Bio130 (weeks 7-9)

True or False: the membrane is self-healing

true

how many membranes do bacteria have

a single membrane

what are cell membranes composed of

lipids and proteins

what are most abundant lipids

phospholipids

other than phospholipids what are the other two molecules which are also amphipatic

cholesterol and glycolipids

Why does the membrane form a lipid bilayer

because it satisfies the amphipatic properties and is energetically favourable, which also allows for the membrane to self-heal (since tears are unfavourable)

what are liposomes

artificial spherical vesicles formed by pure phospholipids when added to water

what lipid movement is very rare to occur without the presence of a protein to facilitate it?

flip-flop

What are all the common lipid movements

1. lateral diffusion

2. rotation

3. flexion (wobbling)

4. flip-flop (move from one leaflet to another)

at what temperature is the membrane more fluid

higher temperatures

what does fluidity of a cell membrane mean?

the ease with which its lipid molecules move within the plane of the bilayer

what increases the fluidity of the membrane?

• shorter hydrocarbon tail (since they have a lower tendency to form van der Waals interactions, making them weaker)

• higher number of unsaturated tails, which would form kinks, making it hard to pack together more closely

how do bacterial/yeast cells moderate their membrane fluidity

they modify the length and saturation of the hydrocarbon tails depending on temperature

how do animal cells moderate the membrane fluidity

by the inclusion of the “sterol” cholesterol

how does cholesterol reduce fluidity

they fill up the spaces made by the kinks, stiffening the membrane, and also making it less flexible and permeable

who were the two scientists that came up with “fluid mosaic model”

Singer and Nicolson

what is another name for the single layer of the bilayer

leaflet

what are the three main types of lipids

phospholipids, glycolipids and sterols

true or false: membranes are only composed on 1 type of lipid

false

what type of phospholipids have a glycerol backbone

phosphoglycerides

what is the typical structure of a phosphoglyceride

• polar head group (phosphate and different groups)

• glycerol (3 carbon)

• hydrocarbon tails (usually 2)

what is the most common sterol found in animal cells

cholesterol (can be up to a 1:1 ratio with phospholipids)

what is the structure of cholesterol

polar head group, rigid planar steroid ring, nonpolar hydrocarbon tail

where is new phospholipids synthesized

in the cytosolic surface of the ER by enzymes which use fatty acids as substrates

how is the phospholipids distributed in the ER

the protein called scramblase removes randomly selected phospholipids from one half and transfers them to the other

how is the asymmetry of the membrane produced

In the golgi protein/ transporter called flippases, move specific phospholipids and proteins to specific leaflets using ATP

in which half are glycolipids added

only the non-cytosolic layer

what percentage of the membrane mass is protein

50%

what are the two types of membrane proteins called

• integral membrane proteins

  • ex. transmembrane, monolayer associated proteins and lipid-linked proteins

  • directly attached to the lipid layer

  • use detergents to extract

• peripheral

  • do not insert in the membrane, non-covalently attached to the membrane

  • use other gentle extraction methods

what are transmembrane proteins

proteins which pass through the membrane at least once, and are amphipathic

in a membrane spanning alpha helix, how many hydrophobic amino acids are present

20-30

what are the examples of the membrane spanning domains

single alpha helix, multiple alpha helices, beta barrel

how can you identify the structures of these membrane spanning proteins

1. x-ray crystallography

2. hydrophobicity plots

on which side are monolayer-associated membrane proteins facing

cytosolic

what type of anchor is used for protein (lipid linked) linked on non-cytosolic side?

GPI anchor

what type of anchor is used for protein (lipid linked) linked on cytosolic side?

lipid anchor (fatty acid and prenyl)

what is a typical structure of a detergent

a single hydrophilic head and tail

what is the full form of FRAP

fluorescence recovery after photobleaching

what happens in photobleaching

protein is fused with GFP or labelled with fluorescent antibody, an area is photobleached and the recovery is measured

why is a lipososme impermeable

since it is artificial lipid bilayer with no proteins to facilitate transport

what does it mean to be permeable

movement via simple diffusion through the lipid bilayer

what are the two more permeable molecules

small non-polar, and small uncharged polar (key: some larger uncharged polar may diffuse)

what are typically impermeable

ions (always) and larger uncharged polar

What are the two types of membrane proteins

channel and transporter (aka carrier)

what is the major difference between channel and transpoters

channels will bind very weakly and undergo very small conformational changes, will this is the complete opposite for transporters

are channel mediated transport active or passive

only passive

what is the electrochemical gradient

concentration + membrane potential (can be additive or work against each other)

What are the key properties of channel proteins

hydrphillic pore, mostly selective and are much faster than transporters (ex. ion channels which can be both non-gated and gated, meaning they require a signal to open)

what are the 3 types of signals which would open gated ion channels

1. mechanical stress

2. ligand-gated: need some kind of molecule to bind (can be intracellular or extracellular)

3. voltage gated: change in voltage across the membrane

do transport/carrier proteins use active or passive transport

both

what type of transport protein performs passive transport

uniport: moves one solute down its electrochemical gradient, ex (GLUT = glucose transporter uniporter)

What are the three types of active transports used by transport proteins

1. gradient driven pumps (symport and antiports, ex. sodium glucose symporter and Na+H+ antiporter)

2. ATP driven pumps (ATPases)

3. light-driven pumps

Key: all three uses energy of some form

what does occluded mean for transporter proteins

blocked

what is an example of a symport, and how does it work

Na+ - glucose, moves sodium down the electrochemical gradient and glucose against (the conformation changes only occur after both sites are occupied or empty)

what is an example of an antiport

sodium hydrogen exchanger, to move sodium down its electrochemical gradient and H+ against, to maintain pH

does excess H+ mean high or low pH

low pH, more acidic

how is the concentration of Na+ maintained in the cell

through the sodium potassium pump (uses ATP)

what are p-type pumps

use ATP, and get phosphorylated during the pumping cycle, many transport ions but some such as flippases transfer phospholipids

What is are examples of a p-type pump

sodium potassium pumps (in animal cells) and H⁺ pumps (in plants)

How do sodium potassium pumps work?

what are ABC transporter pumps

uses 2 ATP to pump small molecules across the cell membrane

what is a v-type proton pump

uses ATP to pump H+ against its gradient into organelles to acidify the lumen

what are F-type ATP synthase

similar in structure to V-type pump, but uses the H+ gradient to drive the synthesis of ATP (in mitochondria, chloroplasts and bacteria)

Explain how the 3 transporters work together to transfer glucose from intestine to the bloodstream

look at diagram

what is membrane potential

difference in electrical charge on two sides of membrane

how is the membrane potential generated in animal cells

potassium leak channels and sodium potassium pumps

how is membrane potential maintained in plant cells?

H+ pumps and H+ driven symport