BI 256: 10/cell membranes

What is a polar molecule?

has unequal sharing of electrons & charge distribution

ex- H2O

What is a nonpolar molecule?

has equal sharing of electrons, no partial sharing

ex- HC

What are the 4 weak bonds seen in cell biology?

• hydrogen bonds

• van der waals interactions

• hydrophobic interactions

• ionic bonds

Hydrogen bond

partial positive of H is attracted to another partial change

Hydrophobic Interactions

nonpolar pushed together from polar interactions, avoid polarity/water

Ionic Bonds

weak in water/cell, dissociate into ions when hydrated

If there are several weak forces, how do activities occur?

summation of weak interactions

What is the basic composition of the cell?

70% water, 30% chemicals

What does the 30% chemicals include?

sugar, 15% proteins, 7% DNA & RNA, ions

What are the main ions seen in the cell?

sodium, potassium, magnesium, calcium, chloride

How are peptide bonds formed?

condensation, removal of water for amino & carboxyl end joining

What are proteins?

long polypeptides held with peptide bonds

What are condensation reactions?

removal of water to join carboxyl & amino ends, energetically unfavorable

What is hydrolysis?

adding water back, breaking protein & peptide bond, energetically favorable

What determines protein shape?

amino acid sequence, folding into the lowest energy configuration

How does polarity affect how proteins fold?

polar side chains out to hydrogen bond with water, hydrophobic core inside

What are fatty acids?

long hydrocarbons with carboxylic group at the end

What determines the identity of fatty acids?

length of hydrocarbon chain

What does it mean to be unsaturated?

more than 1 double bond in tail, kink in chain from double bond, rigid, cis configuration

What does it mean to be saturated?

have no double bond, linear

What are trans fatty acids?

unsaturated fats with a forced configuration, flattened out and industrially made, made with high energy hydrogenation to harden at room temperature

How to unsaturated fats help in the cell?

in membranes, kinks leave space for more fluid/space

What are omega fatty acids?

unsaturated fatty acids, omega is for location of the double bond counting from methyl

Which omega fatty acids are good & bad?

Omega 3 is good, omega 6 in excess inflames gut/intestine & is in highly hydrogenated food

What makes up eukaryotes?

intracellular compartments in membrane

Why do we need a membrane?

cells are water inside & surrounded by it, would be a mess

What is the fluid mosaic model?

membrane is fluid with several components, 50/50 lipids and proteins

What are the 3 types of membrane lipids?

phospholipids, sterols, glycolipids, all amphipathic

What are phospholipids?

most abundant type of membrane lipid, commonly phosphoglyceride

Amphipathic

1 polar end and 1 nonpolar end

What determines membrane structure?

phospholipid structure, depends on presence/absence of glycerol

What can and cannot change in membrane lipids?

change: polar head & tails

not change: phosphate (on 3rd C of →) glycerol

What are the 2 kinds of phospholipids?

phosphoglycerides & sphingolipids

Phosphoglycerides

most common in animal cells

What is the difference between phosphoglycerides and sphingolipids?

phosphoglycerides are built on a glycerol backbone, while sphingolipids are built on a sphingosine backbone

Sphingolipids

phosphocoline head group, different core structure, built on sphingosine backbone

How do phospholipids arrange themselves?

self assemble in water, arrange so hydrophobic tails avoid water

Micelle

oval shape, polar out & nonpolar core, have 1 head group but 1 tail, single layer ball, soaps

What is a liposome?

lipid bilayer forming in on itself to seal layer, cell

What determines the difference between micelles & liposomes?

tail number to determine arrangement

What is the difference betwen liposomes & micelles?

micelle has 1 layer & a hydrophobic/small core, liposomes have 2 layers & a spacious/aqueous core

What is the effect of soap on the cell membrane?

micelle/soap captures lipids & proteins & breaks apart cell membrane

What happens if there is a small/medium tear in the membrane?

membrane will heal itself due to phobic parts trying to avoid water and seal

What are the ways that phospholipids move around?

• rotation around axis

• flexion/moving tails

• lateral diffusion/sliding on 1 plane

• flip flop (most rate, catalyzed w/ enzyme to pull through opposing forces)

What 3 things impact membrane fluidity?

saturation of hydrocarbons, temperature, chain length

How does saturation affect fluidity?

more saturation = more tightly can pack, decreased fluid, straight & hydrogenated

How does temperature impact fluidity?

temperature increase increases movement of tails, cold makes it gel & less fluid

How does wheat stay alive in the winter?

increase fatty acid tail unsaturation to keep fluid, more kinks = more space

How does chain length impact fluidity?

longer tails decrease fluidity with interactions

shorter tails have less interactions & increase fluidity

What membrane component distinguishes prokaryotes & eukaryotes?

cholesterol

eukaryotes HAVE cholesterol

prokaryotes/bacteria DO NOT have cholesterol

How is cholesterol in membranes?

packs between phospholipids, raises melting point at high temps & prevents freezing at low temps, polar head and stiff tail

Which will increase membrane fluidity at colder temperatures?

