Passive Cell Mechanisms

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Chapter: Anatomy and Physiology for Health Professionals: Levels of Organization : Cells

Passive cell mechanisms include diffusion, osmosis, and filtration.


Passive Cell Mechanisms

 

Movements Through Cell Membranes

The cell membrane controls the substances that can enter and leave the cell. It does this by using passive and active mechanisms. Passive mechanisms do not require cellular energy, whereas active mechanisms do.


Passive Cell Mechanisms

Passive cell mechanisms include diffusion, osmosis, and filtration.


Diffusion

Diffusion (also known assimple diffusion) is theprocess by which substances spontaneously move from regions of higher concentration to regions of lower concentration (the concentration gradient). Molecules and ions in various substances move very quickly, colliding with many other types of particles. These collisions occur at the rate of a million times per second. The speed of diffusion is influenced by kinetic energy, molecular size, and temperature. Once parti-cles have diffused to be evenly distributed throughout a substance such as water, they have achieved a stateof equilibrium. Examples of diffusion include ion movement across cell membranes and neurotransmit-ter movement between nerve cells.

Cells allow substances to diffuse into or out of them only if the cell membrane is permeable to the substance and if the concentration of a substance is higher on one side of the membrane than the other (FIGURE 3-13). A molecule or ion will diffuse through the cell membrane if it is lipid soluble, assisted by a carrier molecule, or small enough to pass through membrane channels.


Simple diffusion is further defined as unas-sisted diffusion of very small or lipid-soluble particles. Nonpolar and lipid-soluble substances are diffused through the lipid bilayer. These substances include carbon dioxide, fat-soluble vitamins, and oxygen. Oxygen continuously diffuses from the blood into the cells because its concentration is always higher in the blood than in tissue cells. Oppositely, because carbon dioxide is in higher concentration within the cells, it diffuses from tissue cells into the blood.

Some substances cannot pass through the lipid bilayer of a cell membrane, requiring proteins in the membrane to assist them. This process is known as facilitated diffusion, also known asassisted diffu-sion. It is similar to simple diffusion because it onlymoves molecules from areas of higher concentration toward areas of lower concentration. Substances that require facilitated diffusion include certain amino acids, ions, and molecules such as glucose and other sugars. Facilitated diffusion is a passive transport pro-cess. Transported substances either bind to protein carriers in the membrane (and then move across it) or move through water-filled protein channels.

Therefore, the two types of facilitated diffusion are called carrier mediated and channel mediated. Carriers are proteins that are described astransmem-brane integral. They are specific for the transport­of certain polar molecules or types of molecules such as sugars and amino acids (which cannot pass through membrane channels because of their size).

Therefore, the carrier alters its shape to envelop and later release the transported substance. The carrier shields the substance as it is moved from the non-polar membrane regions. Basically, these carrier changes move the binding site from one location on the membrane to the other.

Just as in simple diffusion, substances transported by carrier-mediated facilitated diffusion move down their concentration gradients. For example, glu-cose is usually in higher concentrations in the blood than in the cells. Therefore, its transport is usually unidirectional­—into the cells. Carrier-mediated trans-port is limited by how many protein carriers arepres-ent. When all glucose carriers are engaged (saturated), glucose transport occurs at its fastest rate.

Channels are transmembrane proteins that moveions, water, and other substances through aqueous channels from one side of a membrane to the other. Because of pore size and amino acid charges in the lining of the channel, they act selectively. Gatedchannels are opened or closed by chemical or electri-cal signals. Leakage channels are always open. They allow water or ions to move through based on con-centration gradients. Similar to carriers, channels can also be inhibited by some molecules, be specific, and show saturation. The concentration gradient is also followed in channels. As substances cross the membrane by simple diffusion, the diffusion rate is not controlled because the lipid solubility of the membrane is not immediately changeable. However, the facilitated diffusion rate is controllable, because membrane permeability may be altered by regu-lating the number or activity of individual channels (or carriers) . Membranes may be freely permeable, selectively permeable, or impermeable. Plasmamembranes have selective permeability because of the size, molecular shape, electrical charge, or lipid solubility of materials as well as other factors. Perme-ability differs because of the lipids and proteins that are present in the plasma membrane and how they are arranged.


Osmosis

Osmosis is a special type of diffusion that occurs whenwater molecules diffuse from an area of higher water concentration to an area of lower water concentration. This requires a selectively permeable membrane such as a cell membrane. Solutions containing higher con-centrations of solutes have lower concentrations of water and vice versa. The ability of osmosis to create enough pressure to raise a volume of water is called osmotic pressure. Water always diffuses towardsolutions of greater osmotic pressure. Via osmosis, water equilibrates throughout the body, so the con-centration of water and solutes in both intracellular and extracellular fluids is nearly the same. Surpris-ingly, although highly polar, water passes via osmosis through the lipid bilayer. This may occur because of random movements of membrane lipids, which open small gaps in the membrane that allow water to move through. Osmosis is very important in the determina-tion of water distribution in the cells, blood, and other fluid-containing body compartments. It basically con-tinues until osmotic and hydrostatic pressures acting upon the membrane are equal.

