Foundations: The Cell

Membranous Organelles

Membranous organelles are specialized structures within eukaryotic cells, each enclosed by their own lipid membranes. These membranes isolate the organelles from the cytosol, allowing for compartmentalization of cellular processes.

• Definition: Membranous organelles are cellular structures surrounded by lipid bilayers, enabling distinct internal environments for specific biochemical functions.

• Examples: Nucleus, mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and peroxisomes.

The Nucleus

The nucleus is the largest organelle in most eukaryotic cells and serves as the control center, housing the cell's genetic material.

• Nuclear Envelope: A double-layered membrane that surrounds the nucleus, separating it from the cytoplasm.

• Nucleoplasm: The fluid matrix within the nucleus, containing dissolved ions, enzymes, and nucleotides.

• Nucleolus (plural: nucleoli): A dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• Chromatin: The non-coiled form of chromosomes present when the cell is not dividing; consists of DNA and associated proteins (histones).

• Functions:

  • Acts as the control center of the cell by regulating gene expression and cell activities.

  • Contains DNA, which provides instructions for protein synthesis.

  • Transmits genetic information during cell division.

• Example: The nucleolus is especially prominent in cells with high rates of protein synthesis, such as liver cells.

Mitochondria

Mitochondria (singular: mitochondrion) are often referred to as the "power plants" of the cell due to their role in ATP production.

• Structure:

  • Enclosed by two membranes:

    • Outer mitochondrial membrane: Encloses the entire organelle.

    • Inner mitochondrial membrane: Folds inward to form cristae, increasing surface area for ATP production.

  • Mitochondrial matrix: The space enclosed by the inner membrane, containing enzymes, mitochondrial DNA, and ribosomes.

• Function: Generates most of the cell's ATP (about 95%) through aerobic respiration, primarily via the electron transport chain and oxidative phosphorylation.

• ATP Synthase: An enzyme complex located in the inner membrane that synthesizes ATP from ADP and inorganic phosphate.

• Key Equation:

• Example: Muscle cells contain large numbers of mitochondria to meet high energy demands.

Endoplasmic Reticulum (ER)

The endoplasmic reticulum (ER) is an extensive network of membranes involved in synthesis, storage, transport, and detoxification. It exists in two forms: rough ER and smooth ER.

• Structure: Composed of tubules, vesicles, and flattened sacs called cisternae.

• Rough Endoplasmic Reticulum (RER):

  • Studded with ribosomes on its cytoplasmic surface.

  • Site of protein synthesis for proteins destined for secretion, insertion into membranes, or lysosomes.

  • Packages newly synthesized proteins into transport vesicles for delivery to the Golgi apparatus.

  • Contributes membrane material for the cell membrane.

• Smooth Endoplasmic Reticulum (SER):

  • Lacks ribosomes; appears smooth under the microscope.

  • Functions in lipid and steroid (carbohydrate) synthesis, lipid metabolism, calcium ion storage, and detoxification of drugs and poisons.

• Example: Liver cells have abundant SER for detoxification, while pancreatic cells have extensive RER for protein secretion.