Biology for Kids: Cells, Organisms, and the Diversity of Life
Life on Earth
What is Biology?
Biology is the science of life. Biology means the study of living things.
We usually take it for granted that we can tell the difference between something that is alive and something that is not alive; between organic and inorganic things.
But scientists don't take anything for granted. We ask questions. We don't just want to guess. Scientists like to figure things out.
So, what is "life"? How does it "work"? What factors do living things have in common? What are their differences? These are all great questions that Biology, the science of the study of life, sets out to try to answer.
What is Biology?
What You'll Learn About Biology on This Page
By the time you've completed studying this page, you should be able to do the following things:
- understand and describe the fundamental characteristics of living things
- locate, describe and explain the functions of structures within the cell, such as the nucleus, cytoplasm, cell membrane, cell wall, chloroplast and vacuole
- describe the characteristics that both animal and plant cells have in common and the differences between these cell types
- describe the characteristic features of plants, animals, fungi, protoctista, bacteria and viruses
- understand the word pathogen and give a simple explanation of what it means
So that you can easily check your understanding, there's a fun quiz to do at the end. All the answers can be found on this page and you'll get your score straight-away.
Ready to get started? Great! First let's agree a definition of what we mean when we say that something is 'alive'.
Charles Darwin's Theory of Evolution and the Scientific Developments That Followed from It Explain All Biology
The Characteristics of Living Things
One of the first things we notice when we stop to observe the differences between the things we understand to be alive and other things that aren't, is that living things do stuff.
Rocks, dirt, puddles of water, don't do much. But birds fly, rabbits run, trees grow, people watch TV. You get the idea.
So, most scientists agree that living things are defined by what they do. In order to be thought of as alive, a thing must do most of the following things:
- Eat. All living things need to consume raw materials (food, sunlight, water) to get the energy and chemicals they require to function. Biologists call this nutrition.
Eating Is Fun and Sociable. It Is Also Essential for Life.
- Respire. Respiration is the process that breaks up big, carbon-rich molecules to release energy.
Sometimes, people get confused about the difference between breathing and respiration. Breathing is a mechanical action of the muscles and lungs that sucks oxygen containing air into your body. Respiration is the chemical action within the cell that uses the oxygen to make energy.
Even Holding Your Breath Underwater, Respiration Continues to Happen!
- Poop. Well, more technically, they excrete. The processes of nutrition and respiration produce waste material that needs to be gotten rid of. That's excretion to a biologist. Poop to anyone else.
Animal Poop. All Living Things Excrete Waste Matter.
- Respond to stimuli. If you tickle a stone, it can't respond. Tickle me, and I'll scream! More seriously, however, the ability to respond to environmental stimuli - whether that's a group of monkeys taking to the trees when one of them sounds an alarm or a leaf turning towards the sun - is a hallmark of living things.
Responding to Stimuli Is Vital for Life...and Sport
- Move. Living things can move. Birds can fly, mammals can run, dig, jump and so on. Plants can bend, unfurl petals, or extend climbing tendrils. Stones just sit there unless something else moves them.
Sometimes Super Fast, Sometimes Almost Imperceptibly Slowly, but Living Things Are Always on the Move
- Internal control. So, living things can regulate the conditions within their bodies - to maintain an optimum temperature, for example, or fight off disease.
A Polar Bear and Her Cub Rest on the Arctic Ice. Internal Regulation Is Part of the Biology of Survival for Animals like These.
- Reproduce. Possibly the most important thing that only living things do, is to reproduce. Some organisms do that by simply splitting in two, others have sex (I can hear you guys giggling at the back) and produce young. Rocks don't do that.
Ladybugs Mating. A Common Reproductive Strategy in the Animal Kingdom.
- Grow. You were once a drooling, gurgling little baby. Now, you're grown up some. Plant a seed, give it time, and it'll grow into a plant. Growth and development are the final factors that define living things.
The Metamorphosis of a Tadpole into a Frog Is Just One Example of the Life Cycle. Growth Is an Essential Characteristic of Life.
