Biology is the study of living things.
Living things are also know as organisms. What makes
something "living"? Here is a list of attributes for what is
considered Life on Earth. Some of these attributes would be
necessary for any sort of Life, and some might be variable:
somewhere else in the universe, there may be things we would
recognize as alive but which don't have cells, or have a different
Organisms are internally organized.
Many complex functional bits
work together to produce the functions of living things. On
Earth, part of that complexity is a combination of chemistry
based on protein activity and a reproducible coding system
for that chemistry based upon DNA. DNA contains genes,
which are the "recipes" for all of those proteins.
Organisms on Earth are also based upon a basic
living unit, a cell. A cell is a contained chamber
with its own internal complexity, even its own chambers, but it is
the smallest unit considered to be alive. Many organisms
(most, in fact) are just a single cell (unicellular organisms),
although the larger ones we see around us are made up of many cells
(multicellular organisms). There are even organisms
that are kind of both: single-celled, capable of living that
way, but found in collections where different cells do different
activities, as cells in multi-celled systems would behave.
These are one type of colonial organisms.
Complexity levels are very common in living
things: cells have organelles inside them; cells
in multi-celled systems can form specialized tissues;
tissues can organize into organs or structures;
organs and structures organize around basic functions in organ
systems, which work with each other.
There is organization beyond the individual
organism as well. Organisms of the same type in an area form
populations; all of the populations in an area form a
community; add in the rest of the area, even the
non-living parts, and it's an ecosystem.
Internal complexity of a bacterial cell, considered "simple."
Internal complexity of a plant cell.
Internal complexity of an animal cell.
Some examples of unicellular colonial organisms.
Organisms transform and use energy.
requires energy: things must be built, activity must be
supported. Energy is available from the environment, but it
isn't in exactly the needed form, so it must be taken in and changed
into a usable form. The form cells use for their own energy is
the energy used to hold chemicals together; these chemicals
transfer energy to the ongoing chemistry of the cell.
Ultimately, the energy originates from the
environment. It could come from energetic but unstable
environmental chemicals, but these are not common. Using this
kind of energy is called chemosynthesis, and can be found in
ecosystems around deep-ocean cracks where water and chemicals are
heated by lava just under the surface. These areas are called
hydrothermal vents. Vents like them, but probably in
much shallower water, are very likely where Life first developed on
the planet from an ocean full of early organic chemicals, a process
The most common form of environmental energy used
to make proper chemical energy is light, and the process of
converting light energy to the energy holding chemicals together is
called photosynthesis. Usually, small non-living
chemicals, carbon dioxide and water, are bound together using
captured light energy into sugar molecules. Much more
on photosynthesis in later chapters.
Respiration is a chemical process that
moves energy from basic energy-holding chemicals (usually sugar) to
chemicals that can be actively used in the energy processes of a
cell. The most common active fuel is called ATP, for
adenosine triphosphate. Respiration comes in many forms, and
does not necessarily use oxygen in the process. Much more on
respiration in later chapters.
In ecosystems, environmental energy is captured
and stored in chemicals by producers. Most producers
are photosynthetic. That chemical energy can be used by
organisms that can't capture environmental energy and must get it
from producers: these are consumers. One special
class of consumers are called decomposers: these
process dead and waste materials so that almost all living material
is recycled to the raw materials later organisms need to build
themselves from. The energy, meanwhile, is lost in bits at
every transformation as random bursts of heat, which can't be
recycled. Ecosystems can recycle materials, but require
constant inputs of energy to continue. Most of Earth's Life
energy comes from the Sun.
Metabolism is the name given to all of the
energy-using chemistry in a system. It sets up the ability of
organisms to be self-organizing, as discussed above.
More on deep-sea chemosynthesis.
The chemistry of deep-sea chemosynthesis.
Video of hydrothermal vents, with surrounding ecosystem.
Basics of photosynthesis
Some non-oxygen respiration types, with products.
Energy in ecosystems.
A food chain.
Organisms interact with their environments.
Since every activity in a cell loses a bit of energy
in the process, an outside source of energy is needed.
Chemicals break down to bits that can no longer be needed, and more
chemicals must be taken in. There might be consumers out there
looking to consume you. For these and many more reasons,
living things have to exist in a give-and-take relationship with the
world around them. They need to be able to sense important
aspects of the world and respond to them.
Organisms make more organisms like them.
It might have occurred to you, when trying to imagine
what makes something alive, that "Living things die." That
turns out not to be useful, since death is just the absence of Life,
and it's not even always true. Organisms can die, but
some may not ever do so.
mentioned that living things on Earth have a coding system based
on DNA. That coding system directs and controls the
protein-based chemistry of living things, but it is also a way of
passing on all of the information needed to make another individual.
This is called a genetic system, and it factors into an
important ability of living things: reproduction.
