Introduction to Cell Biology: Structure and Function
Introduction to Cell Biology
The Origin of Life
In 1992, biochemists revisited Oparin and Haldane’s hypothesis on the processes of chemical evolution during the origin of life. According to them, organic molecules could be formed from atmospheric gases reacting with each other due to solar radiation. These new organic molecules would fall into the oceans to form what they called the primordial soup. The molecules would then associate with each other, forming coacervates. This hypothesis was largely confirmed by Miller, who demonstrated the possibility of spontaneous organic molecule formation.
First Cells
Of the macromolecules known today, only nucleic acids are capable of self-replication. In the early 90s, Altman and Cech demonstrated that RNA can catalyze a series of reactions, including the polymerization of nucleotides. The first RNA enzyme capable of self-replicating was named a ribozyme.
Endosymbiotic Theory
Woese proposed the progenote or protobiont as the common ancestor of all organisms, representing the most primitive living unit. From this common trunk arose the prokaryotes, which would include archaebacteria and eubacteria. Subsequently, eukaryotes appeared.
Origin of Eukaryotic Cells
Prokaryotic cells would be the precursors of peroxisomes, mitochondria, and chloroplasts. The acquisition of these two types of bacteria had vital significance, as eukaryotic cells acquired the ability for aerobic respiration and photosynthesis.
Prokaryotes
In general, prokaryotes are usually very small and have a coated membrane. Their cell wall composition varies, and sometimes a capsule or gelatinous sheath can be found above it. The cytoplasm has two distinct regions: the nucleoid (or bacterial chromosome) where the genetic material is located, and the remaining cytoplasm dominated by ribosomes.
Prokaryotes can have flagella and divide by binary fission.
One characteristic of prokaryotic morphological diversity is their shapes, which can be distinguished as bacilli, cocci, vibrios, and spirilla.
Structure
- Cytoplasm: Contains ribosomes involved in protein synthesis.
- Nucleoid: Contains a simple, circular DNA molecule without a membrane.
- Mesosomes: Invaginations of the cell membrane that contain enzymes involved in respiration and cell division.
- Plasma membrane: A lipoprotein envelope.
- Rigid cell wall: Composed of polysaccharides and peptides, surrounding the plasma membrane.
- Capsule or glycocalyx: Composed of polysaccharides and peptides, surrounding the plasma membrane.
- Fimbria: Numerous structures shorter than pili, likely involved in adhesion to substrates.
- Flagella: One or two flagella that allow locomotion.
- Pili: Structures similar to flagella, but more numerous, involved in DNA exchange.
Structure of Eukaryotes
Plant Cell
- Chloroplasts: Bodies within plant cells where photosynthesis takes place.
- Cell wall: A thick, rigid outer shell that acts as an exoskeleton, developed on the plasma membrane.
Animal Cell
- Centrosome: A structure without a membrane that serves as the microtubule organizing center.
Common Features
- Mitochondria: Organelles present in all eukaryotic cells where oxidative metabolism occurs, producing the majority of ATP molecules.
- Lysosomes and peroxisomes: Provide specialized metabolic compartments for digestion and oxidation within cells.
- Vacuoles: Most cells have vacuoles that perform diverse functions, such as digestion of macromolecules and nutrient storage.
- Cytoskeleton: Fundamental to the structure and function of eukaryotic cells. It is a network of protein filaments extending throughout the cytoplasm, responsible for cell shape, organelle distribution, and cell movement.
- Nucleus: An organelle present in all eukaryotic cells that contains the genetic information (DNA).
- Endoplasmic reticulum and Golgi complex: Specialize in protein and lipid synthesis, transport, and secretion, as well as incorporation into the plasma membrane or lysosomes.
The Nucleus
The appearance of the nucleus depends on the stage of the cell cycle. It is called the interphase nucleus when the cell is not undergoing mitotic division, and the mitotic nucleus when chromosomes are condensed.
