Chapter 31 Transport in plants

Ancestors of land plants: what did they evolve from, what did they absorb

Algae are the ancestors of land plants, they absorbed water, minerals, and CO2 directly from surrounding water.

What did the evolution of xylem and phloem in land plants make possible?

Allowed for long-distance transport of water, minerals, and products of photosynthesis

What purpose do stems serve in plants?

Stems serve as conduits for water and nutrients and as supporting structures for leaves

What is soil for plants?

Soil is a resource mined by the root system

Transport of sap

Transport begins with the absorption of resources by cells, the movement of substances into and out of cells is regulated by selective permeability. To survive plants must balance water uptake and loss. Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure.

What happens when a plant is placed in an environment with a higher solute concentration?

The cell will lose water and undergo plasmolysis

What happens if a flaccid cell is placed in a solution with a lower solute concentration?

The cell will gain water and become turgid

What does turgor loss in plants cause?

wilting, which can be reversed when the plant is watered

3 compartments that control transport

cell wall, cell interior, and a vacuole that takes up 90% or more of the protoplast's volume.

What regulates transport in plants?

Plant cells regulate transport. The plasma membrane is highly folded and directly controls the traffic of molecules into and out of the protoplast.

What is the plasma membrane?

a barrier between two major compartments, the cell wall and the cytosol

The symplastic route

water and solutes move along the continuum of cytosol. uses the continuum of cytosol. within the living part of the cells

Aploplast route

continuum of cell walls and extracellular spaces. outside the cells in between them

Transmembrane route

Out of one cell, across a cell wall, and into another cell

Bulk flow?

Efficient long-distance transport of fluid. It is driven by pressure. Water and solutes move together through tracheids and vessel elements of xylem and sieve tube elements of phloem.

What do mature tracheids and vessel elements not have?

no cytoplasm. this allows for efficient movements. sieve-tube elements have few organelles in their reduced cytoplasm.

Where are most water and minerals absorbed in a plant?

near the root tip. Where the epidermis is permeable to water and where root hairs are located. Root hairs increase the surface area of roots. After a soil solution enters the roots the cortical cell membranes enhance uptake of water and selective material.

Endodermis in plants

The innermost layer of cells in the root cortex. It surrounds the vascular cylinder and is the last checkpoint for selective passage of minerals from the cortex into vascular tissue. water can cross the cortex via the symplast or aploplast or both.

Casparian strips in roots

Strip of the endodermal walls the blocks apoplastic transfer of minerals from the cortex to the vascular cylinder

Bulk Flow

Driven by negative pressure in the xylem. Positive root pressure is relatively weak, it’s a minor mechanism of xylem bulk flow.

Guttation

The exudation of water droplets on tips or edges of leaves

Transpirational pull

Most water is pulled upward into the xylem due to transpirational pull. Water evaporates from a leaf causing an increase in tension on remaining water inside the leaf, eventually pulling water up out of the xylem. Facilitated by the cohesion of water molecules to each other and the adhesion of water molecules to cell walls.

How does xylem sap move?

Bulk flow moves sap against gravity using the transpiration-cohesion-tension (Cohesion-tension) mechanism. Transpiration lowers water potential in leaves and this generates tension that pulls the sap up through the xylem. there is no energy cost to bulk flow of xylem sap. hydrogen bonds hold water to water (cohesion) and water to xylem (adhesion).

Where do plants lose most of their water?

Through the stomata, each stomata is flanked by guard cells that control the diameter of the stomata by changing their shape. Stomata are open in daylight but not at night and they are regulated by CO2, light and circadian rhythm

Effects of transpiration on wilting and leaf temperature

Plants lose a large amount of water through transpiration. If the lost water isn’t replaced by transport water the plant will lose water and wilt. Also results in evaporative cooling which can lower the temperature of a leaf and prevent the denaturation of various enzymes involved in photosynthesis and other metabolic processes.

Xerophytes

Plants adapted to arid climates. They have leaf modifications (stomatal crypts, short-lived leaves, reflective bristles) that reduce the rate of transpiration

How are the products of photosynthesis transported through phloem?

Translocation

Translocation

As organic substances concentrate inside cells at the phloem source (leaf), water is drawn into the cells using positive (turgor) pressure which generates movement by bulk flow towards a sink. (sinks are the top and roots of plants, the ends)

Phloem Sap? Where does it travel?

An aqueous solution that is high in sucrose and is under pressure. From a sugar source to a sink source

What is a sugar source?

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

What is a sugar sink?

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