Lecture 1: Cell Biology PDF

Introduction  Cell biology is the study of cells and how they function, from the subcellular processes which keep them functioning, to the way that cells interact with other cells. Whilst molecular biology concentrates largely on the molecules of life (largely the nucleic acids and prot...

Introduction  Cell biology is the study of cells and how they function, from the subcellular processes which keep them functioning, to the way that cells interact with other cells. Whilst molecular biology concentrates largely on the molecules of life (largely the nucleic acids and proteins), cell biology concerns itself with how these molecules are used by the cell to survive, reproduce and carry out normal cell functions. In biomedical research, cell biology is used to find out more about how cells normally work, and how disturbances in this normal function can result in disease. An understanding of these processes can lead to therapies which work by targeting the abnormal function.  Common Cell Biology Techniques 1. Tissue culture : The controlled conditions in cell and tissue culture allows researchers to carry out experiments with a lower number of variables which may affect the outcome of the test. Tissue culture is a powerful tool which provides an almost limitless supply of test material for researchers to use without resorting to using whole organisms. 2. RNA Interference 3. Microscopy – the basic tool of cell biology is microscopy. Types of microscopic techniques which are used include: A. Bright-field – traditional microscopy, (uses visible light) it gives a general picture of cell function B. Electron Microscopy –( uses a focused beam of electrons) is useful in obtaining detailed information about sub-cellular structures. C. Fluorescence Microscopy – uses fluorescent materials to indicate structures in a specimen D. Immunofluorescence – (uses antibodies) E. Timelapse Microscopy (cellular processes) Imaging cells over a period of time  Cell biology and other biological sciences Due to its wide application in various branches of biological science, many new hybrid biological sciences, have sprung up. Some of them are as the follows: 1. Cytotaxonomy (Cytology and Taxonomy) 2. Cytogenetics (Cytology and Genetics) 3. Cell physiology (Cytology and Physiology) 4. Cytochemistry (Cytology and Biochemistry) 5- Ultrastructure and Molecular biology 6. Cytopathology (Cytology and Pathology) 7. Cytoecology (Cytology and Ecology). Animal cell Plant cell Plasma Membrane  Plasma membrane An external envelope surrounding the cell that separates and protects the cell from the external environment and provides a connecting system of the cell with its environment All cells are enveloped by plasma membrane that is usually about 7.5 nm thick. The membranes of prokaryotic and eukaryotic cells have common overall structure, which consist of assemblies of lipids and proteins held together by noncovalent forces.  Chemical composition 1. Lipid: phospholipid, galactolipid, cholesterol. 2. Proteins: 1. Peripheral (extrinsic): - Separated by mild treatment - soluble in aqueous Solution - usually free from lipid. 2. Integral proteins (intrinsic): - Represent more than 70%. - Separated by drastic procedure. - Insoluble in water. - Need the presence of detergents. - May be attached to other non-proteins (glycoprotein, phosphoprotein, and lipoproteins). 3. Carbohydrate: hexose - hexoseamine – fucose. 4. Enzymes about 30 enzymes..  Molecular organization of membranes About sixty years ago the plasma membrane was described as a lipid bilayer (by relatively indirect evidence). Sandwich model OR Danielli- Davson Model Proposed by Davson and Danielle in 1935 “Cell membrane is lipid bilayer sandwiched with two monolayer of globular proteins”.  Molecular organization of membranes In the early 1960s, J. David Robertson proposed a modified version of the Danielli-Davson model, which assumed that the proteins were extended rather than globular conformation on both surface of the phospholipid bilayer (by. M+ membrane function). He proposed that the uniform thickness of 7.5 nm and the consistent dark light dark staining pattern of the cellular membranes. Could be explained if each protein coat was 2nm thick and the phospholipid bilayer sandwiched in between about 3.5 nm thick. Various objections to the available models stimulated new kinds of studies using mitochondrial and chloroplast membranes as well as plasma membranes.  In the late 1960s, S. Jon Singer and Garth Nicolson proposed the fluid mosaic membrane model; according to which a mosaic of protein molecules was distributed in and on a fluid phospholipid bilayer. The fluid mosaic membrane is widely accepted today, although certain details have been changed In this model, a flexible layer made of lipid molecules is interspersed with large protein molecules that act as channels through which other molecules enter & leave the cell.  