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
Exam Tip
The structures of mitochondria and chloroplasts resemble microorganisms in many ways. The topic of endosymbiosis puts forward a theory that these organelles originated from primitive microorganisms and became incorporated into larger, eukaryotic organisms. This theory is explored in more detail in Topic 2.11
