Characteristics & Needs of Living Things: Sixth Grade Science

Surrounding us in every environment on Earth, an astonishing variety of organisms thrive, from microscopic bacteria in pond water to towering redwood trees and blue whales in the deep sea.

Defining the Living World

When you walk through a park or along a beach, you encounter a wide variety of objects: granite boulders, drifting clouds, flowing streams of water, scurrying squirrels, green moss, and chirping birds. While your intuition immediately recognizes that a squirrel is alive while a boulder is not, defining life in precise scientific terms requires careful criteria. Biology—the scientific study of life, from the Greek words bios meaning life and logos meaning study—establishes that all living things share fundamental biological characteristics and require specific essential resources to maintain life and function.

                           THE CONTINUUM OF LIFE
               Non-Living (Abiotic)             Living (Biotic)
             ========================       =======================
             * Quartz Crystal               * Paramecium (Single Cell)
             * Flowing River                * Oak Tree
             * Gust of Wind                 * Honeybee
             (May grow or move, but         (Possesses all 7 characteristics
             lacks cellular biology!)        of life simultaneously!)

An object may display one or two characteristics of living things without actually being alive. For example, a river moves and grows larger after rain, and an icicle grows as water drips and freezes, yet neither is alive. An organism must exhibit all primary characteristics of life simultaneously.

The Essential Characteristics of Life

Biologists recognize seven fundamental characteristics that define every living organism on Earth, from unicellular microbes to complex multicellular mammals.

                       SEVEN CHARACTERISTICS OF LIFE

       [1] Cellular Organization   ===> Built of one or more cells
       [2] Biochemical Heredity    ===> Contains DNA instructions
       [3] Metabolism & Energy     ===> Converts matter to energy
       [4] Homeostasis             ===> Maintains internal balance
       [5] Response to Stimuli     ===> Reacts to environmental cues
       [6] Growth & Development    ===> Increases size & matures
       [7] Reproduction            ===> Generates offspring

1. Cellular Organization

Every living organism is composed of one or more membrane-bound units called cells. The cell is the smallest, most fundamental unit of life capable of performing all life processes. Organisms can be unicellular (consisting of a single independent cell, such as an amoeba, paramecium, or bacterium) or multicellular (consisting of billions or trillions of coordinated cells organized into specialized tissues and organs, like a human, dog, or dandelion). Non-living objects do not have cellular structures.

2. Universal Genetic Code (DNA)

All life on Earth stores biochemical instructions for building proteins and directing cellular activities in nucleic acids: deoxyribonucleic acid (DNA). When organisms reproduce, they pass copies of their genetic code down to their offspring, ensuring that children inherit traits from their parents.

3. Metabolism and Energy Processing

Living things cannot create energy out of nothing; they must acquire energy and raw materials from their environment to power cellular work, repair damage, and construct new biological molecules. The sum total of all biochemical reactions occurring within an organism is called metabolism.
- Autotrophs (Producers): Organisms that capture energy from sunlight (via photosynthesis) or inorganic chemicals (via chemosynthesis) to produce their own organic food molecules, such as green plants and algae.

- Heterotrophs (Consumers): Organisms that cannot synthesize their own food and must consume other living organisms, organic matter, or detritus to obtain chemical energy, such as animals, fungi, and many bacteria.

4. Homeostasis

Living systems maintain a relatively stable, balanced internal environment despite dramatic fluctuations in the external surroundings. This process of self-regulation is called homeostasis (from the Greek words homeo meaning similar and stasis meaning standing still). For example, the human body maintains an internal core temperature of roughly 37 degrees Celsius (98.6 degrees Fahrenheit). When the external environment becomes hot, your body initiates homeostatic cooling mechanisms: sweat glands secrete moisture that absorbs body heat as it evaporates, and surface blood vessels dilate to release heat. When cold, muscles contract involuntarily (shivering) to generate metabolic heat.

5. Response to Environmental Stimuli

Organisms detect changes in their external or internal environment and react appropriately. A physical or chemical change that triggers a reaction is a stimulus (plural: stimuli), and the resulting action or behavior is a response.
- When an earthworm senses light, it moves away into dark, moist soil to avoid dehydration.

- When a houseplant on a windowsill detects directional sunlight, plant hormone molecules called auxins accumulate on the shaded side of the stem, causing cells to elongate and bending the stem toward the light—a response known as positive phototropism.

