11/8/2011 Objective: Understand that the phospholipid bilayer is fluid and selectively permeable Warm-Up: Define hydrophobic, hydrophilic, and amphipathic.

If you pour a bunch of amphipathic molecules into a beaker of water, how would they arrange themselves? Draw your hypothesis.

Cell Membranes

Membrane Structure The cell membrane is a phospholipid bilayer Each phospholipid has a polar phosphate group (often called the “head”) and non-polar fatty acids (often called the “tails”) The polar phosphate group is hydrophilic The non-polar fatty acids are hydrophobic The extracellular fluid and the intracellular fluid are both aqueous solutions Define “extracellular” and “intracellular”

11/11/2011 Objective: Describe the structure and function of the selectively permeable cell membrane Warm-Up: How are organisms able to move solutes across the cell membrane against their concentration gradient?

Membrane Structure Cell membranes also contain cholesterol molecules, which help stabilize the membrane Transmembrane proteins There are proteins that span the cell membrane, connecting the inside of the cell with the surrounding environment These membrane proteins function as channels that allow certain molecules to pass through the membrane There are also membrane proteins that act as receptors

Cell Membrane Function Membranes act as a barrier between the internal and external environments of the cell Membranes are fluid Phospholipids float around within the membrane Membranes regulate the flow of substances into and out of the cells There are some molecules that can diffuse freely across cell membranes Other molecules cannot diffuse across the membrane

Who’s Allowed In? Generally, small, uncharged molecules can diffuse across the membrane Oxygen, carbon dioxide Small, uncharged, polar molecules can also diffuse freely Water, glycerol Large, uncharged, polar molecules cannot diffuse through the membrane Glucose, amino acids, nulceotides Ions cannot diffuse through the membrane H+, Na+, K+, Ca2+, Cl-, etc

Selectively Permeable Selectively permeable (sometimes called semi-permeable): describes a membrane that only allows specific kinds of substances to pass through Think of the root word “permeate” to help you remember this one “Large and charged”—molecules that are large and/or charged cannot diffuse freely across the selectively permeable cell membrane