Fluid: individual phospholipids and some proteins can move sideways (laterally) in each layer-therefore FLUID extracellular fluid (outside) carbohydrate receptor protein glycoprotein Mosaic: range of recognition protein binding phospholipid protein bilayer transport site different proteins phospholipid cholesterol pore protein resting on the surface or through the phospholipid layer gives it a mosaic appearance protein filaments cytosol (inside)  Fluid mosaic membrane model The lipid bilayer exists in a relatively fluid state, with the consistency of light oil, and that lipid in each monolayer can move laterally within the plane of the membrane. Various protein molecules are distributed on each surface of the lipid bilayer, and other proteins penetrate the thickness of the bilayer but protrude from one or both of its surfaces. The surface proteins are peripheral and the proteins that extend into or through the bilayer are integral.  Singer and Nicolson proposed that the predominant forces responsible for membrane structure are hydrophilic and hydrophobic interactions. Because membrane lipids and many membrane proteins are amphipathic, and thus contain hydrophilic and hydrophobic regions in the same molecular. Phospholipid are amphipathic having both hydrophobic fatty acid chains that repel water and polar heads that are hydrophilic and so attract water. Hydrophobic interaction between the fatty acid chains hold the phospholipid bilayer together Plasma membrane function A) Steroid hormones, such as estrogen and progesterone, probably because they are lipid soluble and thus able to penetrate the central bilayer of the membrane easily, readily cross the plasma membrane and enter the cell. B) Glycoprotein hormones, on the other hand (e.g. Insulin is a protein composed of two chains), bind to the surface of their target cells and, without penetrating the plasma membrane, exert their influence from there. The stimulation of a target cell by most non-steroid hormones therefore depends upon the presence of receptors on the outer cell surface. These receptors are integral plasma membrane components and can be identified by their ability to bind specifically to a particular hormone molecule.  A target cell that is stimulated by a variety of different hormones (for example, fat cells respond to at least nine different hormones) will bear several populations of surface receptors, each specific for one hormone. As a general rule, there are about 10000 receptor molecules for each hormone on each cell, and stimulation occurs when a sufficient number of them become occupied. The binding between the hormone and its receptor is concentration-dependent and appears to be reversible, so that, when the concentration of hormone in the environment around the target cell decreases, the bound hormone molecules probably dissociate from their receptors.  2-The plasma membrane and transport The transport mechanisms that exist in the plasma membrane include both micro- transfer and macro-transfer processes. 1- Micro-transfer Includes the transport of ions and small molecular species such as amino acids and simple sugars; it is carried out by both passive and active mechanisms A) Passive and facilitated transport When uncharged substances (i.e. non-electrolytes) move across the plasma membrane, they tend to flow down the concentration gradient at a rate determined by their relative concentration on each side of the membrane (‘diffusion’).  For charged substances, however, the electrical concentration needs to be considered in addition to the concentration gradient, because if charged substances move across the plasma membrane they will also generate a potential difference. The transport of different ions across the plasma membrane, it has been predicted that hydrophilic channels or pores allowing the transport of water and ions through the plasma membrane must exist. Certain molecules, such as glucose and glycerol, cross the plasma membrane many times faster than can be accounted for by passive diffusion alone. Their transport is thus said to be ‘facilitated’ or ‘catalysed’ and it is presumed to be regulated by specific catalytic components in the membrane.  