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• A cell is the basic structural and functional unit of all living organisms.
• Human cells share three fundamental components: cell membrane, cytoplasm, and nucleus (except mature RBCs).
• Makes up 70–80% of a typical cell’s weight.
• Serves as the medium for biochemical reactions and transport of solutes.
• Proteins (≈15%)
– Form enzymes, structural elements, transporters, receptors, and regulatory molecules.
• Lipids (≈15%)
– Found mainly in the plasma membrane and organelle membranes.
– Act as barriers and signaling molecules.
• Carbohydrates (≈2%)
– Present as glycogen in some cells and as glycoproteins/glycolipids on membranes.
• Nucleic Acids
– DNA and RNA store genetic information and direct protein synthesis.
• Essential ions include Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, HCO₃⁻, PO₄³⁻.
• Maintain osmotic balance, electrical gradients, enzyme function, and signaling.
• Includes amino acids, glucose, fatty acids, nucleotides, and intermediates of metabolic pathways.
• Continuously synthesized, degraded, and recycled as part of metabolism.
• Specialized structures performing distinct functions:
– Nucleus (genetic control)
– Mitochondria (ATP generation)
– ER (protein and lipid synthesis)
– Golgi (sorting and packaging)
– Lysosomes (degradation)
– Peroxisomes (oxidation reactions)
• Together maintain cellular organization and homeostasis.
Cells contain organized internal structures called organelles, each performing specific biochemical functions essential for survival and metabolism.
• Largest and most prominent organelle; contains the cell’s genetic material (DNA).
• Surrounded by a double nuclear membrane with nuclear pores for controlled transport.
• DNA is packaged with proteins as chromatin, which condenses into chromosomes during division.
• Site of DNA replication and RNA transcription.
• Contains the nucleolus, the center for rRNA synthesis and ribosome assembly.
• Studded with ribosomes.
• Site of protein synthesis, particularly for secretory and membrane-bound proteins.
• Helps in folding, processing, and initial glycosylation of proteins.
• Lacks ribosomes.
• Functions in lipid synthesis, steroid hormone production, and detoxification of drugs (via cytochrome P450).
• In muscle cells, forms the sarcoplasmic reticulum for calcium storage.
• Stack of flattened membrane-bound cisternae: cis, medial, and trans regions.
• Receives newly synthesized proteins from ER, modifies them (glycosylation, sulfation), and sorts them.
• Packages proteins into vesicles for delivery to:
– Plasma membrane
– Lysosomes
– Extracellular secretion
• Essential for protein sorting and trafficking.
• Membrane-bound vesicles containing acid hydrolases.
• Enzymes work optimally at acidic pH (~5).
• Responsible for degradation of:
– Damaged organelles
– Foreign particles
– Cellular waste
• Formed by fusion of Golgi-derived vesicles with endocytic/phagocytic vesicles.
• Defects lead to lysosomal storage disorders.
• Small organelles containing oxidative enzymes (e.g., catalase, oxidases).
• Carry out β-oxidation of very long-chain fatty acids.
• Detoxify hydrogen peroxide (H₂O₂) by converting it to water.
• Involved in plasmalogen synthesis (important in myelin).
• Double-membraned organelle; outer smooth membrane and inner membrane with cristae.
• Site of oxidative phosphorylation and ATP generation.
• Contains its own DNA and ribosomes, enabling synthesis of some mitochondrial proteins.
• Involved in apoptosis, TCA cycle, fatty acid oxidation, and urea cycle (partial).
• Number varies with energy demand; highest in muscle, heart, and liver.
• The plasma membrane is described by the fluid mosaic model, which states that the membrane behaves like a flexible, dynamic lipid sheet with proteins embedded in it.
• Composed of amphipathic phospholipids arranged in two layers:
– Hydrophilic heads facing outward
– Hydrophobic tails facing inward
• Lipids move freely within the plane of the membrane, giving it fluidity.
• Proteins are distributed throughout the lipid bilayer like a mosaic.
• Two main types:
– Integral (transmembrane) proteins: span the bilayer, act as transporters, receptors, channels.
– Peripheral proteins: loosely attached to the surface, involved in signaling and structural support.
• Inserted between phospholipids.
• Maintains membrane stability by:
– Reducing fluidity at high temperatures
– Preventing rigidity at low temperatures
• Present as glycoproteins and glycolipids on the extracellular side.
• Important for cell recognition, adhesion, and immune response.
