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- Plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving.
- In other words, plasma membranes are selectively permeable—they allow some substances to pass through, but not others.
- If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed.
- The most direct forms of membrane transport are passive.
- Passive transport is a naturally occurring phenomenon and does not require the cell to exert any of its energy to accomplish the movement.
- It is a process by which an ion or molecule passes through a cell via a concentration gradient, or from an area of high concentration to an area of low concentration without the expenditure of energy.
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Features of Passive Transport
- Passive transport requires no energy input, as compounds are able to move freely across the membrane-based only on a favorable concentration gradient.
- Unlike active transport, it does not require an input of cellular energy because it is instead driven by the tendency of the system to grow in entropy.
- The rate of passive transport depends on the permeability of the cell membrane, which, in turn, depends on the organization and characteristics of the membrane lipids and proteins.
- Passive transport occurs by simple diffusion or via pores in the plasma membrane.
- The passive forms of transport, diffusion, and osmosis move material of small molecular weight.
- Passive transport is independent of membrane proteins and the catabolism of biological molecules for energy.
Types of Passive Transport
The four main kinds of passive transport are simple diffusion, facilitated diffusion, filtration, and/or osmosis.
- Diffusion is a passive process of transport.
- A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across space.
- Materials move within the cell’s cytosol by diffusion, and certain materials move through the plasma membrane by diffusion.
- Diffusion expends no energy. Rather the different concentrations of materials in different areas are a form of potential energy, and diffusion is the dissipation of that potential energy as materials move down their concentration gradients, from high to low.
- Several factors affect the rate of diffusion.
- The 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.
- Mass of the molecules diffusing: More massive molecules move more slowly because it is more difficult for them to move between the molecules of the substance they are moving through; therefore, they diffuse more slowly.
- Temperature: Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion.
- Solvent density: As the density of the solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium.
- In facilitated transport, also called facilitated diffusion, material moves across the plasma membrane with the assistance of transmembrane proteins down a concentration gradient (from high to low concentration) without the expenditure of cellular energy.
- However, the substances that undergo facilitated transport would otherwise not diffuse easily or quickly across the plasma membrane.
- The solution to moving polar substances and other substances across the plasma membrane rests in the proteins that span its surface.
- The material being transported is first attached to protein or glycoprotein receptors on the exterior surface of the plasma membrane. This allows the material that is needed by the cell to be removed from the extracellular fluid.
- The substances are then passed to specific integral proteins that facilitate their passage because they form channels or pores that allow certain substances to pass through the membrane.
- The integral proteins involved in facilitated transport are collectively referred to as transport proteins, and they function as either channel for the material or carriers.
- Filtration is the movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system.
- Depending on the size of the membrane pores, only solutes of a certain size may pass through it.
- For example, the membrane pores of the Bowman’s capsule in the kidneys are very small, and only albumins, the smallest of the proteins, have any chance of being filtered through. On the other hand, the membrane pores of liver cells are extremely large allowing a variety of solutes to pass through and be metabolized.
- Osmosis is the diffusion of water through a semipermeable membrane according to the concentration gradient of water across the membrane.
- Whereas diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the diffusion of solutes in the water. Osmosis is thus a special case of diffusion.
- Water, like other substances, moves from an area of higher concentration to one of lower concentration.
- This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero. Osmosis proceeds constantly in living systems.
- There are three types of Osmosis solutions: the isotonic solution, hypotonic solution, and hypertonic solution.
- Isotonic solution is when the extracellular solute concentration is balanced with the concentration inside the cell. In the Isotonic solution, the water molecules still move between the solutions, but the rates are the same from both directions, thus the water movement is balanced between the inside of the cell as well as the outside of the cell.
- A hypotonic solution is when the solute concentration outside the cell is lower than the concentration inside the cell. In hypotonic solutions, the water moves into the cell, down its concentration gradient (from higher to lower water concentrations). That can cause the cell to swell.
- A hypertonic solution is when the solute concentration is higher than the concentration inside the cell. In a hypertonic solution, the water will move out, causing the cell to shrink.
Channels and Carrier Proteins for Passive Transport (Facilitated Transport Protein)
- Some molecules, such as carbon dioxide and oxygen, can diffuse across the plasma membrane directly, but others need help to cross its hydrophobic core.
- In facilitated diffusion, a form of passive transport, molecules diffuse across the plasma membrane with assistance from membrane proteins, such as channels and carriers.
- A concentration gradient exists for these molecules, so they have the potential to diffuse into (or out of) the cell by moving down it.
- However, because they are charged or polar, they can’t cross the phospholipid part of the membrane without help.
- Facilitated transport proteins shield these molecules from the hydrophobic core of the membrane, providing a route by which they can cross.
- Channel proteins span the membrane and make hydrophilic tunnels across it, allowing their target molecules to pass through by diffusion.
- Channels are very selective and will accept only one type of molecule (or a few closely related molecules) for transport.
- Passage through a channel protein allows polar and charged compounds to avoid the hydrophobic core of the plasma membrane, which would otherwise slow or block their entry into the cell.
- Some channel proteins are open all the time, but others are “gated,” meaning that the channel can open or close in response to a particular signal (like an electrical signal or the binding of a molecule).
Example. Aquaporins are channel proteins that allow water to cross the membrane very quickly, and they play important roles in plant cells, red blood cells, and certain parts of the kidney (where they minimize the amount of water lost as urine).
- Another class of transmembrane proteins involved in facilitated transport consists of the carrier proteins.
- Carrier proteins can change their shape to move a target molecule from one side of the membrane to the other.
- Like channel proteins, carrier proteins are typically selective for one or a few substances.
- Often, they will change shape in response to binding of their target molecule, with the shape change moving the molecule to the opposite side of the membrane.
- The carrier proteins involved in facilitated diffusion simply provide hydrophilic molecules with a way to move down an existing concentration gradient (rather than acting as pumps).
Significance of Passive Transport
Membrane transport is essential for cellular life. As cells proceed through their life cycle, a vast amount of exchange is necessary to maintain function. Transport may involve the incorporation of biological molecules and the discharge of waste products that are necessary for normal function.
- It commonly occurs in the blood-brain barrier as specific molecules, such as sodium thiopental, can diffuse across the membrane.
- Passive diffusion occurs across the placenta as all solute particles are exchanged between mother and fetus.
- The passive forms of transport, diffusion and osmosis, move materials of small molecular weight across membranes.
- Digested food molecules (amino acids, glucose) move down a concentration gradient from the intestine to the blood. Waste products such as carbon dioxide or urea travel by diffusion from body cells into the bloodstream.
- Oxygen moves from high concentration (in the air sac) to a lower concentration (in the blood). Carbon dioxide moves from high concentration (in the blood) to a lower concentration (in the air sac).
- The biological importance of osmosis is that it facilitates the distribution of essential nutrients in the body and the excretion of metabolic waste products. Cells have semipermeable membranes, and osmosis makes it possible for liquid solvents to pass through these cell membranes.
- In animals (including humans), renal filtration removes waste from the blood.
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell. New York: Garland Science.
- Koolman, J., & Röhm, K.-H. (2005). Color atlas of biochemistry. Stuttgart: Thieme.