General Biology: Chapter 4 – Cell Structure and Function

Why Do We Study Cells?

Introduction to Cell Study

Cells are the fundamental units of life, and understanding their structure and function is essential to all biological sciences. The study of cells has been revolutionized by advances in microscopy, allowing scientists to observe living cells and their internal structures.

• Light microscopes can display living cells and are commonly used in biology labs.

• Electron microscopes (scanning and transmission) provide much higher magnification and resolution, revealing the ultrastructure of cells.

• Magnification is the increase in an object's image size compared to its actual size.

• Resolution is the measure of the clarity of an image, or the ability to distinguish two nearby objects as separate.

• The cell theory states that all living things are composed of cells and that all cells come from other cells.

Example: The diagram provided compares the sizes of various biological structures, from human height to atoms, illustrating the scale at which cells and their components exist.

The Relationship Between Surface Area and Volume

Surface Area-to-Volume Ratio

The surface area-to-volume ratio is a key factor that limits the size of cells. As a cell grows, its volume increases faster than its surface area, which affects the ability of the cell to transport materials in and out efficiently.

• Cells must maintain a large surface area relative to volume to allow for adequate exchange of materials (nutrients, waste, gases).

• Smaller cells have a higher surface area-to-volume ratio, which is more efficient for transport.

Total Volume	Total Surface Area	Surface-to-Volume Ratio
27 units3	54 units2	2

Example: A cube with sides of 3 units has a surface area of 54 units2 and a volume of 27 units3, giving a surface-to-volume ratio of 2.

The Plasma Membrane

Structure and Function

The plasma membrane is a critical structure that surrounds all cells, providing a barrier and regulating the movement of substances in and out of the cell.

• The membrane is a phospholipid bilayer with embedded proteins.

• Channel proteins form channels that shield ions and other hydrophilic molecules as they pass through the hydrophobic center of the membrane.

• Some proteins act as pumps, using energy to actively transport molecules across the membrane.

Example: The diagram shows the arrangement of phospholipids and proteins in the plasma membrane, highlighting hydrophilic heads, hydrophobic tails, and the role of channel proteins.

Types of Cells: Prokaryotic and Eukaryotic

Cell Types and Common Features

All cells share certain features, but there are two main types: prokaryotic and eukaryotic cells.

• All cells have a plasma membrane, DNA, ribosomes, and cytoplasm.

• Bacteria and Archaea consist of prokaryotic cells.

• All other forms of life (plants, animals, fungi, protists) are eukaryotic and belong to the domain Eukarya.

• Eukaryotic cells have a membrane-enclosed nucleus and membrane-enclosed organelles that perform specific functions.

• Prokaryotic cells are generally smaller and simpler in structure.

Example: Escherichia coli is a common prokaryotic cell, while human skin cells are eukaryotic.