How Big is Too Big? All living organisms are made up of cells. Cells are really small. Why? Why cant a human be made of just one big cell instead?
All cells diffuse nutrients and wastes in and out at a constant rate. Basically, the more surface area (aka cell membrane), the more nutrients and wastes a cell can exchange. Using the formulas for volume and surface area, we can make a ratio to compare the two.
Surface Area to Volume Assignment Follow along for two examples then complete the rest of the assignment using the formulas provided. Ratio: SA / V
If we were to watch nutrients being absorbed at a constant rate in 3 different size cubes, the nutrients could reach the entire cytoplasm of smaller cells because, for their volume, they had a much larger surface area. Dead: nutrients cant get to all parts of the cell. Better, but still dead. Alive! Nutrients make it to all parts of the cell. 4cm 3 1 cm 3 2 cm 3 Surface Area =(base) (height) added up for all sides S.A.= (4cm)(4cm)(6 sides) = 96 cm 2 x 1 cube = 96 cm 2 S.A.= (2cm)(2cm)(6 sides) = 24 cm 2 x 8 cubes = 192 cm 2 S.A.= (1 cm)(1cm)(6sides) = 6 cm 2 x 64 cubes = 384 cm 2 Volume = (base) (width) (height) V = (4cm)(4cm)(4cm) = 64cm 3 x 1 cube = 64cm 3 V = (2cm)(2cm)(2cm) = 8cm 3 x 8 cubes = 64 cm 3 Surface Area to Volume Ratio V = (1cm)(1cm)(1cm) = 1cm 3 x 64 cubes = 64 cm 3 The smaller the cell, the larger the SA / V ratio, and the easier it is for diffusion to reach all parts of the cell
Shape also affects surface area to volume ratio. 16 7.11 Skinnier, flatter cells large SA / V ratios. Cells with projections, like microvilli, have large SA / V ratios.
In order for an organism to be large, it must be multi cellular. Which of the equal volumes below has the greatest SA / V Ratio and is therefore most likely to survive?
Distribution systems, such as your circulatory system deliver nutrients to and remove waste from each cell. Diffusion is sufficient because of each cells large SA / V ratio.
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