Chapter 2. Cell Theory
Higher category: 【Biology】 Biology Index
1. Overview
2. Structure 1. Endomembrane
3. Structure 2. Metabolic cell organelles
4. Structure 3. Broad meaning of cytoskeleton
5. Structure 4. Cytoplasm
1. Overview
⑴ Cell: Functional and structural unit of all living organisms.
⑵ Cell theory
① George Palade: Father of cell biology. Personally developed the electron microscope.
② Content 1: All living organisms are composed of cells.
○ Cell: A compartment where life processes occur, surrounded by a cell membrane.
○ Cells are composed of organelles.
○ Organelles are composed of common biomolecules.
③ Content 2: All cells come from pre-existing cells.
○ Exception: Theory of chemical evolution
⑶ Number of cells
① The human body is composed of approximately 260 types of cells.
② The adult body is composed of approximately 100 trillion cells.
⑷ Cell size
① Increasing factor: Cell size must be large enough to accommodate DNA, proteins, and other components necessary for cell activity.
② Decreasing factor (Surface-to-volume ratio hypothesis): Cell size must be small enough to increase the efficiency of cellular activities (∝ surface area ÷ volume).
③ The actual size of a cell is determined by the balance (trade-off) between the increasing and decreasing factors.
⑸ Types of cells: Prokaryotic cells and eukaryotic cells.
① Prokaryotic cells: Bacteria, cyanobacteria, etc.
○ The earliest type of cell on Earth.
○ Diameter: Approximately 1 μm to 10 μm.
○ Common 1: Cell membrane, genome, and ribosomes are present.
○ Common 2: Cilia, flagella, cell wall, and a capsule surrounding the cell wall are present.
○ Difference 1: Lack of membrane-bound organelles.
○ Difference 2: Absence of a true nucleus. Instead, they have a nucleoid.
○ Difference 3: Most prokaryotic cells, such as E. coli, have one circular DNA molecule, while some have multiple circular DNA molecules.
② Eukaryotic cells: Animal cells, plant cells, fungi, most protists, etc.
○ Diameter: Approximately 10 μm to 100 μm.
○ Animal cells are round, while plant cells are rectangular.
○ Common 1: Cell membrane, genome, and ribosomes are present.
○ Common 2: Some have cilia, flagella, a cell wall, and a capsule surrounding the cell wall.
○ Difference 1: Presence of membrane-bound organelles.
○ Difference 2: Presence of a nuclear membrane.
○ Difference 3: Composed of multiple linear DNA molecules.
⑹ Types of cells: Animal cells and plant cells.
| Animal Cells | Plant Cells | |
|---|---|---|
| Centriole | Present (involved in cell motility) | Absent |
| Vacuole | Absent | Vacuole occupies 30% of the volume |
| Mitochondria | 1000-3000 | 100-200 |
| Chloroplasts | Absent | Present |
| Cell Wall | Absent | Present |
Table 1. Animal cells and plant cells
Figure 1. Structure of animal cells and plant cells
⑺ Cell structure
① Endomembrane
② Metabolic cell organelles
③ Broad sense of cytoskeleton
④ Cytoplasm
2. Structure 1. Endomembrane
⑴ Nuclear envelope (plasma membrane or cell membrane)
① Composition: Lipids, proteins
② Functions: Material transport, shape maintenance, signal transduction, muscle contraction, nerve stimulation
○ Membrane structure: 【Phospholipid head - Hydrophobic tail】 + 【Hydrophobic tail - Phospholipid head】
○ Spontaneous membrane formation by phospholipid bilayer: Phosphate heads face the aqueous surface, and hydrophobic tails point toward the interior of the membrane.
○ Fluid mosaic model
○ Membrane proteins move within the fluid phospholipid bilayer like a mosaic.
○ Revealed by FRAP (Fluorescence Recovery After Photobleaching) and FLIP (Fluorescence Loss In Photobleaching) experiments.
○ Lateral movement of the membrane: 105 to 107 times per second.
③ Membrane permeability
○ Non-charged, lipid-soluble substances diffuse through the membrane.
○ Selective permeability: Water-soluble molecules cannot pass through.
○ Reason: Hydrophobic molecules can pass through the cell membrane via simple diffusion, as the hydrophobic tails are longer than the hydrophilic heads.
④ Fluidity factor 1: Transition temperature (Tm)
○ The temperature at which a rigid cell membrane abruptly transitions to a fluid state.
○ Related to gel-to-sol transition.
⑤ Fluidity factor 2: Restricted movement of membrane proteins attached to the cytoskeleton (e.g., caveolin).
⑥ Fluidity factor 3: Restricted movement of membrane proteins due to tight junctions.
