Transport system in plant

Importance of transport system in plants

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Transport In plants

Grade – 12 foundation

What is the relationship between the size of organism and its surface area to volume ratio?

Length 1 cm 2cm 3cm

Surface area

Volume

S.A./V

Conclusion :

As the size of organism increases à its SA/V decreases

( As the S.A./ V ratio decreases the need of transport system increases ).

Why do large organisms ( like plants ) need a transport system, but not smaller

ones?

Organisms

Small organisms Large organisms

Have large surface area to volume ratio Have small surface area to volume ratio

Diffusion is sufficient method for exchange materials Diffusion is too slow and insufficient method.

( no need for a specialized transport system ) ( need a specialized transport system )

Water movement in relation to water potential

Diffusion : is the movement of substance from high concentration area to low concentration area.

Biological membranes are partially permeable, this means :

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they allow some substances to pass but not others.

Osmosis : is the movement of water from area of low to area of high solute concentration.

Or it is the movement of water from area of high Ψw to area of low Ψw.

Water potential ( Ψw ) :

Is a term describes the tendency of water molecules to move from area to area.

Note about water potential :

- Pure water has the highest value of water potential ( = 0 )

- As the concentration of solutes increases the Ψw decreases ( become more negative ).

- Water always moves from area of high Ψw to area of low Ψw

Types of solutions

1. Hypertonic solution : has solute concentration higher than the concentration inside the cell.

- Ψw inside the cell is larger than the water potential outside it.

- Water leaves the cell by osmosis.

- Plasmolysis occur : the cell membrane detaches ( moves away ) from the

cell wall of the plant cell when it is in a hypertonic solution ( low Ψw solution )

2. Hypotonic solution has solute conc. lower than the concentration inside the cell.

- Ψw inside the cell is less than the water potential outside it.

- Water enters the cell by osmosis.

- The plant cell swell and become turged ( where as the animal cell bursts ).

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Ψw = -80Ψw = -50

Ψw = -50Ψw = -80

3. Isotonic solution has the same concentration as the cell.

- Ψw inside the cell is equal to the water potential outside it.

- The amoun of water enter the cell is equal to the amount leaves.

- So, no net change in the size occurs.

Transport system in plant

Transport system in plant consists of tubes ( vessels ) of cells, adapted to perform the function of

transport, so it is known as vascular system.

There are two types of vessels in the plant

1. Xylem : transport water and mineral from roots to leaves.

2. Phloem : transport food from leaves ( source )to all parts of the plants ( sink ).

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Ψw = -50Ψw = -50

These vessels run along the plant from roots to stem up to leaves. But they have different distribution

patterns in different plant parts.

Distribution of the vascular system in plant parts

1. Root :

The vascular tissue is located at the central region of the root so it is known as vascular

cylinder or stele. This arrangement enables the root to withstand uprooting.

The vascular cylinder is enclosed within the endodermis.

Xylem : radiates out from the center of the vascular cylinder like spokes of a wheel.

Phloem : fills the areas between spokes.

2. Stem :

Vascular tissue takes the form of bundles.

These bundles arrange in the form of rings ( in dicotelidon plants )

Each bundle contain :

- Xylem : located toward inside of the stem.

- Phloem : located toward outside of the stem.

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This arrangement gives the stem strength and flexibility so it can resist sideway bending by

strong wind.

3. Leaves :

The vasculat tissue of the leaf takes the form of bundles runs across the leaf as veins.

Xylem : forms the upper part of the bundle.

Phloem : forms the lower part of the bundle.

Structure of the xylem tissue

The xylem tissue includes :

1. Fibers :

Are long cells, with thickened walls.

Function : provide structural support for the plant.

2. Parenchyma cells :

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Xylem

Phloem

Living cells with thin cell walls and a lot of spaces inbetween.

Function : Store substances as starch, packing and lateral transport.

3. Xylem vessels :

Made of many cells known as vessel elements arranged above each other in the form of a

column. When these cells mature :

- they become empty ( dead cells )

- their end walls break-down

- their side walls become thickened by lignin.

4. Tracheids :

Long dead cells with tapered ends.

End walls are not removed completely.

So water passes through pits from cell to the next.

Structure of the phloem tissue

Phloem tissue includes :

1. Sieve tubes :

Is a tube consist of many cells Known as sieve tube elements.

These cells arrange above each other to form a sieve tube.

Sieve elements are alive cells without nucleus. Their end walls are perforated by larg number

of pores forming sieve plate.

2. Companion cells :

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Alive cells with nucleus and a lot of mitochondria, these cells located behind the sieve tubes

and provide them with energy.

Companion cells linked to sieve tubes by plasmodismata.

3. In addition to the sieve tubes and companion cells phloem tissue contains parenchyma and fibers.

The movement of water into and across the root

1. Absorption of water and minerals :

Root hairs : are projections of the epidermal cells that spread through the soil articles

providing a large surface area for water absorption.

Water moves from soil to the root hairs by osmosis ( from high water potential to low water

potential ). The continue moving toward the xykem across the root in two ways :

1. Apoplast : water moves along the cell walls and through the spaces beween cells.

2. Symplast : water moves through the cells them selves and from one cell to the other

through plasmodismata.

