4Plasma Membranes - Why should we care? Problems with the cell membrane are involved in many diseasesType 2 Diabetes, organ transplant rejection, Cholera, Cystic fibrosis, cancer
5Membrane Structure & Function A plasma membrane encloses every cell.It protects a cell by acting as a barrier between it’s living contents & the surrounding environment.
6It is selectively permeable thus it regulates what goes into & out of the cell. It receives and produces signals to and from other cells.It identifies the cell as belonging to a particular organism and tissueIt maintains connections between cells in organs and tissue
7At the turn of the last century, researchers noted lipid-soluble molecules entered cells more rapidly than water soluble molecules, suggesting lipids are component of plasma membrane.
8At the turn of the last century, researchers noted lipid-soluble molecules entered cells more rapidly than water soluble molecules, suggesting lipids are component of plasma membrane.
92. Later, chemical analysis revealed the membrane contained phospholipids.
103. Gorter & Grendel (1925) found amount of phospholipid extracted from a red blood cell was just enough to form one bilayer; suggested nonpolar tails (hydrophobic) directed inward, polar heads (hydrophilic) outward.
114. To account for permeability of membrane to nonlipid substances, Danielli & Davson proposed sandwich model (later proved wrong) with phospholipid bilayer between layers of protein.
125. With electron microscopy, Robertson proposed proteins were embedded in outer membrane and all membranes in cells had similar compositions-the unit membrane model.
136. In 1972, Singer & Nicolson introduced the currently accepted fluid-mosaic model of membrane structure.
15The main biological molecules in membranes are lipids and proteins, but include some carbohydrates. The most abundant lipids are phospholipids.
16The molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water.hydrophilichydrophobic
17Membranes are fluidMembrane molecules are held in place by relatively weak hydrophobic interactions.Most of the lipids and some proteins can drift from side to side in the plane of the membrane, but rarely flip-flop from one layer to the other.
18Cholesterol is wedged between phospholipid molecules in the plasma membrane of animals cells. At warm temperatures, it restricts the movement of phospholipids and reduces fluidity.At cool temperatures, it maintains fluidity by preventing tight packing.
19Membranes are mosaics of structure and function A membrane is a collage of different proteins embedded in the lipid bilayer.
20The proteins in the plasma membrane may provide a variety of major cell functions.
21Many transport proteins simply provide corridors allowing a specific molecule or ion to cross the membrane.These channel proteins allow fast transport.
22Some transport proteins do not provide channels but appear to actually move the solute across the membrane as the protein changes shape.
23Membrane carbohydrates are important for cell to cell recognition The membrane plays the key role in cell to cell recognition.Cell to cell recognition is the ability of a cell to distinguish one type of neighboring cell from another.This attribute is important in cell sorting and organization as tissues and organs in development.It is also the basis for rejection of foreign cells by the immune system.
24A membrane’s structure and composition results in selective permeability. A steady traffic of small molecules and ions moves across the plasma membrane in both directions.However, substances do not move across the barrier indiscriminately; membranes are selectively permeable.
25Specific ions and polar molecules can cross the lipid bilayer by passing through transport proteins that span the membrane.Each transport protein is specific as to the substances that it will move.
27Passive transport is diffusion across a membrane Diffusion is the tendency of molecules of any substance to spread out in the available spaceMovements of individual molecules are random.The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen.
28For example, if we start with a permeable membrane separating a solution with dye molecules from pure water, dye molecules will cross the barrier randomly.The dye will cross the membrane until both solutions have equal concentrations of the dye.At this dynamic equilibrium as many molecules pass one way as cross the other direction.
29In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient.Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances.
30Osmosis is the passive transport of water Differences in the relative concentration of dissolved materials in two solutions can lead to the movement of ions from one to the other.The solution with the higher concentration of solutes is hypertonic.The solution with the lower concentration of solutes is hypotonic.Solutions with equal solute concentrations are isotonic.
31Free (unbound) water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer.This diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis.Osmosis continues until the solutions are isotonic.
32The same cell in a hypertonic environment will loose water, shrivel, and probably die. A cell in a hypotonic solution will gain water, swell, and burst.
33For example, Paramecium, a protist, is hypertonic when compared to the pond water in which it lives. In spite of a cell membrane that is less permeable to water than other cells, water still continually enters the Paramecium cell.To solve this problem, Paramecium have a specialized organelle, the contractile vacuole, that functions as a bilge pump to force water out of the cell.
34As a plant cell looses water, its volume shrinks. Eventually, the plasma membrane pulls away from the wall.This plasmolysis is usually lethal.
35Facilitated Diffusion Many polar molecules and ions that are normally impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane.The passive movement of molecules down its concentration gradient via a transport protein is called facilitated diffusion.
36Active transport is the pumping of solutes against their gradients Some proteins can move solutes against their concentration gradient, from the side where they are less concentrated to the side where they are more concentrated.This active transport requires the cell to expend its own metabolic energy.