Cell Biology Review PDF

**GEN BIOLOGY REVIEWER** - ***Cell*** is the basic structural and functional unit of living organism. - Each cell is capable of performing a basic function of life. Such as ***reproduction, respiration, excretion, growth**,* and so on. - Cells are found in all ***plants, animals***,...

**GEN BIOLOGY REVIEWER** - ***Cell*** is the basic structural and functional unit of living organism. - Each cell is capable of performing a basic function of life. Such as ***reproduction, respiration, excretion, growth**,* and so on. - Cells are found in all ***plants, animals***, and ***bacteria.*** - The cell theory is a scientifically and universally accepted theory that was formulated and proposed in the mid-17th century. - ***Robert Hooke*** was the first to use the term \"cell\" when he observed small, box-like structures in a cork slice through his compound microscope. These structures reminded him of the small rooms, or \"cells,\" in a monastery. - **The cell theory, as we know it today was formulated in the years 1838 and 1839. German Scientist *Matthias Schleiden* studied plant cells and postulated that every living thing is made up of cells or the product of cells.** - Nineteen years later, ***Rudolph Virchow*** completed the cell theory by providing the final postulate, which states that every cell is generated from pre-existing cells. - In the year 1839, ***Matthias Schleiden and Theodor Schwann*** were credited with the development of the cell theory. **[Postulate of the cell theory]** ***"All living organisms are made up of one or more cells".*** The first part of the cell theory states that all living things, whether small or big, simple or complex, irrespective of species or kingdoms, are made up of either one or more than one cells. - Living things or organisms that are made up of one cell are called unicellular or single-celled organisms. Bacteria and protozoans like amoeba are examples of single-celled organisms. - Living things or organisms that are made up of more than one cells are termed as multi-cellular or multi-celled organisms. Almost all animal and plant cells are multi-cellular organisms. The human body is made up of billions of multi-cellular cells. All biological processes that occur within the body for it to sustain itself cannot occur without the presence of cells. - They form the basic unit of life that not only provide structure to living cells but are also necessary for all their vital functions that are needed to sustain life. For example, in human beings, the cells that make up the skeletal system are called osteocytes. - Together they form the hardened structure called bone that provides structure, as well as function of the body. Cells are the building blocks of every system in living things -- starting from the cellular level to tissues, organs and organ systems. Every cell is generated from another cell that existed before it. This postulate of the cell theory refers to the process of cell division whereby one cell divides to produce more than one cells. This is the basis of cellular reproduction which can be either asexual or sexual depending on the living organism that the process is occurring in. - Cell division processes can be of different types, like budding or fission seen in yeast cells, or Mitosis and meiosis that is observed in both plant and animal cells. - Generally, during the process of cell division, a single cell divides into two or more cells, distributing its genetic content among its progeny. Thus, the newly formed cells are usually identical to the parent cell. **MODERN CELL THEORY** **"Energy flow occurs within cells".** The energy referred to in this postulate its chemical energy produced from thousands of biochemical reactions that take place inside the cell. From the breakdown of glucose to the production of ATP by the powerhouse of the cell, the mitochondria, all biochemical reactions produce a great deal of ***energy within each cell that flows from one part of the cell, one organelle to another through chemical messengers and molecules.*** **"Hereditary information or DNA is passed on from one cell to another".** - Chromosomes that contain the genetic material is passed on from parent to daughter cell. In unicellular organisms as well, such as bacteria, the DNA material is divided into its progeny simply by splitting of the cell into two, a process known as cytokinesis. - This results in the progeny cells being identical genetically to the parent cells. However, in higher organisms such as some animals and humans, the environment and a process known as ***recombination*** are key players in determining the genetic makeup of every individual being. - Almost every cell is surrounded by a ***cell wall*** and filled with a fluid-like substance known as ***cytoplasm*** or cytosol, in which many different structures called organelles are present that each has their own defined function. - All cells contain a nucleus or a region that holds the genetic content (DNA) of the organism, and all have biochemical processes and catalysts that enable it to sustain itself. ***[A few exceptions to the cell theory exist. These are:]*** 1\. Viruses are considered to be non-living because they cannot replicate or reproduce, despite having their genetic material. 