Transport in plants

How water moves up to the top of a 10 story tree.

• Water and minerals first enter the roots.

• Then move to the xylem.

• Innermost vascular tissue.

• Water rises through the xylem because of a combination of factors.

• Most of that water exits through the stomata in the leaves.

Long distance movement

• Local changes result in longdistance movement of materials

• Most of the force is “pulling” caused by transpiration.

• Evaporation from thin films of water in the stomata.

Transport of water in roots

Water enters the root into xylem Carbohydrates move up or down.

water transport in stem

Water moves up the xylem Carbohydrates move up or down.

Transport of water occurs due to

Cohesion and adhesion

Cohesion

water molecules stick to each other.

Adhesion

water sticks to walls of tracheids or vessels.

Movement of water at cellular level

Water can diffuse down its concentration gradient across a plasma membrane by osmosis.

Osmotic concentration

When two solutions have different osmotic concentrations: Hypertonic, hypotonic and isotonic.

Hypertonic solution

Higher solution concentration

Hypotonic solution

Lower solute concentration

Isotonic

When two solutions have the same osmotic concentration.

Osmosis and cellular changes

• If a single plant cell is placed into pure water.

• Water moves into cell by osmosis.

• Cell expands and becomes turgid.

• If cell placed in high concentration of sucrose (or salt).

• Water leaves cell.

• Cell shrinks – plasmolysis .

plasmolysis

Plasmolysis is the process where a cell, particularly a plant cell, loses water and its cytoplasm shrinks away from the cell wall, causing the cell to wilt, due to being placed in a hypertonic (high solute concentration) solution. 

Osmotic pressure

• Force needed to stop osmotic flow

• Plant cell wall can reach a balance of osmotic pressure driving water in with hydrostatic pressure driving water out

• Provides support for plant

• A flaccid or plasmolyzed plant cell cannot support its weight

Water potential

• are a way to represent free energy of water

• Especially useful for botany

• Includes measurement of solutes (osmolarity) and pressure (turgor pressure)

• high solute concentration causes low water potential

• high turgor pressure causes high water potention

is used to predict which way water will move.

Water absorption

• Most of the water absorbed by the plant comes in through the region of the root with root hairs

• The Surface area further increased by mycorrhizal fungi.

• Once absorbed through root hairs, water and minerals must move across cell layers until they reach the vascular tissues

• Water and dissolved ions then enter the xylem and move throughout the plant

The three transport routes that exisys through plant cells

1. apoplast route

2. symplast route

3. transmembrane route

Apoplast route

movement through the cell walls and the space between cells

• Avoids membrane transport.

Symplast route

cytoplasm continuum between cells connected by plasmodesmata

Transmembrane route

membrane transport between cells and across the membranes of vacuoles within cells

• Permits the greatest control.

Inward movement of water

• Water moves through the apoplast route through the ground tissue of the cortex

• Eventually on their journey inward, water molecules reach the endodermis

• Any further passage through the cell walls is blocked by the waterproof Casparian strips

• Molecules must pass through the cell membranes and protoplasts of the endodermal cells to reach the xylem (Symplast or Transmembrane routes)

Movement of ions

• Plasma membranes of endodermal cells contain a variety of protein transport channels

• Mineral ion concentration in the soil water is usually much lower than it is in the plant (low water pressure )

• Active transport across endodermis is required for increased solute concentration in the stele.

• Proton pumps (symporters) transport specific ions against even larger concentration gradients.

Regulation of water movement

• Water potential regulates the movement of water through a whole plant

• Water moves from the soil into the plant only if water potential of the soil is greater than in the root

• Water in a plant moves along a Ψw gradient from the soil to successively more negative water potentials in the roots, stems, leaves, and atmosphere

Xylem transport

• The aqueous solution that passes through the endodermal cells moves into the xylem

• As ions are actively pumped into root or move via facilitated diffusion, their presence decreases the water potential

• Makes a hypertonic environment

• Water then moves into the plant via osmosis, causing an increase in turgor pressure

Root pressure

• Caused by the continuous accumulation of ions in the roots at times when transpiration from leaves is low or absent . Often at night.

• Causes water to move into plant and up the xylem despite the absence of transpiration

• Root pressure alone, however, is insufficient to explain xylem transport

• Transpiration provides the main force

Guttation

is the loss of water from leaves when root pressure is high.

