closer look @ cell membrane Flashcards
plasma membrane
cell membrane
selectively permeable
allows some substances to cross it more easily than other substances
cell membrane is…
selectively permeable
Amphipathic molecule-
has hydrophilic and hydrophobic regions
main ingredients of membranes
Lipids and proteins
Phospholipid
most abundant lipid in the world, component of cell membrane, amphipathic, Must be two layers (bilayer)-
why must Phospholipid be bilayer?
Hydrophobic tails of phospholipids are sheltered from aqueous surrounding, while hydrophilic heads are
exposed
phospholipid structure
fatty acids=hydrophobic tails of phospholipids, head has glycerol and phosphate group, is charged and therefore hydrophilic
Proteins in cell membrane
Also amphipathic. o Proteins are randomly dispersed with only their hydrophilic
regions protruding into the water
cell fractionation
broke cell apart to examine new model like toothpicks stuck throughout)
Membrane structure
fluid mosaic model
fluid mosaic model
found that there were globular
proteins with both hydrophobic and hydrophilic regions-
extended throughout membrane. . Supported by freeze-fracture technique, which splits membrane
down the middle- shows interior part of bilayer appearing
cobble-stoned
pre f-m model
Used to think “sandwich model”- proteins surround lipids. But
in cell fractionation, found that there were many different type
of proteins with different shapes- how could they lie flat on the
surface?
why mosaic (f-m model)
consists of glycolipids, phospholipids, proteins and cholesterol
why fluid (f-m model)
movement between each part
what holds membranes together?
weak hydrophobic interactions
Membranes must be ___ in order to function and _____ & _____ can drift randomly in the plane of the membrane
fluid, Lipids and proteins
rate of movement of lipids and proteins
Phospholipids can move rapidly
o Proteins move slowly- much larger, some seem to be
immobile
experiment that fused membrane of mouse with membrane
of human cell
proved that proteins were evenly distributed throughout; used fluorescent dye to label the proteins from mouse and a different ones for humans–if they were dispersed throughout, so they were liquid
what stabilizes membrane in animals? explain.
steroid cholesterol. integrated
between phospholipids, keeps them from
shifting too much and from solidifying at low temperatures
Two major groups of proteins
f-m model
Integral + peripheral
transmembrane
spans entire membrane
not all are all the way but to some extent
Integral proteins
Transmembrane proteins, with hydrophobic regions spanning the hydrophobic interior of the membrane and hydrophilic regions sticking out
Peripheral proteins
Not embedded- loosely bound to the surface of the membrane
Functions of proteins:
Transport, Enzymatic, Receptor, Adhesion, Recognition, Attachment to the cytoskeleton and ECM
(mnemonic is TERARA; not necessary to know)
Transport proteins
can serve as a
channel for particular solutes
Enzymatic activity-
protein built in can be an enzyme
Receptor protein-
Signal transduction- can have binding sites
for chemical messengers
Adhesion proteins
Intercellular joining- proteins of adjacent
cells can be joined together as a junction
Recognition proteins-
Cell-cell recognition- some glycoproteins (proteins with carb chains) serve as identification tags
Attachment to the cytoskeleton and ECM
microfilaments can
be bonded to membrane proteins- helps cell shape
ECM
extracellular matrix (stuff outside cell)
Why is it important for cell to be able to distinguish one type of cell from
another?
Sorting of cells into tissues and basis of rejection of foreign cells
oligosaccharides
dozen sugars
Recognition based on surface molecules (carbs) on plasma membrane
Usually branched oligosaccharides (short polysac) with fewer
than 15 units. Some- bonded to lipids, forming glycolipids, most are bonded
to proteins- glycoproteins
What does diversity of oligosaccharides allow?
them to function as markers to distinguish one cell from another
Permeability of Lipid Bilayer
Hydrophilic molecules such as polar molecules, charged molecules cannot pass through the hydrophobic center, but hydrophobic molecules can pass through, such as hydrocarbons,
carbon dioxide, oxygen.
