Chapter 12- Lipids and cell membranes Flashcards
Fatty acids structure
Fatty acids are chains of hydrocarbon atoms that have a carboxylic acid at one end and a methyl group at the other end. They may be saturated (have single bonds) or unsaturated (have one or more double bonds).
How are fatty acid carbon atoms usually numbered?
Fatty acid carbon atoms are usually numbered beginning with the carboxyl terminal carbon atom. Carbon atoms 2 and 3 are also referred to as α and β, respectively. Fatty acids can instead be numbered from the methyl carbon atom, which is called the omega (ω) carbon
Fatty acid names are based on
Their parent hydrocarbons. The “-e” in the hydrocarbon name is replaced with “-oic acid”. For example, hexadecane becomes hexadecenoic acid.
How long are fatty acid chains generally?
Fatty acids in biological systems usually contain an even number of carbon atoms, with the 16- and 18-carbon atom chains being the most common
What are the configuration of double bonds in fatty acids?
When double bonds are present, they are commonly in the cis configuration. In polyunsaturated fatty acids, the double bonds are separated by at least one methylene (CH2) group.
The properties of fatty acids are dependent on which factors?
Chain length and degree of unsaturation
Which factors enhance the fluidity of fatty acids?
Short chain length and the presence of cis double bonds
3 common types of membrane lipids
- Phospholipids
- Glycolipids
- Cholesterol
4 components of phospholipids
one or two fatty acid tails, a platform molecule, a phosphate, and an alcohol. The fatty acids and phosphate are bonded to the platform, and the alcohol will be bonded to the phosphate if it’s present
Platform molecule
This is a molecule that acts as a backbone of attachment for other groups in a phospholipid. The fatty acids and phosphate group will be attached to the platform molecule. Glycerol is an example.
2 common types of platform molecules
Glycerol and sphingosine
phosphoglycerides/phosphoglycerols
Phospholipids with a glycerol platform. They are formed when an alcohol is added on to phosphatidate (diacylglycerol 3-phosphate). Phosphoglycerides are derived from phosphatidate by the formation of an ester bond between the phosphate and an alcohol
Sphingomyelin
A common membrane lipid in which the primary hydroxyl group of sphingosine is esterified to phosphorylcholine.
Glycolipids
Carbohydrate-containing lipids derived from sphingosine. The carbohydrate is linked to the primary alcohol of sphingosine. The carbohydrate components of glycolipids are on the extracellular surface of the cell membrane, where they play a role in cell–cell recognition
2 types of glycolipids
- Cerebrosides
2. Gangliosides
Cerebrosides
The simplest glycolipids, containing only a single sugar (glucose or galactose)
Gangliosides
Contain a branched chain of as many as seven sugar molecules
Cholesterol
A steroid with multiple rings that is modified on one end by the attachment of a fatty acid chain and at the other end by a hydroxyl group. In membranes, the hydroxyl group interacts with phospholipid head groups.
Amphipathic
Membrane lipids are considered amphipathic molecules because they contain hydrophobic and hydrophilic properties. The fatty acid tail components provide the hydrophobic properties, while the polar head group provides the hydrophilic properties
Polar head group
The alcohol and phosphate components of fatty acids. This part of the fatty acid is hydrophilic
Why do lipids form cell membranes?
Membrane (bimolecular sheet) formation is a consequence of the amphipathic nature of the constituent lipid molecules- phospholipids and glycolipids spontaneously form lipid bilayers in aqueous solutions. The hydrophobic effect drives membrane formation, and van der Waals interactions between the hydrophobic tails also
stabilize membranes
Which interactions occur between the polar head groups of fatty acids and water molecules?
There are electrostatic and H-bonding attractions between the polar head groups and water molecules.
