2a - Cell Membrane Structure Flashcards
What is the primary function of the cell membrane?
The cell membrane acts as a boundary for the cell.
How does the cell membrane control the cell’s internal environment?
By controlling what enters and leaves the cell.
What role does the cell membrane play in chemical regulation?
regulates the chemical composition of the cell.
The cell membrane maintains homeostasis by ____ the internal conditions of the cell.
Regulating
is flexible and allows a unicellular organism to move
cell membrane
Studies of the ___ __ __ plasma
membrane provided the first evidence that biological membranes consist of lipid bilayers.
red blood cell
Plasma Membrane models
- Sandwich model (Danielli + Davson)
- Unit membrane model (Robertson)
- Fluid Mosaic model ( Singer + Nicolson)
-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
- protein -> phospholipid bilayer -> protein
Sandwich model
Phospholipid has ___ head while the tails that face the interior of the cell membrane are ___
Hydrophilic
Hydrophobic
What was the main flaw in the Sandwich Model?
The Sandwich Model did not account for the fluidity of the membrane and the presence of integral proteins that span the lipid bilayer, which led to its replacement by the Fluid Mosaic 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
Unit membrane model
Phospholipid bilayer with proteins partially or fully imbedded, electron
micrographs of freeze-fractured membrane
Fluid Mosaic model
describes the structure of cell membranes as a flexible layer made of phospholipid bilayer with proteins embedded in it.
Fluid Mosaic model
Why is the model called “Fluid”?
because the phospholipid bilayer is flexible, allowing lipids and proteins to move laterally within the membrane.
What does the “Mosaic” part of the Fluid Mosaic Model refer to?
patchwork of proteins that are embedded in or attached to the lipid bilayer, resembling a mosaic.
In fluid Mosaic model it span the entire lipid bilayer and are involved in various functions such as transport, signaling, and maintaining the cell’s structure.
Integral proteins
In the Fluid Mosaic Model, it is interspersed within the phospholipid bilayer and helps to maintain membrane fluidity and stability
Cholesterol
- technique used to study the structure of cell membranes by freezing a cell and then fracturing it to reveal the internal organization of the membrane.
- reveals the distribution of proteins within the phospholipid bilayer, showing that proteins are embedded in the membrane rather than just lying on the surface
Freeze-fracture membrane
Steps of freeze-fracture membrane technique
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
In a cell membrane, lipids are organized in a flexible, fluid-like layer called a ___ ___ ___.
liquid crystalline lattice
This means that while the lipids are arranged in a structured way, they can still move around, much like ice cubes floating in a drink.
liquid crystalline lattice
Each membrane has a specific temperature at which it changes from being flexible (liquid crystalline) to becoming more solid (frozen crystalline gel).
Transition temperature
Formed when the temperature drops below this transition temperature, the membrane lipids become less fluid and more rigid
solid-like structure (frozen crystalline gel).
at normal temperatures, the membrane is (1) ___ and __, but as it gets colder, it becomes more (2) ___ and less ___.
(1) Flexible and fluid
(2) Rigid and less flexible
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.
- plasma membrane has consistency of olive oil at body temperature,
due to unsaturated phospholipids.
-phospholipids and proteins move around freely within the layer, like it’s a
liquid.
Fluid
- proteins form a collage that differs on either side of membrane
and from cell to cell (greater than 50 types of proteins)
-proteins span the membrane with hydrophilic portions facing out and
hydrophobic portions facing in.
-mosaic pattern produced by scattered protein molecules when membrane is viewed from above.
Mosaic
proteins span the membrane with hydrophilic portions facing (1) ___ and hydrophobic portions facing (2) ___.
(1) out
(2) in
Occurs at low temperatures when the hydrocarbons in the lipid bilayer are tightly packed, making the membrane less fluid
Gel phase
Lipid Bilayer:
at low temperatures =
at higher temperature=
LOW TEMP - hydrocarbons are tightly packed = less fluid
HIGHER TEMP = moves to fluid phase = movement is allowed
At higher temperatures, the lipid bilayer moves to the ___ phase
Fluid
is energetically unfavorable because it increases the exposure of hydrophobic regions to water.
planar phospholipid bilayer with exposed edges to water
is energetically favorable because it prevents the exposure of hydrophobic edges to water, stabilizing the system.
sealed compartment
spontaneously form vesicles when placed in water to avoid exposing hydrophobic edges, creating a more stable, sealed compartment.
