1. Introduction
Cell membranes present barriers to movements of ions and molecules. Within eukaryotic cells, membranes specify specific sub-cellular components. Cell membranes divide the inside of the body into compartments. Most cells of the body are not in direct contact with the outside world. They are surrounded extra-cellular fluid.
2. Movement across Membranes
2.1 Mechanisms for Ionic and Molecular Transport
Passive transport occurs due to differences in concentration of substances inside and outside of the cell. It provides the energy source to drive the movement in either direction. This is known as the concentration gradient. There is no expenditure of cellular energy in the form of ATP.
Active transport is a process whereby ions and molecules move against their concentration gradient and cellular energy in the form of ATP is needed.
2.2 Rate of Transport
The transport across the lipid component of the membrane is stictly passive and is influenced by its hydrophobic non-polar characteristics.
The transport across the protein component of the membrane primarily involves movement of ions and polar molecules. Some are passive driven by concentration gradient. Others are active and require cellular energy expenditure. Only cetain types of ions and polar molecules are transported.
2.3 Significance
This maintains a suitable pH and ionic concentration within the cell for enzyme activity. It helps in exchanging respiratory gases, absorb nutrients and vitamins and take in and lose water. Toxic excretory products are excreted. Certain enzymes and hormones are secreted. Finally, it helps to generate ionic gradients essential for nervous and muscular activity.
3. Terminologies
3.1 Definitions on Membrane Permeability
The permeability of the cell membrane determines precisely which substances can enter of leave the cytoplasm. If nothing can cross the cell membrane, the membrane is impermeable. If any substance can cross without difficulty, the membrane is freely permeable. However, the permeability of the cell membrane is somewhere between the two extremes and is described as selectively permeable. This membrane permits the free passage of some materials and restricts the passage of others. The distinction may be on the basis of size, electrical charge, molecular shape, lipid solubility of a combination of factors. Cells differ in their permeabilities according to variations in the organization and identity of membrane lipids and proteins. A semi permeable membrane refers to one that is permeable to water alone and will not allow dissolved substances to pass cross it.
3.1.1 Size and Solubility of Molecules in Lipids & Water
Non-polar (hydrophobic) molecules dissolve in lipid bilayer and cross it with ease. If two molecules are equally lipid soluble, the smaller one will cross the membrane faster.
Polar (hydrophilic) molecules will not easily pass through the membrane. Smaller polar uncharged molecules diffuse through the bilayer. All ions, even small ones have difficulty penetrating the hydrophobic bilayer.
3.1.2 Transport proteins (as Carrier-Mediated Transport)
Integral membrane proteins transport specific molecules or ions across the membrane. Ions diffuse across the plasma membranes much faster than would be predicted from their very low solubility in lipids. Presence of transport proteins explains the transportation of these ions. The variation in membrane permeability depends on different number of proteins in the membrane. The greater the number of transport proteins, the greater the substance flux across the membrane. It also depends on the different types of proteins in the membrane.
4. Traffic of Small Molecules
4.1 Diffusion
Ions an molecules are constantly in motion, colliding and bouncing off one another and off any obstacles in their paths. The movement is random. Over time, the molecules in any given space will tend to become evenly distributed. This process is known as diffusion.
Diffusion is the net movement of a substance from a region of high concentration of that substance to a region of low concentration of the same substance down a concentration gradient. It is a form of passive transport.
Diffusion is a passive process. The diffusion of a substance is not affected by gradients of other substances. It results form intrinsic kinetic energy or thermal motion of molecules and atoms. Diffusion continues until a dynamic equilibrium is reached. Diffusion is rapid over short distances but much slower over long distances.
4.1.1 Factors Influencing Diffusion Rate
4.1.1.1 Simple Diffusion
(1) Size of Concentration Gradient. Concentration gradient is a regular, graded concentration change over a distance in a particular direction. The larger it is, the faster diffusion proceeds. When cells become more active, intracellular concentration of oxygen decreases. This change increases the concentration gradient for oxygen between the inside of the cell and the interstitial fluid outside. The rate of oxygen diffusion into the cell then increases.
