BIO 10 Lecture 5

A SEPERATE SELF:

THE PLASMA MEMBRANE

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)

Phospholipid Bilayer

• 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

Cholesterol

• Helps prevent passage of some small molecules

• Adds fluidity to the membrane– 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

Proteins in the Plasma Membrane:

• 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

Glycocalyx

• 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.

Moving Materials In and Out: Diffusion and Gradients

• 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 semi-permeable membrane from an area of lower solute concentration to higher solute concentration.

• Membrane lets water through but not solutes

Osmosis in Cells:

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

Two Main Types of Transport across Cell Membranes

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.

Short Review of Lecture 5

• 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 semi-

permeable? 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?