The Cell in Its Environment: Physical and Biophysical Processes

Substances can move into and out of a cell through its semi-permeable cell membrane. There are three different processes through which materials can move in and out of a cell. They are:

•Through the process of diffusion,
•Through the process of osmosis and
•Through the process of active transport.
Diffusion is the major process through which small molecules move in and out across the cell membrane region of higher concentration to a region of lower concentration.
The concentration of a substance is the amount of that particular substance in a given volume of liquid. Diffusion results as a result of movement and collision of molecules. The collisions of the molecules are what caused the molecules to thrust away from one another and spread out.
Molecules would normally diffuse through the cell membrane into a cell when the contents of the cell are of a lower concentration than the surrounding solution. The diffusion of water molecules through a semi permeable membrane is known as osmosis.
Due to the fact that cell function effectively without sufficient water, the majority of cellular processes depend on osmosis.
In osmosis, the molecules of water travel through the process of diffusion from a region of higher concentration to a region of lower concentration.
The movement of dissolved substances through a cell membrane without the use of cellular energy is referred to as passive transport. Diffusion and osmosis are two types of passive transport.
When there is a need for the cell to assimilate materials that are in higher concentration inside the cell than outside the cell, the movement of the materials requires energy.
On the contrary, active transport is the movement of materials through a cell membrane with the use of cellular energy. The major variation between passive transport and active transport is that active transport necessitates the use of cell’s own energy while passive transport does not.
Cells have many ways of transporting materials through active transport. In one method, especially in the transportation of protein molecules, the cell membrane “lift up” molecules outside the cell and transport them inside the cell.
Another method of active transport is through engulfing. In this case, the cell membrane envelops or engulfs a particle and forms a vacuole inside the cell.
The majority of the cells are highly small. This is because the all particles have to move in and out of the cell through the cell membrane. On entering inside the cell, it is transported to its target through a stream of moving cytoplasm.
In an extremely large cell, streams of cytoplasm ought to travel farther to transport materials from the cell membrane to every part of the cell.
In a nutshell, Osmosis can be defined as:
•Diffusion of a solvent (frequently water molecules) through a semi permeable membrane from an area of low solute concentration to an area of high solute concentration.
•Net movement of water molecules through a semi-permeable membrane from an area of higher water concentration to an area of lower water concentration.
•The tendency of water to move from a hypotonic solution (lower concentration of dissolved substances) to hypertonic solution (higher concentration of dissolved substances) through a semi-permeable membrane
In biological processes, osmosis is very significant since a lot of biological membranes are semi permeable in nature, and it leads to diverse physiological effects.
For instance, when an animal cell is placed into a hypertonic surrounding- a surrounding with lower water concentration- the water will depart from the cell resulting to a shrinking of the cell.
When an animal cell is placed in a hypotonic environment, or an environment that is of higher water concentration, the water molecules will move into the cell causing the cell to swell. If osmosis continues and becomes extreme the cell will finally burst.
In a plant cell, extreme osmosis is barred as a result of the osmotic pressure exerted by the cell wall which stabilizes the cell. In fact, osmotic pressure is the major cause of support in plants.
On the other hand, if a plant cell is placed in a hypertonic surrounding, the cell wall cannot prevent the cell from losing water. This normally leads to a condition known as flaccidity or the shrinking of the cell.


This can be defined as the shrinking of protoplasm away from the cell wall of a plant or bacterium as a result of water loss from osmosis and in so doing leading to gaps between the cell wall and cell membrane.
When a plant cell is immersed into a highly concentrated solution, water diffuses out of the cell, and turgor pressure of the cell is lost. This makes the cell to become flaccid. Additional loss of water will lead to plasmolysis, and finally to cytorrhysis which means the total collapse of cell wall.
Plasmolysis only occurs in severe situations and seldom happens in nature. It is induced in the laboratory by immersing cells in strong saline or sugar solutions to give rise to exosmosis, frequently with the use of Elodea plants or onion epidermal cells.

Wilting and Plasmolysis

Plasmolysis is the separation of plant cell cytoplasm from the cell wall due to excessive loss of water. It is unlikely to happen naturally except in rigorous situations. Plasmolysis is induced in the laboratory by immersing a plant cell in a powerfully saline or sugary solution, so that the plant losses water through the process of osmosis.

