Cell Structure and Function: A Detailed Look

Posted on Feb 5, 2025 in Biology

Cell Structure and Function

All living things are composed of cells, the fundamental units of life. Cells are the anatomical (structural), physiological (functional), and genetic (inheritance) units of living beings. They are capable of nutrition, interaction, and reproduction.

Cell Types

  • Prokaryotes: Cells with no nucleus, DNA is spread throughout the cytoplasm. Example: Bacteria.
  • Eukaryotes: Cells with a nucleus where DNA is contained. Examples: Animal and plant cells.

Cell Parts

  • Plasma membrane: The cell’s border. It isolates the cell and controls the entry and exit of materials. Plant cells, bacteria, and fungi are also surrounded by cell walls (made of cellulose).
  • Cytoplasm: The cell’s medium, where organelles are found. Organelles are structures that perform specific functions.
  • Ribosomes: Very small structures responsible for protein synthesis (manufacturing).
  • Endoplasmic reticulum: A system of pouches that communicate with the plasma membrane. It transports and stores substances (rough endoplasmic reticulum has ribosomes attached).
  • Golgi apparatus: A stack of sacs with thickened ends, surrounded by vesicles. It produces substances to be released outside the cell.
  • Lysosomes: Vesicles that perform digestion.
  • Vacuoles: Cavities where reserve substances are kept; more developed in plant cells.
  • Mitochondria: Elongated organelles, visible with a light microscope. Their function is to obtain energy.
  • Plastos: Cellular organelles exclusive to green plants because they contain chlorophyll. Their function is photosynthesis.
  • Centriole: Cellular organelle exclusive to animal cells. It is involved in cell division (it stretches chromosomes at the ends) and in the formation and movement of cilia and flagella.
  • Cilia and flagella: Cellular organelles for movement. Cilia are short and numerous. Flagella are few and long (e.g., sperm tail).

Nucleus

The nucleus contains the genetic material, DNA and RNA. Its function is to control and direct all cellular activities. It can have one or two nucleoli formed by an accumulation of DNA.

DNA Expression

Transcription: A snippet of DNA is copied to mRNA (messenger RNA). The mRNA moves outside the nucleus. The spiralized fragment (chromosome) is then read by ribosomes, forming proteins according to the genetic information.

Amino acids: Units of proteins. Every three nucleotides encode an amino acid.

Chromosome: Each piece of DNA that encodes a characteristic. Prior to cell division, it spirals and condenses. At the end of division, the fragments rejoin. Each species has a characteristic number of DNA molecules. Half of the chromosomes can pair with the other half, forming pairs.

Diploid number: The total number of chromosomes. The number of mature chromosomes is haploid.

Homologous chromosomes: Pairs of chromosomes.

The last pair encodes sex: XY = male, XX = female.

Cell Division

  • Mitosis: From a stem cell, two daughter cells are formed with the same number of chromosomes as the mother cell (all cells except sperm and ovules). Before cell division, DNA is duplicated.
  1. Prophase: Chromosomes are formed.
  2. Metaphase: Chromosomes align at the cellular equator.
  3. Anaphase: Each chromosome moves to an opposite end of the cell.
  4. Telophase: Two cells are formed as the original cell divides.
Meiosis: From a mother cell, four daughter cells are formed with half the number of chromosomes as the mother cell (only in gametes: sperm and ovum).

Two consecutive cell divisions:

  1. First division:
    • Prophase 1: Chromosomes appear and form pairs of homologous chromosomes.
    • Metaphase 1: Pairs of homologous chromosomes are arranged at the cellular equator (they also rotate).
    • Anaphase 1: Each chromosome migrates to one end of the cell.
    • Telophase 1: Chromosomes reach the ends, and the cell divides.
  2. Second division (Prophase 2, Metaphase 2, Anaphase 2, Telophase 2): Identical to the respective phases of mitosis.
Vascular Tissues in Plants

Vascular tissues are vessels (tubes).

  • Xylem: Vessels formed by woody cells. They transport crude sap (water + mineral salts) from the roots to the leaves. Photosynthesis takes place in the leaves, transforming raw sap into elaborated sap (water + organic matter).
  • Phloem: Vessels formed by liberian cells. They transport elaborated sap from the leaves throughout the plant.