Plant Respiration, Gas Exchange And Fruit Ripening

Respiration

The main energy source of plants for growth is carbohydrates which are formed during photosynthesis (see later). During respiration, the energy trapped in the carbohydrates is released in a usable form for the plant. The process of respiration takes place in the plant cell organelles called Mitochondria.

Mitochondria are sub-cellular compartments that contain enzymes and membrane systems that are required to control the biochemical processes which are involved in converting sugars (trapped energy) to Adenosine triphosphate (ATP) (usable energy) during respiration. ATP is the universal energy-carrying molecule in all living organisms (bacteria, fungi, plants, animals). Every plant cell contains hundreds of mitochondria, the equivalent of small power generators feeding into a grid, providing energy for the cell. The sugars that are produced in the leaves during photosynthesis are transported to other organs such as growing shoot tips, roots or fruits. The sugar is supplied to every cell in the plant otherwise these cells will die. The sugars are essential because they are consumed during respiration releasing energy. During a series of biochemical reactions in the mitochondria, sugar is combined with oxygen, releasing carbon dioxide, water, and energy. The energy, which is released, is captured in the molecule called ATP. ATP is then in turn used to "drive" a number of other cellular processes aimed at maintenance (e.g., repair), functions (e.g., nutrient uptake and transport) and growth of plant tissues. From the sugar produced during photosynthesis, approximately half is consumed during respiration. The number of carbohydrates used per day depends on environmental conditions. Throughout the respiration process, enzymes which are protein catalysts are involved with the reactions.

The respiration process can be summarised as:

Sugar (C6H12O6) + oxygen (6O2) renders (→) carbon dioxide (6CO2) +water (6H20) + energy

Respiration can be separated into distinct phases:

During the first step, the sugar glucose (6 carbon carbohydrates) is oxidised (where oxygen is added) to form two molecules of pyruvic acid (3 carbon organic acids). This process is known as Glycolysis (‘Glyco’ refers to sugar and ‘lysis’ meaning splitting) and is set out schematically below.

During the first phase, no oxygen was involved and is therefore called the anaerobic phase that takes place in the ground substance of the cytoplasm and not in the mitochondria. During the second, aerobic phase of respiration taking place inside the mitochondria, the newly formed pyruvic acid is converted into 2 acetaldehyde molecules. These then enter the organic acid cycle of respiration.

Pyrovate Acetyl-CoA

Eight hydrogens (H) atoms are liberated; two from malate conversion to oxaloacetate; two from succinate conversion to fumarate; two from ketoglutarate conversion to Succinate and two to form isocitrate conversion to ketoglutarate.