Cell Theory 4 basic concepts of cell theory are: Cells are the units of structure (building blocks) of all animals and plants. Cells are the smallest unit of function in all animals and plants. Cells originate only from pre-existing cells by cell division. All cells maintain homeostasis Cell Structure, Cell Transport and Mitosis Cell Cell-membrane, Cytoplasm and Nucleus Cytoplasm Cytosol and Cell Organelles Nucleus Nuclear Envelope, Nucleoplasm and Chromatin (DNA + Histones) Fig 3-2 Table 3-1 represents the summary of structure and function of various parts of a representative cell. IMPORTANT Cell Membrane All cells are covered with a thin covering of a double layer of Phospholipids and associated Proteins present here and there. Each phospholipid has a polar (hydrophilic) head and non-polar (hydrophobic) tails. In the double layer the tails face each other forming a hydrophobic barrier which keeps water dissolved contents inside. Proteins may be Intrinsic embedded in the lipid double layer and Extrinsic associated outside the lipid double layer. Fig 3-3 and Table 3-2 Cell Membrane and Transport In bacteria, plants, and fungi cell membrane is surrounded by a non-living cell wall. But cell membrane is the real boundary. Most cell walls are permeable. Semi-permeable: Cell membranes allow some materials to pass through them and prevent others from doing so. Regulators like hormones can change permeability of a cell membrane. Transport across membrane can be Passive or Active. Passive Transport Active Transport From high to low concentration Independent of conc. Gradient Energy (ATP) not needed ATP needed Mostly Carrier Protein not needed Carrier Protein needed Diffusion, Osmosis, Facilitated Diffusion Primary & Secondary Active T. Bulk Transport Passive Transport Passive Transport includes Diffusion, Osmosis and Facilitated Diffusion. Diffusion: All fluids (liquids + gases) move from area of higher concentration to area of lower concentration (concentration gradient). This movement of substances is called Diffusion. It can be through cell membranes. For example, spreading of fragrance, dissolving of ink drop in water, movement of O2 and CO2 between lungs and blood Osmosis is always the net movement of water through cell membrane from its higher concentration (dilute solution) to its lower concentration (concentrated solution) when the 2 solutions are separated by semi-permeable membrane. For example absorption of water by roots of plants. Facilitated diffusion is faster than normal diffusion but needs a carrier protein though no ATP needed. For example, absorption of glucose and amino acids in intestine 1
Diffusion Osmosis Movement of solute Movement of water Membrane not needed Semi-permeable memb needed High conc low conc Low conc high conc of solution Fig 3.4 Fig 3.6 Active Transport Only active transport can operate against concentration gradient. It is the fastest mode of transport. Fig 3-9, Na-K pump It always consumes ATP directly or indirectly. It always needs one or more transport proteins. For example absorption of minerals by plant roots, absorption of nutrients when their concentration is already higher inside the cells. Vesicular Transport Large molecules like proteins cannot transport through membrane by passive or active transport discussed so far. These are packed into membrane bound sacs and transported across cell membrane. Endocytosis is the bulk transport into the cell. If solid material including prey is brought in as Food Vacuole, the process is called Phagocytosis. Fig 3-11. For example, white blood cells eating bacteria. When cell brings in liquid bound in sac the process is called Pinocytosis. Fig 3-10 Exocytosis: When the cells releases solid or liquid in sacs the process is called exocytosis. For example Amoeba throws excess water outside to maintain required concentration (osmoregulation). IMPORTANT Table 3-3 depicts summary of mechanisms of membrane transport. Recap-1 Chapter-3 1. The liquid part of cytoplasm is --------------- 2. Structures formed of membranes and molecules, present in cytoplasm, doing a special function, are ---------- ---------------- 3. 2 halves of cell membrane are held together by -------------- bonds 4. Cell membrane is fluid due to --------------- molecules 5. Membrane bound --------- molecules regulate membrane transport 6. Movement of solute from high to low concentration is ------------- 7. Net Movement of water across cell membrane (from low concentration solution to high concentration solution) is ---------- 8. ----------- ---------------- can operate against concentration gradient and needs both a carrier protein and ATP. 9. --------- ------------- moves along the concentration gradient and needs a carrier protein but no ATP. 10. Bulk movement of solids into the cell in a vesicle is --------- and of liquids is ------------------. (also called vesicular transport) Cytoplasm Cytoplasm is the living fluid part between cell membrane and nucleus. It has special structures called Cell Organelles in it. Cytosol is the matrix (formless) part of cytoplasm formed of water having dissolved or suspended substances in it. Cell Organelles are organ like each performing specific function/s but formed of molecules and membranes only (sub- cellular). Double Membrane bound Organelles: Mitochondria, Chloroplasts, Endoplasmic Reticulum, Golgi Body, and Nucleus. Single Membrane bound Organelles: Lysosomes, Peroxisomes, Vacuoles Organelles lacking any membrane: Ribosomes, Centrioles, Nucleolus IMPORTANT: Table 3.1 Nucleus and Ribosomes 1 2