It should be clear now that all these characteristics of life things are things that animals, plants and other living things do. They do them in order to stay alive and to reproduce.
One of the most extraordinary facts about living things is that, despite incredible diversity (think of the differences between an insect and an elephant) we are all built from the same basic building blocks.
These building blocks are called cells. But what is a cell? How do they work? Are there different kinds? Let's find out...
All Living Things Are Made of Cells. Cells Are the Building Blocks of Living Things.
Cells, Organisms and Diversity
Cells are small: so small that, in most cases, you can't see them without a microscope. But with a microscope, you can.
Microscopes weren't invented until the seventeenth century, so before that we didn't know about cells. A guy called Robert Hooke, who was looking at things through an early microscope, was the first to recognize that all the living things he looked at seemed to be made up of tiny compartments joined together. He called them cells because he thought they looked like the little rooms monks live in (which are also called cells).
Cells Are so Small That They Can Only Be Seen under a Microscope.
It wasn't until the nineteen-thirties and the invention of the electron microscope that we discovered that cells were really complicated inside, too, with loads of moving parts that made them work. Those parts inside a cell are called organelles and they are seriously, mind-bogglingly, small.
A Transmission Electron Microscope Enables Us to See the Components Within Cells.
Thanks to all this microscopy (looking at things through microscopes) we now know that animal and plant cells are different and that there are many kinds of cells that do different things and that the kind of cells a thing is made from will determine what kind of thing it is.
And now you know why biologists all have squinty eyes and wear thick glasses. Except they don't. I'm just kidding. You can be a biologist and still have good eyesight. Really.
What is a Cell? Take a Tour Inside One to Find Out!
As we are animals, let's start with taking a look at animal cells.
Animal cells - the cells that you are made of - have many components. For now, we'll just concentrate on a few of these. These are the ones that are most important for the life and function of the cell.
We'll look at the nucleus, the cytoplasm, the cell membrane and the mitochondria.
But as a picture speaks a thousand words, take a look at this diagram of a typical animal cell and see if you can find these parts of a cell in among all the others.
Diagram of an Animal Cell Showing all the Major Components and Organelles
The Nucleus of an Animal Cell: Where it Is and What it Does
The nucleus, in an animal cell, is usually found somewhere around the middle, or just off to one side.
Photograph of a Cell Nucleus Seen Through A Microscope
It's quite a large organelle and frequently fairly spherical in shape (so it will usually appear round on a two-dimensional diagram such as the one above).
It does lots of things, but the two most important are:
- the nucleus controls everything else that happens inside the cell. It is often referred to as 'the brain' of the cell.
- It is also where the chemically coded information (the DNA) is stored and copied to make new cells.
Because the nucleus is so big, it is usually the easiest component of a cell to see under a microscope.
Cytoplasm: Where Metabolism Takes Place.
The nucleus sits within a jello-like substance called cytoplasm. This stuff fills up the rest of the cell and includes all the other organelles. It helps give the cell its structure, and it's also the place where most of the chemical reactions that sustain life (which, lumped all together, we call metabolism) take place.
A Variety of Organelles in the Cytoplasm of a Cell.
The Cell Membrane Separates the inside of the Cell from the Environement and Regulates What Goes in and What Comes Out.
The cell is kept together by a surrounding cell membrane, sometime also referred to as the plasma membrane.
The cell membrane is made of fats (called lipids) and proteins.
The cell membrane protects the inside of the cell from the outside world - much as your skin protects the inside of your body from the surrounding environment. Like skin, the cell membrane is also a selectively permeable membrane. All that means is that only certain substances can cross the membrane - usually useful things like nutrients, oxygen and water, go in, and nasty things like poisons and waste materials go out.
In this way, the cell membrane helps to keep the internal composition of the cell in a constant, healthy state.
The Mighty Mitochondria: The Powerhouses of the Cell.
The organelles known as mitochondria (or mitochondrion if you're talking about just one) are very important to the maintenance of life.