A. longer fatty acid tail lenghts

B. increased cholesterol content

C. decreased amount of unsaturated fatty acids

D. all of the above

B. increased cholesterol content

What is FRAP & why is it used?

fluorescence recovery after photo bleaching, used to see motility and fluidity

How does FRAP work?

label lipids and proteins, mainly membrane, bleach with a laser after a fluorescent dye, look for it to come back, to show motility

What can membranes form?

distinct domains, microdomains in membranes

How do membranes form microdomains?

liposome forms domains within itself with cholesterol added

What are lipid rafts?

domains within the cell membrane, enriched with cholesterol, clustered to do something in the cell

What will cluster in lipid rafts?

specific membrane proteins & lipids, cluster to work in signaling or transport

How is cholesterol like an amazon distribution center?

makes sure everything is together in a patch of membrane for efficient tasks and quick output

What is the fluidity like in the lipid raft?

decreased fluidity in the raft to stay together, but whole raft can move along the membrane freely

How is the lipid bilayer asymmetrical?

happens in membrane synthesis, things stay on cytosol or extracellular side

Why are things asymmetrical on the lipid bilayer?

allows things to interact separately, keep things in in, and out outside

How do proteins stay asymmetrical?

extra/intracellular proteins have specific bindings sites for particular phospholipid head groups

What does membrane asymmetry distinguish?

live and dead cells

What is PS?

phosphatidylserine, found on inner layer/cytosol side, used in immune system

What happens to PS when the cell undergoes apoptosis?

PS flips outside through the work of enzymes since it’s energetically unfavorable, signals for phagocytosis

Which enzymes are involved in apoptosis to help PS?

flippase which flips things correctly is inactivated, scramblases turn on to flip PS for signaling

Which parts of the membrane point out?

glycolipids & glycosylated membrane proteins

Glycolipids

sugar groups covalently attached to membrane phospholipids, help in cell-cell recognition

Where are sugars usually located in relation to the cell?

any sugar or carb is always outside the cell, cells usually sugarcoated

Which microbe takes advantage of glycolipids & why?

cholera, wants to get inside cell

How do membrane proteins control cellular access/activity?

cell controls fluidity of membranes & proteins using tis own proteins

Which proteins help control the fluidity of the membrane?

transporters, anchors, receptors, enzymes, make up 50% mass in membrane

What are th 2 main ways membrane proteins associate with the membrane?

integral membrane proteins & peripheral membrane proteins

What are integral membrane proteins?

embedded into the membrane, include transmembrane, monolayer associated, lipid-linked

Transmembrane Proteins

alpha helixes, multi pass/7 times, threaded through membrane by hydrophobic/nonpolar interactions in core of bilayer

Transmembrane Porins

beta barrels that form channels, allow water & small molecules to pass in/out of cell, beta sheets have nonpolar amino acids interacting with bilayer, water/other things can go through barrel, transport water in/out

Monolayer Associated Proteins

embedded on 1 side of bilayer, don’t pass whole way through

Lipid-Linked Transmembrane Proteins

proteins covalently attached to lipids in the membrane

Peripheral Membrane Proteins

can be protein attached, usually associated with other proteins embedded, attached with H bonds, hydrophobic, electrostatic forces, dipole/vanderwaals

T or F: peripheral membrane proteins are weakly associated with the membrane

true

Membrane Protein Topology

hydropathy plot tells if an amino acid/protein part is hydrophilic/phobic,

polar region is negative, nonpolar region is positive

How are amino acid side chains used in membrane protein topology?

chemistry is used to predict hydrophobicity & chance of location

T or F: some membrane proteins have limited movmeent

true, not all are fully motile

How are membrane proteins limited?

• self assembled aggregates needed to stay together

• tethered to intracellular molecules inside

• tethered to extracellular matrix making up tissue

• attached to proteins on other cells/tissue

What does restricting movement do>

restricting movement marks membrane domains, specialized areas of cell membranes

Epithelial Cells

restrict proteins with different functions, tight junction to restrict food & transport when necessary

What is a polar cell?

2 different poles, skin cell, 1 side doing 1 job, other side is doing another job

What is a non-polarized cell?

has no poles, RBC, has no sidedness to it, unilateral function, WBC, all areas of cell membrane are doing the same job