Genetic Control of the Cell Nucleus: is the most distinct structure inside cell visible with light microscope. It has inside it DNA having all the information needed to form and run the cell. The segments of DNA are called Genes. Fig 3-16 Nuclear Envelope: is formed of 2 membranes with a gap between them. It has a large number of Nuclear Pores usually bound by a nuclear complex. The pores are large enough to allow RNA and proteins to pass through. Nucleoplasm: is the matrix (formless) of nucleus and has a different composition than Cytosol. Chromatin fibers: are very long molecules of DNA associated with proteins (Histones and other nuclear proteins). Each chromatin fiber, at the time of cell division, organizes into Chromosomes. Fig 3-17 Nucleolus: is present in the nucleus when the cell is not dividing. No membrane bounds it. It assembles both units of Ribosomes. Nucleus and Ribosomes 2 Transcription Translation DNA ------------------- m-rna ----------------------- Protein A gene (DNA) produces m-rna by transcription. The m-rna comes out of nuclear pores into the cytoplasm. Here Ribosomes attach one by one to m-rna and synthesize a protein. t-rna s bring amino acids from cytosol to m-rna ribosome complex. Fig 3-18, 19 Each ribosome has 2 sub-units larger (60S) and smaller (40S). r-rna and ribosomal proteins form the ribosomal subunits. No membrane covers the ribosomal subunits. Ribosomal subunits join only around m-rna for protein synthesis otherwise remain separate. At the time of cell division one DNA divides and produces 2 DNA molecules by replication. DNA replication leads to produce sperms and eggs (gametes). m-rna = messenger RNA Replication t-rna = transfer RNA DNA ---------------- DNA + DNA r-rna = ribosomal RNA The Endomembrane System 1 Manufacturing and Distributing Cellular Products Endoplasmic Reticulum, Golgi Apparatus, Lysosomes and Vacuoles collectively form Endomembrane System. Endoplasmic Reticulum: is a system of double membranes in the form of tubes and sacs throughout cytoplasm (in between cell membrane and nuclear envelope). Main manufacturing facility. Functions include synthesis of proteins and lipids including steroids, detoxification, and cellular transportation. It transports materials inside the cell by transport vesicles. RER SER 1 Rough ER, ribosomes fixed 1 Smooth ER, no ribosomes 2 Flat sacs 2 Tubular 3 Synthesis of memb proteins 3 Lipids, steroids 4 Synthesis of secretory proteins 4 Detoxification of drugs The Endomembrane System 2 Golgi Apparatus = Golgi Body: is a stacks of flattened sacs called cisternae. A cell may have from a few to a few hundred of Golgi stacks. Golgi Apparatus receives transport vesicles from ER on one side, modifies received chemicals, can store them and packs them in secretory vesicles and releases them on shipping side. Lysosomes: are single memb bound organelles rich in digestive enzymes, help in breakdown of large molecules like proteins, polysaccharides, lipids and nucleic acids. Lysosomes provide a safe place for digestion of large molecules without damaging molecules of the cell. A Lysosome joins a food vacuole to digest the materials inside vacuole. Lysosomes are absent in most plant cells. Vacuoles: are membrane bound sacs and pinch off from ER, Golgi Apparatus and cell membrane. Functions include endocytosis, exocytosis, maintain turgor pressure in a plant cell, trasnportation. 3
Mitochondria Energy Conversion Mitochondria (sing. Mitochondrion): are the powerhouses of cells and the site for cellular respiration. Respiration is oxidation of food and chief source of energy for the cell. These are bound with double membrane, outer smooth and inner folded. Mitochondria have enzymes for breakdown glucose derivatives, fatty acids and amino acids. Mitochondria have Electron-Transport-System that generates ATP molecules by using the energy contained in H s produced during breakdown of glucose. Mitochondria are found in plants, animals, fungi and protists. The Cytoskeleton Cell Shape and Movement Maintaining Cell Shape: The shape of the cell is maintained by Intermediate Filaments, the thick ropes of twisted protein fibers, Microtubules, the hollow organelles and Microfilaments, the solid thinner organelles. These maintain the shape of cells and keep in position the nucleus. The framework of support is highly dynamic that can organize and dismantle really fast. Transport of organelles and molecules: Microtubules are the freeways used by organelles like lysosomes to move from one part of cell to another. Microfilaments are the roads/streets used by smaller things. Genes DNA Chromatin fiber Chromosomes Fig. 3-17 Genes, the segments of DNA, are part of chromatin fiber found in nucleus. Chromatin fiber is formed of DNA and Histone proteins. Most of the time the chromatin fibers exist as a diffuse network (not visible even under electron microscope). However, when the cell starts to divide the chromatin fibers organize