Cell Bio - Membranes

What is the first function of the cell membrane (define)?

define boundaries of cells and organelles and act as permeability barriers

• plasma membrane surrounds the cell and are permeability barriers because

  • membrane interiors are hydrophobic, restricting the passage of polar molecules and ions

  • membranes contain transport proteins that allowing for selective movement of specific molecules

What is the second function of the cell membrane (local)?

localize specific biological functions

membranes associated with specific functions bc molecules responsible are embedded or localized on membranes

What is the third function of the cell membrane (regulate)?

regulate movement of substances into and out of cells and organelles via membrane proteins

membrane proteins carry out and regulate the transport of solute and solvent molecules across the membrane

substances that cannot move directly through the lipid bilayer and must be moved by specific transporter proteins

What is the fourth function of the cell membrane (detect)?

detect and transmit electrical and chemical signals to allow cellular responses

cells receive information at its surface from its environment as chemical or electrical signals

signal transduction: mechanisms by which signals are transmitted from outer surface to interior of cell

some chemical signals bind to membrane proteins

What is the fifth function of the cell membrane (mediate)?

mediate cell-to-cell adhesion and communication

many cell-to-cell contacts are mediated by cadherins (proteins that bind calcium and promote similar types of cells to adhere)

junctions between animal cells:

• adhesive junction: hold cells together

• tight junction: form seals between cells

• gap junction: allow for communication and exchanges between adjacent animal cells

What is a model that describes the membrane structure?

fluid mosaic model envisions membrane as a fluid bilayer of lipids with proteins embedded within and on the surface (cytosolic or extracellular facing) of the bilayer

What lipids are the major lipids in cell membranes?

phosphoglycerides, sphingomyelin, glycolipids, sterols (cholesterol)

Discuss what phosphoglycerides are in the cell membrane.

phosphoglycerides are the most abundant lipids in cell membranes. they consist of two fatty acids attached to glycerol with a phosphate and polar head group also attached

fatty acid tails are ~14 to 24 carbons with one chain saturated and one not

longer and saturated fatty acids pack more tightly and are less fluid at any temperature

phosphatidylethanolamine, phosphatidylserine, and phosphatidylcholine are the most abundant phosphoglycerides in animals cells

• phosphatidylserine has a negative charge on polar head (with a positive charge) while the other two have only a positive charge

Discuss what sphingomyelins are in the cell membrane.

sphingomyelin (a type of phospholipid) are based on sphingosine (which has a hydrocarbon chain, an amine group, and a hydroxyl)

Discuss what glycolipids are in the cell membrane.

glycolipids have small carbohydrate groups attached to sphingosine and lack phosphates

major glycolipids include cerebrosides (uncharged carbohydrate) and gangliosides (negative carbohydrate)

glycolipids occur largely on the outer monolayer of the plasma membrane

Discuss what sterols are in the cell membrane.

eukaryotic membranes contain sterols (cholesterol in animals)

planar ring structure that is more rigid than other membrane lipids, with a polar head group and a hydrocarbon tail

How are bilayers formed?

phospholipids spontaneously form bilayers because its energetically favorable for its hydrophobic regions to clump together and be covered by its hydrophilic regions

amphipathic detergents (1 fatty acid tail) form micelles (sphere)

phosphoglycerides (2 fatty acid tails) form bilayers that spontaneously form vesicles

How do bilayers act?

phospholipids diffuse freely within single layer but flipping between layers is energetically unfavorable and rarely happen without flippase

polar head groups attract hydration shell, resulting in bilayers not readily merging

What does the fluidity of bilayers depend on?

membranes function properly only in the fluid state.

membrane fluidity decreases as temperature falls and increases as temperature rises

• every lipid bilayer has a transition temperature Tm, below which its in gel phase (ordered and not fluid), above which its in a liquid-crystalline phase (disordered and fluid)

  • organisms subject to significant temp changes use homeoviscous adaptation to adjust membrane properties to maintain fluidity (shorter saturated fatty acids=more fluid, longer unsaturated fatty acids=less fluid)

cholesterol also alters membrane properties

• it reduces permeability and acts as a fluidity buffer, increasing fluidity at low temps and decreases fluidity at higher temps

Discuss how lipid bilayers form domains of different compositions.

increased cholesterol, glycolipids, and longer fatty acid tails produce a region with thickened more gel like monolayer called lipid rifts. lipid rifts can be associated with signaling mechanisms and some forms of membrane transport.

How can membrane fluidity be demonstrated?

FRAP (fluorescence recovery after photobleaching) can be used to demonstrate membrane fluidity. a laser bleach dyes the fluorescent dyed membrane and its observed to see how long it takes for the dark spot to disappear.

In the plasma membrane, are the lipids or proteins on both sides of the membranes the same?

All membrane proteins will have a specific orientation in the membrane.

glycolipids are found on the extracellular surface. glycosylation occurs in the Golgi apparatus which explains its orientation

How are membrane proteins associated in the lipid bilayer?

Integral membrane proteins: have hydrophobic regions that pass through or into the hydrophobic center of membrane

peripheral membrane proteins: do not enter hydrophobic center of membrane

lipid anchored membrane proteins: have covalently attached lipid groups that anchor protein to membrane

• lipid anchors control the membrane localization of some signaling proteins

What is the orientation of transmembrane proteins?

transmembrane proteins’ orientation is established in the rough endoplasmic reticulum

the membrane spanning region is most commonly a-helix made of hydrophobic amino acid residue (still has polar backbone)

if transmembrane protein segment is b-sheet, it passes through membrane multiple times forming b-barrels. b-barrels most form pores or channels

hydropathy plots

positive = hydrophobic regions, negative = polar regions. number of separate hydrophobic regions is number of times protein passes through membrane.

What is true of many membrane proteins (and integral membrane proteins)?

many transmembrane proteins are: glycosylated and stabilized by disulfide linkage on the noncytosolic side of membrane (bc most glycosylation occurs in golgi apparatus and bc reducing conditions inside of cytosols prevent disulfide bonds

integral membrane proteins are usually not stable away from membranes so isolating requires detergents (one fatty acid chain lipid)

Can cells confine membrane molecules to specific domains?

cells can confine proteins and lipids to specific domains through tight junctions that restrict diffusion of molecules and divide membrane into apical and basolateral domains with proteins and lipids differences

What can affect membrane shape?

the cortical cytoskeleton gives membranes mechanical strength and restricts protein diffusion (shape of red blood cells membrane comes from its interaction with the cytoskeleton)

In what case do phospholipids form a monolayer?

intracellular neutral lipid droplets are covered by phospholipid monolayer