Cell Structure and Function: Prokaryotic and Eukaryotic Cells

Posted on Jan 10, 2025 in Biology

Cell Structure and Function

The cell is the structural unit of living beings. The cell is the functional unit of living beings; it carries out all the metabolic processes that allow it to live. Every cell comes from another pre-existing cell. The cell is the genetic unit of all living beings.

Levels of Organization

The subject has different organization levels, each more complex structurally and functionally than the previous one: subatomic particles, atoms, molecules, macromolecules and supramolecular complexes, organelles, cells, tissues, organs, organ systems, individual.

Cell Types

  • Prokaryotes: They have no nucleus; their genetic material is distributed throughout the cytoplasm (bacteria). They have no organelles except ribosomes and vesicles. They have a cell wall.
  • Eukaryotes: They have a nucleus that contains the genetic material and numerous cellular organelles (protozoa, algae, fungi, plants, animals). The cytoplasm has a large number of organelles.

Prokaryotic Cell Structure

  • Cell wall: A rigid envelope that shapes the bacteria.
  • Plasma membrane: Controls the entry and exit of substances. When folded inward, it forms mesosomes, where bacterial metabolism occurs.
  • Bacterial chromosome: A circular DNA molecule that contains all the genetic information of the cell.
  • Ribosomes: Small organelles where protein synthesis occurs. They vary according to the cell type.
  • Flagella: Extensions of the cytoplasm involved in movement.
  • Pili: Short and numerous structures that fix bacteria to the substrate.

Eukaryotic Cell Structure

  • Plasma membrane.
  • Nucleus: Contains the genetic material and is separated from the rest of the cell by the nuclear envelope, which allows the exchange of substances with the rest of the cell.
  • Cytoplasm: An aqueous medium that forms a network of protein fibers involved in the movements and division of the cell.
  • Centrosome: Controls the movement of the endoskeleton fibers and intervenes in cell division.
  • Organelles:
    • Endoplasmic reticulum: A set of flattened sacs and tubular conduits. If it has ribosomes, it is rough; otherwise, it is smooth. The rough endoplasmic reticulum synthesizes proteins, and the smooth endoplasmic reticulum synthesizes lipids.
    • Golgi apparatus: A set of flattened and stacked tanks. They accumulate substances from the endoplasmic reticulum and secrete them outside through small vesicles that form on their periphery.
    • Ribosomes: Formed by RNA and protein. Protein synthesis is carried out in them.
    • Mitochondria: Cellular respiration takes place in them so that the cell can obtain energy.
    • Lysosomes: Membranous vesicles from the Golgi apparatus that contain digestive enzymes. They convert complex molecules into simpler ones by hydrolysis.
    • Vacuoles: Membranous vesicles that accumulate water, reserve substances, and pigments inside.

Animal vs. Plant Cells

  • Animal cells: They have centrioles in the centrosome, formed by protein tubules. They have no cell wall or plastids, but they have small vacuoles.
  • Plant cells: They have a cell wall with channels called plasmodesmata that connect with other cells. They have large vacuoles and chloroplasts.

Nucleus

In animal cells, it is usually in the center; in plant cells, it is usually on one side.

Components

  • Nuclear envelope: A double membrane separated by an intermembrane space. The external membrane is connected to the endoplasmic reticulum. Nuclear pores are present, through which the exchange of substances occurs.
  • Nucleoplasm: The internal aqueous environment where the nuclear components are found. It is the site of DNA replication.
  • Nucleolus: Lacking a membrane, it forms ribosomes.
  • Chromatin: Filaments formed by DNA and associated proteins dispersed throughout the nucleoplasm. There are as many chromatin filaments as the cell nucleus has during division.

Chromatin vs. Chromatid vs. Chromosomes

  • Chromatin: DNA when it is not dispersed in the nucleoplasm.
  • Chromatid: DNA when it forms the structure of chromosomes.
  • Chromosomes: Filamentous structures that appear during cell division. They distribute the genetic information contained in the DNA of the stem cell to the daughter cells. Each chromosome is formed by two chromatids that bind at the centromere.

Sister chromatids: Same information. Homologous chromatids: Same or different information. Homologous chromosomes: Same information for one character.

Haploid vs. Diploid Organisms

  • Haploid organisms: They have a single set of chromosomes in their cells (n) – egg, sperm, and prokaryotic cells.
  • Diploid organisms: They have an even number of chromosomes in their somatic cells (2n).