Lecture # 17 Membranes and Proteins

Question 1

What are the main storage lipids?

Answer 1

Triglycerides that are fully saturated.

Question 2

What are the two major structural lipid classes?

Answer 2

Fatty acid containing and isoprenoid-derived.

Question 3

What type of lipid are the main constituents of membranes?

Answer 3

Glycerolipids

Question 4

What type of lipids play important roles in cell recognition and insulation?

Answer 4

Sphingolipids

Question 5

Why is cholesterol such a dynamic compound in the body?

Answer 5

Cholesterol acts as a membrane lipid and is a precursor for steroid hormones; cholesterol changes membrane fluidity at different temperatures.

Question 6

What are the four functions of biological membranes?

Answer 6

1) Permeability 2) Membrane Potential 3) Signaling 4) Connection

Question 7

30. (a) List three different major components of eukaryotic membranes. (b) When a
    preparation of mitochondrial membranes was treated with high salt (0.5 M NaCl), it was
    observed that 40% of the total protein in this preparation was solubilized. What kind of
    membrane proteins are in this soluble extract, and what forces normally hold them to the
    membrane? (c) What kind of proteins constitute the insoluble 60%, and what forces
    hold these proteins in the membrane?

Answer 7

(a) phospholipids, sterols, proteins. (b) Peripheral proteins would be present in the extract, because peripheral proteins loosely associate with integral proteins. They can be removed easy by addition of salt. (c) The other two proteins are integral and lipid anchored. Lipid anchored proteins are anchored to the membrane by modification with a lipid. They can be removed by enzymatic activity (Lipase). Integral proteins span the entire membrane with tight association to it. They can only be removed by disrupting the membrane itself.

Question 8

What are the three types of proteins that associate with the membrane?

Answer 8

integral, peripheral, and lipid anchored proteins.

Question 9

Integral protein

Answer 9

Integral proteins span the entire membrane with tight association to it. They can only be removed by disrupting the membrane itself.

Question 10

Lipid Anchored protein

Answer 10

Lipid anchored proteins are anchored to the membrane by modification with a lipid. They can be removed by enzymatic activity (Lipase).

Question 11

Peripheral Protein

Answer 11

Relative to te integral membrane proteins, these associate loosely with the polar head groups of membranes or other membrane proteins. Can be removed without breaking the membranes itself (Salt, urea, pH etc.) Breaking ionic and hydrophobic bonds.

Question 12

32. (a) When relatively high concentrations of fatty acids are suspended in water, they form
    structures known as ________. (b) When relatively high concentrations of membrane
    phospholipids are dissolved in water, they form structures known as ________. (c)
    Why are the structures listed in your answers to (a) and (b) above energetically favored?

Answer 12

(a) Micelles (b) Bilayers (c) If the hydrophobic portion of either phospholipids or triglycerides associate with water, the water molecules become highly order, and thus experiences a decrease in entropy, which is energetically less favorable. Both of these structures cause the association between the polar molecules with each other and the same for non-polar molecules, which is more energetically favorable.

Question 13

What are the functions of membrane proteins?

Answer 13

1. Selective membrane transport 2. Membrane potential 3. Signaling by receptors 4. Connection to the cytoskeleton and extracellular matrix.

Question 14

Selective Membrane Transport

Answer 14

Channels, gates, and pumps (nutrients, ions, metabolites, and some signals)

Question 15

Membrane Potential

Answer 15

Enzymes can take advantage of the membrane potential (ATP synthesis) ATPase/ATP synthase

Question 16

Signaling by receptors

Answer 16

Receptors detecting signals from outside

Question 17

Connection to the cytoskeleton and extracellular matrix

Answer 17

Fibrous protein interactors; Ankyrins and Adherens

Question 18

(a) Explain why polar glycerolipids are capable of spontaneously assembling into the
bilayer structure found in biological membranes but triacylglycerols are not. (b) What
are the forces that drive bilayer formation?

Answer 18

(a) glycerol lipids have groups (polar head groups) that are incredibly more ionized than the slightly polar glycerol portion of triacylglycerols the high degree of ionization for glycerolipids causes the spontaneous assembling into a bilayer (b) the forces that drive bilayer formation is the energy released as water molecules decrease their association with the lipids.

Question 19

Uniport

Answer 19

transportation of one molecule

Question 20

symport

Answer 20

transports two molecules in the same direction

Question 21

Antiport

Answer 21

transports two molecules in opposite directions

Question 22

(a) What kinds of forces or bonds anchor an
integral membrane protein in a biological
membrane? (b) What forces hold a peripheral membrane protein to the membrane? (c)
What might one do to solubilize each of the two types of membrane proteins?

Answer 22

(a) Hydrophobic interactions (b) ionic and hydrophobic bonds (c) for an integral protein, agents that interfere with hydrophobic interactions ( detergents and organics), while peripheral proteins can be removed by application of salt, urea, pH, etc.

Question 23

The bacterium E.coli can grow at 20 °C or at 40 °C. At which growth temperature would you expect the membrane phospholipids to have a higher ratio of saturated to unsaturated fatty acids, and why?

Answer 23

At higher temperatures, I would expect the phospholipids to have a higher ratio of saturated to unsaturated fatty acids, because saturated fatty acids have a higher melting point and are more stable.

Question 24

Distinguish between simple diffusion (SD), facilitated diffusion (FD), and active transport (AT) across a membrane for the following questions. (More than one may be true.)
(a) Which processes are energy dependent?
(b) Which processes need some kind of carrier protein(s)?
(c) Which processes can be saturated by substrate?
(d) Which processes can establish a concentration gradient?
(e) How much energy does it take to transport an uncharged substrate in, if its starting
inside concentration is 10-fold greater than outside?

Answer 24

(a) AT (b) FD or AT (c) SD, FD, AT (d) AT (e) AT

Question 25

Explain why nonpolar compounds are generally able to diffuse across biological
membranes without the aid of a specific transport system.

Answer 25

Small nonpolar molecules are able to pass through biological membranes, because the membrane is mostly non-polar. Like dissolves like.

Question 26

Both the glycerol-3-phosphate shuttle and the malate-aspartate shuttle move NADH
equivalents to the matrix. Why might a cell use one instead of the other?

Answer 26

The glycerol-3-phosphate shuttle is used in the mitochondria of the skeletal muscle or the brain, while malate-aspartate shuttle is used in the kidneys, liver and heart. Glycerol-3-phosphate is used in times of need of quick energy.

Question 27

For each of the following explain whether they are transported into or out of the
mitochondrion and comment briefly on the mechanism for each:
(1) NADH
(2) FADH
(3) ADP

Answer 27

Neither NADH nor FADH is transported into the mitochondrion directly. The malate-aspartate shuttle and the glycerol-3-p shuttle are used to transport electrons from these compounds down the electron transport chain. ADP is transferred into the mitochondria via specialized ADP transporters.

Question 28

As you read and answer this question, you are (hopefully) consuming oxygen. What single reaction accounts for most of your oxygen use? Although molecular oxygen (O2) does not participate directly in any of the reactions of the citric acid cycle, the cycle operates only when O2 is present. Explain this observation.

Answer 28

The single reaction that accounts for most of oxygen usage is the incorporation of oxygen as a terminal electron acceptor at the end of the electron transport chain. Although O2 does not participate directly in any of the reactions of the citric acid cycle, the cycle only works in the presence of O2, because when O2 is absent, pyruvate is not converted into one of the critical substrates of the tricarboxylic acid cycle.