S2W1 - Membrane Structure and Transport Flashcards
what are two organelles that are specific to animal cells?
- extracellular matrix: specialised material outside the cell
- lysosome: degradation of cellular components that are no longer needed
what are three organelles that are not found in animal cells but are found in plant cells and some other cells?
- cell wall
- vacuole (2 types)
- chloroplast
cell wall two functions
- cell shape
- protection against mechanical stress
vacuoles two functions
- degradation (like animal lysosome)
- storage (small molecules and proteins)
chloroplast function
- site of photosynthesis
distinguish between the cytoplasm, the cytosol, and the lumen
- cytoplasm: contents of the cell outside the nucleus (membrane-bound organelles)
- cytosol: aqueous part of the cytoplasm. does not include membrane-bound organelles, does include ribosomes and cytoskeleton
- lumen: inside of organelles
what cellular functions occur at membranes?
- compartmentalisation
- scaffold for biochemical activities
- selectively permeable barrier
- transport solutes
- respond to external signals
- interactions between cells
describe the model proposed by Singer and Nicolson in 1972
Fluid Mosaic Model of the Membrane
- fluid: due to mobility of lipids and some of the proteins
- mosaic: many different lipids and many different proteins
- lipid bilayer: = 1 membrane consisting of two layers of leaflets
define amphipathic molecules
have different biochemical/biophysical properties on different sides of the molecule
what makes phospholipid molecules amphipathic?
they have a hydrophilic/polar head and hydrophobic tails
state three types of lipids that membranes are composed of
- phospholipids
- sterols
- glycolipids
all have hydrophilic heads as well as hydrophobic tails
phospholipids
- there are different types of membrane phospholipids
- most have a glycerol group (termed phosphoglycerides, of which there are different types)
general structure of a phospholipid
polar head group (hydrophilic):
- different groups
- phosphate
glycerol
hydrocarbon tails
- length: 14-24 carbon atoms
- saturated/unsaturated
kink
- hydrocarbon tail is unsaturated
- contains a cis-double bond
- this causes a bend in the tail
what happens to phospholipids in aqueous environments?
- they spontaneously self-associate into a bilayer
- the polar head group interacts with water
- the two hydrophobic hydrocarbon tails interact with other hydrophobic tails
how are sealed compartments formed by phospholipid bilayers?
- a planar phospholipid bilayer is energetically unfavourable as the hydrophobic tails are exposed to water along the edges)
- the formation of a sealed compartment shields hydrophobic tails from water
define liposomes and describe their uses
artificial lipid bilayers used to:
1. study lipid properties
2. study membrane protein properties
3. drug delivery into cells (nanotechnology)
how can membrane fluidity be visualised?
live cell imaging where laser tweezers are used to manipulate the membrane show that a membrane can be deformed without causing damage
describe the different types of phospholipid movement within cell membranes
phospholipids within each leaflet rapidly:
- diffuse laterally (side-to-side or deeper into the membrane plane)
- rotate
- flex
- RARELY move from one leaflet to other (flip-flop) on their own
why is cell membrane fluidity carefully regulated?
as it is important for function, e.g. membrane proteins for transport, enzyme activity, signaling
two factors affecting membrane fluidity
- temperature
- composition (phospholipid saturation, phospholipid tail length, lipid composition)
how does temperature impact membrane fluidity?
lower temperatures make the membrane more viscous and less fluid, which is unwanted
how does composition - phospholipid saturation - impact membrane fluidity?
- cis double bonds increase fluidity at lower temperatures (reduce tight packing)
- phospholipids can change from being saturated to unsaturated to alter fluidity
how does composition - phospholipid tail length - impact membrane fluidity?
shorter hydrocarbon tails increase fluidity at lower temperatures (lipid tails interact less)
how does composition - lipid composition - impact membrane fluidity?
the addition of cholesterol in animal cell membranes stiffens the membrane, making it less permeable to water
how are sterols typically present in animals and plants?
in animals, mainly cholesterol; in plants, plant sterols and some cholesterol
general structure of a sterol
- polar head group
- non polar rigid planar steroid ring structure
- non polar hydrocarbon tail
how does the addition of cholesterol molecules impact cell membranes?
- decreases the mobility of phospholipid tails (stiffens and thickens the membrane by filling space)
- plasma membrane is less permeable to polar molecules
draw a diagram of a cholesterol molecule surrounded by two phospholipid molecules
polar head
cholesterol-stiffened region
more fluid region
describe how lipid movement to the other leaflet is created in the ER membrane
- phospholipid synthesis adds to the cytosolic half of the bilayer
- scramblase (a phospholipid translocator in the ER membrane) catalyses the rapid flip lop of random phospholipids from one leaflet to another
- this ensures symmetric growth of both halves of the bilayer
distribution of phospholipids in the ER membrane is
random
how does distribution of phospholipids and glycolipids in the cell membrane differ from that in the ER?
the noncytosolic face and the cytosolic face have different lipids
- glycolipids only on noncytosolic face
- phosphatidylserine only on cytosolic face
how do phospholipids and glycoproteins end up on the plasma membrane?
