Membranes Flashcards
What are membranes mainly composed of?
- Phospholipids
- Protein
- Carbohydrates
What are two ways carbohydrates bind to the membrane?
- Binding to phospholipids (glycolipids)
- Binding to proteins (glycoproteins)
What are the two major categories of membrane proteins? Give a description of each.
- Peripheral proteins
• Associate with phospholipid heads (hydrophilic domains) of integral proteins - Integral proteins
• Amphipathic molecules
• Hydrophilic regions face cytoplasm or extracellular fluid
• Hydrophobic domain interacts with phospholipid tails
Describe the structure of phospholipids in the membrane.
• Hydropholic (water-loving) or polar heads - face outside and inside of cell
• Hydrophobic (water-fearing) or non-polar tails - line up in middle to avoid exposure to water
What are the classifications of integral proteins?
- Monotopic - embedded into a single face of membrane
- Bitopic (transmembrane proteins):
• Alpha-helix structure
• Span entire lipid bilayer only once - Polytopic (transmembrane proteins):
• Consist of multiple alpha-helices or a cylindrical beta-sheet (beta-barrel)
• Extend across membrane multiple times
What is membrane fluidity?
Ability of phospholipids and proteins to move freely inside membrane
How is membrane fluidity affected?
- Phospholipid types in membrane
- Temperature of surroundings
- Concentration of cholesterol
Compare saturated and unsaturated phospholipids.
Saturated phospholipids:
• Straight fatty acid chains with no double bonds
• Form a densely packed membrane (lower membrane fluidity)
Unsaturated phospholipids:
• Bend in fatty acid chains with at least one double bond
• Higher membrane fluidity
How does temperature affect membrane fluidity?
Low temperatures = membrane lipids are tightly packed with little movement (rigid membrane)
High temperatures = lipids show increased movement (makes membrane more fluid)
How does cholesterol affect membrane fluidity?
• colder temperature = Increases fluidity
• Warmer temperature = prevents formation of too fluid membrane
How do organisms maintain optimal membrane fluidity?
By regulating:
• Ratio of saturated to unsaturated phosphorus
• Cholesterol concentration
What are the roles of transmembrane proteins?
- Transporters
- Enzymes
- Receptors
Describe the structure of a transmembrane protein.
Transmembrane domain:
• Membrane-spanning region
• Contains nonpolar amino acids (form hydrophobic sections)
Cytosolic and exoplasmic domains:
• Hydropholic
What are the two classifications of transmembrane proteins?
- Single-pass
- Multi-pass
Give two examples of multi-pass transmembrane proteins.
- G protein-coupled receptor:
• 7 transmembrane alpha-helices - Porins:
• Contain multiple beta-strands
• Forms a beta-barrel
Describe beta-barrels.
• Cylindrical beta-sheet
• Amino acids alternate between polar and nonpolar residues
• Nonpolar groups point towards outside of barrel (interact with hydrophobic membrane)
• Polar groups orient towards hydrophilic opening (forms a channel from extracellular to intracellular space - allows passage of small polar solutes)
Describe the structure and function of detergents.
• Hydrophilic head groups
• Hydrophobic tails
• Form spherical micelles
• Used to purify membrane proteins
What are three types of detergents? Compare them and give an example of each.
- Ionic:
• Charged head groups
• Ex: SDS - Non-ionic:
• Uncharged head groups
• Ex: Triton X-100 - Zwitterionic:
• Positively and negatively charged head groups
• Ex: CHAPS
Which types of detergents are most commonly used for protein purification? Why?
• Non-ionic and zwitterionic
• Allow protein to retain its functional form
Why are ionic detergents not as commonly used for protein purification?
Ionic detergents can denature the target proteins
Explain the three stages in which integral membrane proteins are purified.
- Detergent micelle interacts with outer bilayer of membrane
- Membrane is solubilized (formation of protein-detergent complex and lipid-detergent complex)
- Protein is purified using affinity chromatography
What are the functions of carbohydrates in the membrane?
• Bind hormones or neurotransmitters
• Allow cells to recognize eachother
What are the functions of integral and peripheral proteins within the membrane?
Integral proteins:
• Transfer molecules and signals across cell membrane
Peripheral proteins:
• Also participate in cell signaling cascades
• Link membrane with cytoskeleton (provides structural support)
