The Characteristics of Cells Study Notes

Subject: HMH Science Dimensions, Module B, Unit 1: Cells, Lesson 1.
Core Topic: Understanding the characteristics and fundamental nature of cells as the building blocks of life.
Vocabulary Definitions:
- Cell: In the field of biology, a cell is the smallest unit that can perform all life processes. Anatomically, cells are covered by a membrane and contain genetic material ( $\text{DNA}$ ) and cytoplasm.
- Organism: A living thing; defined as anything that can carry out life processes independently.
- Unicellular Organism: An organism that consists of a single individual cell.
- Multicellular Organism: An organism that consists of more than one cell.

Definition of Organisms: All living things are categorized as organisms. This includes a vast range of life forms such as bacteria, mushrooms, mosses, and insects.
The Cell as a Fundamental Unit:
- All organisms are composed of one or more cells.
- A cell is officially recognized as the smallest unit that can be said to be alive.
- Cells serve as the fundamental units of all living organisms, regardless of the organism's size or complexity.
Human Cellular Composition: A human being is an exceptionally complex multicellular organism made up of more than $30$ trillion cells ( $> 3 \times 10^{13}$ cells).

Microscopic Identification: If an object is examined under a microscope, the presence of cells determines if it is a living organism.
Diversity of Cells: Although all organisms are made of cells, these cells can exhibit vastly different shapes and specialized functions depending on the needs of the organism.
Classification by Cell Count:
- Unicellular Organisms: Living organisms composed of just one single cell.
- Multicellular Organisms: Living organisms made up of multiple cells working together.

Origin of Cells: New cells cannot arise spontaneously; they can be produced only by existing cells.
Cell Division:
- The cells of a living organism undergo division to create new cells.
- This process allows the organism to grow and continue carrying out essential life processes.
- Cells can divide to produce two identical cells.
Functions of New Cells:
- Growth: Increasing the size or complexity of the organism.
- Maintenance: Replacing cells that are old, weakened, or dead.

Visibility: Most cells are too small to be seen with the unaided human eye.
The Role of Microscopes:
- Due to their microscopic size, cells can generally only be observed through a microscope.
- Microscopes allow humans to observe cellular parts at a scale that is otherwise invisible.
Types of Microscopes and Specimen Requirements:
- Light Microscopes: These can be used to view both living and dead specimens.
- Electron Microscopes: These provide much higher magnification but can only be used to view dead specimens.

Early Magnification: While glass lenses have been used to magnify images for many years, early iterations lacked the power necessary to identify individual cells.
The Compound Microscope: The invention of the compound microscope in the late $1500s$ marked a significant historical step toward the discovery and study of cells.
Modern Microscopy Scales: Today, microscopes have advanced significantly, allowing scientists to study the world at extremely small scales.
Specialized Electron Microscopy:
- Transmission Electron Microscopes (TEM): Used for high-resolution imaging of the internal structures of cells.
- Scanning Electron Microscopes (SEM): Used to study the detailed surface architecture of specimens.

Visual Evidence: A magnified image of a single layer of red onion skin reveals a distinct pattern of structures.
Composition: An onion is built of layers of skin, which are themselves composed of layers of cells.
Scientific Reasoning: By observing different scales of the onion skin, it is clear that the macro-structure of the onion (the layers we see) is directly related to the micro-structure (the individual cells) that compose it.

Objective: To observe cells and demonstrate the biological difference between living and nonliving things.
Methodology: Conduct an investigation using microscopic equipment to provide evidence that living things consist of cells while nonliving objects do not.
Evidence Collection: Observations should be recorded in an Evidence Notebook to construct explanations regarding how multicellular structures (like onion layers) are organized.