Cellular Organelles 2 (College Board AP® Biology)

Mitochondria & Chloroplasts

Mitochondria

• Mitochondria are the site of aerobic respiration within eukaryotic cells

• Mitochondria are surrounded by a double membrane with the inner membrane highly folded to form cristae

  • The double membrane allows proton gradients to form across the membranes

  • Proton gradients are important in the production of ATP

• The matrix (central part of the mitochondrion) formed by the cristae contains enzymes needed for aerobic respiration

  • The reactions of the Krebs cycle of aerobic respiration take place in the matrix

• Small circular pieces of DNA, mitochondrial DNA (mtDNA) and ribosomes are also found in the matrix (needed for replication of mitochondria)

A single mitochondrion is shown – the inner membrane has protein complexes vital for the later stages of aerobic respiration embedded within it

Chloroplasts

• Chloroplasts are found in green plants and in photosynthetic algae

• Chloroplasts are larger than mitochondria, and are also surrounded by a double membrane

• Membrane bound compartments called thylakoids containing chlorophyll stack to form structures called grana

  • They resemble a stack of pancakes

• Grana are joined together by lamellae (thin and flat thylakoid membranes)

• Chloroplasts are the site of photosynthesis:

  • The light dependent stage takes place in the thylakoids

  • The light independent stage (Calvin cycle) takes place in the stroma

  • Chloroplasts also contain small circular pieces of DNA and ribosomes used to synthesize the proteins needed in chloroplast replication and photosynthesis

Chloroplasts are found in the green parts of a plant; the green color is a result of the photosynthetic pigment chlorophyll