Comprehensive Introduction to the Foundations of Biology

Biology is defined as the scientific study of life.
Life is characterized by a set of common properties.
For an entity to be classified as an organism or as "life," it must possess all of these common properties simultaneously. Having only some of these properties is insufficient to define something as life.
A foundational characteristic of all life is that it is composed of cells.

The cell is the basic and fundamental unit of life.
It is defined as the structural and functional unit of life, meaning it is the smallest entity that contains all the properties required for life.
A large number of cells can combine together to form higher-level structures.
All cells are enclosed by a membrane that functions to "block" the cell. This membrane serves as a regulator, allowing only certain specific substances to enter or exit the cell.
Cells utilize DNA as their source of genetic information. In many cells, this DNA is housed within a nucleus.
There are two primary types of cells, one of which is the prokaryotic cell.

All organisms require energy processing to sustain life.
Energy is derived from food.
Some organisms, such as plants, have the capability to utilize light energy to produce chemical energy.
Some plants are known as "animal-eating plants" because they consume small animals, specifically insects, to obtain energy.
Regulation is a key process in living things. For example, the human body regulates its internal environment to maintain a constant state.
A specific example of human regulation is body temperature, which is maintained at a constant $37\,^\circ\text{C}$ . Temperatures significantly above or below this threshold are considered abnormal.
The body also regulates other internal factors, such as pH, to keep them constant.
Organisms that do not regulate temperature internally in the same way as humans include "The Lisa," which is a cold-blooded animal.

Life is organized across a spectrum of levels, ranging from the largest scale to the smallest scale:
- Biosphere: The largest scale of organization. It includes environments such as the sky, which contains air, light, and materials like spores that sustain themselves in the air environment.
- Ecosystem: A level that includes both the biological community and the surrounding physical environment.
- Community: A level that focuses on the biological groups but does not include the physical environment.
- Population: A group of individuals of the same species.
- Organism.
- Cell: The basic unit of organization.
- Molecule: The smallest level of biological organization.
The exchange of gases, such as oxygen, is a process that occurs within these levels to sustain life.

Evolution is considered the core theme of biology.
Genes are the units of inheritance that transmit information from parents to offspring (the next generation).
DNA is a very long molecule, and a gene is defined as a specific fragment or section of that DNA.
DNA is structured as a double helix.
Specific genes are coded by sequences consisting of four different kinds of building blocks.
The specific sequence of these building blocks determines the protein produced, and these proteins control specific activities within the body.
The "diagnosis of life" or the diversity of life arises from differences in these DNA sequences. Different organisms have different sequences.
Even siblings who come from the same parents will have different DNA sequences. Every individual has some DNA that is unique to them specifically, as well as some DNA unique to their siblings.