Cells: How their

discovery led to the cell

theory

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History Cells were unknown until the

discovery of microscopes in the

1650’s

two scientists working

independently built the first

microscopes

Anton von Leeuwenhoek in

Holland

Robert Hooke in England

Leeuwenhoek

Studied pondwater, sour milk,

and semen

named moving organisms

“animalcules”

scared people and caused a

sensation

Leeuwenhoek’s

drawings of

“animalcules” set off a

flurry of amateur and

sometimes ridiculous

claims, such as:

- pondwater

animalcules causing

madness!

and...

Human sperm cells

contain tiny human

beings!

Today we can look

back and think

“crazy,” but at the time

people took these

ideas very seriously.

Hooke

Studied cork - a kind

of tree bark

named the structures

he saw “cells”

because they

reminded him of the

small rooms monks

slept in.

The Cell Theory

The cell theory has three parts:

All living things contain at least one

cell

Cells are the smallest living units of

matter

Cells can only come from pre-

existing cells

The cell theory-a closer look

All living things contain at least one cell

Many scientists working after Hooke

and Leeuwenhoek observed different

plants and animals

Each of them noted that no matter

what they observed, if it was alive it

had cells.

Cells are the smallest living units of

matter

scientists quickly realized that when

cells were dissected or broken open

they died

This meant that whatever “life” is, it is

something that happens inside cells

Cells can only come from pre-existing

cells

does not answer the question of

where the first cell came from or how

it came to be.

has not been disproved yet- no

scientist has ever built a living cell

from nonliving organic molecules

Spontaneous Generation

From pre-historic times to about 1850,

most people believed that under the

right conditions, living things could

spontaneously appear from non-living

material.

People throughout the

Middle Ages believed

that mice could be

“created”

spontaneously by

putting grain in dark,

quiet place and leaving

it for a few weeks.

The discovery of

cells only

confused people

more- If cells are

alive, then where

do they come

from? Can these

almost invisible

things appear

spontaneously

from the air?

Francesco Redi Born 1626 in Italy

First to challenge the

idea of spontaneous

generation

Did not accept the

common belief that

flies magically

appeared from rotting

meat

Redi’s Experiment

IV=cover

DV=presence of flies

Hypothesis: If a jar

containing rotting

meat is covered,

then it will produce

no flies

Redi’s Conclusions

Flies lay eggs, which

grow into maggots,

which

metamorphose into

flies

If flies can’t lay

eggs, then no new

flies can grow

Fly eggs

Objections to Redi

Many people rejected Redi’s claim that

flies do not spontaneously generate

Their reasoning:

“sealing the jar closed prevented a

magical essence from entering the

rotting meat and brining it to life”

“Scientists seek only to challenge

belief systems and stir things up”

1729. Italian

Believed microbes that spoil food

come from the air and can be killed

by boiling

IV= air, DV=food spoilage

Hypothesis: If air is allowed to reach

food, then microbes will get in and

cause it to spoil

Lazzaro Spallanzani

Spallanzani’s Experiment

Flask 1: boiled broth, open

Flask 2: boiled broth, sealed shut

Results

Flask 1 spoiled

Flask 2 did not spoil

Objections to Spallanzani

Sealing the flask shut

blocked the entrance of a

magical life force in the air

from getting to the broth.

Since few people had seen

these microbes, few people

believed him.

Louis Pasteur

1822. France

Supported that

spontaneous

generation is a myth

Invented

pasteurization

(sterilization by heat)

Pasteur’s Experiment

An improvement on Spallanzani’s work

Used special “swan-neck flasks” that allowed

air in but kept bacteria out

IV= bacteria, DV=spoiling broth

Hypothesis; If boiled broth is kept free of

bacteria, then it will not spoil even if air can

reach it.

The curved neck allows air in but traps

bacteria-carrying dust and dirt particles

Spontaneous generation is

dead!

Redi didn’t believe in it, and did an

experiment using flies

Spallanzani didn’t believe in it, and

experimented with broth

Pasteur disproved it conclusively with

his open-air yet spoilage free flasks.

Life from dead stuff= 0

Biogenesis = 3

Experimental method wins out over

superstition

the cell theory takes hold among

ordinary people

Pasteur is the father of

modern microbiology

supported the last part of the cell theory:

cells only come from pre-existing cells

identified yeasts as the microbes that

change grape juice into wine

showed that heat can be used to

sterilize foods and preserve them in

sealed glass containers, and later cans.

Finally...

in the late 1870’s,

doctors start to wash

their hands and

instruments before

operating...some even

start wearing gloves!

Joseph Lister’s wacky

idea that antiseptics kill

germs catches on...

The cell theory is one of the

most important theories in

biology.

All living things contain at least one cell

Cells are the smallest living units of matter

Cells can only come from pre-existing cells