2. Why are cells so small? Think, Pair & Share 2

3.  A cells size or shape is limited by: ◦ The distance materials must travel inside the cell. ◦ The number of placesat which materials can enter the cell.

4.  Many of the materials entering and exiting cells do so by diffusion.  In small cells, this diffusion of materials is effective in meeting the needs of the cell.  As cells become larger, diffusion becomes less effective due to a smaller surface area to volume ratio.

5. 1. Calculate the Surface Area. 2. Calculate the Volume. 3. Find the Ratio. 2 cm 3 : 1

6. 2 cm 1 cm 2 cm 3 : 1 4 : 1

7. 7 3 mm 1 mm  Calculate the Surface Area and Volume Ratio of the “cells” above. 2 : 1 6 : 1

8.  As a cell gets bigger, its surface area to volume ratio decreases.  If the surface area to volume ratio is too low then diffusion becomes an inefficient way of delivering nutrients and removing wastes as they take longer to travel the greater distance from the cell membrane to the center of the cell. 8

9.  Suppose it takes too long for materials to be of any use if they diffuse over a distance >1cm.  Materials will just reach the centre of the cell. 2 cm

10.  Bigger than the last cell, its centre is now further than 1cm away and nutrients are not able to reach the centre in time to be effective.

11.  What type of cells have a large central vacuole?  What type of cells would you expect to be bigger? Why?

12.  Many organisms and organs require a high surface area to volume ratio.  E.g. ◦ Thin Flat Leaves ◦ Villi ◦ Microvilli 12

13.  Read and Complete Biozone Pg.151-152