AP®BiologyCollege BoardExam QuestionsUnit 2: Cell Structure & FunctionCell Membranes & Transport

Cell Membranes & Transport (College Board AP® Biology)

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Cell Membranes & Transport

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Cell Membranes & Transport

1
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Proteins in the cell surface membranes of mouse cells and human cells were labelled with coloured dyes, with a different colour for human and mouse proteins.

When a cell from each species is fused together, the different-coloured labels are first found in different regions of the hybrid cell’s cell surface membrane. After 40 minutes, the labels are evenly distributed around the entire cell surface membrane. This is shown in the diagram below; the viewpoint is looking down onto the surface of the membrane.

Diagram showing sections of human and mouse membranes with proteins merging into a fused membrane. Phospholipid heads are depicted as circles.
Figure 1. Diagram of sections of human and mouse membranes

Which of the following explains this observation?

  • Groups of protein and phospholipid molecules in the cell surface membrane are attached to each other and move together.

  • Only protein molecules in the outer layer of the cell surface membrane can move freely between phospholipid molecules

  • All protein molecules in the cell surface membrane are fixed to structures within the cell, but phospholipid molecules move freely between them

  • Protein molecules in the outer layer of the cell surface membrane and those which span the bilayer can move freely between phospholipid molecules

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2
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The structure of a molecule affects its ability to pass through the plasma membrane. Membrane proteins are required for facilitated diffusion across the plasma membrane. Which of the following is not true of membrane proteins?

  • Charged ions, including Na+ and K+, require channel proteins to move through the membrane.

  • Membrane proteins are necessary for active transport.

  • Large quantities of water pass through aquaporins.

  • Membrane proteins are in a fixed position within the plasma membrane.

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