6 - Biological Membranes (part?)
• State the rationale used to classify biological molecules as a lipid.
-non-polar -hydrophobic -insoluble in water
• List four distinct types of molecule that are classified as lipids (all lipids seems fatty acids)
1. Fatty acids 2. Triacylglycerol 3. membrane lipids 4. cholesterol
• List the three major types of membrane protein and state how each is associated with the membrane. (see slide 33 of fatty acid slides for diagram)
1.Integral membrane proteins (held in place by hydrophobic interactions from the midrift section of the proteinX membrane with HYDROPHOBIC AMINO ACID SIDE CHAINS ON ITS SURFACE, while polar surfaces protrude in both the internal and external aqueous environments. Ex. K+ channel) 2. Peripheral membrane protein (polar/electrostatic, not embedded in any special way and may associate with negative charge of membrane as it just sits on top of the membrane.) 3. Lipid linked protein (hydrophobic prosthetic group sits inside the membrane, AND there may also be peripheral membrane protein qualities of polar interactions between protein and membrane)
• Describe the structure of a triacylglycerol.
Aka TAG- way of storing fatty acids. Very hydrophobic, 3 fatty acids bonds to glycerol. 'Acyl' refers to hydrocarbon tail. TAGs are NOT even amphipathic- just hydrophobic. Usually TAGs are mixed and in animals= lard because triglycerides are more saturated than oils.
• Identify the alpha- omega- and beta-carbon atoms in a fatty acid.
An alpha carbon is next to the carboxyl group in fatty acids. Beta carbon is the second carbon following the alpha carbon. Omega carbons is the furthest carbon from the carboxyl group, and the ones in the middle
• Explain why lipid bilayers are fluid, yet stable, structures.
Because of their NON COVALENT ASSEMBLY. Their composition varies, but the membrane lipids are always made of acyl chains with polar head groups. Cholesterol gets almost entirely buried in the layer and helps keep the lipid fluid in low temperatures and together in high ones.
• State why the lipids and proteins in a membrane bilayer typically move only laterally.
Because the hydrophobic section within the membrane prohibits the polar heads to cross, but as the membrane is fluid they can easily swap places laterally. Lateral diffusion in a particular leaflet is easy and fast- but transverse diffusion (flip-flop) is not so easy- flipase enzymes increase the rate and allow for differences in lipid composition in leaflets, BUT a significant energy barrier is associated with desolvating a polar head group to move it through the hydrophobic area.
• State why fatty acids are termed amphipathic.
Because they have BOTH polar (acidic) portions and nonpolar (fatty)
• State the function of cholesterol in biological membranes.
Cholesterol is RIGID and PLANAR, and limits the ROTATIONAL MOVEMENT of acyl groups around it to INCREASE VAN DER WAALS INTERACTIONS. But splits them up in the case of tight packing in cold. For one thing, cholesterol can migrate in the fatty acid/nonpolar section of the membrane. Cholesterol helps hot membranes not wiggle around so much and maintain their integrity (decreases motion/ disorder, increasing rigidity), and cold ones stay loose enough not to freeze, lose their fluidity, and shatter (prevents close packing between acyl chains).
• Sketch cis and trans double-bonds, and compare their effects on the shape of a fatty acid.
Cis bonds are more enzymatically useful because they're just straight when you draw them. Trans bonds are more energetically favorable because
• Distinguish between primary and secondary active transport processes.
Depends on the substrates being transported! Primary usually uses ATP as the source of free energy (just go the really basic, need energy way of active transport) Secondary uses an ion gradient of something else as a source of free energy for the transport of the target molecule against its gradient.
• Compare the behaviour of fatty acids, membrane lipids, and TAGs when they are mixed with water.
Fatty acids are nonpolar so they exclude water (form mycelles?
• Identify the most likely locations for hydrophobic and polar amino acid side chains, given the structure of an integral membrane protein.
Hydrophobic amino acid side chains will be in the middle, facing the hydrophobic membrane region, while the peripheral parts will have the polar amino acid side chains to interact with polar aqueous environment.
• Describe the two most common structures seen in proteins which cross a lipid bilayer.
