Cellular Biology: Exploring Organelles and Cell Cycle
Chloroplasts
Chloroplasts are cellular organelles found in eukaryotic photosynthetic organisms. They are essential for photosynthesis. Enclosed by a double membrane, chloroplasts contain thylakoids, vesicles where pigments and other molecules convert light energy into chemical energy. Chlorophyll, the primary pigment, is mainly located within chloroplasts. Key Features: Responsible for the light and dark phases of photosynthesis. Involved in the biosynthesis of fatty acids from carbohydrates and the reduction of nitrate to nitrite.
Endoplasmic Reticulum (ER)
Rough Endoplasmic Reticulum (RER)
The RER is responsible for protein synthesis and transport. Found only in eukaryotic cells, it consists of interconnected channels and sacs distributed throughout the cytoplasm. These flattened sacs circulate proteins before they reach the Golgi apparatus. The RER is physically connected to the smooth endoplasmic reticulum (REL). Its “rough” appearance is due to the presence of ribosomes on its membrane. Located adjacent to the nuclear envelope, the RER receives messenger ribonucleic acids (mRNA) containing protein synthesis instructions. Functions: Synthesizes proteins for packaging or transfer to the plasma membrane, modifies these proteins, and manufactures lipids and integral proteins of cell membranes.
Smooth Endoplasmic Reticulum (REL)
The REL, a network of membranous sacs and tubes, is involved in cellular transport and the synthesis of triglycerides, phospholipids, and steroids. It also contains detoxifying enzymes that metabolize alcohol and other chemicals. Unlike the RER, the REL lacks ribosomes. Functions: Synthesizes hormones in the gonads and adrenal cortex, detoxifies organic compounds in the liver, releases glucose, sequesters and releases calcium ions (Ca2+) in muscle cells (sarcoplasmic reticulum), and synthesizes lipids.
Golgi Apparatus
The Golgi apparatus, part of the endomembrane system, is present in all eukaryotic cells except mammalian red blood cells. It consists of one or more dictyosomes, stacks of flattened sacs and vesicles. The Golgi apparatus has three functional regions: Cis-Golgi region: Receives transition vesicles containing proteins from the RER. Medial-Golgi region: A transition zone where secretory vesicles are formed. Trans-Golgi region: Closest to the cell membrane, with a similar membrane composition.
Nucleus
The nucleus, present in all eukaryotic cells except red blood cells, consists of the nuclear envelope and the nucleoplasm, which contains chromatin and the nucleolus. Its size varies depending on the cell cycle. The nuclear envelope separates transcription (DNA to RNA in the nucleus) from translation (RNA to protein in the cytoplasm). Key Components:
- Chromatin: The complex of DNA, histones, and nonhistone proteins that make up eukaryotic chromosomes. Includes euchromatin (less compact) and heterochromatin (more compact).
- Nucleoplasm: The semi-fluid matrix inside the nucleus containing chromatin and non-chromatin material.
- Nucleolus: Responsible for the production and assembly of ribosomal components.
Cell Cycle
The cell cycle, the series of events leading to cell division, consists of the following stages:
Interphase: The longest phase (95% of the cycle), comprising:
- G1: Cell growth and protein/RNA synthesis.
- S: DNA replication, resulting in two identical chromatids per chromosome.
- G2: Continued protein and RNA synthesis.
M (Mitosis and Cytokinesis): Cell division where a parent cell divides into two identical daughter cells. Mitosis includes prophase, metaphase, anaphase, and telophase. Cytokinesis begins during telophase.