Plant transport

VII. Plant Transport

A. Nutritional Requirements

1. 16 essential elements
2. 3 = C, H, O (C from carbon dioxide in air, H from water, O from water and air -- but O from water becomes a product of photosynthesis; O from CO2 is incorporated into macromolecules during fixation; O2 from air is used in aerobic respiration)
3. 13 elements made available as dissolved mineral salts
[Table 30.1]
- 6 = macronutrients (used in significant quantities)
- 7 = micronutrients (used in trace amounts)

B. Absorptive Structures

1. Bacteria, fungi help plants to take up nutrients, particularly nitrogen ( = mutualism). Plenty of N2 is available in air, but plants cannot fix nitrogen. [What does "fix" mean?]
2. Bacteria reside in root nodules of legumes (string beans, peas, alfalfa, clover); they convert gaseous nitrogen to forms the plants can use = nitrogen fixation. In return, they withdraw organic compounds from plant tissues. [Fig. 30.2]
3. Fungi grow around plant roots in mycorrhizae; they aid in absorbing minerals in exhange for sugars.
4. Root hairs are epidemal extensions that greatly increase the surface area for absorbing water and nutrients [Fig. 30.3]

C. Control of Nutrient Uptake

1. Uptake of mineral ions is regulated -- don't want too much of a good (or bad) thing -- e.g., certain salts, heavy metal ions.
2. Endodermis surrounds vascular cylinder, Casparian strip in endodermis prevents flow of water around cells --> water and ions must move through cell cytoplasm. [Figs. 30.4a, 30.4b,c]
3. This allows membrane transport proteins to control absorption of solutes; most flowering plants (angiosperms) also have an exodermis just inside the roots which also has a Casparian strip.
4. Energy for membrane pumps = ATP from either photosynth or aerobic respiration.

D. Water Transport

1. Transpiration: water moves (roots --> stems --> leaves) through xylem (tracheids and vessel members. Some water used for growth, metabolism; most evaporates into air = transpiration.
2. Cohesion-tension theory [Fig. 30.6]
- transpiration at leaves --> tension on water in xylem --> evaporation --> water molecules pulled up stem to replace molecules lost to air --> water pulled into roots
- during all this pulling, hydrogen bonds hold water molecules together in columns inside xylem tubes = cohesion

E. Control of Water Loss

1. Water-conserving cuticle: secreted by epidermal cells; translucent to allow entry of light photons, restricts water loss and diffusion of CO2, O2 [Fig. 30.7]
2. Stomata (openings mainly on bottom of leaves) regulate inward, outward movement of water vapor, CO2, O2 [Fig. 30.8]
- 2 guard cells define each opening; sunlight --> drop in CO2 inside cells --> potassium uptake by active transp. causes water to enter --> guard cells swell --> stomata open in daytime [Fig. 30.9]
- at night, CO2 levels increase --> guard cells lose potassium, water, unswelling --> stomata close
- in CAM plants (cactus, succulents) stomata open at night to conserve water (CO2 levels drop at night due to fixation by a special C4 pathway -- see p. 119)

F. Transport of Organic Substances

1. Starch, fats, proteins ( = storage forms of organic molecules) unsuitable for transport; these are converted to soluable forms such as sucrose, amino acids)
2. Translocation = movement of organic molecules from photosynthetic sites to organs that need them. Occurs in phoem. [Figs. 30.11, 30.13]
- aphid feeding, shows sugars inside seive tubes being moved under relatively high pressure
3. Pressure Flow Theory [Fig. 30.14]
- translocation driven by pressure, concentration gratients
- molecules move through phloem from sources (mainly leaves) to sinks (leaves, fruits, seed, roots)
- solutes loaded at source by active transport into phloem (companion cells do the loading); water follows by osmosis building up pressure in phloem at source
- pressure causes bulk flow of solution inside phloem; concentration gradient helps = diffusion
- organic compounds unloaded into sink cells (any region where cells are growing or storing food) followed by water (--> low hydrostatic pressure)
- pressure-flow theory accounts for fact that movement of sugar in phloem is much faster than can be accounted for by diffusion (up to 40,000 times faster in cotton plants!)