Introduction to Biology: Characteristics of Life

What is Living?

What makes something living, or biotic, or part of the bios of this planet? What makes me different from the zombies in The Walking Dead — or better yet, Night of the Living Dead? Why is Young Frankenstein’s monster “not alive”… or is he? If you’ve never seen that movie, you should — it’s a classic comedy.

You probably feel like you know what “living” means, but it’s surprisingly hard to put into words. Some people say, “If it’s breathing, it’s alive” or “If it can move, it’s alive.” But what about bacteria or jellyfish? They don’t have lungs. Yet they exchange gases with the environment. They just do it by diffusionPassive movement of molecules from areas of high to low concentration.. They’re small enough that gases slip right through their surfaces.

Life is defined by a set of properties — and it’s much more than just “breathing.”

Order

Every living thing has order — an organized structure. Take a sunflower. Its female reproductive organs are surrounded by male reproductive organs. Bright petals neatly frame them to attract pollinators.

When you’re healthy, your body maintains order without you even thinking about it. But when you’re sick? That order starts breaking down. Instead of your usual well-groomed self, you might look like you’ve just rolled out of bed in a hurricane. Your body diverts energyThe capacity to do work or cause change. toward making white blood cellsThe basic structural and functional units of life.. This leaves little energy for brushing your hair, showering, or even brushing your teeth. Order in biology is not just about appearance. It is about homeostasisThe maintenance of a stable internal environment in the body., which means keeping an internal steady state despite what’s going on outside.

Sensitivity

All living things interact with their environment. They sense stimuliChanges in the environment that are detected by sensory receptors. and respond to them.

For example, sunflowers show phototaxis — movementA fundamental property of life involving motion of the body or its parts. in response to light. In the morning, their faces turn toward the rising sun in the east. By afternoon, they’re all facing west toward the setting sun.

There’s also chemotaxis — movement in response to chemicals. Personally, when I smell fresh coffee brewing, that stimulus draws me straight into the kitchen.

Reproduction

Every living thing can reproduce — either:

• Asexually, creating clones of itself
• Sexually, shuffling genetics so offspring are genetically unique

Fruits and vegetables are often the direct results of reproductionThe process of producing offspring.. The peppers, tomatoes, anise pods, limes, and coconuts you see are products of sexual reproduction involving pollination and seed growthAn increase in size and number of cells..

Onions are a bit different. They’re often grown by division — a form of asexual reproduction. If you dig up onions in your garden, you might find tiny baby onions attached to the main bulb. These are clones of the parent and can be replanted to grow new onions. This is why seed catalogs often sell “onion sets” rather than onion seeds.

Adaptation

Adaptation is how traits in living things become fine-tuned for survival in a specific environment.

In my backyard, I have purple coneflowers (Echinacea) and blackberry lilies. They bloom at the same time of year but don’t compete for pollinators. Coneflowers release their pollen into the wind, attracting large insects like butterflies that like a big “landing pad.” Blackberry lilies, on the other hand, produce sugary nectar. This nectar draws in hummingbirds and hummingbird moths. These creatures have long, straw-like beaks. Each plant has adapted to different pollinators, which increases the chance of successfully spreading their genes.

Growth and Development

Life grows and develops — at every level, from single cells to entire ecosystems. Growth doesn’t always mean getting bigger; it can mean becoming more complex.

Populations can grow in number (think of the human population boom), and individuals can develop in structure. Human embryos start as a ball of nearly identical cells. They then develop into complex organisms with heads, arms, and legs.

Homeostasis

Homeostasis is the ability to regulate internal conditions. This elephant, for example, uses its big ears as radiators. The thin membranes are full of blood vessels near the surface. In hot weather, the elephant flaps its ears to increase blood flow. It releases heat into the environment. It’s like built-in air conditioning.

Energy Processing

Living things process energy. This butterfly drinks nectar from a zinnia flower, then converts the sugars into ATPThe energy currency of cells used for muscle contraction. — the cell’s “energy currency.”

Humans, plants, animals — we all break down carbohydrates, lipidsOrganic molecules including fats, oils, and steroids., and proteinsLarge molecules made of amino acids with various functions in the body. to make ATP. A student once asked me: “If anything that processes energy is alive, why isn’t my computer alive?” Great question — but life isn’t defined by just one property. A living thing has all of these properties, not just one.

Evolution

AAdaptation is one pathway to evolution, but evolution is bigger — it’s change in a population over generations.

Gregor Mendel, the “father of genetics,” studied sweet pea plants in the 1860s, tracking traits over thousands of plants. His work laid the foundation for understanding inheritance long before DNA was discovered.

Evolution happens to populations, not individuals, and requires multiple generations. In humans, that means changes can take decades, centuries, or even millions of years to see.

Properties of Life?

These qualities describe life as we know it today. All living things are made of cells, as stated in cell theoryA well-tested and widely accepted explanation.: all cells come from preexisting cells. Naturally, that raises the question: where did the first cell come from? That’s a mystery for another semester.

All known living things also use DNA as their genetic material. But what if we discover life on another planet? They might have “building blocks” that aren’t cells, and a “genetic language” that isn’t DNA. Our definition of life might have to change — just like our understanding of the universe keeps evolving.