Cellular Organelles: A Quick Review

Introduction to Cellular Organelles

Cellular organelles are specialized structures within cells that perform distinct metabolic functions essential for life. They are found within the cytoplasm, the material between the plasma membrane and the nucleus.

• Cytoplasm: The region between the plasma membrane and the nucleus, largely composed of water with dissolved proteins, salts, sugars, and other solutes.

• Organelles: Internal metabolic structures, some surrounded by membranes, others not.

• Non-membranous organelles: Structures such as cytoskeleton, centrioles, and ribosomes that are not surrounded by membranes.

• Inclusions: Chemical substances like glycosomes, lipid droplets, and pigment granules found in the cytoplasm.

The Nucleus

Structure and Function of the Nucleus

The nucleus is the control center of the cell, housing genetic material and coordinating cellular activities such as growth, metabolism, and reproduction.

• Nuclear Envelope: A double membrane that surrounds the nucleus, selectively permeable and containing pores for molecular transport.

• Outer Membrane: Continuous with the rough endoplasmic reticulum (ER) and studded with ribosomes.

• Pore Complex: Regulates the entry and exit of large molecules into and out of the nucleus.

Contents of the Nucleus

• Chromatin: Composed of 30% DNA (chromosomes), 60% globular proteins (histones), and 10% newly forming RNA chains. Chromatin condenses to form chromosomes during cell division.

• Nucleoli: Sites within the nucleus where active RNA synthesis and ribosome assembly occur.

• Genetic Library: The DNA in the nucleus contains blueprints for nearly all cellular proteins, dictating the types and amounts of proteins synthesized.

Chromatin Organization

• Nucleosomes: DNA wraps around histone proteins to form nucleosomes, which help compact DNA and play a role in gene regulation.

• Gene Expression: DNA must 'unravel' from histones before a gene can be expressed into messenger RNA (mRNA).

• Chromosomes: Condensed, bar-like bodies formed from chromatin during cell division; humans have 23 pairs of chromosomes.

Summary Table: Membranous vs. Non-Membranous Organelles

Key Terms and Concepts

• Histones: Proteins that organize DNA into nucleosomes and regulate gene expression.

• Nucleolus: The site of ribosomal RNA (rRNA) synthesis and ribosome assembly within the nucleus.

• Chromosomes: Structures formed from condensed chromatin during cell division, carrying genetic information.

Example: DNA Packaging

DNA wraps around histone proteins to form nucleosomes, allowing the cell to fit large amounts of genetic material into the small space of the nucleus. This packaging also regulates which genes are accessible for transcription.

Additional info:

• Gene regulation involves chemical modifications to histones and DNA, affecting how tightly DNA is packaged and its accessibility for transcription.

• Mutations or errors in chromatin structure can lead to diseases such as cancer.