WHY DO CELLS DIVIDE?. IT’S ALL ABOUT SIZE THERE IS AN UPPER LIMIT TO HOW LARGE A CELL CAN BE

WHY DO CELLS DIVIDE?WHY DO CELLS DIVIDE?


IT’S ALL ABOUT SIZEIT’S ALL ABOUT SIZE



THERE IS AN UPPER LIMIT TO THERE IS AN UPPER LIMIT TO HOW LARGE A CELL CAN BEHOW LARGE A CELL CAN BE


1. Diffusion


1. Diffusion

2. DNA


1. Diffusion

2. DNA

3. Surface area-to-volume ratio


1. Diffusion– A cell must move all materials needed for

functioning via diffusion through cytoplasm



functioning via diffusion through cytoplasm– It takes a fraction of a second for materials

to move from the plasma membrane to the center of a typical cell with a diameter of 20 µm



functioning via diffusion through cytoplasm– It takes a fraction of a second for materials

to move from the plasma membrane to the center of a typical cell with a diameter of 20 µm

– If a cell were 20 cm in diameter, it would take months to move materials from the plasma membrane to the center of the cell


2. DNA– A cell must respond to changing conditions

rapidly


2. DNA– A cell must respond to changing conditions

rapidly– A cell frequently needs to produce large

amounts of proteins and enzymes


2. DNA– The process by which proteins and enzymes

are made is complicated• DNA makes an RNA copy of the gene to be

copied• RNA leaves the nucleus and travels to a

ribosome• RNA directs protein production at the ribosome


2. DNA– If there is not enough DNA to rapidly carry

out RNA production, the metabolism will slow and the cell will die


2. DNA

Larger cell = greater metabolism


2. DNA

Larger cell = Greater metabolism

Greater metabolism = More DNA needed to direct metabolism


3. Surface area-to-volume ratio– As the size of any object increases, volume

increases at a much greater rate than does surface area


3. Surface area-to-volume ratio– As the size of any object increases, volume

increases at a much greater rate than does surface area

– A cell receives all the materials needed for life through its plasma membrane


3. Surface area-to-volume ratio– Larger organisms (or cells) need more

“stuff”: you require more food than does a two-year-old human


3. Surface area-to-volume ratio– Larger organisms (or cells) need more

“stuff”: you require more food than does a two-year-old human

– As the volume of a cell increases, it needs more and more materials to be passed through the plasma membrane in order to sustain life


3. Surface area-to-volume ratio– At some point, there will not be enough

surface area on the plasma membrane to absorb the required amount of materials necessary for life

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WHY DO CELLS DIVIDE?. IT’S ALL ABOUT SIZE THERE IS AN UPPER LIMIT TO HOW LARGE A CELL CAN BE