JR

biology

Cell theory is one of the fundamental principles of biology, stating that cells are the basic unit of life.

1. Key Tenets of Cell Theory

  1. All known living things are made up of one or more cells. This principle emphasizes the cellular composition of all organisms, from the smallest bacteria to the largest mammals.

  2. The cell is the basic structural and functional unit of all living things. Cells are the smallest units that can perform all the processes of life, such as metabolism, reproduction, and response to stimuli.

  3. All cells arise from pre-existing cells by division. This tenet disproves the concept of spontaneous generation and highlights the continuity of life through cell division.

2. Historical Contributions

  • Anton van Leeuwenhoek (1674): Often considered the "Father of Microbiology," he significantly improved the microscope and was the first to observe and describe single-celled organisms, which he called "animalcules." He also observed blood cells and sperm.

  • Robert Hooke (1665): Coined the term "cell" after observing cork through a microscope. He saw tiny, box-like compartments that reminded him of the small rooms (cells) occupied by monks.

  • Matthias Schleiden (1838): A German botanist who concluded that all plant parts are made of cells.

  • Theodor Schwann (1839): A German zoologist who concluded that all animal parts are made of cells. Together, Schleiden and Schwann are credited with the initial formulation of the first two tenets of cell theory.

  • Rudolf Virchow (1855): A German physician who famously stated "Omnis cellula e cellula" (All cells arise from pre-existing cells), adding the third crucial tenet to the cell theory, thus completing its widely accepted modern form.

Main Phases of the Cell Cycle

  1. Interphase (the longest phase — preparation stage)

    • G1 phase (Gap 1):
      The cell grows and does normal cell functions.

    • S phase (Synthesis):
      The cell copies its DNA (so there are two identical sets).

    • G2 phase (Gap 2):
      The cell prepares for division by making proteins and organelles.

  3. M Phase (Mitosis)
    This is when the cell divides into two identical cells. It includes:

    • Prophase: Chromosomes become visible; nuclear membrane disappears.

    • Metaphase: Chromosomes line up in the middle of the cell.

    • Anaphase: Chromosomes are pulled apart to opposite sides.

    • Telophase: Two new nuclei form.

  5. Cytokinesis
    The cytoplasm splits, and two daughter cells are formed.

Plant and animal cells are both eukaryotic cells, meaning they have a true nucleus and other membrane-bound organelles. However, they have distinct differences in their structure:

Common Parts (found in both plant and animal cells):

  • Nucleus: Contains the cell's genetic material (DNA) and controls cell activities.

  • Mitochondria: Often called the "powerhouses" of the cell, they generate most of the chemical energy needed to power the cell's biochemical reactions.

  • Endoplasmic Reticulum (ER): A network of membranes involved in protein and lipid synthesis.

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

  • Ribosomes: Responsible for protein synthesis.

  • Cytoplasm: The jelly-like substance that fills the cell and surrounds the organelles.

  • Cell Membrane: A selective barrier that regulates the passage of substances into and out of the cell.

Parts Specific to Plant Cells:

  • Cell Wall: A rigid outer layer that provides structural support and protection to the cell.

  • Chloroplasts: Sites of photosynthesis, containing chlorophyll which captures light energy.

  • Large Central Vacuole: A large, fluid-filled sac that stores water, nutrients, and waste products, and helps maintain turgor pressure.

Parts Specific to Animal Cells:

  • Centrioles: Involved in cell division and the formation of cilia and flagella.

  • Lysosomes: Contain enzymes that break down

  • Meiosis Meiosis is a special type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell. This process is essential for sexual reproduction.

Key characteristics of Meiosis:

  • Purpose: To produce gametes (sperm and egg cells) in animals and spores in plants and fungi.

  • Outcome: Four haploid daughter cells (n) from one diploid parent cell (2n).

  • Genetic Variation: Achieved through crossing over (exchange of genetic material between homologous chromosomes) and independent assortment (random alignment of homologous chromosomes).

Main Phases of Meiosis:

  1. Meiosis I (Reductional Division): Homologous chromosomes separate, reducing the chromosome number by half.

    • Prophase I: Chromosomes condense, homologous chromosomes pair up (forming bivalents), and crossing over occurs. The nuclear envelope disappears.

    • Metaphase I: Homologous pairs align at the metaphase plate.

    • Anaphase I: Homologous chromosomes separate and move to