Cellular Organelles and Their Functions

Endoplasmic Reticulum

• A network of interconnected channels called cisterns.

• Types of Endoplasmic Reticulum:

  • Rough Endoplasmic Reticulum (RER):

    • Covered with ribosomes, appearing as tiny dots.

    • Ribosomes are responsible for protein synthesis.

  • Smooth Endoplasmic Reticulum (SER):

    • Lacks ribosomes.

    • Involved in steroid synthesis and detoxification processes.

    • Primarily found in the liver, where detoxification of alcohol and drugs occurs.

Ribosomes

• Responsible for protein synthesis.

• Read instructions from messenger RNA (mRNA) and assemble proteins accordingly.

• Protein Synthesis:

  • Proteins are the language of the cell, facilitating communication within the body.

  • Ribosomes create proteins based on cellular needs.

• Location:

  • Found freely in the cytosol.

  • Located on the surface of the rough endoplasmic reticulum.

  • Present inside the mitochondria

Golgi Complex

• Stacks of flattened membranes resembling pancakes.

• Function:

  • Receives proteins.

  • Modifies proteins.

  • Packages proteins into vesicles (membrane-bound sacs) for delivery.

  • Acts as a processing, packaging, and shipping center, similar to Amazon's role.

Lysosomes

• Membrane-bound sacs containing powerful digestive enzymes.

• Function:

  • Act as the demolition crew of the cell.

  • Responsible for autophagy: breakdown and removal of old or dysfunctional organelles.

    • Example: Removal of worn-out mitochondria.

  • Responsible for autolysis (cell suicide or cell death):

    • Lysosomes release digestive enzymes, destroying the entire cell.

Peroxisomes

• Membrane-bound sacs containing enzymes.

• Uses molecular oxygen to produce hydrogen peroxide ($H<em>2O</em>2$), a toxic compound.

• Function & Detoxification:

  • Hydrogen peroxide acts as a potent disinfectant.

  • Peroxisomes contain catalase, an enzyme that breaks down hydrogen peroxide into water ($H<em>2O$) and oxygen ($O</em>2$).

  • Catalase reaction: $2H<em>2O</em>2 \rightarrow 2H<em>2O + O</em>2$

• Location:

  • Abundant in the liver and kidney.

  • Involved in detoxification processes.

Mitochondria

• The powerhouses of the cell, responsible for energy production.

• Structure:

  • Surrounded by a double membrane (outer and inner membrane).

  • The inner membrane has folds called cristae, which increase surface area.

  • The space between cristae is called the matrix, containing ribosomes for ATP synthesis.

  • Contains its own mitochondrial DNA (mtDNA).

• Function:

  • ATP (adenosine triphosphate) synthesis.

Centrioles

• Cylindrical structures, typically found in pairs that are perpendicular to each other.

• Function:

  • Play a crucial role in cell division.

  • Organize microtubules to form the spindle fibers that separate chromosomes during cell division.

Lipids

Lipids are organic compounds that do not dissolve in water. Lipids include: 

• Fats 

• Phospholipids

• Steroids

• Eicosanoids

• Triglycerides

• Cholesterol

There are several kinds of lipids because their chemical composition is different however, they are grouped together because they share the characteristic that do not dissolve in water. Since their chemical composition is different, then their functions are different. The terms fat and lipids are used interchangeably but that is not accurate.  

Proteins

Proteins are organic compounds composed of amino acids. Proteins are synthesized (assembled) from amino acids using information encoded in genes. Each protein has its own unique amino acid sequence that is specified by the nucleotide sequence of a gene. The genes are present in the DNA located in the cell nucleus.

Nucleic Acids

The nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).  Nucleic acids are composed of nucleotides. Nucleotides are composed of sugar, phosphate, and a base containing nitrogen.

• Phosphate: It is present in both DNA and RNA.

• Sugar: There are two types of sugar.

  1. In RNA the sugar is ribose.

  2. In DNA the sugar is deoxyribose.

• Base containing nitrogen or nitrogenous base.

  1. The bases for RNA are adenine, cytosine, guanine, and uracil. 

  2. The bases for DNA are adenine, cytosine, guanine, and thymine.