into compact threads called Chromosomes. Each species has a fixed # of chromosomes 46 in most human cells. Cellular Reproduction = Cell Division Passes on Genes from Cells to Cells Reproduction of Organisms Chromosome DNA Gene Almost all the eukaryotic genes (about 20000 in human genes) are found in the chromosomes. Some genes are present in Mitochondria and Chloroplast. DNA associate with 4 kinds of Histones and coil to form Nucleosome. A 5 th Histone molecule, keeps the coils in position. Nucleosomes pack and form thicker and thicker threads. The thickest threads are Chromatids. A chromosome has 1 or 2 Chromatids in it. A chromosome with 1 chromatid divides to form a chromosome with 2 Chromatids (sister). One chromatid is passed on to each daughter cell. Fig 3.17 Cell Cycle Cell Cycle: Most cells in body divide though at different rates. There are 2 distinct phases that alternate with each other and form a cell-cycle. M-phase: when a cell is dividing. The daughter cells are half in size. Interphase: Each daughter cell must grow by making new materials including proteins and DNA. Interphase is divided into 3 sub-phases : G1, S and G2. 4
S-phase occurs in the middle part of Interphase and DNA replication takes place. DNA and chromosomes are doubled. G1 and G2 are growth phases of cell with synthesis of proteins and ribosome. G1 takes place before S-phase. But G2 occurs after the S-phase. Fig 3-20 Mitosis interphase The cell division of growth and maintainance Mitosis is the division of growth and replacement of lost or damaged cells. It is equational division. 2n 2n or 1n 1n Fig 3-22 depicts mitosis (division of nucleus) and Cytokinesis (division of cytoplasm). Interphase near its end has inside cytoplasm 2 centrosomes, each with a pair of centrioles. These initiate the organization of spindle fibers. The chromosomes are double with 2 sister chromatids joined only at centromeres but still indistinct. Mitosis has 4 distinct phases Prophase, Metaphase, Anaphase and Telophase. Memory aid: P-MAT Mitosis Prophase pro = first Prophase: is the phase that prepares the cell for mitosis. Centrosomes start moving to opposite ends and spindle formation starts. Chromosomes coil and pack into thick threads and get distinct. In late prophase nuclear envelope degenerates and chromosomes are released in cytoplasm. Spindle fibers either join a spindle fiber from the opposite centrosome or connect to the centromere of a chromosome. Mitosis Metaphase meta = after Metaphase: The spindle is fully formed now. The chromosome pack further and get most distinct. Chromosomes arrange on an imaginary disc = equatorial plate at the middle. The centromeres of chromosomes lie at the plate. Each centromere is joined through spindle fibers to both centrosomes. Mitosis Anaphase ana = apart Anaphase: is the movement of young chromosomes from the middle towards respective poles (centrosomes). It starts suddenly when the centromeres divide. Each chromosome is formed only of 1 chromatid. The motor proteins at centromeres move the chromosomes on the microtubules of spindle fibers. Telophase telo = end Telophase begins when the 2 groups of chromosomes reach the poles. This phase is the reverse of prophase. Chromosomes unpack to diffuse network. Nuclear envelope is reorganized from Endoplasmic Reticulum. Spindle fibers disappear. One nucleus is completely divided into 2 genetically similar daughter nuclei. Cytokinesis kinesis = motion 5
Cytokinesis takes place along Telophase. In an animal cell cleavage furrow appears at the middle and divides the cytoplasm into 2 equal halves, each with a nucleus. In a plant cell a cell-plate is formed at the middle. Golgi apparatus provides most of the materials packed in vesicles. Cell plate starts at the center and proceeds towards parent cell wall. Cell plate joins with the parental cell wall to complete the Cytokinesis. Most plant cells lack centrioles in them and centrosomes organize spindle formation. Cell Diversity and Differentiation Somatic Cells All have same genes Some genes inactivate during development Cells thus become functionally specialized Specialized cells form distinct tissues Tissue cells become differentiated The Cell Life Cycle Cell Division and Cancer Abnormal cell growth Tumors (also called, neoplasm) Benign (Encapsulated) Malignant: 1. Invasion 2. Metastasis Cancer Disease that results from a malignant tumor Recap-2 Chapter-3 1. Smooth E.R. synthesizes ---------------- 2. Rough E.R. synthesizes ----------------- 3. Golgi Apparatus is the --------- ------------ of the cell and produces secretory vesicles. 4. ATP s are synthesized in the --------- chamber of --------------. 5. Synthesis of DNA takes place during ----------- of cell cycle. 6. When cell is not dividing DNA occurs in the form of ------------ 7. When the cell is dividing the DNA occurs in the form of ------------- 8. During ------------ chromosomes arrange on mid-plate in mitosis 9. Division of cytoplasm after mitosis is called ---------------- 10. Divided chromosomes move to opposite poles during ------------- 11. Spindle is formed, chromosomes appear randomly arranged and nuclear envelope breaks during ------------- 12. Nuclear envelope appears, chromosomes unpack during ------------ 6