Cell Biology & Physiology: Epithelial Cells, Organelles, Gene Expression, and Secretion
Page 1: Epithelial Cell Morphology and Polarity
Cuboidal epithelial cells are discussed; cells vary in morphology and shape (e.g., some are round like erythrocytes).
Epithelial cells attach to tissues and exhibit polarity with distinct faces:
Apical surface: faces the 'outside' (the exterior surface such as skin or the lumen of the small intestine).
Basal surface (base): faces connective tissue or other tissue underneath.
In physiology, terminology often groups surfaces as basolateral (base + sides) and apical (the surface facing the lumen/exterior) because many important molecules reside on these specific membranes.
The apical surface and the basolateral surface have different molecular compositions; this affects transport and signaling across the epithelium.
Cells are held tightly together by junctional complexes (desmosomes and tight junctions) which prevent leakage and maintain distinct fluid compartments.
Fluids are compartmentalized: the fluid outside the basal side (between cells) is not the same as the apical fluid; each side has its own milieu.
When studying cells, note the organization of membranes and the surfaces they present to different environments.
Page 2: Intercellular Junctions and Fluid Compartments
Epithelial cells are attached to adjacent cells by junctions (desmosomes, tight junctions) that create a tight seal and alignment.
The basal side faces connective tissue; the apical side faces the lumen or outside world.
Basolateral membranes include the base and lateral sides; apical membranes face the lumen or exterior.
The fluids surrounding and between cells can be different in composition; the apical fluid is not the same as the basal/interstitial fluid.
Understanding junctions helps explain selective permeability, barrier function, and vectorial transport across epithelia.
Page 3: Organelles, Cytoskeleton, and Cellular Architecture
Cells differentiate by varying the abundance of organelles; some cells have more of certain organelles to support specialized functions.
Cytoskeleton: a network of fibers that anchors organelles, supports cell shape, and moves organelles and vesicles around the cell; it interacts with mitochondria and other organelles.
Endoplasmic reticulum (ER): a network of membranous sacs, continuous with the nuclear envelope.
Rough endoplasmic reticulum (RER): studded with ribosomes; site of synthesis for secreted and membrane-bound proteins.
Smooth endoplasmic reticulum (SER): lacks ribosomes; involved in lipid synthesis and other metabolic processes.
Mitochondria: powerhouse of the cell; primary function is to synthesize ATP (adenosine triphosphate), the energy currency of the cell.
ATP structure (conceptual): adenosine plus three phosphate groups; the covalent bond of the last phosphate stores energy for cellular reactions.
ATP is rapidly degraded; ATP turns over on the order of seconds, so cells continuously synthesize ATP.
Nutrients fed into mitochondria include glucose, fatty acids, and amino acids turned into acetyl-CoA, which enters the Krebs cycle to generate ATP with oxygen.
Cells also contain organelles such as lysosomes, peroxisomes, and components of vesicular transport.
Nuclei and nuclear constituents (nuclear envelope, nucleolus) play roles in genetic control and ribosome production.
Page 4: ATP, Metabolism, and Oxygen Use
Primary function of mitochondria is ATP production; ATP provides energy for all cellular reactions.
The interaction between nutrients and mitochondria: glucose, fatty acids, and amino acids are converted to acetyl-CoA and enter the Krebs cycle; oxygen is used to drive electron transport and ATP synthesis; carbon dioxide is produced as a waste product but has additional roles in metabolism.
The overall yield for glucose metabolism is commonly cited as 38 ATP per glucose, though some sources report 36 or 38 depending on assumptions. In this discussion, the net yield is described as 36 ATP per glucose after accounting for ATP used in transport and other early steps.
Net ATP per glucose:
Total theoretical yield:
The respiratory quotient (RQ) and the amount of oxygen consumed per glucose molecule are topics for later discussion; the basic idea is to relate oxygen use to CO₂ production and ATP yield.
ATP participates in active transport, muscle contraction, and other ATP-dependent cellular movements; passive processes do not require ATP.
Page 5: Nucleus, Transcription, and the Central Dogma
The nucleus features a nuclear envelope that protects genetic material and regulates traffic of RNA and proteins in and out of the nucleus; prokaryotes lack a nucleus and regulate traffic differently.
Transcription is the process by which genes in DNA are copied into messenger RNA (mRNA) inside the nucleus.
Messenger RNA (mRNA): carries the genetic message from DNA to the cytoplasm where it will be translated into protein; DNA cannot leave the nucleus, so mRNA serves as the intermediary.
