Unit 6 – Transport in Flowering Plants - Wikispaces6+-+Transport+in... · Unit 6 – Transport in...

Unit 6 – Transport in Flowering Plants 6.1 – Water and ion uptake, 6.2 – Transpiration and translocation

SUFEATIN SURHAN BIOLOGY MSPSBS 2009

SYLLABUS CHECKLIST

Candidates should be able to:

a) relate the structures and functions of root hairs to their surface area and to water and ion uptake;

b) state that transpiration is the loss of water vapour from the leaves to the stomata;

c) describe: how water vapour loss is related to cell surfaces, air spaces and stomata the effects of variation of temperature, humidity and light intensity on transpiration rate how wilting occurs;

d) investigate, using a suitable stain, the pathway of water in a cut stem;

e) explain the movement of water through the stem in terms of transpiration pull;

f) identify xylem and phloem tissues as seen in transverse sections of unthickened,

herbaceous, dicotyledeneous roots, stems and leaves;

g) state the functions of xylem and phloem.

sufe/bio/mspsbs/2009 of 11

For plants, there are some advantages in large size.

They can, for example, compete more readily for

light. As a result, many trees are tall,

some exceeding 100m.

The leaves, as a site for photosynthesis, must be in

these aerial parts to obtain light.

The water, so essential for photosynthesis, is

however, collected by the roots which may be some

considerable distance beneath the soil surface. An

efficient means of transporting this water and

certain minerals to the leaves is necessary.

The sugars formed as a result of photosynthesis in

the leaves must be transported in the opposite

direction for cellular respiration in the roots.

The transport system in plants is known as the

vascular system.

The Vascular System

The vascular system of plants consists of vascular

tissues; the xylem and the phloem.

The xylem and phloem extend uninterrupted from

the roots to the stem and the leaves.

Xylem

The xylem is composed mainly of vessels. These are

long hollow tubes stretching continuously from root

to leaf formed by cells joined end to end, without

any cross walls.

Xylem cells are dead cells which contain no

protoplasm and the walls are strengthened by lignin

deposits.

Lignin is a tough, complex, organic compound that

gives a plant its woody characteristic. Cellulose and

lignin both provide strength and keep the plant

upright.

Xylem vessels conduct a mixture of water and

solutes called xylem sap.

The different types of xylem vessels:

The functions of the xylem include:

Transporting water and mineral salts from the

roots to the stems and leaves.

To provide support for plants.

Phloem

The phloem is composed mainly of sieve tubes and

companion cells.

A sieve tube is made up of a single row of elongated

cells with perforated cell walls (sieve plates).

Phloem cells are living cells (however, no nuclei in

mature cells) and are thin-walled.

Transport of food (in the form of sucrose and amino

acids) occurs by diffusion and active transport.

Companion cells have nuclei and assist sieve tubes

in their transport function.

The function of the phloem vessel is to conduct

manufactured food mainly from leaves to all other

parts of the plant – this is known as translocation.


Positions of xylem and phloem in the root

of a dicotyledon (Transverse section)

Actual tissue structure:

Schematic diagram:

Positions of xylem and phloem in the stem



Schematic diagram:

Positions of xylem and phloem in the leaf




Functions of the main tissues in the stems

and roots

Type of tissue: Epidermis.

Description:

A thin outer layer of living cells.

Functions:

Maintains the shape of the whole structure.

Protects against bacterial or fungal

infections.

Some differentiates into projections called

root hair cells.

Type of tissue: Cortex.

Description:

Known also as packing tissues.

It has several layers of relatively large thin-

walled living cells located between the

epidermis and pith.

Functions:

Cells are highly permeable to water and

dissolved solutes.

Intercellular spaces in this layer allow

oxygen to diffuse into the stem or root for

aerobic respiration.

Cells may be used to store starch in the

form of granules.

Type of tissue: Pith.

Description:

Central part of stem.

May be absent in the stem of some plants

(hollow).

Function:

Storage of starch grains.

Root systems of flowering plants

There are two types of root systems in flowering

plants: Fibrous root system and Tap root system.

Structure of a root

The root is divided into several regions:

The root cap protects the root as it grows

through the soil.

The meristematic region consists of small young

cells that are actively dividing by mitosis to

form new identical cells.

The elongation region is where the new cells

elongate to increase the length of the roots.

The maturation region has minute projections

called root hairs several hundred per cm2 which

increases the surface area for rapid and

efficient uptake of water and mineral salts.


