Cell Structure and Function: Organelles, Vacuoles, and Intercellular Junctions

Cell Structure and Function

Intermediate Filaments

Intermediate filaments are one of the three main components of the cytoskeleton, providing structural support to eukaryotic cells.

• Composition: Composed of fibrous proteins such as keratin.

• Functions:

  • Strictly structural: They stabilize the cell shape.

  • Fix the position of organelles like the nucleus. The nucleus is held in position by intermediate filament cages.

• Example: The nuclear lamina, a network of intermediate filaments, supports the nuclear envelope.

Mitochondria

Mitochondria are membrane-bound organelles known as the "powerhouse of the cell" because they generate most of the cell's supply of ATP, used as a source of chemical energy.

• Main function: Site of ATP production in all eukaryotes.

• Structure: Double membrane with inner folds called cristae, increasing surface area for energy production.

• Additional functions: Involved in cellular respiration and metabolic processes.

• Equation for cellular respiration:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Chloroplasts and Chromoplasts

Chloroplasts and chromoplasts are plastids found in plant cells and some protists, involved in photosynthesis and pigment storage.

• Chloroplasts: Contain chlorophyll (green pigment) and are the site of photosynthesis.

• Chromoplasts: Store hydrophobic pigments such as orange and yellow carotenoids.

• Amyloplasts: Store starch (long-term carbohydrate storage in plants).

• Example: Potato plant stores starch in amyloplasts, visible when stained with iodine.

Peroxisomes

Peroxisomes are small, membrane-bound organelles involved in various metabolic reactions, including the breakdown of fatty acids and detoxification of hydrogen peroxide.

• Function: Fatty acid metabolism and detoxification.

• Byproduct: Produce hydrogen peroxide (H2O2), a reactive byproduct.

• Enzyme: Contain catalase and peroxidase to break down hydrogen peroxide quickly.

Endomembrane System

The endomembrane system is a group of interconnected organelles within eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Components:

  • Nuclear membrane

  • Endoplasmic reticulum (ER):

    • Rough ER: Studded with ribosomes; synthesizes proteins.

    • Smooth ER: Lacks ribosomes; synthesizes lipids, phospholipids, steroids, and triglycerides.

  • Golgi apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

  • Vacuoles and vesicles: Store and transport substances within the cell.

  • Lysosomes: Contain digestive enzymes to break down waste.

• Protein trafficking: Proteins travel from the rough ER to the Golgi apparatus via transport vesicles. The Golgi adds a molecular address label to direct proteins to their destination.

• Destinations: Proteins and lipids can be sent outside the cell, incorporated into the cell membrane, or sent to other organelles.

Vacuoles

Vacuoles are membrane-bound sacs within the cytoplasm of plant and some protist cells, serving various storage and structural functions.

Types of Vacuoles

• Central vacuole: Found in plant cells; maintains turgor pressure, stores nutrients and waste products.

• Contractile vacuole: Found in freshwater protists like Paramecium; expels excess water to prevent cell bursting.

• Food vacuole: Stores ingested food; formed by phagocytosis.

Functions of Central Vacuole

• Turgor pressure: Maintains cell rigidity; loss of water causes wilting.

• Storage of hydrophobic pigments: Stores pigments such as red and blue anthocyanins.

• Sequestration of harmful materials: Isolates toxic substances (e.g., nicotine in tobacco).

• Sequestration of materials for edibility: Example: Arsenic in rice is sequestered to make it edible.

Functions of Contractile Vacuole

• Regulates water balance in freshwater protists by expelling excess water.

• Example: Paramecium uses contractile vacuoles to survive in hypotonic environments.

Functions of Food Vacuole

• Stores ingested food particles for digestion.

• Formed by phagocytosis (engulfing food particles).

Lysosomes

Lysosomes are membrane-bound organelles containing hydrolytic enzymes for intracellular digestion.

• Function: Digest phagocytosed material and break down cellular waste.

• Autophagy: Process of digesting and recycling the cell's own organelles.

• Terminology: 'Lyso' = to break, 'some' = body; 'phago' = to eat, 'auto' = self.

Intercellular Junctions

Intercellular junctions are specialized structures that connect adjacent cells, facilitating communication and adhesion.

Types of Intercellular Junctions in Animal Cells

• Desmosomes: Provide strong adhesion between cells; found in skin tissue.

• Tight junctions: Seal cells together to prevent leakage; found in the stomach and bladder.

• Gap junctions: Allow rapid passage of materials and communication between cells; found in cardiac muscle, smooth muscle in the uterus, and embryonic cells.

Plant Intercellular Junctions

• Plasmodesmata: Channels that join the cytoplasm of adjacent plant cells, allowing transport and communication; functionally similar to gap junctions in animal cells.

Summary Table: Types and Functions of Cell Organelles

Organelle	Main Function	Key Features
Mitochondria	ATP production (cellular respiration)	Double membrane, cristae, own DNA
Chloroplast	Photosynthesis	Contains chlorophyll, double membrane, own DNA
Peroxisome	Fatty acid metabolism, detoxification	Contains catalase and peroxidase
Vacuole	Storage, turgor pressure, waste sequestration	Large central vacuole in plants, contractile in protists
Lysosome	Digestion of macromolecules	Contains hydrolytic enzymes
Golgi Apparatus	Modification, sorting, packaging of proteins/lipids	Stacked membrane-bound sacs
Endoplasmic Reticulum	Protein and lipid synthesis	Rough (ribosomes), smooth (lipid synthesis)