RNA types central to translation:
Messenger RNA (mRNA): carries codons that specify amino acids.
Transfer RNA (tRNA): carries specific amino acids and recognizes codons on mRNA through anticodons.
Ribosomal RNA (rRNA): a major structural and functional component of ribosomes.
Nucleolus: region within the nucleus where ribosomal RNA (rRNA) synthesis and ribosome assembly begin.
The central dogma of life (as discussed): DNA -> RNA -> Protein. DNA is transcribed to RNA in the nucleus; RNA is translated to protein in the cytoplasm by ribosomes.
The endomembrane system (ER and Golgi) is involved in processing and trafficking proteins destined for secretion or for membranes.
Differences between eukaryotes and prokaryotes can influence drug targeting and side effects of medications.
A single amino acid change can lead to disease; the immune and transport systems rely on properly folded and functional proteins.
Page 6: Protein Synthesis, Trafficking, and Secretory Pathways
Messenger RNA binds to ribosomes in the cytoplasm; translation converts nucleic acid language to protein language by assembling amino acids according to codon sequence.
Codons: sequences of three nucleotides on mRNA that specify a particular amino acid. In translation, the codon is recognized by a corresponding transfer RNA (tRNA) carrying the correct amino acid.
Codon length:
There are 20 naturally occurring amino acids; 10 are essential and must be obtained from the diet.
Ribosomes are composed of ribosomal RNA (rRNA) and proteins; they can be free in the cytoplasm or attached to the rough endoplasmic reticulum (RER).
Protein trafficking example: insulin synthesis in pancreatic beta cells
Insulin gene is expressed in beta cells; ribosomes on the rough ER synthesize the insulin precursor protein into the ER lumen.
Insulin is processed, folded with the help of chaperones, and transported via vesicles to the Golgi apparatus for further modification and packaging.
Secretory vesicles then fuse with the plasma membrane to secrete insulin (exocytosis).
Golgi apparatus functions as a packaging and modification center for proteins to be released outside the cell or delivered to specific cellular compartments.
Vesicles: various vesicles carry different contents; some are lysosomes containing digestive enzymes (e.g., lysozyme).
Chaperones (chaperonins) assist in proper protein folding; misfolding can occur due to wrong pH, salt, or faulty chaperone activity, potentially leading to disease.
Central dogma recap in terms of sequencing and organization: DNA -> transcription to mRNA in the nucleus; mRNA -> translation to protein in the cytoplasm; proteins are then modified and trafficked via the Golgi and vesicles.
The genetic basis of disease is reinforced by the example of cystic fibrosis (CF): a mutation in a chloride channel leads to thick mucus and multi-systemic symptoms.
A note on source attribution: figures from textbooks and publishers are cited, and the end slides contain references; the exact materials are for illustration and do not require memorization.
Page 7: Cystic Fibrosis: Genetics and Pathophysiology
Cystic fibrosis (CF) is a syndrome characterized by thick mucus affecting the respiratory, digestive, and reproductive systems.
Pathophysiology: CF is caused by mutations in a chloride channel (CFTR) that impairs chloride transport to the apical surface of epithelia. This reduces water movement to the luminal surface, resulting in thick, sticky mucus.
The observed clinical consequences include recurrent respiratory infections, malabsorption in the digestive tract, and fertility issues due to mucus in the reproductive tract.
The general mechanism described: a chloride channel is absent or nonfunctional, leading to reduced water movement into the lumen and thick mucus.
A common CF mutation discussed is ΔF508 (ΔF508), which represents deletion of phenylalanine at position 508 in the CFTR protein; it is used as an example of a mutation that can disrupt channel function (not necessary to memorize the exact mutation in exams here).
Ethnic prevalence data (illustrative, not for memorization practice):
Caucasians: about 1 in 2,000 are heterozygous for the mutation.
African Americans: about 1 in 17,000.
Asian Americans: about 1 in 31,000.
The description also emphasizes that genetic diseases vary by ethnicity due to allele frequencies and mating patterns within populations.
The idea of the mutation and chromosome mapping discussed: chromosome 7 and the location of the CFTR gene is mentioned as a conceptual example; the notation 7 (chromosome 7) and q (long arm) indicates a region where a mutation may be described (the exact region name is not memorized in this context).
Page 8: The Three Types of RNA and the Code-to-Protein Translation
The three major RNA types involved in translation are described: mRNA, tRNA, and rRNA.
