L16 Lipids and membranes Flashcards
What is a membrane?
important barrier between compartments with different conditions and functions
Where do we find membranes?
- the vacuole
- the plasma membrane
- the endoplasmic reticulum (smooth & rough)
- the Golgi apparatus
- the peroxisome/glyoxosome
- the microbodies
- the chloroplast (incl. other plastids: chromoplasts, gerontoplasts etc, and their internal membranes)
- the mitochondria
- the nuclear envelope
- All kinds of vesicles that transport e.g. enzymes
Major membranes ?
- nuclear envelope
- vacuole membrane
- endoplasmatic reticulum
- mitochondrial membrane
- cytoplasm membrane
Why membranes?
• Barrier function
• Creates different compartments within the cell
-Vacuole pH 5, cytoplasm, nucleus pH 7, mitochondria matrix pH 8
-Stroma pH 8, lumen pH 4
• Isolate different processes (that operate under different conditions) from each other
-Photosynthesis, respiration, gene transcription..
• Membranes are a matrix for adhesion/embedding proteins that facilitate
specific processes
-Photosystems 1 and 2, variety of receptors, ribosomes (protein synthesis), channels and/or pumps,
Structure of biological membranes
2-dimensional fluid
- hydrophilic head groups
- hydrophobic fatty acid tails
-> they are dynamic (flexion, rotation)
Structure of biological membranes
2-dimensional fluid
- hydrophilic head groups
- hydrophobic fatty acid tails
-> they are dynamic (rotation, flexion)
What do lipids contain ?
Contain glycerol core linked to two fatty acid-derived “tails” by ester linkages
and to one “head” group by a phosphate ester linkage
Components of membranes
- Amphipathic lipids (contain both hydrophobic and hydrophilic regions)
- Proteins:
• embedded, or integral, proteins (eg transporters)
• attached proteins - anchored in membrane via prenyl, fatty acid, or phospholipids groups bound to the protein but dissolved in the
hydrophobic core of membrane
• peripheral proteins (loosely attached) - Water
- Cations (e.g. calcium)
Mitochondrial membranes contain mainly _______
phospholipids
Chloroplast membranes contain mainly _______
glycolipids
Sterols
*have a smaill hydrophilic part
examples
- cholesterol
- camposterol
- sitosterol
- stigmasterol
Fatty acids
Difference between saturated and unsaturated
Unsaturated : they have a dense structure and there is a double bond which works like a reaction center available for hydrogens
*they are healthiel (olive oil)
Saturated : not healthy and they are most known for raising your LDL cholesterol
Degree of fatty acid saturation
• The degree of lipid unsaturation affects the melting
temperature of the free fatty acid:
• The more double bonds in the fatty acid then the poorer the packing
of the chains and the lower the melting point of the fatty acid.
• A low melting point means that the membrane consisting of such fatty
acids is in liquid phase even at low temperature
• It melts at low temperature
Melting poins of C18 faty acids
The longer the chain the higher the melting point
- unsaturated fatty acids (olive oil) -> liquid in room temperatures
- saturated fatty acids (butter) -> solid in room temperature
Poly unsaturated fatty acids and oxidation (1/2)
- Unsaturated fatty acids are relatively reactive with the hydrophobic membranes
- Oxygen is particular problem - it reacts with double bonds in the unsaturated fatty acids.
- This requires membranes to be actively protected against oxidation and the free-radical chain reactions
- Anti-oxidants such as tocopherol (vitamin E) are present in membranes to scavenge free-radicals - one tocopherol can protect about 200 fatty acids
*tocopherol has a lot of double bonds
Poly unsaturated fatty acids and oxidation (2/2)
• Lipid hydro peroxides formed can be reduced to hydroxyl lipids by alkyl hydro
peroxide reductases
• About 1% of a typical membrane is oxidized
• oxidation results in a loss of membrane fluidity and hydrophobicity
• Antioxidants have much higher affinity for *radicals
• Take the place of the lipid in this reaction
• Protect the membrane
Membrane proteins
- receptors (trasduce signals from outside the cell to inside)
- channels
- transporters
- pores
- pumps
*they all have different functions
Membrane synthesis and degradation
- Cell expansion (imagine leaf or petal) requires massive synthesis of membrane components and formation of membrane double layers
- Regular turnover of membrane components (degradation and synthesis)
- Enzymatic breakdown by phospholipases and lipoxygenases
Jasmonic acid (JA) synthesis
The major function of JA and its various metabolites is regulating plant responses to abiotic and biotic stresses as well as plant growth and development
Phospholipid breakdown and signalling
- PLC generates IP3 and DAG
- DAG can be phosphorylated into PA by DGK
- PLD generates PA
- PA can be phosphorylated into DGPP
- All signaling molecules
very quick signaling
Loss of membrane integrity
Membrane integrity (loss of function) may occur through different mechanism:
• Changes in PL composition (relative increase in saturated fatty acids)
• Changes in PL/sterol ratio (e.g. phospholipid breakdown)
• Lipid oxidation (a self propagating process)
Such processes lead to increase in micro viscosity (less fluid) and to “leaky regions” due to phase separation
• Such processes may increase the phase-transition temperature to such an extend that the membrane becomes “solid” at room temperature
Biophysical changes during
senescence/ageing
Membranes become increasingly leaky
• The cell starts to eat itself (PCD), nutrients are re-used
Finally accumulation of ROS
• Membrane lipid peroxidation
• Less fluidity, phase separation
• leaky membranes (plasma and vacuole membranes)
Change in PL/Sterol ratio (more PL breakdown than synthesis)
• Change in micro viscosity (less fluid)
• Leaky membranes
Phase transition temperature increases (results of several processes)
• gel phase at ambient temperature
• Cells die, intercellular spaces become flooded, tissue dries out
conclusions slide 52
