Lecture 8: Membrane Transport, Part I Flashcards
Different molecules diffuse through the membrane at different rates due to
lipid bilayer having selective permeability
What allows the lipid bilayer to have selective permeability?
Thin layer of hydrophobic material in aqueous environment
What molecules rapidly diffuse across the membrane?
Small, nonpolar molecules
Ex. O2, CO2, N2, steroid hormones
What molecules diffuse across the membrane?
Small, uncharged polar molecules
Ex. H20, Urea, glycerol, NH3
What molecules rarely diffuse across the membrane?
Large uncharged polar molecules
Ex. Glucose, sucrose
What molecules cannot cross the membrane?
Charged Ions
Ex. H+, Na+ HCO3-, K+, Ca2+, Cl-, Mg2+
Collection of isolated membrane lipids (which contain proteins) thrown into water is example of
hydrophobic interactions as membrane lipids will form sphere around proteins –> causes proteins to be locked in and are very large, polar molecules (often have charges) with no way to escape
What component of lipid bilayer prevents passage of hydrophillic, water soluble molecules?
Hydrophobic interior of lipid bilayer of hydrophobic tails
In living cells, hydrophilic molecules typically pass through membrane using
transmembrane proteins
What are the 2 Types of Transmembrane Proteins cells use to import or export molecules?
- Transporters
- Ion Channels
Similarities between Transporters and Ion Channels
Both multipass proteins to form hydrophilic pores
Arrange alpha helices for hydrophobic outside with nonpolar amino acids and hydrophilic on the inside with polar amino acids
Transporters
Transmembrane protein that undergo conformational changes for each specific molecule, solute or ion
Conformation change takes time so lower rates of transport
Can do passive and active transport
Ion Channels
Transmembrane protein that for hydrophilic pores which can open or close to transport ions and water
After channels open, no need for conformation change so much higher rates of transport
Only does passive transport
Transporter and Channels regulate
ion concentrations within the cell
Sodium (Na+) concentration in mammalian cell
High concentration outside cell
Low concentration inside cell
Potassium (K+) concentration in mammalian cell
High concentration inside cell
Low concentration outside cell
Direction of transport for solutes often depends on
concentration between inside and outside of cell
Simple Diffusion
Describes ability of molecule to travel through lipid bilayer without transmembrane protein (unassisted)
Hydrophobic molecules diffuse very quickly
Large polar charged molecules diffuse very slowly or not at all –> rate of simple diffusion is too slow to meet the cell’s needs
Passive transport/ Facilitated transport (diffusion)
Solutes spontaneously travel from high to low concentration using transmembrane protein (assisted)
No energy is required due to concentration gradient
Done by all channels and some transporters
How is water passive transported?
Water is passively transported by channel proteins called aquaporins to form hydrophilic pore with alpha helices for water to passively pass through
Which transmembrane proteins use passive transport?
All channels
Some transporters
Active transport
Solute travels from low to high concentration
Energy is required and is carried out by transporters
What is common source of active transport that proteins use?
ATP as transmembrane proteins hydrolyze ATP which releases energy and couple reaction as energy pushes against gradient
How does cell know to use passive or active transport for uncharged molecules?
Ex. glucose
Dependent on concentration gradient
