The Cell Membrane- Important Terminology:

fluid mosaic model, lipid bilayer, extracellular matrix, proteins- integral and peripheral, cholesterol, carbohydrate markers for recognition, selectively permeable, diffusion, concentration gradient, passive transport, osmosis, osmotic pressure, facilitated diffusion, active transport

Purpose of the Membrane:

  • protects and supports the cell
  • controls what enters and leaves the cell

Image credited to:

Structure of Membrane:

Terminology: phospholipid bilayer, aqueous solution inside and outside of the cell, hydrophobic, hydrophilic, peripheral proteins, integral proteins, protein channels

Purpose of Proteins in the Membrane:

a. Protein channels and pumps
b. Enzymes
c. Receptors for cell signals (hormones or signals between nerve cells) or cell recognition (think blood type!)
d. Cell adhesion
e. Attachment to cytoskeleton (the "skeleton" of the cell)

Movement Through the Membrane:

1. Diffusion- no energy expended by the cell

2. Active Transport- energy expended by the cell
  • requires energy in the form of ATP (adenosine triphosphate) external image ATP.gif
(Does the structure look familiar- say, maybe like a nucleotide with a few extra phosphate groups?)

  • Sodium- potassium pump- maintains energy potential across membrane (very important for nerve cells)
    • moves substances against a concentration gradient- from low to high concentration; Na ions are pumped out of the cell while potassium ions are pumped into the cell
    • requires a protein channel that is specific for each substance; for example, a channel that transports glucose will not also transport Na ions.
  • Endocytosis- engulfing particles (either solids or liquids)
    • Phagocytosis- engulfing solids; your immune cells do this when they engulf foreign bacteria
    • Pinocytosis- engulfing liquids (your intestinal cells take in water this way- that's why your, um, "waste" is pretty solid)
    • This animation demonstrates how the cell membrane can form vesicles and ingest substances, even whole bacteria cells!
  • Exocytosis- opposite of endo; gall bladder secretes bile into the small intestine to break down lipids
What moves through the membrane?
Everything we need to survive and everything we need to get rid of!
We exchange gases (carbon dioxide and oxygen), bring in food, and let go of wastes through the membrane.

What controls how substances move through the membrane?

  • The size of the molecule matters (selectively permeable)
  • The charge of the molecule

What moves easily through the membrane?

  • water (non-ionized although polar)
  • carbon dioxide
  • oxygen

What does not move easily through the membrane?

  • large molecules such as glucose and proteins
  • molecules or atoms with charges, such as ions (sodium, potassium, calcium)

Diffusion through the Membrane:

  • definition
  • passive transport
  • down a concentration gradient
  • state of equilibrium
  • Oxygen and carbon dioxide


  • definition
  • passive
  • depends on concentration of solutes on each side of the membrane (solute?)
  • hypotonic, hypertonic, isotonic (a solution will be hypotonic in relation to another solution or area- relative terms)
  • hypo ---> hyper
  • plants and unicellular organisms
  • sea water, plant fertilizer
external image osmosisPocus.gif