Cell size
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Cell Size. Unit 3 Part 2 – The Cell. Why are cells small?. Why cells are small? Diffusion limits cell size DNA limits the cell size. Surface area-volume ratio. Diffusion Limits Cell Size.

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Cell Size

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Cell size

Cell Size

Unit 3 Part 2 – The Cell


Why are cells small

Why are cells small?

  • Why cells are small?

    • Diffusion limits cell size

    • DNA limits the cell size.

    • Surface area-volume ratio


Diffusion limits cell size

Diffusion Limits Cell Size

  • 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.


Dna limits cell size

DNA Limits Cell Size

  • 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


Surface area volume ratio

Surface Area-Volume Ratio

  • 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.


As a organism grows can the cells get bigger

As a organism grows, can the cells get bigger?

  • 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.


Why must cells divide

Why must cells divide?

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


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