BIOB10H3-Summer 2024: Mitochondria and Chloroplasts Lecture 9

Mitochondria Structure and Function

• Mitochondria are heterogeneous and dynamic organelles that change shape, divide, and fuse, and are associated with the cell cytoskeleton.
• Mitochondria specialize into distinct tissue-specific phenotypes, which are highly variable in structure and function.
• Mitochondria are surrounded by a double-membrane system, with an inner and outer membrane separated by an intermembrane space.
• The inner membrane forms numerous folds (cristae) that extend into the interior (matrix) of the organelle.
• The matrix is highly concentrated in proteins, contains the genetic system of the mitochondria, and is the site of food-derived molecule conversion into energy (ATP).

Mitochondrial Function

• Mitochondria perform several interconnected functions, including ATP production, biogenesis of heme groups and iron-sulfur clusters, synthesis of phospholipids, and regulation of apoptosis.
• ATP production involves the conversion of food-derived substrates into acetyl CoA, which is then oxidized in the matrix via the Krebs cycle, producing electrons that are transferred to the electron-transport chain.
• The electron-transport chain generates an electrochemical gradient that drives the synthesis of ATP by the ATP synthase.
• ATP synthase is a nanomachine that can work in forward (to produce ATP) or in reverse (to hydrolyze ATP) and is arranged as dimers in the tip of the cristae.

Chloroplasts Structure and Function

• Chloroplasts are large organelles present in plants and photosynthetic bacteria, responsible for capturing light energy to make sugars in a process called photosynthesis.
• Chloroplasts have an outer and inner membrane, with internal thylakoid compartments and a stroma that contains metabolic enzymes, the ATP synthesis machinery, and the chloroplast's genetic system.
• Thylakoid membranes are highly folded into numerous local stacks of flattened vesicles called granum or grana, interconnected by non-stacked thylakoids (stroma lamellae).

Photosynthesis

• Photosynthesis is a process by which plants use CO2 and H2O to make carbohydrates, divided into two categories: light reactions and carbon-fixation reactions.
• Light reactions involve the conversion of light energy into chemical energy, generating NADPH and ATP, which are then used in the carbon-fixation reactions.
• Carbon-fixation reactions involve the conversion of CO2 into glycerolaldehyde 3-phosphate, a critical intermediary sugar for the synthesis of sugars, amino acids, and fatty acids.

Light Reactions

• Light reactions occur in the thylakoid membranes of chloroplasts and involve the transfer of electrons and protons, generating an electrochemical gradient that drives ATP synthesis.
• The process involves the absorption of light energy by chlorophyll, which is used to generate a positively charged chlorophyll ion, and the transfer of electrons to a second reaction center, resulting in the storage of electrons and H+ in NADPH.

Dark Reactions

• Dark reactions involve the carbon-fixation cycle, which uses NADPH and ATP produced in the light reactions to convert CO2 into glycerolaldehyde 3-phosphate.

• The carbon-fixation cycle is a series of reactions that occur in the stroma of the chloroplast, using the energy from ATP and NADPH to drive the conversion of CO2 into organic compounds.### Carbon Fixation

• Carbon fixation produces glycerolaldehyde 3-phosphate using the electron carrier and ATP from light reactions.

Light Reactions vs. Dark Reactions

• Dark reactions produce the sugar glyceraldehyde 3-phosphate.
• Light reactions involve the electron-transfer chain embedded in the thylakoid membrane and produce O2.
• Light reactions generate ATP.

Chloroplasts and Mitochondria

• Chloroplasts convert light energy into chemical energy.
• Mitochondria consume the chemical energy to produce ATP.
• Optimal carbon fixation and plant growth require coordinated actions between chloroplasts and mitochondria.

Chloroplasts and Viral Replication

• Many plant viral pathogens change and/or rearrange chloroplasts for viral replication.
• Viral influence on chloroplast structures and functions usually leads to depleted photosynthetic activity.

Leaf Color Change in Fall

• As daylight hours decrease and temperatures cool, photosynthesis slows down, and chlorophyll production decreases.
• This reveals yellow or orange pigments called carotenoids.
• Anthocyanins are produced mainly in the fall and are dissolved in the cell sap.
• Cool fall temperatures cause the closing of leaf veins, preventing sugars from moving out, which prolongs fall color.