Unit 2: Structure and Function of Cell Organelles
Organelles are the specialized, membrane-bound "little organs" within a eukaryotic cell that perform specific jobs.
Cytosol
The cytosol is the gel-like fluid (mostly water) that fills the cell and surrounds the organelles. It is distinct from cytoplasm, which is the cytosol *plus* all the organelles suspended within it (except the nucleus).
Functions of the Cytosol:
- Site of many metabolic pathways (e.g., Glycolysis).
- Contains free ribosomes for protein synthesis.
- Acts as a medium for signal transduction.
Endoplasmic Reticulum (ER)
A vast network of membranes continuous with the outer nuclear membrane.
- Rough ER (RER): "Rough" because it is studded with ribosomes.
- Function: Synthesis, folding, and modification (e.g., glycosylation) of proteins destined for secretion or insertion into membranes.
- Smooth ER (SER): "Smooth" because it lacks ribosomes.
- Function: Lipid synthesis (steroids, phospholipids), calcium storage (crucial for muscle contraction), and detoxification of drugs and poisons (abundant in liver cells).
Golgi Complex (Golgi Apparatus)
The "post office" of the cell. It's a stack of flattened membrane sacs called cisternae.
- Structure: Has a "receiving" side (cis face, facing the ER) and a "shipping" side (trans face, facing the plasma membrane).
- Function: Receives proteins and lipids from the ER, further modifies them, sorts them, and packages them into vesicles for transport to their final destinations (e.g., lysosomes, plasma membrane).
Mitochondria
The "powerhouse" of the cell. Responsible for cellular respiration and ATP synthesis.
- Structure: Has two membranes:
- Outer Membrane: Smooth.
- Inner Membrane: Highly folded into cristae. This increases surface area.
- Compartments:
- Matrix: The fluid-filled center. Site of the Krebs Cycle.
- Inner Membrane: Site of the Electron Transport Chain (ETC) and ATP synthase.
The Endosymbiotic Theory: Mitochondria (and chloroplasts) are thought to have originated as free-living bacteria that were engulfed by a larger cell.
Evidence:
- They have their own circular DNA.
- They have their own 70S ribosomes.
- They have a double membrane.
- They replicate by binary fission.
Chloroplast
The site of photosynthesis in plant and algal cells.
- Structure: Has two outer membranes plus a third internal membrane system.
- Thylakoids: Flattened, interconnected sacs. The light-dependent reactions occur in the thylakoid membrane.
- Grana: Stacks of thylakoids.
- Stroma: The fluid surrounding the thylakoids. Site of the Calvin Cycle (light-independent reactions).
Ribosomes
The "protein factories" of the cell. They are not membrane-bound. They are made of rRNA and protein.
- Function: Translate the genetic code from mRNA into a polypeptide chain (protein synthesis).
- Types:
- Eukaryotic: 80S (60S + 40S subunits).
- Prokaryotic: 70S (50S + 30S subunits).
- Location: Can be free in the cytosol (make proteins for use *inside* the cell) or bound to the RER (make proteins for *export* or for membranes).
Lysosomes
The "digestive" or "recycling" system of the cell. Found primarily in animal cells.
- Structure: Single-membrane vesicles formed by the Golgi.
- Function: Contain powerful hydrolytic enzymes that work at an acidic pH (~5). They break down old organelles (autophagy) or engulfed food/pathogens (phagocytosis).
Peroxisomes
Small, single-membrane organelles involved in various metabolic processes.
- Function:
- Break down fatty acids.
- Detoxify harmful substances (like alcohol).
- Crucially, they break down hydrogen peroxide (H2O2), a toxic byproduct of metabolism, using the enzyme catalase.
Key Reaction (Catalase):
2 H2O2 → 2 H2O + O2
Nucleus
The "control center" of the eukaryotic cell.
- Structure:
- Nuclear Envelope: A double membrane that encloses the nucleus.
- Nuclear Pores: Regulate the passage of molecules (like mRNA) in and out.
- Nucleolus: A dense region where ribosomes are assembled.
- Chromatin: The complex of DNA and histone proteins.
- Function: Protects the cell's genetic material (DNA) and is the site of DNA replication and transcription (making RNA).
Vacuole
Membrane-bound sacs with diverse functions.
- Plant Cells: Have a large central vacuole that stores water, nutrients, and waste. It is responsible for maintaining turgor pressure against the cell wall.
- Animal Cells: Have small, temporary vacuoles (if any).
Cytoskeleton
A network of protein fibers providing structural support, shape, and movement. (This topic is also in DSC 101, but is included here from the DSM 101 syllabus image).
- Microfilaments: (Actin) Thinnest fibers. Involved in muscle contraction, cell crawling, and cell shape.
- Intermediate Filaments: Mid-sized. Provide mechanical strength and anchor organelles (e.g., Keratin).
- Microtubules: (Tubulin) Thickest, hollow tubes. Act as "tracks" for motor proteins to move vesicles, form the mitotic spindle in cell division, and are the core of cilia and flagella.