Phospholipids: Membrane Components

• Phospholipids consist of one glycerol molecule linked to two fatty acids and a modified phosphate group.

• Phospholipids are amphipathic and their function is to form plasma membranes.

• Phospholipids also tend to form a phospholipids bilayer.

Plasma Membrane Structure and Function

• The plasma membrane is common to all cells, separates cell contents from outside of the cell, allows for communication between cells, allows for cells to attach to each other, and is selectively permeable.

• The plasma membrane’s main components are the phospholipid bilayer protein molecules, carbohydrates, and cholesterol.

• Membrane fluidity decreases as temperature drops.

• The plasma membrane defines the intro-cellular environment.

• There are three mechanisms of membrane transport: diffusion, facilitated diffusion through transports of carriers, and active transport (this mechanism requires energy for transport.)

• Permeability of the plasma membrane is affected by water moving across it, via aquaporins (a type of channel protein.)

• Double bonds can cause a “kink” in the hydrocarbon chain.

• Saturated hydrocarbon chains have fewer spaces and stronger hydrophobic interactions.

• Longer tails make the membrane less permeable (increased hydrophobic reactions.)

Membrane Proteins

• Membrane proteins function as transporters, receptors, and enzymes or in binding and adhesion.

• There are two types of membrane proteins: integral and peripheral.

Carbohydrate Chains

• Carbohydrate chains are attached to proteins or phospholipids, and they function as cellular “fingerprints.”

Membrane Permeability

• Many factors influence the behavior of the membrane such as the number of double bonds in phospholipid tails, length of the tails, the number of cholesterol molecules in the membranes, and the temperatures.

• Cholesterol reduces membrane permeability.

• Adding cholesterol to membranes increases the density of the hydrophobic section.

• Membrane fluidity decreases as the temperature drops.

How Substances Move Across Lipid Bilayer: Diffusion and Osmosis

• Diffusion is the net movement of molecules down a concentration gradient. The movement stops when it hits equilibrium.

• A solution consists of a solvent (liquid) and a solute (dissolved liquid.)

• Diffusion is a type of passive transport.

• Membrane proteins facilitate diffusion.

• Facilitated diffusion is a passive transport involving transmembrane proteins.

• Channel proteins facilitate diffusion. Electrical gradients are a combo of two gradients.

• Osmosis is the diffusion of water across a selectively permeable membrane.

• The movement of water is toward a low water (high solute) concentration.

• Isotonic solutions describe how the amount of solute and water are equal on both sides of the membrane. There is not net gain or loss concentrations of water in the cell.

• Hypotonic solutions describe the concentration of solute in the solution is lower inside the cell.

• Hypertonic solutions describe the concentration of solute being higher in the solution than inside the cell.

More on Osmosis

• Cells placed in a hypertonic solution will shrink, this is called crenation and occurs in animal cells.

• Plasmolysis occurs in plant cells.

• Cells placed in a hypotonic solution will swell which causes turgor pressure in plants.

• This may cause animal cells to lyse (rupture.)

More on Active Transport

• Active transport moves substances against their gradient and requires an input of energy (ATP.)

• Bulk transport is the movement of large particles across the plasma membrane using vesicles.

• There are two varieties: exocytosis and endocytosis.

• Pinocytosis describes how vesicles form around a liquid or very small particles

• Receptor-Medicated Endocytosis describes a specific form of pinocytosis using receptor proteins and a coated pit.

• Phagocytosis describes how the plasma membrane engulfs a smaller cell or food particle. Large, solid material is taken in and digested and is common in unicellular organisms.