Functional Anatomy of Prokaryotic and Eukaryotic Cells: Plasma Membrane and Transport

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Cell Membrane & Transport

Structure of the Plasma Membrane

The plasma membrane is a fundamental structure in both prokaryotic and eukaryotic cells, serving as a selective barrier that regulates the movement of substances into and out of the cell. It is primarily composed of a phospholipid bilayer with embedded proteins, carbohydrates, and, in eukaryotes, sterols such as cholesterol.

Phospholipid Bilayer: Amphipathic molecules with hydrophilic heads facing outward and hydrophobic tails oriented inward, forming a double layer.
Proteins: Integral and peripheral proteins are involved in transport, signaling, and structural support.
Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids) for cell recognition and adhesion.
Sterols: Cholesterol in eukaryotes maintains membrane fluidity; absent in most prokaryotes except Mycoplasma.

Phospholipid bilayer structure Fluid mosaic model of the plasma membrane Cross-section of the plasma membrane showing proteins and bilayer

Comparison of Prokaryotic and Eukaryotic Membranes

While both cell types share the basic phospholipid bilayer structure, there are key differences:

Sterols: Present in eukaryotes (cholesterol), absent in most prokaryotes except Mycoplasma.
Carbohydrates: More prominent in eukaryotic membranes for cell recognition.
Fluidity: Maintained by hydrophobic interactions, not covalent bonds, allowing proteins and lipids to move laterally.

Mycoplasma cell structure Eukaryotic plasma membrane with proteins and carbohydrates

Functions of Membrane Proteins

Membrane proteins perform a variety of essential functions:

Transport: Channel and carrier proteins facilitate movement of substances across the membrane.
Enzymatic Activity: Enzymes embedded in the membrane catalyze biochemical reactions.
Signal Transduction: Receptor proteins bind signaling molecules (ligands) and initiate cellular responses.
Cell-Cell Recognition: Glycoproteins serve as identification tags for immune recognition.
Intercellular Joining: Proteins form junctions between adjacent cells.
Attachment: Proteins anchor the membrane to the cytoskeleton and extracellular matrix, maintaining cell shape.

Membrane protein functions: transport, enzymatic activity, signal transduction Membrane protein functions: cell recognition, intercellular joining, attachment

Traffic Across the Membrane

The plasma membrane controls the movement of ions, nutrients, and waste products. This is essential for cellular respiration, nerve signaling, and maintaining homeostasis.

Selective Permeability: Hydrophobic molecules (e.g., O2, CO2, steroids) pass freely; hydrophilic molecules (e.g., ions, glucose) require transport proteins.
Site of ATP Synthesis in Prokaryotes: Since prokaryotes lack mitochondria, their plasma membrane is the site of cellular respiration and ATP production.

Cellular respiration and ion channels in the plasma membrane Phospholipid bilayer permeability to different molecules

Diffusion and Osmosis

Diffusion is the passive movement of molecules from high to low concentration. Osmosis is the diffusion of water across a selectively permeable membrane.

Diffusion: Driven by concentration gradients; continues until equilibrium is reached.
Osmosis: Water moves toward higher solute concentration to balance solute levels across the membrane.

Diffusion across a membrane Osmosis: movement of water across a selectively permeable membrane Hydration shells around ions in solution Selective permeability in a beaker with salt and water

Tonicity and Its Effects on Cells

Tonicity describes the ability of a solution to cause a cell to gain or lose water. It is always a comparative term between two solutions separated by a membrane.

Isotonic: Equal solute and water concentration; no net water movement. Best for animal cells; plant cells become flaccid.
Hypertonic: Higher solute concentration outside; water leaves the cell, causing animal cells to shrivel and plant cells to undergo plasmolysis.
Hypotonic: Lower solute concentration outside; water enters the cell, causing animal cells to lyse and plant cells to become turgid (optimal for plants).

Isotonic solution: equal solute and water Isotonic effect on animal and plant cells Flaccid vs. turgid plant cells Hypertonic solution: more solute outside Hypertonic effect on animal cells Hypertonic effect on animal and plant cells Hypotonic solution: less solute outside Hypotonic effect on animal cells Hypotonic effect on animal and plant cells Turgid plant cell in hypotonic solution

Passive and Active Transport

Transport across the membrane can be passive (no energy required) or active (requires energy, usually ATP).

Passive Transport: Includes simple diffusion (small, nonpolar molecules), facilitated diffusion (via channel or carrier proteins), and osmosis (water movement).
Active Transport: Moves substances against their concentration gradient using energy. Includes ion pumps (e.g., Na+/K+ pump) and co-transporters (e.g., sucrose-H+ cotransport in plants).

Passive transport mechanisms Active transport: proton pump and cotransporter

Bulk Transport: Exocytosis and Endocytosis

Large molecules and particles are transported across the membrane via vesicular transport mechanisms.

Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell (e.g., secretion of hormones, neurotransmitters).
Endocytosis: The cell engulfs external material by forming vesicles. Includes phagocytosis ("cell eating"), pinocytosis ("cell drinking"), and receptor-mediated endocytosis (specific uptake of molecules).

Protein production and secretion pathway Phagocytosis: engulfing particles

Prokaryotic vs. Eukaryotic Cell Structures

Key Differences

Prokaryotes: Lack membrane-bound organelles; plasma membrane is the site of ATP synthesis.
Eukaryotes: Have membrane-bound organelles (nucleus, mitochondria, ER, Golgi apparatus, etc.).
Cell Wall: Present in most prokaryotes (peptidoglycan) and in plants, algae, fungi (cellulose or chitin), but absent in animal cells.

Summary Table: Prokaryotic vs. Eukaryotic Plasma Membranes

Feature	Prokaryotes	Eukaryotes
Phospholipid Bilayer	Yes	Yes
Sterols (e.g., cholesterol)	No (except Mycoplasma)	Yes
Carbohydrates	Rare	Common (cell recognition)
Site of ATP Synthesis	Plasma membrane	Mitochondria

Example: Mycoplasma is a unique prokaryote that lacks a cell wall but contains sterols in its membrane, similar to eukaryotes.