2\. The very first cell did not arise from a precursor cell. 3\. Mitochondria and chloroplasts, although present within the cell, have their own genetic material and can reproduce independently from the cell that they are present in. **Organelle** **Function** **Factory part (analogy)** ------------------------------------ -------------------------------------------------------------- ------------------------------------------------------------ Nucleus DNA Storage Room where the blueprints are kept Mitochondrion Energy production Powerplant Smooth Endoplasmic Reticulum (SER) Lipid production; Detoxification Accessory production - makes decorations for the toy, etc. Rough Endoplasmic Reticulum (RER) Protein production; in particular for export out of the cell Primary production line - makes the toys Golgi apparatus Protein modification and export Shipping department Peroxisome Lipid Destruction; contains oxidative enzymes Security and waste removal Lysosome Protein destruction Recycling and security **[Nucleus]** Our **[DNA]** has the blueprints for every protein in our body, all packaged into a neat double helix. The processes to transform DNA into proteins are known as transcription and translation, and happen in different compartments within the cell. ***[This is where stores the DNA in a cell.]*** ***[Cytoplasm]*** It is a jelly-like fluid structure where cell organelles are located. **[Endoplasmic Reticulum]** Endoplasmic means inside (endo) the cytoplasm (plasm). Reticulum comes from the Latin word for net. Basically, an endoplasmic reticulum is a plasma membrane found inside the cell that folds in on itself to create an internal space known as the lumen. **[Rough Endoplasmic Reticulum]** The rough endoplasmic reticulum is so-called because its surface is studded with ribosomes, the molecules in charge of protein production. When a ribosome finds a specific RNA segment, that segment may tell the ribosome to travel to the rough endoplasmic reticulum and embed itself. **[Smooth Endoplasmic Reticulum]** The smooth endoplasmic reticulum makes lipids and steroids, instead of being involved in protein synthesis. These are fat-based molecules that are important in energy storage, membrane structure, and communication (steroids can act as hormones). The smooth endoplasmic reticulum is also responsible for detoxifying the cell. **[Golgi apparatus (aka Golgi body aka Golgi)]** It is the mailroom that sends our product to customers. It is responsible for packing proteins from the rough endoplasmic reticulum into membrane-bound vesicles (tiny compartments of lipid bilayer that store molecules) which then translocate to the cell membrane. **[1. Cytosol:]** the proteins that enter the Golgi by mistake are sent back into the cytosol (imagine the barcode scanning wrong and the item being returned). **[2. Cell membrane:]** proteins destined for the cell membrane are processed continuously. Once the vesicle is made, it moves to the cell membrane and fuses with it. Molecules in this pathway are often protein channels which allow molecules into or out of the cell, or cell identifiers which project into the extracellular space and act like a name tag for the cell. **[3. Secretion:]** some proteins are meant to be secreted from the cell to act on other parts of the body. Before these vesicles can fuse with the cell membrane, they must accumulate in number, and require a special chemical signal to be released. This way shipments only go out if they're worth the cost of sending them (you generally wouldn't ship just one toy and expect to profit). **[4. Lysosome:]** The final destination for proteins coming through the Golgi is the lysosome. Vesicles sent to this acidic organelle contain enzymes that will hydrolyze the lysosome's content. **Mitochondria** are membrane-bound organelles found in eukaryotic cells that generate most of the cell\'s energy through a process called cellular respiration. Often referred to as the \"powerhouse of the cell,\" they produce ATP, the primary energy carrier in cells. **[Cytoskeleton]** Within the cytoplasm there is network of protein fibers known as the cytoskeleton. This structure is responsible for both cell movement and stability. The major components of the cytoskeleton are microtubules, intermediate filaments, and microfilaments **Prokaryotes** are ***single-cell organisms (unicellular)*** which do not contain organelles or any internal membrane structures. That means that they have a single chromosome and no nucleus, but instead they have ***[nucleoid]*** (a circular container that has double-stranded DNA). These organisms belong to domain Prokarya. **Eukaryotes** are ***multi-cellular organisms***, and they contain a ***[nucleus]*** and other organelles encapsulated within membranes. From an organization perspective, they belong a domain Eukarya. **Prokaryote Plasma Membrane:** Made of a phospholipid bilayer with proteins; often has a [ ***peptidoglycan***] cell wall (in bacteria). **Eukaryote Plasma Membrane:** Composed of a phospholipid bilayer with proteins and cholesterol for fluidity; plant and fungal cells have ***[cellulose]*** or **chitin** cell walls. **[Plant cells]** that have their genetic material enclosed in the nucleus and have membrane-bound organelles. One of the most distinctive features of plants cells is the presence of cell wall apart from the cell membrane itself. - This cell wall, primarily composed of cellulose, is what provides the whole plant structure support and rigidity. - The primary function of plant cells is to carry out photosynthesis because of the presence of chlorophyll in their chloroplasts. **[Types of Plant Cells]** ### **1. Parenchyma** Among all types of plant cells, [**parenchyma cells**](https://www.bioexplorer.net/parenchyma-cells.html/) are the simplest in terms of structure -- they only have thin walls. These cells are not highly specialized a primarily used for the storage of organic products. ### **2. Collenchyma** Collenchyma cells have relatively thin walls but with some degrees of thickening at some parts of the cell. This type of structure allows the plant cell to utilize their function as structural support. ### **3. Sclerenchyma** Unlike parenchyma and collenchyma cells, sclerenchyma cells have highly lignified (embedded with lignin) cell walls which are thickened dead cells at maturity. ### **4. Water Conducting Cells** Xylem is a plant vascular tissue which helps in transmitting water from roots to all parts of the plant. The cells in this tissue have a hardening agent, unlike collenchyma cells. There are two types of cells within Xylem namely *tracheids* and *vessel members*. Seedless vascular plants contain tracheids whereas flowering plants (Angiosperms) contain both tracheids and vessel members. ### **5. Sieve Tube Members** Phloem is another plant tissue which is responsible for conducting foods produced (via Photosynthesis) in the leaves to all parts of the plant. Within this tissue, three types of cells found namely *companion cells*, *phloem fibers*, and *parenchyma cells*. **[Animal cells]** are also a type of eukaryotic cells that contain a "true nucleus" and membrane-bound organelles enclosed together by a plasma membrane. - Animal cells do not have a cell wall, which typically distinguishes them from other eukaryotic organisms like plants and fungi. **[Types of Animal Cell]** 1. **Nerve Cells** - Nerve cells are specialized cells that electro-chemically send impulses or information to and from the sensory receptors and the central nervous system. 2. **Blood Cells** - Also called at the [**hematopoietic cell**](https://www.bioexplorer.net/hematopoietic-stem-cells.html/), the blood cell is responsible for carrying oxygen to the different tissues while at the same time collecting carbon dioxide from them. Aside from that, blood cells also bring with them hormones and other nutrients and send them to the different parts of the body. ### **3. Muscle Cells** - Muscle cells, also called *myocytes*, are long and tubular cells (sometimes spindle-shaped) that function for the production of force and movement. In animals, muscle cells contain the most number of mitochondria. (Example: cardiac muscle) ### **4. Skin Cells** Located in the epidermal and dermal layer, skin cells function mainly for protection, perception, and transmission of sensation. In addition to that, [**skin cells**](https://www.bioexplorer.net/epidermis-cells.html/) also prevent water loss through dehydration. **5. Bone Cells** Bone cells make up the bones and overall skeleton of animals. While there are different [**types of bone cells**](https://www.bioexplorer.net/types-of-bone-cells.html/), their primary function is to provide structural support and aid in movement. **[Three types of Cell Modification ]** - Apical Modification - Basal Modification - Lateral Cell Modification 1. **Apical (Surface or luminal) modifications: It is specialized to carry out functions that occur at these interfaces, including secretion, absorption, and movement of luminal contents.