Physiology Plant Plant Transport Systems and Metabolism
Plant Transport Systems Transpiration the loss of water from leaves (see fig. 4.1) Occurs mainly through stomata when they are open and gas exchange is happening Action of the guard cells of the stomata regulates the rate of water loss through transpiration and regulates the rate of photosynthesis by controlling CO2 uptake When guard cells become turgid they separate from one another and the stomata open; photosynthesis and transpiration occur as CO2 moves in while O2 and water move out When water loss exceeds uptake, the guard cells become flaccid and close the stomata Absorption of water (including dissolved minerals) from the soil (see fig. 4.2) From the outside to the endodermis Through the walls of the cells of the epidermis and the cortex Through the interiors (protoplasm) of the cells of the epidermis (mainly via root hairs) and the cortex Through the walls of the cells of the epidermis and then through the interiors of the cells of the cortex Through the interior of the cells of the epidermis (mainly via root hairs) and then through the walls of the cells of the cortex From the cortex through the endodermis to the vascular tissue Water and dissolved minerals move from the cortex to the xylem of the stele only through the interior (protoplasm) of the cells of the endodermis; hence, potentially harmful minerals are selectively excluded Movement of water (and minerals) throughout the plant Root pressure (not covered in the text) Water and minerals that have already entered the xylem and are now above the root hair zone cannot reenter the cortex because of the watertight endodermis Continued absorption of minerals and water in the root hair zone causes a build up of water pressure, called root pressure Root pressure forces water and minerals up the root and stem inside the xylem
Transpiration-Cohesion (see fig. 4.1 and 4.3) Because of the cohesion of water molecules to one another and the adhesion of water molecules to the walls of the xylem cells, there is a continuous column of interacting water molecules in the xylem from the roots to the leaves Evaporation of water from leaf tissues (transpiration) causes water to move from the xylem into the leaf tissues As water molecules leave the xylem, they drags other water molecules along with them Cohesive forces pull water upward through the xylem, all the way from the roots Because minerals are dissolved in the water, they also move through the plant Translocation of sugar (see fig. 4.4) Phloem Vascular tissue involved with the transport of sugar and other organic molecules; composed of sieve tubes, which are composed of sieve tube Translocation members carries phloem sap containing food (sucrose) from sugar sources (sites of photosynthesis or starch breakdown) to sugar sinks (sites of sugar consumption or storage) Storing the sugar Certain plant organs, especially stems and roots, can store food for later use Sucrose is transported to storage cells via the phloem Sucrose molecules are bonded together to form long polymers starch Starch is not soluble in water; therefore, it must be converted back to sucrose before it can be transported away from storage cells Storage of food in the form of starch allows plants to have a food source when there is little or no photosynthesis occurring
Metabolism The sum total of all reactions occurring in living organisms Metabolic reactions that synthesize compounds are anabolic and endergonic (require an input of energy) Metabolic reactions that break down compounds are catabolic and exergonic (release energy) A constant input of energy is required to maintain living systems When a phosphate group is added to a molecule it has been phosphorylated and it has gained energy (the process is endergonic) When a phosphorylated compound gives up one of these high-energy phosphates, energy is released and it has lost energy (the process is exergonic) ATP (adenosine triphosphate) is the main energy molecule of living organisms; it is a transportable form of energy in a usable "size" Energy that is released during a catabolic (exergonic) reaction can be captured by coupling that reaction to the anabolic (endergonic) synthesis of ATP from ADP (adenosine diphospate) + PO4 (phosphate): ADP + PO4 + energy ATP ATP can then give up a phosphate and release energy: ATP ADP + PO4 + energy The above exergonic reaction (ATP ADP + PO4 + energy) can be coupled to an energy requiring (endergonic) reaction
Examples of ATP/ADP Coupled Reactions
Glucose + O2
Exergonic
ATP
Endergonic
CO2 + H2O ATP
Exergonic
ADP + PO4 Protein
Endergonic
ADP + PO4 ATP
Exergonic
Amino Acids DNA
Endergonic
ADP + PO4
Main Points of A Closer Look 4.2 Sugar and Slavery
Nucleotides
Sugars are usually stored as starch or other storage carbohydrates; only sugar cane and sugar beet store large amounts of sucrose (table sugar) The establishment of slavery in the Americas was directly related to the introduction of sugar cane to the Caribbean Islands, South America, the West Indies, and the U.S. (Louisiana) Today, sugar beets provide close to 40% of the world's supply of table sugar
This lecture outline was prepared mainly from Plants and Society, by Levetin and McMahon, 2003 (3rd edition), and may contain phrases or entire sentences taken verbatim from that source.