Cytology

L.O:
1.Knowledge of cellular morphology and the structure/ function of cell organelles.

2.A basic understanding of the structure/ function of the plasmamembrane.

3.An awareness of microbiological factors (e.g. prokaryotes vs eukaryotes).

4.Knowledge of physiological terms:  ‘Metabolism’; ‘Catabolism’; ‘Anabolism’.

•           We need to know the structure and functions at a cellular level in order to understand tissues in health and disease; changes in function leads to diseases such as cancer

                      -The principal components of a cell are the:

• Cell membrane
• Cytoplasm
• Cytosol
• Organelles, with a distinct structure
• Inclusions - secretory and storage material
  
  
  -Intracellular matrix can be fluid or solid (when solid, referred to as 'ground substance')
  
  
  -Most cells are in the size range of 5-50 μm diameter. (Iμm, or micron, is 10-6 metres, or 1/1000mm. See Appendix A-2). Red blood cells (erythrocytes) are 7μm in diameter. Liver cells are 12-25 μm in diameter.
  
  
  
  

  THE CELL MEMBRANE

  The Fluid Mosaic model suggests selective permeability and includes functions of: protection, transportation, recognition, and transportation.
  
  There are two components to the cell membrane:
  
  
  1) Lipids (3 types):
  
  
  a) Phospholipid (75% of the lipid of the cell membrane)
  

   • Each molecule consists of:

  o A polar head of phosphate that is hydrophilic

  o Two non-polar tails of fatty acids that are hydrophobic• The molecules are organised as a bi-layer with their polar heads orientated outwards (to the watery cytosol and extracellular fluid)

  
  

  b)  Glycolipids (5%) important in cell recognition and communication
  

  
  

  c)  Cholesterol (20%) strengthens the cell membrane
  
  
  2) Protein:
  
  
  

  1. a)  Integral proteins - extend across the bilayer
  2. b)  Peripheral proteins - Only partially extend through the bilayer.

  1. 
     

  Protein functions:

  -Channels for passage of materials

  -Carrier molecules (see transmembrane transport)
  -Receptors to hormones and neurotransmitters and involved in cell recognition
  -Enzymes
  -Anchorage of the cytoskeleton
  
  
  
  
  Note that it:
  

  1. i)  Determines the shape of the cell;
  2. ii)  Segregates metabolic activities into packages which differ in function, i.e. enables cell specialisation;
  3. iii)  Acts as a boundary to entry and exit;
  4. iv)  Controls entry and exit; Transmembrane transport will be discussed later. It is SELECTIVELY permeable, not semi-permeable, because:
     • •
  5. v)  Can actively transport materials across, by a process involving energy and ATP;
  6. vi)  Has enzymes that are embedded in it. Adenylate cyclase catalyses the formation of cAMP from ATP;
  7. vii)  Determines the degree of cell adhesion to other cells and cell motility (movement);
  8. viii)  Has proteins on the surface that are important in the recognition by cells of whether or not other cells are foreign to the body. Protein in the plasma membrane of erythrocytes results in the occurrence of blood types;

  
  

  The permeability is variable and not just “semi”;
  Permeability may change by the opening and closure of pores.

  9. ix)  May have electrical properties, e.g. muscle cells, nerve cells and sense cells, due to a voltage difference across the plasma membrane. These cells are termed “excitable cells”;
  10. x)  Possesses receptor sites for chemical messengers, e.g. hormones and immnological molecules, very important in cellular communication. The number of these sites determines the responsiveness of a cell to a specific messenger. Cells can control the number and distribution of receptor sites on their surface.
      
