Transport in plants

Explain what the vascular tissue is in plants and explain the three places it is distributed with diagrams

Vascular tissue is the xylem and a phloem in plants and it transports materials around the body. They are found adjacent to each other in vascular bundles.

They are found:

1. In the roots - the xylem is the central star-shaped vessel with phloem between the groups of xylem cells. This arrangement resists vertical stresses and anchors the plant in the soil.

2. In the stems - vascular bundles are In a ring at the periphery with the xylem towards the centre and phloem on the outside. This gives the stem flexible support and resists bending.

3. In the leaves - vascular tissue is in the midrib and in a network of veins giving a flexible strength and resistance to tearing of the leaves.

Explain the xylem structure

There are two main types of cells in the xylem

1. Vessels - they occur only in angiosperms.

   • as lignin builds up in their cell walls the contents die leaving an empty space called the lumen

   • As the tissue develops the end walls of the cells break down leaving a long hollow tube like a drain pipe through which water climbs straight up the plant. Lignin is laid down in a characteristic spiral pattern and unlike cellulose of the phloem cell. It also stains red so the xylem is easy to identify under. A microscope

2. Thracheids - occur in ferns, conifers and angiosperms. Miss has no water conducting tissue and are therefore poorer at transporting water so their growth is limited. They have lignin which is hard, strong and water proof. Water travels down pits and Tracheids are spindle shaped so twisting path upwards not straight

What are the two functions of the xylem

1. Transporting of water and dissolved minerals

2. Providing mechanical strength and support

Explain the water uptake in roots

terrestrial plants must conserve water to reduce dehydration so water is taken up via the roots and transported to the leaves where it maintains turgidity and is a reactant in photosynthesis. However a lot is lost through transpiration and therefore the water must be constantly absorbed through the soil by the root hair cells. The root hair cells are adapted for this role by having a high SA:V and thin cell walls. The water enters the root hair cells via osmosis down the WP gradient. This is because The soil water contains a very dilute solution of mineral ions so it has a high water potential but the vacuole has a concentrated solution of solutes so a low water potential. So water enters via osmosis.

Explain the movement of water through the root

Water moves into the xylem from the root via the cells of the root cortex in 3 ways

1. Apoplast pathway - water moves in the cell walls cellulose fibres which are permeable and separated by the movement of the water

2. Symplast pathway - water moves through the cytoplasm and plasmodesmata (strands of sytoplasm in pits of cell walls)

3. Vacuolar pathway - water moves from vacuole to vacuole

Water can’t enter the xylem from the apoplast because the lignin makes the xylem walls impereable and waterproof so it uses symplast or vacuolar routes and leaves via apoplast.

Explain the structure and role of the endodermis and how it relates to movement of water through cells to xylem from root

vascular tissue in the centre of the root is surrounded by a region called pericycle which is surrounded by a single layer of endodermis cells

STRUCTURE - impregnated with a waxy material - SUBERIN - forming a distinctive band ok the radial and tangerbal walls called the CASPARIAN STRIP.

FUNCTION - The Suberin is waterproof so the casparian strip prevents water moving further in the apoplast and drains it into cytoplasm

HOW THIS RELATES TO MOVEMENT OF WATER

Water moves from the root endodermis into the xylem by osmosis down water potential gradient across the endodermis cell membrane. for this to be effective the water potential of the xylem must be more negative than the endodermal cells. This is achieved in two ways

1. The water potential of the endodermis is raised by water being driven into cytoplasm by casparian strip

2. The water potential is lowered in xylem by active transport of mineral salts mainly sodium ions from the endodermis and pericycle into xylem

This movement of water coming into xylem generates an upwards push called root pressure

Explain uptake of mineral ions from root hair cells

UPTAKE OF MINERALS:

The soil water is a much more dilute solution than inside root hair cells. So minerals are absorbed into cytoplasm by active transport.

Mineral ions can also move along the apoplast pathway in solution. When they reach endodermis where casparian strip prevents further movement. They then enter cytoplasm by active transport into xylem.

Eg. Nitrogen usually enters the plant as nitrate or ammonium ions which diffuse in the apoplast, enter symplast by active Transport and flow into cytoplasm by plasmodesmata

Active transport allows the plant to absorb ions selectively at endodermis.

Explain the movement of water up the xylem

Water always moves down a water potential.

The air has a very low water and soil water is very dilute so has a high water P. So water moves from soil through plant into air.

