Biology 102 Lectures 6 & 7: Biological Membranes

Lecture outline 1. Relationship of membrane structure and function Functions Functions Structure: The fluid-mosaic model of membranes Structure: The fluid-mosaic model of membranes 2. Movement of substances across membranes Principles of Diffusion Principles of Diffusion Passive and active transport of solutes Passive and active transport of solutes Osmosis Osmosis Endocytosis and exocytosis Endocytosis and exocytosis 3. Specialization of cell surfaces

1. Membrane structure and function Biological membranes Biological membranes  Thin barrier separating the inside of the cell (or structure) from the outside environment Functions (focus on plasma membrane) Functions (focus on plasma membrane)  Selectively isolate the cell’s contents from the external environment  Regulate the exchange of substances between the inside and outside of the cell  Communicate with other cells

Fluid-mosaic model of membrane structure

The phospholipid bilayer is the fluid portion of the membrane Double layer Double layer Polar head group: hydrophilic exterior Polar head group: hydrophilic exterior Non-polar hydrocarbon tails: hydrophobic interior Non-polar hydrocarbon tails: hydrophobic interior Unsaturated hydrocarbon chains: maintains fluidity Unsaturated hydrocarbon chains: maintains fluidity

Phospholipid bilayer as a barrier Hydrophilic molecules cannot pass freely through the membrane’s hydrophobic interior Hydrophilic molecules cannot pass freely through the membrane’s hydrophobic interior  Sugars, amino acids, charged ions (i.e. Na + and Cl - ) are some examples  Though polar, H 2 0 is so small it does pass through. Many hydrophobic molecules can pass freely through the membrane’s hydrophobic interior Many hydrophobic molecules can pass freely through the membrane’s hydrophobic interior  Steroid hormones and other lipids are some examples

Cholesterol molecules are part of the lipid bilayer Adds strength Adds strength Adds flexibility Adds flexibility Affects fluidity Affects fluidity  Decreases fluidity at “moderate” temperatures  Harder for phospholipids to move  Prevents solidification at low temperatures  Keeps phospholipids from binding to each other

A mosaic of proteins is embeded in the membrane Glycoproteins: proteins with attached carbohydrates Glycoproteins: proteins with attached carbohydrates

Types of membrane proteins Transport proteins Transport proteins  For passage of materials through the plasma membrane  Channel vs. carrier proteins Receptor proteins Receptor proteins  Bind molecules and trigger cellular responses  Example: hormones Recognition proteins Recognition proteins  Self vs. non-self (glycoprotein-based) recognition  Markers during development

2. Movement of substances across membranes Definitions Definitions  Concentration  Number of molecules in a given volume  Gradient  Differences in concentration between two regions of space. This causes molecules to move from one region to the other (if no barrier to movement) This causes molecules to move from one region to the other (if no barrier to movement)  Diffusion  Net movement of molecules from regions of high concentration to regions of low concentration Considered as movement “down” its concentration gradient Considered as movement “down” its concentration gradient

Diffusion of Dye in Water Time 0 Steep Concentration Gradient Time 1 Reduced Concentration Gradient Dispersing Time 2 No Concentration Gradient Random Dispersal

Passive vs. active transport Passive transport Passive transport  Movement of molecules down their concentration gradients  Requires no net energy expenditure  The gradients themselves provide energy Active transport Active transport  Movement of molecules against their concentration gradients  Requires energy!

Focus: Passive transport 1.Simple diffusion 2.Facilitated diffusion 3.Osmosis  Remember that no energy is required, and molecules move down their concentration gradients

Focus: Passive transport 1.Simple diffusion  Molecules simply cross cell membrane on their own, down their concentration gradients  Possible only for molecules that can cross the lipid bilayer on their own  Lipid-soluble molecules Examples: ethyl alcohol, vitamin A, steroid hormones Examples: ethyl alcohol, vitamin A, steroid hormones  Very small molecules Examples: water, carbon dioxide Examples: water, carbon dioxide  Rate depends upon  Concentration gradient  Size  Lipid solubility

Focus: Passive transport (cont.) 2.Facilitated diffusion  Molecules move down their concentration gradients (as for simple diffusion), but…  Transport proteins assist these molecules in crossing the membrane  No net energy expenditure! (This is a type of diffusion…)

Focus: Passive transport (cont.): Facilitated diffusion via a channel

Focus: Passive transport (cont.): Facilitated diffusion via a carrier protein Carrier protein has binding site for molecule Molecule enters binding site Carrier protein changes shape, transporting molecule across membrane Carrier protein resumes original shape (Inside Cell) (Outside Cell) Diffusion Channel Protein Diffusion Channel Protein Diffusion Gradient Molecule in Transit Molecule in Transit

Focus: Passive transport (cont.) 3.Osmosis  Movement of water from a high [water] to an area of low [water] concentration across a semi- permeable membrane  Note here that water can pass through, but glucose cannot Think about which way water will move (blackboard demo)

The effects of osmosis Compare solute and water concentrations outside vs. inside the cell (sketches) Compare solute and water concentrations outside vs. inside the cell (sketches)

Focus: Active Transport 1.Movement via active transport proteins 2.Endocytosis 3.Exocytosis  Remember that energy is required, and molecules are moved against their concentration gradients

Focus: Active transport 1. Movement via active transport proteins ATP required (has own binding site) ATP required (has own binding site) Note movement of particles (Ca++) against their concentration gradient Note movement of particles (Ca++) against their concentration gradient

Focus: Active transport 2. Endocytosis Three types of endocytosis Three types of endocytosis  Pinocytosis  “cell drinking”  Extracellular fluid taken in  Receptor-mediated endocytosis  Specific for particular molecules  Molecules bind to receptors.  Receptor-molecule complex taken in  Phagocytosis  Large particles engulfed

Focus: Active transport 3. Exocytosis

3. Specialization of cell surfaces Connections between cells Connections between cells  Desmosome: Membranes of adjacent cells glued together by proteins and carbohydrates  Tight junction: Cells sealed together with proteins

3. Specialization of cell surfaces (cont.) Communication between cells Communication between cells  Gap junctions: Channels connect adjacent cells  Plasmodesmata: Continuous cytoplasm bridges between two cells (plants)  Note also cell walls. Only certain cell types have cell walls!