Plant Structure

Plant tissues and their roles: 

Epidermis: The outer layer of the plant. 

Cortex: Contains cells with packing function. 

Endodermis: Layer of cells with waterproof walls, encourages the simplistic pathway, which surround the vascular tissue. 

Schlerenchyma fibres: Cells with thick cell walls which support the plant. 

Phloem:  Made up of sieve tubes and companion cells, which allow for transport of organic solutes through the plant. 

Xylem: Composed of dead cells containing lignin, which allow for transport of water and mineral ions through the plant. 

Cambium: Contains meristematic cells, with the ability to differentiate into any tissue. 

Tissues in root: 

Tissues in stem: 

 

 

Transpiration

Transpiration definition: 

The process by which water moves from the roots of the plant, to the leaves and diffuses out through the stomata. The stomata are pores on the leaves, which open or close depending on light levels. 

Xylem Vessels:

Tube-like structure responsible for transporting water and mineral ions through the plant. The xylem is made up of empty dead cells, allowing water to flow through. The xylem cells are dead because the lignin is toxic to the cells. Lignin is a polysaccharide which provides support to the xylem vessels, allowing the vessels to withstand the low pressure caused by the gravitational pull opposed by the pull transpiration creates. The cells from the xylem also have thick cell walls to withstand this low pressure. 

Transport through the Xylem: 

1. Mineral ions present in the soil are transported to the roots through active transport. When there aren't enough mineral ions in the soil, fungi can provide mineral ions directly to the roots of plants, in exchange for carbohydrates from the plant, making this mutually beneficial. 
2. The high solute concentration inside root cells causes water from the soil to follow by osmosis, allowing water to enter the roots. 
3. Water from the root cells enters the xylem cells through the apolistic or simplistic pathway, due to the high root pressure. The apolistic pathway involves the flow of water through cell walls. The simplistic pathway involves the diffusion of water through the cytoplasm. 
4. Meanwhile, water present in the leaves evaporates into air space within the leaves. The evaporated water in the leaves, then diffuses out of the leaves through the stomata.
5. The diffusion of water out of the leaves creates a pulling force, which pulls water up the xylem. It is the adhesive and cohesive properties of water that allow for this to happen. The water adheres to the xylem walls, and coheres to other water molecules. 
6. The water that is transported through the xylem is used for metabolic processes. 

Factors affecting the rate of transpiration: 

Temperature: Higher temperatures increase the rate of transpiration due to water molecules moving faster, making diffusion out of the plant faster. 

Humidity: Humidity in the surroundings affects the concentration gradient of water outside the plant, in comparaison to within the leaves. The more humid the environment, the slower the rate of transpiration, due to a less significant concentration gradient. 

Light levels: Light intensity affects the opening and closing of the stomata, therefore altering the diffusion of water out of the plant.  The greater the diffusion, the stronger the pulling force to the leaves of the plant, which increases transpiration. 

Wind: The more wind, the higher the rate of transpiration. This is linked to the relative humidity in the air, which is lower when the wind is stronger. 

 

Translocation

Translocation:

The movement of organic solutes through the phloem from source to sink. The organic solutes transported are known as sap. Sap consists of sugars, amino acids and plant hormones. Translocation happens from a high hydrostatic pressure to a low hydrostatic pressure. 

Phloem: 

A tube-like structure made up of sieve tubes connected to companion cells through the plasmodesmata. The sieve tubes are living cells with reduced contents to allow the flow of sap through the cells. Since they have reduced contents they rely on the companion cells which carry out metabolic processes. The sieve tubes are connected to one another by sieve plates. 

The steps involved in translocation: 

1. At the source: H+ is pumped out of companion cells through proton pumps. The higher concentration of protons outside the companion cells cause them to diffuse back into the companion cells, due to the concentration gradient established. As the protons diffuse back through the companion cells into the sieve tubes they carry a molecule of sucrose with them to the companion cells. This is known as Co-transport. 
2. The high concentration of sucrose inside the phloem causes water to follow by osmosis, creating a high hydrostatic pressure. 
3. At the sink, sucrose is actively transported to different regions of the plant where it is needed, and water follows it by osmosis. The water then returns to the source. This creates a low hydrostatic pressure at the sink. 
4. Sap then flows from the high hydrostatic pressure at the source, to the low hydrostatic pressure at the sink.

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