Biology 1 - Lesson 5: Cell Structure and Organelles
Cells are the fundamental building blocks of life, providing the organized environment where biochemical reactions occur. They come in two major categories—prokaryotic and eukaryotic—distinguished primarily by the presence or absence of membrane-bound organelles. This organizational complexity is pivotal for compartmentalizing reactions, regulating metabolic pathways, and integrating signals to maintain homeostasis. This lesson offers a comprehensive look at eukaryotic cell architecture, key organelles, and their functions.
Prokaryotic vs. Eukaryotic Cells
Prokaryotes (Bacteria, Archaea) are typically smaller (0.5–5 µm), lacking a nucleus and other membrane-enclosed structures. Eukaryotes (Protists, Fungi, Plants, Animals) are larger (10–100 µm) and internally compartmentalized with diverse organelles. Despite these differences, both cell types share core features:
- Plasma Membrane: A selective barrier that regulates transport of substances.
- Cytoplasm: The semi-fluid interior where metabolic processes occur.
- DNA: Genetic material encoding proteins. Prokaryotes house it in a nucleoid region, while eukaryotes enclose it in a nucleus.
- Ribosomes: Molecular machines synthesizing proteins, though eukaryotic ribosomes differ slightly in structure from prokaryotic ones.
Organelle Overview
Below is a table summarizing major eukaryotic organelles and their primary roles:
| Organelle | Key Function(s) | Notes |
|---|---|---|
| Nucleus | Stores genetic material, site of transcription | Enclosed by nuclear envelope; contains nucleolus |
| Endoplasmic Reticulum (ER) | Protein synthesis (RER), lipid synthesis (SER) | RER has ribosomes; SER lacks them |
| Golgi Apparatus | Protein/lipid modification, sorting, packaging | Flattened sacs called cisternae; “cis” to “trans” direction |
| Mitochondria | Aerobic respiration, ATP production | Double membrane; has own DNA and ribosomes |
| Chloroplasts | Photosynthesis (in plants, algae) | Double membrane; has own DNA, thylakoids, stroma |
| Lysosomes | Digestion of macromolecules, recycling | Contain hydrolytic enzymes; mainly in animal cells |
| Vacuoles | Storage of substances, maintain turgor pressure | Prominent in plant cells, can store ions, pigments |
| Peroxisomes | Breakdown of fatty acids, detoxification | Contain oxidative enzymes (catalase, etc.) |
| Cytoskeleton | Structural support, transport, cell division | Microtubules, microfilaments, intermediate filaments |
The Nucleus
The nucleus houses chromosomes—DNA-protein complexes known as chromatin when unwound. Gene expression primarily occurs here, with mRNA transcripts exiting via nuclear pores. The nucleolus synthesizes rRNA, assembling it with ribosomal proteins into ribosomal subunits. These subunits then pass into the cytoplasm for final assembly into ribosomes.
Endomembrane System
This system coordinates protein and lipid synthesis, modification, and transport. Proteins produced on RER-bound ribosomes enter the ER lumen, undergo folding or modifications, then move to the Golgi Apparatus for final processing (e.g., glycosylation). They exit via vesicles and can be secreted or targeted to specific organelles.
The diagram above shows how materials flow through the ER–Golgi–vesicle pathway. Lysosomes, specialized for digestion, often emerge from Golgi-derived vesicles loaded with hydrolytic enzymes.
Mitochondria and Chloroplasts
These double-membraned organelles convert energy into forms cells can harness:
- Mitochondria: Responsible for aerobic respiration, forming ATP via oxidative phosphorylation. The inner membrane folds into cristae, maximizing surface area for the electron transport chain.
- Chloroplasts (in photosynthetic organisms): Capture light energy in thylakoid membranes, producing sugars (e.g., glucose). Oxygen is released as a byproduct. Starch grains in chloroplasts can store excess carbohydrates.
Both contain their own genomes and ribosomes, consistent with the endosymbiotic theory that they originated from engulfed prokaryotic cells.
Cytoskeleton
A dynamic network of protein filaments helps maintain cell shape, organize organelles, and facilitate movement:
- Microtubules: Tubulin polymers forming hollow rods. They direct vesicle transport, form the spindle in cell division, and make up structures like cilia and flagella.
- Microfilaments: Actin strands that enable muscle contraction, cell crawling, and changes in cell shape.
- Intermediate Filaments: Provide mechanical strength (e.g., keratins in epithelial cells). They help maintain cell rigidity under stress.
Extracellular Structures
Cells extend beyond their plasma membranes, forming protective or connective layers:
- Cell Walls: In plants (cellulose), fungi (chitin), some protists. They provide structural integrity, limit water intake, and defend against pathogens.
- Extracellular Matrix (ECM) in Animals: Composed of collagen, fibronectin, and other glycoproteins. It plays a pivotal role in cell adhesion, intercellular signaling, and tissue organization.
- Intercellular Junctions: Tight junctions create seals between cells; desmosomes anchor cells to each other; gap junctions allow small molecules or ions to pass for direct communication.
Microscopy and Cell Visualization
The study of cell structure heavily relies on microscopy. Techniques include:
- Light Microscopy: Suitable for observing living cells, though limited in resolution (~200 nm).
- Electron Microscopy: Offers higher resolution (TEM, SEM) but requires extensive sample preparation, often killing cells.
- Fluorescence Microscopy: Uses fluorescent tags or dyes to highlight specific proteins or organelles, enabling visualization of dynamic processes in live cells.
Advances like super-resolution microscopy continue to push the boundaries of what we can visualize within cells, revealing fine details about organelle structure and molecular organization.
Relative Organelle Volumes
Different cell types vary in organelle abundance based on functional needs. Below is a sample bar chart showing approximate relative volumes of major organelles in a hypothetical eukaryotic cell.
Summary
Eukaryotic cells function as dynamic communities of specialized compartments, each carrying out unique yet interdependent tasks. The nucleus manages genetic operations, the endomembrane system handles macromolecule trafficking, and mitochondria and chloroplasts drive energy transformations. Coupled with the cytoskeleton’s scaffolding, this intricate organization underlies cellular behavior, growth, and adaptation. A strong grasp of organelle structure and function not only offers insights into normal physiology but also sheds light on diseases where these processes go awry.
Suggested Reading:
Molecular Biology of the Cell (Alberts et al.) – chapters on organelle functions and compartmentalization
Cell Biology by Pollard and Earnshaw – sections on cytoskeletal organization, endomembrane trafficking, and cell surfaces
