Plasma Membrane Structure and Function

Oligosaccharides CytoplasmCytoplasm Membrane Proteins Cholesterol Outer Surface Fluid Mosaic Model of Membrane Structure Phospholipid Bilayer

Macromolecules found in Membranes LipidsLipids –Phospholipid bilayer –Cholesterol ProteinsProteins –Transport proteins –Receptor proteins –Recognition proteins CarbohydratesCarbohydrates –Oligosaccharides on glycoproteins

Phospholipid Structure Fatty Acid Glycerol Polar group Glycerol Fatty Acid Change from Triglyceride to Phospholipid

The plasma membrane “fluid” is a double layer of phospholipids.

The plasma membrane “mosaic” is a collection of proteins.

Functions of Membrane Macromolecules LipidsLipids –Phospholipid bilayer Forms boundary to isolate cell contents from environmentForms boundary to isolate cell contents from environment Restricts passage of hydrophilic substances across the membraneRestricts passage of hydrophilic substances across the membrane –Cholesterol Increases bilayer strength, flexibilityIncreases bilayer strength, flexibility Reduces membrane fluidityReduces membrane fluidity Reduces permeability to water-soluble substancesReduces permeability to water-soluble substances

Functions of Membrane Macromolecules ProteinsProteins –Transport proteins Regulate movement of water soluble substancesRegulate movement of water soluble substances –Channel proteins have pores that allow passage of ions and small water-soluble molecules –Carrier proteins bind to molecules and change shape for delivery across membrane

Functions of Membrane Macromolecules ProteinsProteins –Receptor proteins Docking site for molecules outside the cellDocking site for molecules outside the cell Trigger internal cellular responseTrigger internal cellular response –Recognition proteins Identification tagsIdentification tags –Oligosaccharides aid in cell-cell recognition Cell-surface attachment sitesCell-surface attachment sites

Transport Processes Movement of substances across the cell membraneMovement of substances across the cell membrane Passive transportPassive transport –Substances move from [high]  [low] –No energy input required –Simple Diffusion, Facilitated Diffusion, Osmosis Active transportActive transport –Substances move from [low]  [high] –Requires energy input –Protein carriers, Endocytosis, Exocytosis

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

Simple Diffusion (extracellular fluid) (cytoplasm) Some molecules diffuse freely across Some molecules diffuse freely across (lipid soluble molecules, CO 2, O 2, H 2 O)

Facilitated Diffusion: Use of Carriers to Assist with Diffusion 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

Osmosis Diffusion of water across a differentially permeable membraneDiffusion of water across a differentially permeable membrane Water moves from [high]  [low]Water moves from [high]  [low]

“Bound” water molecules clustered around sugar: cannot fit through pore “Free” water molecule: can fit through pore

Normal RBCs Isotonic Solution The Effects of Osmosis Equal movement of water into and out of cells Net movement of water out of cells Net movement of water into cells Shriveled RBCs Swollen RBCs Hypertonic Solution Hypotonic Solution

Active Transport with Protein Carrier Inside Cell Outside Cell Energy Provider Energy Provider Spent Energy Provider Spent Energy Provider Transported Molecule Transported Molecule 1234

Endocytosis (extracellular fluid) Pinocytosis vesicle containing extracellular fluid) (cytoplasm) cell (extracellular fluid) (cytoplasm) coated pit forming Receptor-mediated Endocytosis Phagocytosis coated vesicle nutrients receptors food particle pseudopod particle enclosed in food vacuole

Endocytosis Active process for movement of large molecules and organismsActive process for movement of large molecules and organisms –Substance is taken in by vesicle formed from cell membrane –Phagocytosis: solid substance in vesicle –Pinocytosis: liquid droplets in vesicle –Receptor-Mediated Endocytosis: incoming substance binds to receptor

Receptor-mediated Endocytosis I Begins with a shallow pit in plasma membrane Pit is coated with protein (cytoplasm) (extracellular fluid) coated pit protein coating extracellular particles bound to receptors plasma membrane Pit deepens a a b b 0.1 Micrometer

Receptor-mediated Endocytosis II Pit deepens further and begins to pinch off Eventually becomes a coated vesicle d d c c 0.1 Micrometer coated vesicle

Exocytosis Substance is expelled after being enclosed in a vesicle within the cellSubstance is expelled after being enclosed in a vesicle within the cell Used to move large molecules out of the cellUsed to move large molecules out of the cell

Exocytosis (cytoplasm) 1 Vesicle (extracellular fluid) plasma membrane 2 Secreted Material 3

Cell Attachment Structures Desmosome desmosome Protein strands holding cells together Protein filaments in cytoplasm Small intestine Plasma membrane (edge view) Cells lining small intestine Tight Junction Tight junctions formed by strands of protein Plasma membrane (edge view) Cells lining bladder Tight junctions seal membranes of adjacent cells together

Cell Communication Structures Gap Junctions desmosome Gap Junctions: pairs of channels connect insides of adjacent cells Liver Plasma membrane (edge view) Liver cells Plasmodesmata Plasmodesmata connect insides of adjacent cells Plasma membrane (edge view) Root cells Cell wall (edge view) Middle lamella (edge view) Root AnimalsPlants