5 Flashcards
Gateway to the Cell
CELL MEMBRANE
- Acts as a boundary
- Controls what enters and leaves cell
- Regulates chemical composition
- Maintains homeostasis
CELL MEMBRANE
The ________
is flexible and
allows a
unicellular
organism to
move
cell membrane
Studies of the red blood cell plasma
membrane provided the ________ that
biological membranes consist of lipid
bilayers.
first evidence
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
Sandwich Model (Danielli + Davson)
-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)
Which membrane model is correct?
Fluid-Mosaic Model
Basic paradigm of biological membrane structure
Fluid-Mosaic Model
According to the electron micrograph which membrane model is
correct?
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
The lipids in a membrane are organized into a
__________.
liquid crystalline lattice
The lattice becomes a _______ at
the transition temperature.
frozen crystalline gel
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
gel phase–low temperatures
hydrocarbons are tightly packed
at higher temperature
it moves to fluid phase
at higher temperature –> it moves to fluid phase
bilayer “melts”, movement is allowed
Formation of sphere is ________
energetically favorable
planar phospholipid bilayer with edges exposed to water
energetically unfavorable
sealed compartment formed by phospholipid bilayer
energetically favorable
Planar Lipid Bilayers
Spontaneously Form
________
Vesicles
Small tears in
membranes are
_______ this way.
repaired
are commonly used solubilizing agents.
- SDS, a harsh ionic detergent
- Triton X-100, a gentler non-ionic detergent
Membrane
Proteins Can Be
_______ by
Detergents That
Disrupt the
Membrane
Solubilized
A Scheme to Study a
Membrane Protein –
___________
Solubilization,
Purification, and
Reconstitution in
Liposomes
Most of the lipids and some proteins drift ______ on either
side.
laterally
Phospholipids ________ from one layer to the next
do not switch
The lipids and proteins in the cell membrane are _______
in position but constantly moving.
not fixed
The proteins move ______within the cell membrane –
lateral diffusion
laterally
the lipids can move both laterally and rotate 360
degrees – ________
flip-flop diffusion
Cholesterol affects fluidity:
* at body temperature
it lessens fluidity by restraining the movement
of phospholipids; also reduce permeability to small mol
Cholesterol affects fluidity:
* at colder temperature
it maintains fluidity by not allowing
phospholipids to pack close together
The Planar Rings of Cholesterol Make the Membrane ___
More Rigid
Less Permeable
Resistant to Low Temperature Crystalization
*Major membrane component in
animal cells
*Cholesterol is not found in plant
membranes
-Sterols perform the same function
*Same molar amounts as
phospholipids
Cholesterol
THE MEMBRANE PROTEINS CLASSIFICATION:
- 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
span the lipid bilayer with
portions exposed on both
sides of the membrane.
Transmembrane proteins
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) 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 Proteins
protein
fibers and carbohydrates
secreted by cells and fills the
spaces between cells and
supports cells in a tissue.
Extracellular Matrix
Intercellular Junction Proteins
– Bind cells together
–Tight junctions
–Gap junctions
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.
- Ex. Heart muscle, animal embryos
Gap Junctions
