Every cell is surrounded by a liquid or semi-liquid environment. The cell transport system produces a nearly continuous movement of substances, both into and out of the cell, to ensure that the cell receive the correct and enough raw ingredients and discard toxic wastes fast enough for survival.
The concepts of diffusion, osmosis and active transport are required to understand the movement of water and substances between the cell and its surroundings.
Diffusion – net movement of substances (atoms, ions, molecules) from a region of higher concentration to a region of lower concentration, down a concentration gradient.
Movement of substance across a fully permeable membrane
The concentration gradient refers to the difference in the concentration of solutes present in a solution between two regions. The steeper the concentration gradient, the faster the rate of diffusion. As the substances diffuse, the concentration gradient becomes less steep.
When the substances have evenly spread out, we say that a state of dynamic equilibrium has been reached. The substances are still in constant random movement in the space it is confined with but there is no net change in concentration any further. Not applicable to water molecules.
Osmosis – movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, down the water potential gradient.
Water potential is simply the measure of relative tendency of water molecules to move from one area to another. A dilute solution for instance contains more water molecules per unit volume than a concentrated solution, and hence have a higher tendency of water to move to the other areas of the solution and a higher potential.
A water potential gradient is the difference in water potential between two solutions of different water potential, separated by a partially permeable membrane.
A partially permeable membrane is a barrier which allows the passage of small molecules but not the large solute molecules.
When a solution have a higher/lower/same water potential as another solution we are comparing with, it is hypotonic/hypertonic/isotonic with respect to the second solution it is compared with.
The cell wall of a plant cell is permeable and allows most dissolved substances to pass through, which cause the plant cell to behave differently from an animal cell, when placed in solutions with different water potentials.
When the surrounding environment is hypertonic:
- Plant cell – water leave cell by osmosis, resulting in the shrinkage of cytoplasm and cell surface membrane from the cell wall (plasmolysis; can be restored by placing in water or solution with higher water potential)
- Animal cell – water leave cell by osmosis, resulting in the shrinkage of cells and the formation of spikes on surface of cell (crenation; cannot be restored, dehydration and eventual death)
When surrounding environment is hypotonic:
- Plant cell – water enters cell by osmosis and cause the enlargement of vacuole which pushes the cell contents against the cell wall, resulting in swollen/turgid state (turgor important in maintaining shape of soft tissues in cells; pressure exerted by water on cell is turgor pressure), cell does not burst as the cell wall is strong and relatively inelastic which prevents overexpansion by exerting opposing pressure as water enters the cell
- Animal cell – water enters cell by osmosis, cell burst when too much water enters the cell
When surrounding environment is isotonic:
- No change in water movement between the surrounding and the plant/animal cell since there is no difference in water potential
Active transport – is the energy-dependent process of moving substances from a region of low concentration to a region of high concentration, against a concentration gradient.
The rate at which a substance enters a cell across the cell surface membrane is dependent on the permeability of the membrane and the surface area of a cell.
For a given concentration gradient and volume, the greater the area of the cell surface membrane, the faster the rate of diffusion of a substance.
However, as the cell grows in size, even though its surface area also increase, the surface area to volume ratio (SA:V) decreases. A decrease in SA:V results in a decrease in rate of movement of substances across the cell membrane, per unit volume. Take the following figure as an example.
Both combination of cells, on the left and right side of the figure, results in the same volume occupied in the body even though there are more cells on the right hand side of the figure.
Let’s take the volume of the surrounding environment to be 200 cm square. The SA:V on the represented by the left hand side of the figure is 5400:200 (or 27:1) while the SA:V for the right hand side of the figure is 16,200:200 (or 81:1). There will be a faster rate of substance diffusion in the scenario on the right hand side than the scenario on the left hand side.
This is also the reason to which why as small, actively growing cells increase in size, their level of activity and metabolism decreases and they stop growing once a maximum size is reached.
Cells can compensate for their small SA:V by adopting some of these strategies:
- Self-restriction – staying small maximises SA:V for greatest efficiencies in diffusion, bacteria and yeast cells adopt this strategy
- Increasing surface area through physical modifications – elongating and flattening out of the body of the cell (e.g. flat discs of red blood cells) or formation or microfolds, fingers or indentations within the surface of the cell (e.g. human intestinal microvilli)
- Bulk acquisition – take in nutrients through water found in vacuoles in which they are dissolved, allows intake of large quantities of nutrients in bulk (e.g. intestinal epithelium cells)
- Movement of cytoplasmic organelles – streaming motion of parts of cytoplasm transfers nutrients away from the plasma membrane, help maintain a steady concentration gradient for continuous diffusion (e.g. leaf and root hair cells)
- Active relocation – mobile unicellular organisms can search out areas of high concentrations of nutrients (e.g. protists)