Membranes
And
Cell Transport
• Concept 6.2: Membranes organize a
Cell’s Activities (Plasma membrane thin
barrier between cell and environment, but
regulates what can enter/leave cell.)
•
• I. Membrane Structure – key to how
membrane does its job
•
A. Membrane jobs: compartmentalize
teams of enzymes, regulate what
enters/leaves areas
•
B. Parts: (of plasma membranes
and all other organelle membranes)
• . Phospholipid molecules and proteins
•
a. Phospholipids (see Fig. 6.7 p. 115) –
phosphate head (hydrophilic) and 2 fatty acid
tails (hydrophobic) (Heads attracted to water,
tails repelled by water)
1.) Membranes – double layer =
Phospholipid bilayer
- separates 2 watery environments
from each other
•
(ex. Cytoplasm,
•
exterior envir.)
• 2.) See orientation of parts on Fig. 6.8 p.
116
3.) hydrophobic tails face middle of
membrane – help make membrane
selective barrier
• 4.) Nonpolar molecules can cross
phospholipids easily (O2 and CO2)
• 5.) Polar molecules (sugars, many ions)
cannot cross as easily
Membranes Dynamic = semi-rigid,
parts flow within them
• a. Proteins drift freely through the
phospholipids, like “icebergs” float in the
“sea”
II. The Many Functions of Membrane
Proteins (See Fig. 6.9, p. 117)
• A. Membrane contains “Mosaic” of
Proteins (Fluid-Mosaic Model – not in text)
•
1. May also contain some carbs. stuck
into it, too
B. Functions of these Membrane
Proteins
• 1. Enzyme activity – to help carry out some of
the cell’s chemical rxn.
2. Cell-to-cell recognition: (Transplant issues,
blood group, etc. – Ex.)
3. Cell signaling: chem. signals from 1 cell may
be picked up by proteins in another cell
• for action
4. Transport of Materials: Serve as channels to
help some materials cross the plasma
membrane (Ex. Sugars)
Concept 6.3: Membranes
regulate the Traffic of Molecules
• 1. Cell membranes are “ Gatekeepers”
– Control what enters and leaves cell, and what
enters/leaves all internal organelles, too.
– (Ex. Water, nutrients, dissolved gases, CO2,
& wastes)
a. Protein molecules that form chemical
“pumps” and channels to move material
across membrane
TYPES OF CELL TRANSPORT
PASSIVE VS. ACTIVE
TRANSPORT
• Passive Transport:
– Caused by the random motion of molecules
(Because all matter has kinetic energy)
= “Brownian motion”
- Does NOT require any of the cell’s own
energy
- Cell can be dead or alive, and will still happen
http://lessons.harveyproject.org/development/ge
neral/diffusion/diffnomemb/diffnomemb.html
• Active Transport:
– Requires the cell to use energy in the form of
ATP
– Cell must be alive
– http://highered.mcgrawhill.com/sites/0072495855/student_view0/cha
pter2/animation__how_the_sodium_potassiu
m_pump_works.html
B. Diffusion- Movement of materials from
areas of high concentration to areas of low
concentration
1. All living cells have a liquid interior
(cytoplasm) and are surrounded by liquid
2. Diffusion causes many
substances to move across
cell membrane but does not
require the cell to use energy
(called passive transport)
3. Equilibrium- when conc. of
molecules are equal on both
sides of membrane
4.
http://www.nsf.gov/news/news_videos.jsp?org=NSF&cntn_id=10579
7&media_id=57229
EXAMPLES OF DIFFUSION
• Air Freshener (or other obnoxious odor)
spreading through a room
• Dye spreading in a glass of water
•
http://www.youtube.com/watch?v=ASGj_RH4Voc
• O2 or CO2 entering or leaving a cell
•
http://www.indiana.edu/~phys215/lecture/lecnotes/diff.html
C. Osmosis- Diffusion of water molecules
through selectively permeable membrane
1. Most membranes are selectively
permeable (some materials can pass
across membrane and others cannot) like
“sieve”
Types of Solutions to which Cells
may be exposed:
• Isotonic: iso = “same” , tonic refers to
“strength” of solute concentration
- Isotonic solutions have same conc. Of
solutes as the cell’s cytoplasm
• Hypotonic: hypo = “less or below”,
•
- Hypotonic solutions have a lower
conc. of solutes than the cell’s cytoplasm =
more water conc.!
Types of Solutions to which Cells
may be exposed:
• Hypertonic: hyper = “more, above”,
- hypertonic solutions have a higher
solute conc. than the cell’s cytoplasm
= less water
2. Effects of osmosis on cell
a. Isotonic- “same strength”
1.) Effect on animal cells – no change
2.) Effect on plant cells – no change
Ex. Contact lens solutions and IV’s for patients
hypotonic
solution
hypertonic
solution
Isotonic
solution
2. Effects of osmosis on cell
b. Hypotonic- “below strength”
1.) Effect on animal cells – in danger of
bursting = “lysis” (1-celled orgs. – have
contractile vacuole to prevent this!)
2.) Effect on plant cells – cell wall prevents
bursting, “turgor pressure” pushes
outward from water vacuole on cell wall,
allows plants to stand upright! (Think
fresh, crispy carrots!)
hypotonic
solution
hypertonic
solution
Isotonc
solution
2. Effects of osmosis on cell
Hypertonic- “above strength”
1.) Effect on animal cells = shrinking or
shriveling
= “crenate”
2.) Effect on plant cells = plasma membrane
pulls inward away from the cell wall as the water
vacuole shrinks
= “plasmolysis”
Hypertonic
3. Osmotic Pressure- osmosis exerts pressure
on hypertonic side of selectively permeable
membrane
a. Cells filled with salts, sugars, proteins and
other molecules- always hypertonic to fresh
water
b. Osmotic pressure produces movement of
water into cell that is surrounded by fresh
water.
c. Fortunately cells don’t come into contact
with fresh water. Usually bathed in isotonic
solutions (e.g. blood) or protected by cell
wall (e.g. bacteria) or mechanism to pump
out excess water (contractile vacuole)
D. Facilitated Diffusion- membrane proteins
help (faciliate) the movement of certain molecules
across the cell membrane by diffusion (from high
conc. To low conc.) that can’t diffuse through lipid
bilayer directly.)
1. Many different protein
channels for ions, sugars,
salts.
2. Does not require addition
of energy (passive transport)
E. Active Transport- movement of molecules
and ions against a concentration difference (from
low to high)
1. Requires input of energy
2. Small molecules “pumped” across
membrane (e.g. Na+ and K+ ions)
a. Endocytosis- movement into cell by
means of “infolding”
1). Pinocytosis- “cell drinking”
2). Phagocytosis- “cell eating”
b. Exocytosis- removal of large amounts of
material from a cell. Membrane of vacuole
surrounding the material fuses with the cell
membrane