Cell Biology: An Overview of Cell Structure, Function, and Division
Similarities and Differences Between Prokaryotic, Eukaryotic, and Animal/Plant Cells
Eukaryotic Cells:
- Nucleus is distinct, it is covered up by the nuclear membrane.
- It has double-membraned organelles, like chloroplasts, mitochondria, or nucleus.
- It also has single-membraned organelles such as golgi body, lysosomes, or vacuoles.
- Ribosomes → 80 subunits.
- Different compartments.
Prokaryotic Cells:
- Nucleus is not distinct as it is not covered up by a membrane. It is called nucleoid.
- It only has single-membraned bodies.
- Ribosomes → 70 subunits.
- No compartments.
Animal Cell:
- No cell wall.
- They are irregularly round.
- If there is a vacuole, it’s normally small (and many).
- They have plastids.
- They have centrioles.
Plant Cell:
- Cell wall of cellulose.
- They have a regular shape.
- Vacuoles are big, they can even occupy a big part of the cell.
- They have plastids, they do photosynthesis.
- They don’t have centrioles.
Cell Membrane (Plasma Membrane) and Biological Membranes (Biomembranes)
Each cell has a limiting boundary, the cell membrane, plasma membrane, or plasmalemma. It is a living membrane, outermost in animal cells but next to the cell wall in plant cells. It is flexible and can fold in (as in food vacuoles of Amoeba) or fold out (as in the formation of pseudpodia of Amoeba). The plasma membrane is made of proteins and lipids and several models were proposed regarding the arrangement of proteins and lipids. The fluid mosaic model proposed by Singer and Nicholson (1972) is widely accepted.
According to the fluid mosaic model
- The plasma membrane is composed of a lipid bilayer of phospholipid molecules into which a variety of globular proteins are embedded.
- Each phospholipid molecule has two ends, an outer head hydrophilic i.e. water attracting, and the inner tail pointing centrally hydrophobic, i.e. water repelling.
- The protein molecules are arranged in two different ways: a. Peripheral proteins or extrinsic proteins: these proteins are present on the outer and inner surfaces of lipid bilayer. b. Integral proteins or intrinsic proteins: These proteins penetrate lipid bilayer partially or wholly.
Formation:
Formed due to the aggregation of membrane lipids in aqueous solutions. Aggregation is caused by the hydrophobic effect, where hydrophobic ends come into contact with each other and are sequestered away from water. This arrangement maximizes hydrogen bonding between hydrophilic heads and water while minimizing unfavorable contact between hydrophobic tails and water. The increase in available Hydrogen bonding increases the entropy of the system, creating a spontaneous process.
Fluidity:
The fluidity of the phospholipid bilayer is crucial for cellular functions, allowing rapid diffusion of membrane proteins, lipid and protein movement within the cell, membrane fusion, and even distribution of membrane molecules during cell division. Below a transition temperature, the bilayer becomes a gel-like solid, influenced by factors like hydrocarbon chain length and fatty acid saturation.
Temperature-dependent fluidity is important for bacteria and cold-blooded organisms, which adjust their membrane lipid composition accordingly. In animal cells, cholesterol stiffens the bilayer, reducing permeability. Without fluidity, cellular processes such as growth and reproduction would be compromised.
Cell Nucleus (The Hereditary Organelles):
In eukaryotic is the largest organelle seen clearly when it is not divided as is mostly spherical. It is double layered containing chromatin and a nucleolus ( a dark spot inside the nucleus). The nucleus is the place where all the genetic material, as well as being the site of synthesis for ribosomes. And also, there it is stored a substance called nucleoplasm. Nuclear envelope is made up of two layers of membranes: an outer membrane and an inner one. Nuclear pores that are small channels that span the nuclear envelope, are the ones that let substances enter and exit.
PARTS:
A. Nuclear membrane:
A double-layered structure with many pores. Composition similar to the plasma membrane (lipids and proteins) contributing to protein synthesis. Pores that facilitate the transport of large molecules into and out of the nucleus. Maintenance of contact between the hereditary material and the rest of the cell.
B.Chromatin:
Nucleoplasm, a jelly-like substance rich in proteins. Chromatin fibrils which condense into bodies called chromosomes during cell division. Chromatin material shows two regions: Euchromatin: less compact DNA containing frequently expressed genes, appearing light under an electron microscope. Heterochromatin: more compact DNA with infrequently transcribed genes, appearing dark under a microscope. Heterochromatin is less genetically active compared to euchromatin. The number of chromosomes is fixed in an organism, and during cell division, to ensure daughter cells receive identical amounts of hereditary material.
C. Nucleolus:
Membraneless. It has DNA, RNA and proteins. Store house for RNA. Regulates synthetic activity. Functions –> Maintains the cell in a working order. Coordinates the activities of organelles. Takes care of repair work. Participates directly in cell division to produce genetically identical daughter cells, this division is called mitosis. Participates in production of gametes through another type of cell division called meiosis.
