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Lectures 3 and 4
- Lipid bilayer permeable to some molecules:
o Dependent on hydrophobic tails, core of bilayer = hydrophobic so hydrophobic molecules easily pass
through bilayer because they can interact easily with the tail (eg. O2, CO2, N2, benzene). Anything with
high Hydrophobicity is able to go through bilayer
o Small molecules that are uncharged and polar are able to cross but they don’t cross that easily. (eg.
H2O, urea, glycerol)
o Movement of molecules is through simple diffusion through bilayer
High concentration to low concentration (down concentration gradient)
More hydrophobic or non polar the molecule, the faster it is able to diffuse across the bilayer by
- Lipid bilayer is nonpermeable to some molecules:
o Don’t interact well with hydrophobic tails
o Require membrane proteins for transport. Important for cellular processes
o Eg. (Large uncharged polar molecules: glucose, sucrose. Ions: anything with a charge)
- Membrane transport proteins: always multipass Transmembrane proteins to create 3-D pore that allows
molecules to move through protein and not interact with lipid tails
o There is a protein lined path across cell membrane through hydrophobic core created by tails transport
polar and charged molecules mainly
Eg. Ions, sugars, AA, nucleotides, various cell metabolites
o Different cell membranes different transport proteins – dependent on function
o Each transport protein is selective: it transports a specific class of molecules or even 1 specific molecule
within that class
o Highly specific and can distinguish very small differences in shape of molecules
- Passive and active transport
o Passive transport:
1. Channel mediated: gated open or close pass through down concentration gradient
2. Transporter mediated: don’t need NRG just need path across membrane. They specifically bind to a
molecule and then undergo conformation changes that lets it across bilayer and protects it from
- Active transport: against concentration gradient. Needs NRG
- Resting membrane potential: contributes to the ability of ions to move across cell membrane
o Positive outside, negative inside
- Concentration gradient and membrane potential = electrochemical gradient – when charged molecules are
moving across the membrane.
- Types of active transport:
o Active transport is against electrochemical gradient of at least 1 molecule. You need NRG to do this
1. Coupled transporters (active transport) 1 molecule is moving down the electrochem gradient which
provides NRG for another molecule to transfer against the gradient.
2. ATP-driven pumps: use ATP hydrolysis (NRG stored in phosphate bond in ATP to move molecule
against gradient (ATP ADP).
3. Light-driven pumps (bacteria) they use light NRG to move molecules against the gradient.
- TRANSPORTER PROTEINS: passive transport by transporter proteins (Uniporter) move only 1 molecule.
o It binds specific solutes. It binds to molecule. They will transport, interacts in binding pocket, then
undergo conformational change that transports solute across membrane.
o Direction of transport is reversible and dependent on direction of EC gradient.
o Works by facilitated diffusion
o Eg. GULT Uniporter: transports glucose across cellular membrane. (Passive process). Down  gradient,
can work in both directions. Glucose higher in extracellular environment
- Active transport by transporter proteins
1. Coupled transporters
a. Symporter Na+/ glucose symporter (two molecules, move in same direction across
- Na+/ glucose symporter:
o Na+ (higher outside) goes down electrochemical gradient. Symporter moves Na+ into cell, move glucose
against concentration gradient. Cooperative binding the Na+ and glucose leads to a conformational
change in the protein – opens binding pockets of intracellular environment and Na+ and glucose then
leave protein and diffuse into intracellular environment.