Chapter 5: Membrane Structure, Synthesis and Transport
Membrane Structure
phospholipid bilayer
phospholipids are amphipathic (chapter 3)
contain proteins and carbohydrates
The two leaflets (halves of bilayer) are asymmetrical
considered a mosaic of lipid, protein, and carbohydrate molecules
resembles a fluid because lipids and proteins can move relative to each other within the membrane
Proteins bound to membranes
intrinsic membrane proteins
Lipid-anchored proteins
Transmembrane proteins
extrinsic membrane proteins
Approximately 25% of All Genes Encode Transmembrane Proteins
This trend is found throughout all domains of life including archaea, bacteria, and eukaryotes
TEM
Biological sample is thin sectioned and stained with heavy-metal dyes
Dye binds tightly to the polar head groups of phospholipids, but not to the fatty acyl chains
This makes membranes resemble railroad tracks
Fluidity of Membranes
Membranes are semifluid
Most lipids can rotate freely around their long axes and move laterally within the membrane leaflet
“flip-flop” movement does not happen spontaneously
Flippase requires ATP to transport lipids between leaflets
Lipid rafts
Certain lipids associate strongly with each other
A group of lipids floats together as a unit within the larger sea of lipids in the membrane
Composition
Unique set of membrane proteins
High concentration of cholesterol
Factors affecting fluidity
Presence of double bonds
Presence of cholesterol
Length of fatty acyl tails
more short more fluid
more double bonds more fluid
Cholesterol tends to stabilize membranes
more cholesterol less fluid
Effects vary depending on temperature
Synthesis of Membrane Components
Lipids
In eukaryotes, the cytosol and endomembrane system work together to synthesize lipids
Fatty acid building blocks are made via enzymes in cytosol or taken into cells from food
Process occurs at cytosolic leaflet of the smooth ER
Transfer of lipids to other membranes
Lipids in ER membrane can diffuse laterally to nuclear envelope
Transported via vesicles to Golgi, lysosomes, vacuoles, or plasma membrane
Lipid exchange proteins – extract lipid from one membrane for insertion in another
Transmembrane Proteins
Except for proteins destined for semiautonomous organelles, most transmembrane proteins are directed to the ER membrane first
From the ER, membrane proteins can be transferred via vesicles to other membranes of the cell
Glycosylation
Process of covalently attaching a carbohydrate to a protein or lipid
Types
Glycoprotein
Glycolipid
carbohydrate to protein
carbohydrate to lipid
serve as recognition signals for other cellular proteins
plays a role in cell surface recognition
Helps protect proteins from damage
N-linked
O-linked
Attachment of carbohydrate to nitrogen atom of asparagine side chain
Addition of sugars to oxygen atom of serine or threonine side chains
Occurs only in Golgi
Membrane Transport
selectively permeable
This structure ensures that
Essential molecules enter
Metabolic intermediates remain
Waste products exit
Ways to move across membranes
Active transport
Passive transport
Requires energy
Requires no input of energy
Facilitated diffusion
Passive diffusion
Phospholipid bilayer barrier
Barrier to hydrophilic molecules and ions due to hydrophobic interior
Rate of diffusion depends on chemistry of solute and its concentration
Gradients
Ion electrochemical gradient
Transmembrane gradient
Tonicity
Hypertonic
Hypotonic
Isotonic
Equal water and solute concentrations on either side of the membrane
Solute concentration is higher (and water concentration lower) on one side of the membrane
Solute concentration is lower (and water concentration higher) on one side of the membrane
Osmosis
Water diffuses through a membrane from an area with more water to an area with less water
water movement can make the cell shrink or swell
Osmotic pressure
the tendency for water to move into any cell
Animal cells
Osmotic Lysis
Crenation
swelling and bursting of a cell in a hypotonic solution
shrinkage of a cell in a hypertonic solution
Plant cells
Plasmolysis
cell wall prevents major changes in cell size
plants wilting because water leaves plant cells
Turgor pressure
pushes plasma membrane against cell wall
Maintains shape and size
Freshwater protists
have to survive in a strongly hypotonic environment
To prevent osmotic lysis, contractile vacuoles take up water and discharge it outside the cell
maintains a constant cell volume
Transport Proteins
Two classes
Channels
Transporters
open passageway for direct diffusion
Most are gated
example: Aquaporins
Conformational change transports solute across membrane
known as carriers
Principal pathway for uptake of organic molecules, such as sugars, amino acids, and nucleotides
Types
Symporter
Antiporter
Uniporter
Movement against the gradient
Types
Primary active transport
Secondary active transport
pump
Uses a pre-existing gradient to drive transport
generate ion electrochemical gradients
Actively transports Na+ and K+ against their gradients using the energy from ATP hydrolysis
3 Na+ are exported for every 2 K+ imported into cell
Exocytosis and Endocytosis
Exocytosis
Endocytosis
Pinocytosis
Phagocytosis
Receptor-mediated endocytosis
Material inside the cell packaged into vesicles and excreted into the extracellular medium
Used to transport large molecules such as proteins and polysaccharides