# Molecular Biology B.2.2 – Organelles and Compartmentalization ## Key Questions - **How are organelles adapted to their functions?** - **What advantages does compartmentalization provide in cells?** --- ## Organelles Overview Organelles are specialized structures within cells that perform specific functions. They are essential for maintaining cellular processes and ensuring efficiency. However, **not all cellular components are considered organelles**: ### Structures NOT Considered Organelles: - **Cell Wall** - **Cytoskeleton** - **Cytoplasm** --- ## Advantages of Compartmentalization Compartmentalization is a hallmark of eukaryotic cells, allowing for the separation of processes and increased efficiency. ### Key Benefits: - **Protection of Genetic Material:** DNA is housed in the nucleus, safeguarding it from damage. - **Localized Reactions:** Enzymes and substrates are concentrated in specific regions, optimizing reactions. - **Toxin Management:** Toxic substances are isolated in membrane-bound vesicles. - **Dynamic Movement:** Organelles can relocate within the cell to meet functional demands. - **Increased Membrane Surface Area:** More membrane area facilitates metabolic processes and molecular exchanges. Compartmentalization allows eukaryotic cells to perform complex functions that are not possible in prokaryotic cells. --- ## The Endomembrane System The endomembrane system is a network of membranes within the cell that work together to modify, package, and transport lipids and proteins. It includes the **nucleus**, **endoplasmic reticulum**, **Golgi apparatus**, **lysosomes**, and **vesicles**. --- ## The Nucleus The nucleus is the control center of the cell, housing the genetic material (DNA). It is surrounded by a double membrane called the **nuclear envelope**. ### Structure: - **Nuclear Envelope:** - **Outer Membrane:** Contains ribosomes and is continuous with the rough endoplasmic reticulum (RER). - **Inner Membrane:** Contains the nuclear lamina, which anchors chromatin and regulates nuclear growth. - **Perinuclear Space:** The space between the two membranes. - **Nuclear Pores:** Octagonal openings that regulate the exchange of substances (e.g., RNA, proteins) between the nucleus and cytoplasm. - **Nucleoplasm:** Gel-like matrix inside the nucleus. - **Nucleolus:** A dense region responsible for synthesizing ribosomal RNA (rRNA). ### Key Functions: - **DNA Protection:** Chromatin (DNA + proteins) is stored in the nucleus. - **RNA Processing:** In eukaryotes, mRNA matures before leaving the nucleus. - **Ribosome Assembly:** The nucleolus synthesizes rRNA and assembles ribosomal subunits. In mammalian red blood cells, the nucleus is absent, allowing more space for hemoglobin. Conversely, paramecia have two nuclei: a macronucleus for daily functions and a micronucleus for reproduction. --- ## Endoplasmic Reticulum (ER) The ER is a network of membranes that extends from the nuclear envelope to the plasma membrane. It is divided into two types: ### Rough Endoplasmic Reticulum (RER): - **Structure:** Flattened sacs (cisternae) with ribosomes attached to the cytosolic side. - **Functions:** - **Protein Synthesis:** Ribosomes synthesize proteins, which are transported into the lumen for modification. - **Glycosylation:** Proteins are converted into glycoproteins for export. The RER is highly developed in cells with high protein synthesis, such as pancreatic cells. ### Smooth Endoplasmic Reticulum (SER): - **Structure:** Tubular network without ribosomes. - **Functions:** - **Lipid Synthesis:** Produces phospholipids and cholesterol for membranes. - **Detoxification:** Oxidizes toxic substances in liver cells. - **Calcium Storage:** In muscle cells, the SER (sarcoplasmic reticulum) releases calcium for contraction. In hepatocytes, the SER detoxifies drugs and alcohol, while in muscle cells, it regulates calcium for contraction. --- ## Ribosomes Ribosomes are non-membranous structures composed of rRNA and proteins. They are responsible for protein synthesis. ### Types: - **Free Ribosomes:** Float in the cytoplasm and synthesize