• Lipids and many proteins move laterally within the membrane, enabling:
– Rapid distribution of molecules
– Flexibility
– Membrane repair
The membrane is shown as a double layer of phospholipids with their hydrophobic tails facing inward. Proteins of various sizes are embedded within this layer, some spanning across it and others positioned only on one surface. Cholesterol molecules are interspersed among the lipids, while carbohydrate chains extend outward from proteins and lipids on the outer side.
• Lipid rafts are specialized microdomains within the plasma membrane that are richer in cholesterol, sphingolipids, and certain proteins compared to the surrounding membrane.
• Contain tightly packed sphingomyelin, glycosphingolipids, and cholesterol.
• More ordered and less fluid than the rest of the lipid bilayer.
• Host selected proteins such as receptors, G-proteins, and signaling molecules.
• Appear as small, dynamic “islands” that can cluster together during signaling events.
• Their ordered lipid environment provides a stable platform for assembling signaling complexes.
• Resistant to detergent solubilization due to tight lipid packing.
• Act as signaling hubs, organizing receptors and intracellular signaling proteins.
• Facilitate membrane trafficking, assisting in endocytosis and sorting.
• Important in immune cell activation, hormone signaling, and nerve function.
• Help in clustering proteins required for viral entry, making them biologically significant.
A membrane area is shown enriched with cholesterol and sphingolipids, forming a thicker, less fluid patch compared to the surrounding phospholipid bilayer. Specific membrane proteins cluster within this patch while others remain outside, illustrating selective organization of molecules.
• Caveolae are small flask-shaped invaginations of the plasma membrane.
• Rich in cholesterol, sphingolipids, and the protein caveolin.
• Represent a specialized subtype of lipid rafts.
• Common in endothelial cells, adipocytes, muscle cells.
• Involved in endocytosis, especially receptor-mediated internalization.
• Play a role in signal transduction, clustering receptors and signaling molecules.
• Help in mechanotransduction — sensing stretch or tension in the membrane.
• Important in cholesterol transport and lipid regulation.
Small pouch-like membrane invaginations enriched with caveolin proteins form a pocket dipping into the cytoplasm. These pockets contain concentrated signaling receptors and cholesterol-rich lipids.
• Tight junctions are seal-forming contacts between adjacent epithelial cells.
• Located at the apical region of cell–cell boundaries.
• Formed by proteins such as claudins, occludins, and junctional adhesion molecules.
• Create a selective barrier that prevents free movement of solutes between cells.
• Maintain cell polarity by separating the apical and basolateral membrane domains.
• Regulate paracellular transport, allowing controlled ion and water movement.
• Essential for organs like the intestine, kidney, and blood–brain barrier.
Adjacent cells are shown sealed together along their edges, forming continuous ridges that block substances from passing between them, ensuring directional movement of molecules.
• A dynamic network of protein filaments that supports cell shape, organization, and movement.
• Composed of actin.
• Provide structural support, enable cell movement, cytokinesis, and muscle contraction (with myosin).
• Found beneath the plasma membrane forming the cell cortex.
• Provide tensile strength to cells.
• Different types depending on tissue:
– Keratin (epithelium)
– Desmin (muscle)
– Vimentin (connective tissue)
– Neurofilaments (neurons)
• Hollow tubes made of α- and β-tubulin.
• Form the mitotic spindle, cilia, flagella, and serve as tracks for intracellular transport using motor proteins (kinesin, dynein).
• Help maintain cell shape and internal organization.
A meshwork of actin filaments lines the cell’s periphery, thicker intermediate filaments run throughout the cell for stability, and long microtubule tracks radiate from the centrosome toward the membrane, guiding vesicle transport and organizing cell structure.
Cells use several mechanisms to move substances across the plasma membrane depending on size, charge, and concentration gradients.
• Does not require energy.
• Movement occurs down the concentration gradient.
• Includes simple diffusion, facilitated diffusion, and osmosis.
• Requires ATP or other energy source.
• Moves substances against their concentration gradient.
• Utilizes carrier proteins or pumps.
• Large particles are transported via membrane-bound vesicles.
• Includes endocytosis, exocytosis, pinocytosis, and phagocytosis.
• A passive process where substances move from high to low concentration with the help of membrane proteins.
• Does not require ATP.
• Bind specific molecules (e.g., glucose, amino acids) and undergo a conformational change to move them across the membrane.
• Saturable — maximum transport rate depends on number of carriers.
• Form hydrophilic pores allowing ions or water to move rapidly across.
• Highly selective for ions such as Na⁺, K⁺, Ca²⁺, Cl⁻.
• Can be gated (voltage-gated, ligand-gated, mechanically gated).
• Transport is highly selective: each carrier/channel moves a particular molecule or ion.
