In multicellular organisms, simple diffusion is insufficient to supply deep-seated cells with oxygen because diffusion distance scales non-linearly with time. The average time t t\,t taken for a molecule to diffuse a distance x x\,x can be estimated using the formula:
t≈x22D t \approx \frac{x^2}{2D} t≈2Dx2where D D\,D is the diffusion coefficient of oxygen in biological tissue.
If a spherical tissue aggregate increases in radius from 50 μm50\ \mu\text{m}50 μm to 1.5 mm, by what factor does the diffusion time for oxygen to reach the innermost cells increase, and how do multicellular organisms overcome this physical limitation to keep core cells alive?
By a factor of 303030, and the adaptation is the active transport of oxygen across membranes using ATP-driven pump proteins.
By a factor of 900900900, and the adaptation is a specialized mass transport system that brings oxygen-rich fluid close to internal cells.
By a factor of 27,00027,00027,000, and the adaptation is the elongation of cells to increase their surface area to volume ratio.
By a factor of 900900900, and the adaptation is the downregulation of mitochondrial respiration in the core cells to prevent metabolic demand.
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.