⑦ Fluidity factor 4: Cholesterol
○ Cholesterol: At high temperatures, it restricts the movement of phospholipids, and at low temperatures, it prevents phospholipids from clumping together, helping maintain membrane fluidity. It does not allow the passage of hydrophilic substances.
○ Temperature-Dependent Regulation: Adjusts the content of unsaturated fatty acids (increases fluidity) and the length of fatty acids (decreases fluidity) in phospholipids to regulate membrane fluidity.
Table 2. Content of unsaturated fatty acids in phospholipids
⑧ Asymmetry of phospholipids: Phospholipid vesicles produced in the smooth endoplasmic reticulum (SER) are initially symmetrical. Asymmetry is created after vesicles fuse with the plasma membrane through the action of flippases and floppases.
○ Flippase: An enzyme that facilitates the movement of phospholipids from the outer layer to the inner layer.
○ Floppase: An enzyme that facilitates the movement of phospholipids from the inner layer to the outer layer.
○ Examples of phospholipid asymmetry.
○ Disruption of asymmetry leads to cell death signaling. The flip-flop process is involved.
○ Flippase is also involved in the membrane expansion mechanism of the endoplasmic reticulum.
⑨ Patch clamp method
○ Experimental technique used to study electrophysiology by attaching a patch to the cell membrane of living cells.
⑵ Nucleus
① Component 1. Nuclear envelope: Double membrane. Has numerous nuclear pores with selective permeability.
○ Outer membrane of the nuclear envelope is connected to the endoplasmic reticulum, and the inner membrane is supported by intermediate filaments.
○ Nuclear pores: An octameric channel. Monomers such as nucleotides, amino acids, and glucose can move freely.
② Composition 2. Nucleoplasm
③ Composition 3. Nucleolus: A region where chromatin is concentrated.
○ Site of rRNA synthesis, telomerase synthesis, and ribosomal subunit assembly.
○ Lacks a membrane and appears dense.
○ The most easily observed structure within the nucleus.
⑶ Ribosome
① Found in every cell.
② A workstation for protein synthesis.
③ Classified into free ribosomes and bound ribosomes.
⑷ Endoplasmic reticulum (ER): A large network of membranes that extends into the cytoplasm while connected to the nuclear membrane.
① Rough Endoplasmic Reticulum (Rough ER, RER): The endoplasmic reticulum studded with ribosomes. Responsible for protein secretion.
○ The area where vesicle budding occurs is smooth and lacks ribosomes.
○ Function 1. Formation of vesicles and cell membranes.
○ Function 2. Primary glycosylation: N-linked protein glycosylation. Asparagine, etc. are involved.
○ Example: Mannose, blood type
○ Function 3. Removal of the endoplasmic reticulum signal sequence: Occurs before the protein is delivered to the endoplasmic reticulum but is generally considered a function of the rough ER.
○ Function 4. Disulfide bonding
② Smooth ER (SER): The endoplasmic reticulum without ribosomes.
○ Function 1. Lipid and phospholipid synthesis, fatty acid elongation, and unsaturation of fatty acids
○ Typically synthesizes unsaturated fats from saturated fats.
○ In later stages of SER, SER synthesizes sterols.
○ Function 2. Calcium storage (Ca2+ pump) and release (IP3 dependent Ca2+ channel)
○ Example: Sarcoplasmic reticulum (SR) in muscle
○ Function 3. Cytochrom p450
○ Detoxification by adding -OH groups to lipid-soluble substances for extracellular release.
○ The -OH group increases reactivity.
○ Function 4. Control of blood sugar level
○ Involves glycogen phosphorylase.
○ Function 5. Gluconeogenesis
○ Facilitated by glucose-6-phosphatase (present only in the liver).
③ Alcohol breakdown and hangovers
⑸ Golgi apparatus: Absent in red blood cells.
① Structure
○ Cisternae structure: CGN (cis-Golgi network) + medial + TGN (trans-Golgi network)
○ One side continuously receives (cis) while the other side continuously sends vesicles (trans).
② Formation
○ Cisternal maturation model: The trans portion of the Golgi apparatus disappears, and cis becomes trans. Vesicles from the endoplasmic reticulum become cis.
○ Vesicular transport model: Small molecules are transported directly through vesicles without undergoing Golgi maturation, resulting in faster transport.
③ Secretion pathways
○ Constitutive secretory pathway: Continuous secretion of membrane proteins and extracellular matrix (e.g., collagen, proteoglycans).
○ Signal-mediated secretory pathway: Secretion of proteins mediated by hormones.
○ Lysosome pathway
④ Function 1. Protein sorting: Related to lysosomes.
⑤ Function 2. Secretion to intracellular destinations
⑥ Function 3. Protein cleavage → Activation of proteins: Insulin cleavage, digestive enzyme cleavage
⑦ Function 4. Secondary glycosylation: O-glycosylation (O-linked protein glycosylation). Involves serine and threonine.
