Chapter 12- Lipids and cell membranes

Question 1

Fatty acids structure

Answer 1

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).

Question 2

How are fatty acid carbon atoms usually numbered?

Answer 2

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

Question 3

Fatty acid names are based on

Answer 3

Their parent hydrocarbons. The “-e” in the hydrocarbon name is replaced with “-oic acid”. For example, hexadecane becomes hexadecenoic acid.

Question 4

How long are fatty acid chains generally?

Answer 4

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

Question 5

What are the configuration of double bonds in fatty acids?

Answer 5

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.

Question 6

The properties of fatty acids are dependent on which factors?

Answer 6

Chain length and degree of unsaturation

Question 7

Which factors enhance the fluidity of fatty acids?

Answer 7

Short chain length and the presence of cis double bonds

Question 8

3 common types of membrane lipids

Answer 8

1. Phospholipids
2. Glycolipids
3. Cholesterol

Question 9

4 components of phospholipids

Answer 9

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

Question 10

Platform molecule

Answer 10

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.

Question 11

2 common types of platform molecules

Answer 11

Glycerol and sphingosine

Question 12

phosphoglycerides/phosphoglycerols

Answer 12

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

Question 13

Sphingomyelin

Answer 13

A common membrane lipid in which the primary hydroxyl group of sphingosine is esterified to phosphorylcholine.

Question 14

Glycolipids

Answer 14

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

Question 15

2 types of glycolipids

Answer 15

1. Cerebrosides

2. Gangliosides

Question 16

Cerebrosides

Answer 16

The simplest glycolipids, containing only a single sugar (glucose or galactose)

Question 17

Gangliosides

Answer 17

Contain a branched chain of as many as seven sugar molecules

Question 18

Cholesterol

Answer 18

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.

Question 19

Amphipathic

Answer 19

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

Question 20

Polar head group

Answer 20

The alcohol and phosphate components of fatty acids. This part of the fatty acid is hydrophilic

Question 21

Why do lipids form cell membranes?

Answer 21

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

Question 22

Which interactions occur between the polar head groups of fatty acids and water molecules?

Answer 22

There are electrostatic and H-bonding attractions between the polar head groups and water molecules.

Question 23

Micelle

Answer 23

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

Question 24

Liposomes

Answer 24

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

Question 25

Protein-liposome complexes can be used to

Answer 25

Investigate membrane protein functions

Question 26

Planar bilayer membranes are useful for examining

membrane properties such as

Answer 26

Ion permeability in the presence of a voltage difference across the membrane

Question 27

Which molecules are lipid bilayers mostly impermeable to?

Answer 27

Ions, most polar molecules

Question 28

Why are small molecules able to cross a lipid bilayer?

Answer 28

The ability of small molecules to cross a lipid bilayer is a function of their hydrophobicity (lack of polarity)

Question 29

Membrane proteins

Answer 29

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.

Question 30

How can membrane proteins be visualized?

Answer 30

SDS-polyacrylamide gel electrophoresis

Question 31

Schwann cell

Answer 31

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

Question 32

Multiple sclerosis

Answer 32

A demyelination disease. Myelin assembly can be impaired and existing myelin can be damaged.

Question 33

How do proteins associate with the lipid bilayer? (3 ways)

Answer 33

1. Integral membrane proteins are embedded in the hydrocarbon core of the membrane.
2. Peripheral membrane proteins are bound to the polar head groups of membrane lipids or to the exposed surfaces of integral membrane proteins.
3. Some proteins are associated with membranes by attachment to a hydrophobic moiety that is inserted into the membrane.

Question 34

Integral membrane proteins

Answer 34

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

Question 35

Bacteriorhodopsin

Answer 35

Bacteriorhodopsin alpha helices contain predominantly hydrophobic residues, while polar and charged residues tend to be found in the cytoplasmic and extracellular regions

Question 36

Porin

Answer 36

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

Question 37

How is a protein linked to the membrane surface?

Answer 37

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.

Question 38

How does aspirin work?

Answer 38

Aspirin inhibits cyclooxygenase activity by obstructing the channel. It acts by transferring an acetyl group to a serine residue in prostaglandin H2 synthase-1

Question 39

How can hydrophilic proteins associate with the cell membrane?

Answer 39

Hydrophilic proteins can be made to associate with the membrane by attaching one of a variety of possible hydrophobic groups (which are covalently attached).

Question 40

Which protein conformation is able to transverse a lipid membrane?

Answer 40

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.

Question 41

How is the hydrophobicity of each amino acid quantified?

Answer 41

By determining the free energy required to transfer the amino acid from a hydrophobic to a hydrophilic environment

Question 42

Hydropathy plot

Answer 42

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

Question 43

FRAP

Answer 43

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

Question 44

During FRAP, what does lateral diffusion of proteins depend on?

Answer 44

Lateral diffusion of proteins depends on whether they are attached
to other cellular or extracellular components

Question 45

Fluid mosaic model

Answer 45

Describes membranes as two-dimensional solutions of oriented lipids and globular proteins. The lipids serve as a solvent and a permeability barrier

Question 46

How do lipids move as they diffuse through lipid membranes?

Answer 46

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.

Question 47

Why is membrane fluidity important?

Answer 47

Membrane processes depend on the fluidity of the membrane. Cholesterol helps to maintain proper membrane fluidity in membranes in animals

Question 48

Melting temperature of lipid membranes

Answer 48

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

Question 49

Which molecules can cholesterol form complexes with?

Answer 49

Cholesterol can form complexes with sphingolipids, glycolipids, and some GPI-anchored proteins

Question 50

Lipid rafts

Answer 50

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

Question 51

Why are all biological membranes considered to be asymmetric?

Answer 51

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

Question 52

How are the types of bacterial membranes distinguished?

Answer 52

By Gram staining. Bacterial membranes can be categorized as Gram positive or Gram negative

Question 53

Gram positive bacteria structure

Answer 53

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

Question 54

Gram negative bacteria structure

Answer 54

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

Question 55

What is the difference in staining between Gram positive and Gram negative bacteria?

Answer 55

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

Question 56

Do eukaryotic cells have cell walls?

Answer 56

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

Question 57

Nuclear envelope

Answer 57

The nuclear envelope is a double membrane connected to another membrane system of eukaryotes, the endoplasmic reticulum.

Question 58

Receptor-mediated endocytosis

Answer 58

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

Question 59

How are molecules released from the cell?

Answer 59

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

Question 60

Cardiolipin structure

Answer 60

This molecule has a different structure compared with the other
phosphoglycerides. It has a net charge of −2 and an inverted cone
shape

Question 61

Where is cardiolipin usually found?

Answer 61

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.

Question 62

What is required for cardiolipin synthesis?

Answer 62

Proper synthesis and maintenance of cardiolipin levels requires the
enzyme tafazzin, which catalyzes the transfer of linoleate chains
from phosphatidylcholine to immature cardiolipin

Question 63

Barth syndrome

Answer 63

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.

Question 64

Structure of glycolipids

Answer 64

The carbohydrate is linked to the primary alcohol of sphingosine.