Cell Membranes and the Origin of Semiautonomous Organelles

Cell Membrane Structure and Function

Phospholipid Bilayer

The cell membrane is primarily composed of a double layer of phospholipids, which forms a barrier between the cell's internal and external environments.

• Phospholipids have a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails.

• In aqueous environments, phospholipids arrange themselves into a bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward toward water.

• This arrangement creates a selectively permeable barrier, allowing some molecules to pass while restricting others.

Example: The plasma membrane of animal and plant cells is a classic example of a phospholipid bilayer.

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character.

• Fluidity is influenced by the composition of fatty acids in phospholipids and the presence of cholesterol.

• Unsaturated fatty acids (with double bonds) create kinks, increasing membrane fluidity.

• Saturated fatty acids (no double bonds) pack tightly, decreasing fluidity.

• Cholesterol acts as a fluidity buffer, stabilizing the membrane at various temperatures.

Example: Increasing the proportion of unsaturated fatty acids or adding cholesterol can make the membrane more fluid.

Membrane Proteins

Proteins embedded in the membrane perform a variety of essential functions.

• Integral proteins span the membrane and are involved in transport and signaling.

• Peripheral proteins are attached to the membrane surface and assist in signaling or maintaining cell shape.

• Functions include transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

Glycosylation and Membrane Sidedness

Many membrane proteins and lipids are glycosylated, meaning they have carbohydrate groups attached.

• Glycosylation is important for cell-cell recognition and communication.

• Membranes are "sided"—the composition and orientation of proteins and carbohydrates differ between the inner and outer leaflets of the bilayer.

Transport Across Cell Membranes

Types of Transport

Cells regulate the movement of substances across their membranes through several mechanisms:

• Passive Transport: Movement of molecules down their concentration gradient without energy input.

• Facilitated Diffusion: Passive movement of molecules via transport proteins.

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (usually from ATP).

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane.

• Water moves from areas of low solute concentration to areas of high solute concentration.

• Osmotic balance is crucial for cell survival; imbalances can cause cells to swell or shrink.

Endocytosis and Exocytosis

Bulk transport mechanisms move large molecules or particles across the membrane.

• Endocytosis: The cell engulfs material by wrapping the membrane around it, forming a vesicle.

• Exocytosis: Vesicles fuse with the membrane to release contents outside the cell.

Semiautonomous Organelles and the Endosymbiotic Theory

Definition and Properties

Semiautonomous organelles, such as mitochondria and chloroplasts, are specialized compartments within eukaryotic cells that have some autonomy but still depend on the cell for certain functions.

• They have their own DNA and can grow and reproduce independently of the cell cycle.

• They are not formed by the endomembrane system but arise from pre-existing organelles of the same type.

• They depend on the cell for most of their proteins, which are imported from the cytosol.

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts as a result of a symbiotic relationship between ancestral eukaryotic cells and certain prokaryotes.

• Mitochondria and chloroplasts are similar in size to bacteria.

• They have double membranes, their own circular DNA, and free ribosomes similar to those of bacteria.

• They reproduce by a process similar to binary fission.

Example: The engulfment of an oxygen-using nonphotosynthetic prokaryote led to the formation of mitochondria; a similar event with a photosynthetic prokaryote led to chloroplasts.

Evidence Supporting Endosymbiosis

• Presence of circular DNA in mitochondria and chloroplasts, resembling bacterial genomes.

• Division of these organelles is independent of the host cell's division.

• Ribosomes within these organelles are more similar to bacterial ribosomes than to eukaryotic cytoplasmic ribosomes.

• Some proteins required by these organelles are encoded by nuclear DNA and imported from the cytosol.

Summary Table: Comparison of Mitochondria, Chloroplasts, and Bacteria

Feature	Mitochondria	Chloroplasts	Bacteria
Size	Similar to bacteria	Similar to bacteria	Small (1-10 μm)
Membranes	Double	Double	Single
DNA	Circular	Circular	Circular
Ribosomes	70S (bacterial type)	70S (bacterial type)	70S
Reproduction	Binary fission	Binary fission	Binary fission

Key Equations

• Osmosis: Water potential ($\Psi$) determines the direction of water movement: $\Psi = \Psi_s + \Psi_p$ where $\Psi_s$ is solute potential and $\Psi_p$ is pressure potential.

• Diffusion: Fick's Law describes the rate of diffusion: $J = -D \frac{dC}{dx}$ where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.