Cells and

cell theory8TH GRADE SCIENCE

Discovery of cells

In 1665, Robert Hooke used a

pair of lenses (a newly

invented technology) to create

the first microscope.

He used his microscope to

observe a thin slice of cork.

He saw that the cork was

made up of small boxes.

These boxes reminded Hooke

of the small rooms that monks

live in known as cells.

Cell theory

In the mid 1800’s a set of

scientists added to our

knowledge of cells.

Schleiden (1838) – all

plants are made of cells.

Schwann (1839) – all

animals are made of

cells.

Virchow (1855) – All cells

come from pre-existing

cells.

Cell theory

Together they developed

the “cell theory”

1. All living things are

composed of one or

more cells.

2. The cell is the basic

structural and functional

unit of life.

3. All cells come from pre-

existing cells (only cells

can make new cells).

Cell theory

Cells are the basic structural, functional, and

biological unit of all known living organisms.

Cell size

Most cells are too small to be seen without a microscope.

Cell size

Eggs are the largest single cells.

Ostrich eggs can be 15 cm long

and weigh 1.5 kg. (human eggs

are ~1 mm wide).

The largest cells that can live on

their own are Chaos diffluens

and are about 1-5 mm long.

The smallest cells are those of

Mycoplasma gallicepticum a

single celled bacteria whose

cells are about 0.2 – 0.3 µm

wide.

Cell size

To put it in perspective, an adult human is estimated to be

made up of about 35 quadrillion cells. All working together to

sustain the life of the person.

Cell size

Cells are so small that just

your thumb contains

several billion cells.

Cell size

Most cells are small

because cells must rely on

diffusion to get nutrients and

get rid of waste.

The smaller a cell is, the

quicker nutrients and wastes

can move in and out of the

cell.

Diffusion

Diffusion is the natural process by which molecules

move from areas of high concentration to areas of

low concentration until equilibrium is reached.

The speed of diffusion through a cell is directly

proportional to the ratio of the surface area to the

volume of the cell.

Diffusion

For a cube

Surface area = 6 x length x width

Volume = length x width x height

Ratio = 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎𝑉𝑜𝑙𝑢𝑚𝑒

SA = 6 mm2SA = 24 mm2

SA = 96 mm2

V = 1 mm3V = 8 mm3

V = 64 mm3

SA/V = 6:1 SA/V = 3:1

SA/V = 1.5:1

Atom Molecule Cell

Organelle

Cell

Tissue

Organ

Organ

System

Cell