TRANSPORT IN PLANTS

NAME : NOOR MELLINA BINTI ABU KASIMCHAPTER : 1 - TRANSPORT (BIOLOGY FORM 5)TEACHER : PUAN ZAHARAH BINTI THARIMSCHOOL : SMKA (P) AL-MASHOOR

1.7 UNDERSTANDING THE TRANSPORT OF SUBSTANCES IN PLANTS

• Vascular tissue : Transport substances in plants• There are 2 types of vascular tissue : xylem &

phloem• Xylem : - Transport water and dissolved

mineral salts absorbed by the roots up the stems and to the leaves

- Provides mechanical support to woody plant• Phloem : Transport organic substances from

leaves down to the storage organs and from the storage organs (roots) up to the growing regions (buds)

• Vascular tissues are found in the roots, stems and leaves of a plant

VASCULAR TISSUE

VASCULAR TISSUE IN ROOT

• Outermost layer of root : Epidermis - No waxy cuticles

- Absorbs water and dissolved

mineral ions from soil• Specialised epidermal cells grow outwards to

form root hairs• Root hair – Increase the surface area for water absorption• Cortex - Region between the epidermis and vascular cylinder

- Made up of parenchyma cells which stores starch grain• Endodermis – Single layer of cells located

immediately after the cortex

• Pericycle - Next to endodermis - Consist of sclerenchyma tissue :

Provides mechanical support for the roots

• Vascular cylinder : Consist of vascular tissue and pericycle

• In dicotyledonous plant : Xylem has a star-shaped form, phloem fills the areas between the xylem

• In monocotyledonous plant : - Vascular cylinder has a central core called pith which contain parenchyma cell

- Vascular tissues form a ring around the pith, xylem tissue alternating with phloem tissue

Figure : Cross section of dicotyledonous root

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VASCULAR TISSUE IN STEM• Stem has epidermal layer : - Epidermal layer secrete

cuticle (in young plant) - Epidermis absent but is replaced by bark (in older plant)

• Cortex layer : Inside epidermis• Cortex layer is made up of : - Collenchyma cells :

provide support and flexibility to stem - Parenchyma cells : store food • Inner part of stems consist of vascular bundles and pith

which is the central region of stem• Pith : - Used for food storage in young plants - May be absent in older plant, making them hollow• Vascular bundle : - In dicotyledonous plants : - Arranged in

ring around the pith - Xylem are

found towards inside with the phloem on the outside. Cambium lies between

xylem and phloem - In monocotyledonous plants :

Scattered throughout the stem

Figure : Cross section of dicotyledonous stem

Figure : Cross section of monocotyledonous stem

VASCULAR TISSUE IN LEAF

• The leaf consist of broad portion called lamina (leaf blade)

• Leaf blade (lamina) : Contains leaf veins• Petiole : Stalk that connects the leaf blade to

the stem• Vascular tissue : Found inside petiole and leaf

veins• Xylem : - Forms upper part of vascular bundle

in leaf - Transport water and mineral salts to

the leaves• Phloem : - Forms lower part of vascular bundle

in leaf - Transport sucrose and other products

of photosynthesis from leaves

Figure : Leaf

Figure : Cross section of leaf

STRUCTURE OF XYLEM

• Xylem contains 4 types of cells : - Xylem vessels

- Tracheids - Fibres (a type of

sclerenchyma) - Parenchyma

• Xylem vessels & Tracheids : water-conducting cells• Fibres : Provide support to the xylem• Parenchyma : Stores food substances

Figure : Cross section of xylem

Figure : TracheidsFigure : Xylem Vessel

ADAPTATION OF XYLEM VESSELS & TRACHEIDS

• They are elongated cells arranged end to end- The end walls of xylem vessels are open so that the

cells join end to end to form a long continuous hollow tube

- This arrangement allows water to flow upwards continuously from one cell to the next• During growth, the walls of the xylem vessels and

tracheids are thickened with lignin deposits- Lignin : - Making them strong & hardy so that they do

not collapse under the tension created by the upward pull of water during transpiration (transpiration pull)

- Prevent the entry of food substances. Hence, cytoplasm of these cells are disintegrates, leaving a cavity in the centre of the cells. As a result, mature xylem vessels and tracheids are hollow and dead

• The walls of xylem vessels and tracheids have small openings (pits)

- Pits : Allow water and mineral salts to pass sideways between the cells

TRACHEIDS

• Longer and have smaller diameter than xylem vessels

• They are pointed at the ends

• The end walls break down in the pits to allow water to pass from cell to cell

STRUCTURE OF PHLOEM

• Phloem tissue is composed of 4 types of cells : - Sieve tubes - Companion cells - Parenchyma - Fibres