** - **[Cilia]**: are membrane-covered extensions of the entire apical surface. They beat in waves, often moving a surface coat of mucus and trapped materials. Ciliated epithelia include ciliated pseudostratified columnar (respiratory) epithelium and the ciliated simple columnar epithelium of the oviducts. - **[Flagella]**: are also concerned with movement. Spermatozoa, derived from seminiferous epithelia, are the only flagellated human cells. - **[Microvilli]** (brush border or striated border): are plasma membrane-covered extensions of the cell surface. Their cores are composed of parallel actin microfilaments; these are anchored in a dense mat of filaments in the apical cytoplasm called the terminal web. By interacting with cytoplasmic myosin, the microfilaments can contract, shortening the microvilli. - **[Stereocilia]**: are not true cilia but very long microvilli. They are found in the male reproductive tract (epididymis. ductus deferens). where they have an absorptive function, and in the internal ear (hair cells of the maculae and organ of Corti). Where they have a sensory function **2. Basal specialization the basal surface contacts the basal lamina. Because it is the surface closest to the underlying blood supply, it often contains receptors for blood borne factors such as hormones.** - A basal lamina underlies all true epithelial tissues. The basal lamina is a sheet-like structure, usually composed of type IV collagen, proteoglycan, and laminin, a glycoprotein that aids in binding cells to the basal lamina. The basal lamina exhibits electron-lucent and electron-dense layers termed the lamina lucida (lamina rara) and the lamina dense, respectively. Basal lamina components are contributed by the epithelial cells, the underlying connective tissue cells, and (in some locations) muscle, adipose, and Schwann cells. **3. Lateral Modification - Cell modification found on the basal surface of the cell** - ***Lateral Modification Tight Junction*** - Act as barriers that regulate the movement of water and solutes between epithelial layers. Prevent leakage of ECF - ***Lateral Modification*** Adhering Junction - Anchoring junction on the lateral surface of the cell. Very similar to the anchoring junction of the basal surface of the cell. Fasten cells to one another basement membrane. Three types of cell junctions in animal tissues. Gap junctions adhering junctions tight junctions free surface of epithelial tissue - ***Lateral Modification Gap Junction*** - Also known as communicating junctions. Closable channels that connect the cytoplasm of adjoining animal cells. The **[interphase]** is the preparation phase for mitosis and it is also the longest phase in the cell cycle. The interphase takes place in the cytoplasm and the cell nucleus. **[G1 Phase (First Gap)]** The first stage of interphase is called the G1 phase (first gap) because, from a microscopic aspect, little change is visible. However, during the G1 stage, the cell is quite active at the biochemical level. The cell grows and accumulates the building blocks of chromosomal DNA and the associated proteins as well as sufficient energy reserves to complete the task of replicating each chromosome in the nucleus. **[S Phase (Synthesis of DNA)]** The synthesis phase of interphase takes the longest because of the complexity of the genetic material being duplicated. Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. In the S phase, DNA replication results in the formation of identical pairs of DNA molecules, sister chromatids, that are firmly attached to the centromeric region. **[G2 Phase (Second Gap)]** In the G2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase. There may be additional cell growth during G2. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis. **[Features of Mitosis]** 1. In each cycle of cell division, two daughter cells are formed from the parent cell. 2. The cell is also known as equational cell division because the chromosome number in the parent cell and daughter cell is the same. 3. In plants, mitosis leads to the growth of vegetative parts of the plant like root tip, stem tip, etc. 4. Segregation and combination do not occur in this process. **[The processes occurring during mitosis have been divided into different stages.]** 1. **[Prophase]** The condensation of the chromatin (consisting of two sister chromatids) marks the beginning of prophase. These DNA molecules were previously produced and duplicated during the S phase in the interphase. - During prophase, the chromatin coils into visible rod-shaped structures called the chromosomes. - Aside from condensation, the mitotic spindle fibre, which will further help in the segregation of the chromosomes, is also formed due to various cytoplasmic changes in the cell. 2. **[Metaphase]** After prophase, the cell immediately undergoes metaphase where the cell's chromosomes align at the center of the cell equatorial plate (also called as the metaphase plate). 