Fluid Mosaic Model

Chapter 8 8.1 Fluid mosaic model • • The membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. An amphipathic molecule means it has both hydrophilic region and hydrophobic region as phospholipids. Proteins • The proteins are not randomly distributed in the membrane. However, Groups of proteins are often associated in long-lasting, specialized patches, where they carry out common functions. The fluidity of membranes: • • • • A membrane is held together mainly by hydrophobic interactions (which are much weaker than covalent bonds). The phospholipids move sideways like party goers elbowing their way through a crowded room, and very rarely phospholipids can flip-flop across the membrane, switching from one phospholipid layer to the other. Many other proteins seem to be held immobile by their attachment to the cytoskeleton or to the extra cellular matrix. A membrane remains fluid as temperature decreases until the phospholipids settle into a closely packed arrangement and the membrane solidifies. Temperature affect: • • • • The temperature at which membrane solidifies depends on the types of lipids it is made of. The membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails Cholesterol resist the fluidity change -which is caused by temperature change- and this making him a “fluidity buffer” In plant cells there is a lack of cholesterol, so they have a steroid lipid fluidity buffer. Membrane proteins and their functions: • • A membrane is collage of different proteins, often clustered together in groups, embedded in the fluid matrix of the lipid bilayer Phospholipids form the main fabric of the membrane, but proteins determine most of the membrane functions. Types of protein: • • Integral proteins: penetrate the hydrophobic interior of the lipid bilayer. The hydrophobic region of the integral proteins consists of one or more stretches of nonpolar amino acids (usually coiled into alpha helix). Peripheral proteins: they are not embedded in the lipid bilayer at all, they are loosely bound to the surface of the membrane, often to exposed parts of integral proteins. Functions of membrane protein: 8.2 Membrane structure result in selective permeability: • • • The fluid mosaic model helps explain how membrane regulate the cell’s molecular traffic. A steady traffic of small molecules and ions moves across the plasma membrane in both directions. Although the heavy traffic through them may seem to suggest otherwise, cell membranes are selectively permeable and substances do not cross the barrier indiscriminately. The cell can take up some small molecules and ions and exclude others. The permeability of the lipid bilayer: • • • Nonpolar molecules, as are lipids they can all therefore dissolve in the lipid bilayer of the membrane and cross it easily (without the aid of membrane proteins). Polar molecules pass only slowly through a lipid bilayer. A charged atom or molecule and its surrounding shell of water are even less likely to penetrate the hydrophobic interior. Transport proteins: • • • Specific ions and a variety of polar molecules cannot move through cell membrane on their own. They need transport proteins. A transport protein is specific for the substance it translocate, allowing only a certain substances to cross the membrane. The selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane. 8.3 Passive transport is diffusion of a substance across a membrane with no energy • • • • • • • Molecules have a type of energy called thermal energy, due to their constant motion. A result of this motion is diffusion. The movement of particles of any substance so that they spread out into the available space. Rule of diffusion: in the absence of other forces a substance will diffuse from where it is more concentrated to where it is less concentrated. In order to across the membrane a substance must use diffusion, so it can easily cross it. A diffusion of a substance across a biological membrane is called passive transport because the cell does not have to expend energy to make it happen. Water diffuses across the membrane from region of higher free water concentration (lower solute concentration) to a lower free water concentration (higher solute concentration). The diffusion of free water across a selectively permeable membrane whether artificial or cellular is called osmosis. Water balance of cells without cell walls: • tonicity the ability of surroundings solution to cause a cell to gain or lose water. Animal cells: • • • isotonic hypertonic Hypotonic no net movement for water cell loses water cell gains water cell is alive. cell shrivelled and dies. cell swell and lyse, die. ❖ Salt solute is hypertonic Osmoregulation is balancing the water movement. Ways for certain cells to live in noisotonic solutions. Water balance with cell walls: Cell wall : (in plants, prokaryotes, fungi, and some unicellular eukaryotes). Isotonic Hypertonic Hypotonic • Hypo= less cell becomes flaccid(limp) the plant wilts. lose water the cell shrink, die gain water the cell is turgid (normal) hyper= more. Plasmolysis: as the plant cell shrivels, its plasma membrane pulls away from the cell wall at multiple places. Facilitated Diffusion: passive transport aided by proteins. Facilitated diffusion: polar molecules and ions impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane. 2 types of transport proteins: • • Channel proteins: simply provide corridors that allow specific molecules or ions to cross the membrane. Carrier proteins: seem to undergo a subtle change in shape that somehow translocate the solute- binding site across the membrane. 8.4 Active transport uses energy to move solutes against their gradients (concentration ratio). • • • To pump a solute across a membrane against its gradient requires work. (the cell must expend energy) Proteins that work in this process: carrier proteins. ATP hydrolyses supplies the energy for most active transport. One way ATP can power active transport is when its terminal phospholipid group is transferred directly to the transport protein. 8.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis. • • Exocytosis: particles are transported out of the cell. A vesicle containing cell products move toward the cell membrane, the membrane surrounding the vesicle fuses with cell membrane and the contents are out the vesicle which means out of the cell. Endocytosis: the cell membrane folds into a pouch surrounding particles outside the cell. A vesicle is borned, and it carries the contents into the cell and can release them into the cytoplasm. types of endocytosis: