1. Homeostasis and Cell Transport
MORE EFFICIENT THAN JAPANESE
RAILWAYS

2. The plasma membrane
Why is it so awesome?
 Chemical exchange discrimination=selective
permeability
 Scientists conjecture its place in evolution
 It may be only 8nm thick, but it controls inter-cell
traffic.
 They vary according to function.
Proteins determine the membranes’ specific
functions.
Ex: Mitochondrial membranes have a greater
percentage of proteins.

3. The Fluid Mosaic Model
Did it hurt?
Did what
When you
hurt?
fell from
heaven and
started
selectively
permeating
my heart.
<3

4. The Fluid Mosaic Model
 Held together by hydrophobic interactions (weaker than covalent bonds)
 Membrane=mosaic of protein molecules bobbing in fluid of phospholipids
 Maximizes the contact of hydrophilic regions of phospholipids and proteins
 Provides hydrophobic parts a nonaqueous environment
 Lateral movement of lipids and proteins
 Phospholipids move 2μm/s (rarely flip-flop across the membrane)
 Proteins move more slowly (larger)
Some proteins may be driven along cytoskeletal fibers by motor proteins (in its cytoplasmic regions)

 Temperature
 Membrane solidifies once reaching a certain cold temperature
 Cholesterol
 Wedged between phospholipid molecules in animal cell membranes
 At warm temperatures: makes membrane less fluid by restraining phospholipid movement
 At cold temperatures: hinders the close packing of phospholipids; temperature required for
membrane solidification is lowered
 Solidification?
 Permeability changes, enzymatic proteins become inactive
 To avoid: increase unsaturated phospholipids (ex: winter wheat)

5. Components of the Membrane
 Amphipathic molecules + Cell Membranes= BFFs
 Amphipathic molecule: both hydrophilic and hydrophobic regions
 Lipids
 Phospholipid bilayer (amphipathic)
 Proteins
 Membrane proteins(amphipathic)
Integral Proteins: penetrate the hydrophobic core of the lipid bilayer

Peripheral Proteins: not embedded in the bilayer, loosely bound (often to

parts of integral proteins)
 Carbohydrates
 Glycoproteins
 Glycolipids
 Note: membranes have distinct inside and outside faces
 Molecules that start out on the inside ER face end up on the outside face
of the membrane, and vice versa.

6. Lipids
 Most abundant lipids in membranes=phospholipids
 Two lipid layers may differ in lipid composition
 Why lipids?
All membrane lipids are amphipathic.

Unsaturated hydrocarbon tails have kinks keeping from molecules

from packing together (enhancing membrane fluidity)
Hydrophobic (nonpolar) molecules (CO2, hydrocarbons, O) can

dissolve in lipid bilayer
Hydrophobic core impedes transport of ions and polar (hydrophilic)

molecules (water, sugars, charged atoms or molecules)
 Cell adjusts lipid composition in changing temperatures
to maintain fluidity.

7. Proteins
 Has a directional orientation in the membrane
 More than 50 kinds have been found so far
 Functions:
Transport:

some have hydrophilic channels that allow certain molecules or ions to pass through; some

hydrolyze ATP in order to actively pump substances across the membrane
Enzymatic activity:

embedded proteins may protrude so that its active site is exposed to substances

(sometimes, in multiple, enzymes are ordered to carry out sequential steps of metabolism)
Signal transduction:

binding sites may have specific shapes that match with chemical messengers, causing

conformational changes to relay messages to the inside of cells
Intercellular joining:

adjacent cells hook together in various kinds of junctions

Cell-Cell recognition:

glycoproteins (proteins with oligosaccharides) serve as identification tags

Attachment to the cytoskeleton and ECM:

bonds to cytoskeletal structures maintain cell shape and fixes a protein’s location


8. Carbohydrates
 Only found on the exterior surface of the cell
 Important for cell to cell recognition
 Ex: sorting of cells into tissues and organs in embryos
 Usually branched oligosaccharides (fewer than 15
monosaccharides)
Oligo=few in Greek

Glycolipids=oligosaccharides covalently bonded to lipid

Glycoprotein=oligosaccharides covalently bonded to protein

Vary from species to species, individuals among a species, and

one cell type to another within an individual

9. Traffic Across Cell Membranes
PUTTING THE MEMBRANE TO USE
Yeah, it’s that complex. Not really.

10. Diffusion
 Principles:
 A substance will diffuse down its concentration gradient (where it is
more to less concentrated)
Imagine a group of molecules spreading out in space

Result of thermal motion (intrinsic kinetic energy)

Movement: random for individual molecules, directional for population

of molecules (ex: red dye in water)
Increases entropy by producing a more random mixture

Each substance diffuses down its own concentration gradient and is not

affected by other substances’ concentration differences.
 In action:
 Occurs when a substance that is permeable is concentrated on one side
of the membrane
 Ex: uptake of oxygen by a cell performing cellular respiration; dissolved
oxygen diffuses into the cell across the plasma membrane
 Did you know that diffusion was the simplest type of passive transport?

11. Passive Transport
 The diffusion of a substance across a biological
membrane
 Requires no energy
Concentration gradient represents potential energy, drives

diffusion
 Types:
 Diffusion
 Osmosis
 Facilitated Diffusion
 Filtration

12. Osmosis
 The passive transport of water; the diffusion of water
molecules across a selectively permeable membrane
 Water will diffuse across the membrane from the
hypotonic solution to the hypertonic solution
What in the world does that mean?

Hypertonic: higher concentration of solutes

 Hypotonic: lower solute concentration
 Isotonic: equal solute concentration
 Translation: Water will move from areas of higher (water)
concentration to lower (water) concentration, depending on the
amount of solute.
 Direction is determined only by total solute
concentration differences

13. Cell Survival and Osmosis
 Osmoregulation: control of water balance
 Membranes are adapted to environments, ex: paramecium
 Animal/wall-less cell water balance in ___ environments:
 Isotonic: Optimal!=no net movement of water, 
 Hypertonic: lose water to environment, shrivel, die, 
Ex: animals die when lake salinity increases

Hypotonic: water enters faster than it leaves, swell, lyse (burst), 

 Plant cells water balance in ___ environments:
 Isotonic: no net tendency for water to enter, flaccid cells (wilted
plant), 
 Hypertonic: cell loses water to environment, shrinks (membrane pulls
away from wall=plasmolysis), usually dies, 
 Hypotonic: wall helps maintain water balance, turgid (firm)
state=healthy! 

14. Facilitated diffusion
 Diffusion of polar molecules and ions across the membrane by transport
(carrier) proteins
 Facilitated diffusion is a passive process because the solutes still move
down the concentration gradient.
 Transport proteins
Has specialized binding site (like enyzmatic active site) for the solute it

transports
Can be inhibited by “imposters” compete with normal soutes

Some undergo subtle shape change that translocates solute-binding site

across the membrane
Channel proteins: provide hydrophilic “corridors” to allow a specific

molecule/ion to cross membrane (channel proteins)=quick flowing (ex:
aquaporins)
Gated channels: stimuli (electrical signals or chemical signals [ex: nerve cells by

neurotransmitter molecules] or stretching of the cell membrane) cause proteins to
open or close
 Speeds the transport of a solute by providing an efficient passage through
the membrane

15. Facilitated Diffusion: Examples
 Ex: of polar molecule tranport:
 Transport of Glucose: sugars are polar molecules; cannot
simply diffuse across membrane
 Glucose requires a specific carrier protein to cross membrane
(in or out of cell)
 Ex: of ion transport
 Cl -, Na+, K+, Ca 2+ are moved across the membrane through
carrier proteins

16. Ion Movement Across the Membrane
 Movement depends on concentration gradient (chemical) and
voltage differences across the membrane
 Membrane potential: voltage across a membrane
Ranges from -50 to -200 mmV

Acts as energy source that affects the traffic of all charged substances

across the membrane
Favors the passive transport of cations in and anions out (because

cytoplasm of a cell is negative in charge compared to the extracellular
fluid)
 Electrochemical gradient: combination of electrical and chemical
forces that affect ions during passive transport
 Active ion transport
Electrogenic pump: transport proteins that generate voltage across a

membrane, store energy that can be tapped for cellular work
(cotransport)
Proton pump: actively transports H+ out of the cell


17. YEAH
Active Transport !
 The “uphill” movement of solutes up their concentration
gradient across the plasma membrane
 In order to pump a molecule against its [ ] gradient, a cell
must expend its own metabolic energy
 A major factor in the ability of a cell to maintain internal
concentration of small molecules that differ from
concentrations in its environment
 Performed by specific embedded (integral) proteins
ATP usually supplies energy for active transport by transferring its

terminal phosphate group directly to the transport protein, causing a
conformational change (causing solute to translocate across
membrane)

18. The Infamous Sodium-Potassium Pump
 Cells maintain high internal K+ concentration (pump it
in) and low internal Na+ concentration (pump it out)
 For every 3 Na+ pumped in, 2K+ are pumped out.
 Generates voltage (membrane potential) across
membrane
 Steps (fig. 8.15, pg. 149):
1. Binding of cytoplasmic Na+ to the protein stimulates

phosphorylation by ATP
2. Phosphorylation causes the protein to change its conformation

3. The conformational change expels Na+ to the outside, and

extracellular K+ binds.
4. K+ binding triggers release of a phosphate group.

5. Loss of phosphate restores original conformation.

6. K+ is released and Na+ sites are receptive again; cycle repeats


19. Cotransport
 Single ATP-powered pump transports a specific solute
can indirectly drive the active transport of several other
solutes
 Primarily used in the transport of amino acids and sugars
 How?
A substance that has been pumped across can do work as it leaks

back by diffusion
Transport proteins can couple the downhill diffusion of a substance

to actively transport another substance
Ex: return of H+ helps to actively transport sucrose against its

concentration gradient; used by plants to load photosynthesis-
produced sugars into leaf veins

20. The Transport of
Macromolecules
WHEN PROTEINS AND DIFFUSION
CAN’T GET THE JOB DONE.

21. Exocytosis: into cell via vesicles
 How do cells secrete
macromolecules?
Transport vesicle (that budded

from the Golgi apparatus)
surrounding macromolecule
fuses with plasma membrane
Vesicle moves across

cytoskeleton on its way
The two bilayers rearrange

themselves so that the two
membranes fuse and the
vesicle contents spill outside
Ex: export of cell products

(pancreas cells and insulin,
neuron and chemical signals to
stimulate other
neruons/muscle cells)

22. Endocytosis: out of cell via vesicles
 Used when molecules are too large to be moved by simple
diffusion or transport proteins
 Pinocytosis: (drinking)
“Gulps” droplets of extracellular fluid into tiny vesicles

Nondiscriminatory: any and all solutes dissolved in the droplet are taken

into the cell
 Phagocytosis: (eating)
 Engulfing: wrapping pseudopodia around in order to package particle in
vacuole
 Digestion: vacuole fuses with a lysosome with hydrolytic enzymes
 Receptor-mediated: (specific)
 proteins with specific receptor sites (clustered in coated pits, which form
the vesicle) are exposed to extracellular fluid
 Enables cell to acquire bulk quantities of specific substances (ex:
cholesterol binding to low-density lipoproteins [LDLs])

23. In receptor mediated
1)
endocytosis, coated
pits form vesicles in
which the particles
are taken into the
membrane.
In
2)
pinocytosis, extracell
ular fluid is engulfed
by a food vesicle that
takes the solute(s)
into the cell.
In
3)
phagocytosis, solid
particles are
engulfed by
The Three Types of Endocytosis
pseudopodia that
form a food vacuole
called a phagosome.

24. Fin.
I HOPE THAT YOU HAD
EX-CELL-ENT EXPERIENCE.