Cell Structure and Function

Historical Foundations and Cell Theory

Robert Hooke (1665): Examined cork under a primitive compound microscope and coined the term cell to describe the empty box-like structures observed in plant tissue.
Theodor Schwann (1800s): Concluded after extensive studies of animal tissues that all animals are composed of cells, formulating the first major postulate of cell theory: all living organisms are composed of cells.
Louis Pasteur (circa 1859): Conducted seminal experiments with bacteria that disproved the hypothesis of spontaneous generation (the idea that living organisms can arise spontaneously from nonliving matter).
Establishment of Cell Origin: By the end of the 19th century, Pasteur's work established the second major postulate of cell theory: cells arise only from pre-existing cells.

Principles of Modern Cell Theory

Composition: All living organisms are composed of one or more cells and cell products.
Fundamental Unit: The cell is the simplest structural and functional unit of life. No structure smaller than a cell exhibits all properties of life.
Organismal Function: An organism's total structure and functional activities are due to the collective physiological activities of its constituent cells.
Ancestry: Cells come only from pre-existing cells; therefore, all living cells trace their evolutionary ancestry back to the same original ancestral cells.
Biochemical Uniformity: The cells of all species share fundamental similarities in chemical composition, metabolic pathways, and structural organization.

Microscopy and Resolution

Definition of Resolution: Resolution is the ability of an optical system to distinguish between two adjacent points and reveal fine structural detail.
Unaided Human Eye (Naked Eye): Has a resolution limit of approximately $100\,\mu\text{m}$ .
Light Microscope (LM):
- Achieves a resolution limit of approximately $200\,\text{nm}$ .
- Allows visualization of the outer plasma membrane, the nucleus, and the general cytoplasm (including primary organelles).
Electron Microscope (EM):
- Achieves a resolution limit of approximately $1\,\text{nm}$ .
- Enables detailed observation of fine cellular ultrastructures within the cytoplasm, including organelle membranes and molecular complexes.

Cellular Morphology and Functions

Overview of cell shapes including squamous, polygonal, cuboidal, columnar, spheroid, discoid, stellate, fusiform, and fibrous.

Squamous: Thin, flat, scaly shape with a bulge where the nucleus is located (e.g., surface cells of the skin, lining of blood vessels).
Polygonal: Having irregularly angular shapes with four, five, or more sides.
Cuboidal: Squarish-looking in frontal sections; approximately equal in height and width (e.g., liver cells, kidney tubule epithelial cells).
Columnar: Distinctly taller than wide, forming rectangular or column-like shapes (e.g., inner lining cells of the stomach and intestines).
Spheroid: Spherical to oval shape (e.g., egg cells, circulating white blood cells).
Discoid: Disc-shaped with a biconcave profile (e.g., red blood cells / erythrocytes).
Stellate: Star-like shape with multiple broad or pointed extensions radiating from the central cell body (e.g., nerve cell bodies / neurons).
Fusiform: Spindle-shaped; elongated with a thick middle section and tapered ends (e.g., smooth muscle cells).
Fibrous: Long, slender, threadlike shape (e.g., skeletal muscle fibers, collagen-producing connective tissue cells).

Cell Size and Surface Area to Volume Limitations

Comparison of surface area and volume scaling as cell diameter increases from 10 micrometers to 20 micrometers.

Human Cell Dimensional Ranges:
- Most human cells range between $10\,\mu\text{m}$ and $15\,\mu\text{m}$ in diameter.
- Human egg cells (oocytes) are unusually large, reaching $100\,\mu\text{m}$ in diameter (visible to the naked eye).
- Skeletal muscle cells can reach up to $30\,\text{cm}$ in length.
- Certain nerve cells extend over $1\,\text{m}$ in length.
Mathematical Relationship of Growth:
- For a cell modeled as a cube with side length / diameter $D$ :
  - Small Cell ( $D = 10\,\mu\text{m}$ ):
    - $\text{Diameter} = 10\,\mu\text{m}$
    - $\text{Surface Area} = 10\,\mu\text{m} \times 10\,\mu\text{m} \times 6 = 600\,\mu\text{m}^2$
    - $\text{Volume} = 10\,\mu\text{m} \times 10\,\mu\text{m} \times 10\,\mu\text{m} = 1,000\,\mu\text{m}^3$
  - Large Cell ( $D = 20\,\mu\text{m}$ ):
    - $\text{Diameter} = 20\,\mu\text{m}$
    - $\text{Surface Area} = 20\,\mu\text{m} \times 20\,\mu\text{m} \times 6 = 2,400\,\mu\text{m}^2$
    - $\text{Volume} = 20\,\mu\text{m} \times 20\,\mu\text{m} \times 20\,\mu\text{m} = 8,000\,\mu\text{m}^3$
- Scaling Factors:
  - Increasing cell diameter ( $D$ ) by a factor of $2$ increases surface area by a factor of $4$ ( $= D^2$ ).
  - Increasing cell diameter ( $D$ ) by a factor of $2$ increases volume by a factor of $8$ ( $= D^3$ ).
Physiological Constraint on Size:
- Volume grows much faster than surface area as a cell expands.
- A cell that becomes too large will have insufficient plasma membrane surface area to serve the metabolic transport needs (nutrient intake and waste elimination) required by its increased volume.

General Cellular Compartments and Ultrastructure

Diagram of a generalized cell illustrating organelles, cytoskeleton, cell surfaces, and plasma membrane.

