Unit 2: Structure and Function of Cell Organelles

Eukaryotic cells are defined by their internal compartmentalization, with specific metabolic processes occurring within membrane-bound organelles.

1. Cytosol

The cytosol is the semi-fluid, jelly-like substance that fills the cell and surrounds the organelles. It is distinct from cytoplasm, which is the cytosol *plus* the organelles suspended within it.

• Composition: Mostly water, but also contains dissolved ions, small molecules, and proteins.
• Function: It is the site of many key metabolic pathways, including Glycolysis, parts of Gluconeogenesis, and the Pentose Phosphate Pathway.

2. The Endomembrane System

This is a group of organelles that work together to synthesize, modify, package, and transport lipids and proteins. The membranes are either in direct contact or connected via transport vesicles.

2a. Nucleus & Nucleolus

[Image of the structure of the cell nucleus]

• Structure (Nucleus): The cell's "control center." It is enclosed by a nuclear envelope (a double membrane) which is perforated by nuclear pores that regulate traffic with the cytoplasm. Contains the chromatin (DNA + proteins).
• Function (Nucleus): Contains and protects the cell's genetic material (DNA). Site of DNA replication and transcription (synthesis of RNA from a DNA template).
• Structure (Nucleolus): A dense structure inside the nucleus.
• Function (Nucleolus): The site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

2b. Endoplasmic Reticulum (ER)

An extensive network of interconnected membranes (cisternae) in the cytoplasm.

• Rough Endoplasmic Reticulum (RER):
  • Structure: Studded with ribosomes on its outer surface.
  • Function: Folds and modifies proteins synthesized by the bound ribosomes (e.g., adds carbohydrates to form glycoproteins). Synthesizes membrane proteins.
• Smooth Endoplasmic Reticulum (SER):
  • Structure: Lacks ribosomes.
  • Function: Lipid synthesis (oils, phospholipids, steroids), detoxification of drugs and poisons, and storage of calcium ions (Ca²⁺).

2c. Golgi Complex (or Golgi Apparatus)

[Image of the Golgi Complex structure]

• Structure: A stack of flattened membrane-bound sacs called cisternae. It has two faces: a cis face (receiving side, near the ER) and a trans face (shipping side).
• Function: The cell's "post office." It receives proteins and lipids from the ER, further modifies them (e.g., alters carbohydrate tags), sorts them, and packages them into transport vesicles for delivery to other organelles or for secretion from the cell.

2d. Lysosomes

• Structure: A membranous sac containing hydrolytic enzymes (enzymes that break down molecules using water). They maintain a very acidic internal pH (~5.0).
• Function: The cell's "digestive system" or "recycling center."
  • Phagocytosis: Fuses with food vacuoles to digest food.
  • Autophagy: Breaks down and recycles damaged organelles.

2e. Vacuole

• Structure: Large membrane-bound vesicles.
• Function: Varies by cell type:
  • Food Vacuoles: Formed by phagocytosis.
  • Contractile Vacuoles: (In protists) Pump excess water out of the cell.
  • Central Vacuole: (In mature plant cells) A very large vacuole that stores water, nutrients, and waste. It is essential for maintaining turgor pressure against the cell wall.

3. Ribosomes

[Image of a ribosome translating mRNA]

• Structure: Complexes made of rRNA and protein. They are *not* membrane-bound. Composed of two subunits (large and small).
  • Prokaryotic: 70S
  • Eukaryotic: 80S
• Function: The site of protein synthesis (translation). They read the code from messenger RNA (mRNA) and assemble amino acids into polypeptide chains.
• Location:
  • Free ribosomes: Float in the cytosol. Synthesize proteins that will function *within* the cytosol.
  • Bound ribosomes: Attached to the Rough ER. Synthesize proteins destined for secretion or insertion into membranes.

4. Energy-Converting Organelles

These organelles are semi-autonomous, containing their own DNA and 70S ribosomes, supporting the endosymbiotic theory.

4a. Mitochondria

[Image of the structure of a mitochondrion]

• Structure: Enclosed by *two* membranes (outer and inner). The inner membrane is highly folded into cristae, which increase surface area. The space inside the inner membrane is the matrix.
• Function: The "powerhouse" of the cell. The site of cellular respiration.
  • Matrix: Site of the TCA (Krebs) Cycle.
  • Inner Membrane: Site of the Electron Transport Chain and ATP synthesis.

4b. Chloroplast

[Image of the structure of a chloroplast]

• Structure: (In plants and algae) Enclosed by *two* membranes. Inside, it has a system of flattened, interconnected sacs called thylakoids. A stack of thylakoids is a granum. The fluid outside the thylakoids is the stroma. Thylakoid membranes contain chlorophyll.
• Function: The site of photosynthesis.
  • Thylakoids: Site of the "light-dependent reactions."
  • Stroma: Site of the "light-independent reactions" (Calvin Cycle), where CO₂ is fixed into sugar.

5. Peroxisomes

• Structure: A small, single-membrane metabolic compartment.
• Function: Contains enzymes that break down fatty acids. A byproduct of this is hydrogen peroxide (H₂O₂), which is toxic. Peroxisomes also contain the enzyme catalase, which safely breaks down H₂O₂ into water and oxygen.
• Reaction: 2 H₂O₂ → 2 H₂O + O₂ (catalyzed by catalase)

6. Cytoskeleton

• Structure: A dynamic network of protein fibers extending throughout the cytoplasm. Composed of three types of fibers:
  1. Microtubules: Thick, hollow tubes.
  2. Microfilaments (Actin filaments): Thin, solid rods.
  3. Intermediate Filaments: Fibers with intermediate diameter.
• Function:
  • Structural Support: Maintains cell shape.
  • Motility: Involved in cell movement (e.g., amoeboid movement) and forms the core of cilia and flagella.
  • Transport: Acts as "tracks" for motor proteins to move vesicles and organelles around the cell.
  • Cell Division: Forms the mitotic spindle.