Cell Structure: Organelles, Endomembrane System, and Evolution in Eukaryotes

Cell Structure

Introduction

The study of cell structure is fundamental to understanding the organization and function of all living organisms. This topic covers the limitations of cell size, the differences between prokaryotic and eukaryotic cells, the endomembrane system, organelle functions, and the evolutionary origins of mitochondria and chloroplasts.

Limitations to Cell Size: Surface Area to Volume Ratio

Why Cell Size is Limited

• Surface Area to Volume Ratio: As a cell grows, its volume increases faster than its surface area. This limits the efficiency of material exchange (nutrients, waste) across the plasma membrane.

• Formula: For a sphere, $\text{Surface Area} = 4\pi r^2$ $\text{Volume} = \frac{4}{3}\pi r^3$ $\text{Surface Area:Volume Ratio} = \frac{3}{r}$

• Implication: Smaller cells have a higher surface area to volume ratio, allowing for more efficient exchange with their environment.

• Example: Microvilli in intestinal cells increase surface area for absorption.

Prokaryotic vs. Eukaryotic Cells

Structural Differences

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. DNA is located in the nucleoid region. Examples: Bacteria and Archaea.

• Eukaryotic Cells: Have a true nucleus and various membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

• Size: Eukaryotic cells are generally larger (10–100 μm) than prokaryotic cells (0.1–5 μm).

Endomembrane System

Components and Functions

• Definition: A network of membranes inside eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Main Components:

  • Nuclear envelope

  • Endoplasmic reticulum (ER) – rough and smooth

  • Golgi apparatus

  • Lysosomes

  • Vesicles and vacuoles

  • Plasma membrane

• Functions: Protein and lipid synthesis, transport, detoxification, and digestion.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins (especially those for secretion or membrane insertion).

• Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs, and stores calcium ions.

• Structure: Network of membranous tubules and sacs called cisternae; continuous with the nuclear envelope.

Golgi Apparatus

• Function: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

• Structure: Flattened membranous sacs (cisternae) with a cis face (receiving side) and trans face (shipping side).

• Example: Addition of carbohydrates to proteins to form glycoproteins.

Lysosomes

• Function: Digestive organelles containing hydrolytic enzymes that break down macromolecules, old organelles, and foreign substances.

• pH: Acidic environment (pH ~5) optimal for enzyme activity.

• Processes:

  • Phagocytosis: Engulfing food particles or invaders.

  • Autophagy: Recycling the cell’s own organelles and macromolecules.

• Example: White blood cells use lysosomes to destroy bacteria.

Vacuoles

• Function: Storage, waste disposal, protection, and hydrolysis.

• Types:

  • Central vacuole (plants): Stores ions, water, and contributes to turgor pressure.

  • Contractile vacuole (protists): Pumps excess water out of the cell.

  • Food vacuole: Formed by phagocytosis, fuses with lysosomes for digestion.

Protein Targeting and Signal Sequences

How Proteins Reach Their Destinations

• Signal Sequences: Short stretches of amino acids in a protein that act as 'postal codes' to direct proteins to their correct cellular locations.

• Types:

  • Hydrophobic sequences: Target proteins to the endomembrane system.

  • Charged sequences: Target proteins to mitochondria.

  • No signal sequence: Proteins remain in the cytosol.

• Mechanism: Signal recognition particles (SRPs) bind to signal sequences and guide ribosomes to the ER membrane, where translation continues and the protein enters the ER lumen.

Mitochondria and Chloroplasts: The Energy Organelles

Mitochondria

• Function: Site of cellular respiration; converts chemical energy from food into ATP.

• Structure: Double membrane; inner membrane folded into cristae; contains its own DNA and ribosomes.

• Key Processes: Krebs cycle (matrix), electron transport chain (inner membrane).

• Size: 1–10 μm.

Chloroplasts

• Function: Site of photosynthesis in plants and algae; converts solar energy into chemical energy (glucose).

• Structure: Double membrane; internal thylakoid membranes stacked into grana; stroma contains DNA and ribosomes.

• Key Processes: Light reactions (thylakoid membrane), Calvin cycle (stroma).

• Size: 3–6 μm.

Endosymbiont Theory

Origin of Mitochondria and Chloroplasts

• Definition: Proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence:

  • Double membranes

  • Own circular DNA and ribosomes

  • Reproduce independently within the cell

• Significance: Enabled eukaryotes to efficiently produce ATP (mitochondria) and perform photosynthesis (chloroplasts).

Peroxisomes

Detoxification and Metabolism

• Function: Break down fatty acids and detoxify harmful substances by transferring hydrogen to oxygen, forming hydrogen peroxide ($H_2O_2$), which is then converted to water.

• Specialized Peroxisomes: Glyoxysomes in plant seeds convert fatty acids to sugars during germination.

Cell Walls

Structure and Function

• Bacteria: Peptidoglycan cell wall provides shape and protection.

• Fungi: Chitin-based cell wall.

• Plants: Cellulose-based cell wall; provides structural support and protection.

Cytoskeleton

Three Main Elements

• Microtubules: Hollow tubes of tubulin; maintain cell shape, enable cell motility (cilia, flagella), and chromosome movement during cell division.

• Microfilaments (Actin Filaments): Thin rods of actin; involved in muscle contraction, cell movement, and cytokinesis.

• Intermediate Filaments: Fibrous proteins; provide mechanical support and maintain cell shape.

Cell Junctions

Types and Functions

• Tight Junctions: Seal cells together to prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together, providing mechanical stability.

• Gap Junctions: Allow direct communication between adjacent animal cells via channels.

• Plasmodesmata (plants): Channels that traverse plant cell walls, allowing transport and communication.

Extracellular Matrix (ECM) and Cytoskeleton Linkage

Communication and Structural Support

• ECM: Network of proteins (collagen, proteoglycans) outside animal cells; provides structural support and regulates cell behavior.

• Linkage: Integrins connect the ECM to the cytoskeleton, facilitating communication and mechanical support.

Summary Table: Comparison of Cell Types and Organelles

Feature	Prokaryotic Cell	Plant Cell	Animal Cell
Nucleus	No	Yes	Yes
Cell Wall	Yes (peptidoglycan)	Yes (cellulose)	No
Chloroplasts	No	Yes	No
Mitochondria	No	Yes	Yes
Vacuole	No	Large central vacuole	Small or absent
Endomembrane System	No	Yes	Yes