Cell Biology: Structure & Function Lecture Notes PDF

CELL BIOLOGY: STRUCTURE & FUNCTION DEPARTMENT OF MEDICAL BIOCHEMISTRY 1 1  CELL DEFINITION AND THEORY  Cell is the structural and functional unit of which organisms are composed. The major parts of the cell are nucleus and cytoplasm.  THE CELL THEORY  In bi...

CELL BIOLOGY: STRUCTURE & FUNCTION DEPARTMENT OF MEDICAL BIOCHEMISTRY 1 1  CELL DEFINITION AND THEORY  Cell is the structural and functional unit of which organisms are composed. The major parts of the cell are nucleus and cytoplasm.  THE CELL THEORY  In biology, cell theory is the historic scientific theory, now universally accepted that living organisms are made up of cells.  Cells are the basic unit of structure in all organisms and also the basic unit of reproduction.  Notable scientists that contributed to cell theory are:-Robert Hooke, Mathias Scleiden, Theodor schwann and Rudolf Virchow.  The seven versions of the cell theory  All living organisms are composed of one or more cells  The cell is the basic unit of structure and organization of organisms  Cells arise from pre-existing cells  The activity of an organism depends on the total activity of independent cells 2 2  CELL DEFINITION AND THEORY  Energy flow (metabolism and biochemistry ) occurs within cells  Cells contain DNA which is found in the cell nucleus and cytoplasm  All cells are basically the same in chemical composition in organisms of similar species.  CLASSIFICATION OF CELLS  The electron microscope allowed the classification of cells into:- (a) Eukaryotes (b) Prokaryotes  EUKARYOTES  Eukaryotes have nucleus which is covered by nuclear membrane. (Greek: Eue = true, karyon = nucleus). Animals, plants and fungi belong to the eukaryotes.  Eukaryotic cells are much larger than prokaryotes. 3 3  Unlike prokaryotes, eukaryotes have a variety of other membrane-bound organelles (subcellular elements) in their cytoplasm, including:  Mitochondria  Lysosomes  Endoplasmic reticulum and  Golgi complexes.  PROKARYOTES  Prokaryotes have no typical nucleus and subcellular components. (Greek: Pro = before). Bacteria and blue green algae belong to the prokaryotes.  They are relatively small cells surrounded by the plasma membrane with a characteristic cell wall that may differ in composition depending on the particular organism. 4 4  STRUCTURE AND FUNCTIONS OF A CELL AND ITS SUBCELLULAR COMPONENTS  A cell has three major components: (a) Cell membrane (Plasma membrane) (b) Cytoplasm with its organelles (c) Nucleus.  THE CELL MEMBRANE  The cell is enveloped by a thin membrane called cell membrane or plasma membrane.  The plasma membrane defines the periphery of the cell separating the cell contents from the surrounding environment.  It is composed of a lipid bilayer and protein embedded in it.  It has a highly selective permeability properties so that the entry and exit of compounds are regulated e.g membrane transport proteins regulate the influx and efflux of ions and molecules across the membrane.  However, the membrane is also a barrier to the free passage of inorganic ions and most other charged or polar compounds.  Receptor proteins on the membrane also transmits signals into the cell. 5 5 A digrammatic representation of a typical eukaryotic cell 6 6  FLUID MOSAIC MODEL OF CELL MEMBRANE  The membrane structure has been described as a fluid mosaic model and proposed by Nicolson and Singer in 1972 with hydrophilic exterior and hydrophobic core.  A mosaic is a structure made up of many different parts likewise the plasma membrane is composed of different kinds of macromolecules like phospholipid, integral proteins, peripheral proteins, glycoproteins, glycolipids and cholesterol.  According to this model, the bilayer is fluid, because the hydrophobic tails of phospholipids consist of an appropriate mixture of saturated and unsaturated fatty acids that is fluid at normal temperature of the cell.  Proteins are interspersed in the lipid bilayer of the plasma membrane 7 7  THE FLUID MOSAIC MODEL OF CELL MEMBRANE A diagrammatic representation of the fluid mosaic model of cell membrane 8 8  The peripheral proteins exist on the surface of the bilayer and they are attached by ionic and polar bonds to polar heads of the lipid.  The integral proteins are deeply embedded in the bilayer and are attached by hydrophobic bonds or Van der Waals forces.  The integral membrane proteins span the whole bilayer and they are called transmembrane proteins.  The cholesterol content of the membrane alters the fluidity of the membrane, when cholesterol concentration increases the membrane becomes less fluid on the outer surface, but more fluid in the hydrophobic core.  The phospholipids are arranged in bilayer with the polar head groups oriented towards the extracellular side and the cytoplasmic side with a hydrophobic core.  