Nutrition and Transport in Plants (chapter 12)

Structural Adaptation and Function of Xylem

• Dead structures consisting long and hollow tubes with empty lumen and without any cross wall. This reduces resistance to water flow for conducting water and dissolved minerals salts/ions from the roots to the stem and leaves.

• Itha deposited in the cellulose cell wall/lignified walls which provides mechanical support for plant

Characteristics and Function of phloem

• sieve tube (with sieve plates which has pores)

• Inside the sieve tube there is only cytoplasm

• Each sieve tube cell has a companion cell beside it.

• The companion cell has a nucleus, cytoplasm and mitochondria to release energy to load nutrients from mesothelioma to the sieve tube by active transport.

Function:

• Conducts manufactured food in the form of sucrose and amino acids from the leaves to other parts of the plant.

How does water enter the roots

• The sap in the root hair cell is a concentrated solution of sugars & mineral salts.

• Thus, the root hair cell contains sap that has a lower water potential than that of the solution in the surrounding soil.

• Water molecules enter the root hair cell via osmosis through the partially permeable membrane cell surface membrane of the root hair cell.

• Entry of water dilutes the sap. The sap of the roots hair cell is now of a higher water potential than that of the next cell. Water is passed via osmosis,

• Water in cell B is passed to another cell by osmosis. The process continues until the water enters the xylem vessels and moves up the plant.

How does mineral salts or ions enter the root hair cell

• When the concentration of ions in the soil solution is higher than that of the root hair cell sap, diffusion takes place.

• Mineral salts enter the root hair cells and neighbouring cells by diffusion.

• After which it is transported by the xylem

• When the concentration of ions in the soil solution is lower than that of the root hair cell sap, active transport takes place.

• The root hair cell contains many mitochondria, aerobic respiration releases energy for active transport of ions into the cell.

• After which it is transported by the xylem

Transpiration definition

Transpiration is the loss of water vapour through the aerial parts of the plants, mainly through the stomata of the leaves, and is a consequence of gaseous exchange in the plant

How does water moves up the stem

Inside the leaves:

• In the day, the stomata are open to allow entry of carbon dioxide gas.

• Water moves out of the mesophyll cells via osmosis to form a thin layer of moisture over the cell surfaces.

• Water, from the film of moisture, evaporated and forms water vapour in the intercellular air spaces.

• The water vapour in the intercellular air spaces diffuses out of the leaf through the stomata. This process is known as transpiration.

• As the water leaves the mesophyll cells, the water potential of the cell sap decreases. These cells draw water by osmosis from cells further inside the leaf.

• The innermost cells in turn obtain their water from the xylem vessels.

• When the mesophyll cells obtain water from the xylem vessels, the whole column of water moves up from the roots to the leaves. The pull of water up the xylem vessel is known as transpiration pull.

Transpiration pull

Evaporation for water from the cells and water vapour diffuse out via stomata. This them removes water from the xylem vessels. The suction force pulls the column of water up the xylem vessels and this force is known as transpiration pull.

Importance of transpiration

1. Transpiration pull facilitates the movement of water and dissolved mineral salts up the plant, from the roots to the leaves. The water can then be used for photosyntheses and keeps the cell turgid.

2. Evaporation of water and the subsequent removal of water vapour via transpiration leads to the removal of heat as well. This helps with cooling the plant.

Factors affecting Transpiration Rate

1. Air humidity: If the external air is less humid, there is less water vapour present in the air compared to the intercellular air spaces. The less humid the external air, the steeper the water vapour concentration gradient. Water vapour will diffuse out of the stomata at a faster rate. Thus, the rate of transpiration would be faster. → The more humid the external air, the water vapour concentration gradient between the inside of the leaf and the atmosphere will become less steep. Water vapour will diffuse out of the stomata at a slower rate. The, the rate of transpiration would be slower.

2. Air movement/ wind speed: In still air, water vapour that diffuse out of the stomata and accumulates outside. The water vapour concentration gradient between the intercellular air spaces of the leaf and the atmosphere becomes less steep. As a result transpiration rate decreases. → Stronger wind current lead to higher rates of removal of moisture around the leaf as the wind blows away water vapour that accumulates outside the stomata. The water vapour concentration gradient between the inside and the outside of the leaf becomes steeper and rate of transpiration increase.

