Unit 3 Part 2 – The Cell

 Why cells are small? Diffusion limits cell size DNA limits the cell size. Surface area- volume ratio

 Diffusion is fast and efficient over short distances, it becomes slow and inefficient over larger distances. Small cell  good diffusion Big cell  bad diffusion  Speed of Diffusion An organelle in the center of a 20μm diameter cell receives supplies a fraction of a second after it enters the cell. An organelle in the center of a 20 cm diameter cell would have to wait months before receiving supplies that enter the cell.

 The DNA tells the cell which proteins to be made.  Not enough protein the cell will die. Take time for protein to be produced.  As cells increase in size, no extra DNA is made

 As cell size increases, volume increases much faster than surface area. A 1mm cube has a surface area of 6mm 2 and a volume of 1mm 3. A 2mm cube has a surface area of 24mm 2 and a volume of 8mm 3  The larger the volume of the cell, the smaller the surface area gets.

 Before a cell gets too large, it divides into two “daughter” cells.  Before cell division takes place, the cell must replicate all of its DNA.  The cell then divides and solves the problem of the cell becoming too large.

1. Cell size vs. surface area must remain at an efficient ratio 2. Growth - bigger organism needs more cells 3. Repair - regeneration of skin, blood & bone etc. 4. Reproduction - asexual vs sexual