Membranes(1)

Membrane Structure and Function

Overview of Plasma Membrane

• Plasma Membrane: Serves as the barrier separating the cell from its environment.

• Selective Permeability: The membrane's ability to allow certain substances to pass through more easily than others.

Composition of Plasma Membrane

• Phospholipids: Key component, comprising a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails.

  • Structure: Consists of glycerol, phosphate groups, and fatty acid chains.

  • Amphipathic Molecules: Possess both hydrophilic and hydrophobic regions.

Phospholipid Bilayer

• Forms a stable boundary between two aqueous compartments.

• Hydrophilic Heads face outward towards water, while Hydrophobic Tails face inward, shielding themselves from water.

Membrane Fluidity

• Membranes are described as fluid mosaics of lipids and proteins.

• Fluidity: Essential for proper functioning; membranes should be about as fluid as salad oil.

  • Fatty Acid Composition: Membranes rich in unsaturated fatty acids exhibit greater fluidity.

• Phospholipids can move within the bilayer, with rapid lateral diffusion but rare flip-flops.

Effects of Temperature on Membrane Fluidity

• Lower temperatures cause membranes to solidify; solidification temperature varies with lipid types.

• Cholesterol: Modulates fluidity; it restrains phospholipid movement at high temperatures and prevents tight packing at low temperatures.

Membrane Proteins

• Membrane comprises various proteins including peripheral and integral proteins.

  • Peripheral Proteins: Bound to the surface of membranes.

  • Integral Proteins: Span the membrane; transmembrane proteins extend across it with hydrophobic regions.

  • Called Tran membrane proteins

  • Non polar environment where the tails are in a hdrpohobic region

  • 6 MAJOR Functions of Membrane Proteins: Include transport, enzymatic activity, signal transduction, cell-cell recognition, intracellular joining, and attachment to the cytoskeleton and ECM.

Cell-Cell Recognition

• Cells recognize each other through carbohydrate molecules on the extracellular surface of the membrane (glycolipids and glycoproteins).

  • These are covalent bonds between lipids and proteins

• Carbohydrate variations can determine blood types and cell identity.

  • vary among species, individuals, and cell types in individuals

Membrane Sidedness

• Membranes have distinct inside and outside faces, influencing their function and interactions.

• Inside and outside have different functions

• Asymmetrical distribution of components established by the ER and Golgi apparatus during synthesis.

Membrane Permeability

Plasma membrane controls cells exchanging materials and removing things

• Hydrophobic Molecules: Can rapidly cross membranes (e.g., O2, CO2).

  • (Nonpolar molecules) and usually gasses

• Hydrophilic Molecules: Require transport proteins to cross membranes; do not easily diffuse through lipid bilayers.

  • Polar molecules and ions (cannot get through)

    • Selectively permeable regulations the cells hydrophilic - lets some substances through and not others

Transport Proteins

Allow passage of hydrophilic substances across membrane

• Channel Proteins: Provide corridors for specific molecules or ions (e.g., aquaporins for water).

  • Hydrophilic channels

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

  • Specific for the substance that its moving

Passive Transport

All molecules and atoms are in constant random motion

• Diffusion: Movement toward equilibrium, across the membrane without energy expenditure.

  • Caused by random motion

  • Can be directed when there is a membrane with holes “net diffusion”

• Dynamic Equilibrium

  • So many molecules cross the membrane in on direction as in the other. Basically the same amount on both sides of something

• Substances diffuse down their concentration gradient

  • No work involved, molecules are moving by themselves

    • Called passive transport when moving across the cell membrane

• Osmosis: Diffusion of water across a selectively permeable membrane, moving from region of low solute concentration to high.

  • Does this until both sides have the same solute concentration

Water Balance in Cells

• Tonicity: Refers to solution concentration relative to the cell;

  • Isotonic: No net water movement, same

  • Hypertonic: Cell loses water, water is greater than inside the cell, cell shrivels up

  • Hypotonic: Cell gains water, cell expand s

• Cell walls maintain water balance,

  • Can be turgid (stiff) or (limp)

Facilitated Diffusion

• Channel proteins provide corridors that allow specific molecules tar ion to cross the membrane

  • Aquaporins facilitate the diffusion of water

• ION CHANNELS facilitate the diffusion of ions

  • Some ions channels are called gates channels , open or close in response to a stimulus high can be electrical or the the binding of a molecule

  • Move through channels/tunnels

Active Transport

• Moves substances against concentration gradients, requiring energy, usually from ATP.

• Example: Sodium-Potassium pump maintains concentration gradients for cellular activities.

  • Moves 3 positive charges out and bringing only two in

• Carries out by specific proteins

• Uses energy, normally ATP

• Allows cell to maintain concentration gradients that differ from their surroundings

  • Ions bin to transport proteins, atp moves it and changes it shape so it can be released, different ions wil bind to the same protein, and put it back in the cell, making it the shape of the ions that left

Membrane Potential

• More positive on onside than the other scores a membrane by difference in distribution of position and negative ions

• Because of this we have the electrochemical gradients

  • Drives the diffusion of ions across a membrnce

    • By chemical force (ion concentration gradient)

    • An electrical force (the effect of the membrane potential on the ion’s movement)

• Any transport protein that generate voltage across the membrane is called an electrogenic pump

  • Sodium potassium pump is major pump

  • Main pump of plants fungi and bacteria is a proton pump

Bulk Transport

How do the big molecules like proteins and polysaccharides get across the membrane

• Exocytosis: Transport vesicles fuse with the membrane to release contents outside the cell.

  • Requires energy

• Endocytosis: Cell takes in macromolecules by forming vesicles; includes phagocytosis (cellular eating), pinocytosis (cellular drinking), and receptor-mediated endocytosis.

• Phagocytosis is eating, absorbing food in a food vacuoles

• Pinosytosic creates coated proteins Coated pits and coated vesicles

• Receptors use receptors

  • Bad things can take advantage of this like viruses