Characteristics of Life
Essential Functions of Living Organisms
Every living thing shares a set of core functions. If something can't perform these, biologists don't classify it as alive.
Homeostasis is the maintenance of stable internal conditions. Your body temperature stays around 37°C whether you're in a snowstorm or a desert. This works through negative feedback mechanisms, where a change in conditions triggers a response that reverses that change. When your blood glucose spikes after eating, insulin brings it back down. When your body temperature drops, shivering generates heat to warm you back up. The system always pushes back toward a set point.
Metabolism covers all the chemical reactions that keep an organism alive. It has two sides:
- Anabolism builds complex molecules from simpler ones. Protein synthesis is a classic example: amino acids get assembled into functional proteins.
- Catabolism breaks complex molecules down to release energy. Cellular respiration breaks down glucose to produce ATP, the cell's energy currency.
These two processes are constantly running in balance. Without metabolism, there's no energy to power anything else on this list.
Reproduction is how organisms produce offspring. There are two main strategies:
- Asexual reproduction involves a single parent producing genetically identical offspring. Bacteria do this through binary fission, splitting one cell into two copies.
- Sexual reproduction combines genetic material from two parents, producing offspring with genetic variation. This variation is a big deal for evolution because it gives natural selection more traits to act on.
Growth, Development, and Interaction with Environment
Growth means an increase in the size or number of cells. This happens through cell division (mitosis) and cell expansion. Development is different from growth: it refers to changes in form and function over time. A caterpillar undergoing metamorphosis into a butterfly is developing, not just growing. Embryogenesis in mammals, where a single fertilized egg becomes a complex organism with specialized tissues, is another example.
Adaptation is the process by which populations become better suited to their environment over generations. Genetic variations that improve survival and reproduction get passed on. Antibiotic resistance in bacteria is a well-known case: bacteria with a mutation that lets them survive an antibiotic reproduce, and that resistance spreads through the population. Natural selection is the mechanism driving this, favoring traits that increase an organism's fitness (its ability to survive and reproduce in a given environment).
Response to stimuli allows organisms to detect and react to changes around or within them. Stimuli can be chemical (pheromones), physical (touch, light), or biological (the presence of a predator). Responses vary widely:
- Behavioral: a deer fleeing from a wolf (fight-or-flight response)
- Physiological: your pupils dilating in dim light
- Developmental: a plant bending toward a light source (phototropism)
Hierarchical Organization of Living Systems
All living things are built from smaller components arranged in a specific order. Each level builds on the one below it:
- Atoms form molecules (e.g., hydrogen and oxygen atoms form water)
- Molecules form organelles and cells (e.g., phospholipids form cell membranes; mitochondria are organelles within cells)
- Cells form tissues (e.g., muscle cells group into muscle tissue)
- Tissues form organs (e.g., muscle, connective, and nervous tissue combine to form the heart)
- Organs form organ systems (e.g., the heart and blood vessels form the circulatory system)
- Organ systems form organisms (e.g., all your organ systems together make you a functioning human)
This hierarchy is one of biology's most fundamental concepts. Notice that at each level, new properties emerge that weren't present in the level below. A single heart cell can't pump blood through a body. That ability only emerges when cells are organized into the organ we call the heart. This idea is called emergent properties, and you'll see it come up throughout biology.
Levels of Organization
Atoms to Molecules
Atoms are the basic units of matter. Each atom contains protons and neutrons in a central nucleus, with electrons orbiting around it. The elements most important to biology are carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur (sometimes remembered as CHONPS).
Atoms combine to form molecules through chemical bonds. Covalent bonds involve the sharing of electrons between atoms, while ionic bonds involve the transfer of electrons from one atom to another. Molecules in living systems range from small and simple, like water (), to enormous and complex, like DNA. These biological molecules perform specific jobs: enzymes catalyze chemical reactions, hormones carry signals between cells, and nucleic acids store genetic information.
Cells to Organs
Cells are the basic structural and functional units of life. There are two broad categories:
- Prokaryotic cells lack a membrane-bound nucleus and other membrane-bound organelles. Bacteria and archaea are prokaryotes.
- Eukaryotic cells have a true nucleus enclosed by a membrane, plus organelles like mitochondria and (in plants) chloroplasts. Animals, plants, fungi, and protists are all eukaryotes.
Tissues are groups of similar cells working together to perform a shared function. Animals have four main tissue types: epithelial (covering and lining surfaces), connective (support and structure), muscle (movement), and nervous (signaling). Plants have three: dermal (outer covering), ground (photosynthesis, storage, support), and vascular (transport of water and nutrients).
Organs are structures made of multiple tissue types that carry out specific functions. The heart, for instance, contains muscle tissue for pumping, nervous tissue for regulating rhythm, connective tissue for structural support, and epithelial tissue lining its chambers. In plants, a leaf is an organ containing dermal, ground, and vascular tissues working together for photosynthesis.
Organ Systems and Organisms
Organ systems are groups of organs that cooperate to perform a broad function. In animals, the digestive system (mouth, stomach, intestines, liver, pancreas) breaks down food and absorbs nutrients. In plants, the vascular system (xylem and phloem) transports water, minerals, and sugars throughout the plant body.
Organisms are individual living entities made of one or more cells. They can be unicellular, like bacteria or amoebas, or multicellular, like animals, plants, and fungi. Every organism interacts with its environment and with other organisms through relationships like predation, symbiosis, and competition.
Ecological Hierarchy
Populations and Communities
A population is a group of individuals of the same species living in a particular area at the same time. Biologists describe populations using traits like density (how many individuals per unit area), dispersion pattern (how they're spread out), and age structure (the distribution of ages). Populations change over time as individuals are born, die, immigrate in, or emigrate out.
A community consists of all the populations of different species living and interacting in the same area. Species within a community interact through predation, competition, mutualism, and other relationships. Communities also have structure: some species are dominant (most abundant or influential), while keystone species have outsized effects on the community relative to their numbers. A classic example is the sea otter, whose predation on sea urchins prevents urchins from destroying kelp forests.
Ecosystems and the Biosphere
An ecosystem includes all the living organisms in an area (the biotic factors) plus the nonliving components they interact with (the abiotic factors). Biotic factors include producers, consumers, and decomposers. Abiotic factors include sunlight, water, temperature, and soil composition. Energy flows through ecosystems via food chains and food webs, while matter cycles through biogeochemical processes like the carbon cycle and nitrogen cycle.
The biosphere is the sum of all ecosystems on Earth, encompassing every environment where life exists: terrestrial, aquatic, and atmospheric. One key distinction: the biosphere is essentially a closed system for matter (the same atoms get recycled over and over) but an open system for energy (solar energy constantly enters, and heat is lost to space). This is why energy must continuously flow through ecosystems while nutrients can be recycled.