Table of Contents
Introduction to Cell Biology 🧬
Cell biology for the USMLE focuses on how cells are built, how they function, and how their structures relate to disease. You do not need a full research level understanding. You do need to connect specific organelles and pathways to classic clinical presentations and pharmacologic targets. In this chapter you will meet the core cellular components and processes that repeatedly appear in exam questions, especially when linked to genetic disorders, toxins, and drugs.
Cell biology here is about human eukaryotic cells. Bacterial cell structures and viral replication details belong to microbiology and will be covered separately. You will also see some overlap with molecular biology and metabolism, but in this chapter the emphasis stays on the structural and functional organization of the cell.
Eukaryotic Cell Organization 🧫
Human cells are eukaryotic. They have membrane bound organelles, a true nucleus, and a cytoskeleton. For exam purposes, you must recognize which organelles are involved in protein synthesis and processing, energy production, degradation, and structural support, and how defects affect specific tissues.
A very important clinical pattern is tissue specificity. Many cell biology diseases hit high energy tissues first. These include brain, skeletal muscle, heart, and kidney. When you see neurologic plus muscular problems together, think about mitochondrial or peroxisomal involvement as possible explanations.
Nucleus and Nuclear Transport 🧠
The nucleus contains the genetic material and is the site of transcription and ribosome assembly. It is separated from the cytoplasm by a nuclear envelope with nuclear pores. These pores regulate transport in and out of the nucleus.
Nuclear localization sequences are small amino acid sequences on proteins that must enter the nucleus, such as transcription factors and DNA replication proteins. These sequences are rich in positively charged amino acids, especially lysine and arginine. When the sequence is recognized, importins bind and shuttle the protein through the nuclear pore.
Nucleoli are dense regions within the nucleus. They are the sites of ribosomal RNA synthesis and partial ribosome assembly. In rapidly dividing cells such as malignant cells, nucleoli become very prominent because ribosome production is increased.
Nuclear changes are also important in pathology, but detailed patterns such as pyknosis or karyorrhexis belong mainly to general pathology. For cell biology, simply remember that the nucleus is central to gene expression and that drugs or toxins interfering with nuclear transport can alter cell function.
Proteins destined for the nucleus require a nuclear localization signal rich in lysine and arginine to pass through nuclear pores.
Ribosomes and Protein Synthesis 🧵
Ribosomes are the cellular machines that translate mRNA into protein. They are composed of ribosomal RNA and protein, and they exist as small and large subunits that come together during translation. In human cells the cytosolic ribosome is 80S, made of a 40S and a 60S subunit. Mitochondria have ribosomes that resemble bacterial 70S ribosomes, which explains why some antibiotics that act on bacterial ribosomes can also affect mitochondria.
There are two main locations for ribosomes. Free ribosomes float in the cytosol and synthesize proteins that remain in the cytosol or nucleus or are destined for organelles like mitochondria and peroxisomes. Membrane bound ribosomes attach to the rough endoplasmic reticulum and synthesize proteins destined for secretion, insertion into cell membranes, or for lysosomes.
A clinical pattern connected to ribosomes and protein synthesis is the fact that rapidly dividing cells are more sensitive to inhibitors of translation. Chemotherapy agents that block protein synthesis will strongly affect bone marrow, gastrointestinal epithelium, and hair follicles. Some toxins, such as diphtheria toxin, inactivate elongation factors and block protein synthesis, leading to cell death.
Endoplasmic Reticulum and Golgi Apparatus 📦
The endoplasmic reticulum is a network of membranes in the cytoplasm. It comes in two major forms, rough and smooth. The rough endoplasmic reticulum (RER) has ribosomes attached. It is the main site of synthesis for secreted proteins, membrane proteins, and lysosomal enzymes. Many antibodies are made in RER of plasma cells. Defects in RER function can impair production of key secreted proteins and can lead to misfolded protein accumulation.
The smooth endoplasmic reticulum (SER) lacks ribosomes. It is important for lipid and steroid synthesis and for detoxification of drugs and toxins. Hepatocytes have a well developed SER because of their role in drug metabolism. In skeletal muscle cells, a specialized form of smooth ER called sarcoplasmic reticulum stores and releases calcium for contraction.
