Transport in Plants
Part II
Translocation refers to the movement of food made in the leaves during photosynthesis to other parts of the plant. Translocation occurs in the phloem.
The cells where photosynthesis occurs and glucose is made are known as the SOURCE (leaf cells). Glucose is the basic unit of carbohydrates. It is made of a single unit (monosaccharide). It is converted to sucrose, a carbohydrate made up of two units (disaccharide) in the leaf cells. Sucrose moves into the phloem and is then transported to the SINK where sucrose is needed/stored (eg. root cells).
Water enters a plant through the roots. Water moves from cell to cell in the root through osmosis, from a region of high water potential to a region of low water potential until it reaches the xylem.
The absorption of water by the roots is facilitated by the root hairs. Root hairs are fine protrusions found on the epidermal cells (outermost layer) of the root. The root hairs can grow between soil particles and reach the layer of moisture around each soil particle.
For a well-watered plant, the soil surrounding the plant will have a higher water potential compared to the cell sap found in the epidermal cells of roots. Therefore, water moves into the epidermal cells through the partially permeable membrane by osmosis.
When water enters the epidermal cell, the water potential of the cell increases. The epidermal cell now has a higher water potential compared to its neighbouring cell. Therefore water moves into the neighbouring cell through the partially permeable membrane by osmosis.
The process continues as water moves from the outer layer of cells into the inner layer of cells until it reaches the xylem.
Minerals like magnesium are needed for the synthesis of chlorophyll while nitrogen is needed to make proteins. These mineral salts can be found dissolved in the water in the soil.
In a plant growing in an environment rich in mineral salts (eg. well-fertilised farm), the concentration of mineral salts in the soil is higher than the concentration of mineral salts in the epidermal cells. Therefore, mineral salts will move from the soil into the epidermal cells through diffusion.
However, in some cases, the concentration of mineral salts in the soil is lower than the concentration of mineral salts in the epidermal cells. The root will continue to absorb mineral salts through active transport. Mineral salts are absorbed into the root against concentration gradient and with the use of energy.
Roots are specially adapted for its function of absorption of water and mineral salts. Root hair cells have a fine extension that helps to increase the surface area to volume ratio and hence increase the rate of absorption.
The cell surface membrane is a selectively permeable membrane that controls the movement of substance in and out of the cell. The cell surface membrane prevents the cell sap from leaking out of the cell. This is so that the water potential of the cells are determined only by water movement in/out of the cell. As water moves into the different layers of cells in the root, there is a constant water potential difference between cells so that the absorption of water is continuous.
When the surrounding soil has low mineral salt concentration compared to the root, active transport of minerals occur and root hair cells are equipped with mitochondria (power house of cells) which can generate energy through respiration. This energy is used to transport minerals against concentration gradient.
Water absorbed by the roots are transported to the leaves through the xylem. Water is required in leaves to maintain turgidity of leaf cells and also for the process of photosynthesis.
In the leaves, there are openings on the underside of the leaves called stomata. Stomata facilitates gaseous exchange between the leaf and the surrounding air. In the daytime where sunlight is present, photosynthesis is predominant and the plant takes in carbon dioxide and gives out oxygen. At night where sunlight is absent, only respiration occurs and the plant takes in oxygen and gives out carbon dioxide.
Water transported to the leaves may also evaporate and diffuse out of the leaves through the stomata. The process in which a plant loses water as it evaporates and diffuse out of the stomata is known as transpiration.
Although the loss of water appears to be a disadvantage to the plant, transpiration has an important role to play in the transport of water.
Transpiration is the main process that 'pulls' water up along the xylem. It is the main reason why water can travel upwards in the xylem against gravity and reach the leaves that may be a few metres away from the ground. It is like drinking through a straw. When someone sucks through the top of the straw, there is a suction force that pulls the water molecules up along the straw. Transpiration provides an upward force on the stream of water in the xylem.
