hL2 - cell structure and organelles

• what is an example of a prokaryotic cell?

  Bacteria
• describe the structure of a prokaryotic cell:

  • cytoplasm contains the genome (DNA), proteins, ribosomes, amino acids, carbohydrates...
  • mebrane enclosed, sometimes with an outer cell wall and another membrane
  • inner part is not divided into compartments
  • many bacteria are important for body function
  • many bacteria are also involved in diseases
  • dental plaque bacteria produces acids that can dissolve the enamel : caries
• what are gram-positive bacteria:

  • single membrane
  • thick cell wall made of cross-linked peptidoglycan
  • retain the violet dye used in gram staining
• what is gram-negative bacteria?

  Bacteria with a cell wall that has a thin layer of peptidoglycan surrounded by an outer membrane containing lipopolysaccharides.
• defiene organelles:


  structures of various shapes and sizes with specialized functions in the cells
• what is the function of a nucleus?

  • control centre for cellular activities
  • contains genetic info stored on 2 meters of DNA, in man divided into 23 pairs of chromosomes to direct the synthesis of about 20,000 different proteins.
  • controls which proteins are synthesized and in what amounts
  • cells lacking a nucleus disintegrate within months and cannot repair themselves.
• describe the structure of a nucleus:

  surrounded by a double membrane and a nuclear envelope which contains nuclear pores.
• what is the function of a ribosome?

  Protein synthesis

  • consist of proteins and RNA called ribosomal rRNA
  • either freely swimming in cytoplasm of bound to membranes
  • 30nm in size
  • info transcribed from DNA to mRNA moves to the ribosomes where it is translated to proteins consisting of amino acids that are moved to the ribosome coupled to transfer RNA (tRNA)
• what is the function of the rough endoplasmic reticulum?

  • has ribosomes on the surface which synthesise proteins and are released into the rer lumen
  • proteins are modified in the lumen
  • folded proteins are transported through the er to the golgi apparatus.
• what is the function of the smooth endoplasmic reticulum?

  • no ribosomes attached
  • synthesize lipids, carbohydrates and some hormones
  • detoxification of cell from metabolic waste, toxins, drugs, and alcohol.
• what is the function of the golgi apparatus?

  • protein modification and packaging
  • distribution centre of the cell
  • packaged into vesicles and released into the cytosol, other organelles or the extracellular space.
• what is the function of the mitochondria?

  • powerhouse of the cell
  • produces lots of ATP (Adenosine TriPhosphate)
  • often found in close proximity to structures that require a lot of energy
• describe the structure of the mitochondria:

  • membrane double - layer
  • inner membrane surface is enlarged by infolding forming cristae
  • short DNA that contains info of a few enzymes required for energy conversion
• what is the function of lysosomes and peroxisomes?

  • cleaning crew of the cell
  • 0.1 - 1.2 micrometer membrane enclosed vesicles contain digestive enzymes within an acidic environment
  • enzymes break down biomolecules, worn-out cell structures, and many foreign substances.
  • small molecules are recycled into the cytoplasm
  • peroxisomes specialize in the degradation of selected biomolecules by oxidative reactions.
• what is the function of the cytoskeleton?

  Structural support and cell movement.

  • helps cells maintain their structure
  • mechanical support

  consists of 3 independent meshwork:

  1. microfilaments - formed by actin
  2. intermediate filaments - formed by various specific proteins
  3. microtubules - made from tubulins, are a key part of the centrosome guiding cell division.
• what is the function of the cell membrane?

  • provides a flexible barrier separating the inside of the cell from the outside environment.
  • contains cell cytoplasm and organelles
  • outside is in contact with the extracellular space and/or linked with the extracellular matrix
  • basic structure is a phospholipid bilayer containing membrane proteins
  • membrane proteins carry out most of the specialized functions of the cell membranes, especially communication and transport.
• describe the phospholipid bilayer:

  • phospholipid backbone
  • hydrophobic tail, hydrocarbon chains try to shields away from the water by hiding in the centre of the membrane.
  • hydrophobic head, headgroups are in contact with water forming the outside of the bilayer.
• what is the function of the junctions in the cell membrane?

  serve to connect and join two cells together
• what is the function of the enzymes in the cell membrane?

  fixing to membranes localizes metabolic pathways.
• what is the function of the transport in the cell membrane?

  responsible for facilitated diffusion and active transport
• what is the importance of the cell-cell recognition in the cell membrane?

  may function as markers for cellular identification
• what is the importance of the anchorage/ attachment in the cell membrane?

  attachment points for cytoskeleton and extracellular matrix
• what is the importance of the signal transduction in the cell membrane?

  function as receptors for peptide hormones.
• describe passive transport:


  follows a conc gradient from high to low

  no energy required "downhill"
• describe active transport:


  against conc gradient form low to high

  energy required "uphill"
• describe vesicular transport:


  substances are packaged in vesicles
• describe simple transport (passive diffusion):

  • diffusion tends to equalize conc of a substance either side of the cell membrane
  • molecules move downhill form high conc to low
• how does diffusion occur through cell membrane:

  • through transmembrane protein channels
  • also available to ions and ionized molecules e.g., Na+, K+ (nerve conductions)
• explain facilitated transport: diffusion

  • important to control the rate at which specific molecules are allowed to diffuse across the cell membrane, or sometimes to prevent movement altogether.
  • facilitated transport is the process of spontaneous passive transport of molecules across a membrane vie membrane proteins.
• describe bulk transport:


  • bulk import of substances involves the forming of a vesicle form the cell membrane. (endocytosis)
  • bulk export of substances involves their packaging into a vesicle which then fuses with the cell membrane. (exocytosis)
  • vesicle = a membrane bound sphere, formed by 'budding-off' of membrane.