A Tour of the Cell: Structure and Function in Eukaryotic and Prokaryotic Cells

A Tour of the Cell

Introduction

The cell is the fundamental unit of life, and understanding its structure and function is essential in biology. This chapter provides an overview of cell types, organelles, and the tools used to study cells.

Microscopy and Cell Theory

Robert Hooke and the Discovery of Cells

In 1665, Robert Hooke observed cork cells using a primitive microscope and coined the term "cell." His work laid the foundation for the development of cell theory, which states that all living things are composed of cells and that cells are the basic unit of life.

• Cell Theory: All organisms are made of cells; cells are the basic unit of structure and function in living things.

• Hooke's Microscope: Early compound microscope with an eyepiece, barrel, focusing screw, and objective lens.

• Historical Impact: Hooke's observations led to further studies and the eventual formulation of cell theory by Schleiden and Schwann.

• Example: Hooke observed dead plant cells in cork, which appeared as empty chambers.

Microscopy

Types of Microscopy

Microscopy is essential for studying cells, as most are too small to be seen with the naked eye. There are several types of microscopes used in biology:

• Light Microscopy: Uses visible light to magnify specimens up to about 1000x.

• Electron Microscopy: Uses beams of electrons for much higher resolution and magnification.

• Transmission Electron Microscopy (TEM): Passes electrons through thin sections of specimens to reveal internal structures.

• Scanning Electron Microscopy (SEM): Scans the surface of specimens to produce detailed 3D images.

Magnification is the process of enlarging the appearance of an object, while resolution is the ability to distinguish two close objects as separate.

• Magnification: $\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}}$

• Resolution: The minimum distance two points can be separated and still be distinguished as separate points.

Cell Size and Types

Cell Size

Cells vary in size but are generally small due to the need for efficient exchange of materials with their environment. Surface area-to-volume ratio is a key factor limiting cell size.

• Surface Area-to-Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting the rate of exchange.

Prokaryotes vs. Eukaryotes

Cells are classified into two main types: prokaryotic and eukaryotic.

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent (nucleoid region)	Present
Organelles	Few, no membrane-bound organelles	Many, including membrane-bound organelles
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Animal vs. Plant Cells

Animal and plant cells share many features but also have distinct differences.

Feature	Animal Cells	Plant Cells
Cell Wall	Absent	Present
Chloroplasts	Absent	Present
Vacuoles	Small or absent	Large central vacuole

Cell Membrane

Structure and Function

The cell membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins.

• Phospholipid Bilayer: Hydrophilic (water-loving) heads face outward; hydrophobic (water-fearing) tails face inward.

• Proteins: Channel and carrier proteins facilitate transport; some regions are hydrophilic, others hydrophobic.

• Function: Controls movement of substances in and out of the cell.

The Nucleus

Structure and Function

The nucleus is the control center of the cell, containing most of the cell's genetic material (DNA).

• Nuclear Envelope: Double membrane with pores for transport.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA and associated proteins.

Ribosomes

Structure and Function

Ribosomes are the sites of protein synthesis, found free in the cytoplasm or attached to the endoplasmic reticulum.

• Function: Translate mRNA into polypeptides.

• Structure: Composed of rRNA and proteins.

The Endomembrane System

Components and Functions

The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles. It is involved in synthesis, modification, and transport of cellular materials.

• Nuclear Envelope: Separates nucleus from cytoplasm.

• Endoplasmic Reticulum (ER): Rough ER (with ribosomes) synthesizes proteins; Smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and ships proteins and lipids.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

• Vacuoles: Storage and transport; plant cells have a large central vacuole.

Chloroplasts and Mitochondria

Energy Conversion Organelles

Chloroplasts (in plants) and mitochondria (in all eukaryotes) are responsible for energy conversion.

• Chloroplasts: Site of photosynthesis; contain thylakoids and stroma.

• Mitochondria: Site of cellular respiration; contain cristae and matrix.

Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA, ribosomes similar to prokaryotes.

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein fibers that provides structural support, cell movement, and transport within cells.

• Microtubules: Hollow tubes; involved in cell shape, movement (cilia, flagella), and chromosome separation.

• Microfilaments: Thin fibers; involved in cell movement and shape.

• Intermediate Filaments: Provide mechanical support.

The Extracellular Matrix

Structure and Function

The extracellular matrix (ECM) is a network of proteins and carbohydrates outside animal cells that provides structural support and regulates cell behavior.

• Components: Glycoproteins (e.g., collagen), proteoglycans.

• Function: Cell adhesion, communication, and structural integrity.

Cell Junctions

Types and Functions

Cell junctions connect cells to each other and to the ECM, facilitating communication and structural integrity.

• Tight Junctions: Prevent leakage of materials between cells.

• Gap Junctions: Allow passage of ions and small molecules between cells.

• Desmosomes: Anchor cells together.

Plant Cell Walls

Structure and Function

Plant cell walls are rigid structures outside the plasma membrane, providing support and protection.

• Composition: Mainly cellulose.

• Function: Maintains cell shape, prevents excessive water uptake.

Key Terms

• Cell Theory

• Endosymbiotic Theory

• Electron Microscopy

• Cytoskeleton

• Membrane

• Nucleus

• Extracellular Matrix

• Endomembrane System

• Endoplasmic Reticulum

• Golgi Apparatus

• Lysosome

• Cell Junctions

• Tight Junctions

• Gap Junctions

• Plant Cell Wall