Vascular Tissue

Negative Pressure

Pulling (Transpirational Pull)

Positive Pressure

Pushing (Root pressure)

Pits

pores through which fluids are passed from one tracheid to another

Root pressure

the mechanism by which positive pressure in the roots moves water upward in a plant -Water entering the roots creates a positive pressure that tends to push water upward. -Minerals are moved from the soil into the xylem against their concentration gradient by active transport. -The high concentration of dissolved materials in the xylem causes more water to move in by osmosis. -This increases the positive pressure that pushes the water column up. -This process is aided by the adhesion (sticking) of water molecules to the cell walls of the xylem tissue.

Transpiration

the process in which water evaporates from the inside of a leaf to the outside through the stomata

Cohesion

-The columns of water in the xylem have a property called cohesion. -The force of attraction between the water molecules in each narrow xylem tube provides a force that keeps the water column unbroken while it is being pulled up under tension, as shown in

Adhesion

-causes the water molecules to adhere, or stick, to the xylem walls. -Along with cohesion, adhesion keeps the water column from breaking as it is pulled upward.

Transpiration

-the major force responsible for the movement of water and dissolved minerals upward in a plant stem. -Results in negative pressure as water molecules continuously leave the plant through the stomata -This negative pressure exerts tension on the water confined in the xylem's conducting tubes all the way down to the roots.

Transpiration: Process

1. As water evaporates from a leaf cell, the water pressure in the cell falls 2. Water moves by osmosis from the xylem in a leaf vein into a leaf cell 3. The removal of water from the leaf xylem lowers the water pressure in the leaf xylem which causes water to flow from the stem xylem into the leaf 4. This in turn causes water to flow from the root xylem into the stem

Pressure Flow: process

1. Sucrose enters the phloem int he leaf, increasing the concentration of sucrose solution in the phloem 2. Water moves into the phloem as a result of the increased concentration of sucrose. The movement of water leads to an increase in pressure in the phloem 3. The pressure gradient between the source and the sink causes a flow fo solution through the phloem from the source to the sink. Water moves into and out of the phloem along the way 4. Sucrose is removed from the phloem by tissues in the plant stem and root. This causes the concentration of sucrose solution to fall, adn therefore water moves out of the phloem. Pressure in the phloem decreases

...

5. As water continues to transpire from the leaf, pressure continues to drop in the leaf until tension is created 6. THis tension then draws the water confined in the xylem's conducting tubes up to replace the lost water 7. THe tension on the water column extends all the way from the leaves to the roots THe columns of water in the xylem exhibit COHESION - they do not break while they are being pulled up under tension 8. Adhesion helps water molecules adhere to the vessel walls

Sink

Any region in the plant where sugars are used or stored. For example, while the flowers and fruits of a plant are developing, they are sinks because they need organic molecules for their growth and development. Roots are also sinks, since they need nutrients for energy and growth. They also store carbohydrates during the winter.

Source

Any region of the plant where the sugars are entering into sieve tubes. A common source is the palisade and spongy mesophyll tissues in the leaf, where photosynthesis occurs.

Long distance transport in xylem

By root pressure and transpirational pull

Energy for sugars and other nutrients

Energy is needed

Energy for diffusion

Energy is not required because it occurs naturally as a result of the random movement of particles

Phloem vs. Xylem

In xylem, it is mostly negative pressure moving water and dissolved minerals; in phloem, it is positive pressure driving the flow from source to sink.

Transpirational Pull: Pulling Water Up Against Gravity

Negative pressure (pulling) from above is the strongest force for long-distance transport in plants

Transport in the xylem

Roots contain a higher concentration of dissolved nutrients than the surrounding soil, so water moves into the roots by osmosis. Water moves through the root cells or through the intercellular spaces within the roots and enters the xylem Water is then transported in the xylem tissue up through the roots into the stem As xylem tissue carrying water and minerals enters the leaf, the conducting vessels branch into the numerous veins. From the end of each vein, water and minerals can diffuse into the cells of the leaf. Much of the water that reaches the leaf is lost to the atmosphere as it evaporates through the stomata (transpiration)

Active transport

Sugars and nutrients must move across cell membranes

Factors that affect pressure flow

The direction of flow is always from source to sink, and the sinks change depending on the plant's stage of growth and development. For example, in older plants, newly forming leaves are a sink, and the sucrose travels from photosynthesizing leaves to growing leaves. When fruit is forming, sucrose moves there by translocation, and growth in other areas of the plant slows down. Because the flow moves from source to sink, sometimes flow can happen in both directions at once. Different sieve tubes can be conducting phloem sap in opposite directions, from different sources to different sinks.

The Mechanism of Translocation

Translocation in phloem moves sucrose from a source to a sink

Three main factors in cohesion-tension model

Transpiration, Cohesion, Tension

Guttation

Under conditions of high humidity, water cannot evaporate from the leaves. In smaller plants, such as grasses and roses, root pressure can cause water to be pushed up the xylem to the surface of the leaves. Since the air is humid and the rate of transpiration is low, the xylem fluid does not evaporate. Instead, it forms tiny droplets along the edge of the leaves

Pressure Flow in Phloem

When nutrients are pumped into or removed from the phloem, the change in concentration causes a movement of water in that same direction. As a result, internal pressure builds up at the source end of the sieve tube and pushes the sucrose-rich solution toward any sink, where the sucrose is then removed.

Cohesion-tension model

a model of water transport that explains how water is moved from the roots to the leaves of a plant

pressure-flow model

a model that explains how organic molecules move from source to sink through phloem in a flowering plant uses a combination of osmosis and pressure dynamics to explain how materials are pushed from a source to a sink through translocation

Companion cells

carry out life functions to maintain both types of cells

Xylem: structure

consists of cells called tracheids Angiosperms: tracheids and vessel elements

Tracheid

dead cells that taper the ends and overlap one another

Osmosis

diffusion of water molecules across a membrane

Vascular tissue

internal system of tubes that run lengthwise throughout hte stem of a plant, connecting the roots and stems

Vessel elements

long continuous tubes formed from dead, hallow cylindrical cells arranged end to end

Diffusion

net movement of particles from an area of high concentration to an area of low concentration

Sieve tube elements

no nuclei, have plates at both ends that are perforated with holes, making them resemble a sieve. Each sieve tube element has an associated companion cell that does not have a nucleus

Concentration gradient

particles move according to concentration gradients - the difference in concentration between two areas

Translocation

the transport of sucrose and other organic molecules through the phloem of a plant For example, during winter storage, plants transport carbohydrates into their roots, and then back to the trunk and branches in the spring.

Function of vascular tissue

to transport water and dissolved substances throughout the plant

Phloem

transports nutrients such as sugar from leaves to roots but also from roots and mature leaves to new leaves

Xylem

transports water and minerals from roots to leaves

Phloem

two types of cells: sieve tube elements and companion cells