• Sieve tubes : - Transport for organic substance such as sucrose and amino acids

- Sieve tube is a cylindrical column comprising long cells arranged end to end

- When mature, it has no nucleus and its cytoplasm is pushed to the sides of the cell

- The end walls of each cell are perforated by pores to form sieve plate. Long strands of cytoplasm pass through the pores in the sieve plates to allow substances to pass from one cell to another

- Its function is supported by companion cells

• Companion cells : - Normal cell with a nucleus and a large number of mitochondria

- Has active metabolism - Provide sieve tube cells with

protein, ATP, and other nutrients

• Parenchyma : Store food substances

• Fibres : Provide support

Figure : Longitudinal section of phloem

Figure : Cross section of phloem

1.8 THE TRANSPORT OF ORGANIC SUBSTANCES AND WATER IN PLANTS

TRANSPORT OF ORGANIC SUBSTANCES IN PLANTS

• Phloem contains a very concentrated solution (phloem sap) of dissolved organic solutes such as sugars (sucrose), amino acids and other metabolites

• Translocation : Transport of dissolved organic solutes in phloem

• Importance of translocation : - For the survival of plant

- Enables sucrose, product of photosynthesis, to be stored / converted into other sugars when it

reaches its destination• Organic substances in phloem can be

transported both upwards and downwards

TRANSPORT OF WATER IN PLANTS

TRANSPIRATION• Transpiration : Loss of water vapour through

evaporation from the surface of plants• This loss of water is replaced by the absorption of

water from soil by plant roots• Only 1% of water is used for photosynthesis and

to remain turgid• 99% of water is evaporates from leaves and is lost

to the atmosphere through transpiration• Transpiration takes place through stomata of

leaves and lenticles of woody stems• Transpiration stream : Continuous stream of

flowing of water from roots to leaves

IMPORTANCE OF TRANSPIRATION

• Helps in absorption and transport of water and mineral ions from roots to different parts of the plant

• Produce cooling effect in plants

• Helps to supply water to all plant cells for metabolic processes

• Helps to prevent plants from wilting by maintaining cell turgidity

EXTERNAL CONDITIONS THAT AFFECT THE RATE OF TRANSPIRATION

• Light intensity- An increase in light intensity increases the

rate of transpiration- Light stimulates the opening of stomata- The stomata will open wider- Hence, more water vapour evaporates

through stomata

• Temperature- An increase in temperature increases the rate of transpiration- An increase in temperature increases the rate of evaporation of water from surface of mesophylls cell- The rate of diffusion of water through stomata

also increases

• Humidity- High humidity surrounding the leaves reduces the evaporation of water from the stomata - This causes the transpiration to slow

down- Conversely, a rise in temperature lowers the relative humidity of the surrounding

air and this increases the rate of transpiration

• Air movement- As the water vapour that diffuses

through the stomata accumulates near the leaf surface, a faster air movement helps to remove the water vapour- Air movement increases the

concentration gradient between the water vapour in

the leaf and that outside leaf- This increases the transpiration rate- When the air is still, the transpiration

rate decreases or stops altogether

MOVEMENT OF WATER FROM SOIL TO LEAVES

• Movement of water from the roots to the leaves is assisted by :

- Root pressure- Capillary action- Transpiration pull

• Water is absorbed through roots by osmosis• The gradient of water concentration which exist

across the cortex creates a pushing force that results in the inflow of water into the xylem

• At the same time, ions from the soil are actively secreted into the xylem and this causes osmotic pressure to increase

• As a result, water flows continuously into the xylem. This generates a pressure known as root pressure

• Root pressure results in an upward push of water and mineral ions into the xylem of the stem

• Root pressure causes an upward movement of water in plants but it is insufficient to overcome the force of gravity to push the water upwards to the maximum heights of many trees

• Hence, the upward movement of water through the xylem vessels in the stem is helped by the adhesive and cohesive properties of the water molecules

MOVEMENT OF WATER THROUGH THE ROOTS

1. The cytoplasm of root hair cells is usually hypertonic to the surrounding soil water

2. This means that root cells have a higher concentration of solutes than the water in the surrounding soil

3. Hence, water enters the root hair cells via osmosis

4. The root hair cell is now hypotonic to the adjacent cells

5. Water then diffuses into the adjacent cells by osmosis

6. In this way, water moves inwards from cell to cell in the cortex until it reaches the xylem vessels in the root

Figure : Movement of Water Through The Root

MOVEMENT OF WATER IN CORTEX AND ENDODERMIS1. Water flows through cytoplasm, vacuoles and cell

walls of the parenchyma cells in the cortex until it reaches the endodermis

2. Once it reaches the endodermal cells, the water moves through the cytoplasm and vacuoles instead of the cell walls

3. This is because the endodermal cells have special features called Casparian strips which line the sides of the endodermal cells