Micelle
A micelle is formed when a variety of molecules (a fatty acid, a salt of a fatty acid like soap, or phospholipids) are added to water. The molecule must have a strongly polar “head” and a non-polar hydrocarbon chain “tail”. When this type of molecule is added to water, the non-polar tails of the molecules clump into the center of a ball like structure, called a micelle, because they are hydrophobic. The polar head of the molecule presents itself for interaction with the water molecules on the outside of the micelle
Liposomes
Lipid vesicles- they are aqueous compartments enclosed by a lipid membrane. They formed by sonicating a mixture of phospholipids in aqueous solution and can be useful as drug delivery systems
Protein-liposome complexes can be used to
Investigate membrane protein functions
Planar bilayer membranes are useful for examining
membrane properties such as
Ion permeability in the presence of a voltage difference across the membrane
Which molecules are lipid bilayers mostly impermeable to?
Ions, most polar molecules
Why are small molecules able to cross a lipid bilayer?
The ability of small molecules to cross a lipid bilayer is a function of their hydrophobicity (lack of polarity)
Membrane proteins
Membranes vary in protein content, from less than 20% to as much as 75%. Membrane proteins allow transport of molecules and information across the membrane. The types of membrane proteins in a cell are a reflection of the biochemistry occurring inside the cell.
How can membrane proteins be visualized?
SDS-polyacrylamide gel electrophoresis
Schwann cell
This cell has a relatively low membrane protein content. Instead, its plasma membrane is lipid-rich; the lipid serves as an insulator,
allowing rapid transmission of nerve impulses. The wrapping of this type of cell around an axon is referred to as myelination
Multiple sclerosis
A demyelination disease. Myelin assembly can be impaired and existing myelin can be damaged.
How do proteins associate with the lipid bilayer? (3 ways)
- Integral membrane proteins are embedded in the hydrocarbon core of the membrane.
- Peripheral membrane proteins are bound to the polar head groups of membrane lipids or to the exposed surfaces of integral membrane proteins.
- Some proteins are associated with membranes by attachment to a hydrophobic moiety that is inserted into the membrane.
Integral membrane proteins
Proteins embedded in the hydrocarbon core of the membrane. Membrane-spanning α helices are a common structural feature of integral membrane proteins. Example- bacteriorhodopsin, a light-powered proton pump, is an integral membrane protein composed of seven membrane-spanning helices. Integral membrane proteins may also be composed of β strands that form a pore in the membrane- the bacterial protein porin is an example
Bacteriorhodopsin
Bacteriorhodopsin alpha helices contain predominantly hydrophobic residues, while polar and charged residues tend to be found in the cytoplasmic and extracellular regions
Porin
A bacterial integral membrane protein that contains beta sheets. The outside surface of porin, which interacts with the hydrophobic interior of the membrane, is composed of hydrophobic amino acids. The inside is polar and filled with water
How is a protein linked to the membrane surface?
Embedding part of a protein in a membrane can link the protein to the membrane surface. For example, only part of the enzyme prostaglandin H2 synthase-1 is embedded in the membrane. This is because the cyclooxygenase (COX) activity of prostaglandin H2 synthase-1 depends on a channel connecting the active site to the membrane interior. Aspirin inhibits cyclooxygenase activity by obstructing the channel.
How does aspirin work?
Aspirin inhibits cyclooxygenase activity by obstructing the channel. It acts by transferring an acetyl group to a serine residue in prostaglandin H2 synthase-1
How can hydrophilic proteins associate with the cell membrane?
Hydrophilic proteins can be made to associate with the membrane by attaching one of a variety of possible hydrophobic groups (which are covalently attached).
Which protein conformation is able to transverse a lipid membrane?
An α helix consisting of 20 residues can traverse a lipid bilayer. A protein sequence can be examined by measuring the free energy
of transferring each stretch of 20 amino acids, called a window, from
a hydrophobic to a hydrophilic environment.
How is the hydrophobicity of each amino acid quantified?
By determining the free energy required to transfer the amino acid from a hydrophobic to a hydrophilic environment
Hydropathy plot
The free energy is plotted against the first amino acid of the window. Hydropathy plots can identify potential membrane-spanning helices when sequence but little additional information is known for a protein
FRAP
Fluorescence recovery after photobleaching- allows the measurement of lateral mobility of membrane components. The mobility of the fluorescently labeled component is a function of how rapidly the bleached area recovers fluorescence. A membrane component is attached to a fluorescent molecule. On a very small portion of the membrane, the dye is subsequently destroyed by high-intensity light, thereby bleaching a portion of the membrane
During FRAP, what does lateral diffusion of proteins depend on?