Planar lipid bilayer
Small tears in membranes are repaired by the spontaneous formation of ____, where the lipid bilayer seals itself to restore the membrane’s integrity.
spontaneous formation of vesicles
2 commonly used solubilizing agents.
- SDS
- Triton X-100,
a harsh ionic detergent
SDS
gentler non-ionic detergent
Triton X-100
attached to the membrane surface and can be removed by changes in pH, chelating agents, or high salt concentrations, but not by detergents.
Peripheral protein
proteins located on the surface of the cell membrane that are covalently attached to lipids embedded within the cell membrane.
Lipid-linked/anchored proteins
Lipid-linked proteins can be removed by ____ _, which cleaves the lipid tails that anchor these proteins.
phospholipase C,
is a protein that is covalently attached to carbohydrate groups (glycans), which can be involved in cell-cell recognition and signaling.
protein-glycan
solubilize integral proteins by disrupting the lipid bilayer, allowing the proteins to be extracted from the membrane.
Detergents
A Scheme to Study a
Membrane Protein
Solubilization,
Purification, and
Reconstitution in Liposomes
This involves using detergents to disrupt the lipid bilayer and solubilize the membrane proteins
Solubilization
separates the solubilized membrane proteins from other cellular components, isolating them for further study.
Purification
What methods are commonly used for purifying membrane proteins?
chromatography techniques (such as affinity, ion exchange, and size exclusion chromatography)
Structure of the Plasma Membrane
- Extracellular matrix
- Glycoprotein
- Carbohydrate
- Plasma membrane
- Glycolipid
- Phospholipid
- Proteins
- Microfilaments of cytoskeleton
- Cholesterol
10.Cytoplasm
is the outer boundary of the cell, composed of a phospholipid bilayer with embedded proteins, regulating the entry and exit of substances.
plasma membrane
is a network of proteins and carbohydrates outside the cell that provides structural support, adhesion, and communication between cells.
extracellular matrix
are proteins with carbohydrate groups attached, found on the cell surface, and involved in cell recognition and signaling
Glycoproteins
___ on the extracellular surface of the membrane are involved in cell-cell recognition and adhesion, forming glycoproteins and glycolipids
Carbohydrates
are lipids with carbohydrate chains attached, located on the extracellular surface of the plasma membrane, and involved in cell recognition.
Glycolipids
is a molecule with a hydrophilic head and two hydrophobic tails, forming the basic structure of the phospholipid bilayer in the plasma membrane.
phospholipid
___ in the plasma membrane include integral proteins (spanning the membrane) and peripheral proteins (attached to the membrane surface), serving various functions such as transport, signaling, and structural support.
Proteins
are part of the cytoskeleton that provides structural support to the cell, helps maintain its shape, and anchors membrane proteins
Microfilaments
helps stabilize the fluidity of the membrane, making it less permeable to small molecules and maintaining membrane integrity.
Cholesterol
is the gel-like substance within the cell membrane that contains organelles and is involved in various cellular processes.
Cytoplasm
The proteins move laterally within the cell membrane
lateral diffusion
While the lipids can move both laterally and rotate 360
degrees
Flip-flop diffusion
The lipids and proteins in the cell membrane are not ___
in position but constantly ___.
Fixed
moving
Cholesterol affects ___
Fluidity
Cholesterol affects fluidity :
at ____ temperature it lessens fluidity by restraining the movement
of phospholipids; also reduce permeability to small mol
Body
Cholesterol affects fluidity :
at ____ temperatures it maintains fluidity by not allowing
phospholipids to pack close together.
Colder
Phospholipid Movements
- Lateral diffusion (frequent)
- Flip-flop (rare)
- Flexion (bending of fatty acid tails)
- Rotation (lipids rotate around their axis)
The ___ ___of Cholesterol Make the Membrane More Rigid, Less
Permeable, and Resistant to Low
Temperature Crystalization
Planar Rings
Present in animal cell membranes in similar molar amounts as phospholipids.
