Unicellular organisms can still perform all of the functions necessary for life.
History of the discovery of cells
Robert Hooke observed dead cork cells under a simple microscope in 1665.
In 1838, Matthias Schleiden proposed that plant tissues are composed of cells.
In 1839, Theodore Schwann suggested that animal tissues are also made of cells.
Rudolf Virchow proposed in 1858 that all cells come from preexisting cells.
All living things are composed of cells.
The chemical reactions which occur in organisms occur in cells.
Cells come from preexisting cells.
The meaning of "life"
Characteristics of all living things
They have a greater organization than nonliving things.
They can grow.
They respond to changes in the environment.
They tend to keep their internal environment unchanged despite changes in the external environment.
Some things, like fire, may satisfy some of these requirements, but only things which satisfy all five conditions are considered to be alive.
The origin of life
Life first formed 3.5 billion years ago.
A. I. Oparin first theorized how life originated.
There was very little oxygen freely floating in the atmosphere.
Nitrogen, carbon, oxygen, and hydrogen were all present on the primitive Earth.
Energy from lightning, volcanoes, and ultraviolet rays forged the first organic molecules.
Stanley Miller tested Oparin's hypothesis in the 1950s.
Miller constructed an apparatus which simulated the conditions on Earth 3.5 billion years ago.
Within 24 hours, amino acids had formed.
How cells work
Organelles are structures within cells which perform specific functions. In this way, they are similar to organs in animals and humans.
The cytoplasm is the jelly-like solution in cells in which all of the organelles float.
Like all organisms, cells must have ways of eating, breathing, and reproducing.
These processes are often the basis for the corresponding processes in large organisms.
DNA stands for deoxyribonucleic acid.
DNA is a long molecule which directs the production of proteins in a cell.
Proteins affect many of the processes which a cell performs.
Types of unicellular organisms
Prokaryotes are also known as bacteria.
They evolved before the more complex class of organisms, eukaryotes.
Prokaryotes do not have any membrane-bound organelles.
The DNA in prokaryotes is arranged in a circular shape.
Eukaryotes are more complex than prokaryotes because they have membrane-bound organelles.
The DNA in eukaryotes is linear and is wrapped around special proteins called histones.
Heterotrophs versus autotrophs
Any organism which consumes food from the environment is a heterotroph.
Organisms which produce their own food are called autotrophs.
Aerobes versus anaerobes
Organisms which require oxygen to survive are aerobes.
Anaerobes are organisms which do not need oxygen.
Terms to know
aerobe - An organism which uses oxygen during respiration as an acceptor of hydrogen atoms to form water.
anaerobe - An organism which does not use oxygen during respiration.
autotroph - An organism which producec its own food through a method of autotrophic nutrition (such as photosynthesis).
cell - The unit of structure, function, and reproduction of every living organism. Cells perform all of the necessary functions for life, including respiration, reproduction, and responses to changes in the environment.
cytology - The scientific study of cells.
cytoplasm - The fluidic substance inside of all cells.
eukaryote - Cells which contain membrane-bound organelles and are generally more complex than prokaryotes.
DNA - Deoxyribonucleic acid. DNA is a long molecule composed of deoxyribose, phosphate groups, and nitrogenous bases which indirectly dictates the production of proteins in a cell.
heterotroph - An organism which must obtain its food from the environment.
organelle - Specialized structures within cells which perform specific functions.
prokaryote - Cells which do not contain membrane-bound organelles and are in general less complex than eukaryotes. They are also referred to as bacteria.
unicellular - A unicellular organism is one which has only a single cell. The two main types of unicellular organisms are prokaryotes and eukaryotes.