L16 Lipids and membranes

Question 1

What is a membrane?

Answer 1

important barrier between compartments with different conditions and functions

Question 2

Where do we find membranes?

Answer 2

• 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

Question 3

Major membranes ?

Answer 3

• nuclear envelope
• vacuole membrane
• endoplasmatic reticulum
• mitochondrial membrane
• cytoplasm membrane

Question 4

Why membranes?

Answer 4

• 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,

Question 5

Structure of biological membranes

Answer 5

2-dimensional fluid

• hydrophilic head groups
• hydrophobic fatty acid tails

-> they are dynamic (flexion, rotation)

Question 6

Structure of biological membranes

Answer 6

2-dimensional fluid

• hydrophilic head groups
• hydrophobic fatty acid tails

-> they are dynamic (rotation, flexion)

Question 7

What do lipids contain ?

Answer 7

Contain glycerol core linked to two fatty acid-derived “tails” by ester linkages
and to one “head” group by a phosphate ester linkage

Question 8

Components of membranes

Answer 8

1. Amphipathic lipids (contain both hydrophobic and hydrophilic regions)
2. 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)
3. Water
4. Cations (e.g. calcium)

Question 9

Mitochondrial membranes contain mainly _______

Answer 9

phospholipids

Question 10

Chloroplast membranes contain mainly _______

Answer 10

glycolipids

Question 11

Sterols

Answer 11

*have a smaill hydrophilic part

examples

• cholesterol
• camposterol
• sitosterol
• stigmasterol

Question 12

Fatty acids

Difference between saturated and unsaturated

Answer 12

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

Question 13

Degree of fatty acid saturation

Answer 13

• 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

Question 14

Melting poins of C18 faty acids

Answer 14

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

Question 15

Poly unsaturated fatty acids and oxidation (1/2)

Answer 15

• 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

Question 16

Poly unsaturated fatty acids and oxidation (2/2)

Answer 16

• 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

Question 17

Membrane proteins

Answer 17

1. receptors (trasduce signals from outside the cell to inside)
2. channels
3. transporters
4. pores
5. pumps

*they all have different functions

Question 18

Membrane synthesis and degradation

Answer 18

• 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

Question 19

Jasmonic acid (JA) synthesis

Answer 19

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

Question 20

Phospholipid breakdown and signalling

Answer 20

• 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

Question 21

Loss of membrane integrity

Answer 21

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

Question 22

Biophysical changes during

senescence/ageing

Answer 22

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

Question 23

conclusions slide 52