Cell Structure, Membrane Transport, and Endocytosis/Exocytosis in General Biology

Cell Structure and Classification

Distinguishing Prokaryotes and Eukaryotes

Cells are classified into two major types: prokaryotic and eukaryotic. Understanding their differences is fundamental in biology.

• Prokaryotes (e.g., E. coli): Lack a nucleus and membrane-bound organelles. DNA is located in the nucleoid region.

• Eukaryotes (e.g., amoeba, yeast): Possess a true nucleus and various membrane-bound organelles.

• Key differences: Size, complexity, presence of organelles, and method of cell division.

Example: E. coli is a common prokaryote, while yeast is a common eukaryote.

Composition of the Cell Membrane

The cell membrane is a selectively permeable barrier that surrounds the cell, controlling the movement of substances in and out.

• Main components: Phospholipid bilayer, proteins (integral and peripheral), cholesterol (in animal cells), carbohydrates (glycoproteins and glycolipids).

• Phospholipid bilayer: Composed of hydrophilic heads and hydrophobic tails, forming a double layer.

• Proteins: Serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids, involved in cell recognition.

Example: The fluid mosaic model describes the dynamic nature of the cell membrane.

Features of Prokaryotic and Eukaryotic Cells

• Prokaryotic cell features: Cell wall (peptidoglycan), plasma membrane, ribosomes (70S), nucleoid, sometimes flagella or pili.

• Eukaryotic cell features: Nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, cytoskeleton, plasma membrane, ribosomes (80S).

Example: Animal cells lack cell walls, while plant cells have cellulose-based cell walls.

Comparative Table: Prokaryotes vs. Eukaryotes

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Organelles	Absent	Present (membrane-bound)
Cell Wall	Present (peptidoglycan)	Present (cellulose in plants, chitin in fungi)
Ribosomes	70S	80S
Examples	E. coli	Amoeba, yeast, plant cells

Cell Membrane Structure and Function

Plasma Membrane: Structure and Function

The plasma membrane is essential for maintaining cellular integrity and regulating transport.

• Phospholipid bilayer: Hydrophilic heads face outward, hydrophobic tails face inward.

• Proteins: Transmembrane proteins facilitate transport and communication.

• Selective permeability: Allows certain molecules to pass while restricting others.

• Fluidity: Cholesterol and unsaturated fatty acids influence membrane fluidity.

Example: The plasma membrane enables nerve cells to transmit electrical signals.

Glycolipids and Glycoproteins

Glycolipids and glycoproteins are molecules with carbohydrate chains attached, found on the cell surface.

• Function: Cell recognition, signaling, and adhesion.

• Example: Blood group antigens are glycoproteins on red blood cells.

Membrane Transport Mechanisms

Passive Transport

Passive transport is the movement of substances across the membrane without energy input.

• Types: Diffusion, facilitated diffusion, osmosis.

• Diffusion: Movement of molecules from high to low concentration.

• Facilitated diffusion: Uses transport proteins for molecules that cannot cross the lipid bilayer directly.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Example: Oxygen enters cells by simple diffusion.

Equation:

$J = -D \frac{dC}{dx}$

Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Primary active transport: Direct use of ATP (e.g., sodium-potassium pump).

• Secondary active transport: Uses energy from the movement of another substance (co-transport).

Example: The Na+/K+ pump maintains ion gradients in animal cells.

Equation:

$\text{Rate} = k[\text{Nucleophile}][\text{Electrophile}]$

Additional info: This equation is a general rate law, not specific to membrane transport, but included for context.

Osmosis and Tonicity

Osmosis is the movement of water across a membrane. Tonicity describes the effect of a solution on cell volume.

• Isotonic: No net water movement; cell volume remains constant.

• Hypotonic: Water enters the cell; cell may swell and burst.

• Hypertonic: Water leaves the cell; cell shrinks.

Example: Red blood cells in a hypertonic solution undergo crenation.

Endocytosis and Exocytosis

Types of Endocytosis

Endocytosis is the process by which cells engulf external substances.

• Phagocytosis: "Cell eating"; uptake of large particles (e.g., bacteria).

• Pinocytosis: "Cell drinking"; uptake of fluids and small molecules.

• Receptor-mediated endocytosis: Specific uptake via receptor-ligand interactions.

Example: White blood cells use phagocytosis to ingest pathogens.

Exocytosis

Exocytosis is the process by which cells expel materials in vesicles to the exterior.

• Function: Secretion of hormones, neurotransmitters, and waste products.

• Mechanism: Vesicles fuse with the plasma membrane, releasing contents outside.

Example: Neurons release neurotransmitters via exocytosis.

Comparison Table: Endocytosis vs. Exocytosis

Process	Direction	Examples
Endocytosis	Into the cell	Phagocytosis, pinocytosis, receptor-mediated endocytosis
Exocytosis	Out of the cell	Neurotransmitter release, hormone secretion

Symbiosis and Endosymbiosis Theory

Endosymbiosis Theory

The endosymbiosis theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• Key idea: These organelles originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, circular DNA, ribosomes similar to prokaryotes.

Example: Mitochondria are thought to have evolved from aerobic bacteria.

Symbiosis

Symbiosis refers to a close and long-term biological interaction between two different biological organisms.

• Types: Mutualism (both benefit), commensalism (one benefits, other unaffected), parasitism (one benefits, other harmed).

Example: Lichens are a mutualistic relationship between fungi and algae.

Additional info: Some content was inferred and expanded for clarity and completeness, including definitions, examples, and tables.