Why do membranes have selective permeability

Related questions How is the cell membrane affected by temperature? How does the cell membrane change shape? How does a cell membrane affect water movement? Why are cell membranes selectively permeable?

How does a cell membrane become polarized? How does the lipid bilayer form a barrier to molecules? How does cholesterol affect lipid bilayer? How do lipid bilayers form? In other words, plasma membranes are selectively permeable —they allow some substances through but not others Figure 1.

If the membrane were to lose this selectivity, the cell would no longer be able to maintain homeostasis, or to sustain itself, and it would be destroyed. Some cells require larger amounts of specific substances than other cells; they must have a way of obtaining these materials from the extracellular fluids. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that ensure transport. Most cells expend most of their energy, in the form of adenosine triphosphate ATP , to create and maintain an uneven distribution of ions on the opposite sides of their membranes.

The structure of the plasma membrane contributes to these functions. Plasma membranes are asymmetric, meaning that despite the mirror image formed by the phospholipids, the side of the membrane facing the inside of the cell is not identical to the exterior of the membrane. Proteins that act as channels or pumps work in one direction.

Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane. These carbohydrate complexes help the cell bind substances in the extracellular fluid that the cell needs.

If the osmolarity of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell, although water will still move in and out. Blood cells and plant cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances. Figure 9. Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions.

A doctor injects a patient with what the doctor thinks is an isotonic saline solution. The patient dies, and an autopsy reveals that many red blood cells have been destroyed.

Do you think the solution the doctor injected was really isotonic? For a video illustrating the process of diffusion in solutions, visit this site. In a hypotonic environment, water enters a cell, and the cell swells. In an isotonic condition, the relative concentrations of solute and solvent are equal on both sides of the membrane.

There is no net water movement; therefore, there is no change in the size of the cell. In a hypertonic solution, water leaves a cell and the cell shrinks. Remember, the membrane resembles a mosaic, with discrete spaces between the molecules composing it. If the cell swells, and the spaces between the lipids and proteins become too large, and the cell will break apart.

In contrast, when excessive amounts of water leave a red blood cell, the cell shrinks, or crenates. This has the effect of concentrating the solutes left in the cell, making the cytosol denser and interfering with diffusion within the cell.

Various living things have ways of controlling the effects of osmosis—a mechanism called osmoregulation. Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis in a hypotonic solution. The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse. In fact, the cytoplasm in plants is always slightly hypertonic to the cellular environment, and water will always enter a cell if water is available.

This inflow of water produces turgor pressure, which stiffens the cell walls of the plant. In nonwoody plants, turgor pressure supports the plant. Conversely, if the plant is not watered, the extracellular fluid will become hypertonic, causing water to leave the cell.

In this condition, the cell does not shrink because the cell wall is not flexible. However, the cell membrane detaches from the wall and constricts the cytoplasm. This is called plasmolysis. Plants lose turgor pressure in this condition and wilt. Figure The turgor pressure within a plant cell depends on the tonicity of the solution that it is bathed in.

Without adequate water, the plant on the left has lost turgor pressure, visible in its wilting; the turgor pressure is restored by watering it right. Vicente Selvas. Tonicity is a concern for all living things. For example, paramecia and amoebas, which are protists that lack cell walls, have contractile vacuoles. This vesicle collects excess water from the cell and pumps it out, keeping the cell from bursting as it takes on water from its environment.

Many marine invertebrates have internal salt levels matched to their environments, making them isotonic with the water in which they live. Fish, however, must spend approximately five percent of their metabolic energy maintaining osmotic homeostasis. Freshwater fish live in an environment that is hypotonic to their cells. These fish actively take in salt through their gills and excrete diluted urine to rid themselves of excess water.

Saltwater fish live in the reverse environment, which is hypertonic to their cells, and they secrete salt through their gills and excrete highly concentrated urine. In vertebrates, the kidneys regulate the amount of water in the body. Osmoreceptors are specialized cells in the brain that monitor the concentration of solutes in the blood.

If the levels of solutes increase beyond a certain range, a hormone is released that retards water loss through the kidney and dilutes the blood to safer levels.

Animals also have high concentrations of albumin, which is produced by the liver, in their blood. This protein is too large to pass easily through plasma membranes and is a major factor in controlling the osmotic pressures applied to tissues.

Transport across the membrane Design challenge problem and subproblems General Problem: The cell membrane must simultaneously act as a barrier between "IN" and "OUT" and control specifically which substances enter and leave the cell and how quickly and efficiently they do so. Energy story perspective Transport across a membrane can be considered from an energy story perspective; it is a process after all.

Selective permeability One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. Relative permeability The fact that different substances might cross a biological membrane at different rates should be relatively intuitive. Membrane permeability coefficients Below, a variety of compounds are plotted with respect to their membrane permeability coefficients MPC as measured against a simple biochemical approximation of a real biological membrane.

Figure 1. Membrane permeability coefficient diagram. Energetics of transport All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not the transport process is exergonic or endergonic. Passive transport Passive transport does not require the cell to expend energy. Diffusion Diffusion is a passive process of transport.

Figure 2. Diffusion through a permeable membrane moves a substance from an area of high concentration extracellular fluid, in this case down its concentration gradient into the cytoplasm. Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient, independent of the concentration gradients of other materials. In addition, each substance will diffuse according to that gradient. Within a system, there will be different rates of diffusion of the different substances in the medium.

Factors that affect diffusion If unconstrained, molecules will move through and explore space randomly at a rate that depends on their size, their shape, their environment, and their thermal energy. Factors influencing diffusion include: Extent of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes.

Shape, size and mass of the molecules diffusing: Large and heavier molecules move more slowly; therefore, they diffuse more slowly. The reverse is typically true for smaller, lighter molecules. Temperature: Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion.

Lower temperatures decrease the energy of the molecules, thus decreasing the rate of diffusion. Solvent density: As the density of a solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium. If the medium is less dense, rates of diffusion increase. Solubility: As discussed earlier, nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion.

Surface area and thickness of the plasma membrane: Increased surface area increases the rate of diffusion, whereas a thicker membrane reduces it. Distance traveled: The greater the distance that a substance must travel, the slower the rate of diffusion.

This places an upper limitation on cell size. A large, spherical cell will die because nutrients or waste cannot reach or leave the center of the cell, respectively. Therefore, cells must either be small in size, as in the case of many prokaryotes, or be flattened, as with many single-celled eukaryotes.

Facilitated transport In facilitated transport , also called facilitated diffusion, materials diffuse across the plasma membrane with the help of membrane proteins.

Note: possible discussion Compare and contrast passive diffusion and facilitated diffusion. Channels The integral proteins involved in facilitated transport are collectively referred to as transport proteins , and they function as either channels for the material or carriers. Figure 3. Facilitated transport moves substances down their concentration gradients. They may cross the plasma membrane with the aid of channel proteins.

Carrier proteins Another type of protein embedded in the plasma membrane is a carrier protein. Moving against a gradient To move substances against a concentration or electrochemical gradient, the cell must use energy. Carrier proteins for active transport An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement: there are three types of these proteins or transporters.

Figure 5. A uniporter carries one molecule or ion. A symporter carries two different molecules or ions, both in the same direction. An antiporter also carries two different molecules or ions, but in different directions.

Primary active transport In primary active transport, the energy is often - though not exclusively - derived directly from the hydrolysis of ATP. Figure 6.