Processes of Life and Scientific Method

How to Tell If Something Is Alive: Biology Basics You Should Know

A blue whale is alive. A bacterium is alive. A virus… kind of?

Imagine standing at the edge of the ocean. Beneath the waves, blue whales glide — the biggest animals alive. In the water, bacteria float, unseen but vital. And nearby, a virus waits to infect its next host.

Whale, bacterium, virus: all part of our world — yet why can scientists agree that a whale and a bacterium are alive, but not a virus?

Biology is the science that helps make sense of questions like this. It helps explain what life really is: from the largest animals to invisible microbes, and even to organisms like viruses that blur the line between living and nonliving.

This guide will show you what biology really studies, how life is defined in science, and how scientists explore life using the scientific method.

What You’ll Learn:

• How scientists define life
• The 6 characteristics shared by all living things
• How the scientific method helps us explore life

Key Takeaways

• Biology is the science of life, helping us understand what makes organisms alive and how they function.
• Living things are diverse but share six key characteristics: cellular structure, reproduction, growth and development, metabolism, homeostasis, and response to environment.
• Viruses blur the line between living and nonliving because they show some life-like behaviors but lack all essential characteristics.
• The scientific method is the process scientists use to explore life, best remembered through the mnemonic RIP-HOT.

What is Biology?

Biology is the study of life, and life is one of the most complex and unlikely things to happen in the universe, at least as far as we have been able to observe.

Biologists, the scientists who study living things, are not entirely sure how life got started on Earth, or even exactly what separates living things from the nonliving. In fact, the chemical building blocks of life are the exact same elements that make up all nonliving matter, from the rocks under our feet to far away nebulae. The chemistry of natural substances undergoing change through their interactions makes a leap into the realm of biology under certain circumstances. However, the way these molecules organize themselves and interact in living organisms is unique.

Life transforms water and some common chemical elements like carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur into everything a living cell needs to function — compounds like carbohydrates, lipids (or fats), proteins, and nucleic acids. Not only that, life is capable of incredible diversity of design — it comes in a dizzying array of shapes, sizes, colors, functions and modes of being.

Levels of biological organization at the cellular level. Atoms are the basic building blocks of the universe. When they combine, they form molecules which can then form larger structures called macromolecules. Macromolecules can assemble to form organelles—specialized structures that perform specific functions within the cell.

Diversity among living things is astonishing. Consider that some organisms like bacteria are made of only one cell, invisible to the human eye, while a blue whale is made of many trillions of cells and is as big as a commercial jet. Even among more similar organisms like animals, of which there are millions of species, there are many different ways of structuring an eyeball. But given all of this diversity, many of the same processes that drive a redwood tree are also at work in a bedbug.

Classifying diversity. To make better sense of life’s diversity, biologists have classified living organisms based on structures, functions, and other features. All known organisms are grouped into three domains: Bacteria, Archaea, and Eukarya. Domain Eukarya is further divided into four kingdoms: Protist, Fungi, Plantae, and Animalia.

We will begin our exploration of biology with the characteristics all living things have in common.

Processes of Life

Living things differ from nonliving things in that they have to work to exist.

It can be argued that some nonliving things have work to do as well. But living things are not simply molecular machines. Our sun, for instance, creates energy like heat and light as a result of nuclear fusion reactions between gasses in its core. This action is what makes it a star. Eventually, the sun will run out of fuel and cease to exist, just like all living things eventually die. But the sun’s not alive, and here’s why:

Living things are made of cells, the smallest unit of life as we know it. A single cell can be an organism in itself, like bacteria and archaea, or cells can work together to create the tissues of a multicellular organism.

Aside from being made of cells, there are some traditional properties that living beings share in common, whether they’re crocodiles or lichens. These are complex structure, reproduction, growth and development, metabolism, homeostasis, and response to the environment.

The 6 Characteristics of Living Organisms. While most biologists recognize these six fundamental traits of life, exceptions exist. Some organisms may not display all characteristics, and alternative lists may include additional features.

This is not a definitive list—sometimes you see excretion, evolution, movement, or even being carbon-based among the things that make the living alive. As we explore these aspects of life, remember that there are exceptions and gray areas: a mule, for instance, can’t reproduce, but it is obviously alive. Similarly, a virus can reproduce, but since it requires another organism for metabolism and reproduction, most scientists agree it is technically nonliving.