The Golgi apparatus is located near the nucleus and receives proteins from the RER. In the Golgi, proteins are modified, sorted, and packaged into vesicles for transport. Common modifications include glycosylation, sulfation, and phosphorylation. The Golgi is also involved in adding mannose 6 phosphate groups to proteins that are targeted for lysosomes.
A classic exam relevant disorder of protein targeting is I cell disease. In this condition, the Golgi fails to add mannose 6 phosphate to lysosomal enzymes. These enzymes are then secreted instead of being delivered to lysosomes. The result is a buildup of undegraded substrates in lysosomes, coarse facial features, clouded corneas, and early death. This connects the abstract concept of protein sorting to a real clinical scenario.
Lysosomal enzymes require mannose 6 phosphate tagging in the Golgi to be targeted correctly to lysosomes. Failure of this process causes mis-sorting and storage disease.
Lysosomes, Peroxisomes, and Cellular Digestion 🧪
Lysosomes are membrane bound organelles filled with hydrolytic enzymes that function best in acidic pH. They digest material taken up from outside the cell by endocytosis and also break down intracellular components via autophagy. Many inherited storage diseases involve specific lysosomal enzyme deficiencies, which lead to accumulation of substrates. The specific stored material and tissues involved determine the clinical picture.
Peroxisomes are small organelles that participate in oxidative reactions. They break down very long chain fatty acids, certain amino acids, and detoxify hydrogen peroxide. Peroxisomes also participate in the synthesis of plasmalogens, which are important phospholipids in myelin. Peroxisomal biogenesis disorders such as Zellweger syndrome cause severe neurologic defects, hypotonia, and early death due to defective myelination and accumulation of toxic metabolites.
Autophagy is the process by which cells degrade their own components, especially in times of nutrient deprivation or when organelles are damaged. Portions of cytoplasm are enclosed by a membrane to form an autophagosome, which then fuses with a lysosome for degradation. Autophagy plays a role in cell survival, aging, and some neurodegenerative diseases, and it is an example of how the cell uses lysosomal machinery to adapt.
Mitochondria and Energy Production 🔋
Mitochondria are double membrane organelles that produce most of the cell’s ATP through oxidative phosphorylation. They have their own DNA, which is circular and inherited almost entirely from the mother. Mitochondrial DNA encodes some components of the electron transport chain, while most mitochondrial proteins are encoded by nuclear DNA and imported.
The inner mitochondrial membrane contains the electron transport chain complexes that generate a proton gradient. This gradient drives ATP synthase. The mitochondrial matrix houses enzymes for the tricarboxylic acid cycle and for beta oxidation of fatty acids. Because mitochondria are central to energy production, cells with higher energy demands contain more mitochondria.
Mitochondrial diseases typically present with a combination of neuromuscular symptoms, such as muscle weakness, exercise intolerance, and neurologic deficits. One key concept is heteroplasmy, the presence of a mixture of normal and mutant mitochondrial genomes within a cell. The proportion of mutated mitochondria can influence disease severity. Because of maternal inheritance, affected mothers can pass mitochondrial diseases to all children, while affected fathers usually do not.
Some drugs and toxins interfere with mitochondrial function. For example, cyanide inhibits cytochrome c oxidase in the electron transport chain, which rapidly blocks ATP production. Such agents often cause lactic acidosis and rapid organ failure because cells shift to anaerobic metabolism.
Human mitochondrial DNA is maternally inherited, and mitochondrial disorders often show heteroplasmy, which affects disease severity.
Cytoskeleton and Intracellular Transport 🚋
The cytoskeleton is a network of protein filaments that gives cells structure, maintains their shape, and allows movement and transport. It consists mainly of microfilaments, intermediate filaments, and microtubules. Each component has distinct functions and associated clinical correlations.
Microfilaments are made of actin. They play a central role in cell movement, muscle contraction, cytokinesis, and maintenance of cell shape. In muscle cells, actin interacts with myosin to generate contraction. Many cell surface specializations, such as microvilli in intestinal epithelial cells, depend on actin for structural support.
Intermediate filaments provide mechanical strength. They are more stable than microfilaments and microtubules. Different tissues express specific intermediate filament proteins. For example, keratin is found in epithelial cells, desmin in muscle cells, vimentin in mesenchymal cells, and glial fibrillary acidic protein in astrocytes. In pathology labs, immunostaining for intermediate filaments is used for tumor identification.
Microtubules are hollow tubes made of alpha and beta tubulin dimers. They organize the mitotic spindle, form the structural core of cilia and flagella, and act as tracks for intracellular transport. Motor proteins such as dynein and kinesin move cargo along microtubules. Dynein moves toward the minus end, typically toward the center of the cell, while kinesin generally moves toward the plus end, toward the periphery.
Several drugs target microtubules. Colchicine inhibits microtubule polymerization and is used in gout. Vinca alkaloids and taxanes used in cancer therapy disrupt microtubule dynamics and block mitosis. Because microtubules are essential for cell division, rapidly dividing cells are most sensitive to these drugs.
The axonemal structure of motile cilia contains a 9 plus 2 arrangement of microtubules. Dynein arms between microtubule doublets generate sliding forces that result in ciliary beating. Defects in dynein arms cause primary ciliary dyskinesia, with chronic respiratory infections, infertility due to immotile sperm or dysfunctional fallopian tube cilia, and sometimes situs inversus.
Microtubules are essential for mitotic spindle formation, intracellular transport, and ciliary function, so drugs that disrupt microtubules often cause mitotic arrest and ciliary dysfunction.
Cell Membrane and Membrane Transport 🚪
The plasma membrane is a selectively permeable barrier composed mainly of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. Phospholipids have hydrophilic heads and hydrophobic tails, which creates a fluid structure in which proteins can move laterally. Membrane proteins act as receptors, channels, transporters, and structural anchors.
Transport across the membrane can be passive or active. Simple diffusion occurs for small nonpolar molecules such as gases and some lipophilic drugs. Facilitated diffusion uses transport proteins but does not require energy, for example GLUT transporters for glucose. Active transport uses energy to move substances against a concentration gradient. Primary active transport directly uses ATP, as in the Na⁺/K⁺ ATPase. Secondary active transport uses the gradient created by primary transport, for example the Na⁺/glucose cotransporter.
The Na⁺/K⁺ ATPase is a central membrane protein that maintains low intracellular sodium and high intracellular potassium. It pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell per ATP hydrolyzed. This creates electrochemical gradients that are essential for membrane potential, cell volume control, and secondary transport processes. Cardiac glycosides like digoxin inhibit this pump by binding to the extracellular K⁺ site, which indirectly increases intracellular calcium in cardiac cells and enhances contractility.
Endocytosis allows cells to internalize extracellular material. In receptor mediated endocytosis, specific ligands bind membrane receptors, which then cluster in clathrin coated pits and are internalized. Low density lipoprotein uptake by cells is a classic example. Defects in LDL receptor mediated endocytosis cause familial hypercholesterolemia, with high LDL levels and early atherosclerosis.
Cell Cycle and Division ⏱️
The cell cycle is the sequence of phases that a cell passes through as it grows and divides. It includes G₁, S, G₂, and M phases, with some cells entering a resting state called G₀. G₁ is a growth phase in which the cell prepares for DNA synthesis. S phase is where DNA replication occurs. G₂ is another growth and checkpoint phase. M phase includes mitosis and cytokinesis.
Control of the cell cycle involves cyclins and cyclin dependent kinases. These complexes drive the cell through checkpoints, such as the G₁ to S checkpoint, by phosphorylating target proteins. Tumor suppressor proteins, such as retinoblastoma protein and p53, act as brakes that can halt the cycle in response to DNA damage. When these controls fail, unregulated cell proliferation can occur, which is central to cancer development.
Mitosis is ordinary somatic cell division. It results in two genetically identical daughter cells. Meiosis, which will be discussed in genetics, is a specialized division process for gamete formation. Many anticancer drugs target dividing cells by interfering with DNA replication or mitosis. As a result, high turnover tissues are commonly affected as side effects.
