Unit 2: Structure and Function of Cell Organelles
Organelles are specialized, membrane-bound structures within eukaryotic cells that perform specific functions. This unit explores the major organelles.
Endoplasmic Reticulum (ER)
The ER is a vast network of interconnected membranes (cisternae and tubules) that is continuous with the outer nuclear membrane.
1. Rough Endoplasmic Reticulum (RER)
- Structure: Its surface appears "rough" because it is studded with ribosomes.
- Functions:
- Protein Synthesis & Modification: Synthesizes proteins destined for secretion, insertion into membranes, or delivery to certain organelles (like the Golgi or lysosomes).
- Protein Folding: Folds proteins into their correct 3D shapes. Chaperone proteins assist in this.
- Glycosylation: Begins the process of adding carbohydrate chains to proteins (N-linked glycosylation).
2. Smooth Endoplasmic Reticulum (SER)
- Structure: Lacks ribosomes, giving it a "smooth" appearance. More tubular than the RER.
- Functions:
- Lipid Synthesis: Synthesizes lipids, phospholipids, and steroids (e.g., in adrenal gland cells).
- Detoxification: Detoxifies drugs, poisons, and alcohol (abundant in liver cells).
- Calcium Storage: Stores and releases Ca2+ ions, which is crucial for signal transduction and muscle contraction.
Golgi Complex (Golgi Apparatus)
The Golgi complex is the "post office" of the cell. It receives proteins and lipids from the ER, modifies them, sorts them, and packages them into vesicles for delivery.
- Structure: A stack of flattened, membrane-bound sacs called cisternae.
- Polarity: It has a distinct polarity:
- Cis face: The "receiving" side, faces the ER. Vesicles from the ER fuse here.
- Trans face: The "shipping" side, faces the plasma membrane. Vesicles bud off from here.
- Functions:
- Modification: Modifies proteins (e.g., O-linked glycosylation) and lipids.
- Sorting & Packaging: Sorts materials into different vesicles bound for specific destinations (e.g., lysosomes, plasma membrane for secretion).
- Lysosome Formation: Forms primary lysosomes by budding off from the trans-Golgi.
Mitochondria
Known as the "powerhouse" of the cell, mitochondria are responsible for generating most of the cell's supply of adenosine triphosphate (ATP) through cellular respiration.
- Structure:
- Double Membrane: Has a smooth outer membrane and a highly folded inner membrane.
- Cristae: The folds of the inner membrane, which vastly increase the surface area for ATP synthesis.
- Matrix: The fluid-filled space inside the inner membrane. Contains enzymes for the Krebs cycle, mitochondrial DNA (mtDNA), and 70S ribosomes.
- Functions:
- Cellular Respiration:
- Krebs Cycle (TCA Cycle): Occurs in the matrix.
- Electron Transport Chain (ETC) & Oxidative Phosphorylation: Occurs on the inner membrane.
- ATP Synthesis: The primary function.
- Apoptosis: Plays a role in initiating programmed cell death.
The Endosymbiotic Theory: This theory proposes that mitochondria (and chloroplasts) were once free-living prokaryotic cells that were engulfed by a larger host cell.
Evidence:
- They have their own circular DNA (like bacteria).
- They have their own 70S ribosomes (like bacteria).
- They have a double membrane (the inner one being the original, the outer from the host cell).
- They replicate by binary fission, independent of the cell cycle.
Chloroplast
Chloroplasts are the site of photosynthesis in plant and algal cells. They capture light energy and convert it into chemical energy (glucose).
- Structure:
- Double Membrane: Has an outer and inner membrane.
- Stroma: The fluid-filled space inside the inner membrane. Contains enzymes for the Calvin cycle, chloroplast DNA, and 70S ribosomes.
- Thylakoids: A third internal membrane system, consisting of flattened sacs.
- Grana (singular: granum): Stacks of thylakoids. Chlorophyll and other pigments are embedded in the thylakoid membranes.
- Functions:
- Photosynthesis:
- Light-Dependent Reactions: Occur in the thylakoid membranes.
- Light-Independent Reactions (Calvin Cycle): Occur in the stroma.
Ribosomes
Ribosomes are the "protein factories" of the cell. They are responsible for translating the genetic code from mRNA into proteins.
- Structure: They are not membrane-bound. They are composed of ribosomal RNA (rRNA) and proteins. They consist of two subunits, a large and a small one.
- Types (as seen in Unit 1):
- Eukaryotic: 80S (60S + 40S subunits).
- Prokaryotic: 70S (50S + 30S subunits).
- Locations & Function:
- Free Ribosomes: Float in the cytosol. Synthesize proteins that will function in the cytosol, nucleus, mitochondria, or peroxisomes.
- Bound Ribosomes: Attached to the Rough ER. Synthesize proteins destined for secretion, insertion into membranes, or delivery to the Golgi/lysosomes.
Lysosomes
Lysosomes are the "recycling center" or "digestive system" of the cell.
- Structure: Single-membrane vesicles formed by the Golgi apparatus.
- Contents: They contain powerful hydrolytic enzymes (acid hydrolases, e.g., proteases, nucleases, lipases) that break down waste materials.
- Function:
- Acidic Environment: They maintain an acidic pH (~5.0) inside, which is optimal for their enzymes. This is maintained by an H+ (proton) pump in the membrane.
- Phagocytosis: Digest food particles or engulfed pathogens (e.g., bacteria).
- Autophagy: Break down and recycle old, damaged organelles ("self-eating").
Peroxisomes
Peroxisomes are small, single-membrane organelles involved in various metabolic processes, particularly those that produce and break down hydrogen peroxide.
- Structure: Single-membrane vesicles, often with a crystalline core.
- Functions:
- Breakdown of Fatty Acids: Perform beta-oxidation of very long-chain fatty acids.
- Detoxification: Neutralize harmful substances (like alcohol) by transferring hydrogen to oxygen, creating hydrogen peroxide (H2O2).
- H2O2 Metabolism: They contain the enzyme catalase, which breaks down the toxic H2O2 into water (H2O) and oxygen (O2).
Key Reaction (Catalase):
2 H2O2 → 2 H2O + O2
Vacuole
Vacuoles are membrane-bound sacs with diverse functions, varying by cell type.
- Plant Cells:
- Large Central Vacuole: A single, large vacuole can occupy up to 90% of the cell volume.
- Tonoplast: The membrane surrounding the central vacuole.
- Functions:
- Storage: Stores water, nutrients, pigments, and waste products.
- Turgor Pressure: Pushes against the cell wall, providing structural support to the plant.
- Animal Cells:
- Typically have small, temporary vacuoles (if any), often involved in endocytosis or exocytosis.
- Protists:
- Contractile Vacuoles: Expel excess water to maintain osmotic balance (e.g., in *Amoeba*, *Paramecium*).
- Food Vacuoles: Formed by phagocytosis to digest food.