Aquaporins are transmembrane proteins thatconstruct water- specific channels that allow water to move freely and reversibly and water molecules to be diffused in a single file manner. Although believed to exist in all cell types, they are most prevalent in red blood cells and cells involved in water balance (kid-ney tubule cells, etc.). Whenever water concentration differs on opposite sides of a membrane, osmosis occurs. If the solute concentration differs on either side of a membrane, water concentration also differs. When solute concentration increases, water concentra-tion decreases.

The number (not the type) of solute particles determines the extent to which solutes decrease water concentration. This is because one water molecule is (basically) displaced by one molecule or one ion of solute. Osmolarity is defined as the total concentra-tion of all solute particles in a solution. Net diffusion of both solute and water occurs, moving down their con-centration gradients, when the same volumes of aque-ous solutions of different osmolarity are separated by a membrane that is permeable to all molecules in the system. When the water and solute concentration on both sides of the membrane is the same, equilibrium is reached. Osmolarity is based only on a solution’s total solute concentration. It is expressed as osmoles per liter (osmol/L). One osmol is equal to 1 mole of nonioniz-ing molecules.

If a membrane is impermeable to solute parti-cles, water diffuses quickly from the left to the right compartment. This continues until the concentration is the same on both sides of the membrane. In this example, equilibrium results only from the movement of water because the solutes are prevented from mov-ing. The movement of water causes dramatic changes in the volumes of both compartments. This is similar to osmosis across plasma membranes of living cells. In a living plant cell, different from the previous example, the rigid cell wall outside the plasma membrane will eventually reach a point where water that is diffusing in will cause its hydrostatic pressure to equal its osmotic pressure. There will then be no further netwater entry. In general, the higher the amount of non-penetrating (nondiffusible) solutes in a cell, the higherthe osmotic pressure. Also, this means that a greater hydrostatic pressure must occur to resist additional net water entry. The hydrostatic pressure pushes water out, whereas the osmotic pressure pulls water in.

In living animal cells, these major hydrostatic ver-sus osmotic changes do not occur because cell walls are not as rigid. When an osmotic imbalance causes an animal cell to swell or shrink, one of two things occurs: either the solute concentration will be the same on both sides of the plasma membrane or the membrane will stretch until it breaks.

Tonicity refers to a solution’s ability to changethe shape or tone of cells by altering their internal water volume. This is not the same as osmolarity, because tonicity is based on how a solution affects cell volume. This is based on two factors: the solute concentration and the solute permeability of the plasma membrane.

Any solution with the same osmotic pressure as body fluids is called isotonic. The concentrations of nonpenetrating solutes are the same as those found in the cells (5% glucose or 0.9% saline). When a cell is exposed to an isotonic solution, it retains its nor-mal shape with no net gain or loss of water. The body’s extracellular fluids and most intravenous solutions are isotonic.

Any solution with a higher osmotic pressure than body fluids is called hypertonic. This type of solution has a higher concentration of nonpenetrating solutes than in the cells. Cells that receive hypertonic solutions lose water and crenate (shrink). A strong saline solu-tion is an example of a hypertonic solution. Likewise, any solution with a lower osmotic pressure than body fluids is called hypotonic (FIGURE 3- 14). A hypotonic solution is more dilute with a lower concentration of nonpenetrating solutes than cells. A cell receiving ahypotonic solution swells quickly. The most extreme example of a hypotonic solution is distilled water, which contains zero solutes. It causes cells to eventu-ally lyse (burst).


Hypertonic solutions are commonly given intra-venously to patients who are swollen (edematous) because water is retained in their tissues. These solu-tions cause the excess water to be drawn out of the extracellular space. It is then moved into the blood to be eliminated by the kidneys. Hypotonic solutions rehydrate tissues that have become dehydrated. When dehydration is mild, the patient is usually given hypo-tonic fluids such as apple juice or a “sports drink,” and rehydration usually results.


Filtration

Filtration is a passive cell mechanism that forces mole-cules through membranes. It is further defined as any mechanical, physical or biological operation that sep-arates solids from fluids. The fluid that passes through is called the filtrate.


1. Differentiate between osmosis and diffusion.

2. Compare simple diffusion with facilitated diffusion.

3. What is tonicity?

4. Compare hypertonic and hypotonic solutions.

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