Plastids and Chloroplasts: Photosynthesis (BOT 2)

Plastids

• Plastids are organelles found in plant cells with various functions.

Proplastids

• Proplastids are found in young, rapidly dividing cells.
• They are simple structures with few folds.

Chloroplasts

• Chloroplasts are green due to the presence of chlorophyll.
• They are the site of photosynthesis.
• They have a double outer membrane.
• Embedded enzymes control the function of the membranes.
• Chlorophyll has a hydrophobic tail.
• The inner membrane is extensively folded to increase surface area.
• Membrane sheets are called thylakoids.
• Stacks of bag-like vesicles are called grana.
• Photosynthesis (BOT 2) occurs within the chloroplasts.
• Pigment complexes capture sunlight energy.
• Oxygen (O2) is released during photosynthesis.
• Hydrogen ions (H+) accumulate in the thylakoids.
• Substances diffuse out through transport proteins.
• ATP and NADPH are synthesized.
• Enzymes are used in the stroma.
• Carbohydrates are synthesized from carbon dioxide (CO2).
• Rapid photosynthesis leads to temporary storage of starch grains.
• Chloroplasts are 4-6 µm in size.
• There can be up to 50 chloroplasts per cell.
• Photosynthesis produces highly reactive intermediates which must be disposed of by organelles.
• Peroxisomes and glyoxysomes are responsible for disposing of the reactive intermediates.

Amyloplasts

• Amyloplasts are found in non-photosynthetic tissues.
• They have few inner membranes.
• They contain large starch grains.
• Examples of amyloplasts include grains and tubers.
• They can convert to chloroplasts when exposed to light.
• They are dense and involved in gravity sensing.

Chromoplasts

• Chromoplasts are found in flowers and fruits.
• They contain bright red, yellow, and orange lipids.
• They have extensive inner membranes.
• They lack grana.
• They contain pigments, which can be embedded in membranes or in discrete droplets called plastoglobuli.
• They are involved in ripening fruit.
• They synthesize lipid pigments and alter thylakoids.

Leaf Color

• Lipid pigments are present in leaves.
• The presence of green chlorophyll masks the presence of other pigment.
• In cold weather chlorophyll breaks down
• As chlorophyll levels fall, other lipid pigments become visible, and leaves turn yellow or orange.

Leukoplasts

• Leukoplasts is a descriptive term for any colorless plastid.
• They are involved in various types of synthesis.
• They contain various lipids.
• They transport substances to other organelles.

Mitochondria

• Mitochondria are involved in cellular respiration.
• They produce energy in the form of ATP.
• They handle high-energy and reactive intermediates.
• Reactions are isolated from the cytoplasm through compartmentalization.
• Mitochondria are 1-4 µm in size (up to 10 µm).
• They have an outer mitochondrial membrane, which is freely permeable.
• They help determine shape and rigidity.
• The inner mitochondrial membrane is selectively permeable.
• It contains ion pumps and channels.
• Enzymes are embedded in the inner mitochondrial membrane.
• Products are passed between enzymes.
• Highly reactive intermediates are controlled.
• The inner membrane is highly folded into cristae, which provide a large surface area.
• The matrix is the liquid between cristae.
• Reactions occur without highly reactive intermediates.
• Mitochondria resemble bacteria in size, form, and biochemistry.
• They have their own DNA, which is circular and lacks histone proteins.
• They have their own ribosomes, which are smaller and resemble prokaryotes.

Evolution of Chloroplasts and Mitochondria

• Chloroplasts and mitochondria evolved about 2.1 billion years ago.
• Some bacteria can respire, and others are photosynthetic.
• This evolution occurred through endosymbiosis.

Support for the Endosymbiont Theory

• Presence of DNA
• Size of Ribosomes
• Inhibition by antibiotics
• Evolutionary relationship
• Same size
• Plasma-Membrane
• Enzyme secretion
• Replication and protein synthesis
• Bacterial binary fission
• Electron transport chain

Microbodies

• Microbodies are organelles with two classes.

Peroxisomes

• Microbodies isolate hydrogen peroxide (H2O2).
• Hydrogen peroxide can damage anything it encounters.
• Peroxisomes are a good example of compartmentalization.
• They contain enzymes that detoxify peroxide, converting it to water and oxygen.
• Catalase converts 2H2O2 into 2H2O + O2.
• They are associated with chloroplasts and mitochondria.

Glyoxysomes

• Glyoxysomes helps convert fats into sugars.
• They are important for the germination.