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Cell Transport, Cytoskeleton, and Organelles – Comprehensive Study Notes 7

Osmosis and Osmolarity

  • Recap: Osmosis is diffusion of water across a semipermeable membrane where water can pass but solutes may not (depending on permeability).

  • Semipermeable membrane: water moves; solute (e.g., salt) may be blocked if it’s too large to pass.

  • Osmotic pressure: the hydrostatic pressure required to stop osmosis. In other words, the pressure that opposes water movement across a semipermeable membrane.

  • Hydrostatic pressure: the pressure exerted on the membrane from the fluid itself (a counterforce to osmotic movement).

  • Osmolarity: osmotic concentration; amount of non-permeant solutes per liter of solution. Precisely, the osmolarity of a solution is the amount of non‑permeable solute per 1 L of solution.

    • In symbols: ext{Osmolarity}= rac{ ext{osmoles of non-permeant solutes}}{1~ ext{L}} \, \text{(Osmol/L)}

  • Body fluids have varying osmolarities; overall body fluid osmolarity is tightly regulated around 300\ ext{mOsm/L} (homeostasis).

  • Osmolarity versus tonicity: Osmolarity is a measure of solute particles; tonicity refers to the effect of a solution on the volume and pressure of a cell in a given environment.

  • Relevance to filtration and purification: Example of reverse osmosis—applying pressure to move water against the osmotic gradient for purification processes.

  • Quick practical note:

    • Normal saline used clinically is 0.9\%\ \mathrm{NaCl}; considered isotonic with respect to body fluids, so it does not cause net water movement into or out of cells.

Tonicity

  • Definition: Tonicity is the ability of a surrounding solution to affect the fluid volume and pressure inside a cell.

  • Driven by concentration of nonpermeant solutes and the ability of solutes to cross the membrane.

  • If outside solute concentration differs from inside, water moves to balance solute concentration.

  • Terms to know:

    • Isotonic: same solute concentration inside and outside the cell → no net water movement.

    • Hypotonic: outside solution has lower solute concentration (more water) than inside; water moves into the cell; can cause swelling and lysis if excessive.

    • Hypertonic: outside solution has higher solute concentration (less water) than inside; water moves out of the cell; the cell shrinks (crenation).

  • Practical examples:

    • Distilled water is highly hypotonic to cells and can cause cell lysis if used in IVs by mistake.

    • Isotonic solutions keep cells’ volume stable; isotonic IV fluids commonly use 0.9% NaCl.

  • Visual aid concept: isotonic cells maintain a stable biconcave shape (e.g., red blood cells in isotonic solution).

  • Caution: If a cell becomes hypotonic and inflates, it can burst (lyse). If hypertonic, it can crenate (shrink).

Carrier-Mediated Transport

  • Cells use membrane proteins called carriers to move substances that can’t cross the lipid bilayer directly.

  • Carriers are highly specific – they bind particular solutes and undergo conformational changes to shuttle the solute across.

  • Key concept: Saturation (transport maximum, Tm) occurs when all carrier proteins are occupied; increasing solute concentration no longer increases transport rate.

    • Graph idea: y-axis = rate of solute transport; x-axis = solute concentration; curve plateaus at Tm.

  • Carrier transport types (prefix meanings):

    • Uniport: carries one type of solute in one direction.

    • Symport (co-transport): carries two or more solutes in the same direction.

    • Antiport (counter-transport): carries two solutes in opposite directions.

  • Examples:

    • Uniport: calcium pumps (Ca^{2+}) moving calcium out of cells.

    • Symport: sodium-glucose cotransporters (SGLT) moving Na^{+} and glucose into the cell.

    • Antiport: Na^{+}/H^{+} antiport (illustrative example; other antiport systems exist).

  • Major transport mechanisms that use carriers:

    • Facilitated diffusion: diffusion aided by a carrier; no ATP required; high to low concentration.

    • Primary active transport: uses ATP directly to move solutes against their gradients (e.g., sodium-potassium pump).

    • Secondary active transport: uses the gradient created by primary active transport to move another solute; ATP is not directly used by the secondary transport step.

  • Key details on specific systems:

    • Sodium-Potassium Pump (Na^{+}/K^{+} ATPase): maintains gradients across the membrane that are essential for membrane potential and secondary transport.

    • Typical cycle: moves 3 Na^{+} out and 2 K^{+} in per ATP hydrolyzed.

    • The pump consumes energy (ATP) to maintain Na^+ outside and K^+ inside, keeping the cell’s resting membrane potential negative inside.

    • Functions include maintaining the gradient, enabling secondary transport, and contributing to membrane potential and heat production.

    • Secondary Active Transport (e.g., Sodium-Glucose Cotransporter): relies on the Na^{+} gradient established by the Na^{+}/K^{+} pump to drive glucose uptake against its gradient.

    • Example context: kidney and intestine where glucose uptake is coupled to Na^{+} movement.

