An engineered spherical cell is grown in a suspension culture. The cell relies entirely on the simple diffusion of oxygen across its outer membrane to meet its intracellular metabolic demand. The cell's oxygen consumption rate (demand) is directly proportional to its volume, whereas the maximum rate of oxygen diffusion into the cell (supply) is directly proportional to its surface area.
If the radius of the cell increases by a factor of 3, how does the ratio of maximum oxygen supply to metabolic demand change?
It increases by a factor of 3, since the surface area for diffusion increases by a factor of 9.
It decreases to 13\frac{1}{3}31 of its original value, because the cell's volume scales with the cube of the radius while its surface area scales with the square of the radius.
It decreases to 19\frac{1}{9}91 of its original value, because the rate of diffusion is inversely proportional to the square of the cell's radius.
It remains unchanged, because the metabolic demand per unit volume is independent of the cell's surface area.
82 exam-style questions on OCR GCSE Biology 2.1 Supplying the cell, covering 2.1.1 Transport across cell membranes, 2.1.2 Mitosis and the cell cycle, 2.1.3 Importance of cell differentiation, and 2.1.4 Stem cells. Each one has a worked solution and a mark scheme showing where the marks go.