(2) The distance over which diffusion takes place. Concentration gradients are eliminated quickly over short distances. The greater the distance, The longer the time required. In the human body, diffusion distances are small.
(3) The area over which diffusion occurs.
(4) The nature of the membrane across which diffusion takes place.
(5) Temperature. Diffusion is directly related to temperature. The higher the temperature, the faster the diffusion rate.
(6) The molecular size and nature of the diffusing molecule. Diffusion is inversely related to molecular size.
4.1.1.2 Diffusion across a Membrane
(1) The rate of diffusion through a cell membrane depends on the ability of the molecule to dissolve in the lipid layer of the membrane. Only lipid-sloluble molecules can diffuse across the membrane by simple diffusion.
(2) The rate of diffusion through a membrane is directly proportional to the surface area of the membrane. The larger the surface area, the more molecules can diffuse across per unit time.
(3) The rate of diffusion across a membrane is inversely proportional to the thickness of the membrane. The thicker the membrane, the slower diffusion occurs.
4.2 Facilitated Diffusion
It is the diffusion of solutes across a membrane with the help of transport proteins. It is a passive transport as solute is moved down its concentration gradient. It helps the diffusion of many polar molecules and ions that are impeded by the membrane’s phospholipids bilayer and allows more rapid diffusion.
4.3 Active Transport
It is an energy requiring process where a transport protein pumps a molecule across a membrane against its concentration gradient. It helps to maintain steep ionic gradients across the cell membrane. Transport proteins involved in active transport harness energy from ATP to pump molecules against their concentration gradient.
Cells carrying out active transport are characterized by presence of numerous mitochondria, high concentration of ATP and a high respiratory rate. As a consequence of the last factor, any factor which increases the rate of respiration increases the rate of active transport. Any factor decreasing the rate of respiration or causing it to cease, decreases active transport.
5. Traffic of Large Molecules
5.1 Endocyosis & Exocytosis
Exocytosis is a process where a cell secretes macromolecules by fusion of vesicles with the plasma membrane. Vesicles usually bud from the ER or GA and migrate to plasma membrane. It is a mechanism used by secretory cells to export products.
Endocytosis is a process where a cell takes in macromolecules by forming vesicles derived from the plasma membrane. Vesicles are from a localized region of plasma membrane that sinks inwards and pinches off into the cytoplasm. This mechanism is used by cells to incorporate extra-cellular substances. There are 3 types of endocytosis. Phagocytosis (cell-eating) is the endocytosis of solid particles. Pinocytosis (cell-drinking) is the endocytosis of fluid droplets. Finally there is the receptor mediated endocytosis
Receptor-mediated endocytosis is when coated pits form vesicles when specific ligands bind to receptors on the cell surface. It is more discriminating than pinocytosis. It enables cells to acquire bulk quantities of specific substances even if they are in low concentration in the extra-cellular fluid. Membrane embedded proteins with specific receptor sites exposed to the cell’s exterior, cluster in regions called coated pits. A layer of clathrin lines and reinforces the coated pit on the cytoplasmic side. A molecule that binds to a specific receptor site of another molecule is called a ligand.
6. Water Relations
6.1 Osmosis
It is the net movement of solvent molecules from a region where it is highly concentrated (dilute solution) to a region where its concentration is lower (more concentrated solution), through a selectively permeable membrane.
There are effects of solute on water mobility. Solute molecules can reduce the proportion of water molecules that can freely diffuse. Some water molecules form a hydration shell around hydrophilic solute molecules. The bound water cannot freely diffuse across a membrane. In most biological fluids, it is the difference in the proportion of unbound water that causes osmosis, rather than the actual difference in water concentration.
The direction of osmosis is determined by the difference in total solute concentration, regardless of the type or diversity of solutes in the solution. If 2 isotonic solutions are separated by a selectively permeable membrane, water molecules diffuse across the membrane in both directions at an equal rate thus there is no met movement of water. Even though there is no net movement of water across the membrane, the water molecules do not stop moving. At equilibrium, the water molecules move in both directions at the same rate.