Since organisms can die, any population would most
likely eventually die out without replacements, and reproduction
provides those replacements.
are two basic types of reproduction, and students often find this
confusing because they think they know the differences when
they really don't.
In sexual reproduction, there are two sets
of the coding instructions that may be different; these two
sets get mixed up and recombined in offspring. Offspring
are a genetic mixture different from the parent or parents.
Single individuals can, and often do, reproduce sexually all by
themselves, just by changing the code mixtures - reproducing alone
does not imply asexual reproduction. Sexual
reproduction also does not always involves genders, although
male and female roles (based on how the cells work) are common.
In asexual reproduction, a complete set of
coding instructions, just like the parent's, are sent on to
offspring. The offspring may use those codes differently (all
of the cells in your body have the same codes, but they use
different bits to do different jobs), but they still have the same
ones. In a very real way, this is the only type that is
really reproduction, since only asexual reproduction makes
copies of individuals.
Each reproduction type has advantages over the
other. The advantage of asexual reproduction has already been
mentioned: it actually can make copies. The disadvantage
to this is that populations of asexual reproducers tend to be very
much alike; something that will hurt or kill one is likely to
hurt or kill them all. Asexual reproducers counteract this
disadvantage by making huge numbers of offspring: this spreads
offspring out so that some may avoid that bad thing, and it
increases the odds that some "bad copies" might actually be "good,"
with a change that makes them able to survive the factor that's
killing the rest. Most asexual reproducers are single-celled
or tiny, which makes them able to produce large numbers of offspring
Sexual reproduction can't make actual copies, and
that's a definite reproductive disadvantage, but the offspring have
a lot of variation, which makes a new danger less likely to
be equally dangerous to all. Most multicelled organisms,
especially those big enough to be visible without a microscope, are
Connected to reproduction, most organisms also go
through growth and development between their "birth" and the
age at which they themselves can reproduce.
The coding system, really simplified.
Many individual plants sexually reproduce by themselves.
Types of asexual reproduction (how old is
an individual produced by
Populations of organisms can change over time -
one is different, because it can't be applied to individuals but
only general types, kind of the "average" individual in a
population. An individual dog is enough like the "average" dog
to be clearly a dog, but a fox is not a dog, because an "average"
fox is too different. Foxes and dogs are clearly similar,
obviously related, and that relationship implies that if you go back
along their family tree far enough, you'll find a past relative they
share, which belonged to a group with a different "average"
definition - not a dog, not a fox, probably not even "in between,"
but a group that eventually led to both groups of today. The
changes that have happened to the average individual between that
past group and today is evolution.
It was the discovery and analysis of fossils
that led folks to realize that the world has not always been the
same, that organisms, related to those of today but obviously
different existed in the past. They recognized that evolution
had happened and sought to explain how one type could change to
another. One man came up with a particularly satisfying
explanation: Charles Darwin developed his Theory of
Evolution by Natural Selection. This will get discussed
more later, but here are the basics:
In a population, there is variation among the
individuals. The features of the environment (Nature) at any
given time affect different individuals differently. The
individuals with traits that help them survive current conditions
(or help them to be particularly successful at reproduction) are
more likely to make more offspring, many of which will genetically
inherit those helpful traits. Traits suited to an environment
will become more common, and over time (especially in circumstances
where the environment is changing) the "average individual" will be
quite different from its ancestor. Descendant populations in
different places, or working different parts of one environment, can
split off until their genetic systems aren't compatible any more.
A related process is
sexual selection, where the features that change over time
relate to success at reproduction (and can apply to
asexual reproducers as well as sexual ones).
Since evolution is heavily based on passing feature variations to
offspring that become more common or rare in the population over
time, it makes sense that these types of features also change.
There sometimes is a sort of conflict between the two types - think
of a peacock tail, which can increase the odds of getting mates but
also the odds of getting caught by predators.
Evidence for this can be seen in the genetic
systems themselves, in fossils, in observations of it actually
happening in quick-turnover populations and/or populations under
environmental stress. There is more to modern evolutionary
theory than just natural selection, but it is still thought to be a
major driving force in the changes that have been seen over and over
One somewhat strange piece of evidence for
evolution by natural selection is found in some types of organisms
that seem to have not changed from their fossil relatives.
How could they not evolve? First, the fossil just gives you
some basic anatomical parts to compare, and the ancestors could have
changed greatly in their basic chemistry. But maybe they
didn't. These organisms have one thing in common: they
live in parts of the environment that don't really change over even
long periods of time: deep open oceans (where large quick
sharks have swum for a very long time), or surf zones (where
horseshoe crabs have scrambled for hundreds of millions of years).
These types of organisms have been found to actually resist the
mutations that add variation to others.
Fossils - Strengths and Weaknesses
A specimen between fish and land vertebrates
Back the other way: fossils showing ancestry of whales.
A bit about Darwin.
Evolution observed in modern Galapagos finches.