Movement of water and ions into the root

Soil water enters the root hairs by osmosis.

From the root hairs, it moves inwards from cell to

cell by osmosis until it reaches the xylem vessels in

the root.

It is transported from the root to the stem and

leaves in the xylem by root pressure, capillary

action and transpiration pull (main factor).

From the above diagram:

The soil water is at a higher water potential

than the cell sap in the root hair cell. Hence,

soil water is drawn into the root hair cell by

osmosis (along the water potential gradient).

Cell sap of A becomes more dilute, that is, it

has a higher water potential than that of B.

Water is drawn from A into B by osmosis. The

cell sap of B becomes more dilute than that of

C.

Water then enters C. This process of a water

potential gradient being created and water

being drawn inwards from root cell to cell

increases the cell’s turgor pressure.

Due to this pressure water is forced out through

the cell right through the cortex of the root to

the xylem vessels.

In the xylem vessel, water moves in the same way

that a drink moves up a straw when you suck it.

When you suck a straw, the pressure at the top of

the straw is reduced. This causes the liquid at the

bottom of the straw, which is at a higher pressure

to flow up the straw into your mouth.

In the same way, water in the xylem vessel flows up

the xylem vessels to the top of the plant.

The pressure at the top of the plant is reduced by

transpiration.

The water tension developed in the vessels by a

rapidly transpiring plant is thought to be sufficient

to draw water through the root of the soil.

Experimental evidence proving that xylem

transports water:

Mineral salts are dissolved in soil water and exist

in the form of ions. They are often present in low

concentration.

They enter the root hairs mainly by active

transport. Special carrier molecules in the cell

membrane carry the ions into the cell against the

concentration gradient.

Energy used during this transport is supplied by the

mitochondria in the root.

Mineral salts are transported from the roots to the

stem and leaves.

Adaptation of root hair cell to absorption

Root hair is narrow and long. This increases the

surface area to volume ratio which increases the

rate of absorption of water and mineral salts.

The cell sap contains sugars, amino acids and

salts. This makes the cell sap more concentrated

than the soil solution and it is prevented from

leaking out by the plasma membrane. Water can

therefore enter the root hairs by osmosis.

The root hair cell respires to provide energy for

active transport.

Transport of water and mineral salts up

the stem

Three forces cause water to rise up a plant. They

are:

(A) Root pressure.

(B) Capillarity.

(C) Transpiration pull.

(A) Root pressure

Water from the soil enters the root hair cells via

osmosis as a result of osmotic gradient between the

more concentrated root hair cell sap and dilute soil

water.


Water is transported across the cortex of the root to

the vascular bundle in the centre via osmosis also

due osmotic gradient across the cortex.

The continuous inward movement of water into the

xylem causes a hydrostatic pressure in xylem called

root pressure, forcing water up the root towards the

stem.

An experiment showing root pressure:

Capillarity

Once in the xylem of the stem, water is carried

upwards by a second force called capillarity.

Capillarity is the movement of a thin column of

water upward through narrow xylem tubes.

Capillarity depends on two forces:

1. Adhesive forces between water molecules and

xylem vessel walls and

2. Cohesive forces between water molecules

(attraction between water molecules).

Capillarity can be demonstrated by dipping the end

of capillary tubing in water.

Xylem vessels have a microscopic bore (diameter),

which can be responsible for carrying water 20 cm

or more up a plant.

Transpiration pull

Transpiration is the loss of water vapour from the

aerial parts of a plant, especially through the

stomata of the leaves.

It creates a suction force known as ‘transpiration

pull’.

Mesophyll cells take up water from xylem to replace

those lost.

Water in xylem is drawn up to the leaf to replace

the water that has been taken.

Transpirational pull in particular relies on cohesive

forces of water molecules as they move up the

plant.

These cohesive forces ensure that a long, thin,

continuous column of water and mineral salts in

xylem vessels travels up the plant to the leaves.

This is known as the transpiration stream.

Out of these three forces, root pressure and

transpiration pull provide the most force to draw

water up the tree.

Transpiration stream - the flow of water through the

plant from root to leaf:

Water movement from root hair to leaf – a

summary:


A summary of the transport of water into the root of

a plant, up the stem and into the atmosphere:

Importance of transpiration

Transpiration enables water and mineral salts to be

transported to mesophyll cells of the leaves for

photosynthesis to manufacture food.

Transpiration allows water to flow through the

plant, making the cells turgid, thus providing

support.