      

  The plasma membrane is not therefore inert, but is highly active and continually changing depending upon external circumstances and the requirements of the cell. Molecules in the membrane move a around and it is capable of self repair if damaged. Materials enter and leave the cell via the plasma membrane.
  The surface of one or more faces of the cell may be folded to increase the surface area for transport through it, e.g. cells of the kidney tubule. Other cells bear a dense layer of small processes termed Microvilli on their surface, forming a Brush Border, or Striated Border, (Refer to Tortora or other textbooks), that increase the surface area for absorption, e.g. cells lining the small intestine. Note that microvilli differ from:
  

  1. a)  Cilia, which are thinner and contain less cytoplasm and are more motile;
  2. b)  Villi, present in the small intestine, which consist of numerous cells and contain blood vessels
     etc.
     
     

  CELL ORGANELLES AND INCLUSIONS
  
  Nucleus
  Most cells uninucleate. Erythrocytes lack a nucleus. Some cells, e.g. skeletal muscle cells are multinucleate.
  The nucleus is spherical or oval in shape bounded with a double membrane (the nuclear membrane) that has Nuclear pores for molecules, e.g. mRNA to pass through. The nucleus contains chromatin consisting of DNA and protein and nucleoli, the site of RNA synthesis.
  
  Ribosomes
  Consist of granules of RNA (ribonucleic acid) and are the site of protein synthesis.
  

  1. a)  Free ribosomes, present singly or in clusters, for synthesis of protein for use in the cell
  2. b)  Attached ribosomes -to the endoplasmic reticulum - for synthesis of protein for export, or incorporation into the cell membrane.
     
     

  Endoplasmic reticulum
  A double membrane., continuous with the nuclear membrane. Encloses channels or spaces -the cisternae.
  

  1. a)  Smooth (agranular)ER - does not bear ribosomes
  2. b)  Rough (Granular) ER - ribosomes present

  It provides a large surface area for chemical reactions and provides channels for intracellular transport.

  Granular ER is involved in the synthesis and storage of proteins and some conjugation o proteins with other molecules. It is well developed in secretory cells.
  Smooth ER is involved in fatty acid, phospholipid and steroid synthesis. It contains enzymes involved with detoxification
  In skeletal muscle cells calcium ions are stored within it.
  The Golgi Complex (Golgi body/apparatus)
  
  Flattened saucer-shaped sacs containing cisternae and that bud off vesicles from the periphery.
  Involved in the packaging and processing of protein (and lipid), especially for transport out of the cell, i.e. are well developed in secretory cells.
  Note the role of the Golgi Complex (Refer to Tortora or other textbooks) in packaging of material produced by the granular endoplasmic reticulum into membrane-coated vesicles. It is also important in conjugating molecules, e.g. carbohydrate with protein. The products are released from the cell by exocytosis, involving the fusion of the membranes of the vesicles, with the plasma membrane. Some products of the Golgi Complex, may be stored, or utilised by the cell, e.g. in lysosomes.
  
  Lysosomes
  Membrane-bound vesicles containing powerful hydrolytic enzymes that act at low pH.
  Note their role in:
  

  1. a)  Destruction of foreign material, e.g. bacteria. Phagocytes are amoeboid functional structures.
  2. b)  Re-modelling of tissues to ensure that their structure reflects requirements at that moment in time, e.g. of bone.
  3. c)  Re-modelling of tissues to ensure that their structure reflects requirements at that moment in time, e.g. of bone.
     
     

  Peroxisomes
  Contain enzymes involved in oxidation and destruction of organic molecules and hence important in detoxification.
  
  Mitochondria
  Round or elongate structures consisting of:
  

  1. a)  An outer smooth membrane
  2. b)  An inner membrane folded into Cristae to create a large surface area for enzymes to attach to.
  3. c)  A central cavity containing matrix in which DNA is present.

  Mitochondria can replicate.
  Important in the aerobic pathways of respiration, i.e. ATP generation. Note that anaerobic respiration can occur in their absence. Well developed in cells with high energy demands, e.g. cardiac muscle cells.
  
