BIOL 240W Chapter 36

Transpiration provides the pull for the ascent of xylem sap

cohesion of water molecules transmits this upward pull.

The sucking force of negative potential is transmitted via

cohesion.

Cohesion

attractive force between molecules of same substance.

Adhesion

attractive force between water molecules and other polar molecules.

Pressure flow hypothesis for phloem movement

1) phloem near source: active transport of sugar against concentration gradient by co-transport. 2) Phloem near source: water rushes in. 3) Phloem near sink: sugars are withdrawn 4) Phloem near sink: water exists and is recycled in the xylem.

Which of the following statements about xylem transport is true? 1. The xylem cells that conduct water and minerals are alive. 2. All water transported by the xylem is used for photosynthesis in the leaves. 3. Water and minerals move through the root cortex into the xylem and upward through the stem and into leaves. 4. Water and minerals enter the xylem by diffusion.

3. Water and minerals move through the root cortex into the xylem and upward through the stem and into leaves.

Active transport of amino acids in plants at the cellular level requires....?

ATP, transport proteins, and a proton gradient.

What characterizes the rates of photosynthesis and transport in a plant on a dry cloudy day?

Both the photosynthesis and transpiration rates are low.

How bulk flow and active transport work together

Bulk flow works at long distances for sugar transport through phloem, but active transport of sugar is needed at the cellular level.

In roots the _____ forces water and solutes to pass through the plasma membranes of _____ cells before entering the _____.

Casparian strip ... endodermis ... xylem

Which tissue acts as a filter on the water absorbed by root hairs?

Endodermis

True or false? Root pressure can move water a long distance up the xylem because of the higher water potential of the xylem in comparison to the water potential in the surrounding cells.

False

True or false? The rate of sugar transport in a plant depends on the rate of photosynthesis, the rate of transpiration, and the difference in turgor pressure between the source and the sink.

False

Stimuli for opening and closing of stomata

Generally, stomata open during the day and close at night to minimize water loss. Stomatal opening at dawn is triggered by light, CO2 depletion and an internal clock in guard cells.

_____ cells are the cells that regulate the opening and closing of stomata, thus playing a role in regulating transpiration.

Guard

_____ bonds are responsible for the cohesion of water molecules.

Hydrogen

Which of the following statements about bulk flow are correct? I) Bulk flow is driven primarily by pressure potential. II) Bulk flow depends on a difference in pressure potential at the source and sink. III) Bulk flow depends on the force of gravity on a column of water. IV) Bulk flow may be the result of either positive or negative pressure potential.

I, II, and IV

membrane potential in animals vs plants

In animals, membrane potential is established through pumping Na+ by sodium potassium pumps. In plants, membrane potential is established through pumping H+ by proton pumps.

The difference between bulk flow and diffusion

It is driven by differences in pressure potential, not solute potential. It occurs in hollow dead cells, not across the membranes of living cells. It moves the entire solution, not just water or solutes. It is much faster.

Positive root pressure

It's relatively weak (too weak to overcome gravity) and is a minor mechanism of xylem bulk flow.

Phloem is an information superhighway

Many plant viruses spread through the phloem. Functional messenger RNAs can be transported long distances in the phloem. Signals indicating infections are moved through the phloem.

Osmosis

The diffusion of water into or out of the cell. affected by solute concentration and pressure.

Which term describes an area where sugars are used or stored?

Sink

Which of the following statements about the distribution of sap throughout a plant is true?

The mechanism that explains the movement of sugars throughout a plant is called the pressure-flow hypothesis.

Transpiration

The movement and, ultimately, evaporative loss of water from the plant. It's the process that drives transport of water and minerals from roots to shoots via the xylem.

_____ provide(s) the major force for the movement of water and solutes from roots to leaves.

Transpiration

Xylem

Transport water and minerals from roots to shoots.

Phloem

Transports products of photosynthesis from where they are made or stored to where they are needed.

Water potential

a measurement that combines the effects of solute concentration and physical pressure. it determines the direction of movement of water.

Translocation

a process by which the products of photosynthesis are transported through phloem.

Isotonic

a solution surrounding a cell that will cause no net movement of water into our out of cell.

Hypertonic

a solution surrounding a cell that will cause the cell to lose water.

Hypotonic

a solution surrounding a cell that will cause the cell to take up water.

Transpiration occurs within

a thin film of water. Cell walls of mesophyll cells act like a very thin capillary network. cells' walls made of microfibrils.

Root pressure

active transport of solutes into roots stele lowers the stele's water potential. Water flows into the stele, increasing pressure, forcing water up the xylem.

phloem sap

an aqueous solution also moves by bulk flow through sieve tube elements.

A sugar sink

an organ that is a net consumer or storer of sugar, such as a tuber or bulb.

A sugar source

an organ that is a net producer of sugar, such as mature leaves.