- synthesised in the membrane of the ER
- carried in vesicles to the membrane of the Golgi apparatus
- carried in vesicles to the plasma membrane
how is the orientation of the cell membrane different to that of the membranes in the cytosol?
- the plasma membrane has the noncytosolic face towards the extracellular fluid and the cytosolic face towards the intracellular fluid
- the membranes in the cytosol (vesicles, Golgi, ER) have the cytosolic face toward the cytosol and the noncytosolic facing inside the organelle
describe the role of the Golgi membrane in regulation lipid membrane distribution
- delivery of new membrane from ER
- Flippase enzymes in the Golgi membrane catalyse the rapid flip-flop of specific phospholipids to the cytosolic leaflet (eg phosphatidylserine)
- some can bind cytosolic proteins at the plasma membrane (eg phosphatidylserine binds protein kinase c)
how are glycolipids and glycoproteins distributed on the membrane?
- formed by adding sugar groups to lipids/proteins on the luminal face of Golgi
- end up on plasma membrane, inside of organelles, on the noncytosolic face only
- protect the membrane from harsh environments
can proteins flip-flop?
no
two main subsets of membrane proteins
integral membrane proteins
peripheral membrane proteins
three types of integral membrane proteins
- transmembrane (cross the entire membrane, once or multiple times)
- monolayer associated (insert halfway)
- lipid-linked (have a lipid anchor inside the membrane)
peripheral membrane proteins
proteins do not insert into the membrane on either face of the membrane
- bound to other proteins or lipids by non-covalent interactions
distinguish between the extraction methods that need to be used to isolate integral membrane proteins as opposed to peripheral membrane proteins
integral: extraction methods use detergents (lipid bilayer destroyed)
peripheral: gentle extraction methods used (lipid bilayer remains intact)
describe the amphipathic properties of transmembrane proteins
amphipathic
- hydrophilic domains in aqueous environment (AA side chains polar)
- hydrophobic membrane-spanning domains (AA side chains non-polar)
what are the three main types of transmembrane proteins?
- single alpha helix (single-pass)
- multiple alpha helices (multipass)
- beta barrel (rolled beta sheet, multipass)
how many amino acids long would you expect a membrane-spanning alpha helix to be?
20-30
draw a single alpha helix
slide 32
draw multiple alpha helices
slide 32
draw a beta barrel
slide 32
why is it important for transmembrane proteins to not be able to flip flop?
each transmembrane protein has a specific orientation - essential for function
how are the structures of transmembrane proteins identified?
- X-ray crystallography: determines the 3D structure
- hydrophobicity plots: hydropathy index, where +ve ΔG means more hydrophobic, -ve ΔG means mood hydrophilic index
describe mono-layer associated membrane proteins
proteins anchored on the cytosolic face by an amphipathic alpha helix
eg proteins in membrane bending for vesicle budding (Sar1) at the ER
describe two types of lipid-linked membrane proteins
- protein with a GPI anchor (glycosylphosphatidylinositol). Synthesised in the ER lumen and ends up on the cell surface (noncytosolic face throughout whole process)
- protein with another lipid anchor (fatty acid, prenyl). Cytosolic enzymes add the anchor, which directs the protein to the cytosolic face
Techniques: extraction of membrane proteins with detergents
Triton X-100 (has both hydrophobic and hydrophilic region)
By mixing the transmembrane protein with amphipathic detergent monomers and water, you form water-soluble protein-lipid-detergent complexes and water-soluble lipid-detergent micelles, which breaks up the membrane.
Technique: studying the properties of membrane proteins
Following the purification of the protein of interest, you can add phospholipids and remove the detergent to produce a functional protein incorporated into an artificial phospholipid vesicle. This can be used to study the properties of the protein in an isolated environment.
Technique: lateral diffusion of membrane proteins
- there is lateral diffusion of proteins within the leaflet, but no flip-flop
- study of protein movement can be done by Fluorescence Recovery After Photobleaching (FRAP), where the protein is fused to GFP (Green Fluorescent Protein)
describe how FRAP works
- protein fused to GFP or labelled with fluorescent antibody
- photobleach an area (white)
- rate of fluorescence recovery: time taken for neighbouring unbleached fluorescent proteins to move into bleached area
look at the different possible graphs for FRAP on slide 43
can FRAP only be done with transmembrane proteins?
no, you can also do it with other proteins (eg cytosolic) and other molecules (eg lipids)
