Chapter 10 – Cell Size, Mitosis and Cancer

• Cells come in a variety of sizes…• Red blood cells = 9 micrometers• Nerve cells (large animals) = 1m (with small

diameters)

• The cell with the largest diameter is the yolk of an Ostrich egg = 8cm

• Most living cells are between 2 and 200 micrometers.

Why can’t organisms be one giant cell?

• Think about how cells get their food and get rid of waste? By the process of Diffusion.

• Although diffusion is fast over short distances it becomes slow and inefficient over long distances.

• Example: A cell with a diameter of 20cm would have to wait months before nutrients would reach the center of the cell.

• Because of this slow rate of diffusion organisms can’t just be one giant cell. They would die long before nutrients would reach organelles that needed them.

There are two main reasons why cells divide rather than continue to grow indefinitely.

• 1. DNA overload – A larger cell needs more materials (like proteins/enzymes) to function and the DNA has to provide instructions to build those materials. Soon the cell needs more materials than can be produced so the amount of DNA limits the size of the cell.

• Giant Amoebas can have 1000 nuclei (it is a few mm in diameter)

• 2. Rate of exchange of materials – the surface area of the cell membrane versus the volume inside the cell. The more surface area compared to the volume the quicker the cell can get its nutrients and get rid of waste products.

• To demonstrate which cell size is best calculate the surface area, volume and ratio of surface area to volume for 4 cubes (model cells) – 1mm, 2mm, 3mm and 0.01mm.

• 1mm• 2mm• 3mm• 0.01mm

• Which cube would allow substances to diffuse through it in the shortest amount of time?