Biological Membranes PDF

Biological Membranes Membranes Regulates the passage of molecules into and out of the cell organelles Divides cells into compartments, each with specialized metabolism Act as surfaces that holds the enzymes Composition of Membranes Composed of proteins (60%) and...

Biological Membranes Membranes Regulates the passage of molecules into and out of the cell organelles Divides cells into compartments, each with specialized metabolism Act as surfaces that holds the enzymes Composition of Membranes Composed of proteins (60%) and two layers of phospholipid molecules (40%); some contains 8% of sugars Intrinsic proteins – aka integral proteins; are partially immersed in the lipid bilayer Extrinsic proteins – aka peripheral proteins; located outside the membrane, perform important enzymatic functions Glycoproteins – oligosaccharides / sugars that are attached to intrinsic proteins Glycolipids – attached to membrane lipids Phospholipid 1 glycerol molecule + 2 fatty acids + a molecule containing a phosphate group The phosphate end is polar (slightly charged) Polar end is hydrophilic (hydro- “water” phil “love”) Fatty acid chains are nonpolar Nonpolar end is hydrophobic (hydro- “water” phobos “fear”) Arrange hydrophilic heads toward the watery surroundings inside and outside of the cell Hydrophobic tails form the inside of the double layer  Fluid Mosaic Model The current model for the structure of membranes Protein molecules “float” in a fluid phospholipid bilayer Characterizes the plasma membrane and other cell membranes as consisting of a double layer (bilayer) of lipid molecules Proteins are embedded in the lipid bilayer in a way the resembles a mosaic pattern The membrane structure is fluid rather than static Lipids & proteins move sideways within the membrane Properties of Membranes They can grow Membranes are formed molecule by molecule in certain regions of the cell Whole pieces of membrane are moved as vesicles or small bubbles to different sites of the cell When the vesicle of the preformed membranes arrived at the growing membrane, they fuse, allowing the transport of materials Properties of Membranes Permeability Impermeable – nothing passes through; no biological membrane is impermeable to everything Freely permeable – virtually anything can pass through Selectively permeable (differentially permeable) – certain substances can pass through rapidly, others slowly Properties of Membranes They regulate the passage of materials to maintain homeostasis Homeostasis is a relatively constant set of internal conditions Membranes receive information from their surroundings Use cell signaling, hormones, & chemicals Help the cell to respond to its environment Passage of Materials Across Membranes Materials move passively or actively Passive transport Diffusion – the net movement of particles (atoms, molecules, or ions) from a region of higher concentration to a region of lower concentration Moves materials through the cytoplasm and into and out of cells Ex. Sugar cube in a beaker of water Osmosis – Special type of diffusion The diffusion of water across a selectively permeable membrane. Water flows spontaneously from a region of lower solute concentration (more water) to a region of higher solute concentration (less water) Plasmolysis in plant cells Plant cell Low osmotic potential (hypotonic solution) Diffusion of water into the cell continues until the inward pressure of the cell wall prevents a further increase in the osmotic potential of the cell. Turgid- maximum quantity of water allowed by the cell wall. Plasmolysis- when cell membrane begins to shrink and becomes detached from the cell wall. Plasmolyzed cells: lost turgidity and are unable to support the plant. Flaccid-wilting of plant Facilitated Diffusion Materials diffuse from a region of higher concentration to a region of lower concentration through special passageways The presence of large intrinsic membrane proteins allows hydrophilic, charged molecules to diffuse through the membrane Passageways are called carrier proteins “conveyor belts” in the direction of the concentration gradient (from high to low concentration) Active Transport Large intrinsic membrane proteins bind a molecule and force it through the membrane, consuming energy in the process. Requires the cell to use energy (ATP) The assisted movement of a substance from a lower concentration to a higher concentration Substances move against concentration gradient  Exocytosis – The fusion of a vesicle with the cell membrane, releasing the vesicle’s contents to the cell exterior Endocytosis – The invagination of the membrane, forming a vesicle that pinches off and carries external material through the cell Imbibition Special type of diffusion Water is attracted to the surfaces of the cellulose micro fibrils, causing them to move apart and the cell wall to swell. Cellulose imbibes water, allowing the relatively free passage of water. Water potential in plant cells Refers to a measurement that predicts which way water tends to flow. The combination of osmotic potential and pressure potential Osmotic potential- refers to the measurement of water’s tendency to move across a membrane as a result of solute concentration Pressure potential- the pressure a cell wall extends around its protoplast contents.  If the tissue has lower water potential than pure water, it will take up water when placed in pure water= increases weight and volume. Osmotic potential is always zero or negative number; pressure potential is always a positive number. Water potential: +, -, 0 (expanding, shrinking, equilibrium) Cell wall elasticity Cell wall- high elasticity property (minimal volume osmotic potential changes) Young and actively growing cell: more elastic Mature cells: thicker secondary thickenings. Makes them tough and rigid. (mature leaves of vegetables: camote tops, malunggay, gabi and mustard)