Vesicular Transport (Endocytosis and Exocytosis)

  • Vesicular transport uses vesicles (membrane-bound bubbles) to move large particles, fluids, or macromolecules.

  • All vesicular transport requires energy.

  • Endocytosis (into the cell): three main forms

    • Phagocytosis: cell eating; engulfing particulate matter (e.g., bacteria) by forming pseudopods to surround the particle; forms a phagosome; phagosomal contents are degraded in lysosomes after fusion (phagolysosome).

    • Pinocytosis: cell drinking; uptake of extracellular fluid and dissolved solutes.

    • Receptor-mediated endocytosis: highly specific uptake triggered by binding to cell-surface receptors; forms a clathrin-coated pit; vesicle forms and internalizes the bound material; can be followed by cross-cell transport (transcytosis) or delivery to endosomes/lysosomes.

  • Exocytosis (out of the cell): vesicles fuse with the plasma membrane and release their contents; helps in secretion (e.g., insulin from pancreatic islets) and replenishes plasma membrane after vesicle addition.

  • Additional notes:

    • Endocytosis and exocytosis can occur concurrently in cells.

    • Vesicular docking proteins mediate vesicle attachment to the plasma membrane before fusion.

  • Visual cues mentioned: phagosome, phagolysosome, clathrin-coated pits, docking events.

Cytoskeleton

  • The cytoskeleton provides structural support, determines cell shape, and enables movement of materials inside the cell.

  • Three main types of filaments:

    • Microfilaments (actin): form the terminal web in some contexts; support the plasma membrane and aid in vesicle movement; found in microvilli.

    • Intermediate filaments (keratin): high tensile strength; provide mechanical resistance to stress in all directions.

    • Microtubules (tubulin): radiate from the centrosome;柱 support organelle positioning and intracellular transport; form the cilia/flagella structure; form the mitotic spindle during cell division.

  • Microvilli: surface projections supported by microfilaments (actin filaments) that increase surface area.

  • Desmosomes (adhering junctions) involve intermediate filaments to provide mechanical cohesion between cells.

  • Important notes:

    • Cilia and flagella are built from microtubules; 9+2 arrangement is typical in motile cilia/flagella.

    • The mitotic spindle is composed of microtubules and is essential for accurate chromosome separation during mitosis.

Membranous and Non-Membranous Organelles

  • Distinguish membranous vs non-membranous organelles.

  • Membranous organelles (surrounded by membranes):

    • Nucleus (largest organelle): double membrane with nuclear pores; contains nucleoplasm, chromatin (DNA + proteins), and nucleolus (ribosome synthesis site).

    • Endoplasmic reticulum (ER): network of membranous tubules within cytoplasm.

    • Rough ER: studded with ribosomes; site of protein synthesis destined for secretion or membranes.

    • Smooth ER: lacks ribosomes; site of lipid synthesis, detoxification of alcohol/drugs, and calcium storage. Muscle-specific variant is the sarcoplasmic reticulum (Ca^{2+} storage).

    • Golgi apparatus (Golgi complex): membranous structure that receives proteins from rough ER, sorts and modifies them, and packages them into vesicles for transport to lysosomes or the plasma membrane (UPS-like function).

    • Lysosomes: acidic compartments with digestive enzymes; digest ingested materials and old organelles; autophagy is the digestion of aged organelles; autolysis refers to self-destruction via lysosomal enzymes.

    • Peroxisomes: detoxification centers similar to lysosomes but with different enzymes; break down hydrogen peroxide to water and oxygen (catalase); detoxify free radicals, alcohol, and drugs; abundant in liver and kidney.

    • Mitochondria: ATP production; have cristae (folded inner membrane surfaces) where oxidative phosphorylation occurs; oxygen presence enhances ATP yield; mitochondria are key to energy metabolism.

  • Non-membranous organelles (no surrounding membrane):

    • Centrosomes and centrioles: organize microtubules; centrioles have a 9+0 or 9+2-like organization in some contexts; important in forming the mitotic spindle and organizing cilia in some cells.

    • Nucleolus (within nucleus): ribosome synthesis site; not a membrane-bound organelle.

    • Cytoskeleton components (microfilaments, intermediate filaments, microtubules) are non-membranous scaffolds.

  • Nucleus details recap:

    • Nuclear envelope with pores controls traffic between nucleus and cytoplasm.

    • Nucleoplasm contains chromatin and nucleolus within the nucleus.

Nucleus and Genetic Material

  • Nuclear envelope: double membrane; contains nuclear pores to regulate transport of molecules (e.g., mRNA, ribosomal subunits).

  • Nucleoplasm: fluid inside the nucleus.

  • Chromatin: DNA packaged with proteins; chromosomal material in the nucleus.

  • Nucleolus: site of ribosome synthesis; assembly of ribosomal subunits occurs here.

Endoplasmic Reticulum and Golgi—Protein and Lipid Traffic

  • Rough Endoplasmic Reticulum (RER):

    • Studded with ribosomes.

    • Ribosomes synthesize proteins destined for secretion, lysosomes, or the plasma membrane.

