Membranes -Composed of a phospholipid bilayer with embedded proteins -Likened to a “Fluid Mosaic” in structure

Different views of phospholipid structure

The Fluid Mosaic model of membrane structure

Cell membranes consist of four components Phospholipid bilayer – Transmembrane proteins – Interior protein network – Cell surface markers -

Proteins and protein complexes perform key functions Transporters – Enzymes – Cell surface receptors – Cell surface identity (antigens) – Cell-to-cell adhesion proteins – Attachments to cytoskeleton –

Functions of plasma membrane proteins

Diffusion processes Passive Transport – no energy required Simple diffusion Facilitated diffusion Osmosis Active Transport – requires ATP Sodium-potassium pump Coupled transport

Simple diffusion -Occurs with CO2, O2, and some non-polar organic compounds

Facilitated Diffusion Molecules bind to a specific carrier protein to gain access to the cytoplasm Process is still passive as there is no energy (ATP) expended and substances move along a concentration gradient

Osmosis = movement of H2O across a selectively permeable membrane Water moves along a concentration gradient; from area of higher concentration to area of lower concentration

Tonicity of solutions A solution contains a solute(s) dissolved in a solvent Hypertonic solutions contain MORE solute than a comparable solution Hypotonic solutions contain LESS solute than a comparable solution Hence, Hypertonic and Hypotonic relate to 2 solutions that are being compared to one another

Fig. 5.13

The Sodium-Potassium pump: example of active transport

Coupled transport – symport movement across the membrane

Examples of bulk transport