Biology Final Exam Flashcards
transport of water and solutes in plants on three scales
- by individual cells ex. root hairs
- short distance from cell to cell at the level of tissues and organs
- long distance transport between xylem and phloem
how does proton pump work
- the proton pump transport mechanism hydrolyzes ATP and uses the released energy to pump hydrogen ion (H+) out of the cell
- because proton pump removes positive charges in the form of H+ out of the cell it generates a membrane potential, because inside of the cell becomes negative in charge relative to the outside
- their membrane potential help drive K+ in the cell, because K+ diffusing down its electrochemical gradient
- accumulation of the ions by this mechanism represents a passive transport
• there are 3 roots available for lateral transport
- by the first route, such substances move out of one cell, across the cell wall and into the neighboring cells
- by the second route, via the symplast, the continuum of cytosol within a plant issue , requires only one crossing if plasma membrane, after entering one cell
- solutes exist in one cell (through the plasma membrane) then move from cell to cell via plasmodesmata
- the third route, is for lateral transport within a plant tissue or organ is along the apoplast, the extracellular pathway consisting of cell wall
- the solute move in the extracellular spaces between the cells
- As the water and minerals move all the way to the endodermis, the apoplastic roots cannot continue into the stele, due to the presence of the endodermal cosponian scrips
- Only mineral and solutes that are in the symplastic route, or those through crossing the plasma membrane of each cell, can reach to the stele
- Once the water reaches the xylem in the stele, the water enters the vessel element and for tracheid via the apoplast route, because xylem cells are dead cells at maturity
root pressure
When transpiration is very slow or ascent, as at night for example, the root cells may still accumulate ions in the xylem because the vascular tissue of the root is surrounded by the epidermis, ions tend not to lead out of the xylem
• Therefore the water potential of the xylem become more negative and water moves into the xylem by osmosis through the surrounding cells
• In this manner, root pressure is created it forces both water and dissolved ions up by the xylem
• Root pressure causes “guttation” the exudation of water droplets, that can be seen in the morning on tips of grass blades or leaf margins or some herbaceous plants
adhesion
holding together of unlike substances
cohesion
holding together of like substances
cohesion-adhesion method of moving water
• When the cells of the leaf lose water to the air during transpiration, their ions and molecules become more concentrated
• The water potential of the cells becomes increasingly more negative
• Because the intercellular water is in direct connection, through the water permeability of the plasma membranes, a pull is exerted on this water when water evaporates from the walls, increasing the negative water potential
• The water of the wall is in connection with the water is in connection with the water of the xylem cells
• So the negative potential is transmitted to the centrals of the xylem as well which causes a build-up of tension (negative water potential) there
o Because of the extraordinary cohesiveness of water, this tension is transmitted all the way down the xylem of the stem to the xylem of the root
• This lost makes the water potential of the root more negative, this increases the ability of the root to extract water from the soil
• This theory is known as cohesive-tension theory because it depends on the great cohesiveness of water
• It is the cohesion of the water, due to the hydrogen bonds, that makes it possible to pull a column of sap from above without the water separating
• The transpiration pull can extend down the root only through unbroken chains of water molecules
How Stomata Open and Close
• Each stoma is flanked by a pair of guard cells which are kidney-shaped in dicots and dumbbell shaped in most grasses (monocots)
• The guard cells are suspended by subsidiary cells (epidermal cells attached to the guard cells) over an air chamber that leads to air spaces within the leaf
• Guard cells control the diameter of the stoma be changing shape, which widens or narrows the gap between the guard cells
• When guard cells takes in water and swells, the size of the gap increases (stomata opens)
• When guard cells loose water, they become flaccid, thus closing the gap (stomata closed)
• Changes in the turgor pressure results primarily from reversible absorption and less potassium ions by the guard cells
• Stomata opening results from potassium ion accumulation which results in negative water potential and movement of water from nearby cells by osmosis
• Stomata closure parallels an exodus of potassium from guard cells, which leads to osmotic loss of water
• Three factors that contribute to stomata opening
o Light stimulates guard cells to accumulate potassium and become turgid
• Depletion of carbon dioxide within the air spaces of leaf, which occurs during photosynthesis, stimulates opening of the stomata
• Circadian rhythmis (the daily rhythm of opening and closing the stomata)
• Movement in substances in the phloem ( pressure flow mechanism)
• According to the pressure flow hypothesis assimilates move through the sieve tube along the concentration gradient between the sources of the assimilates and the site of utilization or sinks of these assimilates
• Sources are places food substances available for transport such as photosynthesizing leaf.
o Ex. Of sinks are growing and differentiating regions and storage regions
• Sugar manufactured in mesophyll cells of the leaf is actively secreted or pumped into the sieve tube of the vein by neighboring parenchyma cells, such as companion cells
• This decreases the water potential in the sieve tube and cause water to move into the sieve tube from the xylem with the movement of the water into the sieve tube, the sugar is carried passively to the growing root tip, when the sugar is removed from the sieve tube through the expenditure of energy be neighboring parenchyma cells
• This results in increased water potential in the sieve tube and the eventual movement of water out of the sieve tube at the sink
• Note that the role of the sieve tube is relatively passive, as energy is expended by parenchyma cell as the source and sink
• The pressure flow mechanism is an ex. Of “mass form” which depend on the differences in the water potential on either side of the differentially permeable membrane
• These differences in water potential results in osmosis and provides the driving force for the pressure flow mechanism
hormone
- A hormone is a chemical messenger produced in a small amount by one part of the body that is active in a different part of the body
- Reaction to a hormone is usually dependent not so much on the absolute amount f that hormone as on its relative concentration to the other hormones
5 classes of hormones have been possibly identified
o Auxin o Gibberellins o Cytokinins o Abscisic acid o Eythylene
hormones that promote growth
Auxin
o Gibberellins
o Cytokinins
hormones that inhibit growth
Abscisic acid
o Eythylene
• Auxin
- The most common naturally occurring auxin is indolacetic acid (IAA)
- Produced in shoot apex, young leaves and the flowers and fruits
, Charles Darwin and his son Francis reported an experiment they had performed with grass and oat seedlings
- They found that if the coleoptile was kept intact the seedlings would bend toward unidirectional light source
- But if the tip of the seedling was cut off, a covered by a black cap the seedlings would respond to the light
- They concluded that some influence is transmitted from the coleoptile tip to the rest of the shoot to cause bending
- Building on the results, plant physiologist F.W went 1926, performed experiment
- He cut off the tips of the coleoptile and placed them on agar (a gelatin- like material) after about an hour, he removed the tips and cut the agar into small blocks
- When on agar block was placed to one side of the coleoptile, without a tip, the shoot would bend away from that said
- The bending occur even through the seedlings were not exposed to light
