Clegg 1 (second half, more in-depth)
Eukaryotic Organisms
Can be unicellular (e.g., yeasts, protists) or multicellular (e.g., plants, animals).
Have a membrane-bound "nucleus" which contains most (but not all) of the DNA.
Eukaryotic cells are usually larger, ranging from 10 to 100 times larger than prokaryotic cells.
Have complex, organized cytoplasm with many organelles, a cytoskeleton, and an endomembrane system.
Animal Cell Structure
Flagellum: A whip-like structure that facilitates movement.
Centrosome: Involved in organizing microtubules; important for cell division.
Cytoskeleton: Composed of:
Microfilaments: Support cell shape and are involved in motility.
Intermediate Filaments: Provide tensile strength.
Microtubules: Provide rigidity and cell shape, involved in intracellular transport.
Microvilli: Extensions of the cell membrane that increase surface area for absorption.
Peroxisome: Contains enzymes for lipid metabolism and detoxification.
Endoplasmic Reticulum (ER): Includes:
Rough ER: Studded with ribosomes; involved in protein synthesis.
Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.
Nucleus: Contains genetic material, nuclear envelope, nucleolus, and chromatin.
Plasma membrane: Encloses the cell contents.
Ribosomes: Sites of protein synthesis.
Mitochondrion: Powerhouse of the cell, produces ATP.
Golgi apparatus: Modifies, sorts, and packages proteins for secretion.
Lysosome: Contains digestive enzymes for waste processing.
Plant Cell Structure
Nuclear envelope: Transparent barrier around the nucleus.
Nucleus: Contains genetic material, nucleolus, chromatin, and is surrounded by the nucleus envelope.
Endoplasmic Reticulum (Rough and Smooth): Functions in protein and lipid synthesis.
Golgi apparatus: Functions in modification and sorting of proteins.
Mitochondrion: Major site for ATP production.
Peroxisome: Similar function to that in animal cells.
Plasma membrane: Encases the cell contents.
Ribosomes: Present for protein synthesis.
Central vacuole: Stores nutrients and waste products; provides structural support.
Microfilaments and Microtubules: Cytoskeletal elements.
Chloroplast: Site of photosynthesis, contains chlorophyll.
Cell wall: Provides structure and protection.
Plasmodesmata: Channels between adjacent plant cells for communication and material exchange.
The Nucleus
Largest organelle in the cell.
Contains chromosomes that are organized within the structure.
Bound by a double membrane known as the nuclear envelope.
Supported by a nuclear lamina inside the membrane.
Has nuclear pores that selectively regulate traffic in and out of the nucleus.
Nucleolus and Regulation of Genes
Nucleolus: Site where ribosomal RNA is synthesized and ribosomes are assembled.
Other genes are localized to specific areas in the nucleus, but their regulation is still poorly understood.
Nuclear Envelope
Composed of an outer nuclear membrane (ONM) and an inner nuclear membrane (INM).
Continuous with the endoplasmic reticulum (ER) and part of the endomembrane system.
Specific proteins localized to ONM and INM.
Nuclear Lamina
A meshwork of cytoskeletal proteins located inside the nuclear envelope.
Comprised of proteins known as lamin A, B, and C.
Prevents compression of the nucleus.
Progeria and Lamin A Mutations
Mutations in lamin A can cause Progeria, a rare disease characterized by premature aging, inspired F. Scott Fitzgerald’s “The Curious Case of Benjamin Button.”
Nuclear Pore Complex
A multicomponent structure forming channels that allow macromolecules to pass through.
mRNAs bound to proteins exit, and nuclear proteins, like transcription factors, enter.
Nuclear proteins carry a nuclear localization signal recognized by the nuclear pore.
Nuclear Localization Signal (NLS)
Example of a typical NLS: pro - pro - lys - lys - lys - arg - lys - val (ppkkkrkv).
Recognized by a protein called importin, which delivers the protein through the nuclear pore.
c-myc Transcription Factor
A protein that binds to DNA, activating transcription of genes linked to cell replication.
Functions as a proto-oncogene; overexpression can lead to uncontrolled growth and tumor formation.
Identified as a nuclear protein with a specific NLS: PAAKRVKLD.
Medical Mystery
Symptoms of a condition may include excessive thirst, frequent urination, fatigue, weight loss, itchiness, and blurred vision.
Mitochondria
Structure: Approximately 1.5 µm x 2-8 µm; size similar to a bacterial cell; can vary in number per cell depending on the type and metabolic activity.