  The cytoskeleton
  Of filamentous proteins

  1. i)  Microfilaments - of actin
  2. ii)  Microtubules
  3. iii)  Intermediate filaments

  Support and maintain the shape of the cell Involved in cell motility, e.g. amoeboid cells Phagocytosis, pinocytosis and secretion Movement of the cytosol within the cell
  
  Flagella and cilia
  Consist of a projection of the cell containing microfilaments enabling the structures to move. Cilia are short and numerous and involved in movement over the cell surface, e.g. mucus and entrapped particles over the respiratory lining.
  Spermatozoa bear a single structure of similar organisation, bit much larger involved in movement of the whole cell.
  You need NOT know about the centrosome and centrioles.
  
  CELLULAR FUNCTIONSTransmembrane Transport (Refer to Tortora or other textbooks) 
  a) Passive ProcessesDo not require energy, i.e. ATP.
  
  i) DiffusionConsequential to the random movement of molecules or ions.
  Movement occurs along/down a concentration gradient from a high concentration to a low concentration until a point of equilibrium is reached, e.g. diffusion of oxygen from the alveoli of the lung into the lung capillaries.
  The following factors affect the rate of diffusion across a membrane:

  .Temperature, which affects the velocity of random movement of molecules;
  .The surface area available for diffusion;
  .The thickness of the membrane across which diffusion occurs;
  .The concentration gradient, i.e. the difference in concentration of the diffusing substance either side of the membrane;
  .The molecular weight of the substance, i.e. the size of the diffusing molecules.
  
  Osmosis

  If a substance in solution is separated from its solvent, or a solution of different concentration by a selectively permeable membrane, then the solvent tends to pass from the more dilute solution to the more concentrated solution by a process termed osmosis. This is really a special case of diffusion, in which the solute molecules cannot diffuse along a concentration gradient as they cannot pass through the membrane, whereas the solute molecules, being small in size are able go do so.
  The osmotic pressure is the pressure required to prevent the above. A solution of high osmotic pressure (OP) has a high concentration of solute molecules.

  Solutions can be compared with respect to osmotic pressure. Solutions of equal osmotic pressure are described as Isotonic, e.g. the cytosol of red blood cells and plasma.
  If red blood cells are placed in a more dilute solution than plasma, water enters the cells and they swell and burst. The solution is described as Hypotonic.
  If the cells were to be placed in a solution more concentrated than that of plasma, water would leave the cells and they would shrink. This solution would be described as Hypertonic to that of intracellular fluid.
  
  iii) Filtration
  Solvent and dissolved substances pass across a selectively permeable membrane consequential to the pressure difference either side, i.e. they pass from high pressure to low pressure. A common driving pressure is the hydrostatic pressure of blood, e.g. in the kidneys the hydrostatic pressure of blood forces plasma and dissolved substances into the kidney tubules. Cells and plasma proteins do not, however, pass through as they are too large to pass through the filtration membrane.
  
  iv) Facilitated diffusion
  Substances move along a concentration gradient from a high concentration to a low concentration, but the process is aided by a carrier molecule, e.g. glucose entry into intestinal cells.
  
  
  b) Active processes
  These require energy and ATP.
  
  i) Active transportThis is the movement of molecules or ions against a concentration gradient, i.e. from a low
  concentration to a high concentration.
  Energy is needed, supplied by ATP.
  A carrier molecule is involved.
  In primary active transport, the energy is used directly to transport the molecule or ion, e.g. in the sodium pump, pumping sodium out of cells.
  In secondary active transport the energy used to create a concentration gradient of one molecule or ion, and this enables the movement of another. The two may move in the same direction - Co- transport (Symport), e.g. amino acids and sodium ions absorbed into intestinal cells, or in the opposite direction - Countertransport (antiport), e.g. sodium and hydrogen ions in the kidney tubule.
  
  ii) Bulk transport
  This is movement of large particles, e.g. bacteria, or large molecules, e.g. proteins across a cell membrane.
  
  

  Endocytosis: 
  Phagocytosis:
  Pinocytosis:

  Receptor-mediated endocytosis:

  Exocytosis: