Cell size
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Cell Size PowerPoint PPT Presentation


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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

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

  • 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

  • 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

  • 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?

  • 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?

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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