cells

• Cellular structures provide the means to drive cellular processes
• Knowing how cellular structures facilitate specific cellular processes is fundamental to explaining how life 'works'
• Cell theory


  The cell is the smallest and most basic unit of life and that cells grow only from existing cells
• Organelles and cellular structures


  • Understanding their role is crucial to understanding how organisms and life at large work
  • How are the structures of the organelles adapted for their functions?
• Differences between cells of prokaryotes and eukaryotes


  • Nucleoid not enclosed by membrane
  • Plasmid as extra-chromosomal DNA
  • Membrane-bound organelles absent
  • Prokaryotic ribosomes different from eukaryotic
  • Some bacterial cells have peptidoglycan cell walls

  
• Differences between cells of plants and animals


  • Unicellular organisms merely undergo cellular division
  • Cells of multicellular organisms undergo both division and differentiation
• Viruses do not fit the cell model
• Cell theory


  Cells are the smallest unit of life, all cells come from pre-existing cells, and living organisms are composed of cells
• Eukaryotes


  Organisms made of cells that have a true, membrane bound nucleus and membrane bound organelles

  
• Components of a typical eukaryotic cell


  • Cell surface membrane
  • Nucleus
  • Cytoplasm (cytosol, organelles, cytoskeleton)

  
• Eukaryotic organisms


  • Animals
  • Plants
  • Fungi
  • Protoctists
• Cell surface membrane


  Defines the boundary of a cell and retains its contents

  
• Nucleus


  Contains the genetic material that directs cellular activities
• Cytoplasm


  • Aqueous solution of ions and organic compounds
  • Organelles (membrane-bound cellular structures)
  • Cytoskeleton (network of microtubules, intermediate filaments and microfilaments)
• Differences between plant and animal cells


  • Plant cells have cellulose cell wall, plastids, large central vacuole, tonoplast, starch storage, lack of centrioles, few secretions
  • Animal cells lack cell wall, have small scattered vacuoles, no tonoplast, glycogen storage, have centrioles, produce a wide variety of secretions

  
• Ribosomes


  Organelles made of ribosomal RNA and protein, sites of protein synthesis

  
• Ribosomes


  • 70S in prokaryotes, chloroplasts and mitochondria
  • 80S in eukaryotes (free in cytoplasm and bound to rough ER)
  • Have 3 binding sites for tRNA (A, P, E sites)

  
• Centrioles


  Cylindrical structures with 9 triplets of microtubules, involved in cell division

  
• Centrioles are absent in higher plants
• 1. site (peptidyl-tRNA site)


  Holds the tRNA carrying the growing polypeptide chain
• E site (exit site)


  Discharged tRNAs leave the ribosome from the E site
• Ribosomes


  • The main function is to serve as site of protein synthesis
  • Ribosomes bound to endoplasmic reticulum generally make proteins that are destined either for inclusion into membranes, for packaging within certain organelles such as lysosomes or for export from the cell
  • Ribosomes lying free in cytoplasm are sites of synthesis of proteins retained within the cell, i.e. function within cytosol, enzymes that catalyses metabolic processes localized within cytosol

  
• Centrioles


  • They measure around 0.2 m in diameter and 0.3 to 0.5 nm in length
  • From the transverse section, 9 triplets of microtubules are fused together to give a rod-like structure
  • Each centriole is positioned at 90° to each other and are situated next to nucleus
  • Only found in animal cells

  
• Rough endoplasmic reticulum (rER)


  • Cisternae are flattened and interconnected with each other and are continuous with the outer membrane of the nuclear envelope
  • Ribosomes are found on the surface, which are sites of protein synthesis; these many ribosomes give the rough / granular appearance on the rER
  • It can be broken into small pieces and resealed into vesicles known as microsomes during the homogenization procedure

  
• Smooth endoplasmic reticulum (SER)


  • The smooth E.R. lacks ribosomes
  • It consists of tubular cisternae
  • It has a different set of membrane-bound proteins from the rough endoplasmic reticulum

  
• Golgi body / apparatus


  • It consists of flattened saucer-like membrane-bound stacks called cisternae and a system of associated vesicles called Golgi vesicles
  • It consists of a cis face and a trans face
  • At the cis face, new cisternae are constantly formed by the fusion of transport vesicles which budded from the rough ER and smooth ER
  • At the trans face, the Golgi body breaks up to form vesicles such as lysosomes or transport vesicles
• Lysosomes


  • Spherical sacs of 0.2 - 0.5 µm in diameter
  • It contains hydrolytic enzymes such as proteases and lipases
  • It serves as a storage vesicle to keep the degradative enzymes apart from the rest of the cell, hence preventing them from destroying the cell
  • Its contents are acidic
  • There are 2 types: primary lysosome and secondary lysosome
• Nucleus


  • It is found in all eukaryotic cells except in mature red blood cells in mammals and in sieve tubes in plants
  • It is usually spherical or ovoid and averages about 2-10 µm in diameter
  • It has a double membrane, called the nuclear envelope

  
• DNA in the nucleus


  • The nucleus contains most of a eukaryotic cell's DNA and is organized along with proteins, known as histones into long threads called chromatin
  • Two different levels of packing of DNA molecules: Heterochromatin (more condensed, appears darker) and Euchromatin (less condensed, usually not stained)

  
• Nucleolus


  • It is a large and dense region inside the nucleus
  • It consists of a fibrous part and a granular part
  • The fibrous part contains large loops of DNA from several chromosomes containing genes from which rRNA (ribosomal RNA) is transcribed to become part of ribosomes
  • The granular part contains transcribed rRNA fragments and synthesized ribosome subunits which then migrate to the cytoplasm to assemble into ribosomes

  
• Nuclear envelope


  • It is a double membrane including an outer membrane which is continuous with the rough endoplasmic reticulum
  • The double membrane is perforated by nuclear pores (about 50 nm in diameter). Each pore consists of eight pore proteins and a 'plug' protein
• Nucleoplasm


  Similar to how the cytoplasm which fills up the cell outside the nucleus, this is a semi-fluid matrix that fills up the nucleus
• Mitochondrion


  • It has a double membrane bound structure with an outer membrane that is smooth, and an inner membrane that is folded into finger-like projections called cristae
  • The inner membrane is folded to increase the surface area for proteins and enzymes involved in the Electron transport chain (ETC) and oxidative phosphorylation processes in cellular respiration
  • The mitochondrial matrix contains circular mitochondrial DNA, ribosomes (70S), RNA and enzymes involved in the Krebs cycle of respiration

  
• Chloroplast


  • Two membranes enclose the intermembrane space
  • Inside the chloroplast is a system of flattened sacs known as the thylakoid membrane. This membranous system forms stacks called grana

  
• Alk particles


  Present on the inner membrane of mitochondria, each containing a head piece, stalk and base
• Head piece


  Contains ATP (Adenosine triphosphate) synthase for ATP synthesis
• Contents of mitochondrial matrix


  • Circular mitochondrial DNA
  • Ribosomes (70S)
  • RNA
  • Enzymes involved in the Krebs cycle of respiration
• Mitochondria


  • Site for catabolic respiratory activity within the cell, where ATP is synthesized
  • Site for lipid metabolism for cellular respiration
  • Site for lipid synthesis
• Thylakoid membrane


  Flattened sacs inside chloroplasts, forming stacks called grana with intergranal lamellae between

  
• Stroma


  Fluid surrounding the thylakoid membrane, containing chloroplast DNA, 70S ribosomes, enzymes for Calvin cycle and starch grains