Function: Primary role is energy production through processes like the Citric Acid Cycle and Electron Transport Chain; also involved in calcium regulation and programmed cell death.
Growth: Reproduce via binary fission, similar to bacteria.
Mitochondrial Structure
Encased by a double membrane:
Outer membrane: Smooth and semi-permeable, containing numerous transport complexes.
Inner membrane: Folded into structures known as cristae, which increase surface area and house ATP synthesizing enzymes (e.g., ATP synthase).
The matrix of the mitochondrion contains its own DNA and ribosomes.
Mitochondrial DNA
Genetics: Mitochondrial DNA (mtDNA) is circular, approximately 16.5 Kb, encoding rRNA, tRNA, ATP synthase, and Electron Transport Chain proteins.
Proteins necessary for mitochondrial function are encoded by nuclear DNA and imported into mitochondria.
All mitochondria are inherited from one's mother.
Mitochondrial Function
Primary role: ATP production through:
Citric Acid Cycle: Processes pyruvate from glycolysis, producing GTP, NADH, and FADH2 while releasing CO2.
Electron Transport Chain: Electrons from NADH and FADH2 are transferred through proteins in the inner membrane, with the addition of electrons to O2, pumping H+ ions to create a gradient. H+ returns through ATP synthase to generate ATP.
Chloroplasts
Structure: 2-10 µm long and 2-3 µm thick. Chloroplasts contain an outer and inner membrane and a highly folded third membrane system called thylakoid membranes.
Function: The thylakoid membranes are key components in photosynthesis, containing proteins that absorb sunlight.
Inside the chloroplast is called stroma, which contains DNA and ribosomes and approximately 100 genes coding for proteins necessary for photosynthesis.
Photosynthesis in Chloroplasts
Light Reactions: Occur in the thylakoids, generating O2, ATP, and NADPH.
Dark Reactions (Calvin Cycle): Utilize ATP and NADPH to convert CO2 into carbohydrates in the stroma.
Origin of Organelles
Mitochondria and chloroplasts are believed to have originated from the endosymbiotic event, where large eukaryotes engulfed small prokaryotes.
Mitochondria are thought to descend from proteobacteria, while chloroplasts are believed to be remnants of cyanobacteria (blue-green algae).
Peroxisomes
Structure: Very small (0.2 - 1.7 µm) vesicles surrounded by a single membrane, often containing crystalline arrays of enzymes.
Function: In animal cells, primarily detoxifying toxic peroxides such as hydrogen peroxide, which is converted into water and oxygen through catalase reaction (2 H2O2 → 2 H2O + O2).
Diabetes Mellitus
Characteristics: High blood sugar levels, weakness, lethargy, weight loss, and other symptoms.
Type I Diabetes (Juvenile): Caused by the autoimmune destruction of insulin-producing beta cells in the pancreas.
Type II Diabetes: Characterized by decreased responsiveness to insulin.
Insulin
Discovery: Insulin was discovered by Banting and Best in the 1920s, first treated in patients in 1922.
Function: Insulin is a peptide hormone produced by beta cells in the pancreas. It signals cells to import glucose, playing a crucial role in glucose metabolism.
Structure: Composed of 51 amino acids and roughly weighing 5000 Daltons.
Secretion Process: Triggered by elevated blood glucose levels after meals.
The Endomembrane System
Description: A dense network of closed membrane tubules, vesicles, and sacs that divide the cytoplasm into compartments, functioning in transport and processing of cellular materials.
Components: Includes:
Rough ER: Involved in the synthesis of integral and secretory proteins.
Smooth ER: Functions in lipid synthesis and detoxification.
Golgi Complex: Modifies proteins and carbohydrates.
Vesicles: Transfer materials between endomembrane compartments.
Functions of the Endoplasmic Reticulum
Smooth ER Functions
Lipid Synthesis: Creates lipids for membranes.
Detoxification: Uses enzymes (like cytochrome P450) to make hydrophobic toxins more hydrophilic, facilitating their excretion.
Rough ER Functions
Secretory Proteins: Ribosomes synthesize and translocate secreted proteins into the cisternal space.
Membrane Proteins: Integral membrane proteins are produced here.
Chemical Modification: Proteins undergo modifications such as glycosylation, adding sugar residues.
Vesicle Formation: Vesicles pinch off to transport proteins to the Golgi Complex.
Signal Sequence and Protein Transport
Signal Sequence: A short peptide (~20 amino acids) at the N-terminus directs the ribosome to the rough ER, where it is subsequently removed in the lumen of the ER.