                         PHOTOTROPISM IN PLANTS
                          (Response to Stimulus)

                    Sunlight Cues
                     \\ \\ \\
                      \\ \\ \\
                       .---.
                      /     \   Stem bends toward light!
                     |       |  Auxin elongates cells on shaded side
                     |       /
                     |      /
                     |_____/
                     |     |
                    [ POT ]

6. Growth and Development

All living organisms grow and develop over time. In unicellular organisms, growth occurs primarily by an increase in the physical size and volume of the single cell. In multicellular organisms, growth occurs through cellular division (mitosis), increasing the total number of cells in the body.

Growth refers to an increase in physical size, while development describes structural, biochemical, and functional changes that occur throughout an organism's life cycle. For instance, an amphibian develops from a fertilized egg into an aquatic, gill-breathing tadpole, before undergoing metamorphosis into an air-breathing, four-legged adult frog.

7. Reproduction

To prevent extinction, living organisms must produce new individuals of their own species through reproduction. Reproduction can occur through two fundamental mechanisms:
- Asexual Reproduction: A single parent organism produces offspring that are genetically identical to itself (clones). Mechanisms include binary fission in bacteria, budding in yeast, and vegetative propagation in strawberry runners.

- Sexual Reproduction: Two parent organisms contribute specialized haploid sex cells (gametes: sperm and egg) that fuse during fertilization to produce genetically unique offspring with combinations of traits from both parents.

The Basic Needs of Living Things

To maintain their metabolic processes, sustain homeostasis, and ensure survival, all living organisms require four fundamental environmental resources: water, food (energy), living space, and stable homeostatic conditions.

                       FOUR BASIC NEEDS OF LIVING THINGS

        [ WATER ]            Essential solvent for biochemical reactions
        [ FOOD / ENERGY ]    Fuel to power cellular metabolism
        [ LIVING SPACE ]     Territory providing food, water, and shelter
        [ HOMEOSTASIS ]      Conditions (temperature, gases) within tolerable limits

1. Water

Water is the universal solvent indispensable to biological chemistry. The cytoplasm of every living cell consists of roughly 70 to 80 percent water. Water molecules provide the liquid medium in which chemical reactions take place, dissolve vital nutrients and minerals for cellular transport, and carry metabolic waste products out of the body. Most terrestrial animals cannot survive more than a few days without water.

2. Food and Energy Source

Every organism requires a steady supply of energy. Food provides raw organic molecules (carbohydrates, lipids, proteins) that are dismantled during cellular respiration to synthesize adenosine triphosphate (ATP), the universal chemical energy currency of cells.

3. Living Space (Habitat)

Every living thing requires a physical territory or habitat that supplies access to food, water, shelter from predators, and appropriate nesting or burrowing space. Because the physical space and resources of any ecosystem are finite, organisms of the same or different species often compete fiercely for territory.

4. Gas Exchange and Atmospheric Conditions

Most organisms require specific gases from their environment to conduct metabolism. Aerobic organisms (including humans, dogs, fish, and trees) require oxygen to perform cellular respiration, taking in oxygen and releasing carbon dioxide waste. Photosynthetic autotrophs (plants and algae) absorb carbon dioxide gas to build glucose sugars, releasing oxygen into the atmosphere.

Exercises and Solutions

Exercise 1: List the seven fundamental characteristics of living things, and explain why an icicle growing under a roof eaves is not considered a living organism despite increasing in physical size.

Solution 1: The seven characteristics of living things are: (1) cellular organization, (2) universal genetic code (DNA), (3) metabolism and energy processing, (4) homeostasis, (5) response to stimuli, (6) growth and development, and (7) reproduction. An icicle is not alive because its growth occurs purely by the external physical freezing of dripping water molecules; it possesses no cells, contains no DNA, performs no metabolic chemical reactions, maintains no internal homeostatic regulation, and cannot reproduce.

Exercise 2: Define homeostasis, and explain how the human body uses sweating and shivering as homeostatic mechanisms to regulate internal body temperature.

Solution 2: Homeostasis is the biological process by which an organism maintains a stable, balanced internal environment despite changing external conditions. When body temperature rises above normal, sweat glands secrete moisture onto the skin, which absorbs body heat as it evaporates, cooling the blood. When body temperature drops below normal, skeletal muscles contract rapidly and involuntarily in shivering, generating metabolic heat to restore core temperature to roughly 37 degrees Celsius.

Exercise 3: Differentiate between autotrophic and heterotrophic nutrition, providing one specific biological example of each type of organism.

Solution 3: Autotrophs (producers) synthesize their own food by capturing energy from sunlight through photosynthesis or inorganic chemicals through chemosynthesis; an example is a green oak tree or cyanobacteria. Heterotrophs (consumers) cannot make their own food and must consume other organisms or organic matter to acquire chemical energy; an example is a rabbit eating grass or a mushroom decomposing fallen timber.