Cell Structure and Classification

Prokaryotic vs. Eukaryotic Cells

Cells are the fundamental units of life and can be classified into two main types: prokaryotic and eukaryotic cells. This classification is based on the presence or absence of a nucleus and other membrane-bound organelles.

• Prokaryotic cells are small, lack a true nucleus, and have a single internal compartment. Their DNA is clustered in a region called the nucleoid and is not surrounded by a membrane. Prokaryotes include Bacteria and Archaea.

• Eukaryotic cells possess a nuclear envelope that encloses their DNA and have many other membrane-bound organelles. Eukaryotes include protists, fungi, plants, and animals.

The plasma membrane is a selectively permeable barrier that separates the cell from its environment, allowing selective transport of molecules in and out of the cell. The membrane is primarily composed of a phospholipid bilayer with embedded proteins.

Cell Walls and Extracellular Structures

Some organisms have an additional coat called a cell wall, which provides structural support and protection.

• Bacteria have cell walls containing peptidoglycan, a complex of carbohydrates and peptides.

• Plants have cell walls made of cellulose.

• Fungi have cell walls made of chitin.

Cytoplasm and Cytosol

The cytoplasm includes all the space inside the cell but outside the nucleus and other organelles. The cytosol is the fluid portion of the cytoplasm, containing water, dissolved ions, and molecules.

Ribosomes

Ribosomes are the sites of protein synthesis and are found in both prokaryotic and eukaryotic cells. In eukaryotes, ribosomes can be free in the cytosol or bound to the rough endoplasmic reticulum (RER).

Cell Size

Microscopy and Cell Visualization

Electron Microscopy

Electron microscopy uses high-energy electrons to visualize cell structures at very high resolution, revealing details as small as 2 nm.

• Scanning Electron Microscopy (SEM): Produces images of cell surfaces by scanning with a focused beam of electrons.

• Transmission Electron Microscopy (TEM): Passes electrons through thin sections of cells to reveal internal structures.

Membranous Organelles in Eukaryotic Cells

Nucleus

The nucleus contains most of the cell's DNA and is surrounded by a nuclear envelope. The envelope has nuclear pores for transport of materials in and out.

Endomembrane System

The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and the plasma membrane. It is responsible for the synthesis, modification, and transport of proteins and lipids.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, and detoxifies drugs and poisons.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain hydrolytic enzymes for intracellular digestion of macromolecules.

• Vesicles: Small membrane-bound sacs that transport substances within the cell.

Peroxisomes

Peroxisomes contain enzymes that break down fatty acids and detoxify harmful substances. They produce hydrogen peroxide (H2O2) as a byproduct, which is then broken down by the enzyme catalase.

Vacuoles

Vacuoles are large vesicles found in plant and fungal cells. They store water, nutrients, and waste products. The central vacuole in plant cells helps maintain cell shape and stores important substances.

Chloroplasts and Mitochondria

Both chloroplasts and mitochondria are energy-converting organelles with their own DNA and double membranes.

• Chloroplasts: Sites of photosynthesis in plants and algae. Contain the pigment chlorophyll for capturing light energy.

• Mitochondria: Sites of cellular respiration, producing adenosine triphosphate (ATP) from organic molecules.

Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Evidence includes their double membranes, own DNA, and similarities to bacteria.

The Cytoskeleton and Cell Junctions

Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, maintains cell shape, and enables movement.

• Microtubules: Hollow tubes made of tubulin; form tracks for vesicle movement and are involved in cell division.

• Intermediate Filaments: Provide mechanical strength; form the nuclear lamina inside the nucleus.

• Microfilaments (Actin Filaments): Thin filaments involved in cell movement and muscle contraction.

Motor Proteins

• Kinesin and dynein move along microtubules to transport vesicles and organelles.

Extracellular Matrix (ECM)

The extracellular matrix is a network of proteins and carbohydrates outside animal cells that provides structural support and mediates cell signaling.

• Collagen: Main structural protein in the ECM.

• Proteoglycans: Proteins with attached carbohydrates.

• Fibronectin: Connects ECM components to cell surface receptors (integrins).

Cell Junctions

Animal cells have specialized junctions for communication and adhesion:

• Tight Junctions: Seal cells together to prevent leakage of molecules between them.

• Desmosomes: Anchor cells together using protein plaques and intermediate filaments.

• Gap Junctions: Channels that allow direct communication between adjacent cells.

In plants, plasmodesmata are channels that connect the cytoplasm of adjacent cells, allowing transport of substances.

Summary Table: Key Organelles and Their Functions

Key Equations

• ATP Production (Cellular Respiration):

• Photosynthesis:

Additional info:

• Some details, such as the specific roles of certain proteins and the structure of the extracellular matrix, were expanded for clarity and completeness.

• Tables and equations were formatted for clarity and to aid in exam preparation.