The Golgi Apparatus
Functions like the cell's “post office.”
Process:
Vesicles from the ER fuse with the Cis region.
Proteins move through the stacked structure, maturing from Cis to Trans regions.
Vesicles pinch off from the Trans region for secretion or insertion into the plasma membrane.
Eukaryotic Organisms
Can be unicellular (e.g., yeasts, protists) or multicellular (e.g., plants, animals). Their complexity allows for specialization of cells in multicellular organisms.
Have a membrane-bound "nucleus" which contains most (but not all) of the DNA in the form of chromosomes. A minor amount of DNA is also found in mitochondria and chloroplasts.
Eukaryotic cells are usually larger, ranging from 10 to 100 µm (micrometers) in diameter, which is 10 to 100 times larger than prokaryotic cells. This larger size is facilitated by their complex internal membrane systems.
Have complex, organized cytoplasm with many specialized membrane-bound organelles, a dynamic cytoskeleton (providing structural support and facilitating movement), and an extensive endomembrane system for synthesis and transport.
Animal Cell Structure
Flagellum: A long, whip-like appendage that facilitates cell motility; present in some animal cells (e.g., sperm cells) for movement through aqueous environments. It's composed of microtubules in a arrangement.
Centrosome: The primary microtubule-organizing center (MTOC) in animal cells, typically located near the nucleus. It consists of two centrioles arranged perpendicularly and is crucial for forming the spindle fibers during cell division.
Cytoskeleton: A dynamic network of protein filaments extending throughout the cytoplasm, essential for cell shape, internal organization, and movement.
Microfilaments (Actin Filaments): Helical polymers of actin protein, approximately nm in diameter. They are involved in cell motility (e.g., amoeboid movement, muscle contraction), cytokinesis, and maintaining cell shape.
Intermediate Filaments: Ranging from nm in diameter, these are diverse and stable rope-like protein fibers (e.g., keratins, lamins). They provide tensile strength, anchoring organelles, and maintaining cell integrity, especially in tissues subjected to mechanical stress.
Microtubules: Hollow cylinders made of tubulin protein, approximately nm in diameter. They originate from the centrosome, provide rigidity, maintain cell shape, and serve as tracks for motor proteins (kinesin and dynein) for intracellular transport of vesicles and organelles. They also form cilia and flagella.
Microvilli: Finger-like extensions of the plasma membrane that increase the surface area for absorption, particularly abundant in cells lining the small intestine.
Peroxisome: Small, spherical organelles that contain oxidative enzymes. They are involved in lipid metabolism (e.g., breakdown of very long chain fatty acids) and the detoxification of harmful substances, producing hydrogen peroxide () as a byproduct, which is then converted into water and oxygen by catalase.
Endoplasmic Reticulum (ER): A continuous network of membrane-bound tubules and flattened sacs (cisternae) forming an extensive part of the endomembrane system.
Rough ER (RER): Studded with ribosomes on its cytosolic surface, responsible for the synthesis, folding, modification (e.g., glycosylation), and quality control of proteins destined for secretion, insertion into membranes, or delivery to other organelles (e.g., Golgi, lysosomes).
Smooth ER (SER): Lacks ribosomes and is involved in diverse metabolic processes, including lipid synthesis (phospholipids, steroids), carbohydrate metabolism, detoxification of drugs and poisons (especially in liver cells), and storage of calcium ions ().
Nucleus: The most prominent organelle, housing the cell's genetic material (chromosomes) and controlling gene expression. It contains the nuclear envelope, nucleolus, and chromatin.
Plasma membrane: A selectively permeable phospholipid bilayer that encloses the cell's contents, regulating the passage of substances into and out of the cell and mediating cell-cell communication.
Ribosomes: Complexes of ribosomal RNA (rRNA) and protein, responsible for protein synthesis (translation). Free ribosomes in the cytosol synthesize proteins that function within the cytoplasm, while ER-bound ribosomes synthesize proteins for the endomembrane system or secretion.
Mitochondrion: Often referred to as the "powerhouse of the cell," this organelle is the primary site of aerobic respiration, producing adenosine triphosphate (ATP) through the Citric Acid Cycle and Electron Transport Chain. It also plays roles in calcium signaling and programmed cell death.
Golgi apparatus (or Golgi Complex): A series of flattened membrane-bound sacs (cisternae) arranged in stacks. It further modifies, sorts, and packages proteins and lipids synthesized in the ER into vesicles for secretion, delivery to other organelles, or insertion into the plasma membrane.