There are 3 main mechanisms.

1. cohesion-tension theory

   Water vapour evaporates from leaf cells into air spaces and then diffuses out through stomata into atmosphere.

   This draws water across the cell of the leaf in the apoplast, symplast and vacuolar pathways from the xylem.

   As water mols leave xylem they pull up other water molecules behind. The water (hydrogen bonds)

   They show cohesion so this produces a continuous pull tension in the water column. The charges on the water molecules also cause attraction to the hydrophilic lining of the vessels. This is adhesion and contributes to water movement up xylem.

   So adhesion and tension of water resulting in cohesion = cohesion-tension theory.

2. capillary. Capillarity is the movement of water up narrow tubes, in this case the xylem by capillary action. It only operates up to a metre. It may have a role in mosses but only makes a small contribution to water movement in plants more than a few cm high.

3. Root pressure - operates over small distances too and is a consequence of osmotic movement of water into xylem pushing water further up. Caused by osmotic movement of water across root into xylem pressure.

What is transpiration

TRANSPIRATION

In the transpiration stream, water is drawn up by:

• cohesive forces between water mols - hydrogen bonds

• adhesive forces between water molecules + hydrophilic lining of xylem vessels

The transpiration stream is the continual flow of water at the roots up the stem to the leaves and out to the atmosphere.

Plants must balance water uptake with loss otherwise wilting or even dying occurs so plants face a dilemma between keeping stomata open to allow gas exchange but this means the plant loses valuable water through transpiration.

Explain Factors affecting the rate of transpiration - briefly

The rate of water loss is the transpiration rate and this is dependent on

1. genetic factors -> such as those controlling the number distribution and size of stomata.

2. environmental factors eg. temperature, humidity and air movement. These affect water P gradient so they affect transpiration rate.

3. temperature

Explain one of the factors effecting rate of transpiration

1. A temperature lowers the water potential of the atmosphere significantly. It increases kinetic energy of water accelerating rate of evaporation from mesophyll cell walls so speeds up rate of transpiration. The higher the temp. causes water molecules to evaporate away more/leaf more quickly reducing water potential around leaf.

Explain another factor effecting rate of transpiration

2. Humidity

   The air inside a leaf is saturated with water vapor so its relative humidity is 100%.

   The humidity of the atmosphere surrounding the leaf varies but there is a water potential gradient between leaf and atmosphere and when stomata are open water vapor diffuses down WP gradient. 100%80%60%40%

   Transpiration in still air results in the accumulation of saturated air at the surface of leaves. The water vapour gradually diffuses away, leaving concentric rings of decreasing humidity the further from the leaf you go. The higher the humidity, the slower the water vapour diffuses down this gradient of relative humidity which is also gradient of WP away from leaf.

Explain another factor of transpiration

3. Air movement

   movement of the surrounding air blows away the layer the layer of humid air at a leafs surface. The water potential gradient between that increases and so water diffuses through stomata more quickly. The faster the air movement, the faster the concentric shells get blown away → transpiration ⬆

Explain another factor in transpiration

Light intensity

affects transpiration as controls the degree of stomata opening. In most plants as light intensity increases the wider the stomata opens so transpiration increases - middle of the day.

Explain the linking of the factors effecting transpiration

The factors are interdependent and interact with each other

More water is lost on a dry, windy day than on a humid, still day.

This is because the spongy mesophyll cells are saturated with water which evaporates and moves down a gradient of water p and the wind referenced layer of surfaces.

What is a potometer and how is it used to measure the rate of transpiration

Comparing rates of transpiration: POTOMETER (transpirometer)

This potometer doesn't measure transpiration directly but measures water uptake but since 99% of all water uptake is lost through transpiration the rate of uptake ≈ rate of transpiration.

The potometer can be used to measure water uptake by the same shoot under different conditions or can be used to compare the uptake by leafy shoots under different conditions of different species under the same conditions.

Explain how to set up a potometer

How to set it up:

1. Cut a leafy shoot underwater so no air enters the system.

2. Underwater fill the potometer with water ensuring there are no air bubbles.

3. Fit the leafy shoot to the potometer with rubber tubing underwater to prevent air bubbles forming in the apparatus or the xylem.