They are small, sausage-shaped organelles. Do you remember what respiration is? Respiration is the chemical process that releases energy, allowing the cell to do its work. Well, it is right here in the mitochondrion that respiration takes place.
That's why we often refer to the mitochondria as the power stations, or powerhouses of the cell.
TEM (Transmission Electron Micrograph) of a Mitochondrion.
It may be surprising to you to find out that plant cells are more complicated structures than animal cells.
We've looked at four fundamental cellular structures that typify animal cells. Plants have all of those structures and another three as well.
Simplified Diagram of a Plant Cell
The Additional Structures in a Plant Cell
The additional structures in a plant cell are:
- The Cell Wall is a tough, fairly rigid structure made of cellulose that forms a layer outside the cell membrane. It helps to maintain the plant cell's shape and structure and stops it from bursting under pressure.
- The Central Vacuole is a membrane bound structure that in mature plant cells can be very large, taking up almost all the space inside the cell. It is filled with cell sap and is the area where the cell's nutrients and other soluble substances are stored.
- The Chloroplasts are the most distinctive and important elements of a plant cell. They are green, are found in the cytoplasm and are the cell structures which absorb sunlight to be used in the process of photosynthesis. Plant parts which are not green - such as bark, petals and so on, have cells that don't contain chloroplasts.
The cell wall in plants means that they are more rigid structures than most animal cells. They tend to maintain their shape.
This adaptation is possible because plants do not need to move around their environment in the same way as animals. The presence of chloroplasts and the ability to photosynthesize also means that most plants (there are some exceptions, such as fly-traps) don't need to eat. They can produce all the energy they need for life from sunlight, air and soluble nutrients drawn from the soil.
The Cell Song!
Cells, Tissues, organs and Systems in Living Things
So we've looked at cells and you should now have a pretty good idea of the basic structures and functions of animal and plant cells - the building blocks of living things. But the story doesn't end there. These building blocks are pout together by nature to make tissues, organs and systems at increasing levels of complexity.
The cells in living things aren't just thrown together randomly. They have evolved in arrangements known as levels of organisation. Let's take a look at these levels of organisation now:
- Tissues are groups of similar or identical cells which combine to carry out a specialized function. For example, your muscles are all made up of specialized muscle cells which have the particular property of being able to contract.
- Organs are groups of different kinds of tissues which are combined to work together to carry out specific physiological work. For example, your heart is made up of several kinds of muscle, valve and interconnecting tissues which cooperate to create the organ that pumps the blood around your body.
- Systems combine groups of organs together to perform wider functions within an organism. For example, the heart, the blood itself and the blood vessels are all organs which, combined together, make up your circulatory system.
What other tissues, organs and systems can you think of that might be part of a living organism such as an animal or plant?
Biodiversity On Earth
Biodiversity: the Variety of Living Organisms
So far, biologists have identified and classified over ten million different species of living things on Earth - and there are almost certainly many millions more yet undiscovered.
But how do we classify all these different living things?
Categories of Organisms
Because it is so complicated, biologists divide living things up into categories of increasing detail. The first and broadest set of categories after 'Living Things' are known as the Kingdoms.
There are six Kingdoms:
Not everyone agrees that viruses are properly alive, but until a final decision is made, they are still classified as 'organisms'.
Putting organisms together in groups is known as classification and the biologists who study classification are called taxonomists.
The groups are classified according to:
- the kinds of cells they have
- the way they obtain nutrition (the way they eat)
Evolution: the Process that Explains Biodiversity
Animals are multicellular organisms. They are made up of lots of different cells. Their cells can change shape and perform different functions within a tissue. They can move from one place to another. They are frequently controlled by a nervous system.
They feed on other organisms to obtain their nutrition and are able to store energy as fat.
Animals can be further divided into:
The vertebrates have backbones. The invertebrates don't. Whether they are invertebrates (such as insects, crabs, worms and so on) or vertebrates (such as lizards, snakes, rats, birds and humans) they are all classified as animals.