Lysosome: Spherical organelles containing a variety of hydrolytic enzymes that function best in acidic conditions (). They are involved in degrading waste materials, cellular debris, and foreign invaders (e.g., bacteria) through phagocytosis and autophagy.
Plant Cell Structure
Nuclear envelope: A double membrane that encloses the nucleus, perforated by nuclear pores to regulate molecular traffic.
Nucleus: Contains the cell's genetic material (DNA), organized into chromosomes. It includes the nucleolus (site of rRNA synthesis) and chromatin and is surrounded by the nuclear envelope.
Endoplasmic Reticulum (Rough and Smooth): Extensive network of membranes involved in protein (RER) and lipid (SER) synthesis, modification, and transport. The RER in plant cells also participates in cell wall formation by synthesizing components.
Golgi apparatus: Functions in modification, sorting, and packaging of proteins and lipids. In plant cells, it is also crucial for synthesizing polysaccharides for the cell wall (e.g., hemicellulose and pectin).
Mitochondrion: Major site for ATP production through cellular respiration, similar to animal cells.
Peroxisome: Similar function to animal cells, involved in metabolic processes, but in plants, they also play a crucial role in photorespiration, converting fatty acids to sugars, especially important in germinating seeds.
Plasma membrane: Encases the cell contents, controlling entry and exit of substances and mediating cellular responses.
Ribosomes: Present for protein synthesis, both free in the cytosol and attached to the ER.
Central vacuole: A large, dominant organelle in mature plant cells, enclosed by a tonoplast. It stores water, nutrients, ions, secondary metabolites, and waste products; maintains turgor pressure against the cell wall, providing structural support; and can function as a lysosome.
Microfilaments and Microtubules: Cytoskeletal elements that maintain cell shape, provide structural support, guide cell plate formation during cytokinesis, and facilitate organelle movement.
Chloroplast: Unique to plant cells and other photosynthetic eukaryotes. These are the sites of photosynthesis, containing chlorophyll and other pigments that capture light energy to synthesize organic compounds. They have an intricate internal membrane system of thylakoids arranged into grana.
Cell wall: A rigid outer layer composed primarily of cellulose, providing structural support, protection against mechanical stress and osmotic lysis, and defining cell shape. It is permeable, allowing water and solutes to pass through.
Plasmodesmata: Small channels that traverse the cell walls of adjacent plant cells, connecting their cytoplasm and enabling the direct intercellular exchange of water, nutrients, signaling molecules, and even small proteins and RNA.
The Nucleus
Largest organelle in a eukaryotic cell, typically spherical or oval, ranging from µm in diameter. Its prominent size reflects its central role in cellular function.
Contains chromosomes that are organized within its structure. Each chromosome is a single molecule of DNA tightly coiled around histone proteins, forming chromatin.
Bound by a double membrane known as the nuclear envelope, which separates the nucleoplasm from the cytoplasm.
Supported by a nuclear lamina located just inside the inner nuclear membrane. This meshwork of intermediate filaments helps maintain nuclear shape and organizes chromatin.
Has nuclear pores, which are complex protein structures spanning both membranes of the nuclear envelope. These pores selectively regulate the traffic of macromolecules (proteins, RNA) between the nucleus and the cytoplasm, while smaller molecules pass more freely.
Nucleolus and Regulation of Genes
Nucleolus: A dense, non-membranous structure within the nucleus, primarily known as the site where ribosomal RNA (rRNA) is synthesized (transcription) and processed, and where ribosomal subunits (large and small) are assembled from rRNA and ribosomal proteins.
Other genes are localized to specific areas in the nucleus, often associated with particular nuclear structures or chromatin domains. Their regulation is a complex process involving transcription factors, chromatin remodeling, epigenetic modifications, and interactions with nuclear scaffold proteins, which are still areas of active research.
Nuclear Envelope
Composed of an outer nuclear membrane (ONM) and an inner nuclear membrane (INM), separated by a perinuclear space that is continuous with the ER lumen.
The ONM is continuous with the endoplasmic reticulum (ER), and its outer surface is often studded with ribosomes, reflecting its connection to protein synthesis. It is considered an integral part of the endomembrane system.
Specific proteins are localized to ONM and INM. The INM contains proteins that bind the nuclear lamina and chromosomes, helping to anchor chromatin within the nucleus.
Nuclear Lamina
A meshwork of cytoskeletal proteins (intermediate filaments) located just inside the inner nuclear membrane, providing structural integrity to the nuclear envelope and maintaining nuclear shape.
