Lipids, Membranes, and Membrane Proteins

What are membrane lipids made of?

Hydrophobic regions attached by backbones to polar head groups.

What are the most abundant membrane lipids?

Glycerophospholipids

What are the hydrophobic regions of membrane lipids most commonly derived from?

Fatty acids.

What is a fatty acid?

A long-chain aliphatic carboxylic acid found in fats, oils, and membrane lipids.

What is the function of the long hydrocarbon chain in a fatty acid?

It is nonreactive and provides structural stability.

What is the function of the carboxyl group in a fatty acid?

It is reactive and allows fatty acids to be incorporated into membrane or storage lipids.

What determines the nomenclature of fatty acids?

The length and saturation of the fatty acid.

What is a saturated fatty acid?

A fatty acid with a fully saturated alkyl chain.

What is a monounsaturated fatty acid (MUFA)?

An unsaturated fatty acid containing one double bond.

What is a polyunsaturated fatty acid (PUFA)?

A fatty acid containing more than one double bond.

How do double bonds affect fatty acid chains?

They create kinks that decrease packing and change membrane properties.

Why do saturated fats tend to be solid at room temperature?

Their straight chains can pack tightly together.

Where do unsaturated fatty acids in the diet typically come from?

Plants.

What are some health benefits of unsaturated fatty acids?

• Lower inflammation

• Decrease blood lipids

• Improve cardiovascular function

What are omega fatty acids?

Unsaturated fatty acids classified by the position of their double bond.

What are omega-3 and omega-6 fatty acids?

They are types of unsaturated fatty acids classified by the position of their double bond.

Why can too much omega-6 be harmful?

The body processes it into arachidonic acid, which can cause inflammation.

What are the health risks associated with an imbalance of omega-6 and omega-3 PUFAs?

An increased risk of cardiovascular disease.

Why are trans fatty acids harmful?

They are proinflammatory, cause insulin resistance, and raise circulating blood lipids.

What are the three classes of membrane lipids in mammals?

Phospholipids, glycolipids, and sterols.

What are the three main parts of a membrane lipid?

Hydrophobic tail, backbone, and polar head group.

What is the hydrophobic part of a membrane lipid?

The fatty acid or other hydrocarbon region.

What is the hydrophilic part of a membrane lipid?

The polar head group.

What does the backbone of a membrane lipid do?

It holds the hydrophobic and hydrophilic parts together.

What does it mean that membrane lipids are amphipathic?

They have both a hydrophobic region and a hydrophilic region.

What happens when amphipathic lipids are mixed with water?

They spontaneously form micelles or lipid bilayers.

What is a micelle?

An arrangement where nonpolar portions face inward and polar portions face the water.

What is a lipid bilayer?

A double layer of amphipathic lipids with hydrophobic tails facing inward.

Why do lipid bilayers form in water?

Because of the hydrophobic effect and the amphipathic structure of lipids.

What can membrane lipid composition affect?

Membrane fluidity, permeability, melting point, curvature, and rigidity.

What are the three factors that affect membrane fluidity?

• Temperature

• Fatty acid acyl-chain composition

• Sterol abundance

What are the two general states of membrane fluidity?

• Liquid-ordered

• Liquid-disordered.

Which membrane state generally has less permeability?

The liquid-ordered state.

What effect does high cholesterol have on membrane state?

It encourages the liquid-ordered state.

How can carbohydrate groups on membrane lipids help cells?

Proteins on neighboring cells can recognize the carbohydrates and distinguish the cells from invading bacteria.

What is a membrane protein?

A protein associated with or attached to a cell membrane that helps with communication, signaling, transport, adhesion, and more.

What are the three types of membrane proteins?

Integral, peripheral, and lipid-anchored proteins.

How are membrane proteins classified?

By how they interact with the membrane.

What does the fluid mosaic model describe?

It describes how membrane lipids and proteins are arranged in the cell membrane.

What is an integral membrane protein?

A protein firmly bound to the membrane through hydrophobic interactions.

What is a peripheral membrane protein?

A protein loosely attached to the membrane through hydrogen bonds or electrostatic forces.

What is a lipid-anchored protein?

A protein covalently linked to a lipid that is embedded in the membrane.

What type of membrane protein spans the lipid bilayer?

Integral membrane proteins.

What are the membrane-spanning regions of integral proteins like?

They are hydrophobic.

Why are the membrane-spanning regions of integral proteins hydrophobic?

They interact with the hydrophobic interior of the lipid bilayer.

What type of amino acids are found near the ends of transmembrane regions?

Positively charged amino acids.

What amino acid sequence can indicate a protein crosses the lipid bilayer?

About 20 hydrophobic amino acids surrounded by charged regions.

What is the positive-inside rule?

Lys and Arg are more common on the cytoplasm side of the membrane.

Which amino acids are especially common on the cytoplasm side of integral proteins?

Positively charged Lys and Arg.

What features would you expect in an integral membrane protein?

Many hydrophobic α-helices, positively charged amino acids, and aromatic side chains.