Drought stress or freezing

can cause a break in the chain of water molecules through cavitation, the formation of water vapor pocket or air bubble. In effect, a failure of cohesion.

Drought, high temperature, and wind

can cause stomata to close during the daytime.

Turgor loss in plants

causes wilting which can be reversed when the plant is watered

The apoplast

consists of everything external to the plasma membranes of living cells and includes cell walls, extracellular spaces, and the interior of dead cells such as vessel elements and tracheids.

The symplast

consists of the entire mass of cytosol of all the living cells in a plant, as well as the plasmodestmata, the cytoplasmic channels that interconnect them.

Water an minerals moving through apoplast must

cross plasma membrane of endodermal cell to get into xylem which provides some selectivity.

Bulk flow

depends on a pressure gradient. It occurs from higher pressure to lower pressure. It's independent of solute concentration.

transport

diffusion and active transport through the apoplast, symplast, or transmembrane roots is sufficient for short range movement of materials. But long distance transport depends upon bulk flow through the xylem and phloem.

Water is mostly absorbed by

epidermal cells near tips of roots, especially root hairs.

Adhesion to cell walls of tracheids and vessel element

helps to fight gravity

Abscisic acid (ABA)

hormone that is produced in response to water deficiency and causes the closure of stomata.

Bulk flow in xylem

is driven by the negative water potential created by transpiration

Why membrane potential?

it acts like batteries. provide an energy source that cells can harness to do work, like move stuff around.

solute potential

it is directly proportional to its molarity. derived from the concentration of dissolved materials. Solutes bind water molecules and reduce ability to move and do work.

Protoplast

it is the living part of the cell, including the plasma membrane.

The evolution of xylem and phloem

made possible the long distance transport of water, minerals, and products of photosynthesis.

Efficient movement is possible because

mature tracheid and vessel elements have no cytoplasm (part of apoplast), and sieve-tube elements have few organelles in their cytoplasm.

Transport of sugars

movement is always from source to sink but sources and sinks can switch. A storage organ can be both a sugar sink in summer and sugar source in winter.

Which of these are symbiotic associations?

mycorrhizae

Plant adaption--Xerophytes

plants adapted to arid climates. Some plants use a specialized form of photosynthesis called CAM where stomatal gas exchange occurs at night.

Turgor pressure

pressure exerted by the protoplast against the cell wall.

Stomata

regulate the amount of gas exchange and water loss through transpiration. the size changes as turgor pressure inflate or deflate guard cells which are controlled by K+ uptake.

solute transport

selective permeability of the plasma membrane controls the short distance movement of solutes into and out of the cells.

Plasmolysis

shrinkage of cytoplasm and detachment from cell wall in hypertonic solution.

Transport of sugar continued

sugars from source tissues loaded into sieve tube elements by both symplastic diffusion and active loading from the apoplast which depends on energy from the proton pump.

If a flaccid cell is placed in a solution with a lower solute concentration

the cell will gain water and become turgid.

Membrane potential

the difference in electric charge (voltage) across the cell's plasma membrane due to the differential distribution of ions. it affects the activity of excitable cells and the transmembrane movement of all charged substances.

Surface tension

the difficulty to break or stretch the surface of a liquid. Water has high surface tension -- negative pressure potential.

pressure potential

the physical pressure on a solution; can be positive or negative (water in living cells generally under positive pressure).

Casparian strip

the waxy barrier that is in the walls of the endodermis. No apoplastic entry into the vascular bundle.

When an animal cell is placed in a hypotonic solution and water enters the cell via osmosis, the volume of the cell increases until it bursts. This does not happen to plant cells, because _____.

they have cell walls, which provide pressure to counteract the pressure of the incoming water.

water and solutes move together through

tracheids and vessel elements of xylem, and sieve-tube elements of phloem.

the apoplastic route

water and solutes (dissolved chemicals) move along the continuum of cell walls and extracellular spaces.

The symplastic route

water and solutes move along the continuum of cytosol. This route requires substances to cross a plasma membrane once, when they first enter the plant.

The transmembrane route

water and solutes move out of one cell, across the cell wall, and into the neighboring cell, which may pass them to the next cell in the same way.

The word potential in the term water potential refers to

water flows from regions of higher water potential to regions of lower water potential.

Adhesion and cohesion in the ascent of xylem sap

water molecules are attracted to cellulose in xylem cell walls through adhesion. Adhesion of water molecules to xylem cell walls helps offset the force of gravity. Cohesion makes it possible to pull a column of xylem sap because water molecules are attracted to each other through cohesion.

The movement of water across biological membranes can best be predicted by....

water potential.

Once in the xylem

water transport by bulk flow is rapid and prolific. Faster than diffusion or active transport.

Because outside of leaf is often drier than inside

water vapor diffuses down its water potential gradient and exits via stomata. Loss of water by diffusion and evaporation is the pulling force for upward movement.