Premed · Premed · Cell Biology
Lecture 1: Overview of Cells and Methods in Cell Biology
Cell Biology
Learning Objectives
By the end of this lecture, students will be able to:
- Compare and contrast prokaryotic and eukaryotic cells
- Describe the major organelles and their functions
- Explain the cell theory and its historical development
- Identify key methods used in cell biology research
- Describe the principles of cell fractionation and centrifugation
Lecture Content
I. The Cell Theory
The cell theory stands as one of the foundational principles of biology. At its core, this theory holds that all living organisms are composed of one or more cells, that the cell is the basic unit of structure and organization in organisms, and that all cells arise from pre-existing cells, a principle articulated by Rudolf Virchow in 1855.
The development of cell theory was the product of several key historical contributions. Robert Hooke coined the term "cell" in 1665 while observing the compartmentalized structure of cork under a microscope. In the 1670s, Antonie van Leeuwenhoek became the first person to observe living cells, which he called "animalcules." The theory was formally unified for plants and animals by Matthias Schleiden and Theodor Schwann between 1838 and 1839.
Modern additions to cell theory have expanded its scope considerably. We now recognize that cells contain hereditary information in the form of DNA, which is passed to daughter cells during division. All cells share a fundamentally similar chemical composition, and energy flow occurs within cells through a network of metabolic reactions.
II. Prokaryotic vs. Eukaryotic Cells
Life on Earth is organized into two fundamental cell types: prokaryotic and eukaryotic. Understanding the differences between them is essential for appreciating the diversity and complexity of cellular life.
Prokaryotes, which include Bacteria and Archaea, are relatively simple cells that lack a membrane-bound nucleus. Their DNA resides in a region called the nucleoid and is typically organized as one or more circular chromosomes, often accompanied by small circular DNA elements called plasmids. Prokaryotic cells are generally smaller than eukaryotic cells, typically ranging from 0.1 to 10 micrometers in diameter. They lack membrane-bound organelles and instead carry out all metabolic functions in the cytoplasm or at the plasma membrane. A cell wall is present in most prokaryotes, composed of peptidoglycan in bacteria and more varied materials in archaea. Their ribosomes are 70S in size, composed of 30S and 50S subunits. Prokaryotes divide by binary fission and may possess external structures such as flagella, pili, and capsules.
Eukaryotes, encompassing animals, plants, fungi, and protists, are defined by the presence of a membrane-bound nucleus containing linear chromosomes. These cells are generally larger, spanning 10 to 100 micrometers, and feature an extensive internal membrane system known as the endomembrane system. Eukaryotic cells contain numerous membrane-bound organelles, including mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes. Their ribosomes are 80S, made of 40S and 60S subunits. Eukaryotic cells divide through the more complex processes of mitosis and meiosis, and they possess a cytoskeleton that provides both structural support and the machinery for intracellular motility.
<image>Side-by-side comparison of a prokaryotic cell and a eukaryotic animal cell. Panel A: Prokaryotic cell showing cell wall, plasma membrane, nucleoid region with circular DNA, ribosomes, flagellum, pili, and cytoplasm. Panel B: Eukaryotic animal cell showing nucleus with nuclear envelope and nucleolus, rough and smooth ER, Golgi apparatus, mitochondria, lysosomes, plasma membrane, cytoskeleton elements, and ribosomes. Scale bars indicate relative sizes (1 micrometer for prokaryote, 10 micrometers for eukaryote).</image>
III. Major Eukaryotic Organelles — Overview
Eukaryotic cells achieve their remarkable functional complexity through compartmentalization into organelles, each with a specialized role.
The nucleus is the defining organelle of eukaryotic cells. It houses the cell's DNA and is the site of both transcription and DNA replication. The nucleus is bounded by a double membrane called the nuclear envelope, which is perforated by nuclear pores that regulate transport between the nucleus and cytoplasm. Within the nucleus, the nucleolus serves as the site of ribosomal RNA synthesis.
The endoplasmic reticulum (ER) is a vast network of membranes that exists in two functional forms. The rough ER is studded with ribosomes on its cytoplasmic face and is the site of protein synthesis, folding, and initial processing. The smooth ER, which lacks ribosomes, specializes in lipid synthesis, drug detoxification, and calcium storage.
The Golgi apparatus receives proteins from the ER and further modifies, sorts, and packages them for delivery to their final destinations. It has a characteristic polarized structure, with a cis face that receives cargo and a trans face from which processed cargo is shipped.
