The Cell and Its Functions — Short Notes (Guyton & Hall, Chapter 2)

1. CELL STRUCTURE — Overview

• Water — 70–85% of cell mass; solvent for salts, gases, nutrients, waste
• Electrolytes — K⁺, Mg²⁺, PO₄³⁻ (mainly intracellular); Na⁺, Cl⁻, HCO₃⁻ (mainly extracellular); provide the electrical environment needed for cellular reactions
• Proteins — 10–20% of cell mass; two types:
  • Structural proteins — form long filaments; build microtubules (cytoskeleton, cilia, nerve axons, mitotic spindles), collagen, elastin
  • Functional proteins (enzymes) — tubular-globular; mobile in cell fluid; catalyze chemical reactions (e.g., glucose → CO₂ + H₂O + energy)
• Lipids — ~2% of cell mass; mainly phospholipids and cholesterol → form membranes; triglycerides stored in adipocytes (up to 95% cell mass) as energy reserve
• Carbohydrates — ~1% (up to 3% in muscle, 6% in liver); stored as glycogen; dissolved glucose always available in ECF

2. MEMBRANOUS STRUCTURES OF THE CELL

• Lipid barrier → impedes water-soluble substances
• Protein channels in membranes → specialized transport pathways

3. CELL MEMBRANE (Plasma Membrane)

• Thickness: 7.5–10 nm; thin, pliable, elastic
• Composition: 55% proteins, 25% phospholipids, 13% cholesterol, 4% other lipids, 3% carbohydrates

Lipid Bilayer

1. Phospholipids — most abundant; phosphate head = hydrophilic (faces water); fatty acid tail = hydrophobic (faces center); self-orient into bilayer
2. Sphingolipids — derived from sphingosine; mainly in nerve cells; functions: protection from harmful environment, signal transmission, adhesion sites for extracellular proteins
3. Cholesterol — steroid nucleus = fat-soluble; dissolved in bilayer; regulates degree of permeability to water-soluble substances

• Impermeable to: ions, glucose, urea (water-soluble)
• Freely permeable to: O₂, CO₂, alcohol (fat-soluble)

Membrane Proteins

• Integral proteins — span the full thickness of membrane; form ion channels/pores; act as carrier proteins; function as enzymes
• Peripheral proteins — attached to integral proteins or lipid surface; mostly enzymes or control substances that transmit signals

• Attached to proteins → glycoproteins; attached to lipids → glycolipids
• Together form the glycocalyx (carbohydrate coat)
• Functions: (a) negatively charged → repels other negative objects; (b) loose connective tissue attachment (fibronectin); (c) immune reactions (ABO blood group antigens); (d) receptor sites for hormones (e.g., insulin)

4. CYTOPLASM AND ITS ORGANELLES

A. Endoplasmic Reticulum (ER)

• A network of tubules and vesicles throughout the cytoplasm
• Two types:
  • Granular (Rough) ER: surface studded with ribosomes; synthesizes proteins destined for secretion or membrane use
  • Agranular (Smooth) ER: no ribosomes; synthesizes lipids; stores Ca²⁺ (in muscle); contains drug-metabolizing enzymes

• Synthesis of proteins (rough ER): large polypeptide chains pass into ER lumen → undergo modifications (folding, glycosylation, disulfide bonds)
• Proteins extruded into ER are carried in vesicles to Golgi apparatus
• Synthesis of lipids → phospholipids and cholesterol for membrane replenishment
• Detoxification of drugs by smooth ER enzymes (esp. in liver)
• Initial processing and packaging of substances for export

B. Golgi Apparatus

• Closely related to ER; consists of 4 or more stacked flat vesicular sacs
• Concentrated near nucleus, but distributed throughout the cell
• Receives vesicles from ER ("cis" side), processes them, then packages them at the "trans" side

1. Glycosylation — adds carbohydrate moieties to proteins → forms glycoproteins and proteoglycans
2. Packaging for secretion → forms secretory vesicles that migrate to cell surface → exocytosis (triggered by Ca²⁺ entry)
3. Lysosome formation — packages digestive enzymes into primary lysosomes
4. Membrane replenishment — vesicle membranes fuse with cell membrane or organelle membranes to replenish membrane used in phagocytosis/pinocytosis

C. Lysosomes

• Membrane-enclosed vesicles (~250–750 nm); formed by Golgi apparatus
• Contain hydrolytic (digestive) enzymes (acid hydrolases) — active at pH ~5
• Lysosomal membrane: thick, resistant to enzyme digestion; prevents self-destruction
• Primary lysosomes → fuse with phagocytic/pinocytotic vesicles → secondary lysosomes (digestive vacuoles)
• Digest bacteria, foreign particles, damaged organelles (autophagy)
• Residual body: indigestible material remaining after digestion
• Autolysis: in necrotic or damaged cells, lysosomal membranes rupture → enzymes digest entire cell