Diffusion
tendency for molecules of any substance to spread from an area of high concentration to low concentration
down conc gradient
(from more conc to less conc)
Concentration gradient
difference in the number of ions or
molecules of a substance between
two adjoining regions
dynamic equilibrium
equal rate of movement between the two sides
Any substance will spontaneously diffuse down its concentration gradient until…
dynamic equilibrium
is reached. Occurs across cell membrane- ex- oxygen crosses cell membrane for
use in cell respiration
Passive transport
the movement of molecules from a high
concentration to a low concentration w/o the use of energy
Factors that determine diffusion rate:
Gradient steepness, Molecular size, temp, Electric gradient (sometimes), pressure gradient (sometimes)
Gradient steepness (diffusion factors)
faster when gradient is steeper
Molecular size (diffusion factors)
smaller molecules flow down concentration
gradient faster than large ones
temp (diffusion factors)
more heat energy causes molecules to move faster, therefore diffusion is more rapid
Electric gradient (diffusion factors)
difference in electric charge between two
regions. Opposite charges attract, so a
more negative region will attract positive ions
pressure gradient (diffusion factors)
difference in the pressure between two
regions. Depending on where the pressure is, diffusion will be affected
Osmosis-
movement of water from a higher concentration to a
lower concentration across a semipermeable membrane
Tonicity
relative solute concentrations of two fluids
Hypertonic
solution with
higher conc of solutes
hypotonic
solution with
lower conc of solutes
Isotonic
2 solutions of equal solute conc
diffusion in tonicity terms
Water will diffuse across a membrane from a hypotonic solution (greater water concentration) to the hypertonic area. If two solutions are isotonic, water moves across at an equal rate-no net movement. (equilibrium.)
lyse–
burst
What happens if cell w/o walls is in solution that is hypertonic to the cell?
it’ll lose water, shrivel, prob die
Hydrostatic pressure
pressure that a volume of
fluid exerts against a wall or membrane
What happens if a cell w/o walls is placed in hypotonic solution?
water will enter and cell will
swell. Hydrostatic pressure will increase , and an animal cell will lyse
osmoregulation-
control of water balance
What kind of solution do cells wo walls need to live?
To live, cells must live in isotonic solutions, or have special adaptations for osmoregulation
Paramecium
cell membrane that’s not as permeable to water. lives in freshwater–hypotonic (if it didn’t have osmoregulation, they’d die)
Contractile vacuole that pushes out water
turgid
firm and healthy
what happens if a cell w walls is in hypotonic solution?
water flows in but cell wall expands only so
much before it exerts back pressure- opposes further water uptake. Cell is turgid. At some point, osmotic (turgor) pressure will
prevent more water from entering.
osmotic (turgor) pressure
the fluid pressure
that builds up
Osmotic pressure
pressure that must be exerted on the side of
the membrane containing higher solute concentration to prevent
the diffusion of water from the
side containing a lower solute conc
what happens if a cell w walls is in an isotonic environment?
cells become flaccid–limp and the plant wilts
plasmolysis
plasma membrane pulls away from cell wall
what happens if a cell w walls is in an hypertonic environment?
cell will lose water and shrink. Plasmolysis occurs and cell dies
Facilitated diffusion-
transport proteins in membrane help
larger + hydrophilic substances diffuse thru
proteins used for facilitated diffusion- (+ characteristics)
Transport proteins, (like enzymes):
Specialized for each solute o Can be saturated (used up) o Can be inhibited by molecules that resemble normal solute o Unlike enzymes- catalyze a physical process
mechanisms facilitated diffusion
Transport protein can undergo change in shape that brings the solute from one side to the other. Transport protein can be a tunnel that allows channel/specific substances to pass thru. Still considered passive because moving down concentration gradient
and it does not require energy.
- Active transport
any type of movement that requires energy
2 categories- Active transport
Performed by proteins in cell membrane, using ATP, or Exocytosis and Endocytosis
sodium-potassium pump-
(ex of C1 of Active transport) cotransporter–transports three sodium ions outside the cell and brings in 2 potassium ions into the cell
exocytosis
cell secretes macromolecules by fusion of vesicles with
plasma membrane-contents of vesicle then
spill to the outside of the cell
vesicle
transporters
purpose + ex of exocytosis
Used by secretory cells to export products
Ex- Insulin from pancreas, chemical signals from nerve
cells
Endocytosis
cell takes in macromolecules by forming
new vesicles from the plasma membrane
Phagocytosis-
Particle of food come near the cell membrane
Cell engulfs particle by wrapping pseudopods around it, forming a vacuole/vesicle
Vacuole then fuses with
a lysosome for digestion
Pinocytosis
unspecified “gulping” of extracellular fluid and
solutes into
tiny vesicles
Receptor-mediated endocytosis-
proteins with specific
receptor sites bind extracellular substances called ligands. Receptor proteins are clustered in regions on membrane called
clathrin coated pits
Purpose of Receptor-mediated endocytosis-
Enables cell to take in bulk quantities of substances- ex- cholesterol from blood (travels as LDL, or low density lipoproteins) binds to receptors on cells in order to enter cells. (If it doesn’t enter the cells, it will build up in arteries and clog them.
water potential
Water potential is a measure of how likely water is to move from one location (say outside the cell) to another (inside the cell). …water will always move from an area of greater water potential to an area of lesser water potential.
equation water potential
solute potential + Pressure potential = Water potential
equation solute potential
solute potential-iCRT
i=ionization constant/how many ions that solute forms in water
bigger SA:V ratio for a cell means…
more efficient in removing waste by diffusion
SA of rectangular solid
2(wl+hl+hw)
