CELL STRUCTURE CELL - smallest unit of protoplasm capable of independent existence. Considered as the fundamental unit of living organisms Size (variable) - smallest cell – 2-3 u – largest cell – over a meter in length Shape: spherical, cubical, columnar, fusiform, or stellate Major divisions: - Nucleus & Cytoplasm bounded by a Cell Membrane I. NUCLEUS − − Usually single & located at the center or near the center of the cell. Shape conforms to the shape of the cell (oval or spherical) A. NUCLEAR MEMBRANE • • • • Composed of 2 membranes separated by a narrow space called PERINUCLEAR SPACE At various points, the 2 layers fuse are penetrated by NUCLEAR PORES Regulate passage of substances from the NUCLEOPLASM to the CYTOPLASM, & vice versa Encloses a jelly-like fluid known as the NUCLEOPLASM where the chromatin & nucleolus are suspended. B. CHROMATIN • • • • Composed of filaments or strands of DNA During interphase, part of the DNA filaments are coiled & are visible as irregular masses in the nucleus under LM - called chromatin, which is actually the HETEROCHROMATIN Parts that are uncoiled are not seen under the LM - the EUCHROMATIN During cell division, the entire length of the DNA is coiled & form dense, like bodies called CHROMOSOMES C. NUCLEOLUS • • • Maybe one or more in a nucleus Seen as a dark staining body that is eccentrically placed in the nucleus Made up of tightly coiled RNA’s, actually serve as the site for RNA synthesis II. PLASMA MEMBRANE − − − − − Composed of lipid, protein. & CHO Not seen under LM, since it is very thin & can't be resolved by LM Seen as a trilaminar membrane under EM - 2 electron dense lines & a middle paler line Composed of a bimolecular layer of phospholipids with their hydrophilic ends directed to the inner & outer surfaces of the membrane & their hydrophobic ends directed towards the middle The cell membrane of the cell is a phospholipid bilayer containing many components, including proteins and cholesterol, some with carbohydrate groups attached. (continuation – Plasma membrane) - − − CHOLESTEROL is responsible for the fluidity of the membrane GLOBULAR PROTEINS scattered within the lipid layer: a. receptor proteins b. transport or carrier proteins c. Enzymes short chains of sugar (OLIGOSACCHARIDES) are attached to the globular proteins that protrude towards the outer surface of the membrane - this constitute the surface coat (GLYCOCALYX) of the cell membrane. Functions of the surface coat: a. provide a slimy surface that protect the membrane from mechanical & chemical damage b. provide for cell-cell recognitional adhesion SPECIALIZATIONS OF THE PLASMA MEMBRANE A. Free Surface Specializations: a. MICROVILLI - tiny fingerlike projections that increase the cell surface area for absorption b. CILIA - motile processes that are larger than microvilli that propels fluid, mucus, or debris over the cell surface c. FLAGELLA - very long process, usually numbering one per cell; provides motility to the spermatozoa B. Junctional Specializations (LATERAL) TIGHT JUNCTION • adjacent cell membranes fuse at several points around their perimeters. • serving to seal off the intercellular space • prevent entry of substances by this route • for cell to cell adhesion DESMOSOMES * punctate or button- like thickenings of the adjacent cell membranes that are connected by protein filaments • provide cell to cell adhesion GAP JUNCTIONS (NEXUS) • the apposing membranes are connected by hollow cylindrical structures called CONNEXONS that allow passage of substances • allow rapid spread of excitation from cell to cell III. CYTOPLASM - site of most of the cellular metabolic activities A. CYTOSOL -semi fluid matrix where the other elements are suspended B. FORMED ELEMENTS 1. ORGANELLES • organized structures that participate actively in the biochemical processes of cell metabolism a) MITOCHONDRIA – slender threads or short rod-like bodies under LM -enclosed by 2 membranes - outer membrane is smooth, while the inner membrane has shelflike projections called CISTAE-enclosed within the membrane is the mitochondrial matrix that contains enzymes necessary for the production of ATP metabolically active cells contain more mitochondria than less active cells (Ex: liver cells) -Function synthesis of energy (powerhouse of cell) b) Endoplasmic reticulum - irregular clumps or basophilic masses • composed of a network of communicating Fluid-filled tubules & cisterns • ROUGH-SURFACED (ERGASTOPLASM): - ribosomes are attached to the outer surface of its membranes more abundant in cells that export protein products (ex: pancreatic cells) Function: synthesis of proteins • SMOOTH-SURFACED - no ribosomes attached to its membranes; no cisterns Functions: • a. synthesis of lipids & steroids' , b. detoxification of drugs • c. sequestration release of calcium in striated muscles C. GOLGI COMPLEX - composed of flattened membranes arranged one on top of the other, similar to pile of saucers, upsidedown closely associated to it are numerous