Topic2_Biol186_W2026 (2).pdf

Study Guide - Smart Notes

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Introduction to Cell Biology

Cell biology is a foundational discipline in biology, focusing on the structure, function, and processes of cells—the basic units of life. Understanding cells is essential for exploring physiology, genetics, and molecular biology.

The Cell Theory

Historical Context and Principles

All living organisms are composed of cells, which may be unicellular or multicellular.
Cells are the basic units of structure and function in living things.
All cells arise from pre-existing cells through cell division.

Units of Life

Unicellular and Multicellular Organisms

Unicellular organisms consist of a single cell (e.g., bacteria, archaea, some protists).
Multicellular organisms are composed of many cells organized into tissues and organs (e.g., plants, animals, fungi).
Cells vary in size, shape, and function, but all share certain fundamental features.

Paramecium, a unicellular eukaryote

Studying Cells

Microscopy and Cell Visualization

Microscopy is essential for studying cells, as most cells are too small to be seen with the naked eye.

Light microscopy allows visualization of most plant and animal cells, nuclei, and some organelles.
Electron microscopy provides higher resolution, enabling observation of smaller structures such as viruses, ribosomes, and membranes.
Magnification is the increase in apparent size, while resolution is the ability to distinguish two points as separate.

Scale of biological structures and microscopy limits

Types of Microscopy

Brightfield microscopy: Standard light microscopy, can be used with or without stains.
Phase contrast and differential interference contrast microscopy: Enhance contrast in unstained cells.
Scanning Electron Microscopy (SEM): Provides detailed surface images.
Transmission Electron Microscopy (TEM): Reveals internal cell structures.

Light microscopy images of cheek cells SEM image of cilia TEM image showing longitudinal and cross sections of cilia

Cell Fractionation

Cell fractionation is a technique used to isolate specific organelles from cells for further study. This is achieved by homogenizing cells and using differential centrifugation to separate components based on size and density.

Homogenization: Breaking cells open to release organelles.
Differential centrifugation: Sequentially increasing centrifugal force to pellet cellular components.

Differential centrifugation process

Prokaryotes

Structure and Features

Prokaryotes include bacteria and archaea. They lack membrane-bound organelles and a nucleus, but possess all the essential components for life.

Key components: Plasma membrane, cytosol, DNA (nucleoid), ribosomes.
Additional structures: Cell wall, capsule, fimbriae, flagella.
Prokaryotic cells are typically 1–5 µm in size, but some can be much larger.

Structure of a typical prokaryotic cell

Prokaryotic Diversity and Size

Most bacteria are small, but some, such as Ca. Thiomargarita magnifica, can reach centimeter lengths.
Prokaryotes exhibit a wide range of morphologies and ecological roles.

Size comparison of various prokaryotes and eukaryotes

From Prokaryotes to Eukaryotes

Surface Area-to-Volume Ratio

As cells increase in size, their volume grows faster than their surface area, limiting the rate of material exchange with the environment. This principle explains why most cells are small and why multicellular organisms are composed of many cells rather than a few large ones.

	Single Cube	5x5x1 Cubes	5x5x5 Cubes
Total Surface Area	6	150	750
Total Volume	1	125	125
Surface-to-Volume Ratio	6	1.2	6

Surface area to volume ratio in cells

Cell Membranes

Structure and Function

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. It regulates the movement of substances into and out of the cell and facilitates communication with the environment.

Phospholipid bilayer: Hydrophilic heads face outward, hydrophobic tails inward.
Proteins: Serve as channels, receptors, enzymes, and structural components.
Carbohydrate side chains: Involved in cell recognition and signaling.

Structure of the plasma membrane

Eukaryotes: Animal Cell Structure

Key Organelles and Their Functions

Nucleus: Contains genetic material (DNA) and controls cellular activities.
Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.
Mitochondria: Site of cellular respiration and ATP production.
Lysosomes: Contain hydrolytic enzymes for digestion of macromolecules.
Peroxisomes: Break down fatty acids and detoxify harmful substances.
Cytoskeleton: Provides structural support and facilitates cell movement.

Structure of an animal cell with labeled organelles

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Nucleus	No	Yes
Membrane-bound organelles	No	Yes
Size	1–5 µm	10–100 µm
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Conclusion

Cells are the fundamental units of life, exhibiting remarkable diversity in structure and function. Understanding their organization, the differences between prokaryotic and eukaryotic cells, and the principles governing their size and complexity is essential for further study in biology.