Comprised heterogeneously of proteins known as lamin A, B, and C, which polymerize to form a dynamic network.
It prevents compression of the nucleus and plays roles in chromatin organization, DNA replication, and gene expression by providing attachment sites for chromatin.
Progeria and Lamin A Mutations
Mutations in the gene encoding lamin A (LMNA gene) can cause Progeria (Hutchinson-Gilford Progeria Syndrome), a rare, fatal genetic condition characterized by early and rapid aging symptoms, manifesting in childhood. The abnormal lamin protein (progerin) causes defects in nuclear structure and function, leading to cellular dysfunction and premature senescence. This condition famously inspired F. Scott Fitzgerald’s “The Curious Case of Benjamin Button.”
Nuclear Pore Complex
A large, multicomponent protein structure (nucleoporins, NUPs) forming an aqueous channel across the nuclear envelope, allowing regulated passage of molecules.
It acts as a selective gatekeeper: messenger RNAs (mRNAs) bound to proteins (mRNPs) exit the nucleus to be translated in the cytoplasm, and nuclear proteins (e.g., histones, DNA polymerases, transcription factors) essential for nuclear function enter from the cytoplasm. Transport is bidirectional and highly regulated.
Nuclear proteins carry a specific targeting sequence, known as a nuclear localization signal (NLS), which is recognized by import receptors (importins) to facilitate their entry through the nuclear pore.
Nuclear Localization Signal (NLS)
An NLS is a short sequence of basic amino acids (rich in lysine and arginine) within a protein that targets it for import into the nucleus. Example of a typical NLS: pro - pro - lys - lys - lys - arg - lys - val (ppkkkrkv). These positively charged residues are crucial for interaction with import receptors.
Recognized by a soluble cytosolic protein called importin (or karyopherin), which binds to the NLS, escorts the cargo protein to the nuclear pore, and mediates its translocation through the pore into the nucleoplasm. Once inside, the importin-cargo complex dissociates due to interaction with RanGTP, and importin returns to the cytoplasm.
c-myc Transcription Factor
A protein that functions as a transcription factor, binding to specific DNA sequences (E-boxes) in the promoter regions of target genes. Upon binding, it activates the transcription of genes primarily involved in cell proliferation, cell growth, and apoptosis.
Functions as a proto-oncogene; under normal regulation, it controls cell cycle progression. However, overexpression, mutations, or chromosomal translocations involving c-myc can lead to uncontrolled cell growth, inhibited apoptosis, and consequently, tumor formation and cancer. It is one of the most frequently deregulated genes in human cancers.
Identified as a nuclear protein with a specific NLS: PAAKRVKLD, confirming its nuclear localization and function in regulating gene expression within the nucleus.
Medical Mystery
Symptoms of a condition may include excessive thirst (polydipsia), frequent urination (polyuria), fatigue, unexplained weight loss, increased hunger (polyphagia), itchiness, and blurred vision. These are classic symptoms often associated with elevated blood glucose levels, pointing towards conditions like Diabetes Mellitus.
Mitochondria
Structure: Typically rod-shaped organelles, approximately 1.5 µm x 2-8 µm, though their shape and size can be highly dynamic and variable within a cell. Their size is similar to that of a bacterial cell, supporting the endosymbiotic theory. The number per cell varies widely, from a few in quiescent cells to thousands in metabolically active cells (e.g., muscle cells, liver cells).
Function: Primary role is energy production through cellular respiration (oxidative phosphorylation), generating large quantities of ATP. They are also involved in the regulation of cellular calcium homeostasis (storage and release), signaling pathways, heat production, and initiation of programmed cell death (apoptosis) by releasing pro-apoptotic factors.
Growth: Reproduce via binary fission, a process similar to bacterial cell division, where an existing mitochondrion grows and then divides into two, indicating their self-replicating nature.
Mitochondrial Structure
Encased by a double membrane, creating two distinct compartments: the intermembrane space and the mitochondrial matrix.
Outer membrane: Smooth, relatively permeable due to the presence of porin proteins, allowing passage of small molecules and ions. It contains numerous transport complexes for importing proteins synthesized in the cytosol.
Inner membrane: Highly folded into structures known as cristae, which significantly increase its surface area. This membrane is largely impermeable and houses the protein complexes of the Electron Transport Chain (including cytochrome complexes), ATP synthase, and various transport proteins (translocases) responsible for regulating the passage of metabolites to and from the matrix.
