Tour of the Cell

Introduction to Cell Biology

Cells are the fundamental units of life, and their study is central to understanding biology. Biologists use microscopes and biochemical techniques to investigate cell structure and function, as most cells are too small to be seen with the naked eye.

• Microscopy allows visualization of cells and their components at various scales.

• Cells are generally microscopic due to limitations imposed by surface area-to-volume ratios.

Why Are Cells Small?

The small size of cells is essential for efficient exchange of materials with their environment. As a cell increases in size, its volume grows faster than its surface area, limiting the rate at which substances can enter or leave the cell.

• Surface area-to-volume ratio is critical for cellular function.

• Smaller cells have a higher ratio, facilitating efficient transport of nutrients and waste.

Scale of Biological Structures

Biological structures vary greatly in size, from atoms and molecules to entire organisms. The following table summarizes the relative sizes of key biological entities and the tools used to observe them:

The Fundamental Units of Life: Cell Theory

Cell Theory

Cell theory is a foundational concept in biology, stating:

• All organisms are made of cells.

• The cell is the simplest collection of matter that can be alive.

• All cells are related by their descent from earlier cells.

• Cells can differ substantially from one another but share common features.

Types of Cells: Prokaryotic and Eukaryotic

Overview

Cells are classified as either prokaryotic or eukaryotic based on their structural characteristics.

• Prokaryotic cells are found in the domains Bacteria and Archaea.

• Eukaryotic cells make up protists, fungi, animals, and plants.

Basic Features of All Cells

• Plasma membrane: Encloses the cell, controlling the movement of substances in and out.

• Cytosol: Semifluid substance within the cell.

• Chromosomes: Carry genetic information (DNA).

• Ribosomes: Synthesize proteins.

Prokaryotic Cells

Prokaryotic cells are structurally simpler than eukaryotic cells.

• No nucleus; DNA is located in an unbound region called the nucleoid.

• No membrane-bound organelles.

• Cytoplasm is bound by the plasma membrane.

Example: Typical rod-shaped bacterium with structures such as fimbriae, nucleoid, ribosomes, plasma membrane, cell wall, capsule, and flagella.

Eukaryotic Cells

Eukaryotic cells are more complex and generally larger than prokaryotic cells.

• DNA is contained within a nucleus, which is surrounded by a nuclear envelope.

• Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

• Cytoplasm is the region between the plasma membrane and the nucleus.

The Plasma Membrane

Structure and Function

The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell.

• Composed primarily of a phospholipid bilayer with embedded proteins.

• Maintains homeostasis by regulating the internal environment of the cell.

Surface Area-to-Volume Ratio

The efficiency of the plasma membrane is influenced by the cell's surface area-to-volume ratio. As a cell grows, its volume increases faster than its surface area, which can limit the rate of exchange with the environment.

• Formula for surface area of a cube:

• Formula for volume of a cube:

• Surface area-to-volume ratio: (where is the length of a side)

Example: Smaller cells have a higher surface area-to-volume ratio, which is advantageous for efficient transport.

*Additional info: Further details on organelles, endomembrane system, and cell junctions would be covered in subsequent sections or slides.*