# **Cytoplasm and Organelles** This section outlines the structure and function of the cytoplasm and the various organelles found within eukaryotic cells. These specialized compartments allow for efficient and organized cellular activity. ## **Cytoplasm and Cytosol** The *protoplasm* (living matter of a cell) is divided into the nucleus and the cytoplasm. - **Cytoplasm:** The entire region of a eukaryotic cell between the nuclear membrane and the plasma membrane. It comprises the cytosol and all the organelles suspended within it. - **Cytosol:** The semi-fluid, jelly-like matrix of the cytoplasm. - *Composition:* Approximately 90% water, containing dissolved ions, salts, sugars, amino acids, and proteins. - *Function:* Serves as the site for numerous metabolic pathways, including *glycolysis* and protein synthesis. In prokaryotes, which lack organelles, all metabolic reactions occur in the cytosol. ## **Cell Organelles** These are highly organized, discrete structures within the cytoplasm of eukaryotic cells, each performing a specific function. ### **Endoplasmic Reticulum (ER)** An extensive network of interconnected membranous tubules and flattened sacs called *cisternae*. The internal space is known as the *ER lumen*. - **Rough ER (RER):** - *Structure:* Its cytoplasmic surface is studded with **ribosomes**, giving it a "rough" appearance. - *Function:* 1. **Protein Synthesis:** Synthesizes proteins destined for secretion, insertion into membranes, or delivery to certain organelles. 2. **Protein Modification:** Modifies and folds proteins within the ER lumen. 3. **Membrane Factory:** Produces membrane proteins and phospholipids for the cell, which can be transferred via *transport vesicles*. - **Smooth ER (SER):** - *Structure:* Lacks ribosomes, resulting in a smooth surface. - *Function:* 1. **Lipid Synthesis:** Synthesizes lipids, including oils, phospholipids, cholesterol, and steroid hormones. 2. **Detoxification:** Detoxifies drugs and poisons, particularly in liver cells. 3. **Calcium Storage:** Stores calcium ions. In muscle cells, it is a specialized form called the *sarcoplasmic reticulum*. | Feature | **Rough ER (RER)** | **Smooth ER (SER)** | | :--- | :--- | :--- | | **Ribosomes** | Present | Absent | | **Primary Function**| Protein synthesis and modification, membrane production | Lipid synthesis, detoxification, Ca²⁺ storage | | **Structure** | Network of flattened sacs (cisternae) | Network of tubules | ### **Ribosomes** Non-membranous organelles responsible for *protein synthesis*. - *Composition:* Made of ribosomal RNA (rRNA) and protein. - *Structure & Types:* Composed of two subunits (large and small). - **Eukaryotic Ribosomes:** 80S (composed of 60S and 40S subunits). - **Prokaryotic Ribosomes:** 70S (smaller). - **Polysome:** A chain of multiple ribosomes translating the same mRNA molecule simultaneously, allowing for rapid production of many protein copies. ### **Golgi Complex (Apparatus)** A stack of flattened, membrane-bound sacs (*cisternae*) that functions as a cellular "post office." - *Structure:* Has a receiving (*cis* face) and a shipping (*trans* face). - *Function:* 1. **Modification:** Modifies proteins and lipids received from the ER (e.g., glycosylation). 2. **Sorting & Packaging:** Sorts and packages these molecules into vesicles for transport to other destinations. 3. **Formation of other Organelles:** Produces lysosomes and peroxisomes. 4. **Plant Cell Division:** In plants, forms the *phragmoplast* (cell plate) during cytokinesis. ### **Lysosomes** Single-membraned vesicles containing powerful *hydrolytic (digestive) enzymes*. - *Function:* Act as the cell's waste disposal and recycling system. - **Phagocytosis:** Digest food particles or foreign invaders taken in by the cell. - **Autophagy:** Break down and recycle the cell's own old or damaged organelles ("self-eating"). - **Autolysis:** Programmed cell death where lysosomes rupture and digest the entire cell, earning them the nickname *"suicidal bags"*. ### **Microbodies (Peroxisomes & Glyoxysomes)** Small, single-membraned vesicles containing specialized enzymes. - **Peroxisomes:** Contain oxidative enzymes like *catalase* and *peroxidase*. - *Function:* Break down fatty acids and detoxify harmful substances (e.g., alcohol) by forming and then neutralizing hydrogen peroxide ($\text{H}_2\text{O}_2$). Involved in photorespiration in plants. - **Glyoxysomes:** Specialized peroxisomes found in plant seeds. - *Function:* Convert stored fatty acids into carbohydrates for energy during germination via the *glyoxylate cycle*. ### **Vacuoles** Large, membrane-bound sacs with diverse functions. The membrane is called the *tonoplast*. - **Food Vacuoles:** Formed by phagocytosis to enclose food particles (in animals and protists). - **Contractile Vacuoles:** Pump excess water out of freshwater protists. - **Central Vacuole (Plants):** A large vacuole in mature plant cells. - *Function:* Stores water, nutrients, pigments, and waste products in the *cell sap*. Maintains *turgor pressure*, providing structural support to the cell. ### **Mitochondria** The "powerhouses" of the cell, responsible for cellular respiration and ATP generation. - *Structure:* Enclosed by a double membrane. - **Outer Membrane:** Smooth and permeable. - **Inner Membrane:** Folded into *cristae* to increase surface area. Contains the electron transport chain and *ATP synthase* enzymes. - **Mitochondrial Matrix:** The inner compartment containing circular DNA, 70S ribosomes, and enzymes for the Krebs cycle. - **Self-Replicating:** Can divide by fission. - **Endosymbiont Theory:** Mitochondria are thought to have evolved from ancient prokaryotes that were engulfed by a host cell. ### **Plastids (in Plants and Algae)** A family of related organelles. - **Chloroplasts:** The site of *photosynthesis*. - *Structure:* Double membrane envelope surrounding the *stroma* (fluid). - **Thylakoids:** Membranous sacs within the stroma, stacked into *grana*. Thylakoid membranes contain chlorophyll and are the site of light-dependent reactions. - **Stroma:** Contains circular DNA, 70S ribosomes, and enzymes for the light-independent reactions (Calvin cycle). - **Chromoplasts:** Contain pigments that give fruits and flowers their colors. - **Leucoplasts:** Colorless plastids that store starch or oil. ### **Centrioles (in Animal Cells)** Non-membranous organelles that occur in pairs, positioned at right angles. - *Structure:* Each centriole is composed of nine triplets of microtubules arranged in a cylinder. - *Function:* Organize microtubules to form the *mitotic spindle* during cell division and are the foundation of *cilia* and *flagella*. | Organelle | Key Function | Found In | | :--- | :--- | :--- | | **Ribosome** | Protein synthesis | All cells | | **Rough ER** | Synthesis of secretory proteins | Eukaryotes | | **Smooth ER** | Lipid synthesis, detoxification | Eukaryotes | | **Golgi Complex** | Modifies, sorts, and packages molecules | Eukaryotes | | **Lysosome** | Digestion and waste breakdown | Animal cells, some protists | | **Mitochondrion**| Cellular respiration (ATP production) | Eukaryotes | | **Chloroplast** | Photosynthesis | Plants, algae | | **Central Vacuole**| Storage and turgor pressure | Plant cells | | **Centriole** | Spindle formation in cell division | Animal cells |