Water transported to the leaves will be used for photosynthesis and it will also keep the leaf cells turgid. (Refer to Movement of Substances!) Turgidity refers to the pressure of water against the cell wall of a plant cell. When a plant cell has alot of water molecules in its cell sap, the cell becomes roundish and has a fixed shape. However, when a plant cell loses water, the cytoplasm pulls away from the cell wall and the cell becomes flaccid (soft and no fixed shape). Leaf cells have to be turgid so that they have a fixed shape and push against one another. Thus the leaf appears to be flat and 'straightened out'. This increases the surface area of the leaf exposed to sunlight and increases the absorption of sunlight for photosynthesis.
The rate of transpiration (rate of water loss due to evaporation through stomata) is determined by several factors.
1. Humidity. Humidity refers to the amount of water vapour in the air. A high humidity weather (like in Singapore) would mean that the amount of water vapour in the air is high. With large amounts of water vapour in the air, the rate of evaporation of water will decrease, transpiration rate decreases. This is the reason why we feel 'sticky' in Singapore as the sweat produced by our bodies evaporate at a much lower rate. On the other hand, an environment with low humidity level has low amount of water in the air and the rate of evaporation increases, transpiration rate increases. In some countries, the humidity is very low, or more commonly known as dry. Therefore, the moisture on our skin evaporates quickly and often, one's skin/lips become dry and needs to be moisturised regularly.
2. Wind or air movement
With wind, the water molecules that have just evaporated and diffused out of the stomata will be quickly blown away. This reduces the amount of water vapour near the stomata. Therefore evaporation rate increases and transpiration rate increases. However, when there is no wind, the water molecules that have just evaporated and diffused out of the stomata will accumulate near stomata before slowly moving away from the leaf. The amount of water vapour near the stomata increases and evaporation rate decreases, therefore, transpiration rate decreases.
3. Temperature of air
At high temperature, water evaporates at a faster rate, therefore transpiration rate increases. At low temperature, water evaporates at a slower rate, therefore transpiration rate decreases. Simple (:
4. Light
The amount of light affects the size of the stomata which affects the rate of transpiration. How?
In the daytime, when there is abundant sunlight, plants photosynthesize at a rapid rate and require carbon dioxide from the surrounding air as raw material. This means that the stomata need to be open wide for gaseous exchange to occur. When the stomata opening is big, the evaporation rate of water increases and transpiration rate increases. On the other hand, since plants only respire at night (no sunlight) at a slow rate, the need for gaseous exchange reduces and the stomata opening is smaller. Therefore the evaporation rate of water decreases and transpiration rate decreases.
This is a summary slide on the factors affecting transpiration.
On the left hand side: Full sunlight, low humidity, high temperature, high winds -> High rate of transpiration (big blue arrows)
On the right hand side: Low sunlight, high humidity, low temperature, no wind -> Low rate of transpiration (small blue arrows)
Usually the water loss through transpiration is replaced by the water absorbed by the roots. However, due to various reasons (eg. lack of water, too much sunlight), the rate of transpiration (rate of water loss) may become higher than the rate of water absorption.
As a result, the water content in the leaves will decrease and the leaf cells lose their turgidity (becomes flaccid). The leaf cells no longer have a fixed shape and the leaf loses its shape. This is known as wilting. The leaves take on a 'crumpled' shape.
wilted plant
Note: For the second point, it should be the 'rate of transpiration will be reduced due to the limited amount of water loss.'
Wilting has two different and opposite effects on the plant.
Disadvantage: The wilted leaves have less surface area exposed to the sun. Therefore the rate of photosynthesis will decrease and plants are unable to manufacture as much food as before.
Advantage: The stomata of the wilted leaves are 'hidden' and less exposed to the surrounding air. Therefore, the evaporation of water through the stomata is much more reduced. Less water will be lost.
Therefore, at the initial stages of wilting, the plant may be saved through providing more water, putting the plant away from sunlight etc. However, prolonged wilting with no intervention will result in the plant not photosynthesizing enough and then subsequently die :(
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All the best for your common test papers :)