• Occurs only when a concentration gradient exists.
• Movement stops when equilibrium is reached.
A membrane protein is shown with a binding site. A molecule such as glucose binds on the high-concentration side, the protein changes shape, and the molecule is released on the low-concentration side, without any energy usage.
• Ion channels are membrane proteins that create hydrophilic pathways for ions to cross the lipid bilayer.
• Allow rapid movement of Na⁺, K⁺, Ca²⁺, Cl⁻, and other ions.
• Movement is passive, driven by concentration and electrical gradients.
• Highly selective due to precise pore size and charge distribution.
• Essential for neuronal conduction, muscle contraction, hormone release, and maintaining membrane potential.
• Open or close in response to binding of a specific chemical ligand.
• Ligand may be extracellular (e.g., neurotransmitters) or intracellular.
• Nicotinic acetylcholine receptor (opens Na⁺/K⁺ channel).
• GABA-A receptor (Cl⁻ influx → inhibition).
• Glutamate receptors (NMDA, AMPA).
• Mediate synaptic transmission in the nervous system.
• Provide rapid response to chemical signals.
A receptor protein on the membrane binds a neurotransmitter; this binding causes a conformational change, opening the channel so ions can flow across the membrane.
• Open or close in response to changes in membrane potential.
• Critical for action potentials in neurons, skeletal and cardiac muscle.
• Voltage-gated Na⁺ channels (rapid depolarization).
• Voltage-gated K⁺ channels (repolarization).
• Voltage-gated Ca²⁺ channels (neurotransmitter release, muscle contraction).
• Have voltage sensors that detect electrical changes.
• Open and close within milliseconds.
When the membrane depolarizes, the voltage sensor in the channel moves, shifting the protein into an open state and allowing rapid ion entry.
• Small lipid-soluble molecules that transport ions across membranes.
• Produced by microorganisms; often used experimentally.
Carrier Ionophores
• Bind an ion, shield its charge, and carry it across the membrane.
• Example: Valinomycin (selective for K⁺).
Channel-Forming Ionophores
• Create hydrophilic pores allowing ions to flow through.
• Example: Gramicidin.
• Disrupt ion gradients.
• Useful for studying membrane potential, mitochondrial functions, and metabolic pathways.
A lipid-soluble molecule binds to an ion like K⁺, surrounds it, and diffuses through the hydrophobic membrane core, releasing the ion on the opposite side.
• Movement of molecules against their concentration gradient (low → high).
• Requires energy, usually from ATP hydrolysis or an ion gradient.
• Highly specific and involves carrier proteins.
• Essential for maintaining ionic balance, nutrient uptake, and membrane potential.
• Uses direct ATP hydrolysis.
• Example: Na⁺/K⁺ ATPase, H⁺ ATPase, Ca²⁺ ATPase.
• Uses the energy stored in ion gradients created by primary active transport.
• Does not directly use ATP but depends on ATP-driven pumps.
• A classic primary active transport pump.
• Located in almost all cell membranes.
• Maintains resting membrane potential, cell volume, and ionic balance.
• Pumps 3 Na⁺ out of the cell and 2 K⁺ in per ATP hydrolyzed.
• Creates a high extracellular Na⁺ and high intracellular K⁺ environment.
• Electrogenic — contributes to the negative charge inside the cell.
• Drives secondary active transport (glucose, amino acids).
• Maintains osmotic stability.
• Essential for nerve and muscle excitability.
The pump binds intracellular Na⁺, uses ATP to change shape and release them outside, then binds extracellular K⁺ and returns to its original conformation to bring K⁺ inside.
• Transports one type of molecule across the membrane.
• Can be passive (facilitated diffusion) or active (ATP-driven).
• Example: GLUT1 transporter (glucose transport).
• Moves only one substance at a time, in one direction.
• Moves two different molecules in the same direction across the membrane.
• Often used in secondary active transport.
• Na⁺–glucose cotransporter (SGLT) in intestine and kidney.
• Na⁺–amino acid symporters.
• One molecule moves down its gradient, driving the other against its gradient.
• Moves two different molecules in opposite directions.
• Also commonly part of secondary active transport.
• Na⁺–Ca²⁺ exchanger (NCX).
• Cl⁻–HCO₃⁻ exchanger (important in acid–base regulation).
• Mitochondrial ADP–ATP exchanger.
• One substance enters the cell while the other leaves.
• Process by which cells expel materials through fusion of vesicles with the plasma membrane.
• Used for secretion of hormones, neurotransmitters, enzymes, and membrane proteins.