Protein synthesis starts with a gene encoded in DNA; transcription in the nucleus produces mRNA with a language (nucleotides) that is different from the protein language (amino acids).
Translation occurs in the cytoplasm on ribosomes, where mRNA codons are read and matched by tRNA anticodons carrying the appropriate amino acids.
The sequence of amino acids forms a polypeptide chain, which then folds into a functional protein with the aid of chaperones.
The central dogma is reinforced: DNA -> RNA -> Protein.
A few practical notes from the slide:
Ribosomes are primarily composed of rRNA and proteins.
The translation process can be affected by single-nucleotide changes in mRNA that alter the encoded amino acid; some mutations may be neutral, while others disrupt function and lead to disease.
The link between amino acid properties and membrane proteins is highlighted: transmembrane segments require lipophilic (hydrophobic) amino acids to span the lipid bilayer; changes to these residues can disrupt membrane integration and function.
An example of advanced work in tissue engineering (mentioned illustrate): growing tissue structures (e.g., pinna) in animal models demonstrates challenges such as establishing vascular (blood) supply; cornea is naturally avascular, illustrating diffusion of oxygen and nutrients from the surface.
Page 9: Vesicles, Secretion, and Endocytosis
Vesicles and secretion are tightly regulated processes (secretory vesicles, exocytosis) to release substances such as insulin into the bloodstream or other tissues.
Vesicle docking and fusion with the plasma membrane involve linking proteins (often referred to as docking or tethering proteins) that act like anchors to position the vesicle near the membrane until a triggering event causes fusion and release.
The location of secretion (apical vs basal surface) depends on the cell type and function. Some secretions are directed toward ducts, others into connective tissue or the bloodstream.
Endocytosis is the process of bringing substances into the cell. A key player is clathrin, a protein that forms a coat around a budding vesicle; receptors on the cell surface bind target molecules, trigger membrane invagination, and recruit actin/myosin to form the vesicle.
Phagocytosis is a form of endocytosis typically used by leukocytes to ingest bacteria; general endocytosis occurs in many cell types.
The cytoskeleton participates in endocytosis and vesicle trafficking; after vesicle formation, clathrin coat is removed and vesicles fuse with target membranes.
Absorption can occur via endocytosis, not only via other uptake mechanisms.
A thought experiment is introduced to illustrate diffusion across a permeable membrane:
On the right is intracellular fluid (ICF); on the left is extracellular fluid (ECF).
The red line represents the plasma membrane.
The question: If the membrane were permeable to the listed substances, in which direction would each diffuse based on concentration gradients?
The instructor notes this will be revisited on Wednesday after discussing cytoskeleton.
Page 10: Review, Connections, and Practical Implications
The lecture ties together structure and function: morphology, polarity, organelle distribution, and molecular processes underlie the function of cells and tissues.
Practical implications include: understanding why CF leads to multi-systemic symptoms, how mutations in single amino acids can alter protein function and lead to disease, and why targeted therapies must consider cell compartmentalization and trafficking.
The content connects to foundational principles such as energy production (ATP), gene expression (central dogma), membrane transport (active vs passive), protein folding (chaperones), vesicular trafficking, and the interplay between structure and function in physiology.
Ethical/practical implications: genetic diseases are inherited and vary by ethnicity; the management of such diseases emphasizes symptom relief and quality of life, given that many genetic defects cannot be cured by current therapies.
Acknowledgment of sources and figures used in teaching materials; the instructor emphasizes not memorizing every figure but understanding the concepts.
Final reminder: more details on cytoskeleton and specific cellular transport mechanisms will be covered in subsequent lectures (Wednesday).
Quick reference: Key formulas and numerical notes
Net ATP per glucose (cellular respiration): produced, but are net after accounting for ATP used during metabolism.
Codon length in translation:
ATP structure (conceptual): adenosine plus three phosphate groups; hydrolysis of the last phosphate releases energy for cellular work.
Cystic fibrosis mutation example: indicating deletion of phenylalanine at position 508 in the CFTR protein (illustrative of a nonfunctional chloride channel).
Mucus pathology in CF explained by reduced chloride transport to the apical surface, leading to less water movement and thicker mucus.
Ethnic prevalence examples (illustrative, not for memorization): Caucasians ~1/2000 heterozygous; African Americans ~1/17,000; Asian Americans ~1/31,000.
Basic units referenced: 100 μm size for the ovum as a relatable human-scale example; erythrocytes are small and organelles like RBCs lack nuclei in mammals.