The Plasma Membrane - Gateway to the Cell

Photograph of a Cell Membrane

Cell Membrane The cell membrane is flexible and allows a unicellular organism to move

Homeostasis Balanced internal condition of cells Also called equilibrium Maintained by plasma membrane controlling what enters & leaves the cell

Functions of Plasma Membrane 1. Protective barrier 2. Regulate transport in & out of cell (selectively permeable) 3. Allow cell recognition 4. Provide anchoring sites for filaments of cytoskeleton

Functions of Plasma Membrane 5. Provide a binding site for enzymes 6. Interlocking surfaces bind cells together (junctions) 7. Contains the cytoplasm (fluid in cell)

Structure of the Cell Membrane

Phospholipids Make up the cell membrane Contains 2 fatty acid chains that are nonpolar Head is polar & contains a –PO4 group

Cell Membrane Polar heads are hydrophilic “water loving” Nonpolar tails are hydrophobic “water fearing” Makes membrane “Selective” in what crosses

Solubility Materials that are soluble in lipids can pass through the cell membrane easily

Crossing the membrane. Molecules cross the membrane in several ways. Some methods require the cell to expend energy, some do not. How a particular molecule crosses the membrane depends on the molecules size, polarity, and concentration inside versus outside the cell. Small non-polar molecules easily pass through the membrane. Small polar molecules are transported via proteins. Large molecules are moved in vesicles.

Chemical signals are transmitted across the cell membrane A receptor is a protein that detects a signal molecule and performs an action in response. It recognizes and binds to only certain molecules, which ensures that the right cell gets the right signal at the right time. When receptors are activated (they combine with a hormone or neurotransmitter), they change shape. This relays information to the cell.