Ch 7: Membrane Structure and Function

Selective permeability

the plasma membrane allows some substances to cross more easily than others

Amphipathic molecules

molecules that contain hydrophilic and hydrophobic regions (such as the phospholipids and proteins of the cell membrane)

Fluid mosaic model

the theory that a membrane is a fluid structure with a myriad of proteins embedded in it

Peripheral proteins

proteins not embedded in the cell membrane, but loosely bonded to the surface or exposed integral proteins

Integral proteins

• proteins that penetrate the hydrophobic interior of the lipid bilayer

• many are transmembrane proteins

• the regions in the hydrophobic interior of the membrane are nonpolar amino acids typically in alpha helices

• the exposed regions are polar

• some have a hydrophilic channel down the center allowing passage of substances

Transmembrane proteins

proteins that span the membrane (though some stop in the interior)

Glycolipids

• carbs bonded to lipids

• when present in membranes, carbs are short and branched

Glycoproteins

carbs bonded to proteins, which often are present in membranes

Six Major Membrane Protein Functions

1. cell-cell recognition

2. transport

3. enzymatic activity

4. signal transduction

5. intercellular joining

6. attachment to cytoskeleton and ECM

Transport proteins

• hydrophilic molecules avoid contact with the interior of the plasma membrane by passing through these proteins that span the membrane

• some are channel proteins - have hydrophilic channel

• some are carrier proteins - hold on to molecules and pass them through membranes

• specific to certain molecules

Aquaporin

channel proteins that regulate the passage of water through the plasma membrane in some cells

Diffusion

• the movement of molecules so that they spread out evenly in the available space

• caused by the constant motion/thermal energy of molecules

• a spontaneous process

Concentration gradient

• the region along which the density of a substance decreases

• the path diffusion will follow

Passive transport

the type of diffusion that does not require energy

Osmosis

the diffusion of free water across a selectively permeable membrane, causing solute concentrations to become equal

Tonicity

the ability of a surrounding solution to cause a cell to gain or lose water

Isotonic

• solute concentration is the same inside and outside of the plasma membrane

• no net water movement

Hypertonic (solution)

• the solute concentration is higher outside of the plasma membrane

• water leaves the cell, causing shriveling and possible death

Hypotonic (solution)

• the solute concentration is higher inside of the plasma membrane

• water enters the cell, possibly causing it to lyse (burst)

Osmoregulation

the ability of a cell to control solute concentrations and water balance

Turgid

• when a plant cell is very firm, which is healthy

• occurs in a hypotonic environment

Flaccid

• when a plant cell is limp

• occurs in an isotonic environment

Plasmolysis

• the plant cell shrivels and the plasma membrane pulls away from the cell wall

• occurs in a hypertonic environment

• can cause wilting or death

Facilitated diffusion

polar ions and molecules impeded by the plasma membrane diffuse passively with the help of transport proteins - channel or carrier

Ion channels

channel proteins that transport ions

Gated channels

channel proteins that open in response to stimulus - electrical or receptor based

Active transport

• pumping a solute against its concentration gradient, which requires energy (usually ATP)

• these transport proteins are all carrier proteins

• enables cells to maintain an internal concentration of solutes that differs from their environment

Sodium-potassium pump

• an active transport carrier protein

• it exchanges Na for K across the plasma membrane

• accepting the terminal phosphate group from ATP allows this protein to change shape

• exchanges 3 Na for 2 K, creating opposite charges

• electrogenetic pump

Membrane potential

• separation of opposite charges across the plasma membrane causes this

• acts like a battery

• cytoplasmic side is negative (anions)

• EC side is positive (cations)

Electrochemical gradient

• drives diffusion of ions across a cell membrane

• driven by two forces

  • chemical force - ion concentration gradient

  • electrical force - effect of membrane potential on ion movement

Electrogenic pump

a transport protein that generates voltage across a membrane

• sodium-potassium pump in humans

• proton pump in plants, fungi, bacteria

Proton pump

• actively transports protons out of the cell

• electrogenetic pump for plants, bacteria, fungi

  • contributes to membrane potential

Cotransport

when one ATP powered pump indirectly drives the active transport of several other solutes

Exocytosis

the process of the cell secreting biological molecules by fusion of vesicles with the plasma membrane

• transport vesicles from the golgi meet the plasma membrane

• proteins rearrange the lipids so the membranes fuse

• the contents of the vesicle leave the cell, and the vesicle becomes part of the plasma membrane

Endocytosis

the cell takes in biological molecules and particulate matter by forming new vesicles from the plasma membrane

• phagocytosis

• pinocytosis

• receptor-mediated endocytosis

Phagocytosis

a cell engulfs a particle by wrapping it in pseudopodia, forming a food vacuole

Pinocytosis

when a cell “gulps” droplets of extracellular fluid containing necessary molecules into tiny vesicles

Receptor-mediated endocytosis

• receptor proteins bind to ligands, triggering the coated pit area to collapse into a vesicle

• the ligands are released, then the vesicle recycles the receptor proteins back to the plasma membrane

Ligand

any molecule that binds specifically to a receptor site on another molecule

Transport

Enzymatic activity

Signal transduction

Cell-cell recognition

Intercellular joining

Attachment to cytoskeleton and ECM