Cholesterol
Cholesterol is not found in plant cell membranes. Instead, ___ perform a similar function
Sterols
used for sequence-specific interactions in protein–DNA interfaces.
a-helix protein
are linked to lipids or to compatible amino acid side chains in proteins, by N- or O-glycosidic bonds
oligosaccharide chains
Membrane state where lipids are tightly packed and movement is minimal.
Paracrystalline State (Solid)
where lipids have increased movement due to thermal motion, leading to a more fluid structure
Fluid state
Heat induces thermal motion in ___ __ chains, causing a transition from a solid (paracrystalline) to a fluid state.
acyl side
____ flip-flop can take hours to days, while _____ flip-flop occurs in seconds.
Uncatalyzed
flippase-catalyzed
Classification of membrane protein
Peripheral membrane proteins
Integral membrane proteins
Transmembrane proteins
are proteins that dissociate from the membrane following treatments with polar reagents that do not disrupt the phospholipid bilayer
Peripheral membrane proteins
can be released only by
treatments that disrupt the
phospholipid bilayer.
Integral membrane proteins
portions exposed on both
sides of the membrane
Transmembrane proteins
Types of membrane protein
- Channel protein
- Receptor protein
- Carrier protein
- Enzymatic protein
- Cell recognition protein
Allows a particular molecule or ion to cross the plasma membrane freely.
Channel protein
EXAMPLE of channel protein
Cystic fibrosis
is an inherited disorder, is caused by a faulty chloride (CI) channel; a thick mucus collects in airways and in pancreatic and liver ducts.
Cystic fibrosis
Shaped in such a way that a specific molecule can bind to it
Receptor protein
EXAMPLE of receptor protein
Dwarfism
Some types of dwarfism result not because the body does not produce enough growth hormone, but because the ___ ___ growth hormone ___ are faulty and cannot interact with growth hormone
plasma membrane
receptors
Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane.
Carrier protein
EXAMPLE of Carrier protein
The inability of some persons to use energy for sodium-potassium (Na-K”) transport has been suggested as the cause of their obesity.
Catalyzes a specific reaction.
Enzymatic protein
involved in ATP metabolism
membrane protein and adenylate cyclase
release a toxin that interferes with the proper functioning of adenylate cyclase, which eventually leads to severe diarrhea.
Cholera bacteria
- The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve.
- Cells with foreign MHC glycoproteins are attacked by white blood cells responsible for immunity.
Cell recognition protein
- are different for each person, so organ transplants are difficult to achieve.
The MHC (major histocompatibility complex) glycoproteins
Why are organ transplants difficult to achieve
The MHC (major histocompatibility complex) glycoproteins are different for each person
What are attacked by white blood cells
Cells with foreign MHC glycoproteins
2 transport proteins
- Channel proteins
- Carrier proteins
channel for lipid insolublemolecules
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). Carbohydrates are short branched chains of less than 15 sugars.
Cell Recognition Proteins
Attach to cytoskeleton (to maintain cell shape and stabilize proteins) and/or the extracellular matrix (integrins connect to both).
- Extracellular Matrix – protein fibers and carbohydrates
secreted by cells and fills the
spaces between cells and
supports cells in a tissue.
- Extracellular matrix can
influence activity inside the cell
and coordinate the behavior of
all the cells in a tissue.
Attachment protein
protein fibers and carbohydrates
secreted by cells and fills the spaces between cells and supports cells in a tissue.
Extracellular Matrix
– Bind cells together
–Tight junctions
–Gap junctions
Intercellular Junction Proteins
Types of Cell junctions
- Tight junctions
- Desmosomes
- Gap junctions
• Transmembrane Proteins of opposite cells attach in a tight zipper-like fashion
• No leakage
Tight junctions
Tight junctions examples
Intestine, Kidneys, Epithelium of skin
• Cytoplasmic plaques of two cells bind with the aid of
intermediate filaments of keratin
• Allows for stretching
• Ex. Stomach, Bladder, Heart
Desmosomes
Desmosomes example
Ex. Stomach, Bladder, Heart
• Channel proteins of opposite cells join together providing
channels for ions, sugars, amino acids, and other small
molecules to pass.
• Allows communication between cells.
• Ex. Heart muscle, animal embryos
Gap junctions
EXAMPLE of Gap junction
Ex. Heart muscle, animal embryos