• Requires energy (ATP) and is highly regulated.
• Vesicles form inside the cell → move along cytoskeletal tracks → fuse with the membrane → release contents outside.
• Also helps in membrane recycling and growth.
A vesicle moves to the surface, merges with the membrane, and discharges its molecules outside the cell while adding new membrane material.
• Process of internalizing extracellular material by forming vesicles from the plasma membrane.
• Helps in nutrient uptake, receptor internalization, and removal of membrane components.
• Pinocytosis
• Phagocytosis
• Receptor-mediated endocytosis
• Membrane invaginates → surrounds material → pinches off to form an internal vesicle.
• Called “cell drinking.”
• Uptake of fluids and small solutes in small vesicles.
• Occurs continuously in most cells.
• Important for nutrient absorption and routine membrane turnover.
The membrane forms small pits that enclose extracellular fluid, creating tiny vesicles inside the cell.
• Called “cell eating.”
• Uptake of large particles such as bacteria, dead cells, and debris.
• Performed mainly by specialized cells: macrophages, neutrophils, dendritic cells.
• Cell extends pseudopodia → engulfs the particle → forms a large vesicle (phagosome) → fuses with lysosome for digestion.
Large outward extensions from the cell engulf a particle, close around it, and deliver it to a digestive compartment.
• Caused by mutation in CFTR, a chloride channel.
• Impaired Cl⁻ secretion → thick, sticky mucus in lungs and pancreas.
• Leads to recurrent respiratory infections, malabsorption, infertility.
• Demonstrates the importance of ion channels in epithelial transport.
• Abnormal extracellular sodium alters osmotic balance across membranes.
• Water shifts into or out of cells → neurological symptoms (confusion, seizures).
• Rapid correction can cause osmotic demyelination syndrome.
• Digitalis drugs inhibit the sodium pump.
• Increased intracellular Na⁺ reduces Na⁺–Ca²⁺ exchange → ↑ intracellular Ca²⁺.
• Useful in heart failure (↑ contractility) but toxic at high levels.
• Type 2 diabetes: defective insulin signaling → impaired GLUT4 translocation → reduced glucose uptake in muscle and fat cells.
• GLUT1 deficiency: low glucose transport into the brain → seizures, developmental delay.
• Disorders caused by defective ion channels.
Examples:
• Long QT syndrome: abnormal K⁺ or Na⁺ channels → arrhythmias.
• Episodic ataxia: defective voltage-gated K⁺ channels.
• Hyperkalemic periodic paralysis: dysfunctional Na⁺ channels in muscle.
• Drugs like omeprazole inhibit the H⁺/K⁺ ATPase in gastric parietal cells.
• Reduce acid secretion → treat GERD, ulcers.
• Illustrate therapeutic targeting of active transporters.
• Cholera toxin activates CFTR, causing massive Cl⁻ and water efflux into the intestine.
• Leads to severe dehydration and electrolyte imbalance.
• Treated with ORS to restore Na⁺, Cl⁻, glucose-coupled cotransport.
• LDL receptor malfunction → reduced LDL uptake by cells.
• Causes severely elevated LDL → premature atherosclerosis and heart disease.
• Shows the role of endocytosis in lipid clearance.
• Impaired NADPH oxidase in neutrophils → defective killing of ingested organisms.
• Patients suffer from recurrent fungal and bacterial infections.
• Demonstrates importance of phagocytosis in innate immunity.
• Failure of ion pumps or increased capillary permeability causes excessive fluid buildup in tissues.
• Seen in heart failure, renal disease, liver failure.
Because it is made of a fluid phospholipid bilayer with proteins scattered throughout like a mosaic, and both lipids and many proteins can move laterally.
Lipid rafts are cholesterol- and sphingolipid-rich microdomains that organize signaling molecules.
They help in signal transduction, endocytosis, immune activation, and membrane trafficking.
Caveolae are flask-shaped invaginations containing the protein caveolin, whereas lipid rafts are flat microdomains.
Both are cholesterol-rich, but caveolae specialize in endocytosis and mechanosensing.
It stores DNA, directs gene expression, and contains the nucleolus, which forms ribosomes.
Rough ER synthesizes secretory and membrane proteins due to ribosomes attached to its surface.
Smooth ER is responsible for lipid synthesis, steroid hormone formation, and detoxification via cytochrome P450.
It modifies, sorts, and packages proteins into vesicles for secretion, lysosomes, or the plasma membrane.
Lysosomes contain acid hydrolases that break down cellular debris, old organelles, and ingested pathogens.
