Eukaryotic Cell Biology: Structure, Function, and Organization

Eukaryotic Cell Biology

Introduction

Eukaryotic cells are complex, membrane-bound units that form the basis of all protists, fungi, plants, and animals. Their internal organization, compartmentalization, and specialized structures enable diverse cellular functions essential for life.

Microscopy

Types of Microscopy

• Light Microscope (LM): Uses visible light passed through a specimen and glass lenses to magnify images of cells and tissues.

• Electron Microscope (EM): Uses a beam of electrons for much higher resolution, allowing visualization of subcellular structures.

• Magnification: The ratio of an object's image size to its real size.

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

• Contrast: The difference in brightness between the light and dark areas of an image.

Application: Light microscopes are suitable for living cells and tissues, while electron microscopes are used for detailed study of organelles and macromolecular complexes.

Cell Structure

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells: Found in Bacteria and Archaea; lack a nucleus and membrane-bound organelles; generally smaller and simpler.

• Eukaryotic Cells: Found in protists, fungi, animals, and plants; possess a nucleus and extensive internal membranes.

• All cells have: a plasma membrane, cytosol, chromosomes (DNA), and ribosomes.

Major Differences

• Size: Eukaryotic cells are typically 10–100 µm; prokaryotic cells are 0.1–5 µm.

• Internal Organization: Eukaryotes have compartmentalized organelles; prokaryotes do not.

• Genomic Compartmentalization: Eukaryotic DNA is in the nucleus; prokaryotic DNA is in the nucleoid region.

Limitations on Cell Size

• Cellular logistics (e.g., diffusion rates) set limits on cell size.

• As cell size increases, volume increases faster than surface area ($V \propto r^3$, $A \propto r^2$).

• Smaller cells have a higher surface area-to-volume ratio, facilitating efficient exchange of materials.

• Specialized Structures: Cells like intestinal epithelial cells have microvilli to increase surface area.

The Plasma Membrane

Structure and Function

• Composed of a phospholipid bilayer with embedded proteins.

• Functions as a selective barrier, regulating the passage of oxygen, nutrients, and wastes.

• Eukaryotic cells have extensive internal membranes, forming organelles.

The Nucleus and Ribosomes

Nucleus

• Contains most of the cell's genetic material (DNA).

• Enclosed by a double membrane (nuclear envelope) with nuclear pores for molecular exchange.

• The nuclear lamina (protein filaments) supports nuclear shape.

• DNA is organized with proteins into chromosomes; humans have 46 chromosomes (23 in gametes).

• The nucleolus is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Flow of Genetic Information

• DNA is transcribed into messenger RNA (mRNA) in the nucleus.

• mRNA exits the nucleus via nuclear pores and is translated by ribosomes in the cytoplasm into proteins.

Ribosomes

• Composed of rRNA and proteins; sites of protein synthesis.

• Free ribosomes synthesize cytosolic proteins; bound ribosomes (on rough ER) synthesize secretory and membrane proteins.

The Endomembrane System

Endoplasmic Reticulum (ER)

• Accounts for over half the total membrane in many eukaryotic cells.

• Two types:

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium ion storage.

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

• Secretory proteins are glycoproteins (proteins with carbohydrates attached) and are packaged into transport vesicles for delivery to the Golgi apparatus.

Golgi Apparatus

• Consists of flattened membranous sacs (cisternae).

• Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Especially prominent in cells specialized for secretion.

Lysosomes

• Membrane-bound sacs containing hydrolytic enzymes for digestion of macromolecules.

• Optimal function at acidic pH.

• Formed from the ER and Golgi apparatus.

• Involved in phagocytosis (ingestion of external particles) and autophagy (recycling of cellular components).

Vesicles and Vacuoles

• Membrane-bound sacs with diverse functions (e.g., storage, transport, digestion).

• Vacuoles are larger versions, prominent in plant cells for storage and maintaining turgor pressure.

Mitochondria and Chloroplasts

Energy Conversion

• Mitochondria: Sites of cellular respiration; convert chemical energy from food into ATP.

• Chloroplasts: Sites of photosynthesis; convert solar energy into chemical energy (glucose).

Structure

• Mitochondria have a smooth outer membrane and a highly folded inner membrane (cristae), enclosing the mitochondrial matrix.

• Chloroplasts have an outer and inner membrane, with internal thylakoid membranes stacked into grana and surrounded by stroma.

Endosymbiont Theory

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

• Supported by similarities in DNA, ribosomes, and double membranes.

Peroxisomes

• Single-membrane-bound organelles containing enzymes that transfer hydrogen from substrates to oxygen, producing hydrogen peroxide ($H_2O_2$).

• Contain catalase to convert $H_2O_2$ to water, preventing toxicity.

The Cytoskeleton

Overview

• A network of protein fibers that provides structural support, cell shape, and motility.

• Anchors organelles and facilitates intracellular transport.

• Dynamic and adaptable to cellular needs.

Components

• Microtubules: Thick, hollow rods (25 nm diameter) made of tubulin; involved in cell shape, organelle movement, and chromosome separation.

• Microfilaments (Actin Filaments): Thin rods (7 nm diameter) made of actin; support cell shape, enable muscle contraction, and drive cell motility.

• Intermediate Filaments: Medium-sized fibers (8–12 nm diameter) made of various proteins (e.g., keratins); provide mechanical strength and anchor organelles.

Cilia and Flagella

• Structures for cell movement; composed of microtubules arranged in a "9+2" pattern.

• Movement generated by dynein motor proteins using ATP, causing bending of the structure.

• Cilia are short and numerous; flagella are longer and usually singular.

Extracellular Structures and Cell Junctions

Plant Cell Walls

• Composed mainly of cellulose microfibrils synthesized by cellulose synthase.

• Primary cell wall: thin and flexible; secondary cell wall: thicker and more rigid.

• Middle lamella (rich in pectins) glues adjacent cells together.

Extracellular Matrix (ECM) in Animal Cells

• Composed of glycoproteins (e.g., collagen) and proteoglycans.

• Provides structural support, adhesion, movement, and regulation.

• Fibronectin connects ECM to integrins, which link to the cytoskeleton, facilitating mechanical signaling.

Intercellular Junctions

• Plasmodesmata (plants): Channels that connect the cytoplasm of adjacent cells for communication and transport.

• Animal cell junctions:

  • Tight junctions: Prevent leakage of extracellular fluid.

  • Desmosomes: Anchor cells together.

  • Gap junctions: Allow passage of ions and small molecules for communication.

Summary Table: Major Differences Between Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Size	0.1–5 µm	10–100 µm
DNA Location	Nucleoid	Nucleus
Examples	Bacteria, Archaea	Protists, Fungi, Plants, Animals

Key Equations

• Surface Area of a Sphere: $A = 4\pi r^2$

• Volume of a Sphere: $V = \frac{4}{3}\pi r^3$

• Surface Area-to-Volume Ratio: $\text{Ratio} = \frac{A}{V}$

Conclusion

Understanding the structure and function of eukaryotic cells, their organelles, and their interactions with the environment is fundamental to cell biology. The compartmentalization and specialization of eukaryotic cells underpin the complexity and diversity of life.