K-12 General Biology I 1st Quarter: Key Concepts Explained
1. Postulates of the Cell Theory
The cell theory is fundamental to biology and states:
- All living organisms are composed of one or more cells.
- The cell is the basic unit of life in all living things.
- All cells arise from pre-existing cells.
These postulates emphasize the central role of cells as the building blocks of life.
2. Structure and Function of Major and Subcellular Organelles
Cells contain various organelles with specific roles:
- Nucleus: Controls cell activities and houses genetic material (DNA).
- Mitochondria: Produces energy (ATP) through cellular respiration.
- Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids.
- Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.
- Lysosomes: Contains digestive enzymes to breakdown waste.
- Chloroplasts: Found in plants, conduct photosynthesis.
- Ribosomes: Synthesize proteins.
- Cell Membrane: Controls entry and exit of substances.
3. Prokaryotic vs. Eukaryotic Cells
| Feature | Prokaryotic Cells | Eukaryotic Cells |
|---|
| Nucleus | No true nucleus; DNA is in nucleoid. | Has a membrane-bound nucleus. |
| Size | Generally smaller (1-10 μm). | Larger (10-100 μm). |
| Organelles | Lack membrane-bound organelles. | Contain membrane-bound organelles. |
| Examples | Bacteria and Archaea. | Plants, Animals, Fungi, Protists. |
4. Cell Types in Plant and Animal Tissues and Their Functions
Cells form tissues with specialized functions:
- Plant cells: Parenchyma (photosynthesis, storage), Collenchyma (support), Sclerenchyma (rigidity).
- Animal cells: Epithelial cells (cover surfaces), Muscle cells (movement), Nerve cells (signal transmission), Blood cells (transport).
5. Cell Modifications for Specialized Functions
Modifications help cells adapt:
- Microvilli: Increase surface area (e.g., intestinal cells for absorption).
- Root hairs: Extensions of root cells to increase water/nutrient absorption.
6. Phases of the Cell Cycle & Control Points
The cell cycle has phases:
- G1 phase: Cell grows and prepares for DNA replication.
- S phase: DNA replication occurs.
- G2 phase: Further growth and preparation for mitosis.
- M phase: Mitosis and cytokinesis (cell division).
Control points (checkpoints) ensure the cell is ready to proceed, preventing errors.
7. Stages of Mitosis and Meiosis (Given 2n=6)
Mitosis: Produces two identical diploid daughter cells:
- Prophase: Chromosomes condense, spindle forms.
- Metaphase: Chromosomes align at the cell center.
- Anaphase: Sister chromatids separate.
- Telophase: Nuclear envelope forms around each set.
- Cytokinesis: Division of cytoplasm.
Meiosis: Produces four genetically diverse haploid cells through two divisions:
- Meiosis I: Homologous chromosomes separate.
- Meiosis II: Sister chromatids separate.
For 2n=6, cells have 6 chromosomes; meiosis halves this number to n=3 in gametes.
8. Comparison and Significance of Mitosis and Meiosis
| Aspect | Mitosis | Meiosis |
|---|
| Purpose | Growth, repair, asexual reproduction. | Production of gametes for sexual reproduction. |
| Number of Divisions | One. | Two. |
| Daughter Cells | Two, genetically identical, diploid. | Four, genetically diverse, haploid. |
| Genetic Variation | No. | Yes, through crossing over and independent assortment. |
9. Disorders From Cell Cycle Malfunction
Examples include:
- Cancer: Uncontrolled cell division due to failure of cycle checkpoints.
- Genetic disorders: Result from abnormal meiosis, e.g., Down syndrome caused by nondisjunction.
10. Cell Membrane Structure and Function
The cell membrane is composed mainly of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It functions as a selectively permeable barrier, regulating entry and exit of materials, communication, and protection.
11. Transport Mechanisms in Cells
- Diffusion: Movement of molecules from high to low concentration.
- Osmosis: Diffusion of water across a semipermeable membrane.
- Facilitated transport: Molecules move via protein channels without energy.
- Active transport: Movement of substances against concentration gradient using energy (ATP).
12. Exocytosis vs. Endocytosis
- Exocytosis: Process where cells expel materials in vesicles to the outside.
- Endocytosis: Process by which cells engulf external substances by vesicle formation.
13. Enzymes and Factors Affecting Their Activity
Enzymes are biological catalysts composed of protein and sometimes cofactors. They have an active site where substrates bind.
Factors affecting enzyme activity:
- pH: Each enzyme has optimal pH; deviation denatures it.
- Temperature: Activity increases with temperature to a point; high heat denatures enzymes.
- Substrate concentration: Increases activity until active sites are saturated.
Understanding these factors helps us appreciate enzyme roles in metabolism and industry.