Anatomy & Physiology: Cell Structure, Membrane Transport, and Cell Communication (4.1–4.6)

4.1c Common Cell Features and General Functions

• Cell components form the basic organization of the cell and establish the internal environment

  • Plasma membrane: forms outer, limiting barrier separating internal contents from external environment

  • Nucleus: largest structure in cell; enclosed by a nuclear envelope; contains genetic material (DNA)

  • Cytoplasm: cellular contents between plasma membrane and nucleus; includes cytosol, organelles, and inclusions

• Cytoplasmic components

  • Cytosol (intracellular fluid): viscous fluid with high water content; contains dissolved macromolecules and ions

  • Organelles: "little organs" with unique shapes and functions; two categories:

  • Membrane-bound organelles (e.g., rough ER, smooth ER, Golgi apparatus, lysosome, peroxisome, mitochondrion, vesicles, inclusions)

  • Non-membrane-bound organelles (e.g., ribosomes, centrosome, proteasomes, cytoskeleton)

• The Structure of a Cell (illustrative figures)

  • Cytosol, interstitial fluid, nucleus, nuclear envelope, nucleoplasm, nucleolus

  • Membrane-bound organelles listed above; cytoplasm includes the contents of the cell interior incl. vesicles and inclusions

  • Plasma membrane modifications: microvilli, cilia, flagellum

• Key cellular functions (overview)

  • Maintain integrity and shape of the cell; depend on the plasma membrane and internal contents

  • Obtain nutrients and form chemical building blocks

  • Harvest energy for survival

  • Dispose of wastes

  • Avoid accumulation that could disrupt cellular activities

4.2a Lipid Components

• Plasma membrane composition and role

  • Fluid mixture composed of approximately equal parts lipid and protein by weight

  • Regulates movement of most substances in and out of the cell

  • Lipids present: phospholipids, cholesterol, glycolipids

• Phospholipids

  • Represented as a “balloon with two tails”: polar, hydrophilic head; two nonpolar, hydrophobic tails

  • Form a phospholipid bilayer: two parallel sheets with tails facing inward and heads outward

  • Bilayer roles:

  • Hydrophobic tails create the internal membrane environment

  • Hydrophilic heads face the cytosol (inside) and interstitial fluid (outside)

  • Ensures cytosol remains inside the cell and interstitial fluid remains outside

• Cholesterol

  • Scattered within the phospholipid bilayer; helps modulate membrane fluidity and stability

• Glycolipids

  • Lipids with attached carbohydrate groups; contribute to glycocalyx

• Glycocalyx

  • Carbohydrate-rich layer on the cell surface formed by glycolipids and glycoproteins; involved in protection, signaling, and recognition

• Structural/functional image reference

  • Figure 4.5 illustrates membrane components and organization (lipids, proteins, and carbohydrate components)

4.2b Membrane Proteins

• General presence and roles

  • Membrane proteins account for about half the plasma membrane by weight

  • They float and move within the fluid bilayer and perform most of membrane functions

• Two major types

  • Integral proteins: embedded within and often spanning the phospholipid bilayer

  • Hydrophobic regions interact with the interior; hydrophilic regions exposed on each side

  • Many are glycoproteins with carbohydrate portions

  • Peripheral proteins: not embedded; loosely attached to external or interior surfaces of the membrane

• Functional categories of proteins

  • Transport proteins: move substances across the membrane

  • Cell surface receptors: bind ligands to relay signals into the cell

  • Identity markers: allow immune recognition and distinction between healthy vs. abnormal cells

  • Enzymes: catalyze reactions at the membrane or within cells

  • Anchoring sites: secure cytoskeleton to the plasma membrane

  • Cell-adhesion proteins: mediate cell-to-cell attachments

• Transport protein subtypes

  • Channels: pore-forming pathways that allow specific ions or water to cross

  • Carriers (transporters): bind a substrate and undergo conformational change to shuttle it across

  • Pumps: actively move substances against their gradient using energy (e.g., ATP)

• Functional details

  • Channel-mediated diffusion: movement of small ions through water-filled channels; channels can be leak (always open) or gated (open in response to a stimulus)

  • Carrier-mediated diffusion: small polar molecules assisted across by a carrier protein; binding induces shape change; can be uniport (one substrate)

• Figure reference

  • Figure 4.6 shows the arrangement of membrane proteins and their functional categories

4.3 Membrane Transport

• Plasma membrane roles in transport and signaling

  • Acts as a physical barrier; regulates movement into and out of the cell; helps establish electrochemical gradients; participates in cell communication

• Classifications of transport processes

  • Passive processes: do not require cellular energy; substances move down their concentration gradient

  • Active processes: require energy; involve movement up a gradient or vesicular transport

• Diffusion spectrum (passive)