○ Example of O-glycosylation: O-GlcNAcylation
○ Examples of products: Core proteins and chondroitin sulfate, keratin sulfate
⑧ Function 5. Synthesis of pectin and hemi-cellulose in plant cells
○ Cellulose is produced by cellulose synthase in plant cell walls.
⑨ Function 6. Chylomicron formation
⑩ Function 7. Other post-translational modifications after translation
○ O-sulfation
○ Phosphorylation
⑹ Lysosome
① A single-membrane vesicle containing over 50 types of acidic hydrolases.
○ Lipofuscin, etc.
○ Plant cells lack lysosomes, and vacuoles serve their function instead.
② Function
○ Digestion of target molecules, damaged receptors, and damaged organelles.
○ Referred to as a “suicide capsule.”
③ 1st step. In the rough endoplasmic reticulum, mannose, a pentose sugar, attaches to the surface of immature lysosomal enzymes (glycosylation). Flippase is involved in this process.
④ 2nd step. Immature lysosomal hydrolase enzymes move to the cis-Golgi.
⑤ 3rd step. In the Golgi apparatus, mannose is phosphorylated by UTP to form mannose-6-phosphate (M-6-ⓟ).
⑥ 4th step. When M-6-ⓟ reaches the trans-Golgi, it binds to M-6-ⓟ receptors on the membrane surface.
○ The binding of M-6-ⓟ to the receptor directs the hydrolase into the lysosome.
⑦ 5th step. The glycoprotein is enclosed in a vesicle and fuses with a mature endosome derived from the cell membrane.
○ Endosome: A vesicle involved in endocytosis, equipped with an H+ pump.
○ If the enzyme responsible for tagging proteins with M-6-ⓟ malfunctions, acidic hydrolases cannot be transported to lysosomes.
⑧ 6th step. Completion of the lysosome: When the H+ pump activates and lowers the internal pH to 5, M-6-ⓟ is released from the receptor.
⑨ 7th step. Through phagocytosis, bacteria and other substances enter the cell, forming a phagosome.
⑩ 8th step. When the phagosome containing organic material encounters the lysosome, it becomes a phagolysosome.
○ Hydrolytic enzymes in lysosomes completely break down organic matter.
○ The low pH inside lysosomes facilitates hydrolysis.
⑪ 9th step. The remaining debris is expelled from the cell through exocytosis.
⑫ Inhibitors
○ Mannan: Inhibitor of mannose receptor-mediated endocytosis.
○ Bafilomycin: Lysosomal inhibitor.
⑬ Lysosomal diseases
○ Mechanism: Decreased lysosomal hydrolases → Accumulation of degradation products → Increased lysosome concentration → Influx of water → Lysis → Damage to surrounding cells
○ Example 1: Tay-Sachs disease: Newborns experience blindness or hearing loss.
○ Example 2: Pompe’s disease: Occurs in muscles.
○ Example 3: Gouty: Related to acidity
⑺ Vacuole
① Characteristics
○ Vacuoles share the same function and origin as lysosomes and are formed in the Golgi apparatus.
○ Found only in plant cells.
○ Focuses on recycling rather than digestion.
○ Plant cells lack lysosomes and vacuoles take their place.
② Type 1: Phagocytic vacuole (Phagosome)
③ Type 2: Contractile vacuole: Found in protists to expel excess water from the cell.
④ Type 3: Central vacuole
○ Stores a variety of molecules, including water, pigments, digestive enzymes, toxins, and waste products.
○ Maintains internal pressure to support the structure of the plant.
⑻ Peroxisome: Prevents the formation of oxygen radicals by breaking down hydrogen peroxide.
① Single-membrane vesicles that contain about 50 different enzymes, including those involved in the glyoxylate pathway.
② Formation: Derived from vesicles originating from the endoplasmic reticulum. The Golgi apparatus does not produce peroxisomes.
③ Enzymes
○ Superoxide dismutase (SOD): 2O2- + 2H+ → O2 + H2O2
○ Catalase: 2H2O2 → 2H2O + O2. Not found in other cell organelles.
④ Reaction
○ Reactive oxygen species (ROS): Highly reactive chemical species generated from oxygen.
○ Radical: Superoxide (O2·-), hydroxyl radical (·OH)
○ Nonradical: Singlet oxygen (1O2), hydrogen peroxide (H2O2)
○ ROS cascade reaction
Figure 2. ROS cascade reaction
○ O2 + e- → O2·-
○ 2H+ + 2O2·- → H2O2 + O2
○ H2O2 + e- → HO- + ·OH
○ 2H+ + 2e- + H2O2 → 2H2O
○ Fenton reaction: Iron ions can generate ROS.
