Law_Sherrice_Block4_MM3

Cellular membranes are fluid mosaics of lipids and proteins

Phospholipid is amphipathic molecule

hydrophilic region

hydrophobic

Fluid mosaic model

membrane is fluid structure attached to bilayer of phospholipids

Sandwich model

phospholipids between heads (protein)

Membrane proteins are not very soluble bc amphipathic

Proteins are individually inserted into bilayer

Fluidity

Rarely flip-flop, but they move laterally fast

Membrane remains fluid as temp decreases until closely packed and solidifies

unsaturated=more fluid; saturated=viscous

Steroid cholesterol between phospholipids in animal cells

Warm: makes membrane less fluid

Lowers temp required for membrane to solidify

Temp buffer

Membrane Proteins

Proteins determine membrane’s specific functions

Integral proteins penetrate hydrophobic core of bilayer

Peripheral proteins are not in bilayer, loosely bound to surface of membrane

Functions

Transport

through hydrophilic channels or change shape (use ATP) to pump
Enzymatic activity: may be enzyme with active site

Signal transduction

binding site with shape; chemical messenger changes shape and relays message

Cell-cell recognition

glycoproteins serve as ID tags

Intercellular joining

adjacent membrane proteins hook together in junctions, like gap to tight

Attachment to cytoskeleton and ECM

microfilaments or cytoskeleton may be bonded to proteins that helps maintain cell shape and stabilize location

The Role of Membrane Carbs in Cell-Cell Recognition

Basis of rejecting foreign cells in immune system

Membrane carbs: short, branched chains, fewer than 15 sugar units

Covalently bond with lipids: glycolipid

Glycoproteins

Synthesis and Sidedness of Membranes

Synthesis of membrane proteins/lipids in ER: glycoproteins

Golgi: glycolipids

Transmembrane proteins, membrane glycolipids, secretory proteins are transported in vesicles to plasma


Vesicles fuse with membrane

Membrane structure results in selective permeability

Hydrophobic nonpolar molecules like hydrocarbons, CO2, O2 can dissolve in lipid bilayer

Transport Proteins

Hydrophilic pass through transport proteins

Channel proteins (hydrophilic channel)

aquaporins to help water transport

Carrier proteins: change shape

Passive transport is diffusion of a substance across a membrane with no energy investment

Thermal motion(heat): diffusion: spread out evenly

More concentrated to less concentrated

Diffuse down to its concentration gradient

Osmosis

Water diffuses from lower to higher concentration

Water Balance of Cells Without Walls

Tonicity (ability of a solution to cause a cell to gain or lose water)

Isotonic: stable, no net movement of water

Depends on non-penetrating solutes

Hypertonic(more non-penetrating solutes): lose water and shrivel

Hypotonic: water enter cell faster than it leaves and will burst

Osmoregulation: control of water balance

Water Balance of Cells with Walls

In hypotonic, plant cells will maintain water balance

Cell is turgid (very firm)=healthy state of plant cells

If plant’s cells and their surroundings are isotonic, no net tendency for water to enter, and cell becomes flaccid(limp)

In hypertonic, plasmolysis causes plant to wilt and can be lethal

Facilitated Diffusion: Passive Transport Aided by Proteins

Channel proteins allow water/small ion molecules to flow fast

Ion channels function as gated channels (stimulus of electric or chemical causes them to open or close)

Carrier proteins change shape from binding/releasing molecules

Active transport uses energy to move solutes against their gradients

Cell must use energy; uses carrier proteins

Allows maintaining internal concentrations of small molecules that differ from environment

Maintenance of Membrane Potential by Ion Pumps

All cells have voltages(electric potential energy) across plasma membrane

Cytoplasm is negative

Voltage across a membrane is membrane potential (-50 to -200 mV)

Acts like a battery

Electrochemical gradient (ion’s concentration gradient and effect of membrane potential on the ion’s movement)

Sodium-potassium pump: 3 sodium ions out and 2 potassium in

Transport protein generates voltage across a membrane is called electrogenic pump

Proton pump: transport protons out of the cell (plants, fungi, bacteria)

Cotransport: Coupled Transport by a Membrane Protein

Single ATP powered pump that transports a specific solute can indirectly drive the active transport of several other solutes

Proteins can move sucrose into cell against concentration gradient WITH a hydrogen ion

Plants use this process to load sucrose by photosynthesis in specialized cells

Found treatments for dehydration from diarrhea

High concentrations of glucose and salt

Solutes taken up by transport proteins on intestinal cell surface and pass through cells into blood

Osmotic pressure causes a flow of water from intestine through cells into blood

Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

Exocytosis

Cell secretes macromolecules by fusion of vesicles with plasma

Endocytosis

Cell takes in macromolecules and matter by forming new vesicles from plasma membrane

Phagocytosis (cellular eating)

Cell engulfs particle by wrapping pseudopodia around it and packaging it within a membrane closed sac large enough to be a vacuole; particle is digested after vacuole fuses with lysosome containing hydrolytic enzymes

Pinocytosis (cellular drinking)

Cell “gulps” droplets of extracellular fluid into tiny vesicles

All solutes are taken in so nonspecific

Receptor-mediated endocytosis

Enables cell to store bulks even though not very concentrated in extracellular fluid

Receptor proteins are clustered called coated pits

Extracellular substances (ligands) bind to receptors and pit forms vesicle

Receptors are recycled back by same vesicle

Human cells use receptor-mediated endocytosis to take in cholesterol to synthesize membranes and as a precursor for synthesis of other steroids

Cholesterol travels in particles called low-density lipoproteins (LDL) that act as ligands by binding to receptor and enter cell by endocytosis