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Chapter 8
Cell Membranes
Membrane Structure
* Cell membranes made of lipids, proteins, & carbohydrates
* Main component, phospholipids
* Phospholipids are amphipathic, they have a hydrophilic
& a hydrophobic region
* Phospho- region, hydrophilic, mixes w/ water
* -lipid region, hydrophobic, does not mix w/ water
* Phospholipids form a double layer, with phospho- heads
facing out, either to ECF, or interior of cell.
* Hydrophobic tails face inward, away from outer contact
with water.
(Gorter & Grendel, 1925)
Where do the proteins fit?
* Originally thought that there was a layer of proteins
on the outside & inside of the phospholipid bilayer
(Sandwich Model, Davson & Danielli, 1935)
* 1972, Singer & Nicolson proposed a model with the
proteins fitting in with the bilayer
* Hydrophilic portion of proteins would face outward and
hydrophobic portion would fit inside the bilayer (p.
138). Fluid Mosaic Model
* It’s fluid b/c phospholipids & some proteins can move
laterally in the membrane.
* They rarely move across the membrane from one layer to
the other.
* Membrane will stay fluid as temp decreases, until it reaches
a temp where the membrane solidifies
* The more unsaturated fats in the membrane, the more
fluid it is, and the lower temp it can handle before freezing.
* Cholesterol- Molecule found wedged in between
phospholipids
* It reduces fluidity in membrane at body temp.
* As temp decreases, cholesterol keeps membrane from
freezing.
* Certain organisms change composition of their membranes in
the winter time in order to keep fluidity in colder temps.
* Membrane is a mosaic- A collage of many different proteins
1) Integral Proteins- Penetrate hydrophobic area of
membrane.
2) Peripheral Proteins- Not imbedded in lipid bilayer; they
are bound to the surface
* Functions of these proteins- Transport, enzymes, signaling
outside to inside, joining of cells, cell-cell recognition, or
attachment to extracellular membrane & cytoskeleton
Traffic Across Membranes
* Hydrophobic molecules, like CO2 & O2 dissolve easily into the
lipid bilayer & cross to the other side
* Ions don’t easily pass due to their charge
* Water, glucose, & other larger molecules don’t pass thru
easily.
* Transport proteins are the key to movement of all molecules
that don’t pass easily.
* They have a hydrophilic channel that the ions, water, or
other molecules can travel thru, or proteins hold onto their
passenger & move it to the other side.
* The proteins are specific for certain molecules.
What determines the direction they will move?
Passive Transport
* Diffusion- Tendency of molecules to randomly move from
area of high concentration to an area of low conc.
Ex- Membrane separating pure water from a dye solution.
- Water can’t move across this membrane, but dye can.
- Over time, dye leaks across to pure water side, until
both sides have equal conc. of dye molecules.
- Dynamic equilibrium is reached. Dye molecules will
still move, just equal amounts in both directions
* This process does not require input of energy from cell
Osmosis- Movement of water across a membrane according to
rules of diffusion.
Hypertonic Solution- Solution with more solute conc.
Hypotonic- Solution with less solute conc.
Isotonic- Solutions with equal solute conc.
Ex- U-tube w/ selectively permeable membrane. Water can
cross, sugar cannot.
Cells in hypotonic solution (less solute outside cell, more water
* Water enters & lyses/bursts an animal cell.
* Causes plant cell to be turgid with pressure against cell
wall. This is normal.
Cells in an isotonic solution
* Net movement in animal cell is zero, this is normal
* Net movement in plant cell is zero, causes cell to be flaccid,
or limp, b/c no water pressure buildup.
Cells in a hypertonic solution (Outside cell, more solute, less
water)
* Water leaves animal cell, it shrivels & dies
* Water leaves plant cell, it plasmolyzes & dies.
Facilitated Diffusion
* Movement of molecules with help of transport protein
* They are specific for certain molecules
* Found either as hydrophilic channels or as gated channels.
* Gated channels change shape when molecule binds to it.
* This moves the molecule across membrane
* Cell does not need to input energy, it is still movement down a
concentration gradient
Active Transport- Movement of molecules against conc.
gradient, from low conc. to high conc.
* Requires input of energy from cell.
* Ex- Compared to its surroundings, a cell has a higher conc.
of potassium ions & a lower conc. of sodium ions
* Sodium (Na+)-Potassium (K+)Pump
1) 3 Na+ ions bind to protein inside cell membrane; this
stimulates ATP to donate a phosphate group to the
protein
2) Phosphorylation causes protein to change shape
3) Shape change opens Na+ ions to outside of cell; this
leaves space for 2 K+ ions to bind to protein
4) This triggers release of phosphate group
5) Release of phosphate causes protein to change
shape, & K+ is released into cell; Na+ can now bind
and cycle repeats.
* Cytoplasm of a cell has a more negative charge than the
outside of the cell. This creates a voltage across the membrane, a
membrane potential.
* To keep this potential, the transport of cations into the cell b/c
they’re attracted to the negatively charged cytoplasm
* Sodium- potassium pump contributes to this negative potential
because it transports 3 Na- ions out, & only 2 K+ ions
in.
* Another important electrogenic pump is a proton pump that
transports H+ ions out of the cell.
Other types of active transport
a) Cotransport- Protons pumped out of cell w/ help of ATP
- H+ diffuse back in thru a cotransporter protein that
allows sucrose to enter the cell.
b) Exocytosis- Transport vesicle from Golgi fuses w/ cell
membrane & contents of vesicle spill into ECF.
c) Endocytosis- Taking in of macromolecules by forming new
vesicles from the cell membrane
i) Phagocytosis- Cell engulfs a particle by wrapping
pseudopod around it.
ii) Pinocytosis- Cell takes in droplets of fluid by forming
tiny vesicles along the membrane.
iii) Receptor-mediated endocytosis- Specific for certain
molecules
Tues and Wed lab, Fri Test Ch 7 & 8