4. Casparian strip : Impermeable to water (block the movement of water through cell wall)

5. Instead, water moves from cytoplasm and vacuole in the endodermal cells to the xylem cells

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GUTTATION1. At night, the roots of some small plants

continue to actively transport ions and water into xylem

2. This causes root pressure to increase3. Transpiration rate is low during the night4. Root pressure can push water all the way up

the stem and out of special pores called hydathodes at the edge of leaves

5. This natural process is called guttation6. Guttation also occurs on cool humid mornings

when the air is too saturated for the water droplets to evaporate from the leaves

Figure : Leaf Guttation

MOVEMENT OF WATER THROUGH STEM1. The continuous upward movement of water through the

xylem vessels in the stems can be attributed to capillarity2. Capillarity / capillary action – results of the cohesive and

adhesive forces which enable water to enter and move along the very narrow columnsWater molecules adhere to one another by cohesive forcesWater molecules adhere to the walls of xylem vessels by adhesive forces

The cohesion and adhesion of water molecules are due to hydrogen bonding

3. Xylem vessels - Long, narrow and hollow tubes - Joined end to end to form continuous column of water

from the roots to the leaves through the stems 4. The narrow xylem vessel increases the force generated by capillarity.

Capillary action holds the water column together in the capillary-sized xylem vessel

5. Although root pressure and capillary action are not enough to carry water to the top of a tall tree, both effects are important to the water movement in plants

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1. The water on the external surfaces of the mesophyll cells evaporates, saturating the air spaces in the mesophyll with water vapour

2. The air in the atmosphere is less saturated3. So, the concentration of water vapour in

atmosphere is lower than the concentration of water vapour in air spaces of the leaf

4. Hence, water vapour in the air spaces evaporates and diffuses through the stomata

5. The movement of air carries the water vapour away from stomata

6. The loss of water in mesophyll cells makes the cell hypertonic to an adjacent cell

MOVEMENT OF WATER FROM LEAVES TO ATMOSPHERE

7. Water from adjacent cell diffuses into the mesophyll cell by osmosis

8. Eventually, water is drawn from the xylem vessels in the veins9. A tension and pulling force is created to pull water up the

xylem vessels as a result of the evaporation of water vapour from the mesophyll cells

10.This transpiration in the leaves forces the movement of water from soil up the stem

11.The pulling or suction force is known as a transpirational pull

Figure : Movement of Water from Leaves to Atmosphere During Transpiration

THE REGULATION OF TRANSPIRATION BY STOMATA

• The regulation of transpiration in plant is helped by the opening and closing of stomata

• Stomata are found abundantly on the lower epidermis of dicotyledonous leaf and on both upper & lower surfaces of monocotyledonous leaf

• Each stoma is surrounded by two guard cells which regulate gaseous exchange by opening and closing the stoma

• If the stomata are open - Carbon dioxide can enter for photosynthesis

- Water can be lost by transpiration• If the stomata are close - Reduce water loss (stops

transpiration) - Prevents carbon dioxide from

entering the leaf• In general, stomata open during the day and close at

night

Figure : Guard cells and Stoma

THE MECHANISM OF THE OPENING OF A STOMA1. During the day, light stimulates photosynthesis in

the guard cells2. They start synthesizing glucose and generate

energy for active transport3. The guard cells accumulate potassium ions (K+)

from adjacent cells through active transport4. The guard cells become hypertonic and water from

the adjacent cells enter the guard cells by osmosis5. As a result, the guard cells swell up and become

turgid6. Since the inner cell walls of the guard cells are

thicker than the outer cell walls, the guard cell bend outward an the stoma opens. This is because the thinner outer wall stretches more than the thicker inner wall

Figure : Mechanism of The Opening of a Stomata

THE MECHANISM OF THE CLOSING OF A STOMA1. At night, when

photosynthesis does not take place, potassium ion (K+) exit the guard cells and water also leaves the guard cells by osmosis

2. The guard cells become flaccid and the stoma closes

Figure : Mechanism of The Closing of a Stomata

SUMMARY

•Vascular tissues transport substances in plant- Xylem : Transport water and dissolved minerals- Phloem : Transport organic substances (translocation)

•Transpiration is the loss of water vapour from surfaces of plants through evaporation

- Affected by light intensity, temperature, humidity and air movement•Movement of water from roots to leaves is assisted by root pressure, capillary action and transpirational pull•The opening and closing of stomata help in regulation of transpiration in plant

THE END