Plasma Membrane and Membrane Potential

All cells are surrounded by a

plasma membrane

Functions of a plasma membrane

defines the cell boundary and separates it from the environment

is a selective barrier, and determine the composition of the cytoplasm

mediates interaction between the cell and its environment

The functional structure of the membrane is the

phospholipid bilayer

____ are embedded in the bilayer to carry out ____

proteins; specific functions, including selective transport of molecules (like ions Na+ and K+) and cell-to-cell recognition

Phospholipid bilayer is made up of

a hydrophilic head and a hydrophobic tail

Lipid composition of the plasma membrane

five major phospholipids, cholesterol, and glycolipids

Cholesterol function in the plasma membrane

maintain membrane fluidity

Peripheral membrane proteins

associated with membranes though protein-protein interactions; often ionic bonds

Integral membrane proteins

inserted into the lipid bilayer; they can be dissociated only by reagents that disrupt hydrophobic interactions

Transmembrane proteins

integral proteins that span the lipid bilayer with portions exposed on both sides

Fluid Mosaic Model

The mobility of many plasma membrane proteins is restricted by association with the cytoskeleton or specialized lipid domains

Transmembrane proteins anchored to the cytoskeleton have

restricted mobility and may also act as barriers that limit mobility of other membrane proteins

Lipid rafts

are transient structures in which specific proteins can be concentrated to facilitate interactions

Plasma membranes are ___ ____ to small molecules

selectively permeable

Specific transport and channel proteins mediate

passage of glucose, amino acids, and other small molecules and ion

Facilitated Diffusion

direction of movement is determined by concentration gradient; no energy required

Facilitated diffusion is mediated by

proteins

Proteins in facilitated diffusion allows

polar and charged molecule to cross the plasma membrane

Carrier proteins

bind molecules on one side of the membrane, then undergoes conformational changes that allow the molecule to pass through and be released on the other side

Channel proteins

form open pores through the membrane, allowing free diffusion of any molecules of the appropriate size and charge

Porins

type of channel protein that form open pores in the membrane that allow molecules to pass freely

Aquaporins

allow water molecule to cross the membrane rapidly while being impermeable to charged ions; allowing the passage of water without affecting the electricalchemical gradient

Ion channels

rapid transport of highly selective ions, most have “gates” that open only in response to a specific stimuli (binding of a ligand, change in membrane potential)

Ion channels are seen to be responsible for

transmission of electrical signals with their opening and closing

Ligand-gated channels

open in response to binding of neurotransmitters or other signaling molecules

Voltage-gated channels

opens in response to changes in electric potential across the plasma membrane

The role of ion channels in transmitting electric impulses was first shown in

giant squid axons

Leakage channels

open at all times and are selective to an ion

Ion pumps

uses ATP hydrolysis to actively transport ions across the plasma membrane to maintain a concentration gradient

True/False: the ionic composition of the cytoplasm is substantially different form that of the extracellular fluid

True

Because ions are charged, pumping results in

electric gradient across the plasma membrane

There is higher concentration of K+ in the

intracellular

In the Na+/K+ pump the movement of the ions is

3 Na+ out and 2 K+ in

What makes the largest contribution to resting membrane potential?

K+ flow; though the open/leak K+ channels

Active transport

molecules move against their concentration gradients by using ATP or energy from coupled transport of another molecule in the energetically favorable direction

Gradients established by Na+/K+ and H+ pumps provide

a source of energy for active transport

Na+/K+ pumps operate by

ATP-driven conformational change

Glucose transporters

transport 2 Na+ and one glucose into the cell; the flow of Na+ down its gradient provides the energy that drives the uptake of glucose against its gradient

Gradients are necessary for

propagation of electric signals in nerve and muscle cells, to drive active transport of other molecules, and to maintain osmotic balance and cell volume

Hypertonic

more solute in the extracellular; cell shrivels

Hypotonic

more solute in the intracellular; cell burst

Isotonic

equilibrium of solutes on both sides

Vesicles

membraneous sacs that allows for the transport of large particles, macromolecules, and fluid across the membrane

True/False: Vesicular transport does not require cellular energy

false

Exocytosis

transport out of the cell

Examples of Endocytosis

phagocytosis, pinocytosis, receptor-mediated endocytosis

Transcytosis

transport into, across, and then out of the cell

Vesicular trafficking

transport from one area or organelle in a cell to another

Secretory pathway

protein made in ribosomes, enter into ER, packed into vesicles, transported to golgi apparatus, is modifed and packaged, fuse with membrane, and released

Endocytosis

allows cells to take up macromolecules, fluids, and large particles like bacteria

Process of endocytosis

The material is surrounded by an area of plasma membrane, which buds off inside the cell to form a vesicle that contains the ingested material

Phagocytosis

occurs in specialized cell types; cell eating

Process of Phagocytosis

The binding of a particle to a receptor on the cell surface triggers the extension of pseudopodia, which surround the particle and fuse to form a large vesicle called a phagosome that fuse with lysosomes (phagolysosomes) where the material is digested by acid hydrolases

Lysosome

vesicle that contains digestive enzymes

Phagocytosis in multicelluar animals are a

It defends against invading microorganisms and helps eliminate aged or damaged cells

Autophagy

destroys aged or faulty cells to recycle material/energy for new cells; in times of starvation, nonessential macromolecules are also reutilized