Characteristics of Living Organisms

Fundamental Unit of Life: Cells are defined as the smallest units that can be considered alive.
Essential Cellular Components: All cells, regardless of the type of organism, share four common features:
- Cell Membrane
- $DNA$
- Ribosomes
- Cytoplasm
Unicellular Organisms: These are living things composed of a single cell. Representative examples include:
- Bacteria
- Paramecium
- Amoeba
Multicellular Organisms: These are living things composed of multiple cells, such as plants and animals.
Human Cellular Composition: The adult human body consists of a massive colony of cells, totaling between $50$ and $100$ trillion cells.

Definition: Reproduction is the biological process by which organisms produce new individuals.
Sexual Reproduction: This process involves the combination of two cells from different parents to create the first new cell of a new organism. Examples of organisms that reproduce sexually include:
- Maple trees
- Birds
- Humans
Asexual Reproduction: In this process, a single parent produces a new organism by splitting off a portion of itself. Examples of organisms that reproduce asexually include:
- Bacteria
- Hydra

Genetic Code: All cells contain $DNA$ ( $\text{Deoxyribonucleic Acid}$ ), which serves as the blueprint for life.
Inheritance: $DNA$ contains the specific directions for inheritance. Because of this genetic code, offspring inherit all of their traits from their parent or parents.
Biological Consistency: The genetic code ensures that dogs produce dogs and bacteria produce bacteria.

Growth: This refers to the physical increase in the size of an organism.
- Examples include a kitten growing into a cat or a small sprout becoming a large oak tree.
Development: This is the process through which cells in many organisms differentiate, or become different from one another.
Complexity: Differentiation allows a living organism to become more complex over time.
- Metamorphosis Example: A caterpillar develops into a moth.
- Human Development Example: Humans begin life as a single cell and eventually develop into an organism composed of $85$ different types of cells.

Energy Acquisition: To survive, organisms must take in materials and energy from their environment.
Solar Energy: Plants obtain their energy directly from sunlight.
Consumption: Insects and animals acquire energy by eating plants or other animals.
Decomposition: Fungi obtain their energy by breaking down and consuming dead organisms.

Environmental Flux: Since the environment is constantly changing—including fluctuations in temperature, the amount of light, and the availability of water—organisms must be capable of adjusting to these changes.
Stimulus and Response:
- A plant seed is programmed to only begin growing if there is a sufficient amount of water present.
- The leaves and stems of a plant will actively grow toward a light source.

Homeostasis: This is the process by which organisms maintain stable internal conditions despite changes in the external environment.
Temperature Regulation: A bird's body maintains a consistent internal temperature regardless of the external weather conditions.
Water Balance in Plants: Plants maintain stability by taking in water when needed or releasing (giving off) water if they contain an excess amount.

Group Change: While individual organisms stay largely the same throughout their life, the characteristics of a group or population may change over long periods of time.
Natural Selection Principles: Traits that provide an individual with a higher likelihood of surviving and reproducing are more likely to be passed on to the next generation.
Antibacterial Resistance Example: Humans frequently use antibacterial soap to wash their hands. While the soap kills most bacteria, certain individuals within the bacterial population may happen to be naturally resistant to the soap. These resistant bacteria survive the washing process. When these survivors reproduce, they create a new population that is entirely resistant to antibacterial soap.