Chapter
4: Cell Membrane Structure and Function
I. How is the structure of a membrane related to its function?
A. The plasma membrane isolates the cell while allowing communication with its surroundings.
1. Every cell is surrounded by a thin membrane called the plasma membrane
2. The plasma membrane has three general functions.
a. Selectively isolate the cells interior from the external environment.
b. To serve as a gatekeeper and regulate exchange of materials between the interior and exterior of the cell
c. To communicate with other cells
B. Membranes are “fluid mosaics” in which proteins move within layers of lipids.
1. The fluid mosaic model of cellular membranes was developed in 1972 by Singer and Nicolson. The model describes the membrane as constantly shifting groupings of protein “tiles” in a dynamic field of lipids.
2. The lipids and protein distribution of the membrane can change, though the overall content remains stable.
C. The phospholipids bilayer is the fluid portion of the membrane.
1. The inherent chemical properties of phospholipids cause them to spontaneously produce bilayers in aqueous solutions.
2. While the membranes are very stable, the individual phospholipids are very mobile.
3. Small molecules (such as water) and uncharged, lipid soluble molecules can easily pass the membrane.
4. Some animal cell membranes contain cholesterol; it makes the membrane stronger and more flexible, but also less fluid and less permeable to water-soluble molecules.
D. A mosaic of proteins is embedded in the membrane.
1. Thousands of proteins are either embedded in or attached to the plasma membrane.
2. Some of these proteins are bonded to carbohydrates these are called glycoproteins.
3. Some proteins are free to move through the plasma membrane while others are anchored to a network of proteins in the cytoplasm of the cell.
4. There are three major classes of membrane proteins.
a. Transport Proteins regulate the movement of hydrophilic molecules through the membrane.
i. Channel proteins form pores for molecules to pass through.
ii. Carrier proteins bind specific molecules on one side of the membrane, and then change shape to transport the molecule through the membrane for release.
b. Receptor Proteins trigger cellular responses when a specific trigger molecule, such as a hormone, binds to them.
c. Recognition Proteins serve as attachment sites and identification tags.
II. How do substances move across membranes?
A. Molecules in fluids move in response to gradients.
1. Useful definitions
a. Fluid – a substance that can be moved or changed by external forces without breaking (i.e. water or air)
b. Concentration – the numbers of molecules in a fluid in a given unit of volume.
c. Gradient – a difference in a physical property between adjoining areas
2. Movement across a gradient is govern by diffusion
3. Diffusion is the net movement of molecules from areas of higher concentration to those of lower concentration.
a. The steeper the gradient, the faster the diffusion
b. Diffusion continues until the gradient is eliminated
c. Diffusion results in a dynamic equilibrium, molecules are always in motion, it is net movements that count.
B. Movement across membranes occurs by both passive and active transport. (Table 4-1)
1. Passive transport is the movement of molecules across the plasma membrane without the expenditure of cellular energy.
2. Active Transport is the transport of molecules across the plasma membrane that requires the use of cellular energy.
C. Passive transport includes simple diffusion, facilitated diffusion, and osmosis.
1. Diffusion – the net movement of molecules from areas of higher concentration to those of lower concentration.
2. Facilitated Diffusion – diffusion of molecules through the plasma membrane through a carrier protein
3. Osmosis – the diffusion of water across a selective membrane.
a. Isotonic – The cell and its exterior environment have similar solute concentrations
b. Hypertonic – The cell has a lower concentration of solutes than the exterior of the cell (net movement of water out of the cell).
c. Hypotonic – The cell has a higher concentration of solutes than the exterior of the cell (net movement of water into the cell.
D. Active transport uses energy to move molecules against their concentration gradients.
E. Cells engulf particles or fluids by endocytosis.
1. Endocytosis is where the cell surrounds a portion of fluid or a particle with its plasma membrane and engulfs the item. The completed sac is then pinched off inside the cell and called a vesicle.
2. There are three types of endocytosis
a. Pinocytosis is a simple dimpling of the membrane to take in a measure of the extracellular milieu.
b. Receptor-Mediated Endocytosis selectively concentrates for a specific molecule by accumulating the material on specific receptors at a specific site on the cell surface; the concentrated bound receptors are then endocytosed.
c. Phagocytosis is the endocytosis of large particles, including whole microorganisms.
F. Exocytosis is the process of using energy to dump materials out into the extracellular space. Physically this process is the reverse of endocytosis.
III. How are cell surfaces specialized?
A. Various specialized junctions allow cells to connect and communicate.
1. Desmosomes – The membranes of adjacent cells are held together by proteins and carbohydrates. Further protein filaments extend into the interior of the cells allowing for further reinforcement.
2. Tight Junctions – The membranes of adjacent cells nearly fuse along a number of protein ridges creating a leak proof seal.
3. Gap Junctions – Adjacent cells joined by paired protein channels through their membranes, allowing cell-to-cell communication.
4. Plasmodesmata – Openings in the cell walls between plant cells that are lined with plasma membrane and filled with cytoplasm, utilized for cell to cell communication.
B. Some cells are supported by cell walls.
1. Many types of cells have a structural cell wall.
2. Cell walls are composed of are composed of polysaccharides, proteins, or even silica.
3. The purpose of the cell wall is protection of the plasma membrane.