UNIT 6.
BIOMEMBRANES AND TRANSPORT
OUTLINE
 Introduction.
 Lipids structures spontaneously formed in water.
 Fluid Mosaic model.
 Properties of the membranes.
 Membranes asymmetry.
 Membrane proteins.
 Membrane carbohydrates.
 Membrane transport: types of transport.
 Ionophores.
 Summary of the transport systems.
INTRODUCTION:
• Thin laminar structures characterised by their stability and flexibility.
• The plasma membrane separates the cytoplasm from the surroundings.
• They are responsible for:
- Exclusion of certain toxic ions and molecules rom the cell
- The accumulation of cell nutrients
- Energy transduction.
- Cell locomotion, reproduction, signal transduction processes and
interactions with molecules or cells in the vicinity.
• They are not passive borders.
• Biological membranes contain proteins with specific functions.
LIPIDS STRUCTURES SPONTANEOUSLY FORMED IN WATER:
Molecule
containing
one nonpolar tail.
Micelles
Molecule containing two nonpolar
tails.
Bilayer
Water
body
Liposomes
FLUID MOSAIC MODEL:
• It describes membrane dynamics:
- Lipid bilayer is the main structural skeleton.
- Proteins: integral membrane proteins and peripheral
membrane proteins.
• All the kingdoms, species, tissues and organelles are characterised by
the membrane lipid composition.
• Different membranes present different protein/lipids ratio.
FLUID MOSAIC MODEL:
Oligosaccharide side chain
Glycolipid
Lipid bilayer
Nonpolar
acyl chains
Polar groups
Peripheral
protein
Integral protein (one
transmembrane helix)
Peripheral protein
covalently link to a
lipid
Integral protein (several
transmembrane helixes)
PROPERTIES OF THE MEMBRANES:
 The membranes can recover their structures:
The membranes are able to spontaneously reorganise the
structure after suffering any kind of damage.
 The membranes have fluid like properties:
Although phospholipids are interacting by means of hydrophobic
interactions, the lipids area highly mobile in the plane of the
bilayer.
Lateral diffusion or rotation are
the main phospholipid diffusion
processes in a lipid bilayer.
However, the transverse diffusion
(flip-flop) is an extremely rare
event.
PROPERTIES OF THE MEMBRANES:
• The Fluidity of the membranes depends on:
Temperature:
The increase of the temperature promotes molecular diffusion,
and so that, promotes membrane fluidity.
Unsaturations:
The higher saturated phospholipids concentration
the higher interaction between phospholipids. As a
consequence of that, the fluiditydecrease.
PROPERTIES OF THE MEMBRANES:
Length of the hydrophobic chains:
Long
chain
phospholipids
establish
higher
number
of
interactions with other lipids chains. Short chain phospholipids
promotes membrane fluidity (they present higher mobility).
Cholesterol regulates the fluidity:
In animal cells, the fluidity of the membranes can be regulated
by cholesterol. In general, it decreases membrane fluidity
because its rigid steroid ring system interferes with the motions
of the fatty acid side chains in other membrane lipids.
PROPERTIES OF THE MEMBRANES:
Segmentos de
cadenas rígidas
Segmentos de
cadenas flexibles
Segmentos de
cadenas rígidas
Bicapa lipídica.
PROPERTIES OF THE MEMBRANES:
Ca2+ decreases the membrane fluidity:
Ca2+ is able to interact (charge-charge interactions) with
phosphate groups belonging to phospholipids.
These interactions promote highly compact structures
(decreasing membrane fluidity).
PROPERTIES OF THE MEMBRANES:
• Cells can modify the lipids composition at different temperatures to
keep the fluidity of the membrane constant.
• Bacteria cultures at low temperatures: high unsaturated fatty acid
concentration (less saturated fatty acid).
MEMBRANES ASYMMETRY :
• Plasma membrane lipids are asymmetrically distributed between the
two monolayers of the bilayer:
Asymmetric distribution of phospholipids between the inner and
the outer monolayers of the erythrocyte plasma membrane.
MEMBRANE PROTEINS:
• Proteins differ in their association with the membrane:
- They constitute around 50% of the overall membrane mass.
- Proteins are involved in solute transport, adhesion molecules,
enzymatic reactions (some of them are enzymes) or signal receptor.
• Membrane proteins are classified as integral membrane proteins or
peripheral membrane proteins.
MEMBRANE PROTEINS:
• Integral membrane proteins are firmly associated with the lipid bilayer,
and are removable only by agents that interfere with hydrophobic
interactions, such as detergents, organic solvents or denaturants.
MEMBRANE PROTEINS:
• Peripheral membrane proteins associate with the membrane through
electrostatic interactions and hydrogen bonding with the hydrophilic
domains of integral proteins and with the polar head groups of membrane
lipids (the do not cross the bilayer).
• These proteins can be released by relatively mild
treatments: pH o salt concentrations changes (non
covalent intearctions) or phospholipases (covalent
interactions)
Protein diffuse laterally in
the bilayer because of the
bilayer fluidity.
MEMBRANE PROTEINS:
• Not all integral membrane proteins are composed of transmembrane
α helices. Another structural motif common in bacterial membrane
proteins is β barrel.
