Cell structure and organelles, mebranes

• Peptidoglycan cell wall:

  • surrounds the plasma membrane
  • prevents the cell from bursting from osmotic lysis
• glycocalyx:

  • present in some bacteria, surrounds the cell wall
  • protects bacteria from being engulfed via phagocytosis
  • enables bacteria to adhere to surfaces or to one another
• ribosomes in prokaryotes:

  • 70S ribosomes made of small 30S and large 50S subunit
  • protein synthesis
• Ribosomes in eukaryotic cell:

  • non-membranous
  • 80S ribosomes made of 40S and 60S subunit, made up of ribosomal RNA and ribosomal proteins
  • site of protein synthesis, free ribosomes synthesise proteins for use within the cell, bound ribosomes synthesise proteins for insertion into membranes, packaging within organelles or for export out of the cell
• centrioles:

  • pair of cylindrical rod-like structures perpendicular to each other
  • long hollow tubes made of tubulin
  • act as microtubule organising centres for assembly of spindle fibres
• nucleolus:

  • not surrounded by membrane
  • contains dna with genes coding for rRNA
  • site of synthesis of rRNA, site of assembly of ribosomal proteins with rRNA to form subunits of ribosomes
• nuclear envelope:

  • encloses nucleus, is a double membrane, outer is continuous with membrane of endoplasmic reticulum
  • has nuclear pores for movement of molecules (like pre-mRNA)
• rough endoplasmic reticulum:

  • transport of proteins synthesised by ribosomes (that are meant for secretion out of the cell)
  • chemical modification of proteins
• smooth endoplasmic reticulum:

  • contains enzymes that catalyse metabolism of carbohydrates and synthesis of lipids
  • detoxify products of natural metabolism, drugs and overloads of ethanol
• Golgi apparatus:

  • cis face where vesicles fuse to Golgi apparatus, trans face where vesicles bud off and travel to other sites
  • single membraned
  • chemically modifies, sorts and packages molecules before transporting to other parts of the cell or for Secretion out of the cell
  • formation of new cell wall in plant cells via synthesis of pectin and lysosomes
• lysosomes:

  • small spherical membrane-bound vesicles that contain hydrolytic enzymes that are acidic, low pH
  • digestion of material taken in by endocytosis (when lysosomes fuse with other vesicles)
  • autophagy: breakdown of unwanted or worn out organelles
  • Autolysis: release of contents in lysosomes within the cell for cell death
  • release of enzymes outside the cell via exocytosis for breakdown of extracellular structures
• Mitochondrion:

  • double membrane, spherical or rod-shaped, contains semi-fluid matrix
  • inner membrane highly folded to form numerous cristae, provides large surface area for enzymes like ATP synthase and electron carriers in the electron transport chain to be embedded
  • contains 70S ribosomes, circular DNA
  • site of cellular respiration
• chloroplast:

  • lens-shaped, fluid-filled stroma, enclosed by double membrane
  • contains 70S ribosomes, circular dna and enzymes involved in photosynthesis
  • thylakoids in the stroma stacked to form grana, many grana joined by intergranal lamellae
  • site of photosynthesis
• advantages of having different compartments within cells

  • maintain constant internal environment to provide optimal enzymatic reactions
  • sets up and maintains a concentration gradient across membranes
  • allows for regulation of metabolic pathways with enzymes in close proximity to one another
  • protects contents within organelles from the external environment