Lecture 9 cell structure iv -minus_movie
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  • 1. Lecture 9 Cell structure IV And Introduction to Metabolism LIF101 31-1-2014 Friday Dr. Jonaki Sen
  • 2. Eukaryotic Cell wall Many single-celled eukaryotic cells have a cell wall that is a structural component that wraps around the cell membrane. The cell wall protects as well as provides physical support. It is porous thus allowing water and solutes to reach the plasma membrane. Plant cells, fungi and protistans have cell walls.
  • 3. Primary cell wall: Young plant cells in actively growing regions secrete sticky polysacchardies like pectin, glycoproteins and cellulose. The cellulose makes rope-like strands embedded in the gluey matrix. The primary wall makes the cells sticky and are thin so they allow expansion of the cell surface area. Secondary cell wall: As many plant cells mature they start to secrete material on the inner surface of the primary wall. These substances make a rigid secondary wall which reinforces cell shape. 25% of the secondary wall in woody plants is Lignin. This makes cells stronger, waterproof and resistant to insects.
  • 4. Extracellular matrix Animal cells have no cell wall, however, many are embedded in a matrix of cell secretions called the ECM or extracellular matrix. Cartilage cells are embedded in matrix of collagen or elastin fibers embedded in a ground substance of modified polysaccharides In the bone individual bone cells are widely separated from each other and there is an extensive matrix in between.
  • 5. Intercellular Junctions Neighboring cells of an animal or plant often adhere, interact, and communicate through special patches of direct physical contact. The plant cell walls are perforated with channels called plasmodesmata (singular, plasmodesma; from the Greek desmos, to bind). Cytosol passes through the plasmodesmata and connects the chemical environments of adjacent cells. Water and small solutes can pass freely from cell to cell, and recent experiments have shown that in certain circumstances, specific proteins and RNA molecules can also do this. The macromolecules transported to neighboring cells seem to reach the plasmodesmata by moving along fibers of the cytoskeleton. Plasmodesmata
  • 6. Animal cell junctions Tight junctions: Link the cells of epithelial tissues lining the body’s outer surface and inner cavities. They seal adjoining cells such that water soluble substances cannot leak between them. Adhering junctions: Join cells in tissues subject to stretching such as skin, heart etc. Gap junctions: Link the cytoplasm of neighboring cells. They are open channels for the rapid flow of signals of and substances.
  • 7. Prokaryotic cells: Bacteria Bacteria are the smallest cell only 1 micron in diameter and a few micrometers long. Bacteria have a cell wall that is semi rigid and imparts shape to the cell. The cell wall is permeable therefore it allows dissolved substances access to the plasma membrane. There are sticky polysaccharides on the cell wall which help protect the bacteria as well as help it attach to different surfaces. Bacterial cells also have a plasma membrane that has transporters, channels and receptors for. Siganls and substances.
  • 8. Bacterial cells are small enough that they do not need to have a cytoskeleton. Their cytoplasm is continuous with a region of irregularly shaped DNA called the nucleoid which does not have a membrane surrounding it. There is single circular molecule of DNA which is the bacterial genome. They have many ribosomes on which polypeptide chains are assembled.