Bacteriorhodopsin
Integral membrane protein.
Trnasbilayer disposition of
glycophorin in an erythrocyte.
MEMBRANE CARBOHYDRATES:
• Oligosaccharides covalently bounded to lipids (glycolipids)
or proteins (glycoproteins).
• Major monosaccharides: glucose, galactose, mannose,
neuraminic
acid,
N-acetylgalactosamine
or
N-
acetylglucosamine.
• They are exposed on the extracellular surface of the
membrane and they are involved in cell recognition, cell
adhesion or they act as receptors.
MEMBRANE TRANSPORT: TYPES OF TRANSPORT.
• Cells are opened systems exchanging matter and energy with the
surroundings.
• Charged molecules at physiological pH require a proper molecular
environment to establish interactions. So, cells need structures to
promote the mobility of these molecules.
MEMBRANE TRANSPORT: TYPES OF TRANSPORT.
 Classification:
1. Protein non-dependent transport:
- Simple diffusion.
2. Protein-dependent transport:
- Facilitated diffusion: No requires energy. Two types:
 Carrier proteins.
 Channels.
- Active transport: energy dependent. Specialised
carrier proteins are involved. Two types:
 Primary active transport.
 Secondary active transport.
MEMBRANE TRANSPORT: TYPES OF TRANSPORT.c
• Primary active transport: energy from light or ATP hydrolysis.
• Secondary active transport: against electrochemical gradient,
driven by ion moving down its gradient.
MEMBRANE TRANSPORT: TYPES OF TRANSPORT.
• Transport systems of the base of the solutes transported and the
direction of the transport:
- Uniport.
- Simport or Parallel cotransport.
- Antiport or Antiparallel cotransport.
MEMBRANE TRANSPORT: TYPES OF TRANSPORT.
 SIMPLE DIFUSSION:
• Down concentration gradient.
• No saturated by the substrates.
• No energy dependent.
• No carrier proteins.
• Examples: CO2, O2, H2O.
MEMBRANE TRANSPORT: TYPES OF TRANSPORT.
• Transmembrane proteins allow the transpot of charged or high
molecular mass solutes across the cellular membranes.
• Two types: CHANNELS and CARRIER PROTEINS.
• Carrier proteins: high stereospecificity, saturables and they
suffer conformational changes.
• Channels: less stereospecificity and non saturables.
MEMBRANE TRANSPORT: TYPES OF TRANSPORT.
 FACILITATED DIFFUSION:
• Down electrochemical gradient.
• i.e. Glucose transport into erythrocytes:
Topology representation of the glucose transporter (GLUT1)
MEMBRANE TRANSPORT: TYPES OF TRANSPORT.
 FACILITATED DIFFUSION:
• Kinetics of glucose transport into erythrocytes:
Substrate = glucose outside the cell (Sout)
Product = glucose inside the cell (Sin)
Enzyme = transporter
v0 =
Vmáx[S]out
Kt +[S]out
MEMBRANE TRANSPORT: TYPES OF TRANSPORT.
 PRIMARY ACTIVE TRANSPORT:
• Against electrochemical gradient.
• I.e. sodium-potassium pump = Na+-K+ ATPase.
MEMBRANE TRANSPORT: TYPES OF TRANSPORT.
SECONDARY ACTIVE TRANSPORT:
• I.e. Intestinal epithelial cells: glucose is cotransported with Na+ across the apical
plasma membrane into the epithelial cell. It moves through the cell to the basal
surface, where it passes into the blood via GLUT2 (passive glucose uniporter). Na+-K+
ATPase pumps Na+ outward to maintain the Na+ gradient that drives glucose uptake.
MEMBRANE TRANSPORT: TYPES OF TRANSPORT.
 Energetics of pumping by symport:
A solute is transport from a region where the solute concentration is
C1 to another region with solute concentration equal to C2, (bounds are
not broken or established), so ∆G0’ = 0:
∆Gt = RT ln (C2/C1)  Uncharged solutes.
 If the solute is an ion, its transport generates en electrical potential:
Electrogenic transport. The energy required to transport an ion is the
result of the chemical and electrical gradients:
∆Gt = RT ln (C2/C1) + Z F ∆ψ
Z = ion change
F = Faraday constant
∆ψ = transmembrane electrical potential
TRANSPORT ACROSS MEMBRANES. IONOPHORES:
 They collapse ion gradients across cellular membranes.
 They are ion carriers.
 They increase the membrane permeability.
 Cell death is caused by secondary transport inhibition.
 Types:
K
+
Movil carriers (i.e. valinomycin)
Channels (strict ionophores) (i.e.
gramicidine)
MEMBRANE TRANSPORT. SUMMARY:
SIMPLE DIFFUSION (nonpolar
compounds only, down
concentration gradient)
FACILITATED DIFFUSION (down
electrochemical gradient)
PRIMARY ACTIVE
TRANSPORT (against
electrochemical gradient)
IONOPHORE-MEDIATED
ION TRANSPORT (down
electrochemical gradient)
ION CHANNEL (down electrochemical
gradient; may be gated by a ligand or ion)
SECONDARY ACTIVE TRANSPORT
(against electrochemical gradient,
driven by ion moving down its
gradient)