Cellular Processes and Structures: An In-Depth Look
Cellular Processes and Structures
Protein Structure and Transport
Irregular proteins, like surface fiber aggregates, can achieve long filament forms, such as collagen. Quaternary structures involve the interaction of two or more different proteins, such as antibodies. When a protein synthesized in the rough endoplasmic reticulum (RER) is not transported to the Golgi complex, it is directed to a capsule.
Types of Vesicular Transport
- Nuclear Pore (Selective Diffusion): Occurs between continuous compartments.
- Transmembrane: Occurs between different compartments.
- Vesicular: Occurs between membranes.
Endoplasmic Reticulum (ER)
The ER is a network of membranes that defines lumen tanks. It can have ribosomes associated with it or not. It is involved in the production of lipids and proteins. Proteins pass from the cytosol to the lumen. One hypothesis suggests that a nascent protein chain opens a pore, allowing the protein to pass through to the interior.
Reticulum-Golgi-Cell Membrane Pathway
If a vesicle does not enter the Golgi, it passes through all the tanks to be synthesized. This protein is one that exists between non-RER components. Molecular signals direct proteins to the Golgi. The Golgi is involved in glycosylation, packaging, and modification. From the Golgi, vesicles move to the cell membrane.
Lysosomes
Lysosomes originate from the Golgi complex and produce enzymes that degrade proteins. When a bacterium is engulfed, it fuses with a lysosome, resulting in intracellular digestion. Endosomes, formed from phagocytosis or pinocytosis, can fuse with lysosomes, causing digestion. Autophagy is the process of degrading non-functional organelles.
Phagocytosis
Phagocytosis degrades material from outside the cell. It is used for the nutrition of protozoa, defense in the body (white blood cells), and regression of the uterus after birth, where macrophages digest part of the uterine wall.
Pinocytosis
Pinocytosis involves engulfing small drops of liquid. The destination of these droplets is often the lysosome.
Peroxisomes
Peroxisomes are membranous sacs with digestive enzymes and catalase, which converts hydrogen peroxide (H2O2) to water (H2O) and oxygen (O2). They have only one membrane and lack DNA.
Plasma Membrane
The plasma membrane isolates the cell but allows for transport and selection. It defines internal compartments. The fluid mosaic model describes how proteins can move within the cell membrane.
- Passive Transport: Diffusion is the movement from a more concentrated region to a less concentrated region (e.g., a drop of dye in milk, O2 entering and CO2 leaving cells).
- Active Transport: Always requires energy and can occur in three different ways.
Nuclear Components
The nuclear envelope consists of a lamina and pores. Nuclear lamins A, B, and C are globular dimers that polymerize.
Nucleoplasm
The nucleoplasm is an aqueous solution containing proteins, RNA, nucleotides, and ions.
Chromosomes
Chromosomes are the carriers of genetic material (characteristics from the father and mother). They consist of two chromatids joined together by a centromere.
Chromatin
Chromatin’s degree of condensation changes according to the cell cycle.
- Euchromatin: More condensed, contains genes that the cell will use.
- Heterochromatin: Less condensed, contains genes that the cell will not use.
Nucleolus
The nucleolus is a nuclear structure without a membrane. It contains proteins, rDNA, and rRNA. Its size increases with protein synthesis activity.
Functions of Nucleic Acids
- DNA: Stores and transmits genetic information.
- rRNA: Forms the structure of ribosomes, participates in protein synthesis, and promotes codon-anticodon pairing.
- mRNA: Carries genetic information from DNA to ribosomes.
- tRNA: Transports amino acids to the ribosome. It has an anticodon region.
Protein synthesis always starts with the AUG codon and stops with UAA, UAG, or UGA.
Nucleotides
Nucleotides are the subunits of nucleic acids. They consist of a nitrogenous base, a pentose sugar, and a phosphate group.
Translation, Genetic Code, and Protein Synthesis
The genetic code uses four letters (A, U, G, C), but the words are three letters long (codons). In the cytoplasm, tRNA recognizes mRNA and brings the corresponding amino acid. Peptide bonds link amino acids together to form a protein. There are 20 different amino acids, each with a unique radical. A protein’s function is determined by its shape. If the shape is altered, it loses its function.
Protein Formation
Proteins have four levels of structure:
- Primary: The linear sequence of amino acids (no function).
- Secondary: Alpha-helix (found in keratin) or beta-sheet (found in fibroin).
- Tertiary: Globular proteins form a compact, ball-like structure.
- Quaternary: Interaction of multiple polypeptide chains.
Denaturation is the unfolding of a protein, which can be reversible or irreversible. It can be caused by heat or changes in pH.