GIANT AMOEBA EATS NEW YORK CITYSurface-to-Volume Ratios and the

Benefits of Being Multicellular

PREDICTIONSWhy do you think cells are so

small?

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GIANT AMOEBA EATS

NEW YORK CITYYou will likely never see that

headline

As cells get larger, they need more food and produce more waste

Therefore, more materials must be able to move in and out of the cell through the cell membrane

SURFACE-TO-VOLUME RATIOA growing cell needs larger surface

area through which to exchange materials

As cell’s volume increases, its outer surface grows too. But the volume increases at a faster rate than the surface area. So if a cell gets too large, it won’t have enough openings to allow materials in and out

WHY AREN’T THE RATES THE SAME?Because Surface-to-Volume is

actually a ratio

The Surface-to-Volume ratio decreases as cell size increases

Increasing the number of cells but not the size keep a high surface-to-volume ratio

SURFACE-TO-VOLUME RATIO

BENEFITS OF BEING MULTICELLULAR A single cell as big as your are would have

an incredibly small surface-to-volume ratio. The cell would not survive because its outer surface would be too small to allow in the materials it would need

Remember, multicellular organisms grow by?

Why is an elephant larger than a human being?

Producing more cells

It has more cells, not larger cells

MANY KINDS OF CELLSMulticellular organisms are able to

do lots of other things besides grow

This is because different cells are specialized to do different jobs

Humans for example have specialized cells like muscle, eye, and brain cells which let us walk, run, watch a movie, think, and so on.

MATH BREAK The shape of a cell can affect its surface-to-volume ratio.

Examine the cells below and answer the questions that follow

What is the surface area of Cell A? Cell B? What is the volume of Cell A? Cell B? Which of the two cells has the greater surface-to-volume

ratio?

AB

4

2

1

2

22

28 248 8

Cell A 28:8

YOUR PET PARAMECIUM Imagine you have a pet Paramecium, a

type of unicellular organism. The dimensions of your Paramecium are 125 m x 50 x 20. If 7 food molecules can enter through each square micrometer of surface every minute, how many molecules can it eat in 1 minute?

If your pet needs 1 food molecule per cubic micrometer of volume every minute to survive, how much would you have to feed it every minute?