6.2 Water Potential
Water potential is a fundamental term derived from thermodynamics. Water molecules possess kinetic energy, which means in liquid or gaseous form they move about rapidly and randomly from one location to another. The greater the concentration of water molecules in a system, the greater the total kinetic energy of water molecules in the system and the higher is its water potential.
Water potential is a measure of the pressure with which water can move from one region (higher water potential i.e. less negative) to another (lower water potential i.e. more negative). That is to say, the tendency of a cell to lose or absorb water from its environment. It is a measure of the kinetic energy of its water molecules, compared with that of pure water.
Pure water has a water potential of zero, which is the highest. Any solution will have a lower water potential than pure water. Its value will be negative. The more concentrated the solution, the more negative will be its water potential. Overall, it is the measure of the sum of the difference between the solute potential of the vacuolar sap and the pressure potential of the cell.
There are factors affecting water potential of a cell. They are (1) the water content of a cell, (2) the solute content of a cell, (3) the solute potential inside and outside of the cell, (4) The pressure potential or turgor, (5) the external pressure and finally (6) temperature.
6.3 Osmotic / Solute Potential
The effect of dissolving solute molecules in pure water is to reduce the concentration of water molecules and hence to lower the water potential. All solutions therefore have more negative solute potentials than pure water. The amount of this lowering is known as the solute potential.
Solute potential is a measure of the ability of a solute to lower the water potential (makes it more negative). It is, in other words, a measure of the change in water potential of a system due to the presence of solute molecules. It is equal to the osmotic potential and is always negative. The more solute molecules present, the lower (more negative) is the solute potential.
Solute potential is a function of the number of particles. So, 1 M of NaCl (ionized) will have a more negative solute potential than 1 M of sucrose.
6.4 Pressure Potential
If pressure is applied to pure water or a solution, its water potential increases. This is because the pressure is tending to force the water from one place to another. Such a situation may occur in living cells. When water enters plant cells by osmosis, pressure may build up inside the cell, making the cell turgid and increasing the pressure potential.
Pressure potential is the measure of the pressure exerted by the cell wall on the cell’s contents. Pressure potential increases as the cell absorbs water and increases its volume. It has a positive value and tends to force water out of the cell as compared to solute potential which tends to move water into the cell, and thus has a negative (opposite) value.
7. Effects of Solutions of Different Water Potential
7.1 Tonicity of Solutions
Tonicity describes how the size of a cell will change if it were placed in a solution. The volume of a cell at equilibrium determines tonicity. There are 3 ways in which cells avoid swelling. The animal cell keeps the intra-cellular solute concentration low by pumping out ions. The plant cell is saved from swelling and bursting by its tough cell wall. The protozoan avoids swelling by periodically ejecting the water the moves into the cell.
In animal cells, they prevent excessive loss or uptake of water bi either (1) living in an isotonic environment or (2) osmoregulating in a hypo-or-hypertonic environment. They regulate water balance by removing water in a hypotonic environment.
The plant cell wall is usually freely permeable to substances in solution, so it is not important in osmosis. The cell contains a large central vacuole whose contents, the cell sap, contribute to the solute potential of the cell. The 2 important membranes are the cell surface membrane and the tonoplast.
7.2 Plasmolysis
If a plant cell is in contact with a solution of lower water potential than its own contents, the water leaves the cell by osmosis through the cell surface membrane. Water is lost from the cytoplasm and then from the vacuole through the tonoplast. The protoplast (living contents of the cell surrounded by the cell wall) shrinks and eventually pulls away from the cell wall. The cell is said to be plasmolysed. The point at which plasmolysis is just about to happen is called incipient plasmolysis. At this point, the protoplast has just ceased to exert any pressure against the cell wall, so the cell is flaccid. Water will continue to leave the protoplast until its contents have the same water potential as the external solution. After that, no shrinkage occurs.
7.3 Turgidity V.S. Flaccidity
Turgidity occurs in hypotonic environment. Water enters the cell by osmosis. The cell swells. However, the elastic wall will expand so much before it exerts a back pressure on the cell that prevents further uptake of water, thus the cell is firm.
Flaccidity occurs in plant cell where its surroundings are isotonic. There is no tendency for water to enter the cell. The cell is limp.
