Cell Size and Shape
BIO 12 Simulation

Whether they be in multicellular or unicellular organisms, cells vary dramatically in size and shape.  Variation such as that seen in cells in a vascular plant leaf (left, image provided by the Botanical Society of America) has consequences for the functioning of the cell.  In particular, the processes of exchanging matter and energy with the environment are affected in at least two significant ways:

1.  Variation in size and shape influence Surface Area : Volume ratios (SA:V).  Since all matter and energy that are exchanged by the cell must pass through the cell surface, differences in SA:V ratio affect the relationship between surface area for exchange relative to the cellular demand, which is related to the cell volume. 

2. Size and shape also affect the ability of diffusion to supply the demands of the cell.  Diffusion is a slow process, particularly within the cytosol.   Thus, as cells get larger or more spherical, the average distance matter must travel within the cell increases.  In a simple way, you can consider long it takes for a molecule like oxygen to travel to the center of the cell as a way to characterize differences in diffusion distance.  The model below allows you to explore how variation in cell size and shape influence the ability of a mitochondria to get oxygen delivered to it.

The Simulation:

The graph below simulates the activity of a mitochondrion in the middle of a cell.  Diffusion from the cell membrane to the mitochondrion controls its metabolic rate.  The organelle uses oxygen at a rate that increases with concentration.  This simulates a case where the mitochondrion's activity is limited by oxygen, a situation often encountered during periods of high metabolism.   Therefore, diffusion supplies oxygen and the mitochondrion takes it away.  At the start, there is no oxygen in the center of the cell.  Diffusion from the cell membrane transports oxygen to the cell, which increases the activity of the mitochondrion.   This results in an increase in mitochondrial activity (y-axis) and causes the initial positive slope.  With time (x-axis), the use of oxygen by the mitochondrion balances the transport of oxygen by diffusion and the rate of activity reaches an equilibrium. 

1. Each line presents a different cell shape: spherical, cubical and a rod whose length is 10 times longer than its diameter.  Each has the same volume.  Can you explain why they have different initial slopes and different equilibrial concentrations?

2. Use the slider to increase the volume of the cell (in um³).   This increases the volume of all cell shapes, which are equal.  Does an increase of 10 um³ cause the same change in mitochondrial activity when the cell is small and when the cell is large?  Explain.

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