  • 9. Extending from the surface of bacterial cells are one or more threadlike motile structures called “bacterial flagella”. They are not like the eukaryotic falgella as they do not have the 9+2 arrangement of microtubules. The function in however similar as it helps a bacterial cell move rapidly in fluid. There are numerous shorter surface projections called pili (singular pilus). These are many protein filaments that help bacteria attach to surfaces or to each other. Flagella Pili
  • 10. Differences in eukaryotic and prokaryotic cells Nuclear envelope Absent Present Membrane-enclosed organelles Absent Present Peptidoglycan in cell wall Present Absent RNA polymerase One kind Several kinds Histones associated with DNA Absent Present Circular chromosome Present Absent Cytoskeleton Absent Present Prokaryotes Eukaryotes
  • 11. The endosymbiotic origin of mitochondria and plastids The theory of endosymbiosis proposes that mitochondria and plastids were formerly small prokaryotes living within larger cells. The proposed ancestors of mitochondria were aerobic heterotrophic prokaryotes that became endosymbionts; the proposed ancestors of plastids were photosynthetic prokaryotes that became endosymbionts. The evidence supporting an endosymbiotic origin of plastids and mitochondria is overwhelming. 1) The inner membranes of both organelles have enzymes and transport systems that are homologous to those found in the plasma membranes of living prokaryotes. 2) Mitochondria and plastids replicate by a splitting process reminiscent of binary fission in certain prokaryotes. 3) Each organelle contains a single, circular DNA molecule that, like the chromosomes of bacteria, is not associated with histones or other proteins. 4) These organelles contain the transfer RNAs, ribosomes, and other molecules needed to transcribe and translate their DNA into proteins. 5) In terms of size, nucleotide sequence, and sensitivity to certain antibiotics, the ribosomes of mitochondria and plastids are more similar to prokaryotic ribosomes than they are to the cytoplasmic ribosomes of eukaryotic cells.
  • 12. Metabolism
  • 13. Defining Energy Potential Energy : The capacity to do work simply owing to the objects position in space or the arrangements of its parts Kinetic Energy : The energy of motion
  • 14. In the skeletal muscles the ATP gives up some of its potential energy to move the muscle. The transfer of energy from ATP also results in another form of kinetic energy “heat” or thermal energy The potential energy of molecules = Chemical energy 1 kilocalorie or 1000 calories is the amount of energy taken to heat 1000 grams of water from 14.5°C to 15.5°C at standard pressure.
  • 15. What do cells do with Energy? Any organism whether single celled or multicellular has the capacity to obtain energy from its environment. Some obtain it from the sun others extract it from organic and inorganic substances in their surroundings. Within the cell energy is coupled to multiple energy requiring processes: 1) Chemical work- to store, build, rearrange and break apart substances. 2) Mechanical work- to move the flagella and other cell structures or to move the whole cell or parts of it. 3) Electrochemical work- to move charged substances into or out of the cytoplasm.
  • 16. How much energy is available? Energy cannot be created from nothing it must be obtained from somewhere First law of thermodynamics: Energy cannot be created or destroyed. The total amount of energy in the universe remains constant The universe only has a certain amount of energy distributed in various forms. One form of energy is converted to another but energy cannot vanish or be created out of nothing.
  • 17. One way flow of energy Energy available for conversion in the cell is mostly present in covalent bonds. Glucose, Glycogen, Starch and fatty acids CO2 and H2O High chemical energy in their bonds Low chemical energy in their bonds
  • 18. When Glucose is converted to carbon dioxide and water some of the energy is lost as heat. This is very “low quality” energy that cells cannot use for doing work. Bad news: The amount of “low quality” energy is increasing in the universe No energy transfer process is 100 % efficient so some energy is always lost as heat. Thus the total amount of energy in the universe is flowing from sources rich in energy to those that have less and less of it (second law of thermodynamics).
  • 19. Entropy: Measure of disorder in the system The ultimate destination of everything in the universe is a state of maximum entropy. The tendency of entropy to increase is the second law of thermodynamics The second law of thermodynamics does apply to life on earth. The primary source of energy for life on earth is the sun which has been losing energy since it was formed. Plants capture energy from sunlight and convert it to other forms and then lose it to other organisms that feed directly or indirectly on plants. At each step some energy is lost as heat. Overall energy still flows in one direction.
  • 20. Doing cellular work When cells convert one form of energy to another there is a change in the amount of potential energy available to them. The greater the initial amount of potential energy the larger the energy change and more work will be done.
  • 21. Energy In , Energy Out Cells store energy in chemical bonds when bonds are formed. When the bonds are broken this energy is released. Both processes change molecules and such processes are called reactions
  • 22. Endergonic reaction Exergonic reaction