The distribution of the phospholipids is such that choline containing phospholipids are mainly in the external layer and ethanolamine and serine containing phospholipids are in the inner layer. 9 9 The lipid bilayer shows free lateral movement of its component hence, the membrane is said to be fluid in nature. However, the components do not freely move from inner to outer layer or outer to inner layer as flip flop movement is restricted. The basic organization of biological membrane 10 10  CYTOPLASM AND ITS ORGANELLES  Cytoplasm is the internal volume bounded by the plasma membrane. The clear fluid portion of the cytoplasm in which the particles are suspended is called cytosol.  Six important organelles that are suspended in the cytoplasm are:  Endoplasmic reticulum  Golgi apparatus  Lysosomes  Peroxisomes  Mitochondria  Nucleus.  ENDOPLASMIC RETICULUM  This is a network of interconnecting membranes enclosing channels or cisternae.  They are continuous from outer nuclear envelope to outer plasma membrane. 11 11  There are smooth and rough ER.  The Smooth ER is where complex lipid and steroid are synthesized whereas the rough ER appears so because of the attachment of ribosomes, where proteins are synthesized. Drug detoxification also occurs in the ER.  NUCLEUS  This is the most important organelle.  All mammalian cells contain a nucleus except the mature RBC in circulation.  The nucleus is where the DNA, the genetic material of the cell is stored  The DNA consists of the complete set of gene (genome) of the organism in the eukaryote.  The nucleus governs the cell hence it contains the DNA which is the chemical basis of the genes. This genetic material, i.e DNA has long molecules which complex with histone protein to form chromatin which are further organised into chromosomes.  A lighter portion of the nucleus called NUCLEOLUS is where RNA synthesis (transcription) a step in protein synthesis takes place. It is worthy of note that DNA REPLICATION takes place in the nucleus. 12 12  MITOCHONDRIA  This is a spherical oval or rod-shaped body in the cytoplasm.  It is the power house of the cell where energy released from oxidation of food stuffs is stored as ATP for the use of the cell in other metabolic processes.  It has two membranes  The inner membrane folded in cisternae contains the enzymes of the electron transport chain (ETC) where energy is trapped.  The cytochrome P450 in the mitochondria is involved in steroid synthesis.  RBCs do not contain mitochondria and the spermatozoal tails is loaded with it.  Mammalian cells have specific DNA called mitochondrial DNA as distinct from the DNA in the nucleus.  LYSOSOMES  These are tiny organelles where foreign particles are digested.  They are comparable to incinerators where waste are burnt.  Enzymes of the lysosomes when released by the distruption of the membrane can cause tissue damage. 13 13  They help in the destruction of bacteria and other foreign bodies also, removal of excessive secretory products in the cells of the glands.  GOLGI COMPLEX  Golgi complex or golgi apparatus  It is a network of flat, smooth membrane and vesicles.  Its main function is sorting, packaging, maturation and release of protein already synthesized.  Golgi apparatus is present in all cells except in RBCs  PEROXISOME  They contain enzymes such as peroxidases and catalases which are concerned with the metabolism of peroxides.  They are also capable of carrying out β-oxidation of fatty acid. 14 14  THE CYTOSKELETON  The cytoplasm of most eukaryotic cells contains network of protein filaments, that interact extensively with each other and with the component of the plasma membrane. Such an extensive intracellular network of protein has been called cytoskeleton.  The plasma membrane is anchored to the cytoskeleton. The cytoskeleton is not a rigid permanent framework of the cell but is a dynamic, changing structure.  The cytoskeleton consists of three primary protein filaments:  Microfilaments  Microtubules  Intermediate filaments.  THE MICROFILAMENTS  Microfilaments are about 5 nm in diameter. They are made up of protein actin. Actin filaments form a meshwork just underlying the plasma membrane of cells and are referred to as cell cortex, which is labile. They disappear as cell motility increases or upon malignant transformation of cells. The function of microfilaments are as follows: 15 15  To help muscle contraction  To maintain the shape of the cell  To help cellular movement  MICROTUBULES  Microtubules are cylindrical tubes, 20 to 25 nm in diameter. They are made up of protein tubulin.  Microtubules are necessary for the formation and function of mitotic spindle.  They provide stability to the cell.  They prevent tubules of ER from collapsing.  These are the major components of axons and dendrites.  INTERMEDIATE FILAMENTS  Intermediate filaments are so called as their diameter (10 nm) is intermediate between that of microfilaments (5 nm) and of microtubules (25 nm).  