CH 6: B Membrane Components, C. Fluidity, Permeability

Plasma membrane

the structure that separates the cell from its environment.

it has selective permeability: meaning it can choose what enters and exits the cell

Structure of the plasma membrane

made of a phospholipid bilayer

(heads face outward, tails face inward to each other)

there are also other proteins and carbohydrates among the bilayer

Why is the plasma membrane a “mosiac”

because among the membrane there are glycoproteins, glycolipids, cholesterol, peripheral proteins, integral proteins, etc working to help membrane

What hold phospholipid bilayer together?

Van der waals interactions weakly bond fatty acids in the bilayer

this weak bond allows the lipids to move, giving membrane a fluid like consistency

Phospholipid lateral movement

phospholipids moving laterally side to side as a whole

What is flip flopping of phospholipids

rare movement of phospholipid where the head+tails switched around with a counter side head+tail

Why flip flopping infrequent if the membrane heads are polar on both sides?

since the phosphate heads are hydrophilic and the lipid regions are hydrophobic

head would have to pass through hydrophobic region which their molecules want to avoid

Types of membrane proteins

Peripheral proteins

Integral proteins

Transmembrane proteins

Peripheral proteins

proteins on the surface of the membrane

can be binded to the head of phospholid or another protein

Integral proteins

protein that can penetrate membrane, and interact with the hydrophobic lipids

Transmembrane proteins

protein that pierces through the membrane completely (through head and tail) - special type of integral protein

has one portion in the cytoplasm (inside cell)

other portion in the membrane

other portion outside the cell

How are proteins anchored in / on bilayer?

the membrane is made out of hydrophilic and hydrophobic parts, but so are the proteins

you can have protein with both qualities (hydrophilic on ends, hydrophobic in between)

Functions of membrane proteins

Transport

Enzyme Activity

Signal Transduction

Cell-cell Recognition

Intercellular joining

Attachment to cytoskeleton and extracellular matrix

Transport

(membrane protein)

allows certain substances to pass in and out the the membrane

affecting membrane permeability

Enzyme Activity

(membrane protein)

increases the speed of chemical reactions (catalysts)

Signal Transduction

(membrane protein)

uses receptor proteins that can detect signaling molecules like hormones

signaling molecules bind to receptors and send signal (info) inside the cell (so it can perform a specific task)

Cell-cell Recognition

(membrane protein)

when glycoproteins recognize the carbohydrate tag/chain sticking out of other cells, and signals for cell-cell interaction

Intercellular joining

(membrane protein)

the proteins on membranes that hold/connect cells together

Attachment to cytoskeleton and extracellular matrix

(membrane protein)

attachment proteins which attach to a cytoskeleton (inside cell) and then to an extracellular matrix (material outside of cell)

creating a firm structure/support to both

Carbohydrate chains in cell membrane

carbohydrate chains which are covalently bonded to proteins or lipids in the membrane creating either glycoproteins or glycolipids

they can stick outside the cell helping cell-cell recognition

What affects the membrane’s Permeability and Fluidity?

since the membrane is fluid mosaic its very much shaped by its composition and environment

and can change depending on its environment, composistion

What is Membrane fluidity? + importance

refers to the viscosity of the membrane which molecules move through

the fluidity depends on structure of phospholipids, cholesterol in the membrane, temperature

its important because the thinness or thickness affects how proteins, molecules diffuse in the cell which affects how they function

What is membrane Permeability?

the rate at which molecules passively diffuse across the membrane.

ie. how easily they pass through barrier (but selective)

fluidity affects what can pass through and

• transport proteins also choose what can pass through despite membrane’s natural permeability

Temperature’s effect on membrane’s Fluidity

when the temperature is higher it vibrates phospholipids, liquefying the membrane, making it more permeable

when its lower it compresses the membrane, making it less permeable and at risk of rupture (brittle)

thus med temp is ideal (too high cant hold structure)

Cholesterol in the membrane

Cholesterol is used in animal cells, it has hydrophilic hydroxyl group and hydrophobic carbon rings (so amphipathic)

its different regions can thus connect in between phospholipids (OH on head, carbon on tail)

Why is Cholesterol important for membranes? (normal, cold)

at normal temps: connection between phospholipids retrains movement, and thus permeability

at colder temps: the connection between phospholipids prevent compacting, and increases permeability (helping it stay at ideal)

Fatty Acid chain’s effect on membrane fluidity

(C chains and double bonds)

length of fatty acids carbon chains and the number and location of double bonds decide lipid’s fluidity and thus the membrane

length of C chains determines how long the phospholipid, making a thicker membrane = less fluid = less permeable, and opposite = more permeable

unsaturated: more double bonds means kinks in the phospholipid, which when making up a membrane leaves many gaps = more fluid

saturated: less or no double bonds, means no kinks, and phospholipids making membranes sit straight leaving no gaps. This tighter arrangement = morev iscious = less permeability

Review: what has a direct impact on membrane permeability

Transport proteins do. They can allow ions and other necessary ions which cannot permeate the membrane enter. and allow for molecules to leave