○ Fe2+ + H2O2 → Fe3+ + HO● + OH-
○ Fe3+ + H2O2 → Fe2+ + HOO● + H+
○ HO● + HOO● + H2O → 2H2O2
○ Haber Weiss reaction: If there is a free ion in the amount of catalyst, the following reaction will occur:
⑤ Functions
○ Oxidation of fatty acids
○ Removal of H2O2
⑥ Animal cells: 25-50% of beta oxidation occurs in peroxisomes, while the rest occurs in the intermembrane space of mitochondria.
○ Liver cells have more peroxisomes compared to other cells.
⑦ Plant cells: 100% of beta oxidation occurs in peroxisomes.
⑧ Zellweger syndrome: Multiple empty peroxisomes are found.
⑼ Glyoxysome
① A peroxisome that exists only in pre-germinating or germinating seeds, containing a distinct set of enzymes.
② Peroxisomes and glyoxysomes
○ Exist as peroxisomes in light conditions suitable for photosynthesis.
○ Exist as glyoxysomes in conditions where photosynthesis is not possible.
③ Glyoxylate Cycle: A metabolic pathway that converts fatty acids in the endosperm into sugars.
⑽ Mesosome
① Organelle found only in bacteria.
② A tubular or sac-like membrane structure formed by the invagination of part of the cell membrane into the cytoplasm.
⑾ Lipid droplet (LD)
① Surrounded by a phospholipid monolayer.
② Generally 0.1-5 μm in size. 100 μm in fat cells.
③ Formation
○ Triglyceride Formation in LD: Involves DGAT (diacylglycerol acyltransferase) located in the ER.
○ Cholesterol Ester Formation in LD: Involves ACAT (acyl-CoA:cholesterol acyltransferase) located in the ER.
④ Functions
○ Storage of lipid esters
○ Defense against lipotoxicity
○ Lipolytic activity: Involves ATGL (adipose triglyceride lipase), HSL (hormone-sensitive lipase), MAGL (monoacylglycerol lipase)
○ Storage site for preventing protein degradation
○ Under hypoxic conditions, toxic saturated fatty acids are stored because the absence of oxygen leads to reduction, meaning the gain of hydrogen and the loss of oxygen.
⑤ Disposal
○ Lysosomal acid lipase (LAL)
○ Chaperone-mediated autophagy (CMA)
○ Heat shock protein 70 (HSP70): Lysosomal proteolysis mediated by HSP70
○ LAMP-2A (lysosome-associated membrane protein 2A)
3. Structure 2. Metabolic cell organelles
⑴ Mitochondria: Energy-generating cell organelles in eukaryotic cells
① Double-membrane structure: Consists of an outer membrane, an inner membrane, and an intermembrane space.
② Outer membrane
○ Presence of porins (open channels): Substances smaller than 5,000 Da (e.g., ions) can freely pass through.
○ TOM, SAM (sorting and assembly machinery) are present.
○ TSPO (translocator protein)
○ Transports cholesterol into the mitochondria.
○ Involved in porphyrin transport, heme synthesis, and steroid synthesis.
○ Plays a role in apoptosis and proliferation.
○ An outer membrane protein used as a biomarker for neuroinflammation, tumors, etc.
○ The high concentration of H+ in the intermembrane space cannot pass through porins.
③ Inner membrane
○ Resembles the primitive plasma membrane of prokaryotes.
○ Cardiolipin: Restricts substance transport and reduces membrane permeability. Primarily found in regions where the electron transport chain is located.
○ Types of transport proteins: H+-pyruvate symport (active transport), H+-Pi symport, succinate transporter, ADP-ATP antiport, shuttle, ATPase, citrulline and ornithine (freely diffuse)
○ TIM, OXA (oxidase assembly) are present.
④ Matrix
○ Mitochondrial DNA (circular), RNA, ribosomes (70S)
○ 50-75% of β oxidation occurs in animal cells.
⑤ Cristae: Folded structures inside the mitochondria.
⑥ Intermembrane space
○ Mia40, etc., are present.
⑦ Characteristics
○ Mitochondria can replicate themselves.
○ Carbon diffusing into the mitochondria through simple diffusion can be up to 12-C.
⑧ Functions
○ ATP production
○ Fatty acid oxidation
○ Acetyl-CoA synthesis
○ Ketone synthesis
⑵ Chloroplast: Not present in animal cells.
① Double membrane (outer membrane, inner membrane), intermembrane space
② Stroma
○ Chloroplast DNA (circular), RNA, ribosomes (70S)
○ Cytoplasmic proteins have a primary signal protein that facilitates their transport to the stroma, where the signal protein is removed.
③ Granum: The part where thylakoids are stacked (lamellar structure).
○ Stroma Lamella: The part of the thylakoid that is in contact with the stroma.
