Cellular Structures and Functions: A Detailed Look

Posted on Jan 23, 2025 in Biology

Ribosomes

Ribosomes are either free in the cytosol or bound to the rough endoplasmic reticulum (RER), the external nuclear membrane, and inside chloroplasts and mitochondria. They are formed by two subunits: the small subunit and the large subunit. Ribosomes have two active sites:

  • P (peptidyl) site: Where the first aminoacyl-tRNA carrying the first methionine codon is placed.
  • A (aminoacyl) site: Where the second codon is placed.

Ribosomes synthesize proteins.

Peroxisome

Peroxisomes are located near the RER. If they are small, they are called microperoxisomes. They provide oxidative enzymes, and mitochondria use these enzymes with O2. Peroxisomes do not have DNA. Their action is very important in liver and kidney cells. Plant peroxisomes are involved in photorespiration. In seeds, they are called glyoxysomes and can transform fatty acids into sugars.

Vacuole

Vacuoles are typical of plant cells. There may be only one or multiple. They act as storage organelles for substances and regulate cell turgor. They may contain lysosomes. Other types of vacuoles include:

  • Contractile vacuole: Draws water from the cytoplasm.
  • Digestive vacuole: These are phagosomes and are responsible for conducting nutrient digestion.

Nucleus

The nucleus contains genetic information. It can be single or multiple, round, cubic, or poly-lobed. It is located in the center or on the side of the cell. We distinguish between the interphase nucleus and the nucleus undergoing mitosis. Parts of the nucleus include:

  1. Nuclear membrane: Consists of an external and an internal membrane.
  2. Nuclear pores: Form the pore complex, which is an octagon of eight proteins with a central globular granule. It enables the exchange of molecules in two directions.
  3. Nucleoplasm: Formed by the matrix and the chromatin (genetic material).

Chromatin in the interphase is decondensed chromatin (euchromatin or loose chromatin). At the stage of mitosis, it is condensed chromatin or chromosomes. There is also the nucleolus, which contains RNA and proteins. It synthesizes rRNA, disappears during mitosis, and originates from the nucleolus organizer region (NOR) of the chromosome.

Mitotic Chromosomes

Mitotic chromosomes represent the maximum degree of compaction of chromatin in the metaphase stage. They are formed by two chromatids joined by a central constriction, the centromere. In the terminal portions are the telomeres and the satellite, the latter attached to the nucleolus. In anaphase, a chromosome has only one chromatid.

Karyotype

The karyotype is the set of chromosomes of a cell. The photographic image of the karyotype is the ideogram (homologous pairs are placed together). Chromosomes are constant in cells of a species. Chromosomes are grouped by homologous pairs, one from the mother and one from the father. Although they carry information for the same characteristics, it is not the same information. The chromosome number of a cell is diploid (2n), and that of gametes is haploid (n). Types of chromosomes include:

  • Metacentric: Centromere in the middle.
  • Submetacentric: One arm is longer than the other.
  • Acrocentric: One arm is much longer.
  • Telocentric: There is only one arm.

Non-sex chromosomes are called autosomes.

Endoplasmic Reticulum

The endoplasmic reticulum is a network of sacs extended throughout the cytoplasm. It has a space or inner light that communicates with the nuclear membrane and the Golgi apparatus. There are two types:

  • Rough Endoplasmic Reticulum (RER): Has ribosomes attached to the outside of the membrane.
  • Smooth Endoplasmic Reticulum (SER): Does not have ribosomes.

Functions of the Rough Endoplasmic Reticulum

  • Synthesis and storage of proteins.
  • Protein glycosylation.
  • Formation of the cell wall in plant cells.

Functions of the Smooth Endoplasmic Reticulum

  • Synthesis of membrane lipids and steroids.
  • Storage of Ca2+.
  • Involved in detoxification processes (metabolizes alcohol and drugs).

Golgi Apparatus

The Golgi apparatus is located near the centrosome and consists of a series of flattened and stacked sacs forming dictyosomes. It has two faces:

  • Cis face: In continuity with the ER.
  • Trans face: In relation to lysosomes.

Function: Transport of proteins and their glycosylation.