2a. Cell Membrane Structure Flashcards
What is a cell membrane?
- Acts as a boundary
- Controls what enters and leaves cell
- Regulates chemical composition
- Maintains homeostasis
Why is cell membrane call the gateway to the cell?
cell’s outer membrane acts as a gate, preventing intruders from invading the cell.
What is the importance of the cell membrane?
main role is to provide protection to the cell from its surroundings. It controls the passage of some solutes and water into and out of the cell.
The cell membrane in unicellular organisms is
flexible and allows to
move
Studies of the red blood cell plasma
membrane provided ____
the first evidence that
biological membranes consist of lipid bilayers.
The role of the cell membrane in the plant cell is ____
to separate the cytoplasm of the cell from its cell wall.
Sandwich model was proposed by?
Danielli and Davson
3 Plasma Membrane Model
- Sandwich Model
- Unit Membrane Model
- Fluid Mosaic Model
What is fluid mosaic model?
Phospholipid bilayer with proteins partially or fully imbedded, seen with freeze-fractured membrane via electron micrographs
What is sandwich model?
2 layers of globular proteins with phospholipid inside to make a layer and then join 2 layers together to make a channel for molecules to pass
Who proposed unit membrane model?
Robertson
What is the outer layer of PPL bilayer?
glycoprotein coat
What is unit membrane model?
- Outer layer of protein with phospholipid bilayer inside,
- believed all cells same composition,
- does not explain how some molecules pass through or the use of
proteins with nonpolar parts - used transmission electron microscopy
Who proposed fluid mosaic model?
Singer and Nicolson
Process on how to verify the correct membrane model?
1) Rapidly freeze specimen
2) Use special knife to cut membrane in half
3) Apply a carbon + platinum coating to the surface
4) Use scanning electron microscope to see the surface
According to the electron micrograph which membrane model is correct?
Fluid-Mosaic Model
What model is the Basic paradigm of biological
membrane structure
Fluid-mosaic model
The lattice becomes ____ at
the transition temperature.
frozen crystalline gel
The lipids in a membrane are organized into a
_____
liquid crystalline lattice.
What composes in FM model freeze-fracture?
there is a P face and E surface, extracellular surface of membrane
What composes in FM model section?
bimolecular lipid leaflets (trilaminar)
The Plasma Membrane is composed of two layers of lipids. Why is this important?
Having lipids in the plasma membrane means that at least a portion of the membrane repels the water that constantly surrounds it. Allowing
too much water inside the cell could cause the cell to burst.
2 Layers of the FM Model
- Fluid
- Mosaic
Describe the fluid in FM Model
plasma membrane has consistency of olive oil at body temperature, due to unsaturated phospholipids
What happens to the PPL and proteins in the fluid layer
move around freely within the layer, like it’s a
liquid.
Describe the mosaic layer of FM Model
- proteins form a collage that differs on either side of membrane and from cell to cell (greater than 50 types of proteins)
Describe the protein span of the mosaic layer
- proteins span the membrane with hydrophilic portions facing out and hydrophobic portions facing in.
describe the mosaic pattern of the FM Model
- mosaic pattern produced by scattered protein molecules when membrane
is viewed from above.
What happens if it in low temperature?
hydrocarbons are tightly packed and in gel phase
What happens if it in higher temperature?
moves to fluid phase, bilayer “melts”, movement is allowed
Formation of sphere is ____?
energetically favorable
Describe the sphere in energetically unfavorable?
planar phospholipid bilayer with edges exposed to water
Describe the sphere in energetically favorable?
sealed compartment formed by phospholipid bilayer
planar lipid bilayers spontaneously ___
form vesicles
What is the surroundings of the sphere bilayer?
water inside and outside
it is a harsh ionic detergent
SDS (sodium dodecyl sulfate)
a gentler non-ionic detergent and commonly used solubilizing agents
Triton X-100
What happens when you put SDS in the bilayer?
SDS will intercalate first with the phospholipids and will increase the hydrophilicity of the membrane promoting a leak first and complete disruption eventually
What is the difference between triton X-100 and SDS in terms of effects?
Triton-X-100 is a non-ionic detergent suitable for gently solubilizing proteins without denaturing them, while SDS is an anionic detergent that denatures proteins
What happens when you have detergent in the bilayer?
lysing cells to extract protein and other cellular organelles or to permeabilize the living cell membrane for transfection
What are integral membrane proteins?
Integral membrane proteins are embedded in the lipid bilayer and can be removed only by disrupting the bilayer.
how does detergent remove protein membrane?
These amphipathic molecules disrupt the bilayer by forming mixed phospholipid-detergent micelles.
Process to isolate a protein from its membrane
- detergent micelles + monomers added to the membrane
- they will be solubilized membrane proteins
- will now became lipid-detergent micelles
- purification of NaK pump
- Removal of detergent micelles + monomers
- Addition of phospholipids (mixed with detergent)
- functional protein incorporated into phospholipid vesicle
Scheme to Study a
Membrane Protein (3)
Solubilization,
Purification, and
Reconstitution in
Liposomes
Membrane
Proteins Can Be
Solubilized by?