vesicles which become the secretory granules Functions: -a. modify & package protein -b. Maintenance & renewal of the cell membrane -c. synthesis of carbohydrates D) CENTROSOME (CENTRIOLES) • 2 cylindrical structures arranged perpendicular to each other - composed of MICROTUBULES arranged in definite manner (9 triplets) • Functions: - a. formation of the mitotic spindle - b. serve as basal bodies for cilia or flagella • BASAL BODIES - serve as attachments or origins of the microtubules that compose cilia & flagella E) LYSOSOMES • membrane-covered bodies; containing hydrolytic enzymes - Function: for intracellular digestion of foreign bodies & unwanted cytoplasmic structures F) PEROXISOMES • membrane-covered bodies that contain oxidases & catalase • Functions: -a. oxidation of long chain fatty acids producing H202 in the process -b. CATALASE breaks down H202 (a harmful free radical) to H20 & oxygen G) CYTOSKELETON - filamentous structures that form a network in the cytoplasm • MICROTUBULES - largest in diameter • INTERMEDIATE FILAMENTS • MICROFILAMENTS - actin & myosin Functions: A. B. C. D. maintenance of cell shape movement or locomotion of the cell contractility of the cytoplasm flow & distribution of cytoplasm 2. INCLUSIONS - more passive components represented By accumulations of the by-products of metabolism Ex: fat droplets, foreign bodies, glycogen granules, secretory granules, pigments, & crystals CELL: VILLI AND MICROVILLI: JUNCTIONAL SPECIALIZATION (LATERAL) CELL PHYSIOLOGY I. MEMBRANE TRANSPORT A. Passive Transport Processes - movements of molecules "downhill' from a region of high concentration to a region of low concentration that occurs spontaneously without expenditure of energy. 1. Diffusion ✓ process by which molecules tend to scatter throughout available space the driving force is the kinetic energy of the molecules ✓ the molecules collide randomly & and move towards a space where they are less concentrated (move down the concentration gradient) ✓ speed of diffusion is affected by a) size of the molecules & b)Temperature. Diffusion through a membrane: the cell membrane serves as a physical barrier to diffusion • passive movement of molecules through this membrane will occur if: -a. they are small enough to pass through the pores -b. they are soluble in the lipid layer A. Simple Diffusion - Movement of molecules from high concentration to low concentration - Both solute and solvent move B. Osmosis - Movement of solvent (water) across a semipermeable membrane from high to low solvent concentration - Only solvent moves ISOTONIC SOLUTIONS • have the same solute to solvent ratio as the intracellular space • will cause no movement of water into or out of the cell HYPERTONIC SOLUTION HYPOTONIC SOLUTION contain more solutes than water & will cause water to move out of the cells – resulting to CRENATION. contain fewer solutes & more water & will cause water to enter the cell & cause swelling & bursting of cell LYSIS C. Facilitated Diffusion - movement of the molecule into the cell is mediated by a carrier protein in the membrane, because the molecule is either lipid insoluble or too large to pass through the pores. Ex: diffusion of glucose into the cell D. Filtration - is the process by which water & solutes are forced through a membrane by hydrostatic pressure -pressure exerted by the blood pressure in the body • the solute-containing fluid (filtrate) is pushed from a higher pressure area to an area of lower pressure. B. Active Transport Processes - process whereby energy is used to move molecules or ions through the cell membrane • reasons why these substances can't pass by diffusion: 1. substances may be too large to pass through the pores 2. they may be fat-insoluble 3. they are moving "uphill" against a gradient • • Solute pumping - Substances are moved through the membrane by using ATP to energize the carrier proteins (referred to as solute pumps) → Substances transported by this process include: 1. amino acids, 2. some sugars, 3. Most ions, like Na & K (sodium & potassium pumps) Bulk transport - Endocytosis - energy requiring process wherein extracellular substances are taken in or engulfed by enclosing them in small membranous vesicles. ▪ Phagocytosis - taking in of substances in the form of macromolecules or larger particles; whereby the cell must form cytoplasmic extensions called PSEUDOPODS the ingested particle which is enclosed by membrane is called a PHAGOSOME ▪ Pinocytosis - (bulk-phase endocytosis) taking in of substances in the form of fluid - Exocytosis 1. substance to be extruded is enclosed by membrane (secretory granules) 2. Moves to apical cytoplasm 3. Fuses with the cell membrane 4. fused site opens to release the substance outside the cell THE CELL CONSIST OF TWO MAIN PHASES – INTERPHASE, CELL DIVISION 1. Interphase: longer ; cell prepares itself for division G1 Phase: During the first gap phase, the cell grows, carries out its normal functions, and duplicates Its organelles. S Phase: in the synthesis phase, the cell's DNA is copied or replicated to ensure that both new cells will have the same genetic information. G2 Phase: During the second gap phase, the cell continues to grow and prepares for division. It checks its DNA for errors and makes any necessary repairs. 