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When water reaches the endodermis, the aboplast way is blocked, because their cell walls are

thickened with suberin forming a band around the cells called casparian strip.

So water moves through these cells via symplast to reach the xylem vessels.

Note : Minerals enter the root hairs by active transport and then continue moving with water.

2. Transpiration :

Most of the absorbed water in the roots, evaporates and diffuses out from the leaves to the

atmosphere. So, what is the significance of this process ? ?

Transpiration : is the loss of water vapor from Arial parts of the plant ( mainly from leaves )

Cohesion-tension theory :

Wind movement ( mass flow ) removes air saturated with water vapor away and replaces it with

dryer air. This keeps a concentration gradient between the inside and the outside of the leaf.

Water leaves air spaces inside the leaf to outside ( with the concentration gradient ) through

stomata

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Water at high water potential diffuses from

the xylem vessels to replace the lost water.

The pressure in the xylem vessels near the

leaf is lowered and the water moves up from

the root to the leaves.

Cohesion : water molecules are attached to

each other so, they “ stick together “ forming

continuous columns inside the xylem vessels.

This keeps water moving inside the vessel.

Tension : water in the xylem vessels is pulled

by transpiration.

TRANSLOCATION

Organic substances produced in the leaves ( source ) by photosynthesis such as sucrose and

amino acids … etc.

These substances must be transported to other parts of the plant as roots ( sink ).

The mass flow theory explains how these substances are transported.

Is the transport of food inside the phloem from source to sink

Source is any place of the plant Sink is any place of the plant where

where the food enters the phloem

food removed from the phloem

Mass-flow theory :

A. LOADING of substances into the phloem :

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- Sucrose is actively transported from source into the phloem.

- Ψw decreases inside the phloem.

- Water enters the phloem by osmosis.

- This increases the hydrostatic pressure inside the phloem near to the source.

B. UNLOADING of substances from the phloem :

- Sucrose is actively transported from phloem into the sink.

- Ψw increases inside the phloem.

- Water leaves the phloem by osmosis.

- This decreases the hydrostatic pressure inside the phloem near to the sink.

C. MASS FLOW :

- Loading and unloading cause a gradient in the osmotic pressure inside the phloem.

- This cause a mass flow of substances from source to sink.

Properties of translocation :

1. It is an active process ( needs energy input ) during loading and unloading.

2. it occurs in any direction ( according to the location of the source and the sink ).

Growth of the stem

Primary growth : this type of growth resulted from cell division in the apical meristem of the

stem.

Secondary growth : is the growth in which the stem increase in width by the action of a lateral

meristematic tissue called vascular cambium ( in dicotes ).

This cambium is found in between xylem and phloem. When the cambium divides it gives more

xylem to inside and more phloem to out side.

The size of the xylem vessels produced depends on the season of the year. If the amount of

water is high the resulted vessels are larger in size.

As a result of that the size of vessels produced during the spring is large and during summer is

small. This causes annual rings to develop every year in the stem of the plant.

After years of growth many layers of xylem rings are produced ( one ring every year ).

The xylem in the center of the stem stop conducting water and it is known as heartwood. It is

dark in color.

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Sapwood is the xylem rings which is still can transport water, it is light incolor and located

toward the outside of the stem.

Spring wood : in spring the weather conditions are good and the vascular cambium produces

large vessels with thin walls.

Summer wood : in summer the weather conditions are not very good, so the vascular

cambium produces narrow, thick walled vessels.

Each tree ring represents a year`s growth, consists of a cylinder of spring wood surrounded by

a cylinder of denser summer wood.

Environmental factors that affect

the rate of transpiration

However, because water loss occurs by diffusion, the transpiration rate depends on two things: the

gradient in humidity from the leaf's internal air spaces to the outside air, and the diffusion resistance

provided by the stomatal pores.

1. Light Plants transpire more rapidly in the light than in the dark. This is largely because light

stimulates the opening of the stomata (mechanism).

2. Temperature Plants transpire more rapidly at higher temperatures because water evaporates more

rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C.

Also temperature cause air to expand, this cause air to hold much more water molecules and so

increase the rate of transpiration.

3. Humidity The rate of diffusion of any substance increases as the difference in concentration of the

substances in the two regions increases. When the surrounding air is dry, diffusion of water out of the

leaf goes on more rapidly.

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4. Wind When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing

the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier

air.

Measuring the rate of transpiration :

Transpiration rate of plants can be measured by a number of techniques, one is the potometer.

potometer is a device used for measuring the rate of water uptake of a leafy shoot. The reasons for

water uptake are for photosynthesis and transpiration. The potometer consists of a length of capillary

tube. A bubble is introduced to the capillary; as water is taken up by the plant, the bubble moves. By

marking regular gradations on the tube, it is possible to measure water uptake.

A reservoir, typically a funnel with a tap; turning the tap on the reservoir resets the bubble. Some

designs use a syringe instead.

A tube for holding the shoot. The shoot must be held in contact with

the water; additionally, the surface of the water should not be

exposed to the air. Otherwise, evaporation will interfere with

measurements. A rubber bung greased with petroleum jelly suffices.

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