Chapter 11: Membrane structure

What does the cell membrane of an organism do?

It separates a cell from its surroundings, enabling the molecular composition of a cell to differ from its environment

• with eukaryotes having internal membranes as well that separate the inside of an organelle from the cytosol

What does the cell membrane prevent?

Molecules on one side from freely mixing with those on the other side

How is the plasma membrane involved in cell communication?

they contain receptor proteins that are able to receive signals from the external space

How is the plasma membrane involved in import and export of molecules?

They contain channels and transporters that enable the import and export of small molecules

How is the plasma membrane involved in cell growth and motility?

The lipid bilayer is flexible, allowing for a cell to grow, change shape, and move

What do internal membranes have the ability to form?

They are able to form membrane enclosed organelles in eukaryotic cells, with the nucleus and mitochondria being enclosed by two membranes

What connects the hydrophilic head of a phospholipid to its hydrophobic tail?

A glycerol molecule that connects the hydrophilic head to the carboxyl group present in the hydrophobic tails

When do kinks occur in a phospholipids hydrophobic tail?

When there is a double bond between two carbon atoms

What is the most common phospholipid?

Phosphatidylcholine

what are phospholipids bilayers able to do spontaneously?

They can spontaneously close in on themselves to form a sealed compartment

Why is a phospholipids closed structure stable and energetically favorable?

As its able to protect its hydrophobic tails from being exposed to water, which would be energetically unfavorable

Why can the membrane behave as a two-dimensional fluid?

Due to each individual phospholipid molecule being able to move within their own monolayer

how does cholesterol affect a cell membrane?

By inserting itself inbetween each phospholipid, stiffening the membrane and making the hydrophobic tails more rigid and condensed

How are newly synthesized phospholipids added to the cytosolic side of the lipid bilayer?

When enzymes on the cytosolic side of the ER makes new phospholipids from fatty acids, they are inserted into the cytosolic monolayer. Causing scramblases to randomly redistribute the phospholipids between both layers, allowing the membrane to grow evenly as a bilayer.

How are certain phospholipids confined to one side of the membrane?

When membranes leave the ER and incorporate into the Golgi apparatus, it encounters flippases, a type of transporter, that selectively removes lipids from the noncytosolic monolayer and flips them to the cytosolic side. Helping establish and maintain asymmetrical distribution of the phospholipid characteristics within the membrane

How are Phospholipids and glycolipids distributed?

They are distributed asymmetrically in the lipid bilayer

What are the 4 different classes of proteins?

• Transporter and channel proteins

• anchor protein

  • which binds to other proteins internally and externally, preserving them

• Receptor proteins

• Enzymes

What are integral proteins?

A type of membrane protein that are permanently embedded within the cell membrane with there being 3 kinds

• Transmembrane proteins

• Monolayer associated proteins 

• Lipid-linked proteins

What are transmembrane proteins?

They are proteins that extend across the bilayer as either a single a-helix, multiple a-helixes, or as a rolled up B-sheet. With some having parts of their mass exposed on both sides or on one side of the membrane

What are monolayer associated proteins?

They are proteins that are anchored to the cytosolic side of the lipid bilayer by an amphipathic a-helix

What are lipid-linked proteins?

They are proteins that are attached to either side of the bilayer by a covalently attached lipid molecule

What are peripheral proteins?

They are a type of membrane protein that are attached to the membrane by weak non-covalent interactions with other membrane proteins

How is a transmembrane hydrophilic pore form?

It is formed by multiple amphipathic a-helixes assembling together, with their hydrophobic side pointing inward and hydrophilic side pointing outward, forming the pore that allows water molecules to pass through the bilayer

What are porin proteins able to form?

they have the ability to form water-filled channels in the outer membrane of a bacterium

What are the two commonly used detergents?

SDS and triton X-100

What is SDS?

A strong ionic detergent that has ionized groups at its hydrophilic ends

What is triton X-100?

A mild nonionic detergent that has nonionized polar structures at its hydrophilic ends

How does a mild detergent solubilize membrane proteins?

By disrupting the lipid bilayer and interacting with the membrane proteins hydrophobic portion

What is the plasma membrane cortex?

A thin, flexible network of proteins, made primarily of actin filaments, that is essential in controlling the shape of red blood cells and their mechanical properties

How can a cell restrict the lateral movement of its plasma membrane proteins?

The proteins can restricted by being tethered to either:

• the cell cortex

• To the molecules present in the extracellular space

• Or to proteins present on the surface of another cell

What do diffusion barriers in a cell do?

They form a border that dictates which proteins can diffuse at a specific location and which ones cant

How do tight junctions restrict membrane proteins to particular domains in a epithelial cell?

By allowing the proteins to diffuse laterally within their own membrane domain while preventing them from interacting with other protein domains

What does a eukaryotic cells carbohydrate rich layer consist of?

It consists of oligosaccharide side chains attached to membrane glycolipids and glycoproteins, with polysaccharide chains on the membrane proteoglycans

Why are neutrophiles able to migrate out of a blood stream and into infected tissues?

Due to carbohydrates being present on the surface of the cell

How do neutrophils migrate out of the blood steam and into infected tissues?

1. Lectins will recognize the glycolipids and glycoproteins on the neutrophils surface, causing them to adhere to the endothelial cells and crawl along the vessel wall.

2. This forms strong protein–protein interactions that helps the neutrophil squeeze between endothelial cells and enter the infected tissue.

When mild detergents are used to solubilize and reconstitute membrane proteins..

they can be incorporated into artificial lipid bilayers where they can diffuse more freely and rapidly than in a regular cell membrane.