  • Simple diffusion: small, nonpolar solutes move directly through the phospholipid bilayer down their concentration gradient

  • Facilitated diffusion: small, polar, or charged solutes require membrane proteins to cross

  • Channel-mediated diffusion: through water-filled channels; specific for ion types; can be leak or gated

  • Carrier-mediated diffusion: carrier proteins bind substrate and undergo conformational change; moves solutes down their gradient; can be a uniporter

• Vesicular and active transport overview

  • Vesicular transport (bulk transport): large substances move via vesicles; requires energy

• Key terms for transport typography

  • Primary active transport: uses energy from ATP hydrolysis (direct use of ATP) to move substances against their gradient

  • Secondary active transport: movement of one substance against gradient powered by movement of another substance down its gradient; includes symport and antiport mechanisms

  • Exocytosis: vesicle fuses with the plasma membrane to release contents outside the cell

  • Endocytosis: vesicle forms to bring substances into the cell; includes phagocytosis, pinocytosis, and receptor-mediated endocytosis

4.3a Passive Processes: Diffusion

• Diffusion basics

  • Net movement of a substance from an area of greater concentration to an area of lesser concentration

  • Driven by kinetic energy; continues until equilibrium is reached

• Rate determinants

  • Steepness of concentration gradient: steeper gradient => faster diffusion

  • Temperature: higher temperature => higher kinetic energy => faster diffusion

• Simple diffusion specifics

  • Solutes that are small and nonpolar diffuse directly between phospholipid molecules

  • Direction follows the concentration gradient

• Facilitated diffusion specifics

  • Requires membrane proteins for small charged or polar solutes

• Channel-mediated diffusion details

  • Small ions pass through specific protein channels

  • Channel types: leak channels (always open) and gated channels (open transiently in response to stimulus)

  • Essential for functions in muscle and nerve cells

• Carrier-mediated diffusion details

  • Small polar molecules cross via carrier proteins that bind to the substance, causing a shape change and release on the other side

  • Uniporter transports a single substance; transport rate limited by the number of channels/carriers

• Visual reference

  • Figure 4.10b shows carrier-mediated diffusion with a glucose carrier protein

4.3b Passive Processes: Osmosis

• Definition and mechanism

  • Osmosis is the movement of water (not solutes) across a selectively permeable membrane

  • Water crosses via between phospholipid molecules or through aquaporin water channels

• Solute permeability distinctions

  • Permeable solutes: tiny, nonpolar solutes (e.g., oxygen, carbon dioxide, urea) pass through the bilayer

  • Nonpermeable solutes: charged, polar, or large solutes (e.g., ions, glucose, proteins) do not pass through the bilayer easily

• Concentration gradients and water distribution

  • Water moves toward the side with higher solute concentration (lower water concentration)

• Osmotic pressure

  • Pressure resulting from water movement across a semipermeable membrane; greater gradient yields greater osmotic pressure; hydrostatic pressure is the押 internal pressure exerted by a fluid

• Tonicity and its cellular effects

  • Isotonic: cytosol and solution have the same solute concentration; no net water movement (e.g., 0.9% NaCl saline)

  • Hypotonic: solution has lower solute concentration than cytosol; water enters the cell; cell volume increases; potential lysis/hemolysis in erythrocytes

  • Hypertonic: solution has higher solute concentration than cytosol; water leaves the cell; cell volume decreases; crenation may occur

• Illustrative examples

  • RBCs in isotonic saline (0.9% NaCl) remain stable

  • RBCs in pure water (hypotonic) swell and may lyse

  • RBCs in 3% NaCl (hypertonic) shrink via crenation

• Osmosis and tonicity summary

  • The tonicity of a solution dictates the direction of water movement and consequent changes in cell volume and osmotic pressure

4.3c Active Processes

• Overview of active transport and vesicular transport

  • Active transport moves substances against their concentration gradient or moves vesicles away from their origin, requiring energy

• Primary active transport

  • Direct use of ATP via phosphorylation of transport proteins

  • Mechanism: ATP binding, phosphorylation, conformational change, and substrate transport

• Ion pumps and the Na+/K+ pump

  • Ion pumps move ions to maintain intracellular concentrations and cell function

  • Sodium–potassium pump (Na+/K+ ATPase): maintains Na+/K+ gradients across the plasma membrane

  • Stoichiometry and energy flow:

  • 3 Na^+ are pumped out of the cell for each ATP consumed; 2 K^+ are pumped into the cell for each ATP consumed

  • ATP hydrolysis drives the conformational changes that move ions

  • Overall representation: $3 ext{ Na}^+ ext{ out}, \ 2 ext{ K}^+ ext{ in}, \ ext{ATP} <br>ightarrow ext{ADP} + P_i$

• Other ion pumps

  • Ca^{2+} pumps in the plasma membrane help maintain calcium homeostasis and prevent cellular rigidity