The matrix of the mitochondrion is the innermost compartment, a gel-like substance containing its own circular DNA (mtDNA), ribosomes (70S, similar to bacteria), and a diverse array of enzymes, including those for the Citric Acid Cycle, fatty acid oxidation, and pyruvate oxidation.
Mitochondrial DNA
Genetics: Mitochondrial DNA (mtDNA) is a small, circular, double-stranded molecule, typically around 16.5 Kb (kilobases) in humans. It is devoid of introns and encodes a small number of essential genes for mitochondrial function, specifically ribosomal RNA (rRNA), transfer RNA (tRNA), and a few polypeptides that are subunits of the ATP synthase and Electron Transport Chain complexes (e.g., cytochrome c oxidase, NADH dehydrogenase). Most mitochondrial proteins are encoded by nuclear DNA.
Proteins necessary for mitochondrial function, but encoded by nuclear DNA, are synthesized on cytosolic ribosomes and then imported into mitochondria via specific protein translocases in the outer (TOM complex) and inner (TIM complex) membranes.
All mitochondria are inherited exclusively from one's mother (maternal inheritance) through the cytoplasm of the egg cell. Paternal mitochondria from sperm are typically degraded after fertilization. This unique inheritance pattern makes mtDNA a valuable tool for studying human ancestry and tracking maternal lineages.
Mitochondrial Function
Primary role: ATP production through cellular respiration, a highly efficient process that extracts energy from glucose and other fuel molecules.
Citric Acid Cycle (Krebs Cycle or Tricarboxylic Acid Cycle): Occurs in the mitochondrial matrix. It processes acetyl-CoA (derived from pyruvate, fatty acids, and amino acids), oxidizing it completely to CO2. This cycle generates a small amount of GTP (or ATP) directly, but more importantly, it produces a significant amount of high-energy electron carriers: NADH and FADH2, which feed electrons into the Electron Transport Chain.
Electron Transport Chain (ETC): Located in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through a series of protein complexes (I, II, III, IV), collectively known as the ETC. As electrons pass down the chain, energy is released, which is used to pump protons ( ions) from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient (proton-motive force) across the inner membrane. At complex IV, electrons are finally accepted by O2 (the terminal electron acceptor), producing water. The return of ions through ATP synthase (a molecular rotor located in the inner membrane) drives the phosphorylation of ADP to generate the vast majority of cellular ATP (oxidative phosphorylation).
Chloroplasts
Structure: Typically lens-shaped organelles, 2-10 µm long and 2-3 µm thick, larger than mitochondria. Chloroplasts are delimited by an outer and inner membrane, similar to mitochondria. Inside the inner membrane is a highly organized third membrane system called thylakoid membranes. These thylakoids are often stacked into structures called grana (singular: granum), resembling stacks of coins.
Function: The thylakoid membranes are key components in photosynthesis, containing chlorophyll pigments and embedded protein complexes (e.g., photosystems I and II, electron transport chain components, ATP synthase) that absorb sunlight energy and initiate the light-dependent reactions. The internal compartment of the chloroplast, called the stroma, contains its own circular DNA and ribosomes (70S) and approximately 100 genes coding for proteins necessary for various aspects of photosynthesis, including the enzymes for the Calvin Cycle (dark reactions).
Photosynthesis in Chloroplasts
The process by which light energy is converted into chemical energy, driving the synthesis of organic compounds from CO2 and water.
Light Reactions (Light-Dependent Reactions): Occur in the thylakoid membranes. Chlorophyll and other pigments capture light energy, exciting electrons. These electrons are then transferred through an electron transport chain, leading to the splitting of water molecules () to release O2, and the generation of chemical energy in the form of ATP and reducing power in the form of NADPH. This process converts light energy into chemical energy.
Dark Reactions (Calvin Cycle or Light-Independent Reactions): Occur in the stroma of the chloroplast. This cycle utilizes the ATP and NADPH produced during the light reactions to fix atmospheric CO2 and convert it into carbohydrates (e.g., glucose, sucrose) through a series of enzyme-catalyzed reactions. This process is essentially the synthesis of sugars using the energy and reducing power from the light reactions.
Origin of Organelles
Mitochondria and chloroplasts are unique among eukaryotic organelles because they possess their own circular DNA, ribosomes, and replicate by binary fission, similar to bacteria. This evidence strongly supports the endosymbiotic theory, which proposes that these organelles originated from ancient endosymbiotic events.
Mitochondria are thought to descend from free-living aerobic alpha-proteobacteria. These prokaryotes were engulfed by ancestral eukaryotic cells, forming a symbiotic relationship where the host cell gained an efficient mechanism for aerobic respiration, and the symbiont gained protection and nutrients. Over evolutionary time, many genes from the endosymbiont were transferred to the host cell's nuclear genome.