Why can integral membrane proteins function as transporters?

They span the membrane and can provide a pathway for molecules to cross it.

What is an example of an integral membrane protein that transports water?

Aquaporin.

How does aquaporin interact with the membrane?

Its hydrophobic side chains interact with the membrane interior while hydrophilic side chains interact with lipid head groups.

How do peripheral membrane proteins interact with membranes?

Through charged amino acids that interact with lipid head groups or integral proteins.

What type of interactions hold peripheral membrane proteins to membranes?

Noncovalent electrostatic interactions or hydrogen bonds.

Why are peripheral membrane proteins useful in cell signaling?

Their electrostatic interactions are temporary, allowing signals to be rapidly turned on or off.

What type of membrane protein is involved in insulin signaling cascades?

Peripheral membrane proteins.

What is a lipid anchor?

A hydrophobic lipid attached to a protein that inserts into the lipid bilayer.

How are lipid-anchored proteins attached to the membrane?

They are covalently linked to one or more hydrophobic lipid anchors.

What does the lipid anchor do?

It inserts into the lipid bilayer and holds the protein at the membrane surface.

Can lipid attachment be reversible?

Yes. Some lipid-anchored proteins are membrane-bound when lipid-linked and soluble when not.

What stabilizes some lipid-anchored proteins at the membrane?

Ionic attractions between positively charged amino acids and negatively charged lipid head groups.

What is the main difference between integral and peripheral membrane proteins?

Integral proteins are firmly embedded in the membrane, while peripheral proteins are loosely attached to its surface.

What is the main difference between integral and lipid-anchored proteins?

Integral proteins are embedded in the membrane, while lipid-anchored proteins attach through a lipid anchor.

What is the main difference between peripheral and lipid-anchored proteins?

Peripheral proteins attach noncovalently, while lipid-anchored proteins are covalently linked to a lipid.

What types of molecules can diffuse through the lipid bilayer without a transporter?

Hydrophobic molecules and small polar molecules.

What types of molecules cannot easily cross the lipid bilayer?

Ions and large polar molecules.

What happens to membrane permeability as molecule size increases?

Larger molecules have more difficulty crossing the membrane.

What happens to membrane permeability as charge increases?

Greater charge decreases the ability to cross the membrane.

What is simple diffusion?

Movement of a solute down its concentration gradient without energy.

What is a membrane potential?

An electrical gradient across a membrane caused by separation of charges.

What is an electrochemical gradient?

The combined effect of a concentration gradient and electrical gradient on an ion.

What is a passive transporter?

A protein that moves solutes down their electrochemical gradient without energy.

What is an active transporter?

A protein that moves solutes against their electrochemical gradient using energy.

What is the difference between passive and active transport?

Passive transport requires no energy; active transport requires energy.

What do membrane transporters do?

They increase the rate at which specific solutes move across membranes.

What determines whether transport requires energy?

Whether the solute moves with or against its electrochemical gradient.

What must polar or charged molecules lose before crossing the membrane?

Their surrounding water molecules.

Why is crossing the membrane difficult for polar molecules?

The middle of the lipid bilayer is hydrophobic.

How do transport proteins help polar molecules cross membranes?

They lower the activation energy needed to cross the bilayer.

What is a transporter?

A membrane protein that moves specific substrates across the membrane.

What is an ion channel?

A passive transporter that allows ions to move through an aqueous channel.

How are ion channels different from transporters?

Ion channels provide an aqueous pathway, while transporters change conformation to move substrates.

Are ion channels active or passive?

Passive.

What is a uniporter?

A transporter that moves one type of substrate.

What is a symporter?

A transporter that moves two substrates in the same direction.

What is an antiporter?

A transporter that moves two substrates in opposite directions.

What does a secondary active transporter use?

The energy stored in an ion gradient to move another substance against its gradient.

What type of transporter is an ATPase that moves two different ions in opposite directions?

An antiporter.

What is GLUT1?

A passive glucose transporter found in erythrocytes and other cells.

How does GLUT1 transport glucose?

It moves glucose down its concentration gradient by passive transport.

Why is GLUT1 important in erythrocytes?

It provides red blood cells with a constant supply of glucose.

How much faster can GLUT1 transport glucose compared with unassisted movement?

About 50,000 times faster.

How does GLUT1 control glucose movement?

It uses two protein conformations so only one side is open at a time.

What does a negative ΔG mean for glucose moving into a cell?

Energy is released, so the movement is spontaneous.

Why does glucose move into the cell through GLUT1?

Glucose is moving down its concentration gradient.

What are P-type ATPases?

Active cation transporters that are reversibly phosphorylated by ATP.

What is the sodium-potassium pump?

A P-type ATPase that uses ATP to move sodium and potassium against their gradients.

What does the sodium-potassium pump do?

It pumps 3 Na⁺ out of the cell and 2 K⁺ into the cell.

Why is the sodium-potassium pump important?

It establishes ion gradients and the cell's transmembrane electrochemical potential.