Biology Cell Theory and Characteristics

Cell Theory

• Cells serve as the smallest structural and functional unit of all organisms.
• Every organism is made up of one or more cells.
• Cells arise only from preexisting cells through division.

Characteristics of a Cell

• Cells store genetic information and can make and utilize energy.
• They possess movement capabilities and can sense changes in their environment.
• Cells can reproduce, repair themselves, grow, and develop.
• Homoeostasis is maintained through self-regulation.
• Cells synthesize various molecules, including proteins, carbohydrates, fats, and nucleic acids.

Types of Cells

• Prokaryotic Cells: Simple, smaller (<10 µm), lack a true nucleus and membrane-bound organelles, contain circular DNA, and consist of unicellular organisms.
• Eukaryotic Cells: Larger (10-100 µm), have a true membrane-bound nucleus and organelles, contain linear DNA associated with proteins, and can be unicellular or multicellular (includes animal and plant cells).

Prokaryotes vs. Eukaryotes

• Prokaryotes: Asexual reproduction via binary fission, small ribosomes, and single circular DNA.
• Eukaryotes: Reproduction through mitosis and meiosis, larger ribosomes, multiple linear DNA strands.

Three Important Cell Regions

• Cell Membrane: Outermost boundary controlling movement in and out of the cell.
• Nucleus: Control center containing DNA, directing cell function.
• Cytoplasm: Jelly-like substance where organelles are suspended and biochemical reactions occur.

Cell Organelles

• Membranous Organelles: Include endoplasmic reticulum, mitochondria, Golgi apparatus, and lysosomes.
• Non-Membranous Organelles: Include ribosomes and cytoskeletal structures.

Nucleus Structure

• Comprised of the nuclear membrane (double-layer with pores), nucleoplasm (liquid similar to cytosol), nucleolus (synthesizes ribosomes), and chromatin (DNA and proteins).
• Chromatin condenses into chromosomes during cell division.

Chromosome Structure

• Composed of nucleosomes (DNA wrapped around histone proteins), solenoids (coiled nucleosomes), and chromatin fibers.

Anucleated and Polynucleated Cells

• Anucleated cells lack a nucleus (e.g., mature red blood cells).
• Polynucleated cells contain multiple nuclei (e.g., skeletal muscle cells and giant multinucleated cells associated with inflammation).

Nucleus Functions

• Stores DNA and directs cellular activities, including DNA replication and transcription into RNA.
• Contains various proteins that mediate transcription and regulate gene expression.
• Macromolecules are actively transported across the nuclear membrane via nuclear pore complexes.

Disorders Related to the Nucleus

• Defective nucleoli linked to neurodegenerative disorders, including Alzheimer's and Huntington's.
• Autoimmune diseases associated with anti-nuclear antibodies affecting chromatin organization.

Cytoplasm Functions

• Medium for biochemical reactions and cellular processes.
• Contains enzymes, substrates, and organelles involved in functions like protein synthesis.

Endoplasmic Reticulum (ER)

• A network of membranes consisting of rough and smooth ER.
• Rough ER is involved in protein synthesis and sending proteins to Golgi apparatus; smooth ER synthesizes lipids, detoxifies substances, and manages calcium ion storage.

Golgi Apparatus

• Acts as a "finishing factory" for ER products, consisting of flattened vesicles and transporting materials.
• Modifies, sorts, and ships products in vesicles for secretion or storage.
• Errors in Golgi function can lead to various disorders, including congenital glycosylation disorders and certain cancers.

Importance of Golgi Apparatus Function

• Essential for proper protein localization and cellular functions.
• Mistakes in protein sorting can lead to dysfunctional cellular processes.