2.5 Biological membranes

• The fluid mosaic model of membranes describes a phospholipid bilayer in which individual phospholipids can move, giving the membrane a flexible shape.
• Membranes contain extrinsic and intrinsic proteins of different sizes and shapes, which perform various functions such as binding sites, receptors, antigens, and cell signalling.
• Cholesterol is a steroid molecule found in some plasma membranes that connects phospholipids and reduces fluidity to make the bilayer more stable.
• Glycolipids are involved in cell signalling and cell recognition.
• Extrinsic proteins in membranes have binding sites or receptors for hormones and drugs, antigens, and can bind cells together.
• Intrinsic transmembrane proteins in membranes perform functions such as electron carriers, channel proteins, and carrier proteins.
• Membranes within cells provide an internal transport system, are selectively permeable to regulate the passage of molecules into or out of organelles, provide a reaction surface, and isolate organelles from the cytoplasm for specific metabolic reactions.
• The cell-surface membrane isolates the cytoplasm from the extracellular environment, is selectively permeable to regulate transport of substances, and is involved in cell signalling and cell recognition.
• Osmosis out of a cell causes the protoplast to shrink, making the cell flaccid.
• Simple diffusion is a passive process that requires no energy from ATP hydrolysis.
• Exocytosis and endocytosis are active processes involved in bulk transport and transporting large particles.
• More solute in a solution results in a more negative water potential (Ψ).
• Water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established
• High absorbance/ low transmission = more pigment in solution
• Active transport is an active process that uses ATP hydrolysis to transport molecules/ ions from an area of low concentration to an area of higher concentration against a concentration gradient.
• Factors that affect the rate of diffusion include temperature, diffusion distance, surface area, size of molecule, and difference in concentration (how steep the concentration gradient is).
• Facilitated diffusion is a passive process that uses specific channel or carrier proteins with complementary binding sites to transport large and/ or polar molecules/ ions down a concentration gradient.
• Osmosis into a cell causes the protoplast to swell, making the cell turgid.
• Water potential (Ψ) is the pressure created by water molecules measured in kPa.
• Factors that affect membrane permeability include temperature, pH, and the use of a solvent.
• Colorimetry can be used to investigate membrane permeability by disrupting the tonoplast and cell-surface membrane, adding a soluble pigment to the vacuole, selecting a colorimeter filter with a complementary colour, and measuring the absorbance or % transmission value of the solution.