Features
- Components: The nucleus consists of a nuclear envelope (double membrane) and nucleoplasm (or nuclear matrix), which contains chromatin and the nucleolus.
- Form: Highly variable (spherical, oval, polylobulated) depending on the cell type and stage of the cell cycle.
- Size: Proportional to the cell size, occupying about 10% of the cell volume.
- Position: Characteristic of each cell type; in embryonic cells, it is central.
- Number: Usually one nucleus per cell, but there are exceptions. Some cells are enucleated (having lost their nucleus during differentiation), while others have two nuclei (a macronucleus and a micronucleus). Multinucleated cells can arise through two mechanisms:
- Successive division of a primitive nucleus without subsequent cell division, resulting in a cell called a plasmodium.
- Fusion of several uninucleate cells, forming a syncytium.
The nuclear envelope is a double membrane with an intermembrane space.
- Outer nuclear membrane: Usually attached to the endoplasmic reticulum.
- Perinuclear or intermembrane space: Located between the two membranes.
- Inner nuclear membrane: Faces the nucleoplasm.
- Nuclear pores: Circular holes where the two membranes of the nuclear envelope fuse, regulating the exchange of molecules between the nucleus and the cytosol. They allow free circulation of water-soluble molecules and regulate active transport mechanisms.
Chromatin
In the nucleus of eukaryotic cells, DNA is associated with proteins, forming a packed and compact structure called chromatin.
Features
Chromatin consists of DNA and proteins. The proteins can be of two types:
- Histones: Very basic proteins.
- Nonhistones: A variety of proteins with diverse functions.
Ultrastructure
Chromatin has a fibrillar structure, consisting of fibers coiled around each other in a spiral. These fibers are called chromatin fibers or nucleosomes.
Each fiber resembles a string of beads. Each bead, called a nucleosome, has a spherical shape. The nucleosomes are connected by a fibril that corresponds to the thickness of a DNA double helix.
Nucleoplasm
Also called karyoplasm or nuclear matrix, it is a semi-fluid matrix located inside the nucleus.
Nucleolus
A constant structure in the cell nucleus, usually one per nucleus.
It is a more or less rounded, highly refractive organelle, usually located near the nuclear envelope.
Functions: rRNA synthesis and processing, and packaging of ribosomal subunits that are subsequently exported to the cytosol.
Metaphysical Structure of Chromosomes
The chromosome consists of two parallel chromatids, resulting from the duplication of genetic material, except for the centromere region.
- Centromere: The primary constriction that divides the chromosome into two arms of equal or different sizes. It occupies a variable but fixed position along the chromosome. Centromeres contain constitutive heterochromatin (compacted and genetically inactive chromatin).
- Kinetochore: Protein structures located on both sides of the centromere on each chromatid, serving as attachment points for microtubules.
- Secondary constrictions: Narrower regions in the chromosome arms.
- Telomeres: Protective structures located at the ends of each eukaryotic chromosome, preventing the loss of genetic information during replication.
- Bands: Segments of chromatin that stain with different intensities.
Types of Chromosomes
- Metacentric: Centromere is in the middle, with two arms of equal or similar length.
- Submetacentric: Centromere is slightly off-center, with one arm slightly larger.
- Acrocentric: Centromere is near one end.
- Telocentric: Centromere is at one end of the chromosome.
Number of Chromosomes
The number of chromosomes is constant for all cells belonging to the same organism.
- Most organisms are diploid, meaning they have two sets of chromosomes (homologous pairs). Homologous chromosomes contain genetic information for the same traits. Gametes are haploid.
- Some organisms are haploid (e.g., algae and ferns), while others have more than two sets of chromosomes (polyploid).
The number of chromosomes is not related to the organism’s developmental complexity.
The set of all chromosomes in a cell is called the karyotype. There are two types of chromosomes:
- Autosomes: Somatic chromosomes common to both sexes of the same species, involved in the development of somatic characteristics.
- Sex chromosomes (gonosomes): Responsible for sex determination. In humans, these are the X and Y chromosomes.