4.1 Plasma Membrane Structure and Function
CHAPTER OUTLINE
4.1 Plasma Membrane Structure and Function
The plasma membrane separates the internal environment of the cell from the external environment. It regulates the entrance and exit of molecules from the cell. The plasma membrane is a phospholipid bilayer in which protein molecules are either partially or wholly embedded. The combination of proteins, steroids, and phospholipids is called the fluid-mosaic model of membrane structure.
Functions of the Membrane Proteins
Integral proteins largely determine a membrane’s specific functions and can be of the following types:
Channel proteins
Channel proteins have a channel that allows a substance to simply move across the membrane.
Carrier proteins
Carrier proteins combine with a substance and help it move across the membrane.
Cell recognition proteins
Cell recognition proteins are glycoproteins that help the body recognize self, among other functions.
Receptor proteins
Receptor proteins have a shape that allows a specific molecule to bind to it and the binding causes the protein to change its shape and bring about a cellular response.
Enzymatic proteins
Enzymatic proteins carry out metabolic reactions directly.
4.2 The Permeability of the Plasma Membrane
The plasma membrane regulates the passage of molecules into and out of the cell by being selectively permeable, meaning that certain substances can move across the membrane while others cannot. Some molecules passively cross the membrane following their concentration gradient. Large molecules and some ions and charged molecules are unable to freely cross the membrane and must go through channel proteins, with the assistance of carrier proteins, or in vesicles.
Diffusion and Osmosis
Diffusion is the movement of molecules from a higher to a lower concentration until equilibrium is achieved and they are distributed equally. A solution contains both a solute, usually a solid, and a solvent, usually a liquid.
Osmosis
The diffusion of water across a selectively permeable membrane due to concentration differences is called osmosis. Osmotic pressure is the pressure that develops in a system due to osmosis.
Isotonic Solution
In an isotonic solution, the solute concentration and the water concentration inside and outside the cell are equal, and therefore there is no net gain or loss of water.
Hypotonic Solution
In a hypotonic solution there is a lower concentration of solute outside than inside the cell, which can cause the cell to swell as water enters it. The swelling of a plant cell in a hypotonic solution creates turgor pressure.
Hypertonic Solution
In a hypertonic solution there is a higher percentage of solute outside than inside the cell, which can cause the cells to shrink or shrivel.
Transport by Carrier Proteins
Carrier proteins are specific; each can combine only with a certain type of molecule or ion, which is then transported through the membrane.
Facilitated Transport
Facilitated transport explains the passage of such molecules as glucose and amino acids across the plasma membrane. It requires a specific carrier protein, but not an expenditure of energy because the molecules are moving down their concentration gradient.
Active Transport
During active transport, molecules or ions move through the plasma membrane, accumulating either inside or outside the cell. Both carrier proteins and an expenditure of energy are needed because molecules are being transported against their concentration gradient.
Bulk Transport
Molecules that are too large to be transported by carrier proteins are moved into and out of the cell by vesicle formation.
Exocytosis
During exocytosis, a vesicle fuses with the plasma membrane as secretion occurs.
Endocytosis
During endocytosis, cells take in substances by vesicle formation.
Phagocytosis
When the material taken in is large, the process is called phagocytosis.
Pinocytosis
Pinocytosis occurs when vesicles form around a liquid or around very small particles.
Receptor-Mediated Endocytosis
Receptor-mediated endocytosis is a form of pinocytosis that is quite specific because it uses a receptor protein shaped so that a specific molecule can bind to it.
4.3 Modifications of Cell Surfaces
Most cells do not live isolated from other cells, they live and interact within an external environment that can dramatically affect cell structure and function.
Cell Surfaces in Animals
We will focus on two different types of animal cell surface features: (1) the extracellular matrix (ECM) that is observed outside cell, and (2) junctions that occur between some types of cells.
Extracellular Matrix
A protective extracellular matrix (ECM) is a meshwork of proteins and polysaccharides in close association with the cell that produced them. Collagen and elastin fibers are two well-known structural proteins in the ECM.
Junctions Between Cells
Certain tissues of vertebrate animals are known to have junctions between their cells that allow them to behave in a coordinated manner. Adhesion junctions serve to mechanically attach adjacent cells. Tight junctions bring cells close by connecting plasma membranes between adjacent cells. A gap junction allows cells to communicate.
Plant Cell Walls
In addition to a plasma membrane, plant cells are surrounded by a porous cell wall.
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