• Secondary active transport

  • Symport: two substances moved in the same direction by a carrier protein; energy source is the gradient of one substance moving down its gradient (often Na^+ down its gradient)

  • Antiport: two substances moved in opposite directions; energy is derived similarly from a gradient

  • Example: glucose uptake into a cell often coupled to Na^+ movement (Na^+ gradient drives glucose transport via a symporter)

• Vesicular (bulk) transport

  • Exocytosis: secretion of large substances; vesicle fuses with the plasma membrane and releases contents; requires ATP (e.g., neurotransmitter release)

  • Endocytosis: vesicle forms to bring material into the cell; energy-dependent

  • Types of endocytosis:

  • Phagocytosis (cellular eating): engulfs large particles; pseudopodia extend, enclosing the particle in a phagosome; lysosome digestion; performed by select cells (e.g., phagocytic white blood cells)

  • Pinocytosis (cellular drinking): uptake of extracellular fluid and dissolved solutes via small vesicles; performed by most cells

  • Receptor-mediated endocytosis: ligands bind to receptors (often clathrin-coated pits); highly selective uptake of specific bulk quantities (e.g., cholesterol via LDL-receptor)

• Clinical view: Familial Hypercholesterolemia

  • Inherited disorder with defects in LDL receptor or LDL particles; impairs receptor-mediated endocytosis of cholesterol

  • Leads to greatly elevated cholesterol levels and high risk of atherosclerosis and heart attack

4.5a Direct Contact Between Cells

• Direct cell-to-cell contact is essential for certain cellular functions

  • Immune cell interactions (recognition and response)

  • Sperm-oocyte recognition during fertilization (egg glycocalyx interacts with sperm)

  • Cellular regrowth after injury (tissue regeneration)

  • Contact inhibits overgrowth to regulate tissue homeostasis

4.5b Ligand-Receptor Signaling

• Primary means of cellular communication

  • Ligands are signaling molecules that bind to cell surface macromolecule receptors

  • Examples include neurotransmitters and hormones

  • Roles include regulation of growth, reproduction, and various cellular processes

• Three receptor types

  • Channel-linked receptors: open ion channels in response to ligand binding; can initiate rapid electrical changes in excitable cells (e.g., muscle, nerve)

  • Enzymatic receptors: enzyme activity is activated (often receptor tyrosine kinases) leading to phosphorylation of intracellular targets

  • G protein-coupled receptors (GPCRs): activate intracellular second messengers via G proteins; broad roles in signaling

• Illustrative figures

  • Channel-linked receptor (Fig. 4.21a)

  • Enzymatic receptor (Fig. 4.21b)

4.6c Structures of the Cell’s External Surface

• Cilia and flagella

  • Cilia: hair-like projections that move substances along the cell surface

  • Flagella: longer and wider; propel the entire cell (e.g., certain sperm cells)

• Microvilli

  • Extensions of the plasma membrane that increase surface area for absorption and secretion

• Visual reference

  • SEM images illustrating microvilli and related structures (Fig. 4.31)

4.6d Membrane Junctions

• Tight junctions

  • Form strands or rows of proteins linking adjacent cells

  • Prevent substances from passing between cells (transcellular transport required)

• Desmosomes

  • Link neighboring cells via protein connections; provide strong intercellular adhesion

• Hemidesmosomes

  • Anchor the basal layer of epithelial cells to the basement membrane

• Gap junctions

  • Form tiny, direct communication channels (connexons) that allow ions and small molecules to pass between cells (e.g., in cardiac muscle)

• Visual reference

  • Figure 4.32 shows the arrangement and components of tight junctions, desmosomes, hemidesmosomes, and gap junctions

Clinical and real-world connections

• LDL receptor defects and disease mechanism

  • LDL receptor impairment disrupts receptor-mediated endocytosis of cholesterol, leading to high circulating cholesterol and risk of atherosclerosis

• Relevance to pharmacology and therapy

  • Understanding membrane transport and receptor signaling informs drug design (e.g., drugs targeting GPCRs, ion channels, or receptor-mediated endocytosis pathways)

Summary of key numerical references and formulas

• Erythrocyte size: approximately $7{-}8 ext{ μm}$ in diameter

• Human oocyte diameter: approximately $120 ext{ μm}$

• Isotonic saline example: $0.9\%\ NaCl$

• Na+/K+ pump stoichiometry: $3\ Na^+\text{ out}$ and $2\ K^+\text{ in}$ per ATP hydrolysis; ATP → ADP + P_i

• Typical osmotic scenarios:

  • Hypotonic: water moves into the cell; potential lysis (hemolysis in erythrocytes)

  • Hypertonic: water moves out of the cell; potential crenation

• LDL cholesterol uptake example: receptor-mediated endocytosis via LDL receptors and clathrin-coated pits

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