3. **[Anaphase]** The gradual shift from metaphase to anaphase is characterized by the breaking of the link between the sister chromatids that are about to migrate to the separate poles of the cell. 4. **[Telophase]** Telophase the last stage of mitosis, is accompanied by the process called cytokinesis (the division of the cytoplasm of the parental cell) that gives rise to two daughter chromosomes. **Meiosis** is a type of cell division that involves the reduction in the number of the parental chromosome by half and consequently the production of four haploid daughter cells. This process is very essential in the ***[formation of the sperm and egg]*** cells necessary for ***[sexual reproduction.]*** **[Stages of Meiosis]** 1. **[Metaphase I.]** In this stage, the homologous pairs of chromosomes randomly align at the metaphase plate. Such configuration becomes the source of genetic material as the chromosomes from the male and female parents appear similar but are not exactly identical. **[Prophase I:]** - Most complex stage. - Chromosomes condense and pair with homologues. - Divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. **[Metaphase I:]** - Homologous chromosome pairs align randomly at the metaphase plate, leading to genetic variation. **[Anaphase I:]** - Homologous chromosomes are pulled apart to opposite ends, while sister chromatids remain attached. **[Telophase I:]** - Chromosomes reach opposite poles and de-condense. - Nuclear membranes reform, and cytokinesis occurs, resulting in two haploid daughter cells. 2. **[Meiosis II]** - Cells undergo through meiosis I to meiosis II without the replication of the genetic material. It is important to note that the cells that undergo meiosis II are the daughter cells produced during meiosis I. Meiosis II is shorter than meiosis I but still is divided into four stages: prophase II, metaphase II, anaphase II, and telophase II. **Prophase II:** - Similar to mitotic prophase. - Nuclear envelope disintegrates; chromosomes condense. - Centrosomes move apart and spindle fibers attach to chromosomes. **Metaphase II:** - Chromosomes align at the metaphase plate. - Spindle fibers attach to centromeres, ensuring proper segregation. **Anaphase II:** - Sister chromatids separate and move toward opposite poles, unlike in Anaphase I. **Telophase II:** - Cytoplasm divides, forming four haploid cells. - Each cell has a unique set of chromosomes due to crossing over and random alignment in Meiosis I. **[Importance of Mitosis in Living Process]** - **Genetic stability-** Mitosis helps in the splitting of chromosomes during cell division and generates two new daughter cells. Therefore, the chromosomes form from the parent chromosomes by copying the exact DNA. Therefore, the daughter cells formed as genetically uniform and identical to the parent as well as to each other. Thus mitosis helps in preserving and maintaining the genetic stability of a particular population. - **Growth-** Mitosis help in increasing the number of cells in a living organism thereby playing a significant role in the growth of a living organism. - **Replacement and regeneration of new cells-** Regeneration and replacement of worn-out and damaged tissues is a very important function of mitosis in living - organisms. Mitosis helps in the production of identical copies of cells and thus helps in repairing the damaged tissue or replacing the worn-out cells. But the degree of regeneration and replacement in multicellular organisms vary from one another. For example, mitosis process is used in order to regrowth the legs of newts and crustaceans. However, the degree of regrowth may vary. **TYPE OF DISORDER** **ABNORMALITY** **CHARACTERISTICS** ---------------------- -------------------------------------------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Down Syndrome Trisomy 21 (presence of a third copy of chromosome 21) Individuals with this syndrome are often called as "*mongoloids*" due to their physical attributes similar to the Mongolian descent: flat face, slanting eyes, short neck, below average intelligence, etc. Edwards Syndrome Trisomy 18 (presence of a third copy of chromosome 18) Individuals with *Edwards syndrome* are characterized by their small and abnormally shaped head, small jaw, clenched fists and fingers that tend to overlap. Patau Syndrome Trisomy 13 presence of a third copy of chromosome 13) Among the three autosomal trisomies, this is the rarest yet the most severe. Individuals are characterized by having polydactyly (having extra fingers or toes), holoprosencephaly (failure to create double lobes of the cerebral hemispheres), facial clefting, heart ailments, etc. Turner Syndrome Absence of one X chromosome Only affecting females, *Turner syndrome* is characterized by the offspring having short stance, inability to develop during