Characteristics of living things Of course. Here are the characteristics of living things, often summarized by the acronym MRS GREN or similar variations. These are the fundamental traits that biologists use to distinguish living organisms from non-living things.
The Core Characteristics (MRS GREN)
Movement
- Definition: The ability of an organism to change its position or place.
Examples:
- Animals walk, run, fly, or swim.
- Plants move by growing towards light (phototropism) or their roots towards water (hydrotropism).
- Even microscopic organisms like bacteria use flagella to swim.
Respiration
- Definition: The chemical process of breaking down nutrients to release energy for life processes.
- Key Point: It’s not just breathing! It occurs at the cellular level (cellular respiration).
- Equation (simplified): Glucose + Oxygen → Carbon Dioxide + Water + Energy (ATP)
Sensitivity (Response to Stimuli)
- Definition: The ability to detect and respond to changes in the internal or external environment.
Examples:
- A venus flytrap closing when touched.
- A plant growing towards light.
- You pulling your hand away from a hot stove.
- Bacteria moving towards a nutrient source (chemotaxis).
Growth
- Definition: A permanent increase in size and/or dry mass by increasing the number or size of cells.
Examples:
- A seed sprouting and becoming a tree.
- A baby growing into an adult.
- Even single-celled organisms grow before dividing.
Reproduction
- Definition: The ability to produce new individuals of the same species, ensuring the continuation of the species.
Two Main Types:
- Asexual: Involves one parent, producing genetically identical offspring (e.g., bacteria splitting, plants sending out runners).
- Sexual: Involves two parents, combining genetic material to produce genetically unique offspring (e.g., humans, animals, many plants).
Excretion
- Definition: The removal of waste products from metabolism and other cellular activities.
- Important: Waste is toxic if it builds up.
Examples:
- Humans excrete carbon dioxide (from respiration) and urea (from protein breakdown) in urine.
- Plants release oxygen as a waste product of photosynthesis and can excrete waste into their leaves, which fall off.
Nutrition
- Definition: The intake and use of nutrients for energy, growth, and repair.
Two Main Types:
- Autotrophic Nutrition: Organisms make their own food (e.g., plants use photosynthesis to convert sunlight into chemical energy).
- Heterotrophic Nutrition: Organisms consume other organisms for food (e.g., animals eating plants or other animals, fungi decomposing matter).
Additional Key Characteristics
- While MRS GREN covers the basics, most biologists also include these essential traits:
Homeostasis
- Definition: The maintenance of a stable internal environment despite changes in the external environment.
- Examples: Regulating body temperature, blood pH, and water balance.
Made of Cells
- Definition: All living things are composed of one or more cells, which are the basic units of life.
- Unicellular: Single-celled (e.g., bacteria, amoeba).
- Multicellular: Many cells (e.g., humans, trees, dogs).
Contain Genetic Information (DNA/RNA)
- Definition: All living things use the universal genetic code of DNA (or sometimes RNA) to store instructions for life and pass traits to offspring.
Adaptation and Evolution
- Definition: Over long periods, populations of organisms evolve traits that make them better suited to their environment through the process of natural selection.
Important Considerations
- Not every characteristic is displayed at every moment. A dormant seed, for example, shows very few signs of life until conditions are right. It is still alive because it has the potential to carry out all these functions.
- Non-living things may show one or two characteristics. A car moves, a fire grows and consumes “food” (fuel), and a crystal grows. However, it does not do all of these things together and is not made of cells, so it is not considered alive.
- Viruses are the classic exception. They contain genetic material (DNA or RNA) and can evolve, but they cannot reproduce on their own and do not have a metabolism. They are obligate parasites that must use a host cell’s machinery. Therefore, most scientists do not classify them as living things.
Beyond MRS GREN: A Deeper Dive
- While MRS GREN is a useful teaching tool, a more comprehensive understanding includes these critical concepts:
Cellular Organization: The Fundamental Unit
This is arguably the most fundamental characteristic.
- Prokaryotic Cells: Simpler, smaller cells without a nucleus or membrane-bound organelles (e.g., bacteria and archaea).
- Eukaryotic Cells: More complex cells containing a nucleus and specialized organelles (e.g., plants, animals, fungi, protists).
- Unicellular vs. Multicellular: Life exists as single, self-sufficient cells or as complex organisms with differentiated cells working together as tissues and organs.
Homeostasis: The Pursuit of Balance
This is the constant, active process of maintaining a stable internal state.
- Mechanism: It’s typically a feedback loop.
- Example (Thermoregulation): If your body temperature rises (stimulus), sensors detect it. Your brain (control center) triggers a response (sweating, vasodilation) to cool you down. Once temperature normalizes, the response stops.
- Other Examples: Regulating blood sugar levels, water balance (osmoregulation), and pH.
Heredity and Evolution: The Blueprint and Change Over Time
- Genetic Code: All life uses the same molecular language: DNA (Deoxyribonucleic Acid). DNA holds the instructions for building and operating an organism, written in a code using four chemical bases (A, T, C, G). This code is universal across all known life.
- Reproduction’s Role: Reproduction is the mechanism for passing this genetic code to the next generation.
- Evolution: This is the overarching consequence. When reproduction is not perfect (due to mutations in DNA), it introduces variation. Through natural selection, organisms with traits better suited to their environment are more likely to survive and reproduce, passing those advantageous genes onward. This leads to populations adapting and evolving over time.
Metabolism: The Chemistry of Life
This is the umbrella term that encompasses Nutrition, Respiration, and Excretion. It’s the sum of all chemical reactions in an organism.
- Catabolism: Breaking down molecules to release energy (e.g., cellular respiration).
- Anabolism: Building up complex molecules from simpler ones, requiring energy (e.g., synthesizing proteins from amino acids).
- Energy Currency: The energy released from catabolism is stored and transported as ATP (Adenosine Triphosphate), the universal energy currency of the cell.
Applying the Characteristics: The Virus Debate
- This is the perfect test case for our list. Why are viruses a “grey area”?
They DO:
- Contain Genetic Material: They have DNA or RNA.
- Evolve: Their genes mutate and they are subject to natural selection.
- Response to Stimuli (arguably): Some can sense host cells.
They DO NOT:
- Have a Cellular Structure: They are just packets of genes (Nucleic Acids) in a protein coat (capsid).
- Carry out Metabolism: They have no machinery to generate energy or synthesize proteins.
- Grow: They do not increase in size or complexity; they are assembled.
- Reproduce Independently: They must hijack a host cell’s metabolic machinery to replicate.
- Conclusion: Because viruses lack key characteristics (especially independent metabolism and reproduction), they are generally not classified as living things. They are often described as “obligate intracellular parasites” that exist on the boundary between living and non-living.
A Unified View of Life
Think of these characteristics not as a checklist, but as an interconnected web of processes that define a living state:
- A living thing is a highly organized, cell-based system that
- maintains a stable internal environment (homeostasis) through.
- the capture and use of energy and nutrients (metabolism).
- to grow, respond to stimuli, and ultimately.
- replicate using a genetic blueprint (reproduction & heredity).
- which, over time, allows for change and adaptation (evolution).