Chapter 9 Plant Transport

What is a low power plan?

a scientific drawing that shows structures in proportion but with no cellular detail

How are the xylem vessels and phloem tissues arranged in the root of a dicot plant?

there is a central core of xylem vessel in an X-shape, with phloem tissue between the arms of the X-shape

Describe the cells surrounding the vascular bundle in a root and their function

There are meristem cells (that differentiate to xylem and phloem) called the pericycle, and surrounding this is the endodermis, which regulates water movement into the xylem vessels

How is the arrangement of vascular bundles in roots structurally advantageous?

it provides strength to withstand the pulling forces to which the roots are exposed when the stem is blown in the wind

Where are vascular bundles found in stems of dicot plants and why?

on the outer edge of the stem for structural support

Describe the arrangement of cells and tissue in vascular bundles in stems

• xylem vessel - inside of vascular bundle

• phloem tissue - outside of vascular bundle

• cambium - in between xylem and phloem

• fibrous cap - on outside

What is cambium?

meristem cells found in vascular bundles in stems

What is the purpose of vascular bundles in the stem having a fibrous cap?

protection

True or false? xylem vessels in roots have larger lumens than xylem vessels in stems

false - the lumen is wider in stems

Fill in the blanks: In dicot leaves, vascular bundles from the ___ and ___ of a leaf, which branch and get smaller as they spread away. The ___ is located on top of the ___.

midrib, veins, xylem, phloem

Why is the position of vascular bundles in the leaf structurally beneficial?

The veins provide structural support so that the leaf can remain flat and open to absorb maximum sunlight for photosynthesis

When viewing vascular bundles, which differential stain can be used?

Toluidine blue

Correct the statement: the stain Toluidine blue stains the cytoplasm of xylem vessels blue

cytoplasm → lignin

blue → red

True or false? Plants have higher metabolic demands than animals due to photosynthesis

false - plants metabolic demands are lower

What is mass flow?

the bulk movement of materials directed by a force

What is a dicotyledonous (dicot) plant?

a plant that has 2 seeds leaves, and makes seeds that contain 2 cotyledons (organs that act as food stores)

Why do plants need a transport system?

• metabolic rate - although oxygen can be transported by diffusion (as the respiration rate is low enough), water, assimilates, mineral ions and hormones need transporting

• size - materials must be moved throughout the whole plant

• SA:V ratio - although leaves have a large SA and can rely on diffusion the rest of the plant has a low SA:V ratio

Label parts D, E and F

xylem, cambium, phloem

Label A, B and C

epidermis, cortex, pith

In what part of a plant (root, stem or leaf) would this transverse section be found?

stem

In what part of a plant (root, stem or leaf) would this transverse section be found?

leaf

Label parts A and B

A - xylem, B - phloem

Label parts C and D

C - lower epidermis, D - spongy mesophyll

Label parts E and F

E - palisade mesophyll, F - upper epidermis

In what part of a plant (root, stem or leaf) would this transverse section be found?

root

Label part A

Label parts B, C and D

B - xylem, C - phloem, D - pericycle

Label parts A and E

A - endodermis, E - cortex

What are the 2 functions of xylem vessels?

• transport of water and mineral ions

• structural support

Why do xylem vessels give plants structural support?

the lignified cellulose cell walls have a high tensile strength

What are xylem vessels made from, and describe their basic structure?

made from dead xylem cells which are aligned end to end to form a continuous hollow column

What is the substance that is used to waterproof xylem cells?

lignin

Why is it important that the lignified cell walls have a high tensile strength?

they hold the xylem vessel open, keeping it from collapsing even when water is in short supply

Fill in the blanks: as xylem vessels develop, they are impregnated with ___ which makes their cell walls waterproof, but ___ the cells causing the end walls and contents to decay. During this process, patterns form in the cell walls which can be ____ (spiral) rings or ___ (broken network of rings). These rings prevents the vessel from being too rigid and allows some ___.

lignin, kills, anular, reticulate, flexibility

What structures form in places where lignification is incomplete?

pits / bordered pits

What is the function of pits?

allows the lateral movement of water (for maintaining turgidity in the plant)

What 3 adaptations of the xylem vessels ensure that the flow of water is not impeded?

