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Cell Structure and Membrane Transport — Comprehensive Study Notes 6

Cell Shape Terminology

  • Columnar: column-shaped; name reflects the columnar appearance.
  • Polygonal: poly- means many; cell with many sides; usually irregular, not symmetrical.
  • Stellate: star-like with radiating extensions.
  • Spheroid to ovoid: sphere-like to oval-shaped; sphere is round; ovoid means egg-shaped.
  • Discoidal: disc-shaped (like a Frisbee) with a central indentation; red blood cells are discoidal in shape.
  • Fusiform: spindle-shaped; thicker in the middle with tapered ends; sometimes called "spindle-shaped".
  • Fibrous: thread-like; elongated and thin.
  • Visual reminder: cells can look different depending on viewing angle (top view vs. cross-section).

Common cell shapes (quick visual reference)

  • Squamous: very flat; tile-like.
  • Cuboidal: cube-like.
  • Columnar: taller than wide.
  • Polygonal: five-sided or more.
  • Stellate: star-like with rays.
  • Sphere/Spheroid/Ovoid: round to oval.
  • Discoidal: disc-shaped (RBCs).
  • Fusiform: elongated with widened middle and tapered ends; sometimes called spindle-shaped.
  • Fibrous: thread-like.

Size and importance of shape in lab contexts

  • Most human cells: diameter ≈ 10\,\mu\text{m}.
  • Egg cells (oocytes): diameter ≈ 100\,\mu\text{m}.
  • Some nerve cells can be extremely long (up to ≈ 1\,\text{m}).
  • Important concept: there is a limit on cell size due to surface area-to-volume constraints; cells may rupture if overwhelmed by volume.

Cell Size and Surface Area–Volume Relationship

  • In general for a sphere:
    • Surface area: \text{SA} = 4\pi r^2
    • Volume: \text{V} = \frac{4}{3}\pi r^3
  • If the diameter (and thus radius) doubles (i.e., r \to 2r):
    • New surface area: \text{SA}' = 4\pi (2r)^2 = 16\pi r^2 = 4\text{SA}
    • New volume: \text{V}' = \frac{4}{3}\pi (2r)^3 = 8\cdot\frac{4}{3}\pi r^3 = 8\text{V}
  • Take-home: when a cell grows, its volume increases eightfold whereas surface area increases fourfold, stressing the boundary and risking rupture if volume outpaces boundary capacity.

Practical numeric example

  • If a cell grows from d=10\,\mu\text{m} to d=20\,\mu\text{m}, radius rises from r=5\,\mu\text{m} to r=10\,\mu\text{m}.
  • That growth pattern follows the same scaling: SA ↑ by factor 4; V ↑ by factor 8.

Three Core Cell Components

  • Plasma membrane (cell membrane): boundary of the cell; also called cell membrane; sometimes called plasma membrane.
  • Cytoplasm: everything inside the plasma membrane excluding the nucleus; contains organelles, cytoskeleton, inclusions.
  • Nucleus: contains DNA (deoxyribonucleic acid).
  • Important terminology:
    • Cytosol: the liquid portion inside the cell (intracellular fluid, ICF).
    • Cytoplasm: cytosol plus organelles.
    • Extracellular fluid (ECF): all fluid outside cells; includes interstitial fluid, plasma, lymph, cerebrospinal fluid (CSF).
  • Quick definitions:
    • Interstitial fluid / tissue fluid: fluid surrounding cells in tissues; part of ECF.
    • Plasma: liquid component of blood outside cells; part of ECF.
    • Lymph: extracellular fluid in the lymphatic system; part of ECF.
    • CSF: cerebrospinal fluid in brain and spinal cord; part of ECF.

Cytoplasm vs Cytosol (tricky terminology)

  • Cytoplasm = cytosol + organelles.
  • Cytosol = intracellular fluid (ICF) only.