○ Grana Lamella: The part of the thylakoid that is in contact with the grana.
④ Thylakoid: The unit where photosynthesis occurs. Cardiolipin is present in its membrane.
○ Cytoplasmic proteins have a secondary signal protein that facilitates their transport to the thylakoid membrane, where the signal protein is removed.
○ ATP synthase is located in the inner mitochondrial membrane and thylakoid membrane.
○ Mitochondria: ATP synthesis begins when the environment shifts from a basic to an acidic environment.
○ Chloroplasts: ATP synthesis begins when the environment shifts from an acidic to a basic environment.
⑤ Functions
○ NADPH, ATP production
○ Glucose synthesis
○ Fatty acid synthesis
⑦ Leucoplast: Lacks chlorophyll, converts glucose into starch, present in plant cells, and found in white parts of plants such as radish and green onion.
⑧ Plastids (colored plastids)
○ Contains carotenoids (orange) and xanthophylls (yellow).
○ Found only in plants; abundant in carrots, peppers, tomatoes, etc.
○ Fatty acid synthesis and the pentose phosphate pathway occur in plastids.
⑶ Cell fractionation
① Cell homogenate → Nuclei + Supernatant: 1000 g, 10 minutes
② Supernatant without nuclei → Chloroplasts + Supernatant: 3000 g, 10 minutes
③ Supernatant without chloroplasts → Mitochondria + Supernatant: 20000 g, 10 minutes
Figure 3. Cell fractionation
A: nuclei, B: chloroplasts, C: mitochondria
4. Structure 3. Broad meaning of cytoskeleton
⑴ Cytoskeleton: Present only in eukaryotes. Classified into microtubules, intermediate filaments, and microfilaments.
Figure 4. Three types of cytoskeleton
Figure 5. Distribution of cytoskeleton within the cell
⑵ Microtubules: Diameter of 25 nm. Present in both animal and plant cells.
① Function: Maintains cell shape, facilitates cellular movement, and transports vesicles within the cell.
○ Centrioles, flagella, cilia
② Composition 1. Unit: Tubulin heterodimers comprising α-tubulin and β-tubulin
○ (-) End (negative end): End where α-tubulin terminates.
○ Higher net depolymerization rate.
○ (+) End (positive end): End where β-tubulin terminates.
○ Higher net polymerization rate.
○ Both polymerization and depolymerization rates are faster at the (+) end than the (-) end.
○ Neurons have (-) end at the cell body and (+) end at the axon.
○ Structure: Composed of 13 protofilaments formed by tubulin dimers, arranged in a cylindrical structure.
③ Composition 2. Motor proteins capable of moving along microtubules
○ Dynein: Moves from distant points toward the centrosome (centriole) and is used for protein transport within the nucleus.
○ Kinesin: Moves from points near the centrosome (centriole) to distant points, facilitating the transport of exocrine proteins.
○ Myosin
④ Polymerization experiment: Nucleation - Elongation - Steady state
○ α-tubulin: GTPase activity
○ β-tubulin: Can bind to GTP or GDP.
○ At the (+) end, β-tubulin’s GTP is hydrolyzed to GDP, allowing new β-tubulin (GTP)-α-tubulin dimers to bind.
○ GTP cap: β-tubulin at the (+) end is in the GTP state, while the remaining β-tubulins are in the GDP state.
○ Microtubule formation over time: Tubulin heterodimers. Requires GTP, Mg2+ as materials. Can grow up to 25,000 nm.
Figure 6. Microtubule formation over time. Similar to microfilament formation
○ Treadmilling: The phenomenon where the (-) end shortens while the (+) end elongates.
○ Complete Disassembly: GTP hydrolysis in microtubules occasionally exposes subunits with GDP at the end.
○ Activity Increase: Enhanced by GTP and γ-tubulin.
⑤ Tension-extension experiment
○ Catastrophe
○ Rescue
⑥ Type 1: Centriole
○ Absent inprokaryotes, fungi, and plant cells. Its exact necessity is not fully understood.
○ Structure: A barrel-shaped ring composed of 9 microtubule triplets.
○ Centrosome: During the S phase, it duplicates to form two centrioles positioned perpendicularly. Afterward, microtubules elongate to form the spindle fibers.
○ Basal Body: A modified form of the centriole with 9 microtubule triplets. Elongates to form flagella and cilia.
○ Flagella and cilia: Feature a 9 + 2 microtubule doublet structure.
○ Function: Cell motility, movement of fluids
○ Principle: Dynein anchored to one doublet walks along an adjacent doublet, causing the two doublets to bend.
Figure 7. Cross-section of flagella and cilia in eukaryotes
Figure 8. Principle of flagellar movement in eukaryotes
○ Primary ciliary dyskinesia (PCD)
○ A genetic condition where cilia are absent, resulting in a lack of ciliary movement in the respiratory tract.
