Lecture 15 - Membranes and Transport

What determines lipid structure formation?

The type and concentration of lipid molecules.

What are micelles?

Structures formed by amphipathic molecules with a single lipid layer (monolayer).

What types of molecules form micelles?

Fatty acids and detergents.

What is a key feature of micelles?

They have one leaflet with hydrophobic cores and hydrophilic surfaces.

What is a lipid bilayer?

Two layers (leaflets) of lipids with hydrophilic heads facing outward and hydrophobic tails inward.

Why do lipid bilayers form naturally?

To shield hydrophobic tails from water while exposing hydrophilic heads.

What are vesicles (liposomes)?

Small spherical bilayers with an internal aqueous cavity.

What can vesicles (liposomes) do?

Enclose and transport dissolved molecules.

What is the main difference between micelles and bilayers?

Micelles are single-layer structures; bilayers have two layers.

Why are liposomes biologically important?

They can mimic cell membranes and transport molecules.

What are cell membranes made of?

Complex lipid-based structures (mainly phospholipids with proteins).

What is the main function of cell membranes?

They define the boundary of the cell.

What does “selective permeability” mean?

The membrane controls what enters and leaves the cell.

Why are membranes important for cell survival?

They retain essential metabolites and ions inside the cell.

What is compartmentalization in eukaryotic cells?

Separation of different cellular processes into organelles.

How do membranes help energy production?

They separate proton gradients from energy-consuming reactions.

Why is separating proteases important?

It prevents unwanted protein degradation inside the cell.

What role do membranes play in signaling?

They detect external signals and transmit information into the cell.

How are membranes involved in nerve function?

They transmit electrical signals in neurons.

Why are membranes essential for complex organisms?

They allow controlled communication, energy use, and organization within cells.

What is the Fluid Mosaic Model?

A model describing membranes as fluid lipid bilayers with embedded proteins.

What is the typical thickness of a membrane?

About 3–10 nm.

What is the basic structure of most biological membranes?

A lipid bilayer.

What is different about archaebacterial membranes?

They can form monolayers instead of bilayers.

How do membranes form spontaneously?

They self-assemble in aqueous environments.

What force stabilizes membrane formation?

The hydrophobic effect.

Why do phospholipids form bilayers in water?

Hydrophilic heads face water, hydrophobic tails avoid water.

What does “fluid” mean in the fluid mosaic model?

Lipids and proteins can move laterally within the membrane.

What does “mosaic” refer to?

The patchwork of proteins embedded in the lipid bilayer.

What types of proteins are found in membranes?

Integral and peripheral proteins.

Where are sugars found in membranes?

On the extracellular (outer) side.

What role do membrane carbohydrates play?

Cell recognition and signaling.

Do all membranes have the same composition?

No, membrane composition varies between organisms, tissues, and organelles.

What major components vary in membranes?

Phospholipids, proteins, sterols, and (in plants) galactolipids.

Phospholipids, proteins, sterols, and (in plants) galactolipids.

Because each organelle has specialized functions.

What is an example of membrane lipid variation in plants?

Presence of galactolipids.

What are sterols in membranes?

Lipid molecules (like cholesterol) that affect membrane fluidity and stability.

Why are membrane proteins important?

They carry out transport, signaling, and enzymatic functions.

What are lipid transfer proteins?

Proteins that move lipids between membranes.

Why are lipid transfer proteins needed?

Because lipids are not freely soluble in water and cannot easily move between membranes.

What is the main role of lipid transfer proteins?

To maintain and regulate membrane composition.

What does membrane asymmetry mean?

The two lipid leaflets of the membrane have different compositions and functions.

Do both sides of the membrane have the same lipids and proteins?

No, each leaflet has a distinct composition.

Where are membrane sugars located?

On the outside (extracellular) surface of the membrane

Where are peripheral proteins typically found?

On specific sides of the membrane (not evenly distributed).

What is the typical electrical difference across a cell membrane?

The inside is more negative than the outside (~-60 mV).

Why is membrane charge asymmetry important?

It helps regulate signaling, transport, and cell recognition.

What is phosphatidylserine normally doing in healthy cells?

It is located on the inner leaflet of the membrane.

What does it mean when phosphatidylserine appears on the outside of a cell?

It signals apoptosis (cell death).

What happens when phosphatidylserine is exposed externally?

It triggers macrophages to remove the cell.

How is phosphatidylserine involved in blood clotting?

It helps activate platelets.

What are the two physical states of membranes?

Fluid phase and gel (solid-like) phase.

Why must membranes stay fluid?

Because membrane proteins and transport processes need movement to function.

What happens to membranes at low temperatures?

They become more rigid (gel-like/harden).

How do cells increase membrane fluidity in cold conditions?

