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Cell Size

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Cell Size

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

- Small cell

- 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 6mm2 and a volume of 1mm3. A 2mm cube has a surface area of 24mm2 and a volume of 8mm3

- 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.

Cell size vs. surface area must remain at an efficient ratio

Growth - bigger organism needs more cells

Repair - regeneration of skin, blood & bone etc.

Reproduction - asexual vs sexual