The Plasma Membrane and Transport Since nearly all cell organelles are composed of membranes, we will refer to the cell’s surface as the plasma membrane.

Figure 3.2 Fluid Mosaic Model

Fluid Mosaic Model Animation

Fluid Mosaic Components of the Cell Phospolipid bilayer - two lipid (fat) layers arranged “tail to tail”; hydrophilic “heads” and hydrophobic “tails” Proteins - integral and peripheral; may be enzymes, receptors for hormones, involved in cell recognition, cell adhesion, cytoskeleton attachment, transport, or generating action potentials Cholesterol - gives stability to membrane Glycocalax - “sugar coating”; glycoproteins, glycolipids; “self” recognition, determine blood type, immunity to infection, cancer defense, cell-to-cell interactions, guides embryonic cells

Specializations of Plasma Membrane M icrovilli - tiny finger-like projections that increase surface area (absorption in small intestine) Membrane Junctions

Tight junction – impermeable, “leakproof” fusion of plasma membranes; i.e. between small intestine Desmosome – anchoring “buttonlike” junction, prevents cells from being pulled apart; i.e. between skin cells Gap junction – a channel of connexons that allows chemical substances to pass between cells; i.e. in heart and embryonic cells

Tight Junction

Desmosome

Gap Junction

Membrane Transport

Passive Membrane Transpo rt Driven by kinetic energy of solutes, no ATP required Diffusion Facilitated diffusion Osmosis Lipid soluble =substances dissolve in lipids Water soluble =substances dissolve in water Water soluble substances will not dissolve in lipids and vise-versa Solute - what is being dissolved Solvent - what is doing the dissolving

Simple Diffusion Particles move from an area of high concentration to an area of low concentration (or down their concentration gradient)

Figure 3.6a Diffusion Particles diffuse directly through the plasma membrane Lipid-soluble Some must diffuse through channel proteins Lipid-insoluble

Figure 3.6b Facilitated Diffusion Facilitated diffusion – large, polar molecules such as simple sugars Combine with protein carriers (channel proteins)

Facilitated Diffusion

Diffusion - Osmosis Diffusion of water across a semipermeable membrane Osmolarity – total concentration of solute particles in a solution

Figure 3.7a Effect of Membrane Permeability on Diffusion and Osmosis

Figure 3.7b Effect of Membrane Permeability on Diffusion and Osmosis

Filtration The passage of water and solutes through a membrane by hydrostatic pressure Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area (i.e. in the capillaries of the kidneys)

Tonicity Isotonic – solutions with the same solute concentration as that of the cytosol Hypertonic – solutions having greater solute concentration than that of the cytosol Hypotonic – solutions having lesser solute concentration than that of the cytosol

100% Distilled Water 80% H 2 O 70% Water 30% Dissolved Substances 80% H 2 O 80% Water 20% Dissolved Substances 80% H 2 O What would happen to the animal cells in each beaker?

100% Distilled Water 80% H 2 O Why did the cell get so big? Hypotonic Solution Which way will the water move?

80% Water 20% Dissolved Substances 80% H 2 O Why did the cell stay the same size? Isotonic Solution Which way will the water move?

70% Water 30% Dissolved Substances 80% H 2 O Why did the cell get so small? Hypertonic Solution Which way will the water move?

Active Transport

Uses ATP to move solutes across a membrane against its concentration gradient Requires carrier proteins Example: Sodium-Potassium Pump - 3 Na+ out, 2 K+ in; phosphorylation opens pump to outside; dephosphorylation opens pump to inside

Figure 3.9 Sodium-Potassium Pump Animation

Figure 3.10

Vesicular or Bulk Transport Transport of large particles and macromolecules across plasma membranes Two types: Exocytosis - “out of the cell” Endocytosis - “into the cell”

Endocytosis 3 types of endocytosis: (1) Phagocytosis “cell eating”– pseudopods engulf solids and bring them into the cell’s interior

Bacterial Phagocytosis by Macrophage

Vesicular Transport (2) Bulk-phase endocytosis (formerly pinocytosis “cell drinking”)– the plasma membrane infolds, bringing extracellular fluid and solutes into the interior of the cell (3) Receptor-mediated endocytosis – uses clathrin-coated pits (clathrin is a protein) for specific uptake of macromolecules

Figure 3.12

Examples in Human Body Hyponatremia An abnormally low concentration of sodium in the blood. Too little sodium can cause cells to malfunction, and can be fatal. Symptoms: abnormal mental status, brain swelling, convulsions, headache, nausea, muscle spasms or cramps Causes: burns, diarrhea, diuretics, kidney disease, sweating, vomiting, drinking too much water

Examples in Human Body Tetrodotoxin a potent neurotoxin with no known antidote (pufferfish, porcupine fish, ocean sunfish, triggerfish) blocks action potentials in nerves by binding to sodium channels in nerve cell membranes, preventing any affected nerve cells from firing; paralysis and death within 20 min to 8 hours after ingestion