Philosophers and scientists alike have always and likely will perpetually be turning this question over, but biologists generally agree that animate beings are alive because they possess the following six characteristics:

1. Cells and Structure

Prokaryotes and Eukaryotes. All cells are either prokaryotic or eukaryotic. Prokaryotes—unicellular organisms in the domains Bacteria and Archaea—lack a nucleus. Eukaryotes have a nucleus and may be either unicellular or multicellular.

All living things we know about are made of at least one cell, which is the basic unit of life. Cells are complex in their own right, made up structures called organelles that perform specific jobs to keep the cells on task. But many organisms are multicellular, consisting of many millions of cells that perform specific functions.

Levels of biological organization at the organismal level. Similar cells group together to form tissues. Tissues then combine to form organs, which carry out specific functions. Multiple organs working together form an organ system, and several organ systems function together to make up a complete organism.

Not only do all living things that we know of have cells and cellular structures, but many multicellular organisms display larger-scale structure. For instance, humans, butterflies and stingrays all have bilateral symmetry, while sea urchins, lily flowers, and starfish have radial symmetry.

Symmetry. Symmetry—a balanced arrangement of body parts—is an example of a larger-scale structure that is present in many multicellular organisms.

2. Reproduction

Asexual vs sexual reproduction. Asexual reproduction involves a single parent producing offspring that are genetically identical to itself. In contrast, sexual reproduction involves the combination of genetic material from a male and female parent, resulting in offspring with a unique mix of genes.

All life reproduces itself, and that process can look different depending on whether the reproduction is sexual or asexual. Single-celled organisms often reproduce asexually by duplicating their own genetic material to create another identical cell. Animals, plants and fungi commonly reproduce sexually, which requires two individuals to contribute genetic material to the creation of a third.

3. Growth and Development

Life cycle of a butterfly. Butterflies progress through four distinct stages: egg, larva, chrysalis (pupa), and adult. Eggs hatch into caterpillars (larvae), which grow by feeding on plants. Inside the chrysalis, the insect transforms into an adult butterfly. The adult then emerges, ready to reproduce and begin the cycle again.

Living things get bigger and change over the course of their lives, and all the instructions for how to do this are sorted out in the genetic instructions of the organism. Sometimes this can seem pretty straightforward, like a pine sapling’s growth into a giant tree, and sometimes the path is circuitous, like a caterpillar’s path to becoming a butterfly.

4. Metabolism

Photosynthesis. Photosynthesis is a metabolic process used by plants, algae, and some bacteria to convert sunlight, carbon dioxide, and water into glucose (sugar) and oxygen. It transforms solar energy into chemical energy stored in the bonds of glucose. This energy can then be used by the plant—or by other organisms that consume it—to power life’s activities.

The function of living things depends on a series of chemical reactions that make it possible for organisms to do work like growing and moving around. It even takes energy to maintain a functional body. In order to do this, organisms must convert the energy they consume into energy they can use, and the mechanisms for doing this vary widely across types of organisms.  Animals eat food, plants convert sunlight into sugars, fungi consume decaying organic matter, and some bacteria even absorb the energy from the radioactive decay of rocks!

5. Homeostasis

Regulating body temperature. When body temperature rises, sensors (receptors)—mainly nerve cells in the skin and brain—detect the change and send signals to a temperature-regulating control center in the brain. The control center processes this information and activates effectors such as sweat glands. As sweating cools the body, the temperature drops back to a normal range—completing the cycle and maintaining homeostasis.

All living things must maintain a stable internal environment while adjusting to what’s going on outside. This is a dynamic process and requires energy — after all, you have to maintain a body temperature of 98.6 °F, no matter whether you’re running a footrace on a hot day or waiting for the bus in the snow. Homeostasis is the process by which your body constantly self-regulates through a system of biochemical checks and balances to keep your cells and organs functioning correctly.

6. Response to the Environment

Phototropism. Phototropism is when a plant grows or re-orients itself toward light. For example, a sunflower turns to follow the sun across the sky, maximizing light exposure for photosynthesis. This is one way plants sense and adapt to their surroundings to support survival.

Living things display “irritability,” or the ability to be aware of and to react to stimuli in the environment through behavioral or physiological means. A dog salivating when you present him with his favorite treat, a sunflower turning to face the sun as it moves across the sky, and your tendency to flinch when you hear a loud noise—all of these are examples of organisms responding to their environment.