Major Structural Domains:
- Plasma (Cell) Membrane: Outer membrane defining cell boundaries.
- Cytoplasm: Contains cytosol (intracellular fluid, ICF), cytoskeleton, organelles, and inclusions.
- Extracellular Fluid (ECF): Fluid located outside the cell membrane, also known as tissue fluid or interstitial fluid.
Organelles and Subcellular Components:
- Nucleus: Contains genetic material; enclosed by a double-membrane nuclear envelope pierced with nuclear pores.
- Rough Endoplasmic Reticulum (Rough ER): Parallel membrane cisternae covered with ribosomes on rough ER; synthesizes proteins.
- Smooth Endoplasmic Reticulum (Smooth ER): Tubular membrane network lacking ribosomes; synthesizes lipids and detoxifies chemicals.
- Ribosomes Free in Cytosol: Unattached protein-synthesizing complexes floating in the cytosol.
- Golgi Complex: Stacked membranous sacs that process and package proteins.
- Golgi Vesicle & Secretory Vesicles: Membrane-bound spheres formed by the Golgi complex to transport or secrete cellular products.
- Mitochondrion: Double-membrane organelle containing internal folds; produces ATP through aerobic respiration.
- Lysosome: Vesicle containing hydrolytic enzymes for intracellular digestion.
- Centrosome & Centriole: Microtubule-organizing center essential for chromosome segregation during cell division.
- Cytoskeleton: Structural network composed of microtubules, microfilaments, and the terminal web.
- Inclusions: Stored cellular products, such as a lipid droplet.
- Surface Specializations:
  - Microvillus: Finger-like extensions on the apical surface of cell that increase absorptive surface area.
  - Lateral Surface of Cell: Regions contacting adjacent neighboring cells.
  - Basal Surface of Cell: Bottom face resting on the underlying basement membrane.

Transmission electron micrograph showing the ultrastructure of a white blood cell with labeled organelles.

Leukocyte Ultrastructure Example: Transmission electron micrographs reveal internal leukocyte organization, showing the plasma membrane enclosing a prominent nucleus with nuclear envelope, nearby mitochondria, Golgi complex, Golgi vesicles, and scattered ribosomes (scale bar $2.0\,\mu\text{m}$ ).

The Structure and Function of the Plasma Membrane

Ultrastructural Appearance: Under transmission electron microscopy, the plasma membrane appears as a pair of dark parallel lines surrounding the cell with a thickness of approximately $7.5\,\text{nm}$ .
Boundary Function: Defines the boundary of the cell, separating the intracellular fluid (ICF) inside the cytoplasm from the extracellular fluid (ECF) outside.
Physiological Functions:
- Controls intercellular communication and interactions with other cells.
- Controls the passage of materials in and out of the cell via selective permeability.

Membrane Lipid Composition

Detailed diagram of the fluid mosaic plasma membrane showing phospholipid bilayer, cholesterol, glycolipids, glycoproteins, and transmembrane proteins.

Overall Molecular Composition: Lipids constitute approximately $98\%$ of all molecules in the cell membrane.
Phospholipid Bilayer ( $75\%$ of Membrane Lipids):
- Amphipathic molecules arranged in a double layer.
- Polar Heads: Hydrophilic phosphate heads directed outward toward the aqueous ECF on one side and ICF on the other.
- Nonpolar Tails: Hydrophobic fatty acid tails directed inward toward the center, forming a water-repellent barrier.
Cholesterol ( $20\%$ of Membrane Lipids):
- Located scattered among the phospholipid tails.
- Regulates membrane fluidity: higher concentrations stiffen the membrane structure, while lower concentrations prevent fatty acid tails from packing tightly together, maintaining fluidity.
Glycolipids ( $5\%$ of Membrane Lipids):
- Phospholipids with short, branching carbohydrate chains attached.
- Located exclusively on the extracellular face of the plasma membrane.
- Contribute to the glycocalyx, a carbohydrate surface coating involved in cellular identification and protection.

Membrane Proteins and Their Functional Roles

Schematic illustrating six key functional types of membrane proteins: receptors, enzymes, ion channels, gated ion channels, cell-identity markers, and cell-adhesion molecules.

Protein Proportion: Membrane proteins constitute about $2\%$ of the total number of molecules in the plasma membrane, but account for approximately $50\%$ of total membrane weight due to their larger mass.
Structural Classes of Membrane Proteins:
- Integral (Transmembrane) Proteins:
  - Penetrate completely through the phospholipid bilayer.
  - Possess hydrophilic regions in contact with water on both sides and hydrophobic regions spanning the lipid core.
  - Most are glycoproteins.
  - Can drift freely within the lipid layer or be tethered securely to the internal cytoskeleton.
- Peripheral Proteins:
  - Do not protrude into the hydrophobic phospholipid layer.
  - Adhere to one face of the membrane (typically the internal cytosolic face).
  - Usually tethered to an integral transmembrane protein or to cytoskeletal filaments.
Functional Categories of Membrane Proteins:
- (a) Receptor: Binds specific chemical messengers (e.g., hormones, neurotransmitters) sent by other cells to trigger internal signal responses.
- (b) Enzyme: Catalyzes chemical reactions at the cell surface, such as digesting nutrients or degrading chemical messengers to terminate their signals.
- (c) Ion Channel: A pore/tunnel that allows specific ions or water to move continuously across the membrane down concentration gradients.
- (d) Gated Ion Channel: A regulated channel that opens and closes only in response to specific chemical, electrical, or mechanical stimuli.
- (e) Cell-Identity Marker: A surface glycoprotein (part of the glycocalyx) acting as a cellular ID tag enabling the immune system to distinguish the body's own cells from foreign cells.
- (f) Cell-Adhesion Molecule (CAM): Binds a cell to adjacent cells or to extracellular materials, enabling tissues to maintain physical integrity.