Cell Structure and Organization

Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell, maintaining the internal environment and regulating the movement of substances in and out of the cell.

• Structure: Composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipid Bilayer: Amphipathic phospholipid molecules are arranged into two layers (bilayer). The hydrophilic (water-loving) head regions face the extracellular and intracellular fluids, while the hydrophobic (water-fearing) tails face inward, away from water.

• Function: Maintains cellular integrity, mediates communication, and controls transport.

Example: The plasma membrane contains protein channels for ions and receptors for signaling molecules.

Cytoplasm, Cytosol, and Cytoskeleton

The cytoplasm is the combination of the water-based solution inside the cell (cytosol) and the organelles suspended within it. The cytoskeleton is a network of protein filaments that provides structural support and facilitates movement within the cell.

• Cytosol: The fluid portion of the cytoplasm, containing dissolved substances and proteins.

• Cytoskeleton: Provides cell shape, organization, and movement. It consists of three main types of protein filaments:

  • Microfilaments: Composed of actin. They support cell shape, enable cell movement, and are involved in muscle contraction and cell division.

  • Intermediate Filaments: Made of various proteins (e.g., keratin). They provide mechanical strength and maintain cell integrity.

  • Microtubules: Hollow cylinders made of tubulin. They are involved in organelle movement, cell division (spindle fibers), and form structures such as cilia and flagella.

Example: Microtubules form the mitotic spindle during cell division.

Cell Organelles

Nucleus

The nucleus is the control center of the cell, containing most of the cell's genetic material (DNA). It is surrounded by a double membrane called the nuclear envelope and contains the nucleolus, where ribosome synthesis occurs.

• Function: Stores genetic information, coordinates cell activities such as growth and reproduction.

• Nucleolus: Site of ribosome production.

Example: Multinucleated cells, such as skeletal muscle fibers, contain more than one nucleus.

Ribosomes

Ribosomes are small, non-membrane-bound organelles responsible for protein synthesis. They can be found free in the cytoplasm or bound to the rough endoplasmic reticulum (ER).

• Free Ribosomes: Synthesize proteins for use within the cell.

• Bound Ribosomes: Synthesize proteins for export or for use in membranes.

Example: Ribosomes attached to the rough ER produce proteins for secretion.

Endomembrane System

The endomembrane system is a group of organelles that work together to modify, package, and transport lipids and proteins. It includes the nucleus, rough and smooth ER, Golgi apparatus, lysosomes, and vesicles.

• Rough Endoplasmic Reticulum (Rough ER): Studded with ribosomes; involved in protein synthesis and modification.

• Smooth Endoplasmic Reticulum (Smooth ER): Lacks ribosomes; involved in lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for delivery.

• Lysosomes: Contain digestive enzymes for breaking down waste and cellular debris.

• Peroxisomes: Smaller than lysosomes; involved in detoxification and breakdown of fatty acids.

• Proteasomes: Barrel-shaped protein complexes that degrade unneeded or damaged proteins.

Example: The Golgi apparatus packages proteins for secretion.

Mitochondria

Mitochondria are double-membraned organelles known as the "powerhouse" of the cell. They generate ATP through cellular respiration.

• Function: Energy production via the breakdown of glucose and other nutrients.

Example: Muscle cells contain many mitochondria to meet high energy demands.

Surface Modifications and Membrane Junctions

Surface Modifications

• Cilia: Hair-like structures that move substances across the cell surface.

• Flagella: Longer, whip-like structures used for cell movement (e.g., sperm cells).

• Microvilli: Finger-like projections that increase surface area for absorption (e.g., intestinal cells).

Membrane Junctions

• Tight Junctions: Seal adjacent cells together, preventing leakage of extracellular fluid.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow direct communication between cells through channels that permit the passage of ions and small molecules.

Example: Tight junctions are found in epithelial cells lining the intestines.

Summary Table: Cell Organelles and Functions

Summary Table: Membrane Junctions and Surface Modifications

Key Equations

• ATP Production (Cellular Respiration):

General equation for aerobic respiration:

• Microtubule Structure:

Microtubules are polymers of tubulin dimers:

Additional info:

• Some content was inferred and expanded for clarity, including definitions and examples of organelle functions, cytoskeleton components, and membrane junctions.

• Tables were recreated to summarize organelle and junction functions for exam review.