 Located in almost all animals  Carries electrical impulses on the specialized cell membrane of the nerve cells  Nerve cells coordinate the opening and closing of ion channels in the cell membrane to create an action potential.

 Made of two layers of phospholipids  a.k.a. a lipid bilayer  Proteins and cholesterols are embedded in the membrane

1. Passive transport  Simple diffusion  Facilitated diffusion 2. Active transport 3. Endocytosis and exocytosis

a) Simple diffusion  Materials move from areas of high concentration to areas of low concentration  Energy from the cell is not needed  When concentrations are equal, molecules still move but no net change results  Things like water, oxygen, and carbon dioxide move by simple diffusion  The diffusion of water across a selectively permeable membrane is called osmosis

Osmotic pressure = a force driven by differences in solute concentration; can cause cells to shrink or swell

Hypertonic = “above strength,” more solute present in the solution Hypotonic = “below strength,” fewer solutes present in the solution Isotonic = “same strength,” same amount of solute in solution as in cell

b) Facilitated diffusion  Channel proteins and carrier proteins help larger molecules cross the membrane  Many channels are gated for regulation  Carrier proteins change shape to move molecules through  More specific than simple diffusion but still does not require energy  Things like proteins and sugars move by facilitated diffusion

 Cells sometimes need to move substances against the concentration gradient  Often carrier proteins act as pumps  Energy is required  Supplied by ATP  Amino acids, sugars, and ions are often moved by active transport

a) Sodium ions normally are in high concentration in extracellular fluid  Sodium ions diffuse into cells  This reduces the amount of water inside the cell  As a result water diffuses into the cell  This is bad  Too much water means cells can swell and burst  High extracellular sodium concentration is also necessary for glucose transport The solution…..

 Sodium potassium pump in action Sodium potassium pump in action

1. Sodium ions flow into neuron, causing depolarization; starts at the dendrites 2. If enough Na + enters neuron (and K + moves out), the membrane reaches threshold potential 3. An action potential is initiated at the axon hillock (has higher density of Na + /K + pumps) 4. The signal passes and the membrane repolarizes 5. The signal is sent along the axon causing a nerve impulse

 Myelin sheath = electrical insulator that is discontinuous along the axon of the neuron; increases capacitance  Axon hillock = base of the axon; has higher density of Na+/K+ pumps so action potential is more easily initiated here  Neural synapse = gap between two neurons along signal pathway

1. Electrical synapse – uses ion channels connected axon to dendrite without a gap to communicate between neurons.

1. Chemical synapse – uses vesicles to move chemicals (neurotransmitters) across a small gap between neurons

 Vesicles move substances into and out of cells  Substances that are big molecules  Proteins and polysaccharides are moved in = ENDO  Proteins and waste are moved out = EXO Exocytosis moves neurotransmitters into the synaptic cleft Summary Animation