Ch. 5- Membrane Structure and Function

Components of the Plasma Membrane  Phospholipid bilayer  Protein Molecules that are either partially or wholly embedded  Cholesterol  Steroids  Only animal cells include an Extracellular membrane  Each membrane is different by the type of proteins embedded

Fluid- Mosaic Model  The phospholipid bilayer is fluid and the fluidity prevents the membrane from solidifying when external temperature drops.  At body temp, the bilayer has the consistency of olive oil.  Cholesterol prevents the membrane from becoming too fluid at higher temperatures.

Carbohydrate Chains  Phospholipids and proteins have sugar chains attached called glycolipids and glycoproteins.  These chains are on the cells exterior and have various functions including cell-to- cell adhesion, reception of signaling molecules, and cell-to-cell recognition.

Proteins  Channel Proteins- allows particular molecules or ions to cross the membrane freely  Carrier Proteins- selectively interact with specific molecules or ions to cross the membrane  Cell recognition membranes- glycoproteins, and help the body recognize when being invaded by pathogens

Proteins  Receptor Proteins- have a specific shape so only specific molecules can bind to it  Enzymatic proteins- carry out metabolic reactions directly  Junction proteins- tight junctions join cells so that a tissue can fulfill its function.

Permeability of the Plasma Membrane  The plasma membrane is selectively permeable, meaning it only allows certain substances into the cell while keeping others out.  Substances that are hydrophobic can diffuse across the membrane with no energy cost. Substances that are polar require and expenditure of energy.

Passive Transport  Diffusion is the movement of molecules from high concentration to low concentration- down their concentration gradient- until equilibrium is reached.  A solution contains a solute and a solvent.  The solute is the substance being dissolved and the solvent is the substance doing the dissolving.

Passive Transport  Osmosis is the diffusion of water across a semi permeable membrane from high to low concentration.  Isotonic solution- the concentration of both solute and water inside and outside the cell are equal, so there is no net water loss or gain.  Hypotonic Solution- The concentration of solute is lower outside the cell than the inside. Therefore water will move into the cell, potentially causing it burst.  Hypertonic Solution- The concentration of solute is higher inside the cell. This causes water to move out of the cell, potentially causing the cell to shrink.

Active Transport  Active Transport requires the use of proteins, usually called pumps, and energy to move molecules against their concentration gradient.  Example is the sodium- potassium pump

Active Transport  Bulk Transport moves large molecules like proteins, polysaccharides and nucleic acids across the cell membrane.  Exocytosis:  The secretion of hormones, neurotransmitters, and digestive enzymes out of the cell.  Intracellular vesicles fuse with the plasma membrane to secrete these.  Pancreatic cells specialize in exocytosis.

Active Transport  Endocytosis:  Cells take in substances by forming vesicles around the material.  There are 3 ways endocytosis occurs:  Phagocytosis  Cell takes in large materials and is common in unicellular organisms.  Pinocytosis  Vesicles form around a liquid or a very small particle  Receptor-Mediated Endocytosis  A form of pinocytosis that is specific because it uses receptors to recognize compatible molecules and bring them into the cell.