Plant cells are also affected by the movement of water into and out of their cells but the presence of a cell wall prevents plant cells being damaged or bursting. If a plant cell is put into water, water will enter by osmosis but the plant cell wall resists the entry of further water once the cell is full. A plant cell that is full becomes firm and rigid – a condition known as turgor (Figure 1.21).
Water potential the tendency of water molecules to move from an area of higher concentration to an area of lower concentration.
Active transport
Normal saline is a solution of 0.90% w/v (weight by volume) of sodium chloride and is isotonic with human cells. It is used frequently in intravenous drips (IVs) for patients who cannot take fluids orally and are in danger of becoming dehydrated.
Many of the substances a cell needs occur in low concentrations in the surroundings outside the plasma membrane. For example, plants must take in nitrate ions from very dilute solutions in the soil to build their proteins, and muscle cells actively take in calcium ions to enable them to contract. To move these substances into the cell against a concentration gradient, the cell must use metabolic energy released from the breakdown of ATP to ADP and Pi (Subtopic 2.8). This is called active transport (Figure 1.22). Specific proteins in the plasma membrane act as transporters or ‘carriers’ to move substances through. Many of the carrier proteins are specific to particular molecules or ions so that these can be selected for transport into the cell.
A plant cell in a solution that is less concentrated than the cell solution absorbs water by osmosis 1
2
3 pressure of cell contents
water in
pressure of wall resisting uptake of water (pressure potential)
pressure of cell contents
The wall pushes back on the cell contents. The force of the wall pushing on the cell contents is called the pressure potential. The cell is turgid.
Water diffuses into the cell through the partially permeable plasma membrane. The cell contents expand. The contents will push out on the wall.
water potential outside equals water potential inside
As more water enters, water potential of the cell solution rises. Eventually, water potential inside equals water potential outside and there is no further net uptake of water. The cell has reached equilibrium. The cell is fully turgid.
A plant cell in a solution that is more concentrated than the cell solution loses water by osmosis 1
2
water out
Water diffuses out of the cell through the partially permeable plasma membrane.
3
water out
The contents do not push out on the walls. The cell is said to be flaccid.
4
water out
The plasma membrane is eventually pulled away from the wall in places. The cell is plasmolysed. External solution now fills the gap between wall and plasma membrane.
Some parts of the membrane are resistant to pulling away from the wall. If the membrane is torn at these points, the cell dies.
Figure 1.21 Responses of plant cells to solutions of different concentrations. Plant cells are not damaged as water enters by osmosis because their cell wall protects them.
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