• no end walls

• no cell contents

• lignin to prevent collapsing

What are the 2 elements of phloem tissue?

sieve tube elements and companion cells

Describe the sub cellular structures found in sieve tube elements

there is no nucleus or sub cellular structures and very little cytoplasm

Describe the basic structure of the sieve tubes of phloem tissue

elongated sieve tube elements are lined up need to end to form the sieve tubes

Do sieve tube elements contain lingin?

no

True or false? sieve tube elements have thick walls to maintain a pressure gradient

false - they are thin so substance can diffuse/move by osmosis in/out

Describe the structure and function of the end walls of sieve tube elements

there are perforated end walls, called sieve plates, which allow the movement of sap between cells

Describe the sub cellular structure of a companion cells

• large nuclei

• dense cytoplasms

• numerous mitochondria (for ATP for active loading of sucrose)

How are the companion cells and sieve tube elements linked together?

plasmodesmata

Give 3 reasons why water is important for both the structure and metabolism of plants

• turgor/hydrostatic pressure (due to osmosis) supports stems and leaves

• turgor drives cell expansion (allowing plant root to grow through hard surfaces)

• loss of water by evaporation keeps plants cool

• water is a reactant in photosynthesis

• water is needed for the transport of mineral ions and sugars (in aqueous solution)

What is water potential?

The total potential energy of the water molecules in a system. It is a measure of the tendency for water molecules to diffuse from one place to another down a concentration gradient

What is the water potential of pure water?

0

Do solutions have a more positive or negative water potential than pure water?

more negative

Choose the correct words: water moves from a more/less negative water potential to a more/less negative water potential

less, more

How are root hair cells adapted as exchange surfaces?

• microscopic size allows penetration of soil particles

• large SA:V ratio due to root hairs

• thin surface layer (cell wall and membrane) for rapid diffusion/osmosis

• concentration of solutes in cytoplasm maintains a water potential gradient between the soil and cells

Describe how water moves into a root hair cell

Mineral ions diffuse or are actively transported into the root hair cell (RHC) using energy from ATP. This lowers the water potential of in the cytoplasm of the RHC, creating a steep water potential gradient with the soil. Water moves into the RHC by osmosis, down the water potential gradient from a higher water potential in the soil to a lower water potential in the root hair cell across a selectively/partially permeable membrane

Why may the rate of transport of water into the root hair cells decrease in waterlogged soil, even though water is more readily available?

In waterlogged soil, there is less oxygen for aerobic respiration, so less ATP is produced for the active transport of mineral ions, so the steep diffusion gradient is less well maintained

How (briefly) does water move from the root hair cells, through the cortex cells?

The water potential of the RHC is higher than that of the cortex cell, so water (and dissolved mineral ions) move across the cortex by osmosis down a series of water potential gradients

Name the 2 pathways by which water can move through cortex cells

apoplast and symplast pathway

Fill in the blanks: When water move through the apoplast, it moves between _____ strands of the ___ ___ and through ____ spaces. As it doesn’t pass through any plasma membranes, it moves by ____ due to ____ forces between the water molecules, and it can carry mineral ions and salts

cellulose, cell wall, intracellular, mass flow, cohesive

What is the symplast? How does water move through the symplast pathway?

The symplast is the continuous cytoplasm of the plant cells that are connected through the plasmodesmata. Water moves by osmosis along a series of concentration gradients.

What is the layer of cells surrounding the xylem in the root calls?

endodermis

What is found in a waxy, strip around endodermal cells?

casparian strip

What waterproof substance is the Casparian strip made of?

suberin

What is the function of the Casparian strip?

The Casparian strip blocks the apoplast pathway, forcing water into the symplast pathway before it reaches the xylem vessel

Why is the Casparian strip needed? Give 2 reasons

• It moves mineral ions into the endodermal cells, which is needed to move the water into the xylem

• it is a physical defence mechanism as the selectively permeable membranes of the endodermal cells block pathogen

How is water moved from the endodermis into the xylem?

Mineral ions are actively transported into the xylem, lowering the water potential. Water moves down a water potential gradient from the endodermal cells, into the xylem vessel

True or false? the Casparian strip also blocks the back flow of water out of the xylem

true

Name the 3 processes that enable water to move up the stem in the transpiration stream

root pressure, capillary action, transpiration pull

What is root pressure?

the pressure of water being forced into the xylem at the roots caused by the active transport of mineral ions into the xylem from the endodermal cells

True or false? root pressure is the main factor that drives water up the stem

false - it is the least significant factor

Fill in the blanks: Capillary action is caused by the water molecules being ___ (property of water), so they form _____ ___ with the walls of the ____ (narrow/wide) xylem vessels, becoming attracted to them by ____. This pulls water up the sides of the vessels.

charged/polar, hydrogen bonds, narrow, adhesion

Fill in the blanks: Transpiration pull - As water molecules are ___ (property of water), they are attracted to each other by forces of ___, which hold the water molecules together in a ___ ____.

charged/polar, cohesion, continuous column

By what process of movement is water moved up the stem?

mass flow

What is transpiration?

the evaporative loss of water vapour from the aerial parts of a plant

What 3 processes does transpiration involve?

