Cholesterol stabilises the cell membrane.
The lipid bilayer enables the membrane to hold in the water-soluble contents of the cell and prevent them leaking out. This property also makes it difficult for water-soluble substances to pass in and out of cells. In general, substances pass in and out of cells by four main processes:
Diffusion: Is the net movement of molecules or ions from a region of their higher concentration to a region of their lower concentration. The difference in concentration of two regions is called the concentration gradient of diffusion gradient.
Diffusion is important because it is important for the cell to stay alive. Cells take up oxygen in order to live, and the respiration is always being used up in respiration, eh concentration inside the cell of oxygen is lower than in the blood and tissue fluids. This means that oxygen molecules diffuse from outside the cell to the inside. The same thing applies with carbon dioxide in the other direction.
The conversion of the diffused substance will help maintain a concentration gradient, therefore favour continued diffusion. For diffusion to take place, any membranes of partitions in the system must be readily permeated by the molecules of ions in question. The membrane is permeable to both oxygen and carbon dioxide.
The rate of diffusion is the quantity of substance which diffuses from one region to another in a certain time. It depends on the difference in the concentration of the substance in the two regions. The steeper the gradient, the faster the rate of diffusion. It also depends upon the surface area across which the substance is diffusing. The greater the surface area, the faster the rate of diffusion.
Finally it depends upon the distance over which the substance has to diffuse. The greater the diffusion distance, the slower the rate of diffusion.
The rate of diffusion is directly proportional to the concentration difference and surface area and inversely proportional to the diffusion distance:
Surface area x concentration difference
Rate of diffusion = Diffusion distance
This is known as Fick’s Law.
Charged ions do not readily pass through the plasma membrane because they are relatively insoluble in lipid. Within the plasma membrane certain proteins assist such particles to diffuse in or out of the cell. These are called channel proteins. The protein is arranged so that it forms a water-filled pore in the membrane with a hydrophilic lining so water-soluble substances and water can pass through it easily. Channel proteins are particularly concerned with transporting ions into and out of cells. The channels are selective, allowing certain ions to pass through but not others. Some channels can open and close like gates, these gated channels only open when they receive a signal such as a mechanical disturbance of the membrane, a change in the voltage or the binding of another molecule of ion with the protein. Channel proteins speed up the rate at which ions diffuse across the plasma membrane, this movement is like simple diffusion and it is passive and does not involve the transfer of metabolic energy which means it can only take place down a concentration gradient.
Facilitated diffusion: A diffusing molecule combines with a carrier protein which transports it across the membrane and deposits it on the other side. The diffusion occurs down a concentration gradient and no metabolic energy is required. This is the main way by which glucose and amino acids are taken up into cells. Because their molecules re polar, they cannot diffuse through the lipid bilayer and they are too large to pass through the channel proteins. Carrier proteins are likened to enzymes because the relationship between the protein and the transported molecule is specific and the mode of attachment is like that of a substrate and an enzyme. Carrier proteins are susceptible to poisons, and several different molecules may compete for the transport by the same carrier.
Osmosis: The membrane is permeable to water molecules but impermeable to larger molecules, this means that the membrane is partially permeable. Osmosis is the net movement of water molecules from a region of their higher concentration to a region of their lower concentration through a partially permeable membrane.
Water potential: the potential energy of the water molecules, where there are fewer water molecules, the water potential is lower than when there are more water molecules. The steeper the water potential gradient, the greater the tendency for water to diffuse in a determined direction.
The3 symbol used for the water potential and other energy potentials in cells is ψ. It is customary to express ψ in kilopascals or megapascals. When water flows down a water potential gradient, the net movement of water molecules is always from a less negative value to a more negative value.
Osmotic pressure: tendency of a solution to gain water. The greater the solute concentration, the greater the osmotic pressure. A solution with a high osmotic pressure has low water potential.
Hypotonic solution: If a cell is surrounded by pure water, or by a solution with a higher water potential than that of the cell’s contents, water flows into the cell and it swells up. The external solution is hypotonic to the solution in the cell.
Hypertonic solution: If the cell is surrounded by a solution with a high solute solution and a lower water potential than that of the cell’s contents, the water flows out of the cell and it shrinks. The external solution is said to be hypertonic to the solution in the cell.
Isotonic solution: If the cell has the same solute concentration and water potential as the surrounding solution, there will be no flow of water. The external solution is said to be isotonic with the solution in the cell.
