4.1 Studying Cells

Introduction to Cells

Cells as Basic Life Units

Cells are the smallest functional units of life. Every living organism, from bacteria to humans, is made of one or more cells. They carry out the processes that define something as "alive."

Those processes include:

• Metabolism — the chemical reactions that break down or build up molecules
• Energy production — synthesizing ATP to power cellular work
• Waste removal (excretion) — getting rid of byproducts the cell doesn't need
• Growth and reproduction — increasing in size and dividing to form new cells

Every cell contains genetic material (DNA), which gets passed to new cells during division (mitosis or meiosis). In multicellular organisms, cells become specialized for particular jobs. Neurons transmit electrical signals, muscle cells contract to produce movement, and red blood cells carry oxygen. These specialized cells work together to keep the whole organism functioning.

Light vs. Electron Microscopy

Microscopes are what made cell biology possible. Without them, cells are simply too small to see. The two major categories of microscopes differ in what they use to create an image and how much detail they can reveal.

Light microscopy passes visible light through a specimen and uses glass lenses to magnify it.

• Magnification ranges from about 40x to 1,000x
• You can observe living cells and see structures like the nucleus, cytoplasm, and larger organelles
• Resolution (the ability to distinguish two close objects as separate) is limited to about 200 nm because of the wavelength of visible light
• Specimens are often stained with dyes like hematoxylin and eosin to improve contrast and make structures easier to identify

Electron microscopy uses a beam of electrons instead of light, which allows for far greater magnification and resolution.

• Transmission Electron Microscopy (TEM) shoots electrons through an ultra-thin specimen, reaching up to 1,000,000x magnification. This is how scientists visualize tiny structures like ribosomes and the internal details of mitochondria.
• Scanning Electron Microscopy (SEM) bounces electrons off the surface of a specimen, producing detailed 3D images at up to 500,000x magnification.
• The tradeoff: samples must be extensively prepared (fixed, dehydrated, and sometimes coated in metal), and the microscope chamber operates under a vacuum. That means you cannot view living specimens with electron microscopy.

Key Principles of Cell Theory

Cell theory is one of the most foundational ideas in biology. It provides a unifying framework that applies to every living organism on Earth.

The three core principles:

1. All living organisms are composed of one or more cells (from unicellular bacteria to multicellular plants and animals).
2. The cell is the basic unit of structure and function in living organisms.
3. All cells arise from pre-existing cells through cell division.

Cell theory was developed collaboratively over several decades:

• Matthias Schleiden (1838) concluded that all plant tissues are composed of cells.
• Theodor Schwann (1839) extended this idea to animal tissues.
• Rudolf Virchow (1855) proposed that all cells come from pre-existing cells, famously stated as "Omnis cellula e cellula" (every cell from a cell).

This framework connects to many other areas of biology. Genetics builds on the idea that DNA is housed and replicated within cells. Evolution relies on the concept that all cells share common ancestry. Physiology depends on understanding how specialized cells form tissues, organs, and organ systems.

Cell Structures and Their Functions

Even at an introductory level, you should know the major components found inside cells and what each one does.

• Cell membrane — a selectively permeable barrier surrounding every cell. It controls which substances enter and leave, maintaining the cell's internal environment.
• Cytoplasm — the gel-like fluid filling the cell's interior. Organelles are suspended in it, and many chemical reactions take place here.
• Nucleus — the control center of the cell. It houses the cell's DNA and directs activities like growth, metabolism, and reproduction.
• Ribosomes — the sites of protein synthesis. They can be found floating freely in the cytoplasm or attached to the endoplasmic reticulum.
• Chloroplasts — found only in plant cells (and some protists). These organelles carry out photosynthesis, converting light energy into chemical energy stored as glucose.
• Vacuoles — membrane-bound sacs used for storage. Plant cells typically have one large central vacuole that holds water, nutrients, and waste products, helping maintain the cell's shape through turgor pressure.
• Cell wall — a rigid layer outside the cell membrane in plant cells, fungi, and bacteria. It provides structural support and protection. Animal cells do not have a cell wall.