Mitochondria & Plastids - the Semi-Autonomous Organelles

Both have a double membrane, their own circular DNA and 70S ribosomes, and divide on their own - so they are called semi-autonomous organelles (supporting the endosymbiotic origin).

Mitochondria - the 'powerhouse'

  • Bounded by two membranes; the outer membrane is smooth, the inner membrane is folded into cristae that increase surface area and bear the F1 (oxysomes) particles (ATP synthase).
  • The inner cavity holds the matrix, which contains enzymes, circular DNA and 70S ribosomes.
  • Site of aerobic respiration - the citric acid (Krebs) cycle in the matrix and the electron transport chain / oxidative phosphorylation on the cristae → produces ATP; hence the 'powerhouse of the cell'.

Plastids (in plants only)

  • Chloroplasts - green, contain chlorophyll; site of photosynthesis. Double membrane; the stroma holds enzymes, circular DNA and 70S ribosomes; stacked membranous thylakoids form grana, connected by stroma lamellae. Thylakoids hold the photosynthetic pigments.
  • Chromoplasts - contain fat-soluble carotenoid pigments (yellow/orange/red - e.g. in petals and fruits).
  • Leucoplasts - colourless, store food: amyloplasts (starch), elaioplasts (oils/fats) and aleuroplasts (proteins).

Visual - Mitochondrion & Chloroplast

Mitochondrion with cristae and matrix; chloroplast with grana, thylakoids and stroma

Mitochondrion (outer membrane, inner membrane folded into cristae, matrix with DNA + 70S ribosomes) and chloroplast (grana of thylakoids, stroma lamellae, stroma with DNA + 70S ribosomes).

Ribosomes, Cytoskeleton, Cilia & Flagella, Centrosome

Ribosomes

  • Granular structures made of ribosomal RNA (rRNA) and proteins; they have no membrane; the site of protein synthesis.
  • Eukaryotic ribosomes = 80S (a 60S + a 40S subunit).
  • Prokaryotic ribosomes, and those inside mitochondria and chloroplasts, = 70S (50S + 30S).
  • Several ribosomes attached to one mRNA form a polysome (polyribosome).

Cytoskeleton

  • A network of proteinaceous filaments in the cytoplasm - microtubules, microfilaments (actin) and intermediate filaments.
  • Functions: mechanical support, motility, and maintenance of cell shape.

Cilia & flagella

  • Hair-like membrane-covered outgrowths of the cell; cilia are small and numerous (movement of fluids/the cell), flagella are longer and fewer (locomotion).
  • Their core, the axoneme, has microtubules in a 9 + 2 array - nine peripheral doublets around a central pair.
  • They arise from a basal body, which has a 9 + 0 array (nine peripheral triplets, no central microtubule).
  • Note: eukaryotic flagella (microtubule 9+2, covered by membrane) differ structurally from the bacterial flagellum (a protein filament + hook + basal body).

Centrosome & centrioles

  • The centrosome (in animal cells) contains two cylindrical centrioles at right angles, surrounded by pericentriolar material.
  • A centriole has a cartwheel organisation of nine peripheral microtubule triplets with no central microtubule - i.e. a 9 + 0 pattern.
  • Centrioles form the basal bodies of cilia/flagella and the spindle fibres during cell division.

Rank-splitters: 80S = eukaryotic cytoplasmic ribosome; 70S = prokaryote + mitochondrion + chloroplast; cilia/flagella axoneme = 9+2, basal body & centriole = 9+0; ribosomes and nucleolus have no membrane.