D. Peroxisomes

• Similar to lysosomes but contain oxidases (not hydrolases)
• Oxidize various substances using O₂ → produce H₂O₂ → immediately broken down by catalase
• Especially important in liver: oxidize alcohol → detoxification
• Oxidize long-chain fatty acids

E. Secretory Vesicles

• Formed by Golgi apparatus in highly secretory cells
• Diffuse to cell membrane → fuse → exocytosis (stimulated by Ca²⁺)
• Secrete hormones, enzymes, neurotransmitters, etc.

F. Mitochondria

• Present throughout cytoplasm; number varies from a few hundred to thousands depending on cell energy needs
• Structure:
  • Outer membrane: smooth
  • Inner membrane: folded into cristae → greatly increases surface area
  • Matrix between cristae: contains citric acid cycle enzymes, dissolved O₂, CO₂, water, and ATP
  • Two compartments: intermembrane space and matrix
• Mitochondria contain their own DNA → can reproduce themselves (self-replicating)
• Function: "powerhouse of the cell" — oxidative metabolism
  • Nutrients (glucose → pyruvate, fatty acids, amino acids) enter the matrix
  • Citric acid cycle (Krebs cycle) in matrix → releases H atoms
  • Oxidative phosphorylation on inner membrane (electron transport chain + ATP synthase) → produces ATP
  • ~95% of all cellular ATP is produced here

G. Cell Cytoskeleton

1. Microtubules (~25 nm diameter) — made of tubulin polymers; provide structural support; form mitotic spindle during cell division; form cores of cilia and flagella
2. Microfilaments (~6 nm) — composed of actin; involved in cell movement, muscle contraction, cell division (cytokinesis)
3. Intermediate filaments (~10 nm) — mechanical support; various types (desmin, vimentin, cytokeratins)

5. NUCLEUS

• Control center of the cell — contains the genetic code (DNA)
• Structure:
  • Nuclear envelope = double membrane with large pores (~10 nm) allowing substances to pass between nucleus and cytoplasm
  • Nucleolus = dense body inside nucleus; contains large amounts of RNA and proteins; site of ribosome synthesis; may be multiple; absent during mitosis
  • Nucleoplasm = fluid of nucleus; contains DNA organized into chromosomes
• Human cells have 46 chromosomes (23 pairs); total DNA = ~6 feet long when unraveled, packed into a nucleus only 6 μm in diameter
• Chromatin: DNA + histone proteins; condensed into chromosomes during mitosis
• DNA function: controls protein synthesis and cell reproduction

6. FUNCTIONAL SYSTEMS OF THE CELL

A. Endocytosis — Ingestion by the Cell

1. Pinocytosis (cell drinking):
   • Cell membrane invaginates around extracellular fluid + dissolved particles → forms pinocytotic vesicle (~100–200 nm)
   • Continuous in most cells, especially macrophages
   • Driven by: proteins with RGD sequences, clathrin-coated pits
2. Phagocytosis (cell eating):
   • Large particles (bacteria, dead cells, foreign matter) engulfed by pseudopods → form phagocytic vesicle (phagosome)
   • Mainly performed by: macrophages and neutrophils
   • Phagosome fuses with lysosome → phagolysosome → digestion

• Specific proteins on membrane surface bind ligands → invagination → endosome
• LDL cholesterol enters cells this way

B. Lysosomes Digest Ingested Substances

• Pinocytotic/phagocytic vesicles fuse with lysosomes → digestion
• Products (amino acids, fatty acids, glucose, ions) diffuse into cytoplasm for use
• Lysosomal storage diseases: genetic absence of specific lysosomal enzyme → accumulation of undigested substances (e.g., Gaucher disease, Tay-Sachs disease)

7. SYNTHESIS BY ER AND GOLGI APPARATUS

1. mRNA attaches to ribosomes on rough ER
2. Polypeptide chain grows and enters ER lumen
3. Proteins folded, disulfide bridges formed, glycosylation begins
4. Proteins packaged into transport vesicles → migrate to Golgi apparatus

1. Proteins further glycosylated and sorted
2. Packaged into:
   • Secretory vesicles (for exocytosis)
   • Lysosomes
   • Vesicles for intracellular membranes