Some cells divide frequently, such as skin cells and cells of the gastrointestinal mucosa. Others are stable and can reenter the cell cycle when needed, such as hepatocytes. Some are considered permanent and rarely divide after birth, such as neurons and cardiac myocytes. These differences explain why some tissues regenerate well after injury while others form scars.
Cell cycle progression depends on cyclin and cyclin dependent kinase complexes, and disruption of tumor suppressors at checkpoints promotes uncontrolled cell proliferation.
Cell Junctions and Extracellular Matrix 🧱
Cells are not isolated. They connect with each other and with the extracellular matrix to form tissues. Cell junctions specialize in adhesion, communication, and barrier formation. Their dysfunction leads to characteristic clinical problems, especially in skin and mucous membranes.
Tight junctions, also called zonula occludens, form a seal between adjacent cells and limit paracellular movement of substances. In the intestinal epithelium, tight junctions help regulate what passes between the lumen and the bloodstream. Alterations in tight junctions contribute to barrier defects in inflammatory bowel diseases.
Adherens junctions and desmosomes are responsible for strong mechanical adhesion between cells. Adherens junctions connect actin filaments of neighboring cells and are important in maintaining epithelial integrity. Desmosomes connect intermediate filaments, particularly keratin, between cells, and are abundant in skin. Autoantibodies against desmosomal proteins cause pemphigus vulgaris, which presents with flaccid blisters and a positive Nikolsky sign.
Hemidesmosomes connect epithelial cells to the basement membrane. Autoantibodies against hemidesmosomes lead to bullous pemphigoid, with tense blisters and a negative Nikolsky sign. Recognizing which junction is involved helps in interpreting clinical and histologic descriptions.
Gap junctions consist of connexin proteins and form channels between cells that allow passage of ions and small molecules. They are important in synchronized activity, such as coordinated cardiac muscle contraction. Mutations in connexins can cause certain cardiac conduction or hearing defects.
The extracellular matrix is composed of structural proteins such as collagen and elastin, ground substance made mostly of proteoglycans and glycosaminoglycans, and adhesive proteins such as fibronectin and laminin. Cells interact with matrix through integrins. Defects in matrix components, such as collagen in Ehlers Danlos syndrome or elastin in some forms of emphysema, lead to tissue specific structural problems.
Cellular Responses to Stress and Injury 🩹
Cells constantly face stress from changes in workload, nutrients, and environmental factors. They adapt through reversible changes or, if the stress is too severe, they undergo injury and death. While the full discussion of pathology is covered elsewhere, some cell biology concepts are important here.
Adaptations include hypertrophy, hyperplasia, atrophy, and metaplasia. These involve changes in cell size, number, or differentiation state in response to stimuli. At the cellular level they reflect changes in gene expression, protein synthesis, organelle content, and cytoskeletal organization.
When stress exceeds adaptive capacity, cell injury occurs. Early or reversible injury includes cellular swelling, loss of microvilli, and membrane blebbing, mainly due to failure of ion pumps and water influx. If injury continues, mitochondrial damage, loss of membrane integrity, and activation of degradative enzymes lead to irreversible injury and cell death.
Apoptosis is a form of programmed cell death that uses caspase activation and maintains membrane integrity until late stages. It removes damaged or unnecessary cells without provoking inflammation. Necrosis is uncontrolled cell death associated with membrane rupture and inflammation. Many chemotherapy drugs induce apoptosis in cancer cells by activating intrinsic pathways that involve mitochondrial cytochrome c release.
Reactive oxygen species are byproducts of normal metabolism but can damage lipids, proteins, and DNA when present in excess. Cells possess antioxidant systems such as superoxide dismutase, catalase, and glutathione peroxidase to neutralize them. Imbalance between production and removal of reactive oxygen species leads to oxidative stress, which contributes to many diseases including atherosclerosis, neurodegeneration, and certain drug toxicities.
Reversible injury often features cellular swelling due to Na⁺/K⁺ ATPase failure, while irreversible injury is marked by loss of membrane integrity and mitochondrial damage, ending in cell death.