Intermediate filaments are formed from fibrous protein which varies with different tissue type.  They play role in cell-to-cell attachment and help to stabilize the epithelium.  They provide strength and rigidity to axons. 16 16  FUNCTIONS OF CYTOSKELETON  The cytoskeleton gives cells their characteristic shape and form, provides attachment points for organelles, fixing their location in cells and also makes communication between parts of the cell possible.  It is also responsible for the separation of chromosomes during cell division.  The internal movement of the cell organelles as well as cell locomotion and muscle fiber contraction could not take place without the cytoskeleton.  It acts as “track” on which cells can move organelles, chromosomes and other things. 17 17  MEMBRANE TRANSPORT  One of the functions of the plasma membrane is to regulate the passage of a variety of small molecules across it.  Biological membranes are semi-permeable membranes through which certain molecules freely diffuse across membranes but the movement of the others is restricted because of size, charge or solubility.  The permeability of substances across the cell membrane is dependent on their solubility in lipids and not on their molecular size  Water soluble compounds are generally impermeable and require carrier mediated transport.  An important function of the membrane is to withhold unwanted molecules while permitting the entry of molecules necessary for cellular metabolism.  Transport mechanisms are classified into two:-  Passive transport  Active transport 18 18 19 19  PASSIVE TRANSPORT  Passive transport is the process by which molecules move across a membrane without energy (ATP).  The direction of passive transport is always from a region of higher concentration to one of lower concentration.  There are two types of passive transport as follows: 1. Simple diffusion 2. Facilitated diffusion.  SIMPLE DIFFUSION  Lipid soluble, i.e. lipophilic molecules can pass through cell membrane, without any interaction with carrier proteins in the membrane. Such molecules will pass through membrane along the concentration gradient, i.e. from a region of higher concentration to one of lower concentration. This process is called simple diffusion. 20 20  FACILITATED DIFFUSION  The movement of water soluble molecules and ions across the membrane requires specific transport system. They pass through specific carrier proteins. A carrier protein binds to a specific molecule on one side of the membrane and releases it on the other side. This type of crossing the membrane is called facilitated diffusion or carrier-mediated diffusion.  An example of facilitated diffusion is the movement of glucose and most of the amino acids across the plasma membrane.  These diffusion processes are not coupled to the movement of other ions, they are known as uniport transport processes  ACTIVE TRANSPORT  If a molecule moves against a concentration gradient, an external energy source is required; this movement is referred to as active transport.  Substances that are actively transported through cell membrane include, Na+, K+, Ca++, H+, CI–, several different sugars and most of the amino acids. 21 21  Active transport is classified into two types according to the source of energy used as follows : i. Primary active transport ii. Secondary active transport.  In both instances, transport depends on the carrier proteins; like facilitated diffusion. However, in active transport, the carrier proteins function differently from the carrier in facilitated diffusion.  Carrier protein for active transport is capable of transporting substance against the concentration gradient.  PRIMARY ACTIVE TRANSPORT  In primary active transport, the energy is derived directly from hydrolysis of ATP.  Sodium, potassium, calcium, hydrogen and chloride ions are transported by primary active transport. 22 22  Primary active transport of Na+ and K+ (sodium-potassium pump)  Na+-K+ Pump, a primary active transport process that pumps Na+ ions out of the cell and at the same time pumps K+ ions from outside to the inside generating an electrochemical gradient.  Carrier protein of Na+-K+ pump has three receptor sites for binding sodium ions on the inside of the cell and two receptor sites for potassium ions on the outside.  The inside portion of this protein has ATPase activity  The pump is called Na+-K+ ATPase because the hydrolysis of ATP occurs only when three Na+ ions bind on the inside and two K+ ions bind on the outside of the carrier proteins. The energy liberated by the hydrolysis of ATP leads to conformational change in the carrier protein molecule, extruding the three Na+ ions to the outside and the two K+ ions to the inside.  Physiological importance of Na+-K+ pump  The active transport of Na+ and K+ is of great physiological significance. The Na+- K+ gradient created by this pump in the cells, controls cell volume.  