Chloroplasts are believed to be remnants of photosynthetic cyanobacteria (blue-green algae). These prokaryotes were similarly engulfed by early eukaryotic cells (either primary endosymbiosis in the case of green algae and plants, or secondary/tertiary endosymbiosis in other photosynthetic eukaryotes), providing the host cell with the ability to perform photosynthesis.
Peroxisomes
Structure: Very small (0.2 - 1.7 µm) single-membrane-bound vesicles that are roughly spherical. They often contain a dense, crystalline core composed of an array of enzymes, most notably catalase.
Function: In animal cells, peroxisomes play a crucial role in various metabolic functions, including the breakdown of certain fatty acids (beta-oxidation of very long chain, branched-chain, and D-amino acids) and the detoxification of toxic substances such as alcohol and formaldehyde. These oxidative reactions generate hydrogen peroxide (), which is a reactive oxygen species. Peroxisomes contain large amounts of the enzyme catalase, which rapidly converts the potentially harmful hydrogen peroxide into water and oxygen (2
2 + O2), thus protecting the cell from oxidative damage. In plants, peroxisomes are involved in photorespiration and the conversion of fatty acids to carbohydrates during seed germination (glyoxysomes).
Diabetes Mellitus
Characteristics: A group of metabolic diseases characterized by chronically high blood sugar levels (hyperglycemia), resulting from defects in insulin secretion, insulin action, or both. Common symptoms include excessive thirst (polydipsia), frequent urination (polyuria), fatigue, weight loss, increased hunger (polyphagia), and blurred vision. Long-term complications can include cardiovascular disease, kidney damage, neuropathy, and retinopathy.
Type I Diabetes (Juvenile-onset or Insulin-dependent Diabetes): An autoimmune disease where the body's immune system mistakenly attacks and destroys the insulin-producing beta cells in the islets of Langerhans of the pancreas. This leads to an absolute deficiency of insulin, requiring lifelong insulin therapy.
Type II Diabetes (Adult-onset or Non-insulin-dependent Diabetes): Characterized by either decreased responsiveness to insulin by target cells (insulin resistance) or insufficient insulin production by the pancreas, or both. It is often associated with lifestyle factors like obesity, lack of physical activity, and genetic predisposition.
Insulin
Discovery: Insulin was famously discovered by Frederick Banting and Charles Best in 1921 at the University of Toronto. Its first successful treatment in patients, specifically in a 14-year-old boy named Leonard Thompson, occurred in 1922, revolutionizing the treatment of diabetes.
Function: Insulin is a vital peptide hormone produced by the beta cells of the endocrine pancreas. Its primary function is to regulate glucose metabolism by signaling target cells (skeletal muscle, adipose tissue, liver) to import glucose from the bloodstream, convert it into glycogen (in liver and muscle) or fat (in adipose tissue) for storage, and inhibit glucose production by the liver. It is the only hormone that lowers blood glucose levels.
Structure: Composed of 51 amino acids arranged into two polypeptide chains (A and B) linked by disulfide bonds. It has a molecular weight of approximately 5808 Daltons.
Secretion Process: Triggered primarily by elevated blood glucose levels after meals. When glucose levels rise, pancreatic beta cells take up glucose, metabolize it, which leads to an increase in ATP production. This ATP-mediated signal closes ATP-sensitive potassium channels, depolarizing the cell membrane, opening voltage-gated calcium channels, and stimulating the exocytosis of insulin-containing vesicles into the bloodstream.
The Endomembrane System
Description: A complex and dynamic network of closed membrane tubules, vesicles, and sacs that divide the cytoplasm into functionally and structurally distinct compartments. It represents a coordinated system for the synthesis, modification, transport, and degradation of proteins and lipids within the eukaryotic cell.
Components: Includes all membrane-bound organelles except mitochondria, peroxisomes, and chloroplasts. The key components that function in a coordinated manner are:
Rough ER (RER): Involved in the synthesis, folding, and initial glycosylation of integral (transmembrane) proteins and secretory proteins (destined for secretion, lysosomes, or other endomembrane compartments, including the ER itself).
Smooth ER (SER): Functions in lipid synthesis, carbohydrate metabolism, calcium storage, and detoxification of drugs and poisons.
Golgi Complex (Golgi Apparatus): A central hub for further modification, sorting, and packaging of proteins and lipids received from the ER. It directs materials to their final destinations.
