5. Describe the routes for water transport in plants. 6. Describe the potential

Question

5. Describe the routes for water transport in plants. 6. Describe the potential and limits of root pressure to move xylem sap 7. Define the term transpiration. 8.Explain how transpirational pull moves xylem sap up from the root tips to the leaves. 9. Describe the role of guard cells in photosynthesis-transpiration 10. Explain how and when stomata open and close. 11. What is considered a sugar source or a sugar sink? 12. Describe the process of sugar loading and unloading using the concept of pressure flow Remember: water potential is a measure of the potential energy of water relative to the potential energy of pure, free water that is not bound to solutes or surfaces. The potential energy of water refers to water's ability to perform work as it moves to a state of lower free energy

Explanation / Answer

5.Water moves from the soil and the route followed is: epidermis of roots ---crossing the root cortex---enters into the vascular cylinder---flows up the shoot system via xylem.Water moves passively into the roots and then into the xylem.It is the forces of cohesion and adhesion that results in formation of a water column in the xylem.Water also  moves from the xylem into the mesophyll cells in the leaves and evaporates from the leaf via diffusion through the stomata.

6.When the process of transpiration is low at night, roots are in the process of absorbing water.Also, minerals accumulates in the xylem of the stele and this lowers the water potential within the vascular cylinder.As a result of this, pressure build up in the xylem of roots and this results in an upward push of xylem sap.

7.Transpiration can be defined as  process in which water moves through a plant and evaporates outside from the of the plant through the leaves in form of water vapour and through the stem in liquid form which is known as guttation.

8.Water first evaporates from the spaces that are present in the spongy parenchyma,Next,in the corner of the spaces,the water film goes on to form strong menisci.It is this surface tension of the menisci that is able to pull water through the apoplast.This is followed by the cohesion of water molecules as a result of which the entire water column is pulled up.Also, it is to be noted that the negative pressure potential or the tension that is created in the xylem also causes water to move up through the xylem from root tips to the leaves.

9.Chloroplast in guard cells carry out photosynthesis in light and synthesize glucose and the concentration of glucose increases in the guard cell.The epidermal cells surounding the guard cells have less amount of glucose and other nutrients and as a result of this ,water moves from from epidermal cells into guard cells via osmosis.K+ ions also accumulate in guard cells prior to water flow via osmosis.This makes the guard cells to become turgid and stomata open leading to transpiration. At night , K+ ions and water moves out of the guard cells due to low CO2 consumption as no phostosynthesis occurs and guard cells become flaccid closing the stomata and no transpiration occurs.

10.

Stomatal opening :The stomata open during the day time in presence of light.Firstly the potassium ions move into the guard cells and this leads to accumulation of water via osmosis in the guard cells.As a result of this,the guard cells become turgid and expand and their iiner walls that are rigid are pull apart ,opening the stomata.

Stomatal closing:The stomata close during the night time in absence of light.The decrease in consumption of CO2 and movement of potassium ions and water out of the guard cells makes it become flaccid ,leading to closing of stomata.

11.Sugar source refers to the nutrient rich regions of the plant such as the leaves where sugar is synthesised via photosynthesis to be supplied to the rest of the plant parts which are referred to as sinks and that are in need of nutrients for growth such as the growing tissues.

12.Pressure flow refers to the mechanism by which sugars are transported from the sources to the sinks via phloem.The sugar molecules that are synthesised at the sources i.e leaves move into the sieve elements of the phloem cell via active transport.This is known as sugar loading. Next, water moves via osmosis into the sieve elements and this results in creation of turgor pressure in the sieve elements.This tugor pressure generated forces the sugars and fluids down the phloem tubes to the sinks. On reaching the sink,the sugars are actively removed from the phloem,This is known as sugar unloading.This is followed by flow of water osmotically, so that conditions of high water potential and low turgor pressure are maintained to keep the pressure flow process in continuation.

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