It carries the active transport of sugars and amino acids. 23 23 MECHANISM OF SODIUM POTASSIUM PUMP Mechanism of sodium potassium pump (primary active transport) 24 24  SECONDARY ACTIVE TRANSPORT  Secondary active transport uses an energy generated by an electrochemical gradient. It is not directly coupled with hydrolysis of ATP.  Secondary active transport is classified into two types:  Co-transport or symport, in which both substances move simultaneously across the membrane in the same direction e.g. transport of Na+ and glucose to the intestinal mucosal cells from the gut.  Counter transport or antiport, in which both substances move simultaneously in opposite direction e.g. transport of Na+ and H+ occurs in the renal proximal tubules and exchange of Cl- and HCO3 - in the erythrocytes. 25 25 Uniport, symport and antiport transport of substances across the cell membrane 26 26  TRANSPORT OF MACROMOLECULES ACROSS THE PLASMA MEMBRANE  The process by which cells take up large molecules is called endocytosis and the process by which cells release large molecules from the cells to the outside is called exocytosis.  ENDOCYTOSIS  There are two types of endocytosis: Pinocytosis (cellular drinking) Phagocytosis (cellular eating).  PINOCYTOSIS  Pinocytosis is the cellular uptake of fluid and fluid contents and is a cellular drinking process.  Pinocytosis is the only process by which most macromolecules, such as most proteins, polysaccharides and polynucleotides can enter cells.  These molecules first attach to specific receptors on the surface of the membrane. 27 27  The receptors are generally concentrated in small pits on the outer surface of the cell membrane. These receptors are coated on the cytoplasmic side with a fibrillar protein called calthrin and contractile filaments of actin and myosin.  Once the macromolecules (which is to be absorbed) have bound with the receptors, the entire pit invaginates inward, and the fibrillar protein by surrounding the invaginating pit causes it to close over the attached macromolecule along with a small amount of extracellular fluid.  Then immediately, the invaginated portion of the membrane breaks away from the surface of the cell forming endocyte vesicle inside the cytoplasm of the cell.  PHAGOCYTOSIS  Phagocytosis involves the ingestion of large particles such as viruses, bacteria, cells, tissue debris or a dead cell.  It occurs only in specialized cells such as macrophages and some of the white blood cells.  Phagocytosis occurs in much the same way as pinocytosis. 28 28 Three stages in the absorption of Stages in exocytosis macromolecules by endocytosis 29 29  CELL FRACTIONATION  To obtain purified preparations of organelles, the tissue is first carefully broken up in a homogenizing apparatus using isotonic 0.25 M sucrose solution.  Sucrose solution is used because it is not metabolized in most tissues and it does not pass through membranes readily and thus, does not cause inter organelles to swell.  Then homogenate is centrifuged at a series of increasing centrifugal force.  The subcellular organelles, which differ in size and specific gravity, sediment at different rates and can be isolated from homogenate by differential centrifugation.  The dense nuclei are sedimented first, followed by the mitochondria, and finally the microsomal fraction at the highest forces. After all the particulate matter has been removed, the soluble remnant is the cytosol.  Organelles of similar sedimentation coefficient obviously cannot be separated by differential centrifugation.  For example, mitochondria isolated in this way are contaminated with lysosome and peroxisomes.  These may be separated by isopycnic centrifugation technique. 30 30 Subcellular fractionation of cell by differential centrifugation 31 31  MARKER ENZYMES  The purity of isolated subcellular fraction is assessed by the analysis of marker enzymes.  Marker enzymes are the enzymes that are located exclusively in a particular fraction and thus become characteristic of that fraction.  Analysis of marker enzymes confirms the identity of the isolated fraction and indicates the degree of contamination with other organelles.  For example, isolated mitochondria have a high specific activity of cytochrome oxidase but low catalase and acid phosphatase, the catalase and acid phosphatase activities being due to contamination with peroxisomes and lysosomes respectively. 32 32 S/N FRACTIONS/ ENZYMES ORGANELLE 1 Plasma membrane 5 Nucleotidase, Na-K+ ATPase 2 Nucleus DNA Polymerase, RNA Polymerase 3 Endoplasmic Reticulum Glucose-6-Phosphatase 4 Golgi bodies Galactosyl Transferase 5 Lysosome Acid phosphatase 6 Mitochondria Succinate dehydrogenase, Cytochrome C oxidase 7 Peroxisome Catalase 8 Cytosol Lactate dehydrogenase, Glucose-6-phosphate dehydrogenase 33 33