For example, Ventilation Oxygen is a molecule that can freely diffuse across a cell membrane. Oxygen diffuses out of the air sacs in your lungs into your bloodstream because oxygen is more concentrated in your lungs than in your blood. Oxygen moves from the high concentration of oxygen in your lungs to the low concentration of oxygen in your bloodstream. Carbon dioxide, which is exhaled, moves in the opposite direction - from a high concentration in your bloodstream to a low concentration in your lungs. Facilitated Transport 04_08 B) Active transport Also present in the plasma membrane are transport systems that require energy expenditure by the cell. These systems can operate against the concentration gradient and are thus able to achieve a several-fold increase in concentration on one side of the membrane. The best-documented active transport mechanism carries sodium ions (Na+) out of the cell and accumulates potassium ions (K+) within it. In this coupled pumping mechanism (Sodium-potassium pump), a membrane-bound magnesium-activated adenosine triphosphatase (ATPase) hydrolyses to adenosine diphosphate or (ADP) to provide energy for every three Na+ and two K+ ions transported. Active Transport 1 The transport protein 2 Energy from ATP 3 The protein releases binds both ATP and Ca2+. Changes the shape of the ion and the the transport protein remnants and moves the ion of ATP (ADP and P) and across the membrane. closes. (extracellular fluid) ATP ADP ATP Ca2+ (cytosol) P recognition ATP site binding site Active Transport  Most models of active transport postulate that ‘carriers’ translocate the ion (or amino acid, sugar, etc.) from one membrane surface to the other and in so doing lose their energy. It is now regarded more likely, therefore, that ‘carriers’ for active transport are integral transmembrane components. Some examples of active transport in plants include: Ions moving from soil into plant roots Transportation of chloride and nitrate from the cytosol to the vacuole Sugars from photosynthesis moving from leaves to fruit Calcium using energy from ATP to move between cells Minerals traveling through a stem to various parts of the plant Water moving from plant roots to other plant cells via root pressure  2- Macro-transfer processes The transport of macromolecules across the plasma membrane comes under the general heading of cytosis; endocytosis describes movement into the cell, and exocytosis movement into the extracellular space ❖ Exocytosis---Cellular secretion ❖ Endocytosis— ❖ Phagocytosis— “Cell eating” ❖ Pinocytosis– “Cell drinking” ❖ Receptor-mediated endocytosis- specific particles, recognition. A) Endocytosis Endocytosis includes phagocytosis, the process whereby macrophages engulf and remove cellular and non-cellular debris from the body Pinocytosis and micropinocytosis also come under the heading of endocytosis. The mechanisms involved in these processes are similar to those of phagocytosis but they are primarily concerned with the uptake of soluble materials dissolved in tissue fluids In fluid endocytosis the concentration of the substance within the enclosed vesicle is the same as that in the extracellular environment. Endocytosis Phagocytosis is one example of endocytosis. (extracellular fluid) food particle pseudopods food vacuole (cytosol) Phagocytosis Phagocytosis 1 Phagocytosis 2 Pinocytosis Receptor-mediated Endocytosis 04_12c  B) Exocytosis They are the mechanism whereby most exocrine and many endocrine secretory cells release their secretory products, nerve fibers release their stores of acetylcholine, and osteoclasts (which can degrade bone matrix extracellularly) secrete lysosomal enzymes. All of these processes require the transported material to be packaged in the cytoplasm within a membrane-limited vesicle, such as a secretory granule or a lysosome. A demand for the release of content (e.g. a stimulation to secrete) induces the vesicle to move to a specific region of the cell boundary (in exocrine cells it is usually the apical membrane) where the vesicle membrane fuses with the plasma membrane.  A consequence of exocytosis is the addition of intracellular membrane to the plasma membrane; when this occurs repeatedly and becomes a significant contribution. secreted plasma material membrane (extracellular fluid) vesicle (cytosol) 0.2 micrometer Material is enclosed in a vesicle that fuses with the plasma membrane, allowing its contents to diffuse out.  3-The plasma membrane and Malignancy Malignant tumors arise in the body spontaneously but they may also be induced by radiation, chemical agents and virus infection. The most a fundamental abnormality of malignant cells is that they are neoplastic, i.e. they divide without stimulus and are not subject to the normal influences of growth control. As malignant tumour cells proliferate they generally display invasive behavior, penetrating the surrounding tissues. When they reach circulatory systems or body cavities, groups of cells may detach from the tumor and become transported throughout the body. This process of cell dissemination is known as metastasis and it leads to the formation of multiple secondary growths. Thank you