2A: Cell Membrane structure Flashcards
• Acts as a boundary
• Controls what enters and leaves cell
• Regulates chemical composition
• Maintains homeostasis
Cell Membrane
provided the first evidence that biological membranes consist of lipid bilayers.
Studies of the red blood cell plasma membrane
is flexible and allows a unicellular organism to move
Cell membrane
- Important component of Physiology
- Makes life possible
Homeostasis
3 factors of Cell Membrane
- Homeostasis
- Protection
- Setting boundaries
Semi permeable and allows maintenance of homeostasis
Cell membrane
- Allows flexibility
- Provided the first evidence that biological membranes consists of lipid bilayer
Plasma membrane
Allows unicellular organisms to move
Flexibility
Plasma Membrane models
Sandwich model
Unit membrane model
Fluid mosaic 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
Sandwich Model
(Danielli + Davson)
Protein coat of Sandwich model
Hydrophilic
Lipid layer of Sandwich model
hydrophobic
- 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 ( Robertson)
Phospholipid bilayer with proteins partially or fully imbedded, electron micrographs of freeze-fractured membrane
Fluid Mosaic Model
(Singer + Nicolson)
- Hydrophilic heads facing outwards, hydrophobic tails facing inwards.
- used the freeze-fructure membrane to test the model
Fluid-Mosaic model
- Rapidly freeze specimen
- use special knife to cut membrane into half.
- Apply carbon + platinum
- Viewed through scanner electron microscope
Freeze-fracture of membrane
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
low temperatures
- Gel phase or gel like cell membrane
- Hydrocarbons are tightly packed
Higher temperature
- Moves to fluid phase
- bilayer “melts” movement is allowed
- tails move
Formation of sphere is
energetically favorable
Spontaneously Form Vesicles
Planar Lipid Bilayers
commonly used solubilizing agents
SDS and Triton X-100
- Harsh ionic detergent
- sodium dodecyl sulfate
- used in DNA
SDS
a gentler non-ionic detergent
Triton X-100
- Disrupts the membrane from lipids
- Disrupts the protein to isolate DNA
Detergent
Structure and functions of Plasma
- Selectivity
- Barrier
Movement of phospholipid
- Most of the lipids and some proteins drift laterally on either side.
- Flipflop happens rarely
- Phospholipids do not switch from one layer to the nest
Cholesterol
Affects fluidity
Cholesterol affects fluidity: at body temperature
- it LESSENS FLUIDITY by restraining the movement of phospholipids
- reduce permeability to small mol
- Make the Membrane More Rigid
- Less Permeable
- Resistant to Low
Temperature Crystalization
Planar Rings of Cholesterol
Major membrane component (Animals)
Cholesterol
- Have the same function as cholesterol
- Plants’ counterpart of cholesterol
Sterols
3 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
proteins can be released only by treatments that disrupt the phospholipid bilayer.
Integral membrane
span the lipid bilayer with portions exposed on both sides of the membrane.
Transmembrane proteins
Types of membrane proteins
Channel protein
Carrier protein
Cell recognition protein
Receptor protein
Enzymatic protein
- allows ions to cross freely
Channel Protein
Ex. of disorder of channel
Cystic fibrosis
- Selectively interacts with a specific molecule of ion so that it can cross the plasma membrane.
Carrier protein
- The MHC (major histocompatibility complex)
Cell recognition
- Shaped in such a way that specific molecule can bind to it.
Receptor protein
- Essential for cell recognition
- rejects incompatible cells
MHC (Major histocompatibility complex)
- Catalyzes specific reaction.
enzymatic protein
Transport proteins
Channel Proteins
Carrier 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) onextracellular 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)
Attachment Proteins
– protein fibers and carbohydrates secreted by cells and fills the spaces between cells and supports cells in a tissue.
- can influence activity inside the cell and coordinate the behavior of all the 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 Ex. Intestine, Kidneys, Epithelium of skin
Tight Junctions
• Cytoplasmic plaques of two cells bind with the aid of
intermediate filaments of keratin
• Allows for stretching
• Ex. Stomach, Bladder, Heart
Desmosomes
• Channel proteins of opposite cells join together providing channels for ions, sugars, amino acids, and other small molecules to pass.
• Allows communication between cells.
Gap Junctions
Cholesterol1 colder temperatures
- it MAINTAINS FLUIDITY by not allowing phospholipids to pack close together.
