TRANSPORT- ACTIVE TRANSPORT - PART 4

 Active transport Active 

Transport is the movement of substances from a low concentration region to a high concentration region across a living cell membrane using energy from the cell. 

 The molecules move in an opposite direction to the direction they would go during diffusion. Thus, the molecules move against the concentration gradient. 

Examples of Active Transport 

1. Absorption of digested food into the blood in the intestines of mammals. 

2. Movement of glucose into the phloem tissues of plants. 

3. Re-absorption of glucose, amino acids and ions in the kidney of mammals. 

4. Absorption of mineral salt from the soil by plant roots. 

Differences between diffusion and active transport 

Diffusion                                                                                    Active transport 

Does not require energy                                                         Requires energy 

Involves movement of all substances                                    Involves movement of liquids only 

Molecular movement is against concentration gradient        Molecular movement is along with concentration gradient 

In plants, the transport of various substances such as water and mineral salt from the soil through the roots and stems to the leaves and the movement of food produced through photosynthesis from the leaves to other parts such as the storage organs and growing part of the plants keep the plants alive. 

 Absorption and transport of water and mineral salt in plants 

1. Water enters the root hair and then the cells of plants by osmosis. This is because vacuoles in the root hair contain higher concentration of solutes that the surrounding soil solution. 

 2. Water then moves from the root cortex from one cell to another by three different pathways – mostly through the cellulose cell wall, some through the cytoplasm and the rest pass through one vacuole to another. 

 3. Water finally enters the xylem vessels.

The absorption of mineral salt occurs in two major ways – diffusion and active transport

 Diffusion: Diffusion occurs only when the concentration of a particular mineral element is higher in the soil solution than in the root hair. Many mineral elements are found in higher concentration in cell saps than in the soil solution. These must be taken up actively by the plants 

Active transport: This requires the use of energy produced during respiration, which is in the form of ATP, to absorb mineral ions into the root hair cells against their concentration gradient. The minerals ions then diffuse through the cortex cells and enter the xylem with the water. 

Transport of manufactured food (Translocation) 

Translocation is the movement of dissolved products of photosynthesis. 

Translocation occurs mostly in phloem tissue within the sieve tubes. Translocation is necessary because the leaves which synthesize organic materials are far away from other parts of the plant which need a constant supply of the manufactured food. 

 Sugar and amino acids normally move from their manufacture sites in the leaves to growing regions such as buds, or to storage organs such as fruits, seeds or tubers. They may also move from storage organs to growing regions. 

Experiment to show that the phloem tissue is responsible for translocation (the ringing experiment) 

 Remove a ring from the bark of a young plant. 

 Leave it for some 24 hours. Observation It would be observed that the area just above the ring starts to bulge, because food substances, which have to move down to the roots, accumulate there. The whole plant dies of starvation after a few weeks. 

Conclusion: This shows that the phloem tissues are responsible for the transport of food substances in plants.