Mitochondria are the primary sites of ATP production through oxidative phosphorylation. They are bounded by a double membrane and contain their own DNA, which is inherited maternally. Lysosomes function as the cell's digestive compartments, breaking down macromolecules in an acidic environment maintained at approximately pH 4.5 to 5.0. Peroxisomes carry out oxidative reactions including detoxification reactions and fatty acid beta-oxidation. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, provides structural support and drives cell motility. The plasma membrane acts as a selectively permeable barrier and participates in signal transduction. Plant cells possess additional organelles including chloroplasts for photosynthesis, a cellulose-based cell wall, a large central vacuole, and plasmodesmata that connect adjacent cells.
IV. Model Organisms in Cell Biology
Much of what we know about cell biology comes from the study of a small number of model organisms, each chosen for particular experimental advantages.
Escherichia coli serves as the preeminent prokaryotic model for studies in genetics and molecular biology. The budding yeast Saccharomyces cerevisiae is a simple eukaryote that has been invaluable for understanding the cell cycle and genetic regulation. The nematode Caenorhabditis elegans, with exactly 959 somatic cells in the adult, has been central to our understanding of development and apoptosis. The fruit fly Drosophila melanogaster has contributed enormously to our knowledge of genetics, development, and cell signaling. The mouse, Mus musculus, provides a mammalian model for studying physiology and disease. Finally, cultured mammalian cell lines such as HeLa, CHO, and HEK293 cells enable detailed in vitro studies. The HeLa cell line, derived from Henrietta Lacks in 1951, holds the distinction of being the first immortal human cell line and remains widely used today.
V. Key Methods in Cell Biology
Modern cell biology relies on a diverse toolkit of experimental methods. Cell culture techniques allow researchers to grow and study cells outside the organism. This includes maintaining primary cultures taken directly from tissue as well as established cell lines that can be propagated indefinitely. Cells are grown in specialized media supplemented with serum and antibiotics, in incubators that maintain conditions of 37 degrees Celsius and 5% CO2. An important consideration is the passage number of cultured cells, as most normal cells have a limited replicative lifespan known as the Hayflick limit.
Cell fractionation is a powerful approach for isolating specific organelles and cellular components. The process begins with homogenization, in which cells are broken open by mechanical, detergent-based, or osmotic methods. The resulting homogenate is then subjected to differential centrifugation, a series of sequential spins at increasing speeds. Low-speed centrifugation at approximately 1,000g pellets nuclei and unbroken cells. Medium-speed spins at roughly 20,000g bring down mitochondria, lysosomes, and peroxisomes. High-speed spins at about 80,000g pellet microsomes (fragments of the ER) and small vesicles. Very high-speed spins at approximately 150,000g sediment ribosomes and large macromolecules. For finer separations, density gradient centrifugation using sucrose or cesium chloride gradients can be employed. Rate-zonal centrifugation separates particles by size and shape, while isopycnic (equilibrium) centrifugation separates them by buoyant density.
<image>Diagram of differential centrifugation workflow. Panel A: Starting with a cell homogenate, sequential centrifugation steps at increasing g-forces (1,000g, 20,000g, 80,000g, 150,000g) showing pellet and supernatant at each step, with labels identifying the organelles enriched in each pellet fraction. Panel B: Sucrose density gradient centrifugation showing a tube with layered sucrose concentrations and distinct bands of separated organelles after ultracentrifugation.</image>
VI. Biochemical and Genetic Approaches
A range of biochemical and genetic tools complement the structural methods described above. For protein analysis, SDS-PAGE separates proteins by molecular weight, while Western blotting uses antibodies to detect specific proteins. Immunoprecipitation and co-immunoprecipitation techniques allow researchers to isolate protein complexes and identify interaction partners.
For nucleic acid analysis, PCR amplifies specific DNA sequences, and gel electrophoresis separates DNA or RNA by size. Southern blotting detects specific DNA sequences, while Northern blotting detects specific RNA molecules.
Genetic approaches fall into two complementary strategies. Forward genetics begins with an observed phenotype and works backward through mutagenesis screens, mapping, and cloning to identify the responsible gene. Reverse genetics starts with a known gene and disrupts it through knockouts, knockdowns, or RNA interference to determine the resulting phenotype. Reporter genes such as GFP and luciferase provide convenient tools for tracking gene expression in living cells.
Modern proteomics and genomics have dramatically expanded the scale of cell biology research. Mass spectrometry enables the identification and quantification of thousands of proteins, while next-generation sequencing technologies such as RNA-seq and ChIP-seq provide genome-wide views of gene expression and protein-DNA interactions.
<image>Flowchart comparing forward and reverse genetics approaches. Panel A (Forward genetics): Starting from an observed phenotype, arrows lead through mutagenesis screening, mapping, and cloning to identify the responsible gene. Panel B (Reverse genetics): Starting from a known gene sequence, arrows lead through targeted disruption (knockout, RNAi, CRISPR) to observing the resulting phenotype. Both panels converge on linking gene to function.</image>
