Cellular Biology: Structure, Function, and Division

Posted on Jan 27, 2025 in Biology

In 1838, Mathias Schleiden and Theodor Schwann proposed the following:

  • All living things are formed by one or more cells.
  • The cell is the simplest and smallest living thing that exists.
  • All cells come from pre-existing cells.
  • Every single cell carries out its own actions, although there is coordination between cells.

Cell Structure

  • Membrane: The border that separates the cell from the environment. Through the membrane, the cell and the external environment exchange products: substances for nutrition enter, and waste products exit.
  • Cytoplasm: The inner space where chemical reactions needed to perform the vital functions take place.
  • Genetic material: The control system that directs all the cell’s functions. It is formed by molecules that contain information to direct cellular activity and contain the characteristics of each cell.

Three Vital Functions

  • Nutrition: Cells take molecules (nutrients) from the external environment and transform them into energy used to function or renew cellular constructions.
    • Autotrophic nutrition: Cells create organic matter from inorganic matter.
    • Heterotrophic nutrition: Cells use organic matter.
    • Cytoplasm: Metabolism.
    • At the end of nutrition, waste products are produced and released into the external environment.
  • Interaction: Cells adapt to changes and continue performing vital activities, activating mechanisms needed to survive.
  • Reproduction:
    • Unicellular: Produces a new living thing.
    • Multicellular: Substitutes dead cells and increases the number of cells.

Cells are measured in micrometers (µm), commonly known as microns (µ). 1 µm = a thousandth part of a millimeter = 0.001 mm

  • The size of cells varies except in cells of the same type.
  • The shape varies depending on the function of the cell.

Prokaryotic Cells

Prokaryotic cells are simpler and more primitive. Eukaryotic cells evolved from prokaryotic cells. Anaerobic and photosynthetic bacteria combined with prokaryotic cells through endosymbiosis and turned into mitochondria and chloroplasts, respectively.

  • They don’t have membrane structures, membranous organelles, or a nucleus.
  • They are small, from three to 30 microns.
  • They have a cell wall surrounding their plasma membrane.
  • They have a singular circular chromosome.
  • They have isolated ribosomes.
  • Energy is produced in the plasma membrane.
  • Organisms from the Monera kingdom have a prokaryotic cell structure.

Examples of Prokaryotic Cells

  • Archaea: The most primitive prokaryotes. They live in extreme environmental conditions. They are called extremophiles.
  • Cyanobacteria: Primitive prokaryotes that perform photosynthesis. They are the first autotrophic organisms in charge of enriching the atmosphere with oxygen.
  • Bacteria: Organisms that live in almost any environment. They can be autotrophic or heterotrophic. They can be classified according to their shape into cocci, bacilli, spirilla, and vibrio.

Eukaryotic Cells

Eukaryotic cells are more complex.

  • Their cytoplasm contains structures, called organelles, that perform different functions.
  • Genetic material is contained in the nucleus; this way, it is protected.
  • They have a cytoskeleton: a microscopic network of protein filaments and microtubules that help cells move and maintain the cell’s shape and organization.

Organelles

Organelles are membranous structures found in the cytoplasm, except for ribosomes, which are not membranous structures.

  • Ribosomes are responsible for protein synthesis.
  • Organelles process nutrients.
  • Organelles produce energy.
  • Lysosomes: Small vesicles that contain substances that digest molecules captured by cells.
  • Endoplasmic reticulum: A set of tubules and vesicles that manufacture and transport substances, such as lipids and proteins. There are two types: the smooth endoplasmic reticulum and the rough endoplasmic reticulum, which has ribosomes.
  • Vacuoles: Structures that store substances. Plant cells have more and larger vacuoles than animal cells.
  • Golgi apparatus: Organelle formed by vesicles and flattened sacs. It takes substances from the endoplasmic reticulum, modifies them, and introduces them into the vesicles for secretion.
  • Mitochondria: Cylindrical organelles made of a double membrane found in all eukaryotic cells.
  • Chloroplast: Egg-shaped organelle with a double membrane and thylakoids where photosynthesis happens.

Movement Structures

Cells move using their appendix (cilia and flagella) or by changing the viscosity of their cytoplasm.

  • Cilia and flagella: Cilia and flagella are mobile organelles formed by protein fibers from the cytoskeleton. Centrioles coordinate cilia and flagella movement. Centrioles are formed by protein tubules. They are involved in cellular division.
  • Changes in the viscosity of cytoplasm: Proteins in the cytoskeleton can change the viscosity of the cytoplasm by grouping together or separating themselves. They produce pseudopodia and modify the shape of the cell. Pseudopodia are used to surround and capture some materials from the environment in a process called phagocytosis.

Nucleus

The nucleus is the control center of the cell, and it contains genetic material. There are two structures: the interphase nucleus (the cell is not divided) and nuclear division (the cell is dividing).

Nuclear Division

When cell division begins, chromatin condenses into chromosomes, which are X-shaped structures. Each one is made up of two filaments, chromatids; they are joined by a centromere, both are identical, the genetic information is duplicated. The centromere separates into two parts, called arms, each chromatid. The number of chromosomes in gametes varies from somatic cells. The haploid number (n) is the number of chromosomes in gametes, and the diploid number (2n) is the number of chromosomes in somatic cells. There are two sets of haploid cells, one from each parent.

Cell Division

In unicellular organisms, cell division creates new individuals. In multicellular organisms, it replaces tissues and allows growth.

Mitosis

Mitosis has four stages:

  1. Prophase
    • Chromatin fibers thicken and shorten to form chromosomes.
    • The nucleolus disappears.
    • Protein fibers appear between the poles of the cell and form the mitotic spindle.
    • The nuclear membrane disappears, and chromosomes can move around the cytoplasm.
  2. Metaphase
    • Chromosomes bind to the mitotic spindle fibers using their centromeres.
    • This union takes place in the equator of the cell.
    • Sister chromatids belonging to each chromosome point to the opposite poles of the cell.
  3. Anaphase
    • The mitotic spindle fibers break into two halves. The centromere breaks. The two chromatids separate into two unconnected half fibers.
    • The mitotic spindle fibers contract, pulling the chromatids towards the opposite poles. Chromatids become independent chromosomes.
  4. Telophase
    • The mitotic spindle fibers disappear when the chromatids reach the poles.
    • A nuclear membrane surrounds the chromatids, forming two new nuclei.
    • Chromatids expand and turn into chromatin.
    • The nucleolus reappears.

When mitosis ends, the cytoplasm divides, and the organelles are shared; this process is cytokinesis. It can take place in two ways:

  • Animal cell: Cytoplasm stretches, thins out, and separates.
  • Plant cell: A wall forms, which divides the cytoplasm in half.