Variety of Animal Life
Just like the animals, plants are multicellular organisms.
We've already seen that plant and animal cells have many similarities. They also have many differences. So, plant cells are surrounded by a tough cell wall which is composed of a substance called cellulose. This makes plant cells inflexible and they are not able to move.
Plants are the only biological group that (with a couple of exceptions) don't gain nutrition by eating other organisms. Plants use a process called photosynthesis to make food from light energy and mineral nutrients.
Animals store energy as glycogen. Plants store energy as starch and sugar.
In a similar way to the animals, plants can be divided into subgroups. The main subgroups are the flowering plants and the non-flowering plants.
Variety of Plant Life
People are often surprised to discover that fungi are not plants.
In fact, the fungi share many characteristics in common with both plants and animals. All living things have common elements because they all evolved from common ancestors.
A few of the fungi are very simple organisms. They have only one cell and are said to be unicellular. Most are more complex and are built up from long filaments called hyphae which mesh together to create a mycelium network.
Hyphae is the plural of hypha. A single hypha has more than one nucleus unlike other cells which only have one. Like plants, they also have cells walls but unlike plants these are made of a substance called chitin - the same stuff that insect skeletons are made of!
Fungi don't photosynthesize. They grow on their food as they are unable to move about like animals. By extracellular secretion, they give out enzymes which break the food down before it is absorbed. This process is known as saprophytic nutrition.
Moulds, mushrooms, toadstools and yeast are all different kinds of fungi.
Variety of Fungal Life
The category protoctista include single-celled animals and plants.
- protozoa are single-celled animals
- algae are single-celled plants
Most protoctista need a watery environment to thrive and can be found in the soil, rivers, lakes, ponds and even blood, saliva and urine.
Bacteria are single-celled organisms that are not animals, plants or fungi. They are much smaller than the other living things. Most bacteria a one thousand times smaller than a human cell. They also have some special features of their own:
- They do not have a nucleus. Their DNA is wound into a single chromosome and circular plasmids which float freely inside the cell.
- They have no other organelles.
- They have an especially rigid cell wall outside the membrane which is made of a complex sugar and protein mixture called mucopolysaccharide.
- Outside the cell wall, bacteria also have a layer of gooey slime known as the capsule.
- Many bacteria can move and the most common way is by flicking a long flagellum which is a bit like a tail extending from the cell wall.
Some bacteria can photosynthesize but most eat other organisms. When they eat things that are already dead, this is known as decomposition. When they eat things that are still alive this is often a form of disease and is known as bacterial pathogenesis.
Viruses are very strange. Many biologists do not categorize them as living things while others do.
They don't even have cells. They are even smaller than bacteria and made of some DNA wrapped in a coating of proteins.
Viruses do not show the signs commonly associated with life, but they can 'come to life' once they invade another organism's cells and take it over by introducing their own DNA. If they do that, the cell then stops functioning normally and starts to build more viruses.
For this reason, viruses, whether truly alive or not, are known as intracellular parasites.
Viruses can be extremely dangerous and threaten living things with serious disease and death.
What are Pathogens?
Pathogens are any microorganisms which cause illness and disease. Pathogen is the scientific term for what we commonly call 'germs.'
Pathogens can be either viruses, bacteria, protoctista or fungi.
However, many of these types of organism are not pathogenic and can even be beneficial to other organisms.
Quiz Time!view quiz statistics
- Cell wall
- Cell membrane
A Last Word
I hope that you've enjoyed this overview of biological life, looking at cells, organisms, evolution and the astonishing variety of living things.
Biology is a fascinating science because it explores the most important questions about ourselves, what we are, how we evolved and what it means to be alive. It also opens the doorway of understanding on the wide world of other living things and shows the intimate and awesome ways that all life is connected together in a vast biological web,
If you have any questions or comments, please don't be shy! There's a comments box at the end of the page and I do reply to all comments made.
© 2015 Amanda Littlejohn