Vesicles: Small, membrane-bound sacs that bud off from one endomembrane compartment and fuse with another, serving as crucial transport carriers for materials between these compartments (e.g., ER to Golgi, Golgi to lysosomes, Golgi to plasma membrane, or for secretion).
Also includes lysosomes, vacuoles (in plants and fungi), and the plasma membrane.
Functions of the Endoplasmic Reticulum
Smooth ER Functions
Lipid Synthesis: Major site for the synthesis of various lipids, including phospholipids (for cellular membranes), steroid hormones (e.g., in adrenal gland cells and gonads), and fatty acids.
Drug Detoxification: Abundant in liver cells, it contains enzymes (e.g., cytochrome P450 enzymes) that catalyze the addition of hydroxyl groups to hydrophobic drugs and toxins, making them more hydrophilic (soluble in water). This facilitates their excretion from the body via urine.
Calcium Storage: Sequesters and releases calcium ions (), particularly critical in muscle cells where the sarcoplasmic reticulum (a specialized SER) plays a vital role in muscle contraction by regulating intracellular levels.
Rough ER Functions
Secretory and Lumenal Protein Synthesis: Ribosomes attached to the RER synthesize proteins destined for secretion outside the cell (e.g., hormones, digestive enzymes), and proteins that will reside within the ER, Golgi, lysosomes, or peroxisomes. As translation proceeds, these proteins are directly translocated into the RER lumen.
Membrane Protein Synthesis: Integral membrane proteins (transmembrane proteins) are synthesized on the RER, with their hydrophobic regions becoming embedded in the ER membrane during translation. They are then transported to their final membrane destinations within the endomembrane system.
Chemical Modification and Protein Folding: Proteins entering the RER lumen undergo critical post-translational modifications, including disulfide bond formation (catalyzed by protein disulfide isomerase) and initial glycosylation (addition of oligosaccharide chains). Chaperone proteins (e.g., BiP, calnexin, calreticulin) in the RER lumen assist in proper protein folding and assembly. Misfolded proteins are retained and targeted for degradation.
Vesicle Formation: Once proteins are correctly folded and modified, they are packaged into transport vesicles that pinch off from the RER and move towards the Golgi Complex for further processing and sorting.
Signal Sequence and Protein Transport
Signal Sequence: A short, hydrophobic peptide (
amino acids) typically located at the N-terminus of proteins destined for the ER lumen or insertion into the ER membrane. This sequence acts as an address label, directing the ribosome-mRNA complex to the rough ER. During protein synthesis, as the signal sequence emerges from the ribosome, it is recognized by a Signal Recognition Particle (SRP).
Mechanism: The SRP binds to both the signal sequence and the ribosome, temporarily halting translation. The SRP-ribosome-mRNA complex then binds to an SRP receptor on the ER membrane. This docking allows the ribosome to associate with a protein translocon (a protein channel) in the ER membrane. The SRP is then released, and translation resumes, with the polypeptide chain now translocated through the translocon into the ER lumen. Inside the ER lumen, the signal sequence is typically cleaved off by a signal peptidase, and the protein folds into its tertiary structure.
The Golgi Apparatus
Functions like the cell's “post office,” serving as the central sorting and modification station for products of the ER. It consists of flattened membrane-bound sacs called cisternae, typically organized into three functional regions: cis-Golgi network (CGN), medial-Golgi, and trans-Golgi network (TGN).
Process:
Entry (Cis region): Transport vesicles containing proteins and lipids bud off from the ER and fuse with the cis-Golgi network (CGN), the side of the Golgi apparatus closest to the ER. This is where initial sorting and phosphorylation of some proteins occur.
Processing and Maturation (Medial and Trans regions): Proteins and lipids move through the stacked cisternae, from the Cis to the Medial to the Trans regions. Within each cisterna, various enzymes perform sequential modifications to proteins and carbohydrates (e.g., further glycosylation, addition of specific sugar residues, sulfation). The cisternal maturation model suggests that cisternae themselves mature as they move from the cis to the trans face, changing their enzyme content and processing cargo.
Exit and Sorting (Trans region): The trans-Golgi network (TGN) is the exit site, where the final sorting and packaging of modified proteins and lipids occur. Vesicles pinch off from the TGN, carrying their cargo to their specific final destinations, which can include secretion outside the cell (constitutive or regulated secretion), insertion into the plasma membrane, or delivery to other organelles like lysosomes or vacuoles. Specific sorting signals on the proteins guide them to appropriate vesicles and destinations.