Cytology and Histology Chapter 1

Cytology and histology Chapter 1 : Introduction the chemistry of life  Cytology Histology Science which studies the Science which studies the cell tissue  The levels of structural complexity Living beings : 1 organism = set of≠ syste...

Cytology and histology Chapter 1 : Introduction the chemistry of life  Cytology Histology Science which studies the Science which studies the cell tissue  The levels of structural complexity Living beings : 1 organism = set of≠ systems 1 system = set of ≠ organs 1 organ = set of ≠ tissues 1 tissue = set of identical cells The cell is the morphological unit of an organism  Cytology = cell biology = study of the cell Morphology = Form Study of cell life, continuity and structure of cells and function Chemical composition = components & molecules constituting the cells  Atoms, molecules and compounds All matter in our universe is built of particles called atoms An element contains only one type of atom (carbon, sulphur or hydrogen) Substances containing two or more types of atom combined are called compounds (Ex : water is a compound containing both hydrogen and oxygen atoms) There are 92 naturally occurring elements The wide variety of compounds making up living tissues are composed almost entirely of only four: carbon, hydrogen, oxygen and nitrogen Small amounts (about 4% of body weight) of others e l e m e n t s are present, including sodium, potassium, calcium and phosphorus Atoms, molecules and compounds  Atomic structure Atoms are mainly empty space, with a central nucleus containing protons & neutrons surrounded by clouds of electrons Neutrons carry no electrical charge, but protons are positively charged & electrons are negatively charged Because atoms contain equal numbers of protons and electrons  no net charge Protons, neutrons and electrons differ also in terms of their mass. Electrons are so small that their mass is negligible, but the neutrons & protons carry one atomic mass unit each Atomic structure  Molecules and compounds Molecules consist of two or more atoms that are chemically combined The atoms may be of the same element (e.g. a molecule of O2 contains two oxygen atoms) Most substances, however, are compounds and contain two or more different elements ( e.g. a water molecule (H2O) contains two hydrogen atoms and an oxygen atom) Compounds containing carbon and hydrogen are classified as organic and all others as inorganic Living tissues are based on organic compounds, but the body requires inorganic compounds too  Covalent and ionic bonds When atoms are joined together, they form a chemical bond that is generally one of two types: covalent or ionic The vast array of chemical processes on which life is based is completely dependent upon the way atoms come together, bind and break apart Example : - The water molecule is a crucial foundation of all life on Earth. If water was a less stable compound, and the atoms came apart easily, human biology could never have evolved - On the other hand, the body is dependent upon the breaking down of various molecules (e.g. sugars, fats) to release energy for cellular activities  Covalent and ionic bonds Covalent bonds are formed when atoms share their electrons with each other. It forms a strong and stable link between its constituent atoms. A water molecule is built using covalent bonds. Ionic bonds are weaker than covalent bonds and are formed when electrons are transferred from one atom to another (NaCl)  Electrolytes An ionic compound, e.g. sodium chloride, dissolved in water is called an electrolyte because it conducts electricity (Na+, K+, Ca2+..) Example : NaCl - When NaCl is dissolved in water the ionic bond breaks and the two atoms separate - The atoms are charged, because they have traded electrons, they are no longer called atoms, but ions - Na+ is a cation (positive charge) and Cl− is an anion (negatively charged) Many biological compounds, are not ionic, and therefore have no electrical properties when dissolved in water. Example : carbohydrate  Acids, bases and pH pH is the measuring system used to express the concentration of hydrogen ions ([H+]) in a fluid It is an indicator of its acidity or alkalinity Biochemical processes of life continually produce or consume hydrogen ions Living cells are very sensitive to changes in [H+] Sophisticated homeostatic mechanisms in the body constantly monitor and regulate pH  An acid substance releases hydrogen ions when in solution  A basic (alkaline) substance accepts hydrogen ions, often with the release of hydroxyl (OH−) ions  pH values of body fluids The pH of body fluids are generally maintained within relatively narrow limits except pH of the gastric juice Blood pH is kept between 7.35 and 7.45, and outwith this narrow range there is severe disruption of normal physiological and biochemical processes Normal metabolic activity of body cells constantly produces acids and bases, which would tend to alter the pH of the tissue fluid and blood Chemical buffers, which can reversibly bind hydrogen ions, are responsible for keeping body pH stable.  