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.

2Division 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