6.2 Cell Structure, Organelles & Membrane Transport

Cells are the fundamental structural and functional units of all living organisms. Cell biology focuses on cellular structure, organelle functions, membrane dynamics, and transport mechanisms that maintain cellular homeostasis.

Prokaryotic versus Eukaryotic Cells

All cells are classified into two broad categories: prokaryotic and eukaryotic.

• Prokaryotic Cells (Bacteria and Archaea): Simple, single-celled organisms that lack a membrane-bound nucleus and membrane-bound organelles. Their genetic material consists of a single circular DNA molecule located in an unenclosed region called the nucleoid. Prokaryotes possess a plasma membrane, cytoplasm, 70S ribosomes, and a cell wall (containing peptidoglycan in bacteria). They reproduce rapidly via binary fission.
• Eukaryotic Cells (Protists, Fungi, Plants, and Animals): Complex cells containing a true membrane-bound nucleus that houses linear chromosomes wrapped around histone proteins. Eukaryotes feature specialized membrane-bound organelles, 80S ribosomes, and an endomembrane system. They reproduce through mitosis and meiosis.

Eukaryotic Organelles and Cellular Functions

Eukaryotic cells rely on compartmentalized organelles to perform specialized metabolic tasks:

• Nucleus: The control center of the cell. Enclosed by a double-membrane nuclear envelope containing nuclear pores that regulate macromolecular transport. Contains chromatin (DNA and proteins).
• Nucleolus: A dense region within the nucleus dedicated to ribosomal RNA (rRNA) synthesis and ribosome assembly.
• Ribosomes: Non-membrane-bound complexes of rRNA and proteins that carry out protein synthesis (translation). Free ribosomes synthesize cytosolic proteins, while bound ribosomes attached to the rough ER synthesize membrane-bound, lysosomal, or secreted proteins.
• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; functions in the synthesis, folding, and post-translational modification of proteins destined for membranes or secretion.
• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; synthesizes lipids (phospholipids and steroids), metabolizes carbohydrates, detoxifies drugs and toxins (abundant in liver cells), and stores calcium ions ($Ca^{2+}$).
• Golgi Apparatus: A stack of flattened membranous sacs (cisternae). It receives proteins and lipids from the ER at its cis face, modifies them (e.g., glycosylation), sorts, packages, and tags them into vesicles shipped from its trans face to their final destinations.
• Mitochondria: The "powerhouses" of the cell, generating ATP through aerobic cellular respiration. Mitochondria feature a double membrane; the inner membrane is folded into cristae to increase surface area for electron transport chains, enclosing the central matrix (site of the Krebs cycle). Mitochondria contain their own circular DNA and ribosomes, supporting the Endosymbiotic Theory.
• Lysosomes: Membranous sacs containing acidic hydrolytic enzymes that digest macromolecules, damaged organelles (autophagy), and phagocytosed pathogens.
• Peroxisomes: Metabolic compartments containing enzymes such as catalase that break down fatty acids and neutralize toxic hydrogen peroxide ($H_2O_2$) into water and oxygen.
• Vacuoles: Storage sacs. Plant cells contain a large central vacuole that maintains turgor pressure against the cell wall and stores nutrients and waste.
• Cytoskeleton: A dynamic protein network providing structural support, cell motility, and intracellular transport. It consists of:
  • Microfilaments (actin): Support cell shape, muscle contraction, and cleavage furrow formation during cytokinesis.
  • Intermediate Filaments (e.g., keratin): Provide mechanical stability and anchor organelles.
  • Microtubules (tubulin): Hollow tubes that guide organelle movement, form spindle fibers during cell division, and construct cilia and flagella.

Plasma Membrane Structure: The Fluid Mosaic Model

The plasma membrane is a selectively permeable barrier separating the intracellular cytoplasm from the extracellular environment. Its structure is described by the fluid mosaic model, where proteins float within a fluid phospholipid bilayer.

• Phospholipid Bilayer: Arranged with hydrophilic phosphate heads exposed to aqueous environments inside and outside the cell, and hydrophobic fatty acid tails buried in the interior.
• Cholesterol: Embedded within animal membranes to act as a fluidity buffer, preventing membranes from becoming overly fluid at high temperatures or solidifying at low temperatures.
• Membrane Proteins: Integral (transmembrane) proteins span the bilayer to function as ion channels, transporters, and receptors. Peripheral proteins adhere loosely to membrane surfaces for cell signaling.
• Glycoproteins and Glycolipids: Carbohydrate chains covalently attached to membrane proteins or lipids on the extracellular surface, functioning in cell-cell recognition and tissue formation.

Cellular Transport Mechanisms

Movement of substances across the plasma membrane occurs via passive or active transport.

Passive Transport (No Cellular Energy / ATP Required)

Substances move down their concentration gradient (from high to low concentration):

• Simple Diffusion: Direct movement of small, nonpolar molecules ($O_2$, $CO_2$) across the lipid bilayer.
• Facilitated Diffusion: Passive movement of polar or charged molecules (glucose, ions) assisted by transmembrane protein channels (e.g., aquaporins for water) or carrier proteins.
• Osmosis: The passive diffusion of water across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration.

Active Transport (ATP Required)

Substances move against their concentration gradient (from low to high concentration):

• Primary Active Transport: Directly hydrolyzes ATP to move ions. The $Na^+/K^+$ ATPase pump hydrolyzes 1 ATP to pump $3\ Na^+$ ions out of the cell and $2\ K^+$ ions into the cell, maintaining resting membrane potential.
• Secondary Active Transport: Uses energy from an electrochemical gradient established by primary active transport to move another substance (e.g., $Na^+$-glucose symporter).
• Vesicular (Bulk) Transport: Moves large particles or fluids via membrane-bound vesicles:
  • Endocytosis: Inward folding of membrane to engulf material. Includes phagocytosis ("cell eating"), pinocytosis ("cell drinking"), and receptor-mediated endocytosis.
  • Exocytosis: Secretory vesicles fuse with the plasma membrane to release contents outside the cell.

Tonicity and Osmotic Effects on Cells

Tonicity describes the ability of an extracellular solution to cause water to move into or out of a cell by osmosis.

• Hypertonic Solution: Higher solute concentration outside the cell. Water flows OUT of the cell. Red blood cells shrivel (crenation); plant cells undergo plasmolysis.
• Hypotonic Solution: Lower solute concentration outside the cell. Water flows INTO the cell. Red blood cells swell and may burst (lysis); plant cells become turgid (ideal state).
• Isotonic Solution: Equal solute concentration inside and outside the cell. No net water movement. Red blood cells remain normal; plant cells become flaccid.