Evolution and environmental change - seen.
How natural selection led to the evolution of human "childhood
Darwin rap-? (video)
What exactly are fossils?
These are parts of living things that have managed to stick around
for very long times. Fossils are usually hard parts, such as
bones, shells, and teeth, that are difficult to eat, and fossils are
usually the remains of organisms that have been buried in some way
before their hard parts could be broken down by natural processes.
The most common fossils are animal remains (plant remains, even
trees, tend to get broken down fairly quickly) that have sunken to
the bottom of a large body of water and been covered with sediment.
Sediment builds on sediment, layer after layer, compressing and
picking up minerals, becoming sedimentary rock (this is
oversimplifying it). Tectonic plates, huge rafts of
earth crust that float past, over, and under each other, drive
former ocean bottom up out of the water and make it part of the
continents, where people can see it, layer upon layer of fossil
organisms. Deeper layers tend to be older, and layers of ocean
organisms from the same time periods have similar species in them,
allowing a master list of layers to be developed from fossil beds in
Some fossils form when
small animals or plants are covered in special sap, which traps the
organisms, protects them from any breakdown, and mineralizes into
amber. Bacteria millions of years old have been isolated
from amber fossils and reawakened. Organisms may be buried in
mudslides, or sandstorms, or in tar pits, to become fossils.
Ash from volcanic eruptions can bury organisms (or quickly dry and
bury footprints), although lava tend to incinerate everything.
Some of the best fossils come from special bodies of water that had
lost most of their oxygen: there were few things to eat the
soft parts of the bodies that sank to the bottom, and the sediments
were particularly soft and sludgy, often molding parts like skin
surfaces and feathers.
website all about fossils.
Many pictures of fossils.
Fossil feathers may even give evidence of color.
Fossils that include soft-body parts.
A bit on fossil footprints of human ancestors.
Some very old fossil footprints.
We have to wonder how many
of the rules that define Life on this planet would apply to
legitimately living things that evolved somewhere else.
However, there are even things on this planet that don't quite fit
the rules, and so are thought to be alive by some people but not so
by others. These are the viruses.
Viruses may be descendants of pre-cellular molecular collections
from the very earliest abiogenesis. They invade "regular"
cells, particular viruses limited to particular types of hosts and
cells, and make more viruses to infect more cells.
RULES THEY FOLLOW:
They definitely reproduce, but they use the
genetic expression chemistry of the host cell to do it. Often,
all that enters the host cell is viral DNA, and the cell taps into
that code the same way it would do an abnormal code of its own,
making the proteins of new viruses from the gene codes and making
more viral DNA. Eventually, new viruses are packaged,
either in their protein structures or with protein-enhanced bits of
the cell's own membrane, and the viruses are freed to infect more
They evolve. Reproduction is asexual, but
the making of new virus DNA tends to be sloppy (cells have ways to
check the process and correct mistakes that the viruses can't use),
so the codes often change, producing altered virus "copies."
They may even pick up and integrate codes from the host, carrying
them to new hosts. Most alteration of offspring would be
sub-standard, but among the millions of copies will be viruses with
RULES THEY MAYBE DON'T FOLLOW:
Going through the rules covered here, in order -
Although viruses are internally organized, the
method of organization depends totally on processes stolen from the
host cell. Some people think this breaks the rule, but many
clearly living organisms do the same thing - an adult tapeworm, for
example, is totally dependent on the digestive processes of the host
to gain the nutrients it needs. A virus' complexity is very
limited: outside, it has a few proteins that help it get to,
recognize, and enter the next host cell; inside, it has its
DNA and maybe a protein or two to get its processes started in the
Viruses are not truly cellular. Many have no
cellular structures at all, and the ones that have membranes
generally just steal them from the host cell, adding some proteins
in the process.
Inside a host, a virus totally depends on the
metabolic fuels and processes of the host to construct new viruses.
Between hosts, a virus is kind of like a trap, pre-set to react when
the proper host cell is contacted, but there are no internal
metabolic processes going on. Viruses of important disease are
very difficult to develop effective treatments for partly for this
reason: treatments often interfere with a disease organism's
particular chemistry, but viruses generally use the chemistry of the
host cell, which you don't want to poison, and have no active
chemistry when they are out and about. Our body defenses are
limited to attacking recognizably altered host cells (and they may
not be altered enough to be recognized) and gobbling up free
Interaction with the environment for a virus is
limited to the response it gives from bumping into the right
chemicals on the appropriate host cell. It doesn't respond to
anything else, really.
So, are viruses really Life? It's just a
definition, of course, and the viruses don't seem to care too much.
Introduction to viruses.
Are virus-like entities our distant ancestors?
The Big Picture Book of Viruses.
Virus Effects, with
flu H1N1 specifically covered.
of Virus Studies.
Animation of Virus Activity
Animation of HIV (AIDS) Virus Activity with Technical Narration.