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How are these two mechanisms executed?
However, there are a few differences between these two procedures.
Difference Between Passive and Active Transport
The main difference is the fact that passive transport does not require any energy, whereas active transport requires energy for movement.
The energy for this form of transport is obtained from ATP molecules, that is, this process is executed with the help of chemical energy.
There are a few similarities as well, like both of these are mechanisms of movement for substances in and out of the cell.
This is the reason why every cell is said to be dynamic in nature.
Facilitated diffusion or facilitated transport takes place across a cell membrane without the need of energy but with the help of a specific carrier protein.
The last type of movement is osmosis, wherein there is movement across a semi-permeable membrane along a potential gradient.
Active transport is the process by which there is uptake of glucose by the cells present in the intestines of humans.
Thus, they are a vital part of cell biology.
5) CAM physiology - Stomata open during the evening/night instead of during the day (when the temperature is at its highest) as the transpiration rate will be lower during cooler hours.
Temperature - As temperature rises, so does the rate of transpiration. This is because heat is vital for the evaporation of water vapour from the cell walls of spongy mesophyll cells. A rise in temperature leads to an increase in the evaporation rate thereby increasing transpiration rate. Higher temperatures also increase the rate of diffusion between air spaces inside the leaf and the air outside. Finally, an increase in temperature causes a reduction in humidity in the air outside the leaf which causes an increase in concentration gradient and therefore an increase in transpiration rate.
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The plant hormone abscisic acid causes the closing of stomata.
Humidity - Water diffuses out of the leaf, down its concentration gradient, from a high concentration gradient inside the leaf to a lower concentration gradient in the air. The lower concentration gradient in the air is vital for transpiration. Humidity is the water vapour in the air, therefore a rise in humidity means a larger concentration of water vapour in the air and results in a decrease in transpiration rate.
2) Thick waxy cuticle covering the epidermis
Once water has been taken up by the roots it is pulled upwards into the leaves where it then evaporates. This flow of water from the roots to the leaves is called the transpiration stream. This transpiration stream occurs in xylem vessels and the movement of water is passive. Mature xylem vessels are long dead structures made up of cells arranged from end to end. The cell walls between the adjacent xylem cells are broken down and the cytoplasmic content dies to form a continuous tube. The cells also lack a plasma membrane which allows water to enter the vessels freely. In addition, they also contain pores in the outer cell walls which allows the movement of water out of the vessels and into the surrounding cells of leaves. The outer cell walls contain thickenings which resemble spirals or rings impregnated with lignin which makes the vessels strong and able to withstand low pressures. Low pressure (suction) is created in the xylem vessels when water is pulled out of the transpiration stream via evaporation of water vapour from the spongy mesophyll cell walls in the leaves. Heat from the environment is necessary as it provides the energy required for the evaporation of water. The low pressure causes more water from the roots to be pulled upwards through the xylem tubes, this is called transpiration pull. Transpiration pull works due to the cohesion of water molecules. Hydrogen bonds form between the water molecules allowing the formation of columns of water which are not easily broken by the low pressure. In addition, adhesion also plays a role in maintaining transpiration pull. The water molecules adhere to the walls of the xylem vessels preventing the columns of water from breaking. So to conclude, the structure of xylem vessels, transpiration pull, cohesion, adhesion and evaporation are all important in the carrying of water by the transpiration stream.
4) Water storage tissues in roots, leaves and stems
The concentration of mineral ions inside the plant's roots is a lot higher than that found in the soil. Therefore, mineral ions have to be transported into the roots via active transport. Protein pumps exist in the plasma membranes of root cells. There are many types of these protein pumps for the absorption of many different mineral ions. Active transport requires ATP production by mitochondria (aerobic cell respiration, oxygen is needed) and therefore the root cells also contain many mitochondria. The branching of roots and the formation of root hairs increases the surface area for the absorption of mineral ions by active transport.
Animation created by Wes Bellanca and Memorie Yasuda. Earthguide
Mineral ions in the soil move to roots via fungal hyphae (mutualism), mass flow of water in the soil carrying ions and the diffusion of mineral ions.
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