Lateral diffusion of proteins depends on whether they are attached
to other cellular or extracellular components
Fluid mosaic model
Describes membranes as two-dimensional solutions of oriented lipids and globular proteins. The lipids serve as a solvent and a permeability barrier
How do lipids move as they diffuse through lipid membranes?
Lipids rapidly diffuse laterally in membranes, although transverse diffusion (or flip-flopping) is very rare without the assistance of enzymes. The prohibition of transverse diffusion accounts for the stability of membrane asymmetry.
Why is membrane fluidity important?
Membrane processes depend on the fluidity of the membrane. Cholesterol helps to maintain proper membrane fluidity in membranes in animals
Melting temperature of lipid membranes
The temperature at which a membrane transitions from being highly ordered to very fluid is called the melting temperature (Tm). The melting temperature is dependent on the length of the fatty acids in the membrane lipid and the degree of cis unsaturation
Which molecules can cholesterol form complexes with?
Cholesterol can form complexes with sphingolipids, glycolipids, and some GPI-anchored proteins
Lipid rafts
Cholesterol and lipid/protein complexes which concentrate in small, defined regions of the membrane. Lipid rafts help to moderate membrane fluidity and appear to function in signal transduction
Why are all biological membranes considered to be asymmetric?
The outer and inner leaflets (faces) of all biological membranes have different components and different enzymatic activities from each other. Example- asymmetry of the Na+/K+ pump in the plasma membrane
How are the types of bacterial membranes distinguished?
By Gram staining. Bacterial membranes can be categorized as Gram positive or Gram negative
Gram positive bacteria structure
Some bacteria and archaea are enclosed by a single membrane surrounded by a thick cell wall. These cells are sometimes referred to as Gram positive
Gram negative bacteria structure
Some bacteria are surrounded by two membranes, with a cell wall (made of proteins, peptides, and carbohydrates) lying between them. The space between the two membranes is called the periplasm. These cells are Gram negative
What is the difference in staining between Gram positive and Gram negative bacteria?
Gram-positive bacteria retain the crystal violet stain in their thick cell walls, while Gram-negative bacteria have a thinner cell wall that does not retain the stain well
Do eukaryotic cells have cell walls?
Eukaryotic cells, with the exception of plants, do not have cell walls. They are surrounded by a single membrane, known as the plasma membrane (or cell membrane). Eukaryotic cells also have membranes inside the cell that allow compartmentalization of function. Example- the nuclear envelope
Nuclear envelope
The nuclear envelope is a double membrane connected to another membrane system of eukaryotes, the endoplasmic reticulum.
Receptor-mediated endocytosis
This is the process by which cells can acquire molecules from their environment. The protein clathrin helps to internalize receptors bound to their cargo. Example- the internalization of iron-bound transferrin in association with the transferrin receptor
How are molecules released from the cell?
Fusion of internal membranes with the plasma membrane allows the
release of molecules, such as neurotransmitters, from the cell. SNARE proteins facilitate membrane fusion by forming tightly coiled
four-helical bundles
Cardiolipin structure
This molecule has a different structure compared with the other
phosphoglycerides. It has a net charge of −2 and an inverted cone
shape
Where is cardiolipin usually found?
Cardiolipin is most often found in the membranes of bacteria,
archaea, and the inner membranes of mitochondria. In mitochondria, the cardiolipin is involved in the structure and function of the respirasome, which is essential in ATP synthesis.
What is required for cardiolipin synthesis?
Proper synthesis and maintenance of cardiolipin levels requires the
enzyme tafazzin, which catalyzes the transfer of linoleate chains
from phosphatidylcholine to immature cardiolipin
Barth syndrome
Results from mutations that reduce the catalytic activity of the tafazzin enzyme. Symptoms include dilation of the heart chambers, exercise intolerance, and impaired growth. These individuals have malformed mitochondria with distorted inner membranes and poorly functioning respirasomes due to improper assembly of the protein complexes.
Structure of glycolipids
The carbohydrate is linked to the primary alcohol of sphingosine.