When red blood cells are emerged in either a hypertonic or hypotonic solution, the effects are disastrous.
If a cell is to survive, it needs to be permanently in an isotonic solution or have the necessary mechanisms (osmoregulation) enabling it to survive in a hypertonic or hypotonic solution.
Plant cells have a lower water potential compared to that of their surroundings, due to the presence of various solutes in the fluid in the vacuole. Water flows through the plasma membrane and tonoplast into the vacuole by osmosis, so the cell swells but it does not burst because the cellulose wall stretches and develops tension and resists further expansion of the cell.
As water flows into the vacuole by osmosis, the tension caused by the cell wall causes an internal hydrostatic pressure to develop, called the pressure potential. The pressure potential reaches its maximum when the cell wall is stretched as much as it can be and the cell cannot take any more water. The cell is now described as fully turgid or full torpor is achieved.
When the cell is immerged in a solution with a lower water potential than that inside the vacuole. First, the size of the cell decreases as the water flows out of the vacuole and the cytoplasm pulls away from the cell wall leaving a gap between the wall and the plasma membrane. This withdrawal is called plasmolysis. The point when the cytoplasm just starts pulling away from the cell wall is called incipient plasmolysis. Full plasmolysis is reached when the cytoplasm has completely withdrawn from the cell wall. Plasmolysis rarely occurs in nature.
If there is plenty of water in the environment, a plant’s cells are usually surrounded by a watery solution with a lower solute concentration than that inside the cells; the cell walls will be saturated with such a solution. Water tends to enter the cells by osmosis, making them turgid.
Turgor is important in maintaining the shape and form of the plants and keeping the leaves flat and in an opened-out position, if turgor is reduced, the plant droops.
Self check 4:
In drawing q, the red blood cells have shrunk compared to drawing p and the outer membrane has wrinkled, this means that osmosis has occurred from inside the cell towards the outside. The surrounding solution has a lower salt concentration than that of the cells, therefore water has migrated form inside the cells to the outside via osmosis.
Active transport: an energy-requiring movement of molecules or ions against a concentration gradient. The energy for active transport comes form respiration which involves the synthesis of ATP. Cells which are known to engage in active transport have exceptionally large numbers of mitochondria.
Active transport requires carrier proteins coupled with a source of ATP. Ion gradients created by active transport can be used to provide energy for the transport of other ions and molecules.
Active transport allows cells to take up nutrients even when their concentration gradient outside the cell is very low. It also enables cells to remove unwanted substances when their concentration outside the cell is much greater. The carrier proteins only work in one direction. Indirectly, active transport is a way of transporting water across cell membranes, the pumping of chemicals creates a concentration gradient of solute across the plasma membrane and the water molecules follow passively by osmosis.
Sodium-potassium pump: Cells have high concentrations of potassium ions, but low concentrations of sodium ions; this is due to the sodium-potassium pump, which moves these two ions in opposite directions across the cell plasma membrane. The same carrier, ATPase transports both ions, pumping three sodium ions out of the cell for every two potassium ions pumped in.
The calcium pump: The concentration of calcium ions inside the cell is generally much higher than outside. This unequal distribution of calcium ions is maintained by a calcium pump, which actively expels calcium ions across the plasma membrane. If the calcium pump stops working, calcium cells diffuse rapidly into the cell down the steep concentration gradient. If the pump is momentarily stopped, it is a way of transmitting signals to the cells.
Exocytosis: A vesicle containing the material to be expelled moves towards the surface of the cell and fuses with the plasma membrane. The vesicle opens outside and the contents leave the cell. The vesicle membrane becomes part of the surface membrane.
Endocytosis is another active process by which substances are taken into a cell by infoldings in the surface membrane. An example is neutrophils. There are two different types of endocytosis, which differ in the size of the vesicles.
Phagocytosis involves relatively large particles being taken up into large vesicles. The plasma membrane invaginates to form a phagocytic vesicle which encloses the particles. The vesicle then fuses with a lysosome which digests the particles. This is called intracellular digestion. The soluble products of the digestion process are then absorbed into the surrounding cytoplasm. Any indigestible material may be removed by the vesicle moving to the surface of the cell and fusing with the plasma membrane.
Pinocytosis means cell drinking. Tiny channels pinch off small vesicles which pinch off even smaller ones. These pinocycotic vesicles mean that liquids can be brought into the cell and distributed within it.
