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 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.

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