picture of it.
Relate the structure of the leaf to its function of photosynthesis.
Most leaves are thin and flat, a shape that allows maximum absorption of light energy and efficient internal diffusion of gases. The leaves of plants form an intricate green mosaic, bathed in sunlight and atmospheric gases. The epidermis of the leaf is relatively transparent and allows light to penetrate to the interior of the leaf, where the mesophyll is located. Water required for photosynthesis is obtained from the soil and transported in the xylem to the leaf, where it diffuses into the mesophyll and moistens the surfaces of mesophyll cells. The veins carry the dissolved sugar produced during photosynthesis to all parts of the plant. Bundle sheath and bundle sheath extensions provide additional support to prevent the leaf from collapsing. Stomata permit the exchange of gases between the atmosphere and the inside of the leaf. Carbon dioxide, a raw material of photosynthesis, diffuses through stomata from the surrounding atmosphere into the leaf, and the oxygen produced during photosynthesis diffuses out of the leaf through stomata.
List at least five modified leaves, and give the function of each.
Spines- Modified leaves that are hard and pointed, are found on many desert plants, such as cacti. Bracts- Leaves associated with flower clusters. The flowers are clustered in the center of each inflorescence, and what appear to be four white-to-pink petals are actually bracts. Tendrils- A leaf or stem that is modified for holding on or attaching objects. Vines are climbing plants whose stems cannot support their own wright, so they often have tendrils to keep them attached. Bulb- A rounded, fleshy, underground bud that consists of a short stem with fleshy leaves. For example, onions and tulips form bulbs. Figure 8-15. I took a picture on my phone.
Discuss transpiration and its effects on the plant.
Transpiration is the loss of water vapor from aerial plant parts. Most transpiration occurs through open stomata. Also, the loose arrangement of the cells of the spongy mesophyll provides ta large surface area within the leaf which water vapor can evaporate. Several environmental factors influence the rate of transpiration. Most water is lost from plant surfaces at higher air temperatures. Light increases the transpiration rate, in part because it triggers the opening of stomata and in part because it increases the leaf's temperature. Wind and dry air increase transpiration because the air is already saturated, or nearly so, with water vapor. Transpiration is responsible for water movement in plants, and without it, water from the soil would not reach the leaves. It also cools the leaves and stems. When water changes from a liquid state to a vapor, it absorbs a great deal of heat. A second benefit of transpiration is that it moves essential minerals from the roots to stems and leaves. Transpiration can be harmful too. On hot summer days, plants frequently lost more water by transpiration than they take in from the soil. Their cells experience a loss of turgor, and the plant wilts.