4. Remove the porometer and shoot from the water seal joints with vaseline tidy carefully.

5. Introduce an air bubble or meniscus moves into capillary tube.

6. Measure the distance it moved in a given time.

7. Use the water reservoir to bring the air bubble or meniscus back to start.

8. Repeat + calc. a mean.

9. Different conditions may be used e.g. altered light intensity or air movement.

Explain briefly the types of flowering plants

ADAPTATIONS of FLOWING PLANTS to differing water availability:

1. mesophytes - plants that live in conditions of adequate water supplies

2. xerophytes - plants that live in conditions where water is scarce

3. hydrophytes - aquatic plants

Explain what’s mesophytes and their adaptations

1. Mesophytes:

   Most land growing plants in temperate climates are mesophytes. They lose a lot of water but it is readily replaced from uptake from soil so they don't have any special means of conserving water.

   If the plant loses too much water, it will its leaves drop the stomata close and leaf surface area for absorbing light is reduced so photosynthesis is limited.

   Most crop plants are mesophytes. They are adapted to grow best in well drained soils and moderately dry air. Water uptake during the night replaces water loss through day. Excessive water loss is prevented as stomata close at night.

   Mesophytes must survive through winter ground is frozen and liquid so...

• many shed their leaves before winter so they don't lose water by transpiration when water availability is low.

• The aerial parts of many non-woody plants die off in winter so they aren't exposed to frost or cold but their underground organs such as bulbs and corms survive.

• Most annual mesophytes (plants that flower + produce seeds) over-winter as dormant seeds with low metabolic rate that such a low amount of water is lost.

Explain what xerophytes are and their adaptations

Xerophytes

They are plants with xeromorphic characteristics. They have adapted to living with low water availability and have modified structures which prevent excessive water loss. They may live in hot dry desert areas or cold regions where soil is frozen and wind is high.

marram grass

position of hinge cell

vascular bundle

interlocking hairs to trap water vapor

adaxial epidermis

thick cuticle

sclerenchyma

adaxial epidermis with sunken stomata

Marram grass:

• found in sand dunes - xerophytes

• where no soil, rainwater drains rapidly, high winds, salt spray, no shade from sun.

So they have these adaptations:

1. Rolled leaves - large thin-walled epidermal cells (hinge cells) at the bases of the grooves become plasmolysed when they lose excessive water from transpiration and the leaf rolls inwards with the adaxial surface inwards. This reduces surface to air, so reduces transpiration.

2. Sunken stomata - they occur in grooves at the adaxial surface but not the outer abaxial surface. They are pits or depressions that is trapped in the air to reduce WP gradient between in & out so rate of diffusion of water through stomata is reduced.

3. Hairs - stiff interlocking hairs trap water vapor and reduce the water potential gradient between in and out.

4. Thick cuticle - the cuticle is a waxy covering over the outside (abaxial layer) - wall is waterproof so water loss is reduced. The thicker the cuticle the lower transpiration rate through cuticle

5. Fibres of sclerenchyma are stiff so the leaf's shape is maintained even when the cells become flaccid

Explain what are hydrophytes and adaptations

Hydrophytes

grow partially or wholly submerged in water eg. water lily which is rooted in mud at the bottom of a pond and has leaves floating.

Hydrophytes are adapted:

1. water is a supportive medium so they have little or no lignified support tissues

2. surrounded by water so no need for transport tissue system is poorly developed

3. leaves have little or no cuticle as no need to prevent water loss

4. stomata are on upper side of floating leaves as lower surface is in water

5. stems and leaves have large air spaces continuously down to roots forming a reservoir for O2 + CO2 for buoyancy

Explain what translocation is in the phloem

TRANSLOCATION + PHLOEM

Translocation = transport of soluble organic molecules/assimilates eg. sucrose & amino acids through phloem from source to sink

In the summer the leaves of the potato are acting as their source. The potato tubers that grow during summer act as a sink. During the autumn the and winter the tubers start to grow. They become a sink. As shoots grow in spring the potato tubers are source

of energy unit leaves of shoot carry out photosynthesis. During the summer more leaves grow and potatoes produce flower + seeds as well as tubers which grow underground. Now all parts of plant is sink while the parts of plant above ground grow. At night the tubers are source.

Evidence from translocation

1. Ringing experiments

   o The phloem is on the outside and when you cut a ring out it leave

2. Radioactive tracers + autoradiography

   Radioactive source of C14 in mask used in photosynthesis → glucose → sucrose

3. Aphids

   o The aphids have a stylet the pierces the phloem