The Plasma Membrane: Structure and Composition

  • The plasma membrane defines the boundary between the cell's interior and the external environment.
  • It is a phospholipid bilayer with embedded proteins and other lipids.
  • Composition (typical proportions):
    • Phospholipids: ≈ 75\% of the membrane lipids.
    • Cholesterol: contributes to membrane structure and stiffness.
    • Glycolipids: contribute to the glycocalyx; together with glycoproteins contribute to the 5% glycolipids noted in the membrane.
  • Orientation of phospholipids: hydrophilic (water-loving) heads face the aqueous inside and outside; hydrophobic (water-fearing) tails face inward toward each other.
  • The membrane displays a bilayer structure with two faces: intracellular face and extracellular face.
  • Key takeaway: membrane lipids form a stable barrier; the hydrophobic core restricts passage of most water-soluble (hydrophilic) substances.

Membrane Proteins: Types and Functions

  • Two broad categories:
    • Transmembrane proteins: span the entire membrane; often have hydrophilic regions on the surfaces and hydrophobic regions within the bilayer; many are glycoproteins; some are anchored to cytoskeletal elements.
    • Peripheral proteins: attached to the interior or exterior surface; do not span the membrane.
  • Functions of membrane proteins:
    • Receptors: bind chemical signals (e.g., neurotransmitters like acetylcholine) to elicit responses inside the cell.
    • Enzymes: catalyze reactions at the membrane surface, sometimes producing second messengers.
    • Channel proteins: form pathways for substances to cross the membrane.
    • Carriers: bind solutes and transport them across the membrane (often against gradient).
    • Pumps: membrane proteins that actively transport substances using energy (ATP).
    • Cell identity markers: glycoproteins that identify cell type (e.g., muscle cell vs. bone cell).
    • Cell adhesion molecules (CAMs): link cells together or to extracellular matrix.
  • Channel types (focus for this course):
    • Leak channels: always open; allow continuous movement down the gradient.
    • Ligand-gated channels: open in response to binding of a chemical ligand (e.g., a neurotransmitter).
    • Voltage-gated channels: open in response to a change in membrane potential (voltage across the membrane).
    • Mechanically gated channels: respond to mechanical stress; not a primary focus in this course.
  • Important note on gating: ligand-gated channels open when a chemical binds; voltage-gated channels open when the membrane potential changes (inside becomes less negative or positive).

The Glycocalyx

  • Definition: the carbohydrate portions of glycoproteins and glycolipids on the cell surface.
  • Each person has a unique glycocalyx (except identical twins).
  • Roles:
    • Immune system recognition: helps immune cells distinguish self vs. non-self; contributes to immune defense against pathogens.
    • Cancer relevance: changes in the glycocalyx can reflect unregulated cell growth and influence immune detection.
  • Tie-in: glycocalyx contributes to cell identity markers and interacts with the extracellular environment.

Extensions from the Cell Surface

  • Microvilli: small, finger-like projections that increase surface area; up to ~40x increase; important for absorption (e.g., in the small intestine); also called the brush border.
  • Cilia: hair-like, motile structures used to move substances across cell surfaces; act as an antenna to sense the environment; found in respiratory tract, fallopian tubes, ventricles of the brain, epididymal ducts; composed of microtubules with a typical 9+2 arrangement; power stroke and recovery stroke move mucus or CSF.
  • Flagellum: longer, whiplike projection that moves the cell itself; in humans, the only flagellated cell is the sperm cell.
  • Pseudopods: false feet; amoeboid motion; used for locomotion and phagocytosis (engulfing particles like bacteria).

