Examples of proteins anchored in the plasma membrane by lipids and glycolipids

Some proteins (e.g., the lymphocyte protein Thy-1) are anchored in the outer leaflet of the plasma membrane by GPI anchors added to their C terminus in the endoplasmic reticulum. These proteins are glycosylated and exposed on the cell surface. Other proteins are anchored in the inner leaflet of the plasma membrane following their translation on free cytosolic ribosomes. The Ras protein illustrated is anchored by a prenyl group attached to the side chain of a C-terminal cysteine and by a palmitoyl group attached to a cysteine located five amino acids upstream. The Src protein is anchored by a myristoyl group attached to its N terminus. A positively charged region of Src also plays a role in membrane association, perhaps by interacting with the negatively charged head groups of phosphatidylserine

The Cell: A Molecular Approach. 2nd edition.

Cooper GM.

Self Test

1). Why is the phospholipid molecule so appropriate as the primary structural component of plasma membranes?
    a). Phospholipids are completely insoluble in water.
    b). Phospholipids form strong chemical bonds between the molecules, forming a stable structure.
    c). Phospholipids form a selectively permeable structure.
    d). Phospholipids form chemical bonds with membrane proteins that keep the proteins within the membrane.
Answer: c

2). Which increases the fluidity of the plasma membrane?
    a). having a large number of membrane proteins
    b). the tight alignment of phospholipids
    c). cholesterol present in the membrane
    d). double bonds between carbon atoms in the fatty acid tails.
Answer: d

3). Which best describes the structure of a plasma membrane?
    a). proteins embedded within two layers of phospholipids
    b). phospholipids sandwiched between two layers of proteins
    c). proteins sandwiched between two layers of phospholipids
    d). a layer of proteins on top of a layer of phospholipids
Answer: a

4). What locks all transmembrane proteins in the bilayer?
    a). chemical bonds that form between the phospholipids and the proteins
    b). hydrophobic interactions between nonpolar amino acids of the proteins and the aqueous environments of the cell
    c). attachment to the cytoskeleton
    d). the addition of sugar molecules to the protein surface facing the external environment
Answer: b

5). The movement of sodium ions from an area of higher concentration to an area of lower concentration is called _____.
    a). active transport
    b). osmosis
    c). diffusion
    d). phagocytosis
Answer: c

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Apply Your Knowledge

1). If during the action of the sodium-potassium pump, 150 molecules of ATP are used, how many sodium ions are transported across the membrane?
Answer: 450 sodium ions.
Solution: Every time the sodium-potassium pump undergoes its conformational change, 3 sodium ions are transported across the membrane and 1 ATP molecule is required. If 150 ATP molecules have been used by the sodium-potassium pump, then the pump has undergone its conformational change 150 times, and has transported 150 x 3 Na+ ions/time, or 450 total sodium ions.

2). If a cell's cytoplasm were hyperosmotic to the extracellular fluid, how would the concentration of solutes in the cytoplasm compare with that in the extracellular fluid? Assuming the membrane was permeably only to water, in which direction would water move?
Answer: The contents of the cell would contain more solutes than the extracellular fluid, which would be hypoosmotic compared to the cell. Assuming that the membrane was permeably only to water, water would move into the cell, toward the area with more solutes.

3). Cholera, a disease caused by a bacterial infection of Vibrio cholerae, results in severe diarrhea leading to dehydration. A toxin released by the bacterium causes the release of chloride ions (Cl-) from cells lining the small intestines and inhibits the uptake of sodium ions (Na+) by these cells. Explain how this disruption of cellular ion concentrations would result in extreme dehydration.
Answer: Under normal conditions, the chloride ions remain in the cells lining the lumen of the small intestine. This creates an overall negative charge within the cells that drives positively charged sodium ions into the cells creating a hyperosmotic condition in the cells. Water moves into the cells by osmosis. The diarrhea that occurs in cholera results from the bacterial infection, becomes thick and clogs the airways.

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تست

Self Test

1). What type of microscope would you need to view a cellular structure that is 5 nm in size?
    a). a light microscope
    b). an electron microscope
    c). a compound microscope
    d). no microscope can resolve down to 5 nm
Answer: b

2). Which of the following is not found in prokaryotic cells?
    a). ribosomes
    b). cell wall
    c). nucleus
    d). photosynthetic membranes
Answer: c

3). Which of the following statements is incorrect.
    a). DNA in the nucleus is usually coiled into chromosomes.
    b). The nucleolus is the site of ribosomal RNA synthesis.
    c). Some substances can pass into and out of the nucleus.
    d). Red blood cells can not synthesize RNA.
Answer: a

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