puberty, infertility, heart ailments, and other physical, social, and mental disabilities. Metafemale Presence of extra X chromosome (XXX) This syndrome is also referred to as the *Triple X syndrome* as affected individuals (known as metafemales) are characterized by the presence of an extra X chromosome. Most of the time, metafemales do not exhibit unusual physical attributes but they can be described as having behavioral problems, wide-set eyes, and ovarian failure. Klinefelter Syndrome Presence of extra X chromosome (XXY) *Klinefelter Syndrome* only occurs in males (due to the presence of the X chromosome) and it specifically affects their physical and intellectual development. In most cases, affected individuals have small reproductive organs and produce less testosterone than normal. This can then lead to delayed puberty and expression of some female physical attributes like the enlargement of breasts. Jacob's Syndrome Presence of extra copy of the Y chromosome (XYY) Individuals with the *Jacob's syndrome* appear to be physically normal in general. Because of their excess Y chromosome, they are taller than average, weaker muscle tone, and may have some learning and speech problems. **Functions of the Cell Membrane** - The cell membrane regulates what enters and exits the cell, maintaining homeostasis. - Contains receptors that allow cells to receive signals from hormones and other signaling molecules, facilitating cell communication. - Provides structural integrity to the cell, helping maintain its shape and organization. **Lipid Components in Cell Membranes** **Phospholipids:** - Structure: Composed of a hydrophilic (water-attracting) \"head\" and two hydrophobic (water-repelling) \"tails.\" - Form the fundamental structure of the cell membrane, creating a bilayer that serves as a barrier to most water-soluble substances. **Cholesterol**: - A sterol molecule interspersed within the phospholipid bilayer. - Maintains membrane fluidity and stability, preventing the membrane from becoming too rigid or too fluid, especially in varying temperatures. - A crucial lipid that helps maintain membrane fluidity in animal cells. **Glycolipids**: - Lipids with carbohydrate chains attached. - Contribute to cell recognition and communication by serving as receptors and helping to form protective layers on the cell surface. **Sphingolipids**: - Composed of a sphingosine backbone and fatty acids. **Lipid Rafts**: - Microdomains within the membrane that are rich in cholesterol and sphingolipids. - Serve as organizing centers for the assembly of signaling molecules and play a role in cell signaling and membrane trafficking. **[Transport Processes]** - **Endocytosis:** The process of engulfing molecules into the cell via the cell membrane. - **Exocytosis**: The process that forces molecules out of the cell. **Diffusion -** The net passive movement of particles from an area of higher concentration to an area of lower concentration. **Osmosis -** The diffusion of water molecules across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. **Homeostasis -** The process that maintains balance and equilibrium in a cell. **[Types of Transport]** - **Passive Transport -** Does not require energy; includes diffusion and osmosis. - **Active Transport** - Requires energy to move molecules against their concentration gradient. **Biomolecules** Biomolecules are organic molecules essential for life and are typically categorized into four main types: - Carbohydrates - Lipids - Proteins - Nucleic Acids **Enzymes** - Enzymes are specialized proteins that act as catalysts in biochemical reactions. - Speed up chemical reactions by lowering the activation energy required for the reaction to occur. **[Enzyme Components]** **Allosteric site** - This is a site on the enzyme that is distinct from the active site. **Active site** - The active site is the specific region of the enzyme where the substrate binds. **Binding site** - While this term can refer to any site where molecules attach, it is not specific to enzymes and does not accurately describe the role in enzymatic reactions. **Regulatory site** - Similar to the allosteric site, the regulatory site is involved in modulating the enzyme\'s activity. Molecules that bind to this site can influence the enzyme\'s function, but they do not interact with the substrate directly. **Redox Reactions** - In a reduction reaction, a molecule **gains electrons**. This process often results in a decrease in the oxidation state of the molecule. Gaining electrons can lead to the molecule becoming more negatively charged or less positive. **Oxidation Reaction** - In an oxidation reaction, a molecule **loses electrons**. This results in an increase in the oxidation state of the molecule. Losing electrons often makes the molecule more positively charged or less negative.