Ion channels: regular secondary structures- alpha helices (when transmembrane, generally hydrophobic) and beta sheets/barrels cross membranes because these regular structures satisfy the hydrogen bond potential of the polypeptide backbone. They aren't even active transport though = fully passive! Transporter (carrier) proteins: don't have membrane spanning pores and can be active OR passive. Works by doing conformational changes, transporting selectively from one side to other!
• Explain the effect of chain length and temperature on the melting point of a fatty acid.
Longer chain length of saturated fatty acids raises melting point . Unsaturation ruins everything and has a drastic effect on decreasing melting point, even if it's a really really really long chain.
• Describe how changes in fatty acid composition affect the fluidity of a biological membrane.
Longer fatty acid tails mean more van der waals forces, making the membrane less fluid and adhere to itself more. To bear lower temperatures better (have lower melting point?) we want a higher degree of unsaturation and shorter acyl chains (make it harder for them to stick together and die). Below the transition temperature, acyl chains can pack together in van der waals contact in a gel-like solid state, and above this transition temperature, the membrane enters a disordered LIQUIDE crystalline phase (not looking so good), where LIPID MOLECULES and ACYL CHAINS move freely and rapidly. The transition temperature depends on acyl-chain unsaturation, and length! This can be super sharp for an artificial membrane due to homogenous preparation (its all made of the same thing?), although it is not usually sharp for natural membrane since they are mixtures and need to be able to still operate above transition temperature.
• Sketch and label a diagram to illustrate the structure of a typical biological membrane.
Page 37 of fatty acid membrane stuff package.
• Define the terms passive and active transport.
Passive: Spontaneous, delta G is negative. Active: Energy must be provided to make transport occur.
• State why the exact dimensions of a lipid bilayer are variable.
Polar head groups can have seriously different dimensions, and acyl trails can be 16-20 carbons in length. The whole membrane is around 5-6 nm thick, from the top of one polar head to the other polar head.
• Explain why polar substances require proteins to cross a lipid bilayer.
Polar substances cannot cross into the hydrophobic environment/intermembrane space without a large energy penalty, so they don't.
• Define the terms saturated, monounsaturated and polyunsaturated fatty acid.
Saturated means no double bonds in the carbon chain. Monounsaturated means one double bond. It kinks it once. Polyunsaturated means multiple double bonds. All the kinks come together to make it curved.
• Identify substances that do not require proteins to cross a lipid bilayer.
Simple diffusion can occur in: -gases (CO2, O2) -hydrophobic molecules (benzene) -small polar molecules (H2O, ethanol) This semi-permeability creates distinct difference between what's inside and outside of the cell. Rate of simple unmediated diffusion depends on the SIZE OF MOLECULE (smaller= faster), CONCENTRATION GRADIENT (larger gradient= increased rate of diffusion), and LIPID SOLUBILITY (more solubility in lipids= increases diffusion rate)
• Draw the structure of a fatty acid, given its structural notation.
Structural notation puts (# of carbons): (double bonds) ^(delta number of carbon and the next one)
• Describe the structure of cholesterol and state how it "fits" into a lipid bilayer.
Structure: accounts for ~35% of mammalian membranes cuz it helps maintain fluidity and rigidity. because it's RIGID AND NON-POLAR and mostly hydrophobic (27 carbons to 1 OH weakly amphipathetic (singly OH group). With all those rings, no rotation is allowed. It doesn't really form membranes by itself- it just adds itself.
• State how transporter proteins differ from ion channels and porins.
Transport proteins reduce the activation energy barrier for transport. Porins (contain relatively non-specific, water-filled pore in center of a beta barrel. All porins are trimers, and each subunit contains a pore. They don't really care and are non-selective) and ion channels (where the channel is formed between subunits, they are HIGHLY selective, depends on size of pore and properties of the side chains placed there) enable passive transport via membrane-spanning pores.
• Compare the structure of a triacylglycerol with that of a glycerophospholipid lipid.
Triaglycerols are nonpolar, with a tiny polar glycerol part and a looong fatty acid. Glycerolphospholipids - amphipathic because they have such large polar groups (ethanolamine, serine, choline) and hydrophobic tails. Differences exist with tails and heads that change affecting size and melting points.