Introduction to the Cell: Structure, Compartments, and Organelles

Introduction to the Cell

Overview

This study guide provides a comprehensive overview of the structure and organization of living cells, focusing on cellular compartments, organelles, and their functions. It is designed for students beginning their study of cell biology and biochemistry, and covers foundational concepts such as cell theory, differences between prokaryotic and eukaryotic cells, and the roles of major organelles.

Cell Theory

Fundamental Principles

• Cells are the basic units of life.

• All living organisms are composed of cells.

• Cells arise from pre-existing cells.

• All cells evolved from a common ancestor.

Historical Context

• Matthias Jacob Schleiden (1838): Plants consist entirely of cells and cell products.

• Theodor Schwann (1839): Extended Schleiden's conclusion to animals.

Prokaryotic and Eukaryotic Cells

Key Differences

Feature	Prokaryotes	Eukaryotes
Size	0.1–5 μm	10–100 μm
Nucleus	Absent	Present (membrane-bound)
Organelles	Absent	Present (membrane-enclosed)
Ribosomes	70S	80S
Division	Binary fission	Mitosis and meiosis
Flagella	Bacterial type	9+2 microtubule type

Examples

• Prokaryotes: Thiomargarita magnifica (up to 1 cm)

• Eukaryotes: Ostreococcus tauri (2 μm), slime molds (several mm)

Structural Organization of Eukaryotic Cells

General Features

• Larger than prokaryotes

• Extensive endomembrane system

• Compartmentalization into internal organelles

• Membrane-bounded nucleus

• Division by mitosis; sexual recombination via meiosis

• Complex cytoskeleton

Extracellular Matrix

Components

• Collagen fibers (yellow)

• Laminins (silver)

• Proteoglycans (red)

Cell Membrane

Structure and Function

• Composed of phospholipids, glycoproteins, integral proteins, protein channels, and peripheral proteins.

• Regulates entry and exit of substances via:

  • Diffusion

  • Permeation

  • Facilitated transport

  • Active transport

  • Vesicular transport

Vacuolome of the Eukaryotic Cell

Features

• Interconnected membrane system

• Exoplasmic domain: "digestive system" of the cell

• Controlled membrane fusion and fission

• Selective packaging and export of cargo

Endocytosis: How Large Things Enter the Cell

Types

• Phagocytosis: "Cellular eating" of large particles

• Pinocytosis: "Cellular drinking" of fluids

• Receptor-mediated endocytosis: Selective uptake of molecules

Cellular Digestion: Phagosomes and Lysosomes

Process

1. Phagocytic uptake (phagosome)

2. Acidification

3. Fusion with lysosomes containing digestive enzymes

4. Maturation, sorting, and neutralization

5. Exocytosis and membrane retrieval

Mitochondria and Chloroplasts: The Power Stations

Mitochondria

• Double-membrane bound organelle

• Own replicating circular DNA

• Own ribosomes

• Internal membrane folded into cristae

• Fission and fusion dynamics

• Aerobic respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ $\Delta G = -2880 \text{ kJ/mol}$

Chloroplasts

• Double-membrane bound organelle

• Own replicating circular DNA

• Own ribosomes

• Internal membrane folded into thylakoids

• Photosynthetic energy fixation: $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

Nucleus, ER, and Golgi Apparatus: Production and Distribution

Nucleus

• Contains chromosomes and is the defining feature of eukaryotic cells

• Bounded by a double membrane

• Separates DNA replication and transcription from translation

mRNA Production and Processing

• DNA packaged with histones to form chromatin

• Genes transcribed to produce mRNA

• mRNA processed (splicing, capping, polyadenylation)

mRNA Export

• Macromolecules enter/exit nucleus via nuclear pores

• Active transport requiring energy

Translation at Ribosomes

• mRNA translated by ribosomes on the rough ER

• Proteins released into cytoplasm or ER lumen

• Polysomes form when large amounts of the same protein are needed

Protein Folding and Sorting

• Proteins modified in ER lumen:

  • Folding and assembly

  • Glycosylation

  • Disulphide bridge formation

  • Proteolytic cleavage

  • Prenylation

• Transferred to Golgi in vesicles

Golgi Apparatus

• Three sections: cis, medial, trans

• Further modification and sorting of proteins

• Targeting to final destination (plasma membrane, lysosome, etc.)

• Packaging by clathrin scaffolding

Cytoskeleton

Components

• Microtubules: α/β-tubulin cylinders, dynein, kinesin

• Microfilaments: actin chains, interact with myosin

• Intermediate filaments: keratins, lamins, vimentin

Functions

• Structural support

• Cell stability and movement

• Vesicle transport via motor proteins (dyneins, kinesins)

Vesicle Export and Exocytosis

Process

• Vesicles fuse with plasma membrane to release contents

• Exocytosis is the basis of secretion and waste disposal

• Regulated by specific fusion proteins

Eukaryotic Flagellum and Cilia

Structure

• Protrude from cell surface

• Anchored by basal bodies

• Main components: microtubules, dynein, nexin

• Basis for flagellar movement (e.g., sperm cells)

Animal and Plant Cells: Similarities and Differences

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

Summary of Key Points

• All life consists of cells.

• Prokaryotes and eukaryotes differ in size, complexity, and organelle content.

• Genetic material in eukaryotes is contained within the nucleus.

• Strict division between inside and outside environments is maintained by vesicle flow.

• Eukaryotic cells can ingest large particles by phagocytosis and liquids by pinocytosis.

• Lysosomes are the digestive centers of the cell.

• Chloroplasts and mitochondria are energy-producing organelles.

• The cytoskeleton supports cell stability and movement.

• Transcription and translation are spatially separated in eukaryotic cells.

• Animal and plant cells are similar, with a few key differences.

Further Reading

• Goodsell, Escherichia coli, Biochem. Molecular Bio. Educ., vol 37:6, pp325-332, 2009

• Goodsell, Eukaryotic Cell Panorama, Biochem. Molecular Bio. Educ., vol 39:2, pp91-101, 2003

• De Duve, A Guided Tour of the Living Cell, Scientific American Books, 1984

• Burki et al., The New Tree of Eukaryotes, Trends Ecol. Evol., vol 35:1, p43-55, 2019