Chromatin and Chromosomes:
Chromatin and chromosomes are structures containing DNA in different stages of the cell cycle. Chromatin, found during most of the cell cycle, consists of DNA and proteins and is responsible for packaging DNA within the nucleus. Chromosomes, which appear during cell division, are highly condensed structures formed from chromatin and ensure the proper separation of genetic material between daughter cells. The key difference lies in their levels of condensation and function within the cell cycle.
CHROMOSOMES:
- Tighly packaged DNA,
- Found only during cell division,
- DNA is not being used for macromolecule synthesis.
CROMATIN:
- Unwind DNA,
- Found throughout interphase,
- DNA is being used for macromolecule synthesis.
CHROMOSOMES:
typically visible during metaphase of cell division when they condense into X-shaped structures. Before this, they are doubled and joined by a centromere, forming sister chromatids. Chromosomes condense during mitosis or meiosis, becoming compact and transportable. The 30 nm chromatin fibers further fold to form compact metaphase chromosomes. DNA condenses and proteins in the scaffold maintain chromatin in compact chromosomes.
Chromosome Number and Kary
OTYPE: Chromosome number refers to the precise count of chromosomes typical for a species, with somatic cells typically being diploid (2n) and gametes haploid (1n) after meiosis. Karyotyping is the process of preparing and analyzing the complete set of metaphase chromosomes in a species or individual to determine chromosome complement and detect abnormalities.
CELL WALL: present outside the plasma membrane of bacteria and plant cells. Bacterial walls are made of peptidoglycan. But in plants there are some differences: Non living layer. Made up of cellulose+pectin and lignin + substance consisting of microfibrils. Some functions: protects the inner part, as it is rigid it gives shape and turgidity and it allows the passage of water and other chemicals. It has some cytoplasmic strands which connect one cell with another called plasmodesmata.
CYTOPLASM AND CELL ORGANELLES: Catch and release energy: mitochondria, chloroplasts. Involved in synthesizing and transporting: golgi, ribosomes… Mobility: filia, flagella. Saves hereditary information: nucleus.
MITOCHONDRIA AND CHLOROPLAST: A mitochondrion is a double-membrane-bound organelle found in most eukaryotic organisms. Contains outer and inner membranes composed of phospholipid bilayers and proteins. Outer membrane, the intermembrane space, , which is folded inside to form projections called cristae and the matrix, which is a fluid. Inner mitochondrial membrane contains proteins with different types of functions: those that perform the electron transport chain redox reactions and ATP synthase, which generates ATP in the matrix. Mitochondria are the “powerhouses” of the cell, breaking down fuel molecules and capturing energy in cellular respiration. Chloroplasts are found in plants and algae. They are responsible for capturing light energy to make sugars in photosynthesis. analogia: They are organelles surrounded by a double membrane unit and are the main powerhouses of eukaryotic cells.
*METABOLIC PROCESSES: mitochondria: oxidation of metabolites (Krebs cycle, beta-oxidation of fatty acids) and obtaining ATP by oxidative phosphorylation. In the chloroplast in the light (photosynthetic electron transport) and dark (Co2 fixation and Calvin cycle) phases
CELL DIVISION: is a series of events that takes place in a cell as it grows and divides. A cell spends most of its time in what is called interphase, and during this time it grows, replicates its chromosomes, and prepares for cell division. The cell then leaves interphase, undergoes mitosis, and completes its division.
CELL CYCLE: series of events that takes place in a cell as it grows and divides. 2 main phases: Interphase (non dividing period) and dividing phase (M phase). INTERPHASE: interval between 2 successive cell divisions ( there is no division). G1 : Lot of protein and RNA are synthesized. S or synthetic phase: Lot of DNA is synthesized. Two chromatids are joined by the centromere to form a single chromosome. G2 ): More protein is synthesized + duplication of many organelles (mitochondria, golgi bodies, centrioles…). M-PHASE OR DIVIDING PHASE: mitosis occurs so that during this period the chromatids separate and form daughter chromosomes.
CELL DIFFERENTIATION: Process where cells become specialized, losing their ability to develop into any cell type (pluripotency) as they gain specific functions. This process involves significant changes in cell shape, size, and energy needs. Differentiation is not linear or irreversible; it selectively expresses certain genes at different stages. Cells can be manipulated back to a stem cell-like state through reprogramming specific gene expression.
MITOSIS: Cell division for growth and replacement where in the two daughter cells are identical and similar to the mother cell in all respects. 4 phases: (a process of cell duplication, in which one cell divides into two genetically identical daughter cells.)
PROPHASE: Early: centrioles move to opposite poles, centrosomes in long threads. Middle: chromosome condensation, each chromosome made up from 2 chromatids, each chromatid with daughter DNA Late: centrioles reach poles, spindle fibers extend to equator. METAPHASE: Chromosomes move towards the equator, each chromosome is attached to the spindle fiber. ANAPHASE: centromeres divide, chromatids separate,chromosomes goes to opposite poles (half), cytokinesis begins. TELOPHASE: chromosomes begin to form, nucleus is visible.
