Biology Chapter: Cell Structure

Cell Organelles and Their Functions

• Mitochondria: Known as the "Powerhouse of the cell," responsible for producing most ATP through aerobic respiration. They are about 0.2 µm in diameter and 2-6 µm long, featuring double membranes with inner cristae that increase surface area for cellular respiration.

• Lysosomes: Digestive organelles that act as vesicles to break down ingested material. Slightly smaller than mitochondria, they contain about 60 enzymes that facilitate chemical reactions and the disposal of degraded material.

• Centrioles: Rod-shaped structures that occur in pairs (Diplosomes/Centrosome), positioned at right angles to each other. They play a crucial role in chromosome movement during cell division, forming the mitotic spindle.

• Cytoskeleton: An extensive network of protein structures providing internal support, cell shape, and facilitating intracellular transport and cellular movement. Comprised of three types of filaments.

Cell Division Process

• Interphase: The phase between cell divisions that includes metabolic activity and DNA replication. There are three distinct phases within interphase:

  • G1 Phase: The cell grows and metabolizes, but there is no DNA replication.
  • S Phase: Chromosomal replication occurs, resulting in identical genetic copies.
  • G2 Phase: Pre-Mitotic phase where cell growth continues and proteins are synthesized.

• Mitosis: The nuclear division process yielding two identical daughter cells, divided into four stages:

  • Prophase: Chromatin condenses into chromosomes; centrioles move to opposite poles of the cell.

Transport Mechanisms

• Filtration: Movement of liquid through a filter, driven by hydrostatic pressure, separating larger molecules from smaller ones in a solution.

• Dialysis: Involves selective permeable membrane allowing small molecules to pass while retaining larger ones.

• Active Transport: Requires metabolic energy (ATP) to move substances against a concentration gradient, occurring via processes like:

  • Endocytosis: Absorbing extracellular substances into cells by engulfing them.
    • Phagocytosis: "Cell eating," engulfing solid particles for immune defense.
    • Pinocytosis: "Cell drinking," taking in liquids with dissolved nutrients.

• Exocytosis: The process of moving substances out of the cell, including the secretion of hormones and removal of waste.

Structure of the Cell

• Cell Components:
  • Plasma Membrane: Encloses cytoplasm, provides a selective barrier for substance movement.
  • Nucleus: Control center of the cell, containing nucleolus responsible for rRNA synthesis and gene regulation.
  • Cytoplasm: Factory area containing organelles and cytosol where cellular activities occur.

Cell Life Cycle

• Cell Division: The process for cell reproduction, including mitosis and meiosis.
• Mitosis: Results in two identical daughter cells through one nuclear division and chromosomal duplication.
• Meiosis: Reduces chromosome number by half, crucial for gamete formation; involves cytokinesis post-division.

Summary of Transport Mechanisms

• Passive Transport: Movement without energy expenditure, classified into:
  • Diffusion: Movement from higher to lower concentration.
  • Osmosis: Diffusion of water across membranes.
  • Filtration and Dialysis: Specialized processes for separating substances in a liquid.