MBIO 1010 / Topic 2a: Cell Membrane

What does the bacterial cell envelope include?

Cytoplasmic membrane, cell wall, and periplasm.

What does the periplasm look like in gram-positives? How about in gram negatives? What happens here?

• For Gram positives, between cell wall and cell membrane.

• For Gram negatives, between outer membrane and inner membrane.

Metabolic activity occurs here.

What does the cytoplasmic membrane (a.k.a. cell membrane, plasma membrane) look like, and what does it do?

Thin, highly selective semi-permeable barrier surrounding cell, (1) concentrating enough amounts of metabolites and (2) excretes waste products.

What are the two structures that constitutes the bacterial cytoplasmic membrane?

Phospholipids and proteins.

What specific type of phospholipids make up the bacterial cell membrane?

Ester phospholipids.

What is an ester phospholipid composed of?

• One glycerol backbone.

• Two fatty acid tails.

• One phosphate group with a variable side chain.

What is the non-polar, hydrophobic region of a phospholipid? What is the polar, hydrophilic region of a phospholipid?

Fatty acids are non-polar and hydrophobic, while the glycerol backbone and the phosphate group are polar and hydrophilic.

What does amphipathic mean?

Of polar and non-polar character.

Why do phospholipids in water form bilayers?

Due to their amphipathic nature, with the hydrophobic region pointing inward and the hydrophilic region pointing outward to the cytoplasm or external environment.

What is the measurement of the phospholipid bilayer?

8–10nm.

What is the outer layer (facing external environment) and inner (facing cytoplasm) layer also called?

Outer leaflet and inner leaflet, respectively,

What is a true membrane?

A membrane with two phospholipid layers.

What are the two types of proteins found in the cell membrane? What mainly differentiates the two?

Integral proteins are firmly associated in the membrane. Peripheral proteins are loosely associated in the membrane.

What are the two regions of an integral protein?

Non-polar, hydrophobic region in the core of membrane. Meanwhile, polar, hydrophilic region by the borders of the membrane.

What does a peripheral protein have sometimes?

A fatty acid tail, making it a lipoprotein.

Membrane proteins often _____ in ______ to carry out a function.

Membrane proteins often group in clusters to carry out a function.

The cell membrane is in a ___-___ state.

The cell membrane is in a semi-fluid state.

What keeps the bilayer from falling apart?

The hydrophobic core.

Lipids in the outer leaflet are _____ from lipids in the inner leaflet.

Lipids in the outer leaflet are different from lipids in the inner leaflet.

What varies between lipids in the outer and inner leaflets?

Head groups and fatty acid tails.

In what direction can integral proteins move and not move? Why?

They can move laterally, but they can’t flip-flop sides. This is due to the hydrophilic head not wanting to pass through the hydrophobic core. It’s water-loving, not water-frightened.

Are eukaryotic cell membranes more similar to bacterial or archaeal cell membranes? Why so?

The eukaryotic cell membrane is more similar to the bacterial cell membrane because of their ester-linked phospholipids. Archaeal cell membranes have ether-linked phospholipids.

From an evolutionary standpoint, what does the presence of ester-linked phospholipids mean for Eukarya and Bacteria?

Eukarya brought the carbonyl bond back, baby.

What are the two types of ether phospholipids in archaeal membranes?

• Some archaeal membranes are made of glycerol diether phospholipids, with each phospholipid having two phytanyl tails made of isoprene units.

• Other archaeal membranes are made from diglycerol tetraether phospholipids, with each phospholipid having two biphytanyl tails.

Archaeal membranes with diglycerol tetraether phospholipids are essentially what?

Two phospholipids joined by the tails.

If the eukaryotic and bacterial membrane is a bilayer made of two layers of phospholipids, and if the archaeal membrane is essentially made of two phospholipids joined together, then the archaeal membrane is made of how many layers?

One layer, i.e. the membrane is composed of a phospholipid mono-layer.

What is a property that lipid monolayers win against bilayers? On that note, where can you find this win at its finest?

Heat resistance, commonly found in hyperthermophilic Archaea, which grows best at temperatures above 80ºC.

What makes monolayers more heat resistant?

Connected phospholipids constituting a monolayer holds the membrane together more and less breakable by heat.

What are the three primary functions of a cell membrane in both eukaryotes and prokaryotes?

• Act as a permeability barrier.

• Act as a protein anchor.

• Act as an energy conservation.

How do small, non-polar molecules transport through the membrane? How about large, polar molecules?

• Small, non-polar diffuses across the membrane itself.

• Large, polar moves across transport proteins.

What is the energy conserved in the cell membrane?

PMF.

Just a review, differentiate passive transport, active transport, diffusion, simple diffusion, and facilitated diffusion.

Two types of nutrient and waste transport in and out of the cell: passive transport (no energy required, molecules go from high to low) and active transport (energy required, molecules go from low to high).

Passive transport uses diffusion, either simple (without transport protein) or facilitated (with transport protein).

For what situation is facilitated diffusion done instead of simple diffusion?

Speeding of the rate of diffusion for molecules that can’t easily cross the membrane.

What is the saturation effect?

The occurrence wherein transport protein(s) are all occupied, leading to a “slowing down” of rate of solute entry.

Transport proteins are highly _____ and will usually carry a ____ molecule or kind of molecule.

Transport proteins are highly specific and will usually carry a specific molecule.

What are the three common types of active transport? What drives each?

• Simple is driven by PMF.

• Group translocation is driven by phosphoenolpyruvate (PEP).

• ABC is driven by periplasmic proteins and ATP.

What are the two types of simple transport? What’s common between them? What makes the two different?

Symport and antiport. In both, the movement of one molecule gives energy to move the other molecule. Symport has two molecules move in same direction, wherein antiport they move in opposite directions.

Give one example each for symport and antiport.

• For symport, E. coli’s lactose permease, wherein proton is brought in with lactose at the same time.

• For antiport, E. coli’s sodium-proton antiporter, wherein a proton is brought in and a sodium ion is pushed out.

What happens in the group translocation transport?

A series of proteins carries out a single transportation event, modifying the substrate as it is brought in.

Give an example of group translocation transport.

E. coli’s phosphotransferase system (PTS), wherein PEP donates a phosphate group to a phosphorelay system (the series of proteins), with the phosphate group being passed enzyme-to-enzyme until donated by integral protein enzyme II_c to glucose entering the cell.

What does “ABC” mean in “ABC transport”? Explain to me what happens here.

ATP Binding Casette, wherein:

1. Periplasmic solute-binding protein binds to a specific substrate and carries it to the membrane for transport.

2. Integral protein gets the substrate from the periplasmic protein and gets it across the membrane.

3. ATP binding protein hydrolyzes ATP to get energy to push the substrate into the cell.

What four kinds of substrates do ABC transport usually transport? What’s the special ability of the ABC transport, and what does this bring about to cellular organisms?

Some sugars, amino acids, inorganic nutrients, and trace materials. ABC transport can concentrate substrates into the cell even when there is extremely low concentration outside of the cell (less than 1µM). This leads to successful bacterial growth when environments look like they’re out of nutrients.