6. Cell Membrane & Transport

Which molecule most easily diffuses across a biological membrane's lipid bilayer, without help of a protein?

According to the map above, what kind of large molecule transport is also called the process of cell 'eating'?

Which of the following is most associated with diffusion?

a. symports

b. antiports

c. carrier proteins

d. endocytosis

A Gram-negative cell is moving uric acid across the cytoplasmic membrane against its chemical gradient. Which of the following statements is true?

a. The exterior of the cell is probably electrically negative compared to the interior of the cell.

b. The acid probably moves by a passive means such as facilitated diffusion.

c. The acid moves by an active process such as active transport.

d. The movement of the acid requires phagocytosis.

A scientist who is studying passive movement of chemicals across the cytoplasmic membrane of Salmonella enterica serotype Typhi measures the rate at which two chemicals diffuse into a cell as a function of external concentration. The results are shown in the following figure. Chemical A diffuses into the cell more rapidly than does B at lower external concentrations, but the rate levels off as the external concentration increases. The rate of diffusion of chemical B continues to increase as the external concentration increases.

a. How can you explain the differences in the diffusion rates of chemicals A and B?

b. Why does the diffusion rate of chemical A taper off?

c. How could the cell increase the diffusion rate of chemical A?

d. How could the cell increase the diffusion rate of chemical B?

What factors may prevent a molecule from moving across a cell membrane?

Contrast the following active processes for transporting materials into or out of a cell: active transport, group translocation, endocytosis, exocytosis.