Membrane Transport: How Substances Cross the Membrane

  • The plasma membrane is selectively permeable: allows some substances to pass while restricting others.
  • Transport categories:
    • Passive processes: do not require ATP.
    • Filtration: movement of water and solutes through gaps between cells or pores under pressure (e.g., in capillaries); high-to-low pressure.
    • Diffusion: movement of solutes from high concentration to low concentration (down the concentration gradient); can occur through the membrane if permeable to the solute; affected by temperature, gradient steepness, surface area, and permeability; small molecules diffuse faster; larger molecules diffuse slower (note raised in class to consider organization on notes; the instructor suggested removing molecular weight as a factor, but it is often taught as a factor).
    • Osmosis: diffusion of water through a selectively permeable membrane; water moves from area of higher water concentration to lower water concentration (equivalently, from lower solute concentration to higher solute concentration); can occur through lipid bilayer or via aquaporins (water channels).
    • Active processes: require ATP.
    • Active transport: pumping solutes against their gradient using energy (e.g., Na+/K+ pump).
    • Vesicular transport: endocytosis and exocytosis; packaging in vesicles to move large particles or bulk fluids.
    • Carrier-mediated transport: use specific membrane proteins to move substances; may be passive or active depending on transporter type.

Key Concepts in Passive Transport

  • Filtration:
    • Driven by pressure differences; small solutes and water move through capillary pores; large cells like RBCs cannot pass through filtration slits.
  • Diffusion:
    • Movement down the concentration gradient; factors include temperature (↑ temp ↑ rate), steepness of gradient (↑ gradient ↑ rate), membrane surface area (↑ area ↑ rate), and membrane permeability (↑ permeability ↑ rate).
  • Osmosis:
    • Critical for maintaining fluid balance and intravenous therapies; governed by water movement relative to solute concentrations; aquaporins provide selective water transport channels.
    • Osmotic balance relates to tonicity in clinical settings (IV fluids, dehydration, edema).
    • Visual aid from the lecture: water (blue) moves toward higher solute concentration (red) across a semipermeable membrane; over time, side with higher solute gains more liquid as water moves in.

Extracellular Fluid (ECF) and Interstitial Context

  • ECF includes plasma, interstitial fluid, lymph, and CSF.
  • The composition of ECF influences diffusion and osmosis across membranes; disturbances can cause edema, dehydration, or shifts in cellular volume.

Practical Takeaways for Exam Preparation

  • Be able to identify and explain: columnar, polygonal, stellate, spheroid/ovoid, discoidal, fusiform, fibrous cell shapes; give examples where these appear (e.g., squamous epithelial cells, RBCs, muscle cells).
  • Remember the three core cell components and definitions of cytosol vs cytoplasm.
  • Know plasma membrane composition and the distinct roles of phospholipids, cholesterol, and glycolipids; be able to identify the orientation of the bilayer and why the membrane is selectively permeable.
  • Distinguish transmembrane vs peripheral proteins and list their functions; memorize the three channel types most emphasized (leak, ligand-gated, voltage-gated).
  • Understand glycocalyx: composition, uniqueness, and immunological implications, including cancer context.
  • Recognize the major extensions of the cell surface (microvilli, cilia, flagellum, pseudopods) and their functional differences.
  • Grasp the basic ideas of diffusion, filtration, and osmosis; identify factors that influence diffusion rates; understand aquaporins and the clinical relevance of osmosis (IV fluids, tonicity).
  • Recall the extracellular fluid compartments and the concept of ICF vs ECF for test questions.
  • Retrieval practice tips:
    • Practice recall without notes, then check against notes.
    • Use study groups to verbalize and reteach terms; teach each other to reinforce memory.
    • Create prompts with sparse cues and fill in details from memory.

Quick Visual References to Remember

  • Phospholipid bilayer with hydrophilic heads facing water and hydrophobic tails inward.
  • Transmembrane proteins spanning the bilayer; hydrophilic regions outside/inside; hydrophobic region in the core.
  • Glycocalyx on the exterior cell surface as a unique chemical signature.
  • Microvilli (brush border) dramatically increase surface area; cilia vs. flagella distinction in mobility and function.
  • Diagrammatic growth of a cell showing SA vs V scaling: ext{SA} \propto r^2, \quad \text{V} \propto r^3, \quad r \to 2r \Rightarrow \text{SA}' = 4\text{SA}, \text{V}' = 8\text{V}

Note on study strategy mentioned in the lecture: the recall (retrieval) practice is essential. Use sparse recall prompts, discuss with peers, and test yourself repeatedly to convert from recognition to recall.