They perform β-oxidation of very-long-chain fatty acids and detoxify hydrogen peroxide using catalase.
Because they carry out oxidative phosphorylation, producing the majority of cellular ATP.
Passive transport of molecules down their concentration gradient with the help of carrier or channel proteins; no ATP required.
A primary active transporter that pumps 3 Na⁺ out and 2 K⁺ into the cell using ATP, helping maintain membrane potential and osmotic stability.
• Uniport: one molecule moves in one direction.
• Symport: two molecules move in the same direction.
• Antiport: two molecules move in opposite directions.
Process of internalizing extracellular material by forming vesicles from the plasma membrane.
• Pinocytosis: uptake of fluids and small molecules (“cell drinking”).
• Phagocytosis: ingestion of large particles like bacteria (“cell eating”).
Process of vesicles fusing with the plasma membrane to release materials outside the cell.
Cystic fibrosis, caused by mutations in the CFTR chloride channel.
It allows selective uptake of molecules such as LDL, transferrin, and hormones.
Defects cause familial hypercholesterolemia.
A. Cytoskeleton organization
B. Plasma membrane structure
C. Nuclear pore complex
D. Ribosomal assembly
Answer: B
A. Phosphatidylcholine and RNA
B. Sphingolipids and cholesterol
C. Actin and myosin
D. ATP and NADH
Answer: B
A. Clathrin
B. Caveolin
C. Tubulin
D. Vinculin
Answer: B
A. Rough ER
B. Smooth ER
C. Golgi apparatus
D. Lysosomes
Answer: B
A. Mitochondria
B. Peroxisomes
C. Lysosomes
D. Smooth ER
Answer: C
A. Lysosomes
B. Peroxisomes
C. Mitochondrial matrix
D. Cytosol
Answer: B
A. Nucleus
B. Golgi apparatus
C. Mitochondria
D. Rough ER
Answer: C
A. ATP
B. Carrier proteins or channel proteins
C. Vesicle formation
D. Movement against the gradient
Answer: B
A. 2 Na⁺ in and 3 K⁺ out
B. 3 Na⁺ in and 2 K⁺ out
C. 3 Na⁺ out and 2 K⁺ in
D. 2 Na⁺ out and 3 K⁺ in
Answer: C
A. One molecule moving across the membrane
B. Two molecules moving in opposite directions
C. Two molecules moving in the same direction
D. Transport without proteins
Answer: C
A. Phagocytosis
B. Endocytosis
C. Pinocytosis
D. Exocytosis
Answer: C
A. Glucose absorption by SGLT
B. LDL uptake into cells
C. Water reabsorption in kidney
D. Sodium reabsorption in intestine
Answer: B
A. Blocking voltage-gated channels
B. Directly hydrolyzing ATP
C. Carrying ions across membranes or forming pores
D. Enhancing RNA synthesis
Answer: C
A. Increases fluidity at all temperatures
B. Prevents membrane melting at high temperature and freezing at low temperature
C. Only stiffens the membrane
D. Only increases permeability
Answer: B
A. Golgi apparatus
B. Smooth ER
C. Lysosome
D. Peroxisome
Answer: A
A phospholipid bilayer with embedded proteins, cholesterol, and surface carbohydrates.
Because lipids and many proteins move laterally, forming a dynamic “mosaic” of components.
Phospholipids.
Stabilizes membrane fluidity — prevents excessive fluidity at high temperature and rigidity at low temperature.
Cholesterol- and sphingolipid-rich microdomains for signaling and membrane trafficking.
Caveolin.
Rough ER and Smooth ER.
Synthesis of secretory and membrane proteins.
Lipid synthesis, steroid formation, and detoxification (cytochrome P450).
Modification, sorting, and packaging of proteins.
Vesicles containing acid hydrolases for degradation of cellular waste.
Oxidation of very-long-chain fatty acids and detoxification of hydrogen peroxide.
They originate from endosymbiotic bacteria and can synthesize some of their proteins.
ATP production through oxidative phosphorylation.
Passive transport of molecules down their gradient using carrier or channel proteins.
Movement of molecules against their concentration gradient using energy (ATP).
Pumps 3 Na⁺ out and 2 K⁺ in per ATP hydrolyzed.
Transport of a single molecule in one direction.
Transport of two molecules in the same direction.
Transport of two molecules in opposite directions.
Internalization of extracellular material by membrane invagination.
Pinocytosis takes in fluid; phagocytosis takes in large particles.
Release of cellular contents by vesicle fusion with the membrane.
Cystic fibrosis.
Selective uptake of molecules using receptors — e.g., LDL uptake.
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