Ch. 6 - Lipids, Membranes, and the First Cells

Freeman - Biological Science 8th Edition

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Freeman 8th Edition

Ch. 6 - Lipids, Membranes, and the First Cells

Problema 4

Capitolo 6, Problema 4

Which of the following groups of amino acid residues would likely be found in the portion that crosses the lipid bilayer?
a. Acidic
b. Basic
c. Polar uncharged
d. Nonpolar

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Understand that the lipid bilayer is a hydrophobic environment, meaning it repels water and is composed mainly of nonpolar molecules.

Recognize that amino acids with nonpolar side chains are more likely to be found in the hydrophobic core of the lipid bilayer because they do not interact with water.

Identify the nonpolar amino acids from the image: Glycine (Gly), Alanine (Ala), Valine (Val), Leucine (Leu), Isoleucine (Ile), Methionine (Met), Cysteine (Cys), Phenylalanine (Phe), Tryptophan (Trp), and Proline (Pro).

Consider that polar, acidic, and basic amino acids are more likely to be found on the exterior of the protein or in aqueous environments because they can form hydrogen bonds or ionic interactions with water.

Conclude that the group of amino acid residues most likely to be found in the portion that crosses the lipid bilayer is the nonpolar group.

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Lipid Bilayer Structure

The lipid bilayer is a fundamental component of cell membranes, composed of two layers of phospholipids. The hydrophobic (water-repelling) tails face inward, while the hydrophilic (water-attracting) heads face outward. This arrangement creates a barrier that separates the interior of the cell from the external environment, influencing which molecules can pass through.

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Lipids

Amino Acid Properties

Amino acids can be classified based on their side chains (R groups) into categories such as polar, nonpolar, acidic, and basic. Nonpolar amino acids have hydrophobic side chains that do not interact favorably with water, making them more likely to be found in the lipid bilayer, while polar and charged amino acids tend to be hydrophilic and interact with the aqueous environment.

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Amino Acids

Transmembrane Proteins

Transmembrane proteins span the lipid bilayer and often contain regions that interact with both the hydrophobic core of the membrane and the aqueous environments on either side. The portions of these proteins that cross the bilayer typically consist of nonpolar amino acids, allowing them to stabilize within the hydrophobic environment of the membrane.

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Proteins

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How do the phospholipids in archaea differ from those in other cells?

a. They have tails made of unsaturated fatty acids instead of saturated fatty acids.

b. They do not contain hydrocarbon chains.

c. They have isoprenoid tails instead of fatty acid tails.

d. They have two hydrocarbon chains instead of three hydrocarbon chains.

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Draw and label the plasma membrane of a cell that is placed in a solution with concentrations of calcium ions and lactose that are greater than those on the inside of the cell. Use arrows to show the relevant gradients and the activity of the following membrane proteins:

(1) A pump that exports protons

(2) A calcium channel

(3) A lactose carrier

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Cooking oil lipids consist of long, unsaturated hydrocarbon chains. Would you expect these molecules to form membranes spontaneously? Why or why not? Describe, on a molecular level, how you would expect these lipids to behave in water.

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If a solution surrounding a cell is hypertonic relative to the inside of the cell, how will water move?

a. It will move into the cell via osmosis.

b. It will move out of the cell via osmosis.

c. It will not move, because equilibrium exists.

d. It will evaporate from the cell surface more rapidly.

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What two conditions must be present for osmosis to occur? Integral membrane proteins are anchored in lipid bilayers.