Download Lecture 8: The cell membrane

Membranes
• A cell’s plasma membrane defines a cell and
separates it from its environment. Yet at the
same time, it must allow for interaction between
the cell and its environment.
Simple Membranes and the First Cells
 Biological Evolution began with the first selfreplicating molecule
 The first cell arose when this molecule became
enclosed in a membrane
CHEMICAL EVOLUTION
Light energy
Heat
H
H C O
1. Simple molecules in
atmosphere of ancient
Earth
2. Reduced carboncontaining compounds
H H HOCH2 O OH
C H H C
H C C O
H C C H
H
HOOH
3. First carboncarbon bonds
Membranes are Composed of Phospholipids
• Amphipathic lipids can form a bilayer
structure in an aqueous solution
One phospholipid:
Phosphate group
(hyd rophilic)
Glyce rol
Fatty Acids
(hyd rophobi c)
Lipid Bilay er:
Liposomes: Artificial membranebound vesicles
Water
Water
62.5 nm
Liposomes will form spontaneously in solution
The Fluid Mosaic model of
Plasma Membranes
• Phospholipid Bilayer
• Cholesterol
• Glycocalyx
• Proteins
Evidence for the drifting of membrane proteins
Component 1: Phospholipid bilayer
Phospholipid bilayer
Summary of relative permeabilities
Hydrophobic molecules
O2, CO2, N2
Small, uncharged polar
molecules
H2O, glycerol
Large, uncharged polar
molecules
Ions
Glucose,
sucrose
H+,Na+,NCO3–,
Ca2+,CL-,Mg2+,K+
Phospholipids may have saturated
or unsaturated fatty acid tails
Double bonds cause H C
kinks in hydrocarbons. 2
CH2
H2C
CH2
H2C
CH
Kink
CH
H2C
H2C
H2C
CH2
CH2
Unsaturated fatty acid Saturated fatty acid
Double bonds present,
fewer H atoms
No Double bond,
maximum H atoms
Kinks change the fluidity and permeability
of membranes.
Lipid bilayer
with no
unsaturated
fatty acids
Low fluidity
Low permeability
Lipid bilayer
with many
unsaturated
fatty acids
High fluidity
High permeability
Component 2: Cholesterol fills spaces
between phospholipids.
Polar
Nonpolar
The more cholesterol within the membrane, the less permeable it is
Cholesterol keeps the membrane fluid at low temps, prevents it
from becoming too fluid at warmer temps
Component 3: Membrane Proteins
• Integral proteins
• Transmembrane proteins
• Peripheral Proteins
Transmembrane proteins are amphpathic
Glu
Polar amino acids
Tyl
Met
Pro
Ile
Pro
Gly
Ser
Asp
Non- Polar
amino acids
Some functions of membrane proteins
Component 4: Glycocalyx
• Sugar molecules
attached to the outer
surface of the plasma
membrane
• Glycoproteins:
oligosaccharides
attached to proteins
• Glycolipids:
oligosaccharides
attached to lipids
• Used for cell
identification: antigens
Movement of Substances Across Membranes
• Diffusion
• Molecules will diffuse from
areas of high concentration to
areas of lower concentration
• Diffusion is a spontaneous,
passive process (doesn’t
require the input of external
energy) .
Each type of molecule will travel down its concentration gradient
across a permeable membrane
Water will move down (diffuse) its own
concentration gradient--> osmosis
solvent
solute
water is also moving in the direction of
lower solute concentration to higher
solute concentration
Water enters vesicle if internal solution
is hypertonic to the external solution.
Start with:
Hypertonic
solution
Hypotonic
solution
Isotonic
solution
Arrows
represent
direction
that water
moves via
osmosis
Result:
Membrane
shrinks
Membrane
swells or
even bursts
No change
Figure 8.12 The water balance of living cells
Movement of Substances Across Membranes
• Movement across membranes is affected by
the presence of membrane proteins.
• 3 types of transporter proteins:
• Channel Proteins
• Carriers
• Pumps
Facilitated diffusion follows the concentration gradient
and requires no input of external energy
Channel Protein
Carrier Protein
Active transport through pumps moves molecules against
their concentration gradients and requires energy
The sodium-potassium pump
Summary of Transport across membranes
Electrogenic pumps generates a membrane potential
(voltage) across the membrane
The generation of a gradient can be used to do work
or cotransport other molecules across a membrane
Getting Big Stuff Out of Cells
Exocytosis
Getting Big Stuff into cells
The three types of endocytosis in animal cells
Phagocytosis
Pinocytosis
Receptor-mediated
endocytosis
When transport doesn’t work:
Cystic Fibrosis
• Cystic fibrosis is a defect in
the Cl- transporter, CFTR
• About 1 baby in 2000 of
Northern European racial
origin is affected
• Symptoms
• thick, sticky mucus that
clogs the lungs and leads
to lung infections.
• Obstruction of the
pancreas, preventing
digestive enzymes from
reaching the intestines