Chemical constituents of the cell Inorganic molecules : - small molecules formed by ionic bounds - Essential for the functioning of the body - Examples : CO2, mineral salts Organic molecules (carbon compounds): - Macromolecules - Structural units for organism - Bounded together covalently - Are not hydrosoluble - Examples : Carbohydrate, Amino acids and proteins, Lipids, Nucleotides, Enzymes, hormones  Inorganic compounds Water : Most abundant constituent of cell More abundant in plants ( ~ 80%)than in animals (65%) The amount of water varies from one tissue to another : liver (70%), skeleton (22%) Very essential to cellular metabolism Functions : - Natural solvant (for ion per example) - Furnisher of hydrogen bounds - Suspending medium - Elimination of certain products …  Inorganic compounds Mineral salts : - Exist in 2 forms : 1- Low or not soluble - They are locked into rigid structures (e.g : cell wall): magnesium or calcium - Example : calcium hydroxyapathite is the main mineral constituent of human bone 2- Ionized : - They dissociate into ions: cations ( Na+, K+, Ca2+, Mg2+) and anions Cl-,SO4-, CO3H-, NO3-… - The concentration of ions in the cytoplasm of cells is often constant  very specific regulatory mechanism Functions : - Involved in chemical reactions - Maintain the physical and chemical conditions necessary for carrying out chemical reactions - Essential to the maintenance of cell life  Organic compounds : Carbohydrates Carbohydrates (sugars and starches) are composed of carbon, oxygen and hydrogen When two sugar molecules combine to form a bigger sugar molecule, a water molecule is expelled and the bond formed is called a glycosidic linkage Glucose, the cells’ preferred fuel molecule, is a monosaccharide Blood glucose levels are tightly controlled  Organic compounds : Carbohydrates  Functions of sugars include: providing a ready source of energy to fuel cell metabolism providing a form of energy storage, e.g. glycogen forming an integral part of the structure of DNA and RNA acting as receptors on the cell surface, allowing the cell to recognise other molecules and cells Organic compounds : Amino acids & proteins Amino acids always contain carbon, hydrogen, oxygen and nitrogen & many in addition carry sulphur In human biochemistry, 20 amino acids are used as the principal building blocks of protein The amino acids used in human protein synthesis have a basic common structure, including an amino group (NH2), a carboxyl group (COOH) and a hydrogen atom. What makes one amino acid different from the next is a variable side chain When two amino acids join up the reaction expels a molecule of water and the resulting bond is called a peptide bond  Organic compounds : Lipids The lipids are a diverse group of substances whose common property is an inability to mix with water ( hydrophobic) They are made up mainly of carbon, hydrogen and oxygen atoms & some contain additional elements, like nitrogen or phosphorus The most important groups of lipids include: - phospholipids, integral to cell membrane structure - fats (triglycerides), stored in adipose tissue as an energy source prostaglandins are important chemicals derived from fatty acids and are involved in inflammation steroids, including important hormones produced by the gonads and adrenal glands. Cholesterol is a steroid that stabilises cell membranes and is the precursor of the hormones mentioned above, as well as being used to make bile salts for digestion  Organic compounds : Nucleotides Nucleic acids are the largest molecules in the body and are built from nucleotides. They include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) ATP is a nucleotide built from ribose (the sugar unit), adenine (the base) and three phosphate groups attached to the ribose  Organic compounds : Enzymes Enzymes are proteins that act as catalysts for biochemical reactions that is, they speed the reaction up but are not themselves changed by it Enzymes are selective and will usually catalyse specific reaction The molecule(s) entering the reaction is called the substrate and it binds to a very specific site on the enzyme, called the active site When the substrate is bound to the active site the reaction proceeds, and once it is complete the product of the reaction breaks away from the enzyme and the active site is ready for use again  Organic compounds : Enzymes Enzyme action is reduced or stopped altogether if conditions are unsuitable such as increased or decreased temperature, any change in pH. Some enzymes require the presence of a cofactor, an ion or small molecule that allows the enzyme to bind its substrate anabolic reaction : When an enzyme catalyses the combination of two or more substrates into a larger product Catabolic reaction : involve the breakdown of the substrate into smaller products, as occurs during the digestion of foods