Chapter 10: Membrane Structure

What structure defines the boundary of all living cells?

The plasma membrane.

What do internal membranes allow eukaryotic cells to do?

Create distinct compartments (organelles).

Why are membranes biologically important?

They create barriers, maintain ion gradients, and support membrane protein function.

What are the three major components of biological membranes?

Lipids, proteins, and carbohydrates.

How thick is a typical lipid bilayer?

~5 nm.

Is the membrane rigid or dynamic?

Dynamic (fluid bilayer).

What types of molecules are membranes generally impermeable to?

Most water-soluble (polar) molecules.

What does “amphipathic” mean?

Having both hydrophilic (polar) and hydrophobic (nonpolar) regions.

What parts of a phospholipid are hydrophilic vs hydrophobic?

Polar head: hydrophilic
Fatty acid tails: hydrophobic.

Why are phospholipids amphipathic?

They have polar head groups and nonpolar hydrocarbon tails.

What are the most abundant membrane lipids?

Phospholipids

What is the backbone of phosphoglycerides?

Glycerol

What type of lipid is cholesterol?

A sterol

What structural feature makes cholesterol rigid?

A steroid ring structure.

What is cholesterol’s polar group

A hydroxyl (–OH)

How many hydrocarbon tails does cholesterol have

One short tail

What are glycolipids?

Lipids with carbohydrate groups attached to the polar head

Where are glycolipids added to membranes

In the Golgi lumen

Where are glycolipids found in the plasma membrane

Outer (non-cytosolic) leaflet only

What are major functions of glycolipids

Cell recognition, cell–cell interactions, protection of cell surface.

Glycolipids are enriched in what membrane structures

Lipid rafts

Do phospholipids need energy to form bilayers

No, bilayer formation is spontaneous in water

Why do phospholipids spontaneously form bilayers

To avoid unfavorable interactions between hydrophobic tails and water

What thermodynamic principle drives bilayer formation

Avoiding a decrease in entropy of surrounding water

What does bilayer formation create biologically

A sealed compartment

What is a liposome?

A spherical lipid bilayer formed in water

Is the lipid bilayer a solid or a fluid

A two-dimensional fluid

What types of motion do lipids undergo

Rapid lateral diffusion and rotation

What type of lipid movement is energetically unfavorable

Flip-flop between leaflets

Which lipid can flip-flop relatively easily

Cholesterol

How do shorter fatty acid tails affect fluidity?

Increase fluidity.

How do double bonds in fatty acid tails affect fluidity?

Increase fluidity (kinks reduce packing).

How does temperature affect membrane fluidity?

Higher temperature → more fluid.

How does cholesterol affect membrane fluidity?

Modulates fluidity (prevents extremes).

Why do cold-adapted organisms use kinked or shorter lipid tails?

To maintain membrane fluidity at low temperatures.

What is a lipid raft?

A thicker, specialized membrane domain with distinct composition.

What lipids are enriched in lipid rafts?

Cholesterol and glycolipids.

Are lipid rafts fluid or fixed?

They diffuse as a single unit.

What functions are lipid rafts associated with?

Cell signaling and membrane organization.

Are the two membrane leaflets identical?

No, membranes are asymmetric.

How do lipid compositions differ between leaflets?

Specific lipids are enriched in specific leaflets.

Where do glycolipid carbohydrates always face?

Away from the cytosol.

Are membrane proteins symmetrically oriented?

No, their orientation is fixed and asymmetric.

Why is membrane asymmetry functionally important?

It supports signaling, transport, and recognition.

What are membrane proteins?

Proteins embedded in or associated with membranes.

Roughly what percentage of cellular proteins are membrane proteins?

~30%.

What defines an integral membrane protein?

It is embedded in the lipid bilayer.

What is the most common structure for transmembrane segments?

α-helices.

How many amino acids are typically required to span a membrane?

~20–25 hydrophobic amino acids.

Do peripheral membrane proteins cross the bilayer?

No

How can peripheral proteins associate with membranes?

Amphipathic α-helices

Lipid anchors
Binding to other membrane proteins

What is a lipid-anchored protein?

A protein covalently attached to a lipid that anchors it to the membrane.

What do lipid anchors control?

Which membrane leaflet the protein associates with.

What type of amino acids dominate transmembrane α-helices?

Nonpolar (hydrophobic) amino acids.

What is a hydropathy plot used for?

Predicting transmembrane segments.

What does a highly positive region on a hydropathy plot indicate?

A hydrophobic region likely to span the membrane.

What are multipass membrane proteins?

Proteins that cross the membrane multiple times.

Why are α-helix interactions important in multipass proteins?

They help form channels and transporters.

What is a β-barrel?

A cylindrical structure formed by β-strands that spans membranes.

Where are β-barrel proteins commonly found?

Outer membranes of bacteria, mitochondria, and chloroplasts.

What does glycosylation mean?

Covalent attachment of carbohydrates to proteins.

On which side of the membrane are glycoprotein carbohydrates found?

Non-cytosolic side.

Why are detergents used to study membrane proteins?

To solubilize them from membranes.

What structure do detergents form in water?

Micelles

How do detergents keep membrane proteins soluble?

They surround hydrophobic regions.

Do membrane proteins diffuse laterally?

Yes

What experiment demonstrates membrane protein diffusion?

Cell fusion (heterocaryon) experiments.

What lies just beneath the plasma membrane?

The cortical cytoskeleton (actin-rich).

What does the cortical cytoskeleton do?

Provides mechanical strength and restricts protein diffusion.

Why is this especially important in red blood cells?

They must deform without lysing in capillaries.