By using shorter and more unsaturated fatty acids.

Why do unsaturated fatty acids increase membrane fluidity?

Their double bonds prevent tight packing.

What happens to membranes at high temperatures?

They become too fluid and lose stability.

How do cells reduce membrane fluidity at high temperatures?

By using more saturated fatty acids.

Why do saturated fatty acids decrease membrane fluidity?

They pack tightly together.

What is the key adaptation of organisms regarding membrane fluidity?

They adjust lipid composition depending on temperature.

What is the main goal of membrane fluidity regulation?

To maintain optimal membrane function under different temperatures.

What are sterols?

Lipid molecules found in cell membranes that help regulate membrane fluidity and stability.

What is the main sterol found in animal cell membranes?

Cholesterol.

How do sterols affect membranes at high temperatures?

They restrain phospholipid movement, decreasing membrane fluidity.

How do sterols affect membranes at low temperatures?

They prevent phospholipids from packing too tightly, increasing membrane fluidity.

Why are sterols called membrane "buffers"?

They counteract temperature-induced changes in fluidity.

What is the overall effect of sterols on membrane stability?

They help maintain membrane function across a wide range of temperatures.

Do sterols generally increase or decrease membrane rigidity?

Increase membrane rigidity.

What effect do sterols have on membrane permeability?

They affect (and generally reduce uncontrolled) movement of molecules across the membrane, helping stabilize it.

Where is cholesterol most abundant in animal cells?

The plasma membrane.

Is cholesterol abundant in mitochondrial membranes?

No, mitochondria contain very little cholesterol.

Unsaturated vs Saturated vs Cholesterol

Unsaturated fatty acids = Increase fluidity
Saturated fatty acids = Decrease fluidity
Cholesterol = Fluidity buffer

• Hot temperatures → decreases fluidity

• Cold temperatures → increases fluidity

Think of cholesterol as a thermostat for the membrane, preventing it from becoming too fluid or too rigid.

What are the main functions of membrane proteins?

Catalyze reactions, transport molecules, and transmit signals.

What are the two major types of membrane proteins?

Peripheral membrane proteins and integral membrane proteins.

What are peripheral membrane proteins?

Proteins loosely attached to the membrane surface.

How are peripheral proteins attached to membranes?

Through weak ionic and electrostatic interactions with polar head groups.

Are peripheral proteins embedded in the membrane?

No, they remain on the membrane surface.

What is an exception to typical peripheral protein attachment?

Lipid-anchored proteins, which are attached through a lipid molecule.

What are integral membrane proteins?

Proteins tightly associated with the membrane.

Why are integral proteins strongly attached to membranes?

They contain hydrophobic regions that interact with the lipid bilayer.

What does it mean for an integral protein to be transmembrane?

It spans the entire lipid bilayer.

Why can integral membrane proteins remain embedded in the membrane?

Their hydrophobic amino acids interact favorably with membrane lipids.

Are membrane proteins symmetrical on both sides of the membrane?

No, they are asymmetrical.

What does membrane protein asymmetry mean?

Different domains of the protein face different sides of the membrane.

Why is membrane protein asymmetry important?

It allows proteins to perform different functions inside and outside the cell.

What type of amino acids are most common in transmembrane segments?

Hydrophobic (nonpolar) amino acids.

Why are transmembrane segments rich in hydrophobic amino acids?

Because they must interact with the hydrophobic core of the lipid bilayer.

Approximately how many amino acids are needed to span a membrane?

About 20 amino acids.

Which amino acids are commonly found at the membrane interface?

Tyrosine (Tyr) and Tryptophan (Trp).

Where do Tyr and Trp tend to cluster?

At the boundary between the polar and nonpolar regions of the membrane.

Why are Tyr and Trp well suited for the membrane interface?

They can interact with both the hydrophobic membrane core and the polar membrane surface.

Where are charged amino acids usually found in membrane proteins?

In aqueous (water-exposed) domains.

Why are charged amino acids rarely found in the membrane interior?

Because the hydrophobic membrane core is energetically unfavorable for charged residues.

What part of a membrane protein is most likely to contain charged amino acids?

The regions exposed to the cytoplasm or extracellular fluid.

What are lipid-anchored proteins?

Membrane proteins that are attached to the membrane through a covalently linked lipid.

Are lipid-anchored proteins permanently attached to membranes?

No, the attachment can be reversible.

What is another name for lipid-anchored membrane proteins?

Lipoproteins.

How do lipid-anchored proteins associate with membranes?

A lipid group inserts into the membrane and anchors the protein.

What modification is often required for membrane delivery of lipid-anchored proteins?

Prenylation.

What is prenylation?

The addition of an isoprene-derived lipid group to a protein.