• osmosis of water from the xylem vessels into mesophyll cells

• evaporation from the surface of the spongy mesophyll cells into intracellular (air) spaces

• diffusion of water molecules out of the intracellular spaces through the stomata

What is the transpiration stream?

the effect of water being pulled up the plant in a continuous column to replace water lost by evaporation from the leaves

What is the cohesion-tension theory?

the ‘pull from above’ in transpiration creates tension in the column of water (lignification prevents the vessel from collapsing under this pressure), and this relies on a continuous cohesive column of water to be maintained

If a xylem vessel is damaged, how can the cohesion-tension theory still be maintained?

water moves into adjacent xylem vessels through pits

What can the intracellular spaces between the spongy mesophyll cells and stomata also be called?

substomatal cavity

How do guard cells increase turgor?

guard cells passively become turgid when there is adequate water in the plant. This can be increased further by the guard cells actively transporting potassium (K+) ions into the cells, lowering the water potential, so more water moves in by osmosis

How does the structure of the guard cells allow to swell into ‘bean-shapes’?

• cellulose hoops prevent the cells swelling in width

• inner cell walls are less flexible than the outer walls

What happens to guard cells during a water deficiency?

A plant hormone (abscisic acid) is produced in the root, which travels up the transpiration stream, and causes the potassium ions to move out of the guard cells, increasing the water potential, so water moves out by osmosis and the guard cells become flaccid, closing the stomata

How does increasing light intensity affect the rate of transpiration?

it increases the number of stomata open for gas exchange, so increases rate of water vapour diffusing out, so increases transpiration rate

How does increasing relative humidity affect the rate of transpiration?

decreases rate of transpiration and it lowers the steepness of the water potential gradient between the inside and outside of leaf

What is relative humidity a measure of?

the amount of water vapour in the air (humidity) compared to the total concentration of water the air can hold

How does increasing temperature affect the rate of transpiration?

increases transpiration rate as

• increases kinetic energy of water molecules, so they evaporate faster

• increases the concentration of water vapour the external air can hold before it becomes saturates (so decreases relative humidity)

which both increase the diffusion gradient

How does increasing air movement affect the rate of transpiration?

higher air movement outside the leaf carries away water vapour maintaining a high water vapour potential gradient, increasing the rate of transpiration

How does a low soil water availability affect the rate of transpiration?

the plant can’t replace the water lost, so the stomata close and leaves wilt, reducing the rate of transpiration

How does the presence of a cuticle affect the rate of transpiration?

the waxy coating is hydrophobic so water cannot move through it, decreasing transpiration rate

Which of these factors can also affect the rate of transpiration?

• A - turgidity of plant

• B - number of leaves

• C - density of plant cells

• D - number, size and position of stomata

B and D

True or false? you can directly measure transpiration

false

What assumption has to be made when using a potometer?

The rate of water uptake (measured by the potometer) is equal to the rate of water lost through transpiration

Is measuring transpiration with a potometer 100% accurate?

no as some water is in photosynthesis and used/produced in other reactions, and it assumes the cells are turgid

Describe how to set up the shoot cutting in the potometer

• cut the shoot underwater and at an angle (to provide large SA)

• seal the stem in the potometer using waterproof gel such as vaseline (removes air bubbles and makes air tight)

• dry the leaves

How do you calculate the rate of transpiration?

volume of water uptake (pi x r2h) / time

How can you investigate how the rate of airflow affects the rate of transpiration?

use a fan or hairdryer (but hairdryer may add heat)

How can you investigate how the humidity affects the rate of transpiration?

spray water in a plastic bag and wrap it around the plant

How can you investigate how the light intensity affects the rate of transpiration?

change distance of plant from a lamp (use a glass screen to remove heat)

How can you investigate how the temperature affects the rate of transpiration?

change the temperature of the room

Why might some potometers have a reservoir?

water can be let back into the capillary tube, pushing the air bubble back to the start of the scale, so multiple readings can be done

How can you make the air bubble more visible in the potometer?

place the end of the capillary tube in coloured dye

What is translocation?

the movement of assimilates through the phloem tissue

What are assimilates?

substances made by the plant such as sugars