BIOLOGICAL SIGNIFICANCE OF MITOSIS: Equational division, 2 daughter cells are identical. Only made of reproduction in unicellular organisms. Process by which growth takes place in animals and plants. Plays the role in repair by growth: regeneration of damaged parts and replacement of cells during normal wear and tear.
MEIOSIS: It occurs in the gonads for sexual reproduction to produce gametes- The resultant cells, egg (gametes) and sperms (in male), possess half the chromosome number of the parent cell. (a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells.) MEIOSIS I: PROPHASE: 5 phases: Leptone: Chromosomes condense into distinct threads. Zygotene: Homologous chromosomes pair up (synapsis) to form bivalents.Pachytene: Chromosomes contract and thicken, forming tetrads. Crossing over occurs. Diplotene: Homologous chromosomes begin to separate, chiasmata form.Diakinesis: Homologous chromosomes of bivalents move apart, nuclear membrane disappears, spindle formation completes.
METAPHASE: Bivalents align at the metaphase plate, spindle fibers attach to centromeres. ANAPHASE: Spindle fibers shorten, homologous chromosomes separate and move to opposite poles, with no division of centromeres. Each pole receives a mixture of paternal and maternal chromosome parts. TELOPHASE: Chromosomes reach poles and form nuclei, resulting in two daughter cells with half the chromosome
number (haploid). Nuclei reappear, nuclear membrane forms, and the daughter cells enter meiosis II.
MEIOSIS II: Prophase II: Chromosomes shorten and reappear, each with two chromatids attached to a single centromere. Spindle formation begins, nucleolus and nuclear membrane start to disappear. Metaphase II: Chromosomes align along the equator. Spindle apparatus formation completes, each chromosome’s centromere attaches to spindle fibers. Anaphase II: Centromeres divide, separating chromatids into daughter chromosomes. Daughter chromosomes move towards opposite poles. Telophase II: Daughter chromosomes reach poles and organize into haploid daughter nuclei. Nucleolus and nuclear membrane reappear. Cytokinesis: May occur after meiosis I and meiosis II, resulting in four haploid cells. Alternatively, it may occur only after meiosis II.
MEIOSIS SIGNIFICANCE: Maintains chromosome number. Forms gametes for fertilization. Generates genetic diversity through crossing over and mixing of chromosomes. Essential for adaptation and evolution. GENETIC CONSEQUENCES OF MEIOSIS: Specifically, meiosis creates new combinations of genetic material in each of the four daughter cells. These new combinations result from the exchange of DNA between paired chromosomes. Such exchange means that the gametes produced through meiosis exhibit an amazing range of genetic variation.
Cytokinesis is the cytoplasmic division of a cell at the end of mitosis or meiosis, bringing about the separation into two daughter cells. Cytokinesis occurs in mitosis and meiosis for both plant and animal cells. The ultimate objective is to divide the parent cell into daughter cells. In plants , this occurs when a cell wall forms in between the daughter cells. In animals , this occurs when a cleavage furrow forms.
FUNCTIONS: The plasma membrane, provides protection for a cell. It also provides a fixed environment inside the cell. And that membrane has several different functions. One is to transport nutrients into the cell and also to transport toxic substances out of the cell. The Golgi complex prepares proteins and lipid (fat) molecules for use in other places inside and outside the cell. The endoplasmic reticulum can either be smooth or rough, and in general its function is to produce proteins for the rest of the cell to function.
Mitochondria, their main function is to generate the energy necessary to power cells. They also generate the majority of our ATP. The nucleus serves both as the repository of genetic information and as the cell’s control center. The nucleolus is a spherical structure found in the cell’s nucleus whose primary function is to produce and assemble the cell’s ribosomes. The nucleolus is also where ribosomal RNA genes are transcribed.
CONCEPTS: ANTIGEN: An antigen is any substance that causes your immune system to produce antibodies against it. This means your immune system does not recognize the substance, and is trying to fight it off. An antigen may be a substance from the environment, such as chemicals, bacteria, viruses, or pollen. ANTIBODIES: Antibodies are proteins that protect you when an unwanted substance enters your body. Produced by your immune system, antibodies bind to these unwanted substances in order to eliminate them from your system. Another word for antibody is immunoglobulin. ALLERGY: Allergy occurs when a person’s immune system reacts to substances in the environment that are harmless to most people.
IMMUNODEFICIENCY: Immunodeficiency results from a failure or absence of elements of the immune system, including lymphocytes, phagocytes, and the complement system. These immunodeficiencies can be either primary, such as Bruton disease, or secondary, as the one caused by HIV infection. AUTOIMMUNITY: Autoimmunity is the presence of antibodies (which are made by B lymphocytes) and T lymphocytes directed against normal components of a person (autoantigens). These components are called autoantigens or self-antigens and typically consist of proteins (or proteins complexed to nucleic acids). REJECTION: happens when your body’s immune system treats the new organ like a foreign object and attacks it.
*Allergy occurs when a person’s immune system reacts to substances in the environment that are harmless to most people. When an allergen enters the body and is wrongly identified by the immune system as a dangerous substance. In response, the immune system makes antibodies to attack the allergen.