Diameter of plasma membrane: ~ 5 nm
Diameter of DNA: 2 nm
Diameter of cell: 5,000-50,000 nm

The Plasma Membrane has Two
Main Functions:
• Forms a physical, largely impermeable barrier between the cell and its surroundings
– Keeps “the goods” concentrated while keeping harmful materials out
• requires transport in two directions
– Provides a “ bag
” in which DNA can evolve separately from the DNA of other organisms
– All cells have plasma membranes; some also have cell walls
• Facilitates communication between the cell and other cells and the environment

• Major components:
– Phospholipid Bilayer
– Cholesterol (animals only)
– Proteins
– Glycolayx (carbohydrate groups)

• Phospholipids : two fatty-acid chains and a polar phosphate group attached to glycerol.
• Arrangement of phospholipids in water (two layers, heads pointed out, tails pointed in).
• Permeability of bilayer : lipid center is a barrier to passage of most hydrophilic molecules, but it allows nonpolar, hydrophobic molecules to pass.
– Water, although hydrophilic and polar, can move across the membrane because it is so small

– If not for cholesterol, the membrane would harden at low temperatures
• Much like peanut oil left in the refrigerator
– This would be incompatible with life in many creatures

• Two types:
– Integral (span entire membrane)
– Peripheral (lie on either side)
• Functions:
– Structural support — attach to cytoskeleton
– Recognition — helps immune system determine self from foreign
– Communication — receptors and binding sites
– Transport — allow molecules to pass

• Sugars or carbohydrates protruding from lipids and proteins in the membrane
• Functions:
– Binding sites for proteins in communication and recognition
• Forms basis of ABO blood typing in humans
– Lubricate cells
– Attach cells to extracellular matrix
• Enables tissues to form

• Diffusion:
– Is the spontaneous movement of molecules from regions of higher to lower concentrations
– Concentration gradient = difference between the highest and lowest concentration of a solute
• Like bike coasting downhill, the tendency is for molecules to travel from high to low concentration.

• Permeability verses semi-permeability
– A permeable membrane is one that lets everything in or out
– A semi-permeable membrane is one that lets some things in but not others
– An impermeable membrane is one that does not let anything in or out (like a plastic bag)
• Cells have semi-permeable membranes
– They are highly selective
• Osmosis
– Is the net movement of water across a semipermeable membrane from an area of lower solute concentration to higher solute concentration.
• Membrane lets water through but not solutes

1. Passive Transport
– Simple diffusion
• Water, gases, fat soluble molecules
• Membrane is permeable to these molecules, so they travel down their concentration gradients and enter without energy output by the cell.
– Facilitated diffusion
• Larger polar molecules (e.g. ions, sugars)
• Membrane is impermeable, so even if they want to travel down the concentration gradient, they can
’ t without help from a membrane channel (transport protein)
• However, energy is not expended in this process because the molecule is still following its concentration gradient

2. Active Transport
– Molecules have to pass across the membrane up their concentration gradient
– They cannot use the energy of diffusion, but must expend energy in the form of
ATP

Example: Sodium-Potassium Pump

Movement of Big Molecules or
Whole Organisms
• Exocytosis
– movement of materials out of the cell by fusion of vesicles with the plasma membrane
• Endocytosis:
– infolding of the plasma membrane to bring large molecules into the cell.
• Pinocytosis:
– water and solvents are enclosed in invaginating vesicle
• Receptor-mediated endocytosis:
– more specific, with receptor capturing ligand and concentrating it into an invaginating pit.
• Phagocytosis:
– how the human immune system ingests whole bacteria and one-celled creatures eat using pseudopodia.

• What are the two main functions of the plasma membrane?
• What are its 4 main components?
• What does it mean to say that a membrane is semipermeable? Why is it important that the plasma membrane has this quality?
• What is the difference between passive and active transport of small molecules across the plasma membrane?
• How can large molecules move in and out of a cell?
• Why do you think steroids affect all cells in the body, not just those in the skeletal muscles?