Water & Nutrient Circulation

Water is essential for living things. Without water, both plans and animals would eventually die. Gaining sufficient water can be a problem for terrestrial (land) organisms, including plants. Plants especially need water for:

• Making food in photosynthesis - Water is one of the raw materials needed by leaves to make food using photosynthesis. So if a plant runs out of water, it also experiences a shortage of food. 

• Cooling leaves by evaporation - When a plant heats up, it loses water by evaporation through the leaf stomata. Evaporation cools a plant just like sweating cools animals. 

• Providing support for leaves/stems - Soft parts of plants like leaves, stems and flowers are kept firm by water pressure in their cells. When a plant loses too much water, it wilts. 

• Transporting food around a plant (sap) - A plant needs to circulate minerals and sugars to different parts. These substances are dissolved in water and transported along a fine tube (phloem).

• Providing a solution for chemical reactions - Inside cells, many chemical reactions occur. Water is needed to dissolve the chemicals involved, so that the chemicals can freely move and participate in chemical reactions. 

During photosynthesis, most of the water that comes up to the leaves from the roots through the veins, is eventually lost to the atmosphere as water vapour through a process called transpiration. Water evaporates out of the leaves through tiny pores on the underside of leaves called stomata. Simultaneously, carbon dioxide from the air diffuses through the stomata, into the leaf.

Plants have adaptations that help reduce water loss. 

• Thick waxy cuticle that helps retain water

• Stomata are mostly on the underside of leaves out of direct sunshine. 

• If the plant loses too much water, the stomata close up, which traps the water vapour inside, but this slows down photosynthesis. 

Stomata (singular -stoma) are pores in the epidermis of leaves. Each stoma has two guard cells, which control their opening and closing. Stomata control the entry and exit of gases into and out of the leaf.

Guard cells are sausage-shaped and have irregularly thickened walls (inner walls thicker than outer walls).

When these cells are full of water, they open the stomata. They close them when the water leaves them and they become flaccid. In general there are more stomata in the lower surface than the upper surface (particularly in dicotyledons). This is to reduce water loss by transpiration. Up to 98% of water that enters the roots can be lost by transpiration. Plants show adaptations to reduce water loss. e.g. marram grass has leaves that are curved rather than flat and stomata are on the inside of the curve.

Root Hair Cells

Roots not only anchor plants to the ground, they also absorb minerals needed for healthy growth and the water needed for food-making, cooling, support, internal transport, and chemical reactions.

To absorb lots of water the root system spreads out widely among the soil particles. Near the tip of each fine root are lots of tiny projections called root hairs. Millions of very thin root hairs provide a large surface area for absorbing water molecules. The cell membrane (surface) of root hair cells is permeable - it allows water molecules to move freely in and out of the cell.

Osmosis

The problem is how to get more water molecules moving into the root instead of moving out. Plants use a process called osmosis.Osmosis is the movement of water from an area of high water concentration to an area of low water concentration through a semi-permeable membrane (lets very small/simple molecules pass through it but not large/complex molecules). This involves the diffusion of water through a membrane from an area of high concentration to an area of low concentration of water.

high concentration —>  low concentration (via diffusion)

The liquid inside the root cells has lots of glucose sugar molecules dissolved in it. These large molecules cannot escape out of the membrane. The sugar molecules make water inside the root cell less concentrated than the water in the soil surrounding it, so more water molecules move into the plant than out. This pressure helps water rise a short way up the stem.

The sap in phloem is mainly a 30% sugar solution. Sap can move up and down the phloem tubes. Phloem cells long tubes that sit end on end. The end walls are like sieves. Next to these cells are companion cells that control the functioning of phloem. Starch is broken down into glucose molecules for use in respiration and transported throughout the plant.

Oxygen is a waste product of the first stage of photosynthesis and comes from the water molecules that are absorbed through the roots and carried up the xylem to the leaves.

Some trees can be as tall as 100 metres high! So how does water get from the roots where it is absorbed to the leaves where it is used? The water is transported through bundles of very fine tubes called xylem vessels. The tubes are filled with water and run from the root up to the leaf veins where it is needed for photosynthesis.

The leaves are losing lots of water owing to evaporation caused by the heating of the Sun. As water molecules evaporate from cells inside the leaf, the concentration of water in those cells drops. So water moves by osmosis from the xylem vessels into the leaf cells.

Note that: Only water moves up xylem. Mature xylem cells are dead, elongated cells with thickened walls. They form continuous tubes up the stem of the plant into the veins of the leaves.