Eukaryotic Cells: Structure, Organelles, and Cell Division
Eukaryotic Cells
Originated after prokaryotic cells. They are more complex and with some advantages over prokaryotic cells. Cytoplasm with organelles: structures that perform different functions. Genetic material is in the nucleus, where it is protected and provides better stability for cells. They have a cytoskeleton: a microscopic network of protein filaments and microtubules. Maintain the cell’s shape and internal organization. Helps cells move.
Organelles
Are membranous structures found in the cytoplasm. Types:
- Ribosomes: they are not membranous structures. They are small spherical shaped organelles without a membrane. They can be found scattered throughout the cytoplasm or attached to the rough endoplasmic reticulum. Function: responsible for protein synthesis.
- Organelles that process nutrients:
Endoplasmic Reticulum
Very complex set of tubules and vesicles. Function: responsible for manufacturing and transforming substances (lipids, proteins). There are 2 types: rough endoplasmic reticulum (with ribosomes) and smooth endoplasmic reticulum.
Golgi Apparatus
Formed by grouped vesicles and flattened sacs. Function: takes substances from the endoplasmic reticulum, modifies them, and are introduced into vesicles for secretion.
Lysosomes
Small vesicles that contain substances capable of digesting molecules captured by cells.
Vacuoles
Structure that stores different substances. There are more and larger in plant cells.
Mitochondria
Cylindrical with a double membrane. Matrix: the inside, made up of genetic material, ribosomes, and enzymes. Obtain energy through the process of cell respiration.
Chloroplasts
Egg-shaped with a double membrane and a series of disc-shaped sacs (thylakoids) which contain the pigment (gives the green color). Place where the photosynthesis takes place: synthesizes organic molecules from inorganic ones with chemical energy from the Sun. Present just in cells of photosynthetic organisms (plants and some protists).
Movement Structures
Types: Cilia and Flagella: Mobile organelles. Formed by protein fibers from the cytoskeleton. Movement coordinated by the centriole (protein tubules arranged, also involved in cellular division). Changes in the Viscosity of Cytoplasm: Pseudopia: proteins in the cytoskeleton change the viscosity of cytoplasm by grouping or separating themselves, modifying the shape of the cell.
The Nucleus
Contains genetic material. The control center of the cell. Usually located in the center of the cell, but sometimes can be found in peripheral areas. Structure: varies depending on the moment in life of the cell (interphase nucleus and nuclear division).
Interphase Nucleus
The cell is not dividing. With a porous double membrane surrounding the nucleoplasm. Inside the nucleoplasm: Chromatin: substance formed by the double helix of DNA joined to histones. Nucleolus: spherical organelle involved in the synthesis of ribosomes.
Nuclear Division
The cell is dividing. Cells division begins. Nucleus changes completely. Chromatin condenses into chromosomes. Chromosomes: x-shaped structures, differ in number depending on the species. Made of 2 identical chromatids joined by a centromere. The genetic information is duplicated. Variation in number depending on the type of cell (Gametes: haploid number / Somatic cells: diploid number. With two sets of haploid cells).
Animal Cells and Plant Cells
Wall (AC, no / PC, yes) Chloroplasts (AC no, PC yes) Centrioles (AC yes, PC no) Cilia and Flagella (AC in some PC cases, no).
Cell Division
Unicellular organism: creates new individuals. Multicellular organism: creates new individuals + replaces tissues + for growth.
Mitosis
All cells (except gametes) reproduce. Daughter cells. Some exact copy of genetic information transmitted from the parent cell to the daughter cells. Genetic information duplicates before mitosis takes place, in order to have two copies in the parent cell. Same cell genetic information is provided to both daughter cells. Cells obtained in mitosis are diploids. Four stages in mitosis:
- Prophase
- Metaphase
- Anaphase
- Telophase
Once mitosis ends, cytokinesis (division of cytoplasm and sharing of cell organelles) takes place.
Meiosis
Cells divide to create gametes. Produce not identical cells. Gametes have a haploid number of chromosomes. Half the genetic information. Gametes join together during fertilization. Form a zygote with a diploid number of chromosomes (a set of haploid chromosomes from each parent). Zygote grows into an organism after completing many cell divisions. Meiosis produces four haploid daughter cells, with half the chromosomes of the original parent diploid cell.
Similarities and Differences Between Meiosis and Mitosis
Similarities: Both processes start by the condensation of chromatin to form chromosomes. Both create mitotic spindle fibers (to which chromosomes attach and are moved to the cell’s opposite poles). They end the same way: chromatids expand to turn into chromatin and the new daughter nuclei are formed. Differences: Mitosis: A diploid parent cell creates two diploid daughter cells with exactly the same genetic information. Only one process takes place. Meiosis: A parent cell creates four haploid daughter cells, with different genetic information. There are two successive mitosis processes: With genetic recombination and a normal one.
Biological Importance of Cell Division
Aim of mitosis and meiosis. Complete cell reproduction without any mistakes. Make sure cells have the necessary genetic information. Mitosis: Creates cells for growth and asexual reproduction. Makes sure genetic information from the parent cell stays the same in the daughter cells (as any changes in the genetic information would lead to severe consequences). Example of problem during mitosis: cancer (makes cells of the affected tissue grow in a harmful uncontrolled manner). Meiosis: Used for sexual reproduction. Makes sure daughter cells get modified genetic information from the parent cell (using genetic recombination). Genetic recombination is an important process, as it provides new genetic combinations to offspring. Very important for species to evolve in the past and in the future.