Transpiration removes latent heat from plant

(cooling effect).

Factors affecting transpiration rate

There are two types of conditions which affect the

rate of transpiration:

1. The internal conditions; which are the structural

features of the leaves of the plant.

2. The external conditions; which are the

conditions of the surrounding / environment.

The internal conditions:

Factor Fast rate of

transpiration Slow rate of transpiration

Number of stomata in

a leaf High Low

Thickness of cuticle in a leaf

Thin Thick particularly in lower surface

Surface area of

leaf

Large (broad leaf)

- more exposure to sunlight

- traps more light and heat energy

Small (narrow leaf)

- less exposure to sunlight

- traps less light and heat energy

The external conditions:

1. Humidity (Amount of water vapour in the air).

Slow rate of transpiration Fast rate of transpiration

Humid / high humidity:

- High amount of water vapour both in the air spaces inside the leaf and in the air outside the leaf (saturated)

- Concentration gradient for water vapour becomes low

- Less water vapour diffuses out of the leaf in a unit time (slow transpiration rate)

Dry / Low humidity:

- Less amount of water vapour in the air outside the leaf (unsaturated)

- High amount of water vapour in the air spaces inside the leaf (saturated)

- This creates a steep concentration gradient for water vapour in the air spaces inside the leaf and in the air outside

- More water vapour diffuses out of the leaf in a unit time (fast transpiration rate)

2. Temperature.


Low

- Water molecules have less kinetic energy

- Slower evaporation rate

- Also, warm air can hold more water vapour than cold air

High

- Water molecules gain kinetic energy

- Faster evaporation rate

- Exception: At very high temperatures, stomata close to prevent excessive water loss


3. Air movement.


Still air

- Air surrounding the leaf becomes saturated

- High amount of water vapour both in the air spaces inside the leaf and in the air outside the leaf

- Concentration gradient for water vapour becomes low

- Less water vapour diffuses out of the leaf in a unit time (slow transpiration rate)

Windy

- Wind carries away water vapour from the surface of the leaf (outside the stomata)

- This means there is less water vapour outside the stomata (unsaturated, dry air)

- There is a steep concentration gradient for water vapour between the air spaces in the leaf and the air outside the leaf

- More water vapour diffuses out of the leaf in a unit time (fast transpiration rate)

- Exception: At very high wind velocities, stomata close to prevent excessive water loss

4. Light intensity.

*This factor does not affect the rate of

evaporation but affects the size of stomata

through which the water vapour passes.


Low or dark (at night)

- Stomata close

High (during the day)

- Stomata open

- Exception: Stomata close at very high light intensities to protect the leaves from damage by UV rays

Experiments involving transpiration

For transpiration experiments, two chemicals are

usually used:

1. Cobalt chloride paper: It changes from blue to

pink in the presence of water.

2. Anhydrous copper sulphate: It changes from

white to blue in the presence of water.

To demonstrate transpiration from the

surface of a leaf

PROCEDURE:

1. A Piece of dry cobalt chloride paper is placed on

either side of a leaf attached to a plant.

2. It is secured in place by rubber bands as shown in

the figure above.

3. The glass slides prevent the cobalt chloride papers

from coming in contact with the water vapour in the

air.

4. After a short while, note the colour of the papers.

OBSERVATION:

After 10 minutes, the cobalt chloride paper on the

upper surface was still blue and that on the lower

surface had turned pink. This experiment shows that

transpiration is higher on the lower surface of the leaf

compared to the upper surface.

To measure the rate of transpiration

using a Potometer

A potometer can be used to compare the rates of

transpiration under different conditions.

A potometer consists of a bottle fitted with a three

holed rubber stopper.

The holes carry:

1. A funnel with a long stem

2. A graduated bent capillary tube

3. A leafy shoot


The rate of water uptake by a cut leafy shoot can be

determined by measuring the distance travelled by

the air bubble in the capillary tube in a given time

interval.

Initial reading on capillary tube = a cm3

Final reading on capillary tube = b cm3 (after t mins)

Rate of transpiration = (b – a) cm3

t mins

The rate of water uptake is only approximately

equal to the transpiration rate since some water

may be utilised by the plant for other processes

such as photosynthesis and maintenance of turgidity

of plant cells.

To measure the rate of transpiration by

potometer (weighing method)

The difference between the weight of the flask and

plant at the beginning and at the end of the

experiment gives the weight of the water loss by

evaporation.

