Plasma membranes are selectively permeable. Selectively permeable means that only what the cell allows in or out will pass through the plasma membrane. This post will describe the circumstances under which certain substances will be allowed into or out of a cell and the mechanisms by which entry or exit occurs.
Gradients occur when the concentration of a solute is higher on one side of a membrane than it is on the other. In the picture above, there are gradients of Na+ and Cl-. Gradients can be either electrical or chemical. In an electrical gradient, the net charge on one side of the membrane may be positive while the other side is negative. In a chemical gradient, there may be more Cl- on one side of the membrane than there is on the other. The gradient may also have both electrical and chemical properties, which is called an ion electrochemical gradient. The illustration above is an example of this duality.
Gradients always exist in our cells; our cells work hard to maintain their gradients, which are a constant state of disequilibrium. For example, even though our bodies are composed of large amounts of water, allowing too much water into our cells would cause them to burst. Our cells maintain a gradient so that most of the water stays outside the cell and only what the cell needs is allowed inside the cell. Many other substances are kept at disequilibrium so as to maintain a beneficial inner environment of the cell.
Tonicity is a way to describe the relative amounts of solutes on either side of a membrane. Tonicity is always a relative term; we cannot use it unless we make clear what is relative to our descriptor. There are three ways we can describe the tonicity of a solute:
- Isotonic- both sides of the membrane have the same amount of the substance.
- Hypertonic- the hypertonic side of the membrane has more solute than the other side.
- Hypotonic- the hypotonic side of the membrane has less solute than the other side.
After reading the definitions, do you see why tonicity is a relative term? We cannot simply say, “That is a hyportonic solution.” The statement doesn’t make sense. We would be more accurate to say, “The Na+ concentration outside the cell is hypotonic to the inside of the cell.” By this statement, we can also say, “The Na+ concentration inside the cell is hypertonic to the outside of the cell.” Whenever you discuss tonicity, be sure it is clear what your relative terms are so you don’t miscommunicate what you think you are saying!
Before moving on, we need to discuss just two more terms: diffusion and osmosis.
- Diffusion refers to the movement of a SOLUTE across a plasma membrane. Solutes move from the area of higher concentration to the area of lower concentration (from the hypertonic compartment to the hypotonic compartment) until an isotonic solution is reached.
- Osmosis refers to the movement of WATER across a plasma membrane. Water does the opposite of what a solute does in diffusion: water will move from the area of lower solute concentration to the area of higher solute concentration (from the hypotonic compartment to the hypertonic compartment.) Remember that water is the solvent, and is interacting with the solute, or surrounding it. Some of the molecules of water are surrounding the solute, and are not free- they are busy with the solute. In the hypotonic compartment, less of the water is busy with the solute; the water molecules that are not busy are called “free water”. There is more free water on the hypotonic side than there is on the hypertonic side; this is why free water seems to move in a counterintuitive direction. When you think about it, it’s not so counterintuitive after all.
3 Main Ways to Move Solutes Across Membranes
There are three primary ways by which solutes move across membranes. These are: passive diffusion, facilitated diffusion, and active transport. Each has its own mechanisms and energy requirements.
1) Passive Diffusion- does not require energy, with a gradient
Passive diffusion happens when a solute moves across a membrane without the aid of a transport protein. Whether or not a solute will be able to pass through the plasma membrane via passive diffusion is determined by the chemical properties of a solute.
In general:
Highly permeable: small, uncharged molecules and gases. For example, CO2, N2, O2, ethanol. Even though water is polar, it is sufficiently small to be moderately permeable.
Not-so-permeable: large polar molecules, ions, macromolecules. For example, amino acids, ATP, proteins, polysaccharides, DNA, RNA, Na+.
2) Facilitated Diffusion- does not require energy, with a gradient
Molecules that cannot pass through a membrane via passive diffusion may make it through the membrane with a little help from a protein. This process is called facilitated diffusion. Facilitated diffusion may occur through a channel or a transporter. Both channels and transporters are transmembrane proteins that are specially equipped to move solutes across a membrane.
Channels: Channels allow a solute to go right through the membrane. Aquaporin is a type of channel used for the facilitated diffusion of water across the membrane. Most channels can open or close under certain circumstances to allow or disallow diffusion. Many things may cause a channel to open or close, depending on the cell type. A channel that can open and close is called “gated”. Most channels are gated.
Three general categories of things that can cause a channel to open or close are neurotransmitters, electrical currents, or mechanical force.
- For example, ligand-gated channels are controlled by hormones or neurotransmitters.
- Regulatory proteins may bind to channels to control them.
- Phosphorylation, or the binding of a phosphorous molecule, may cause the gate to open or close.
- Voltage-gated channels respond to changes in electrical charge on the membrane.
- Mechanosensitive channels are sensitive to membrane tension, such as cells in our inner ear that are sensitive to different frequencies of sound, and these ion channels open in response to sound and transmits signals to the brain identifying the frequency.
Transporters: Transporters are also known as carriers. When a solute enters a transporter, the transporter changes shape to move the solute to the other side of the cell. Transporters play key roles in the uptake of organic molecules, hormones, and neurotransmitters, and also play a role in waste export. For example, transporters remove lactic acid, a byproduct of muscle cells being exercised, before the lactic acid buildup becomes toxic.
There are three categories of transporters:
- Uniporter- move one solute in one direction
- Symporter- move two solutes in one direction
- Antiporter- move two solutes in two directions
3) Active Transport- requires energy, against a gradient
Active transport moves a solute across a membrane against its gradient, or from an area of low concentration to an area of high concentration. The transport is accomplished via the use of protein pumps.
Pumps: Pumps are a type of transporter that must use energy to function. Pumps can also be classified by uniporter, symporter, or antiporter.
The most well-studied type of pump is the sodium/potassium pump. Watch the short video below for a visual description of how pumps work and what it looks like as compared to channels and other transporters.
Endocytosis/Exocytosis
Eukaryotic cells have two more means by which materials can enter or exit the cell.
Endocytosis is a process by which materials such as nutrients or bacteria are brought into a cell. In endocytosis, the plasma membrane pinches inward to form a vesicle that separates from the membrane and brings the material inside. Endocytosis serves many functions, For example, lipids and iron bind to proteins outside the cell and are brought inside to be used. Plants enclose bacteria in the membrane, which forms root nodules. Macrophages are cells in the immune system that engulf bacteria and destroy it.
Exocytosis is a process by which the cell packages materials into a vesicle which fuses with the plasma membrane and releases the cargo outside the cell. Exocytosis also serves many functions. Insulin is secreted into the bloodstream via exocytosis. Digestive enzymes are secreted from the pancreas into the small intestine, and many components of the extracellular matrix are also processed via exocytosis.
View the short video below to see visual representations of endocytosis and exocytosis.
Need more help studying these topics? Try the below resources for animations and quizzes.
Sodium-Potassium Exchange Pump 1
Sodium-Potassium Exchange Pump 2
Endocytosis & Exocytosis
Cotransport (Symport & Antiport)
How Facilitated Diffusion Works
Diffusion Through Cell Membranes
Osmosis
Mechanisms of Actions of Lipid-Soluble Messengers
Primary Active Transport
Secondary Active Transport
Proton Pump
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