Detergents That
Disrupt the
Membrane
Structure of the Plasma Membrane (9)
- extracellular matrix
- glycoprotein
- carbohydrate
- glycolipid
- cytoplasm
- cholesterol
- proteins
- phospholipid
- microfilaments of cytoskeleton
Membrane movement
- lateral movement (frequent)
> flexion
> rotation - flip-flop (rare)
movement of phospholipids
- Most of the lipids and some proteins drift laterally on either side.
- Phospholipids do not switch from one layer to the next.
Movement of lipids and proteins in the cell membrane
not fixed in position but constantly moving.
While the lipids can move both laterally and rotate 360 degrees
flip-flop diffusion
The proteins move laterally within the cell membrane
lateral diffusion
Cholesterol affects ____
fluidity
Cholesterol at colder temperature
it maintains fluidity by not allowing phospholipids to pack close together.
Cholesterol at body temperature (2)
- it lessens fluidity by restraining the movement
of phospholipids - reduce permeability to small mol
our body needs some cholesterol to make _____
hormones, vitamin D, and substances that help you digest foods
The Planar Rings of Cholesterol Make
the Membranes what? (3)
- More Rigid
- Less Permeable
- Resistant to Low Temperature Crystalization
a waxy, fat-like substance that’s found in all the cells in your body.
cholesterol
plant membrane have no cholesterol but have
sterols
A major membrane component in animal cells
cholesterol
Cholesterol stabilizes the cell membrane by ____
forming interactions with the phospholipid tails and heads
This makes their structure more rigid and helps provide stability
Paracrystalline state (solid)
a. heat produces thermal motion of acyl side chains (solid –> fluid transition)
b. (fluid state) lateral diffusion in plane of bilayer
c. transbilayer diffusion
Classification of membrane proteins
- peripheral membrane proteins
- integral membrane proteins
- transmembrane proteins
proteins that dissociate from the membrane following treatments with polar reagents that do not disrupt the phospholipid bilayer.
peripheral membrane proteins
Integral membrane proteins
can be released only by
treatments that disrupt the phospholipid bilayer.
can be released only by
treatments that disrupt the phospholipid bilayer.
integral membrane proteins
span the lipid bilayer with portions exposed on both sides of the membrane.
Transmembrane proteins
Peripheral MP
proteins that dissociate
from the membrane following treatments with polar reagents that do not disrupt the phospholipid bilayer.
Transmembrane proteins
span the lipid bilayer with portions exposed on both sides of the membrane.
6 types of membrane proteins
- transport protein
- receptor protein
- enzymatic protein
- cell recognition protein
- attachment proteins
- intercellular junction proteins
channel for lipid
insoluble molecules
and ions to pass freely
through
Channel Proteins
bind to a substance and
carry it across membrane, change
shape in process
carrier proteins
Bind to chemical
messengers (Ex.
hormones) which
sends a message into
the cell causing
cellular reaction
receptor proteins
Carry out enzymatic
reactions right at the
membrane when a
substrate binds to the
active site
enzymatic proteins
Glycoproteins (and
glycolipids) on
extracellular surface
serve as ID tags
(which species, type of
cell, individual).
cell recognition proteins
are short branched chains of less than 15 sugars
Carbohydrates
Attach to cytoskeleton (to maintain cell shape and stabilize proteins) and/or the extracellular
matrix
attachment proteins
protein fibers and carbohydrates secreted by cells and fills the
spaces between cells and supports cells in a tissue.
extracellular matrix
Extracellular matrix can
influence the ____
activity inside the cell and coordinate the behavior of all the cells in a tissue.
types of cell junction (2)
–Tight junctions
–Gap junctions
Bind cells together
intercellular junction proteins
Transmembrane Proteins of opposite cells attach in a tight zipper-like fashion; No leakage
tight junction
Examples of tight junctions
Ex. Intestine, Kidneys, Epithelium of skin
Describe the tight junction in epithelial cells
TJ bar the movement of dissolved materials from the lumen through the spaces in between
Cytoplasmic plaques of two cells bind with the aid of intermediate filaments of keratin; allows for stretching
desmosomes
Examples of desmosomes
stomach, bladder, heart
a transmembrane protein that binds with other cadherins to form junctions known as desmosomes between cells
desmogleins
Channel proteins of opposite cells join together providing
channels for ions, sugars, amino acids, and other small
molecules to pass.
gap junctions
allows communication between cells
gap junctions
only intercellular junctions in plants between walls of 2 adjacent plant cells
plasmodesmata
examples of gap junctions
heart muscle, animal embryos
the motion of protein molecules under the influence of an electric field that is applied across the fluid that they are dispersed in
mobility of membrane proteins
Many Membrane Proteins Move _____
Freely in the Plane of the Membrane
(FRAP)
Fluorescence Recovery After Photobleaching
fusion of membrane protein process
- cell fusion creates heterocaryon
2 types of transport protein
- channel protein
- carrier protein