The Scientific Method

Scientists investigate biology (and virtually every other field of inquiry) using a tool called the scientific method. Science is a process for getting us closer to an objective truth about the world, and although scientific labor is varied and complicated enough to make it difficult to unify and generalize anything about it, the scientific method is a helpful guide for scientists working to better understand some aspect of the Universe.

Humans are naturally curious, but it took our species a couple thousand years to establish a workable strategy for getting to the bottom of our questions about the natural world. Aristotle had some ideas about the importance of empirical investigation a couple millenia ago, and by the year 1000, the Iraqi mathematician Ibn al-Haytham was using it to study optics. At the end of the middle ages in Europe, Galileo and Kepler used a version of the scientific method to study our solar system, and the practice became widespread after Issac Newton’s work in the 17th century. The scientific method as we know it today was refined in the 1930s.

Over time, we came up with a set of steps rooted in asking questions, proposing answers and experimenting upon the object of inquiry using as few assumptions as possible.

The scientific method as we know it today is based on empiricism, the idea that theories can only be proved through observation and experimentation. Using the scientific method, scientists formulate plausible explanations for an observed phenomenon.

For instance, if a biologist in northern Alaska notices the rivers in a remote area turning orange and becoming extremely acidic, they would set out to discover why using the five basic steps of the scientific method:

The scientific method. The scientific method is a systematic process that includes five key steps: making observations, asking a question, forming a hypothesis, making a prediction, and conducting an experiment.

• Step 1: Observe. (The rivers in a remote area of northern Alaska are turning orange with oxidized iron and the water is becoming very acidic.)
• Step 2: Ask a question. (Why are the rivers turning acidic and orange?)
• Step 3: Propose a possible, testable hypothesis. (Maybe the melting of the permanently frozen ground, or permafrost, in northern Alaska due to higher summer temperatures is producing iron that oxidizes, or rusts, in oxygen-rich river water.)
• Step 4: Make a prediction. (The source of the iron is waste products of microbes that live in the newly-formed wetlands that used to be permafrost.)
• Step 5: Experiment to test the hypothesis. (Test melted permafrost waters around orange rivers for types of bacteria and sources of iron.)

After experimentation, scientists use the results of the experiments to create new hypotheses. In the case of the mysterious orange rivers, the bacterial waste might be the culprit for the oxidized iron in the rivers, but it doesn’t account for the acidity. Back to the drawing board!

An important aspect of the scientific method is that scientists have to be ready to have their hypothesis disproved — in fact, that’s part of the point. The purpose of the scientific process is to come up with plausible explanations for observed phenomena, and then to try to disprove that hypothesis by any means available to science until the refined hypothesis proves itself to be indestructible.

Science is, of course, infinitely complex, and plenty of scientific discoveries and even paradigm shifts have been due to dumb luck or working outside the scientific method. Einstein, for instance, did not come up with his theory of general relativity using the scientific method, and Marie Curie couldn’t have made her insights about radioactivity if it were her only tool.

To say the scientific method as it is known today has proven useful to Science would be a gross understatement. It has saved untold lives, gotten people into space, driven most of the technology we enjoy and also fear. It is not a perfect tool, but it has helped us learn a lot about our world that we didn’t know before.

Let’s help you remember the key topics with this memory trick or mnemonic.

Remembering the acronym “RIP-HOT” can help you recall the sequential steps of the scientific method:

R: Recognize a problem

I: Investigate and gather information

P: Propose a hypothesis

H: Hypothesis testing through experimentation

O: Observe and record data

T: Test the hypothesis by analyzing results

First, you “Recognize” a problem or question.

Then, you “Investigate” and gather relevant information.

Next, you “Propose” a hypothesis or educated guess.

After that, you engage in “Hypothesis testing” through experimentation.

You “Observe” and carefully “Record” data during the experiment.

Finally, you “Test” the hypothesis by analyzing the results obtained from the experiment.

Conclusion: Living Systems Explained

From blue whales to bacteria to viruses, the living world is full of complexity, but biology gives us the tools to understand it.

By exploring how scientists define life, recognizing the six key characteristics shared by all living things, and learning how the scientific method works, you now have a clearer view of what connects living organisms and what sets them apart.

Whether it’s a massive whale gliding through the ocean or a microbe floating in a drop of water, biology helps explain not just what life looks like but also how it works. And while viruses may blur the lines, biology equips us with the knowledge to keep asking, exploring, and discovering.

Ready to dive deeper? Let’s keep exploring life, one idea at a time.