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Cells
Why are cells small?
Surface area to volume ratio
and cell size

Cell size is limited by the surface area
to volume ratio

As an object increases in volume, so
does it’s surface area but not at the
same rate.
Biological significance

Cell volume determines amount of
activity in a given amount of time.

The surface area determines the
amount of material that can be
moved into and out of a cell

Cells must maintain a large surface
area to volume ratio to function

This explains why large animals have
many small cells rather than a few
large ones
How Cells Get Around
the SA/V Problem

Avoidance - stay small
 Small size maximizes surface area to volume
 Allows most efficient import/export possible.
 Small cells can gather nutrients and reproduce
extremely rapidly
 Examples - bacteria, yeasts
Geometric Solutions

Increase surface area
1.
Elongate or flatten out
 A sphere has a low SA/V ratio. Cells that are
drawn out (e.g. cylinder), or flattened have
much more membrane per unit of cytoplasm.
 Examples - Eubacteria, red blood cells
2.
Fold the surface membrane

Extending the outer surface of a cell into folds,
fingers or indentations increases the total
surface area by a factor of several times

Examples – amoeba, intestinal cells (microvilli)
Decrease effective volume

Hollow out centre of cell

A cell with a large water-filled vacuole in the
center has much less active cytoplasm than its
measurements would suggest. Its metabolic
demands are therefore not as great.

Example - Mature plant cells
Increasing rate of supply
1.
Seek out areas where nutrient
concentration is high

Mobile cells can avoid areas with
sparse nutrients and actively seek
areas where nutrient concentration is
high
2. Actively acquire bulk nutrients
 By taking in food in vacuoles, cells
increase their total imports and
provide themselves with small,
extremely rich bubbles of nutrients.
 Examples – amoeba, paramecium,
intestinal villi
3. Improve transportation of nutrients
within cell
 By moving nutrients rapidly away
from the membrane, concentrations
gradients across the membrane can
be maintained.
Improving efficiency to
reduce demand
1. Division of labour within cell
 The complex organelles of
Eukaryotic cells allow them to have
specialized areas in the cytoplasm
to. The cell is much more efficient.
2 Division of labour between cells
 Form tissues – by joining together cells can
form an organism that can have a large size.
Each cell can specialize and maintain mutual
dependent relationship with the other cells of
the organism (homeostasis)
 Example - Plants, animals, fungi, some algae