Albia Dugger Miami Dade College Cecie Starr Christine Evers Lisa Starr Chapter 26 Plant Nutrition and Transport (Sections )

26.5 Water-Conserving Adaptations of Stems and Leaves Plants must conserve water for photosynthesis, growth, membrane functions, and other processes A cuticle and stomata restrict the amount of water vapor that diffuses out of the plant’s surfaces – but also restrict access to CO 2 for photosynthesis, and oxygen for aerobic respiration

Cuticle Cuticle prevents water loss from evaporation It consists of epidermal cell secretions: waxes, pectin, and cellulose fibers embedded in cutin, an insoluble lipid polymer The cuticle is translucent, so it does not prevent light from reaching photosynthetic tissues

Stomata Two specialized cells (guard cells) define each stoma When guard cells swell with water, a gap (stoma) forms between them When guard cells lose water, the gap closes guard cell One of a pair of cells that define a stoma across the epidermis of a leaf or stem

Stomata in Action

Fig. 26.8a, p. 422 Stomata in Action

Fig. 26.8a, p. 422 A Cuticle (gold) and stoma on a leaf. The stoma is an opening between two specialized epidermal cells called guard cells. stoma Stomata in Action

Fig. 26.8b, p. 422 Stomata in Action

Fig. 26.8b, p. 422 B This stoma is open. When the guard cells swell with water, they bend so that a gap opens between them. The gap allows the plant to exchange gases with air. The exchange is necessary to keep metabolic reactions running. guard cells Stomata in Action

Fig. 26.8c, p. 422 Stomata in Action

Fig. 26.8c, p. 422 C This stoma is closed. The guard cells, which are not plump with water, are collapsed against each other so there is no gap between them. A closed stoma limits water loss, but it also limits gas exchange, so photosynthesis and respiration reactions slow. Stomata in Action

Fig. 26.8d,e, p. 422 Stomata in Action

Fig. 26.8d,e, p. 422 D How do stomata open and close? When a stoma is open, the guard cells are maintaining a relatively high concentration of solutes by pumping solutes into their cytoplasm. Water diffusing into the hypertonic cytoplasm keeps the cells plump. E When water is scarce, a hormone (ABA) activates a pathway that lowers the concentrations of solutes in guard cell cytoplasm. Water follows its gradient and diffuses out of the cells, and the stoma closes. water solutes ABA signalsolutes Stomata in Action

Factors Affecting Stomata Water availability, the level of carbon dioxide inside the leaf, and light intensity affect whether stomata open or close Examples: Light causes guard cells to pump potassium ions into their cytoplasm; the stoma opens to begin photosynthesis Root cells release abscisic acid (ABA) when soil water becomes scarce; binding in guard cells closes stoma

Smog and Stomata Stomata close in response to some chemicals in polluted air Closure protects the plant from chemical damage, but also prevents uptake of carbon dioxide for photosynthesis, and so inhibits growth

Smog and Stomata

Fig. 26.9a, p. 423 Smog and Stomata

Fig. 26.9b, p. 423 Smog and Stomata

Fig. 26.9c, p. 423 Smog and Stomata

Key Concepts Water Loss Versus Gas Exchange A cuticle and stomata help plants conserve water Closed stomata stop water loss but also stop gas exchange Some plant adaptations are trade-offs between water conservation and gas exchange

ANIMATION: Stomata

26.6 Movement of Organic Compounds in Plants Phloem distributes the organic products of photosynthesis through plants Phloem is a vascular tissue with organized arrays of conducting tubes, fibers, and strands of parenchyma cells Sieve tubes that conduct dissolved organic compounds in phloem consist of living cells

Sieve Plates Sieve-tube cells are positioned side by side and end to end Their abutting end walls (sieve plates) are porous

Sieve Tubes in Phloem

Sugar Transport Companion cells actively transport the organic products of photosynthesis (sugars) into sieve tubes Sugars travel through sieve tubes to all other parts of the plant, where they are broken down for energy, remodeled into other compounds, or stored for later use Sucrose is the main carbohydrate transported in phloem

Translocation Organic compounds in phloem flow from a source (region where companion cells load molecules into sieve tubes) to a sink (region where molecules are being used or stored) Translocation Process that moves organic molecules through phloem

Pressure Flow Theory A pressure gradient drives the movement of fluid in phloem pressure flow theory Explanation of how flow of fluid through phloem is driven by differences in pressure and sugar concentration between a source and a sink

Steps in Pressure Flow Theory 1.Companion cells load sugars into sieve-tube members by active transport 2.Solute concentration in sieve tubes increases, so water moves in by osmosis – increased fluid volume increases internal pressure (turgor) 3.High pressure pushes fluid toward sink regions 4.Pressure and solute concentrations decrease as fluid moves from source to sink 5.Sugars are unloaded at sink regions; water follows by osmosis

Translocation by Pressure Flow

Fig , p. 425 SINK (e.g., developing root cells) Both pressure and solute concentrations gradually decrease as the fluid moves from source to sink. As a result of the increase in solute concentration, the fluid in the sieve tube becomes hypertonic. Water moves in from the surrounding xylem, increasing phloem turgor. WATER interconnected sieve tubes Solutes are unloaded from the tube into sink cells, which become hypertonic with respect to fluid in the tube. Water moves from the sieve tube into sink cells. The pressure difference pushes the fluid from the source to the sink. Water moves into and out of the sieve tube along the way. Solutes move into a sieve tube against their concentration gradients by active transport. SOURCE (e.g., mature leaf cells) flow Trans- location by Pressure Flow

Fig , p. 425 SINK (e.g., developing root cells) WATER interconnected sieve tubes SOURCE (e.g., mature leaf cells) flow Solutes move into a sieve tube against their concentration gradients by active transport. 1 The pressure difference pushes the fluid from the source to the sink. Water moves into and out of the sieve tube along the way. 3 Solutes are unloaded from the tube into sink cells, which become hypertonic with respect to fluid in the tube. Water moves from the sieve tube into sink cells. 5 As a result of the increase in solute concentration, the fluid in the sieve tube becomes hypertonic. Water moves in from the surrounding xylem, increasing phloem turgor. 2 Both pressure and solute concentrations gradually decrease as the fluid moves from source to sink. 4 Stepped Art Trans- location by Pressure Flow

ANIMATION: Pressure Flow Hypothesis To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

Sieve-Tube Pressure Honeydew exuding from an aphid after the insect’s mouthparts penetrated a sieve tube

Key Concepts Distribution of Organic Molecules Through Plants Phloem distributes organic products of photosynthesis from leaves to living cells throughout the plant Organic compounds are actively loaded into conducting tubes at sources, then unloaded in sinks

Mean Green Cleaning Machines (revisited) With elemental pollutants such as lead or mercury, the best phytoremediation strategies use plants that take up toxins and store them in aboveground tissues With organic toxins such as TCE, the best phytoremediation strategies use plants with biochemical pathways that break down the compounds to less toxic molecules

Animation: Translocation in Phloem