W11 L2 - Membrane Structure and Function

Flashcards about membranes, covering their structure, history, dynamics, composition, and proteins. Includes key experiments, models, and techniques for studying membranes.

Why are membranes important in biochemistry?

Membranes compartmentalize biochemistry, allowing reactions to occur by concentrating substrates in small volumes.

What principles govern any reaction that occurs in biochemistry?

The energetics of the reaction and the probability of the reactants coming together.

In 1925, what did Hugo Frick calculate, and what was his theory?

Frick calculated the thickness of the membrane to be 3.3 nm, and his theory was that membranes were just one layer thick.

What did Benjamin Franklin discover about oil droplets on water?

A spherical droplet of oil spreads out on water to form a single molecule thick layer, and the surface area of the droplet is identical to the area covered.

How did Gorter and Grendel prove that the membrane is a bilayer and not a monolayer?

They broke down the membrane, spread it on the surface of water, and measured the surface area, finding it to be twice the expected area if it were a single layer.

Why do lipids form a bilayer in water?

Due to entropy; it is more stable as a bilayer than sitting around freely in a solution.

What happens when lipids are put into water?

Water orders around the hydrophobic parts, costing energy.

What shapes can lipids form based on their structure, and what structures do they create?

Conical-shaped lipids form micelles, while cylindrical-shaped lipids form bilayer sheets.

What is the trilamellar model?

The model stating that membranes are composed of a three-layer structure: protein, lipid bilayer, protein.

What technique disproved the trilamellar model?

Freeze fracture, which revealed that proteins are embedded in the membrane.

Describe the experiment involving mouse and human cells coated with different dyes.

Mouse cells were coated with a green dye, human cells with a red dye, and then fused together using a virus. Over time, the dyes fully mixed, turning the cell yellow, demonstrating the dynamic movement of the membrane.

What is fluorescence recovery after photo bleaching (FRAP), and what does it demonstrate?

A technique where a fluorescent molecule dyes the membrane, a small part is hit with an intense laser to bleach out the fluorescence, and recovery shows the dynamic mingling of lipids.

What are the two types of dynamic movement of lipids in a membrane?

Flip-flop (very slow) and lateral diffusion (fast).

What is the Singer-Nicholson model of the membrane?

A model where the membrane is a bilayer with proteins embedded, including integral membrane proteins, GPI-anchored proteins, sterols, and peripheral proteins.

Name the different types of membrane proteins.

Polytopic, monotopic, amphitrophic, and peripheral proteins.

How does membrane composition vary?

It varies between different cell types and between the inner and outer leaflets of the membrane to allow flexibility and movement.

How does temperature affect membranes, and how can bacteria adjust?

Lower temperatures lead to a liquid ordered state with less movement, while higher temperatures cause the membrane to become leaky. Bacteria can modify their membrane by changing the ratio of long and short-chain fatty acids.

What is bacterial rhodopsin, and how do proteins cross membranes?

An example of an integral membrane protein with seven transmembrane helices. Proteins typically cross membranes using alpha helices, around 20-25 residues long.

Where do charged residues tend to occur in membrane proteins?

On the outsides of the membranes, interacting with water, while hydrophobic areas have very few charged residues.

What is a hydropathy plot, and how is it used?

A graph showing the hydrophobic index of a protein sequence, used to identify potential transmembrane helices based on patches of hydrophobicity.

Why is identifying transmembrane proteins important?

Because they are interactions between the inside and outside world, and over 50% of drugs target membrane proteins.

What is atomic force microscopy, and how does it work?

A technique that uses a fine tip to measure small forces between the sample and the tip, creating images of the membrane surface at a molecular level.

Why do we have membranes?

Because they are semi-impermeable and control what passes over them