○ Impairs respiratory function.
○ Flagella of prokaryotes
○ Moves via rotational force driven by a proton gradient.
○ Mechanism is similar to ATPase.
○ Approximately 1/10th the width of eukaryotic flagella.
○ Unlike eukaryotic flagella, it is not covered by the plasma membrane.
○ Composed of flagellin rather than tubulin, as in eukaryotic flagella.
Figure 9. Structure and principle of flagella in prokaryotes
⑦ Microtubule destabilizing agent: Destabilizes microtubules during their synthesis, impairing their function.
○ Vinca alkaloid: Vinblastine, Vincristine, Vinorelbine, Vindesine, Vinflunine
○ Colchicine: Used for creating polyploid plants in crop breeding. It cannot be used as a anticancer drug because it inhibits neutrophils.
⑧ Microtubule stabilizing agent: Stabilizes microtubules during their disassembly, impairing their function.
○ Taxol: Extracted from the pacific yew tree. Used as an anticancer agent.
○ Paclitaxel, Docetaxel, Eleutherobins, Epothilones, Laulimalide, Sarcodictyins
⑨ Microtubule breaking agent
○ Nocodazole: Destroys microtubules.
⑶ Intermediate filament: Diameter of 8 to 12 nm.
① Functions
○ Maintaining cell shape and fixing organelle position: Nuclear lamins, etc.
○ Desmosome: Supports cells by connecting one cell to another.
○ Hemidesmosome: Supports cells by connecting them to the extracellular matrix (ECM).
○ Present only in animal cells.
② Subunits
○ Structure: N-terminus + α-helix + C-terminus. 32 subunits form one filament.
○ Intermediate filaments contain specific proteins depending on the cell type.
○ Type 1. Nuclear lamins: Maintain nuclear envelope structure.
○ In animal cells, the sub-nuclear membrane layer is composed of lamin proteins.
○ In many protists, fungi, and plants, the inner surface of the nuclear envelope is not associated with lamin.
○ Nuclear lamin helps anchor the nucleus in place.
○ Type 2. Keratin: Observed in the cytoplasm, abundant in epidermal cells.
○ Keratin intermediate filaments connect the epidermis and dermis at the basal layer of the skin’s epidermis, protecting the dermis.
○ Skin, hair, and nails are mostly composed of dead cells filled with keratin.
○ Type 3. Vimentin: Abundant in mesenchymal cells, used as a marker.
○ Type 4. Desmin: Abundant in muscle cells.
○ Type 5. Neurofilaments: Observed in axons of neurons.
○ Type 6. GFAP (glial fibrillary acidic protein): Abundant in glial cells.
③ Feature 1. Resistant to destruction, except during cell division and epidermolysis bullosa.
④ Feature 2. Calcium is involved in stabilizing intermediate filaments.
⑤ Tension-extension experiment
○ Catastrophe
○ Rescue
⑷ Microfilament: Diameter of 7 nm.
① Composed of actin filaments and myosin filaments.
○ Actin filaments: Account for 10-15% of cellular proteins.
○ Myosin filaments: Much less abundant.
② Function: Performs functions not only in muscle cells but also in non-muscle cells.
○ Maintains cell shape: Concentrated below the plasma membrane. Mainly associated with cadherins and actin filaments’ attachment.
○ Signaling between the cell surface and the nucleus: Mainly associated with integrins and actin filaments’ attachment.
○ Tight junctions, adhesion belt, focal contacts, etc.
○ Fixes the centrosome during cell division.
○ Muscle actin filaments: Utilize ATP. The motor protein myosin connects to actin.
○ Non-Muscle Cell Movement: Examples include amoeboid movement using pseudopodia.
○ 1st. The interior of the amoeba is in a sol (fluid) state, while the exterior is in a gel (solid) state.
○ 2nd. The amoeba flows its internal liquid toward the direction of pseudopodia formation, followed by gelation.
○ 3rd. In the direction opposite to pseudopodia formation, actin contracts, detaching from the substrate.
○ Cytoplasmic streaming in plant cells
○ Contractile ring in animal cell division
③ Structure 1. Monomer: Actin monomer
○ (-) end (minus end): Higher net depolymerization rate.
○ (+) end (plus end): Higher net polymerization. Higher polymerization and depolymerization rates than the minus end.
○ Microfilaments are composed of two intertwined actin subunits, forming a double helix with a repeating pattern every 37 nm.
④ Structure 2. Fimbrin: Connects actin filaments to each other, making them more resistant to tension and less elastic.
○ Example: Intestinal villi are formed by actin.
⑤ Polymerization experiment: Nucleation - Elongation - Steady state.