  • Smooth Endoplasmic Reticulum (SER):

    • Lacks ribosomes.

    • Synthesizes lipids; detoxifies drugs and alcohol; stores calcium in muscle cells (sarcoplasmic reticulum variant).

  • Golgi apparatus: receives proteins from RER, modifies and sorts them, then packages them into vesicles for delivery.

    • Vesicles may become lysosomes or fuse with the plasma membrane to release their contents or contribute membrane components.

    • Analogy: UPS or FedEx—sorting, modifying, and shipping proteins to their destinations.

Lysosomes, Peroxisomes, and Mitochondria

  • Lysosomes:

    • Contain digestive enzymes; digest endocytosed material, worn-out organelles, and cellular debris.

    • Autophagy: lysosomal digestion of aged organelles.

    • Autolysis: self-destruction via lysosomal enzymes when needed.

  • Peroxisomes:

    • Detoxify harmful substances; break down hydrogen peroxide (H{2}O{2}) to water and oxygen via catalase; neutralize free radicals, detoxify alcohol and drugs.

    • Abundant in liver and kidney.

  • Mitochondria:

    • The powerhouses of the cell; generate ATP via aerobic respiration when oxygen is available.

    • Contain cristae, which are folds of the inner membrane that increase surface area for energy production.

    • In contexts with low/no oxygen, pyruvate can be converted to lactate (anaerobic glycolysis pathway relevance).

Cytoskeleton and Motor Organization in Cells

  • Reiterate: cytoskeleton supports structure, shapes cells, and moves materials.

  • Key structures and roles:

    • Microfilaments (actin): support the plasma membrane and help form structures like the terminal web; essential for vesicle movement and changes in cell shape.

    • Intermediate filaments (keratin): high tensile strength; provide resistance to mechanical stress in all directions.

    • Microtubules (tubulin): radiate from the center (centrosome); involved in organelle positioning, intracellular transport, cilia/flagella structure, and mitotic spindle formation during cell division.

  • Relationship to organelles:

    • Cilia and flagella are built from microtubules.

    • The mitotic spindle is composed of microtubules and is crucial for chromosome separation in cell division.

Summary of Key Concepts and Implications

  • Osmosis and osmolarity define how water moves and how cell and tissue fluids maintain homeostasis; osmosis is water diffusion; osmotic pressure is the driving opposing force to water movement; hydrostatic pressure is the opposing fluid pressure.

  • Tonicity (isotonic, hypotonic, hypertonic) determines whether cells swell, shrink, or stay the same when placed in a solution; exact solute concentrations and the impermeability of certain solutes drive these outcomes.

  • Carrier-mediated transport provides selective, saturable routes for solutes that can’t cross the lipid bilayer directly; saturation (transport maximum) limits the rate.

  • The Na^{+}/K^{+} pump maintains ion gradients essential for resting membrane potential, secondary transport, and overall cellular homeostasis; energy is expended via ATP hydrolysis to move Na^{+} out and K^{+} in (3:2 ratio).

  • Secondary active transport relies on gradients created by primary active transport (like the Na^{+}/K^{+} pump) to drive uptake of other solutes (e.g., glucose with Na^{+}).

  • Vesicular transport (endocytosis and exocytosis) moves large particles and macromolecules; receptor-mediated endocytosis enables highly specific uptake; exocytosis releases substances and can replenish membrane area.

  • The cytoskeleton provides structural integrity and dynamic functionality, enabling vesicle trafficking, maintaining cell shape, and facilitating movement during processes such as division and locomotion.

  • Organelles are compartmentalized to optimize cellular processes:

    • Nucleus houses genetic material and transcription/assembly processes; nuclear pores regulate exchange with cytoplasm.

    • ER and Golgi coordinate protein and lipid synthesis, modification, and trafficking.

    • Lysosomes and peroxisomes handle degradation and detoxification; mitochondria generate cellular energy and regulate metabolic pathways.

  • Real-world relevance and clinical context:

    • Isotonic saline solutions are used clinically to avoid disturbing cell volume.

    • Understanding osmosis and tonicity is critical when considering IV fluid therapy, dehydration, electrolyte balance, and tissue hydration.

    • Dysfunctions in ion pumps, transporter saturation, or vesicular trafficking can contribute to a variety of diseases, including muscular, nervous, and metabolic disorders.

Practice prompts (conceptual applications)

  • If a cell is placed in a solution with 0.9% NaCl, what is the expected water movement and why? Mention tonicity and the role of impermeant solutes.

  • Describe how the Na^{+}/K^{+} pump contributes to secondary active transport of glucose in the renal tubule or intestinal epithelium.

  • Explain why distilled water is hazardous if injected IV in terms of tonicity and cell lysis.

  • Outline the steps of receptor-mediated endocytosis and explain why clathrin-coated pits are important.

  • Differentiate between microfilaments, intermediate filaments, and microtubules in terms of composition (protein), primary role, and a cellular example of each.