proteins for intracellular use. - **Bound Ribosomes:** Attached to the RER or nuclear envelope, synthesizing proteins for export or membrane insertion. ### Ribosome Sizes: - **Prokaryotic Ribosomes:** 70S (50S + 30S subunits) - **Eukaryotic Ribosomes:** 80S (60S + 40S subunits) The "S" refers to the sedimentation rate during centrifugation, which depends on size and shape. --- ## Golgi Apparatus The Golgi apparatus is a stack of flattened membranes (cisternae) involved in modifying, packaging, and transporting molecules. ### Structure: - **Cis Face:** Receives vesicles from the ER. - **Trans Face:** Releases vesicles for secretion or delivery to other organelles. ### Functions: - **Protein and Lipid Modification:** Forms glycoproteins and glycolipids. - **Vesicle Formation:** Produces lysosomes and secretory vesicles. - **Plant Cell Division:** Forms the cell plate during cytokinesis. In sperm cells, the Golgi forms the acrosome, which contains enzymes to penetrate the egg. --- ## Lysosomes Lysosomes are membrane-bound vesicles containing hydrolytic enzymes that digest cellular waste. ### Functions: - **Intracellular Digestion:** Breaks down macromolecules and damaged organelles. - **Endocytosis:** Fuses with vesicles to digest external substances. Many students confuse lysosomes with peroxisomes. Lysosomes digest macromolecules, while peroxisomes detoxify harmful substances. --- ## Peroxisomes Peroxisomes are small organelles containing enzymes for metabolic processes. ### Functions: - **Oxidation:** Breaks down fatty acids and amino acids. - **Detoxification:** Converts hydrogen peroxide (H₂O₂) into water and oxygen using catalase. In seeds, peroxisomes (glyoxysomes) convert stored fats into sugars during germination. --- ## Vacuoles Vacuoles are large membrane-bound structures found primarily in plant cells. ### Functions: - **Turgor Pressure:** Maintains cell rigidity by storing water. - **Storage:** Accumulates nutrients and waste products. - **Digestion:** Contains hydrolytic enzymes. The vacuole's membrane is called the tonoplast, and its contents are known as the vacuolar sap. --- ## Mitochondria Mitochondria are the powerhouse of the cell, producing ATP through cellular respiration. ### Structure: - **Outer Membrane:** Highly permeable due to porins. - **Inner Membrane:** Contains folds (cristae) where the electron transport chain and ATP synthase are located. - **Matrix:** Contains enzymes for the Krebs cycle and mitochondrial DNA. ### Functions: - **ATP Production:** Through oxidative phosphorylation. - **Metabolic Processes:** Krebs cycle and β-oxidation of fatty acids. The number of mitochondria in a cell correlates with its energy demands. Muscle cells, for example, have abundant mitochondria. --- ## Plastids (Plant Cells Only) Plastids are organelles involved in photosynthesis and storage. ### Types: - **Leucoplasts:** Store starch (e.g., amyloplasts), lipids, or proteins. - **Chromoplasts:** Contain pigments for coloration. - **Chloroplasts:** Perform photosynthesis. ### Chloroplast Structure: - **Thylakoids:** Membrane-bound sacs where the light-dependent reactions occur. - **Stroma:** Gel-like matrix where the Calvin cycle (light-independent reactions) takes place. ### Functions: - **Photosynthesis:** Converts light energy into glucose. - **Fatty Acid Biosynthesis:** Produces lipids for membranes. - **Nitrogen Reduction:** Converts nitrates into amino acids. In spinach leaves, chloroplasts are abundant to maximize photosynthesis. --- ## Key Takeaways - **Compartmentalization** enhances efficiency and protects cellular processes. - Organelles are specialized for distinct functions, such as energy production (mitochondria) or protein synthesis (RER and ribosomes). - Plant cells have unique organelles like **chloroplasts** and **vacuoles** for photosynthesis and storage. - The **endomembrane system** coordinates the synthesis, modification, and transport of molecules. 1. What is the main function of the Golgi apparatus? 2. How do lysosomes differ from peroxisomes? 3. Why do muscle cells have more mitochondria than skin cells?