Cell Theory and Cell Composition

A. CELL THEORY:

• Cells are well established as:

  1. The basic building blocks of plants & animals.

  2. The smallest functional units of life.

  3. Produced by the division of preexisting cells (continuity of life has cellular basis).

  4. Basic homeostatic units.

B. CELL BASICS:

• Cytology: study of cell structure and function.

• The human body contains trillions of cells, with over 200 different types.

  • All cell types share certain basic characteristics.

3 Main Cell Regions:

1. Cell (Plasma) Membrane (PM):

   • Serves as a barrier between the inside and outside of the cell, surrounded by interstitial fluid (ISF).

2. Nucleus:

   • The control center of the cell; contains nuclear DNA.

3. Cytoplasm:

   • The material between the plasma membrane and nucleus; contains cytosol (fluid) and organelles.

C. THE CELL (PLASMA) MEMBRANE:

1. Plasma Membrane Functions:

   • Maintains the cell's physical boundaries.

   • Regulates exchange with the environment.

   • Senses and responds to the environment (via receptors).

   • Provides structural support.

2. Plasma Membrane Structure: Fluid Mosaic Model

   • Phospholipid Bilayer:

     • Composed of phospholipids with a hydrophilic "head" and hydrophobic fatty acid "tails."

     • Arranged so that heads face outward and tails face inward, creating a fluid and flexible barrier.

   • Embedded Molecules:

     • Proteins, carbohydrates, and cholesterol.

   • Glycocalyx:

     • Sugar coating on PM, important for intercellular communication and identification.

Membrane Specializations:

• Tight Junctions: Impermeable junctions preventing passage between cells.

• Desmosomes: Anchoring junctions helping resist mechanical stress.

• Gap Junctions: Allow direct passage of small molecules between adjacent cells.

D. MEMBRANE TRANSPORT PROCESSES:

• Selective Permeability: Some substances pass freely, others require specific transport processes.

Types of Membrane Transport:

1. Passive Transport (no ATP required):

   • Simple diffusion, osmosis, filtration.

   • Moves substances down a concentration gradient.

2. Carrier-Mediated Transport:

   • Facilitated diffusion (no ATP, uses carrier proteins).

   • Active transport (requires ATP, can move substances against concentration gradient).

3. Vesicular Transport (requires ATP):

   • Endocytosis (cell uptake, includes phagocytosis and pinocytosis).

   • Exocytosis (substance release from the cell).

Important Concepts:

• Osmosis: Diffusion of water across a selectively permeable membrane (water moves from low to high solute concentration).

• Filtration: Movement driven by hydrostatic pressure (e.g., blood capillaries).

E. CYTOPLASM:

• Material between PM and nucleus, consisting of:

  • Cytosol: mostly water with dissolved solutes.

  • Organelles: metabolic machinery of the cell.

  • Inclusions: nonfunctional materials (e.g., glycogen, fat droplets).

F. CYTOPLASMIC ORGANELLES:

• Categorized into non-membranous and membranous:

  1. Non-Membranous Organelles:

  • Cytoskeleton, microvilli, centrioles, cilia, flagella, ribosomes, proteasomes.

  1. Membranous Organelles:

  • Nucleus, mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes.

Non-Membranous Organelles Details:

• Cytoskeleton: Network of protein fibers providing structure and transport pathways.

• Microvilli: Increase surface area for absorption.

• Ribosomes: Site of protein synthesis.

Membranous Organelles Details:

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes for protein synthesis.

  • Smooth ER: Involved in lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, packages, and directs proteins for secretion.

• Mitochondria:

  • Powerhouses of the cell, producing ATP. Have their own DNA.

• Lysosomes: Contain digestive enzymes to recycle cell waste.

G. NUCLEUS:

• Control center; contains DNA organized into chromosomes.

• Nuclear envelope surrounds nucleus, with pores for material exchange.

• Nucleoli: Produce ribosomal RNA (rRNA).

H. THE GENETIC CODE:

1. DNA Structure:

   • Double-stranded helix, consisting of nucleotide bases (A, T, C, G).

   • Genetic information is encoded by the sequence of these bases.

2. DNA Triplet Code:

   • 3 bases encode for a specific amino acid.

3. Gene:

   • Functional unit of heredity that codes for proteins.

4. Genome:

   • Complete set of genes in an organism (humans have approximately 23,000 genes).

I. PROTEIN SYNTHESIS:

• Requires DNA, RNA, enzymes, and ribosomes.

• 2 Main Processes:

  1. Transcription: DNA is converted into mRNA in the nucleus.

  2. Translation: mRNA is used to create proteins at the ribosomes.

J. CELL LIFE CYCLE:

• Involves cell division (mitosis + cytokinesis):

  • Mitosis: Division of somatic cells.

  • Meiosis: Division of sex cells.

  • Apoptosis: Controlled cell death, important for development and maintenance.

  • Interphase: Preparation phase for cell division including DNA replication.

  • Mitosis Phases:

  1. Prophase: Chromosomes condense, nuclear envelope disappears.

  2. Metaphase: Chromosomes align at cell equator.

  3. Anaphase: Sister chromatids separate.

  4. Telophase: Nuclear envelope reforms around daughter nuclei.