5 Chapter 5
Learning Objectives
- Identify major structural components of the cell membrane
- Identify three types of passive transport
- Identify the characteristics of active transport
Structure and Function of Plasma Membranes
A cell’s plasma membrane defines the cell, outlines its borders, and determines the nature of its interaction with its environment. Cells exclude some substances, take in others, and excrete still others, all in controlled quantities. The plasma membrane must be very flexible. In addition, the plasma membrane’s surface carries markers that allow cells to recognize one another, which is vital for tissue and organ formation during early development and later plays a role in the immune response.
Each phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains.
Among the most sophisticated plasma membrane functions is the ability for complex, integral proteins, receptors to transmit signals. These proteins act both as extracellular input receivers and as intracellular processing activators. These membrane receptors provide external attachment sites for hormones and growth factors and activate internal response cascades when bound. Occasionally, viruses hijack receptors and use them to gain entry into cells.
Two layers of phospholipids compose the plasma membrane, which is a phospholipid bilayer. Proteins are indicated as blue structures embedded in the membrane to control the passage of molecules through the phospholipid bilayer. Cell membranes are modeled with a fluid mosaic model. Fluidity of cells is due to the molecular structure of the phospholipid bilayer, cytoskeleton, and extracellular matrix. Structure determines function.
Membrane proteins can be either integral or peripheral. Most integral proteins are transmembrane proteins, spanning the cell membrane from interior to exterior. Transmembrane proteins are sometimes specialized to be transport proteins. These allow facilitated diffusion of polar, charged, and large molecules. Aquaporins are channel proteins that allow water to pass the membrane at a very high rate to maintain homeostasis. Another transmembrane protein allows sodium and potassium ions to pass. Their specific shape allows particular molecules to pass the membrane.
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Component
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Location
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Phospholipid
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Main fabric of the membrane
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Cholesterol
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Attached between phospholipids and between the two phospholipid layers
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Integral proteins (for example, integrins)
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Embedded within the phospholipid layer(s) may or may not penetrate through both layers
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Peripheral proteins
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On the inner or outer surface of the phospholipid bilayer, not embedded within the phospholipids
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Carbohydrates (components of glycoproteins and glycolipids)
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Generally attached to proteins on the outside membrane layer
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Virus binding to the CD4 receptor, a glycoprotein on T-cell surface. (credit: modification of work by NIH, NIAID)
Each individual’s immune system uses glycoproteins to recognize foreign cells. Glycoproteins on transplanted organs can trigger rejection immune reactions in recipients. Potential rejection issues must be managed for life following an organ transplant.
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. This means membranes are selectively permeable, because some substances pass through but not all.
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. In passive transport, substances move from an area of higher concentration to an area of lower concentration. A physical space in which there is a single substance concentration range has a concentration gradient. Diffusion is a passive process of transport. A single substance moves from a high concentration to a low concentration area until the concentration is equal across a space. You are familiar with diffusion of substances through the air. For example, think about someone opening a bottle of ammonia in a room filled with people. The ammonia gas is at its highest concentration in the bottle. Its lowest concentration is at the room’s edges. The ammonia vapor will diffuse, or spread away, from the bottle, and gradually, increasingly more people will smell the ammonia as it spreads. Simple diffusion allows non-polar molecules, hydrocarbons, carbon dioxide (CO2), oxygen gas (O2), and small, uncharged molecules to pass between lipids.
In facilitated transport, or facilitated diffusion, materials diffuse across the plasma membrane with the help of membrane proteins. A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy. However, these materials are polar molecule ions that the hydrophobic cell membrane portion repels. Facilitated transport proteins allows polar molecules to diffuse into the cell.
Osmosis is the movement of free water molecules through a semipermeable membrane according to the water’s concentration gradient across the membrane, which is inversely proportional to the solutes’ concentration. While diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the solutes’ diffusion in the water. Aquaporins play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules.
Three different scenarios involving red blood cells (RBC) are shown. Left: A RBC placed in a hypotonic solution, where the concentration of solutes in the surrounding fluid is lower than those in the cell, will cause water to rush into the RBC and lead to lysis of the cell. Middle: there is no net water movement into or out of the cell as the concentration of the solutes inside the cell equal or is isotonic to that of the surrounding fluid. Right: a RBC placed in a hypertonic solution, where the concentration of solutes in the surrounding fluid is greater than that in the cell will cause water to rush out of the cell and into the surrounding fluid. This will cause the RBC to shrivel. Credit: Tag, A., Rao, A., Hawkins, A and Fletcher, S. Department of Biology, Texas A&M University.
1. Hypotonic solution has a lower concentration of molecules, while the cell has a higher concentration of molecules. In this condition, water moves into the cell to the point of bursting the cell.
2. Isotonic solution has balanced condition, resulting in no net movement of water into or out of the cell. Some movement of water occurs, but balance is maintained.
3. Hypertonic solution has a higher concentration of molecules, while the cell has a lower concentration of molecules. In this condition, water moves out of cell into solution. The cell shrinks and shrivels.
Active transport mechanisms require energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell from an area of low concentration to an area of high concentration—the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane. One great example is the sodium-potassium pump. Other mechanisms transport much larger molecules.
To move substances against a concentration or electrochemical gradient, the cell must use energy. This energy comes from ATP generated through the cell’s metabolism. Active transport mechanisms, or pumps, work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances that living cells require in the face of these passive movements. A cell may spend much of its metabolic energy supply maintaining these processes.
In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles in bulk transport. Some cells are even capable of engulfing entire unicellular microorganisms. For a cell to take up and release large particles, energy is required. A large particle, however, cannot pass through the membrane, even with energy that the cell supplies. Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different endocytosis variations, but all share a common characteristic: the cell’s plasma membrane forms a pocket around the target particle. The pocket pinches off, resulting in the particle containing itself in a newly created intracellular vesicle formed from the plasma membrane.
Phagocytosis is the process by which a cell takes in large particles, like other cells or relatively large particles. For example, when microorganisms invade the human body, a type of white blood cell will remove the invaders through this process. The microbes will be surrounded, engulfed and destroyed.
In phagocytosis, the cell membrane surrounds the particle and engulfs it. (credit: modification of work by Mariana Ruiz Villareal)
Exercises
Key Takeaways
- Cell membranes are primarily composed of a phospholipid bilayer embedded with a variety of proteins.
- Passive transport moves molecules from high concentration to low. Simple diffusion involves free movement across the bilayer, facilitated diffusion requires a protein that spans the membrane, and osmosis involves the diffusion of water.
- Active transport requires both energy and a membrane protein, and it is capable of moving particles from low concentration to high.