○ 1st. ATP-bound G-actin attaches to the plus end of F-actin.
○ 2nd. ADP-bound G-actin has weak interactions between monomers and easily dissociates from the polymer.
○ 3rd. Dissociated ADP-bound G-actin binds to ATP, replacing ADP, and attaches to the growing end of actin filaments.
○ Polymerization experiment: Faster initial polymerization rate in the presence of seed.
Figure 10. Microfilament polymerization experiment
○ Critical concentration: Concentration of G-actin at equilibrium with actin filaments.
○ Actin filaments can form above the critical concentration.
○ The critical concentration at the plus end (Cc+), which is the concentration of G-actin where polymerization rate equals depolymerization rate of G-actin at the plus end, is low (strong attachment).
○ The critical concentration at the minus end (Cc-), which is the concentration of G-actin where polymerization rate equals depolymerization rate of G-actin at the plus end, is high (weak attachment).
○ When the concentration of G-actin is higher than Cc+ but lower than Cc-, the positive end undergoes net polymerization, and the negative end undergoes net depolymerization.
○ Treadmilling phenomenon: This refers to the apparent movement of a microfilament caused by simultaneous polymerization at the positive end and depolymerization at the negative end under the aforementioned concentration conditions.
Figure 11. Treadmilling phenomenon
⑥ Tension-extension experiment
○ Catastrophe
○ Rescue
⑦ Polymerization promoters and inhibitors
○ Polymerization promoter: Profilin
○ Polymerization inhibitor: Thymosin, Cytochalasin
○ Depolymerization inhibitor: Jasplakinolide
⑸ Cell wall
① Found outside the plasma membrane in plant and bacterial cells.
② Abundant in cellulose.
③ Functions to support the plant body, which stands upright against gravity.
④ Plasmodesmata
○ Cell junctions between plant cells that can send and receive signals and substances.
○ The cell membrane is connected to it.
○ Plant virus moves to adjacent cells via plasmodesmata.
⑤ Primary cell wall: Cellulose + pectin + hemicellulose.
⑥ Secondary cell wall: Primary cell wall + lignin. Thicker.
⑹ Extracellular matrix (ECM): Only present in animal cells. Functions in support, adhesion, movement, and regulation.
① Basal lamina
○ Sheet-like structure surrounding epithelial cells.
○ Primarily composed of laminin, a multi-binding protein.
② Collagen: Strong fibers that connect cells.
○ Present in all animals except protozoa: In vertebrates, collagen accounts for one-third of the total protein.
○ Formation
○ 1st. Released in the form of procollagen.
○ 2nd. Some amino acids at the amino-terminal and carboxy-terminal ends are cleaved by procollagen peptidase.
○ 3rd. Forms an insoluble complex (collagen) after cross-linking.
○ Structure
○ One α chain consists of 100 amino acids: Composed of glycine, proline, and hydroxyproline.
○ Three α chains combine into a triple helix to form a collagen molecule: length 280 nm, diameter 1.5 nm.
○ Every third amino acid in all α chains is glycine → essential for the formation of the triple helix.
○ Cross-linking between collagen molecules forms fibrils: more abundant in mature collagen.
○ Fibrils aggregate to form collagen fiber: diameter 1 to 20 µm.
○ Each α chain adopts a left-handed helix: the natural α chain would form a right-handed helix, but its unique amino acid composition prevents this.
○ Each bundle of three α chains forming the triple helix adopts a right-handed configuration.
○ Type
○ Type I: Secreted by osteocytes and others. Found in tendons and ligaments.
○ Type II: Secreted by chondrocytes.
○ Collagen in cancer cells
○ Normal fibroblasts express both COL1A1 and COL1A2.
○ Therefore, the collagen produced by normal fibroblasts consists of two α1 chains and one α2 chain.
○ In cancer fibroblasts, COL1A1 is expressed while COL1A2 is suppressed by DNA methylation.
○ Therefore, the collagen produced by cancer fibroblasts is composed of three α1 chains.
③ Elastin
○ Engaged in elasticity, bending, etc.
○ Observed in ligaments, vascular walls, lung tissue, epidermis, and tendons.
○ Rarely present in limb ligaments.
○ Found in significant proportions in elastic ligaments, such as the ligamentum flavum.
④ Integrin: Transmembrane proteins
○ Induces cell-matrix adhesion through the integrin motif containing the RGD sequence.
○ RGD: Arginine - Glycine - Aspartic Acid
○ Consists of 18 α subunits and 8 β subunits, both having various forms.
Table 3. Subunits of integrin
○ Function 1: Anchors cells by binding fibronectin outside the cell and actin microfilaments inside the cell.
○ Function 1-1: Connectivity with fibronectin
○ Fibronectin and integrin are connected by non-covalent binding.
