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summary of chapter 10-18 chapter 14.15 = lecture notes
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Comprehensive University level notes on Molecular biology
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MOLECULAR BIOLOGY OF THE CELL 6TH EDITION BRUCE ALBERTS TEST BANK 2025 CHAPTER 3: PROTEINS
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Samenvatting Celbiologie
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Chapter 10 – membrane structureKnow the structure of a cell (organelles)
the plasma membrane surrounds the cell and maintains the essential differences between
the cytosol and the extracellular environment.
In the cell is the cytosol → plasma + the organelles
Nucleus → DNA is stored
- Surrounded by a nuclear envelope
the black dots are ribosomes
Endoplasmic Reticulum surrounds the nucleus
(proteins synthesis)
Golgi apparatus (factory →molecules are modified)
Peroxisomes (important for oxidated processes)
Membranes have a common general structure: very
thin film of lipid and proteins molecules
- Held together by noncovalent interactions.
The lipid bilayer
- Impermeable barrier to the passage of most water-soluble molecules
Lipid molecules in the cell are amphiphilic
- Hydrophilic (polar) end and a hydrophobic (nonpolar) end
The hydrophobic parts are on the inside and the hydrophilic parts are on the outside. It is
energetically favorable.
Plasma membrane: 50% of mass is protein
- Why so many proteins? → to transport molecules in and out of the membrane and
needed for cell-to-cell signaling (hormones)
phospholipids
- Three-carbon glycerol backbone
- Two long-chain fatty acids (ester bonds) → hydrophobic
- The head group determines the name of the molecule → hydrophilic
They can have straight chains or bended chains
- Always in cis-configuration
- Bended chains = cis-double bond (unsaturated bonds)
Glycerophospholipids
- Main phospholipids in most animal cell
- By combining several different fatty acids and head groups, cells make different
glycerophospholipids.
,able to recognize the 4 phospholipid structures
- Phosphatidylserine is overall negative charged!
o Find it only in the inner leaflet of the membrane
Sphingolipids → built from sphingosine.
- Long fatty acid tail with an amino group and two hydroxyl groups at one end.
Sphingosine has a different structure → no glycerol backbone
Fatty acid chain used to have carboxyl acid group
Phospholipids spontaneously form Bilayers
Lipid molecules in water → form spontaneously bilayers.
- Crucial for evolution of life
- Maximize exposure of their hydrophilic heads to water
o Micelles (tails inward)
o Double-layered sheets/ bilayers (hydrophobic tails sandwiched between the
hydrophilic head groups)
Bilayers will spontaneously close to form the liposome
The edges of the bilayer are still exposed to water, which is energetically
unfavorable. That’s why the bilayer will form a sealed compartment, which
shields the hydrophobic tails from the water and forms a liposome.
(energetically favorable)
,The lipid bilayer is a two-dimensional fluid
know the properties of a lipid bilayer
Flip-flop → the hydrophilic head groups must transiently enter and pass through the
hydrophobic core of the bilayer → energetically unfavorable and rarely occurs.
Lateral diffusion, rotating and flexion (extend the chain)
Lipid bilayer behaves like a liquid.
Water molecules slowly diffuse the membrane.
The fluidity of a lipid bilayer depends on its composition.
- Fluidity goes up, when the temperature goes up (like butter)
- The number of double bonds (more cis-bonds present), fluidity goes
up
o If chain is bend, then it’s more difficult to form hydrophobic
interactions
- If the chain length goes down, less interaction, fluidity goes up.
Cholesterol
- Only eukaryotic cell molecule
- Has rigid ring structure
- Hydrophobic
- Fills up the gaps between phospholipids
o Less permeable (enhances the
barrier function)
Cholesterol enhances permeability-barrier
properties & inhibits phase transitions.
- Polar head group
- Cholesterol stiffened region
- More fluid region
Cholesterol is not in prokaryotes
E. coli = 70% phosphatidylethanolamine
Glycolipids are found on the surface of all eukaryotic plasma
membranes
Glycolipids → sugar-containing lipid molecules
- Found on the outside of the membrane (facing away from the cytosol)
- Hydrogen bonds between the sugars and van der Waals forces between the chains.
- Function: cell recognition, communication both within and between cells
- Gangliosides → most complex one (oligosaccharides) → negative charge
, Membrane Proteins
Membrane proteins
- Amphiphilic (having hydrophobic and hydrophilic regions)
Transmembrane proteins → part of their mass extruding from the membrane on both sides
- Hydrophobic regions pass through the membrane and interact with the hydrophobic
tails of the lipid molecules in the interior of the bilayer, where they are sequestered
away from water.
- Can be inserted by a helix, beta-sheet or just a lipid molecule (GPI anchor)
- Can be indirectly associated (bound to the membrane but not part of it)
Proteins can be released from the membrane by:
- Phospholipase C
- High salt solution
- Detergents
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