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.

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 (composed of 50S and 30S subunits).
• 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.

Cell Division→

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 binary fission.
• Endosymbiont Theory: Mitochondria are thought to have evolved from ancient prokaryotes that were engulfed by a host cell. Evidence includes their circular DNA, 70S ribosomes, and double membrane.

Plastids (in Plants and Algae)

A family of related organelles found exclusively in plant cells and algae, each enclosed by a double membrane.

• Chloroplasts: The site of photosynthesis.

  • Structure: Enclosed by a double membrane envelope surrounding the stroma (fluid matrix). Within the stroma are stacks of membrane-bound discs called thylakoids, organized into stacks called grana. Thylakoid membranes contain photosynthetic pigments (chlorophyll).
  • Function: Convert light energy into chemical energy (glucose) via photosynthesis.
  • Endosymbiont Evidence: Like mitochondria, chloroplasts contain circular DNA and 70S ribosomes, supporting the Endosymbiotic Theory.

• Chromoplasts: Contain pigments (carotenoids) other than chlorophyll.

  • Function: Provide yellow, orange, and red colours to flowers and fruits, attracting pollinators and seed dispersers.

• Leucoplasts: Colourless plastids found in non-photosynthetic tissues (roots, seeds).

  • Function: Store starch (amyloplasts), oils (elaioplasts), or proteins (proteinoplasts).