○ Since it is reversible binding, it is involved in attaching cells to the basement membrane or moving them to other locations.
○ Involved in substrate adhesion during amoeboid movement.
○ Function 1-2: Connectivity with actin: Connected to multiple actin filaments, providing cushioning against impact.
○ Function 2: Transmits intracellular changes to the outside of the cell.
○ Signal transduction by integrin.
Figure 12. Signal transduction by integrin
⑤ Fibronectin: Binds with integrin. Involved in signal transduction.
⑥ Vitronectin: Performs functions similar to collagen and elastin.
⑦ Transmembrane proteoglycan: Involved in cell-matrix adhesion.
○ The transmembrane proteoglycan is connected to a transmembrane receptor that lacks an RGD sequence and has low affinity.
○ Transmembrane proteoglycan binds to matrix proteoglycan, collagen, growth factors, etc.
⑧ CAM (cell-cell adhesion molecule): Involved in cell-cell adhesion.
○ Ca2+ independent.
○ Example: N-CAM acts on nerve cells as an Ig-superfamily CAM.
⑨ Cadherin: Involved in cell-cell adhesion.
○ Ca2+ dependent.
○ Acts as a dimer.
○ Example: E-cadherin (epithelial cadherin)
Figure 13. Comparison of CAM and cadherin
⑩ Glycoprotein
○ Important for cell-cell recognition.
○ Exposed on the outer side of the cell membrane.
○ Example 1. Lectin
○ Example 2. Selectin
○ Example 3. Galectin
○ Example 4. Hemagglutinin
○ Example 5. Neuraminidase
⑪ Laminin
○ Protein present in the basal lamina of the basement membrane.
○ The α chain, β chain, and γ chain form three short arms, enabling the binding between the ECM and cells.
⑫ Dystrophin
○ A connecting protein between the six proteins that regulate the calcium ion channels of the muscle cell membrane and the cytoskeleton.
○ Connects actin filaments and membrane glycoproteins, stabilizing the structure of muscle cells.
⑬ Fibrillin
⑭ Tenascin
⑮ Matrigel
○ Commercialized ECM that induces cell differentiation and morphological changes.
○ Forms a grid-like network in epithelial cells and endothelial cells.
⑺ Junctions
Figure 14. Types of junctions
① Type 1. Tight junction
○ Function 1. Seals the space between cells, preventing the passage of substances.
○ Function 2. Involved in the polarization of membrane proteins (e.g., in the digestive tract).
○ Transmembrane tight junctional proteins present in the cell membranes of adjacent cells form continuous bands, leading to membrane fusion.
② Type 2. Anchoring junction: Forms cell-cell and cell-matrix connections through the cytoskeleton.
○ Composed of intracellular attachment proteins and transmembrane linker glycoproteins.
○ Type 2-1. Anchoring junction using intermediate filaments.
○ Type 2-1-1. Desmosome: A junction between cells.
○ Prevents tearing even under strong shear stress (e.g., in the skin, muscles).
○ Involves plaque and cadherin.
○ Cadherin: Stabilized by calcium ions. N-cadherin (nerve), P-cadherin (platelet), E-cadherin (skin).
○ Keratin: Intermediate filament. Holds plaques together.
○ Desmosomes also increase during epithelial cell differentiation.
○ Characteristic: Unlike tight junctions, membrane fusion does not occur.
○ Type 2-1-2. Hemidesmosome: A junction between cells and the extracellular matrix.
○ Type 2-2. Anchoring junction using actin filaments.
○ Cell-cell junction: Adhesion belt, etc.
○ Cell-matrix junction: Focal contact, etc.
③ Type 3. Communicating junction
○ Type 3-1. Gap junction
○ Adjacent cell membranes meet to form a cylindrical structure: A connection pathway between cells is established through cylindrical proteins.
○ The cell membranes are not directly connected.
○ Function: Exchange of substances between cells. Only allows small molecules (e.g., ions) to pass.
○ Connexon: Each channel is composed of 4-6 connexins.
○ Connexin: Molecular weight ranges from 24,000 to 46,000.
○ Gap junctions open and close depending on the structural changes of connexin proteins.
○ When connexons formed in the membrane of each cell combine, they create a pathway connecting the cytoplasm of the two cells.
○ Gap junctions can also be found in electrical synapses.
○ Type 3-2. Chemical synapse
○ Type 3-3. Plasmodesmata: Exists only in plant cells.
5. Structure 4. Cytoplasm
⑴ Function 1. NADPH production: Pathway where malate becomes NADPH in the pentose phosphate pathway, C4 plants, etc.
⑵ Function 2. Synthesis of fatty acids
⑶ Function 3. Synthesis of isoprenoids and sterols
Input: January 15, 2019, 14:53
Modified: May 2, 2020, 21:23