3. Temperature of air: Higher temperature leads to increase in the rate of evaporation of water from cell surfaces. Thus, the rate of transpiration is faster with higher temperatures. Higher temperature also causes the water vapour to diffuse out of the stomata faster.

4. Light: Light intensity affects the size of the stomata. Under high light intensities in the day causes the stomata to open and become wider, More water vapour will diffuse out of the stomata and the rate of transpiration is faster. Under low light intensities, the stomata closes, less water is lost from the leaf. Hence decrease rate of transpiration.

Wilting definition

Wilting occurs when the rate of transpiration exceeds the rate of absorption of water by the roots and the cells lose their turbot and become flaccid.

• Leaves will fold up and surface area of the leaf will be reduced. It helps to reduce rate of transpiration as the surface being exposed to sunlight is reduced. the guard cells become flaccid and close, leading to reduced water lost.

Disadvantages of prolong wilting

• Loss of water upon wilting decreases rate of photosynthesis,

• When the leaves collapses, the leaf surface are exposed to sunlight would be reduced.

• Closing of stomata limits supply of carbon dioxide to the plant and this also reduces phototsynthesis.

Translocation

Translocation refers to transport of manufactured food substances in leaves such as sugar (sucrose) and amino acids, to other parts of the plant via the phloem.

translocation experiments

1. Aphids experiment- Aphids feed on the cell sap of the plant. During feeding, the proboscis penetrates the stem at the phloem. When the body of the aphids is cut off, laving behind the proboscis in the plant tissues, the liquid will exude from the cut end of the proboscis. Analysis of the exuded liquid indicated sucrose and amino acids

2. Isotope studies: The plant is grown in the environment containing radioactive isotope. When photosynthesis takes place, the sugar is formed will contain radioactive carbon. The cut stem section when exposed onto X-ray photographic film will tureen cloudy. The x-ray film shows the phloem being radioactive.

3. Ringing experiment: A complete ring of bark including the phloem and cambium is removed. The xylem is left untouched. Swelling should occur at the region above the ring. This is because of increases tissue growth above the cut region due to excessive nutrients available as they cannot be transported down the stem as the phloem is removed

Upper epidermis

• Single layer of closely packed cells

Cuticle:

• a waxy transport layer which protects the enclosed leaf tissue and prevents excessive evaporation of water

• It is transparent and allows sunlight to pass through.

Palisade mesophyll

• Consists of closely packed, long and cylindrical cells.

• The cells contain numerous chloroplast which enable them to absorb maximum sunlight for photosynthesis

Spongy mesophyll

• Irregular shaped cells.

• Numerous intercellular air spaces among them to allow for rapid diffusion pod gases through the leaf.

• Contains chloroplasts but fewer than palisade mesophyll.

• Mesophyll cells are covered with a thin film of moisture so that carbon dioxide in it.

• Vascular bundles containing xylem and phloem are found here.

Lower epidermis

Single layer of closely packed cells covered by outer layer of cuticle.

Stomata :

• Minute opening called stomata formed by guard cells which contain chloroplast where gas exchange and transpiration occurs here.

Photosynthesis definition

Photosynthesis is the process in which light energy absorbed by chlorophyll is converted into chemical energy. The chemical energy is used to synthesise carbohydrates from water and carbon dioxide. Water and carbon dioxide are raw materials for photosynthesis. Oxygen is released in the process

Carbon dioxide enters the leaf through the stomata

• During the day, carbon dioxide in the leaf is rapidly used up due to photosynthesis.

• Carbon dioxide concentration in the leaf is lower than that in the air resulting in a diffusion gradient. \Carbon dioxide thus dissolve from the surrounding air, through the stomata, into the air spaces.

• A thin film of moisture covers the surfaces of the mesophyll cells so that carbon dioxide can diffuse in it.

• The dissolved carbon dioxide than diffuse into the cell