Cohesive water forces

• Water has an inherent tensile strength that arises from the cohesion of its molecules

• The tensile strength of a water column varies inversely with its diameter

• Because tracheids and vessels are tiny in diameter, they have strong cohesive water forces

• The long column of water is further stabilized by adhesive forces

Effect of cavitation

• Tensile strength depends on the continuity of the water column

• A gas-filled bubble can expand and block the tracheid or vessel (process called cavitation)

• breaks the tensile strength of a water column.

• Damage can be minimized by anatomical adaptations

• Presence of alternative pathways.

• Pores smaller than air bubbles.

Cavitation

A gas-filled bubble can expand and block the tracheid or vessel.

Mineral transport

• Xylem cells are essential for the bulk transport of minerals

• Ultimately the minerals are relocated through the xylem from the roots to other metabolically active parts of the plant.

• Phosphorus, potassium, nitrogen, and sometimes iron may be abundant in xylem.

• Calcium, an essential nutrient, cannot be transported elsewhere once it has been deposited in a particular plant part.

Rate of transpiration

• Over 90% of the water taken in by the plant’s roots is ultimately lost to the atmosphere

• At the same time, photosynthesis requires a CO2 supply from the atmosphere

• Closing the stomata can control water loss on a short-term basis

• However, the stomata must be open at least part of the time to allow CO2 entry

Guard cells

• Only epidermal cells containing chloroplasts

• Have thicker cell walls on the inside and thinner cell walls elsewhere

• Bulge and bow outward when they become turgid.

• This causes the stoma between two guard cells to open.

Stomatal opening

• Turgor in guard cells results from the active uptake of potassium (K+ ), chloride (Cl- ), and malate

• Addition of these solutes causes water potential to drop.

• Water enters osmotically and cells become turgid

Stomal opening and closing

• Closed when CO2 concentrations are high inside leaf

• Open when blue wavelengths of light promote uptake of K+ by the guard cells

• Closed when temperature exceeds 34°C and water relations unfavorable

• Crassulacean acid metabolism (CAM) plants conserve water in dry environments by opening stomata and taking in CO2 at night

Plant adaptations to drought

• Many morphological adaptations allow plants to limit water loss in drought conditions

• Dormancy.

• Loss of leaves – deciduous plants.

• Covering leaves with cuticle and wooly trichomes.

• Reducing the number of stomata. • Having stomata in pits on the leaf surface.

Plant responses to flooding

Plants have adapted to flooding conditions which deplete available oxygen

• Flooding may lead to abnormal growth.

• Oxygen deprivation most significant problem.

• Plants have also adapted to life in fresh water

Growth in saltwater

Plants such as mangroves grow in areas flooded with salt water

• Must supply oxygen to submerged roots and control salt balance.

• Pneumatophores – long, spongy, airfilled roots that emerge above the mud.

• Provide oxygen to submerged roots.

• Succulent leaves contain large amount of water to dilute salt.

• May secrete salt or block salt uptake.

Pneumatophores

long, spongy, airfilled roots that emerge above the mud.

Growth in saline soil

• Halophytes are plants that can tolerate soils with high salt concentrations

• Some produce high concentrations of organic molecules in their roots

• This alters the water potential enhancing water uptake from the soil.

Phloem transport

• Most carbohydrates produced in leaves are distributed through phloem to rest of plant

• This process, called translocation, provides building blocks for actively growing regions of the plant

Phloem transports

• hormones.

• mRNA.

• a variety of sugars.

• amino acids.

• organic acids.

• proteins.

• ions.

Pressure-flow hypothesis

Most widely accepted model describing the movement of carbohydrates in phloem

• Dissolved carbohydrates flow from a source to a sink.

• Sources include photosynthetic tissues.

• Sinks include growing root and stem tips as well as developing fruits.

• Food-storage tissue can be sources or sinks.

Leaf

Source Some water passively follows sucrose into phloem.

Phloem-Loading

occurs at the source

• Carbohydrates enter the sieve tubes in the smallest veins at the source.

• Sieve cells must be alive to use active transport to load sucrose.

• Water flows into sieve tubes by osmosis.

• Turgor pressure drives fluid throughout plant.

Shoot

Sink Water flows out passively into xylem.