2. Cell Division: This is the phase where the cell actually splits into two new cells. It consists of two main processes: 1. Mitosis: In this step, the cell's nucleus (which contains the DNA) divides into two identical nuclei. This ensures that each new cell will have a complete set of genetic instructions. 2. Cytokinesis: After mitosis, the rest of the cell, including the cytoplasm and organelles, divides into two separate cells. This results in two new daughter cells, each with complete set of organelles and genetic material. A. Preparation: DNA Replication - Requisites for cell division: - occurs during interphase 1. replication of DNA: -starts with the uncoiling of the DNA helix - separates into 2 nucleotide strand - each strand serves as a template for building a new strand - the sequence of nucleotides is copied basing on complementary pairs - 2 DNA molecules are formed identical to the original 2. duplication of centrioles 3. storage of energy GAP 1 (G1 PHASE) - • Interval between mitosis and the onset of DNA replication, • Synthesis of RNA and proteins takes place, • cytoplasm increases in volume • G1 checkpoint: - damage to the chromosomal DNA is repaired - cell checks if its environment is favorable before committing itself to S-phase • Non-cancerous cells are stimulated to proliferate by extracellular growth factors secreted by other cells through the signal transduction cascade. G0 PHASE - If not exposed to such signals during G1 the cell diverts from the cycle and enters the mitotically inactive 'GO' state • Some cells in GO are merely quiescent and can rejoin the cycle for reparative growth or normal cell replacement. SYNETHESIS (S) PHASE - • The standard number of DNA double-helices per cell, corresponding to the diploid number of single-strand chromosomes, is described as 2C. • The 2C complement is retained throughout G1 and into S-phase, when new chromosomal DNA is synthesized and the cell becomes 4C. • From the end of S-phase, through G2 and into M-phase each visible chromosome contains two DNA molecules, known as sister chromatids, bound tightly together. • In human cells therefore, from the end of S-phase to the middle of M there are 23 pairs of chromosomes (i.e. 46 observable entities), but 4C (92) nuclear DNA double-helices. GAP 2 (G2) PHASE - • Interval between the S phase and the next mitosis, • DNA repair takes place • cell prepares for mitosis, • the cell contains two identical copies of each of the 46 chromosomes (sister chromatids) M PHASE (MITOSIS) - process through which two identical diploid daughter cells are formedfrom a single diploid cell. 1. Prophase - • The chromosomes, each consisting of two identical chromatids, begin to contract and become visible within the nucleus. • The 'spindle apparatus' of tubulin fibers begin to assemble around the centrosomes at opposite poles of the cell. & • The nucleoli disperse. • DNA coils tightly • Chromosomes shortens and thickens • Microtubules assemble from tubulin building blocks in the cytoplasm to form the spindles. • nucleolus is no longer visible 2. Pro-metaphase - • The nuclear membrane dissociates. • Proteinaceous kinetochores develop around the centromeres of the chromosomes. • Tubulin fibers enter the nucleus and assemble around the kinetochores and linking up with those radiating from the centrosomes. 3. Metaphase - Tension in the spindle fibres causes the chromosomes to align midway between the spindle poles, so creating the metaphase plate. 4. Anaphase - • The centromeric DNA shared by sister chromatids is duplicated • the chromatids separate and are drawn towards the spindle poles 5. Telophase - • The separated sister chromatids (now considered to be chromosomes) reach the spindle poles. • a nuclear membrane assembles around each group • The condensed chromatin becomes diffuse and nucleoli reform. • cell looks like a dumbbell with a set of chromosomes at each end • spindle falls apart • nucleoli and the membranes around the nuclei re-form at each end of the elongated cell • Division of the genetic material is now complete. 6. Cytokinesis - The cell membrane contracts around the mid-region between the poles, creating a cleavage furrow which eventually separates the two daughter cells. KARYOKINESIS • division of the nucleus • starts from prophase to late anaphase CODON • Triplet • a 3-nucleotide chain sequence that codes for a specific amino acid • is found in mRNA CYTOKINESIS • division of the cytoplasm • starts from late anaphase to telophase SIMPLE DIFFUSION FACILITATED DIFFUSION OSMOSIS ISOTONIC, HYPERTONIC, AND HYPOTONIC SOLUTIONS ENDOCYTOSIS EXOCYTOSIS CELL CYCLE (HUMANS) DNA REPLICATION: HAPLOID VS. DIPLOID (mitosis) CELL CYCLE (ANIMALS)