Diffusion and Osmosis

Diffusion is the process by which molecules move from areas of high concentration, to areas of low concentration. When the molecules are even throughout a space – it is referred to as equilibrium state.
Concentration gradient: This is the difference between concentrations in a given environment.
Molecules will always travel down the concentration gradient, toward areas of smaller concentration. Example: food coloring that disperses out in a glass of water the dispersal of the fume from an air freshener sprayed in a room.
Semi Permeable membranes: These are membranes that allow the passage of some things while at the same time disallowing the passage of other things through it. Example of a semi-permeable membrane is the cell wall. It allows water and oxygen to pass freely through the cell’s membrane, by diffusion
Osmosis means the diffusion of water across a membrane. Water will normally travel in the direction where there is a high concentration of solute. In other words where there is a lower concentration of water.
Salt is a solute and when it is concentrated inside or outside the cell, it will pull the water up to its direction. This is also the reason why we are thirsty after eating salty food.

Type of Solutions:

1. Isotonic Solutions:
Iso means the same. When the concentration of solute (salt) is the same on both sides, the water will travel back and forth but it won’t have any effect on the overall amount of water on the two sides.
2.Hypotonic Solutions
“HYPO” means less. Therefore a hypotonic solution is a solution that contains less solute (salt) molecules outside the cell which leads to intake of water from the outside into the cell.
This leads to the cell gaining water and growing bigger. In plant cells, the central vacuoles will get filled up and the plant turns firmer and more rigid. Thus the plant’s cell wall prevents the plant from bursting.
In animal cells however when this happens, the cell possibly will be in risk of bursting. Animal cell organelles known as the contractile vacuoles would normally pump water outside of the cell to avert this danger.
3.Hypertonic Solution:
HYPER means more. Thus A hypertonic solution is a solution that contains more solute or salt molecules outside the cell than inside the cell. This would normally result to water being taking outside of the cell.
In plant cells, the central vacuole loses water to the surrounding leading to the shrinkage of the cells and wilting of the plant.
In animal cells, this also results to the shrinkage of the cell. The situation could lead to death both in plant and animal cell.
This is why it is unsafe to drink sea water . Dehydration would normally be hastened up by intake of salty water. This is also why “salting farm lands is a common war practice with the aim of causing the death of crops in the farm to create famine.
Diffusion and Osmosis are both types of passive transport. What this means is that to transport materials in and out of the cell through either osmosis or diffusion, there is no energy requirement.
Occasionally, big molecules are unable to pass through the plasma membrane, and are assisted to pass across by carrier proteins. This process is referred to as a facilitated diffusion.
Diffusion is which is the net movement of a substance like an atom, ion or molecule from a region of high concentration to a region of low concentration can as well be defined as the movement of a substance down a concentration gradient.
A gradient is an alteration in the value of a quantity like concentration, pressure and temperature with the change in another variable like distance.
For instance, an alteration in a concentration across a distance is referred to as a concentration gradient. An alteration in the pressure across a distance is referred to as a pressure gradient while an alteration in temperature through a distance is referred to as a temperature gradient.
The word diffusion is obtained from the Latin word, “diffundere”, which means “to disperse”. If a substance is being dispersed, it is being transported from a region of high concentration to a region of low concentration.
A distinctive characteristic of diffusion is that it leads to mixing or mass transport, without needing bulk motion or bulk flow. Therefore, diffusion ought not to be mistaken for convection, or advections, which are other transport phenomena that make use of bulk flow to transport particles from one place to another.
Osmosis on the other hand is the spontaneous net movement of solvent molecules via partly permeable membrane into a region of higher solute concentration, in the direction that have the tendency of equalizing the solute concentrations on the either sides.
The osmotic pressure is used to explain the pressure needed to sustain equilibrium without any net movement of solvent. Osmotic pressure is a colligative property. This means that the osmotic pressure depends on the molar of the solute but not on its characteristics.
In biological systems generally, biological membranes are semipermeable and are not permeable to bigger and polar molecules, like ions, proteins, and polysaccharides. They are only permeable to non-polar and/or hydrophobic molecules such as lipids and small molecules such as oxygen, carbon dioxide, nitrogen, and nitric oxide.
Permeability of materials across the cell membrane depends on solubility, charge, or chemistry, in addition to the size of the solute. Water molecules pass through the plasma membrane, tonoplast membrane (vacuole) or protoplast by diffusing across the phospholipid bilayer throough aquaporins.
This means small trans-membrane proteins comparable to those in charge of facilitated diffusion and ion channels. Osmosis makes available the key way through which water is transported in and out of cells.
The turgor pressure of a cell is principally maintained by osmosis crosswise the cell membrane stuck between the cell center and its comparatively hypotonic environment.

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