The total volume of water added to the flask to

restore the water level is the amount of water

absorbed.

Wilting

When a plant loses more water (as water vapour)

through its leaves than it absorbs through its roots;

wilting will occur.

Wilting can be caused by lack of water in the soil,

low humidity, windy condition and high

temperature. When these occur:

1. The turgor pressure of the plant cells

decreases;

2. The plant cells then becomes flaccid;

3. The leaves droop and the stem becomes limp

(for herbaceous plant).

Since the guard cells also become flaccid during

wilting, stomata are closed causing less inward

diffusion of carbon dioxide. Hence, rate of

photosynthesis decreases as a result of wilting.

Wilting of non-woody stem:

Xerophytes

Xerophytes are plants that live in conditions where

water is scarce.

These plants include those in hot, dry, desert

conditions and plants deprived of water during

winter when the soil water freezes.

Xerophytic adaptations enable a plant to reduce its

water loss in a number of ways:

Feature Effect/s

Small or rolled or spiny or no

leaves

Reduced surface area exposed for evaporation of water


Rolled or hairy leaves

Traps water vapour outside the stomata (hairs) / within the enclosed space (rolled leaves)

Low concentration gradient for water vapour

Less water vapour diffuses out of leaves = low transpiration rate

Thick layer of waxy cuticle covering the

leaves

Provides thicker waterproof barrier to reduce water loss through evaporation from the surfaces of the leaves

Reduced number of stomata in

leaves

Less water vapour lost

Sunken stomata

Water vapour is trapped within the pockets / pits

Creates moist air (high humidity, saturated) outside the stomata

Low or no concentration gradient for water vapour

Less water vapour diffuses out of leaves = low transpiration rate

Translocation

All products of photosynthesis are distributed by

phloem cells to non-photosynthetic parts of the

plant.

The entire mass of solutes and water is transported,

either along the concentration gradient or against

it. This movement of synthesized products is called

translocation.

Translocation is rapid and much faster than simple

diffusion. The products are moved to other regions

of the plant for utilization or storage.

These regions include:

a) All living cells that respire,

b) Actively growing regions, e.g. root tip and stem

apex,

c) Flowers, fruits and seeds,

d) Storage organs, mainly in the roots.

Translocation in plants can be affected (slowed

down) if:

a) The temperature is lowered,

b) There is no oxygen supply and

c) There are poisons are present (inhibit enzymes

during respiration).

Translocation results in both downwards (to roots

and storage organs) and upwards (to fruits and

growing buds) transport of substances.

Process of translocation occurs in the following

ways:

1. Carbohydrates made from photosynthesis are

converted first into sucrose (an inert sugar).

2. Sucrose and amino acids are actively

transported into phloem sieve tubes by

companion cells (energy in the form of ATP from

aerobic respiration).

3. These are then transported from one sieve tube

cell into another through the pores in the sieve

plates.

Evidence that phloem translocate organic

food

Using radio isotopes in translocation studies

Carbon dioxide labeled with radioactive carbon

(14CO2) is fed to a green leaf in the presence of

light.

This radioactive carbon would then turn into

radioactive labelled sugars which would then be

transported about the plant.

After a short while, sections can be cut off the

stem.


If the sections are placed on an X-ray photographic

film, this will show that all radioactivity is in the

phloem tubes. This means that the food that the

leaves have made is being transported in the

phloem.

Using Bark ringing experiments

The removal of a ring of bark from around the trunk

of a woody plant takes away all tissues external to

the xylem.

When the ring was cut, sugars from the leaves

(moving to the roots) accumulated in the outer

stem, above the ring.

The tree did not wilt showing that xylem was not

harmed. However, the plant eventually died

because the roots were deprived of sugars. This

shows that photosynthetic products such as sugars

are translocated in phloem cells.

Using aphids in translocation studies

Aphids are insects that feed on plant juices. They

have special mouthparts called proboscis or stylets

that can penetrate the leaf or stem.

The stylets are inserted into the phloem tubes of

the plant and via these structures, the plant juices

or sap in the phloem tubes are sucked up.

A feeding aphid can be anesthetised with carbon

dioxide and the mouthparts cut off.

The sap when analysed contains sugars, amino acids

and other organic materials.

Unit 6 – Transport in Flowering Plants - Wikispaces6+-+Transport+in... · Unit 6 – Transport in...

Documents

Transcript of Unit 6 – Transport in Flowering Plants - Wikispaces6+-+Transport+in... · Unit 6 – Transport in...