8. MITOCHONDRIA EXTRACT ENERGY FROM NUTRIENTS

1. Glucose → pyruvate → acetyl-CoA (glycolysis in cytoplasm)
2. Fatty acids → β-oxidation → acetyl-CoA
3. Acetyl-CoA enters citric acid cycle (Krebs cycle) → CO₂ + NADH + FADH₂ + GTP
4. NADH/FADH₂ → electron transport chain on inner mitochondrial membrane → H atoms oxidized by O₂ → H₂O
5. Energy released → ATP synthesis (oxidative phosphorylation via ATP synthase)

ATP — Functional Characteristics

• Structure: adenine + ribose + 3 phosphate groups; last two linked by high-energy phosphate bonds (~7,300 cal/mol each)
• ATP → ADP + Pᵢ + energy (used for all cellular work)
• ADP recharged back to ATP by mitochondria
• Acts as the universal energy currency of the cell
• 1 glucose molecule → ~38 ATP (net) under aerobic conditions
• Used for: muscle contraction, active transport, protein synthesis, cell division, secretion

9. LOCOMOTION OF CELLS

A. Ameboid Movement

• Cell extends pseudopods (false feet) in the direction of movement
• Actin filaments polymerize at leading edge → push membrane forward
• Cytoplasm streams into pseudopod
• Stimulated by: chemotaxis (chemical gradient toward higher concentration)
• Important in: macrophages, neutrophils (move toward bacteria/inflammation)
• Also important in: embryonic cell migration, wound healing

B. Cilia and Ciliary Movement

• Cilia: hair-like projections on cell surfaces (e.g., respiratory epithelium, fallopian tube)
• Structure: 9 + 2 arrangement of microtubules (9 peripheral doublets + 2 central)
• Mechanism: dynein arms (ATPase) walk along adjacent doublet → causes bending
• Ciliary beat: effective stroke (stiff, rapid) + recovery stroke (flexible, slow) → net fluid movement
• Flagella: longer; similar structure; propel sperm
• Primary cilia: single (9+0); no motility; act as sensory antennae

10. KEY SUMMARY TABLE

Fig. 2.1 — Cell Structures Visible Under Light Microscope

The basic cell compartments as seen under light microscopy — what you can identify without electron microscopy.

Fig. 2.2 — Reconstruction of a Typical Cell (All Organelles)

The most important diagram of the chapter — master this and you know the layout of every organelle.

Fig. 2.3 — Structure of the Cell Membrane (Lipid Bilayer)

Shows the fluid mosaic model — phospholipid bilayer with embedded integral proteins, peripheral proteins, and carbohydrate coat on the outer surface.

Fig. 2.11 — Mechanism of Pinocytosis (A→B→C→D)

• A — Coated pit forms; clathrin lines the inner membrane
• B — Proteins bind to receptors in the coated pit
• C — Actin and myosin pull the pit inward (invagination)
• D — Vesicle pinches off; clathrin begins dissolving → pinocytotic vesicle formed inside cytoplasm

Fig. 2.12 — Lysosomal Digestion of Pinocytotic/Phagocytic Vesicles

The complete intracellular digestion pathway from ingestion to excretion of waste.

Fig. 2.15 — ATP Formation in the Mitochondria

• Glucose → glycolysis (cytoplasm) → 2 ATP + pyruvic acid
• Fatty acids + Amino acids → Acetoacetic acid → Acetyl-CoA
• Acetyl-CoA → mitochondria → Krebs cycle → 36 ATP
• O₂ enters mitochondria; CO₂ + H₂O produced as byproducts
• Total: 38 ATP per glucose molecule

Fig. 2.16 — Chemical Structure of Adenosine Triphosphate (ATP)

• The ~ bonds (red wavy lines) between the last two phosphate groups = high-energy phosphate bonds (~7,300 cal/mol each)
• Hydrolysis of ATP → ADP + Pᵢ + usable energy for all cell work

Fig. 2.17 — Ameboid Movement of a Cell

• Cell moves in direction of pseudopodium (leading edge)
• Exocytosis at the leading pseudopodal tip → new membrane + receptor binding to surrounding tissue
• Endocytosis at the trailing end → vesicles stream forward to form new pseudopodal membrane
• Powered by actin-myosin + ATP

Fig. 2.18 — Structure and Function of the Cilium

• Ciliary stalk with membrane, filaments (axoneme)
• Basal plate → anchors to cell membrane
• Basal body → origin of the cilium
• Rootlet → anchors into cytoplasm
• Cross section → 9 peripheral double tubules + 2 central single tubules = 9+2 arrangement
• Inset: Forward stroke (rapid, whiplike, propels fluid) vs. Backward stroke (slow, flexible, no fluid movement)