Cellular Respiration: Glycolysis and Krebs Cycle

Cellular respiration is a multi-step process that breaks down glucose to produce energy in the form of ATP. The first stage of this process is glycolysis, which occurs in the cell's cytoplasm.

Definition: Glycolysis is the metabolic pathway that breaks down glucose into two pyruvate molecules, occurring in the cytoplasm of cells.

During glycolysis, a single glucose molecule is broken down through a series of ten enzyme-catalyzed reactions. This process yields two pyruvate molecules, two ATP molecules, and two NADH molecules.

Highlight: The glycolysis process and ATP production is a net gain of energy, producing 2 ATP molecules for each glucose molecule processed.

Following glycolysis, the pyruvate molecules enter the mitochondria, where they are converted into acetyl CoA. This marks the beginning of the Krebs cycle, also known as the citric acid cycle.

Vocabulary: The Krebs cycle, or citric acid cycle, is a series of chemical reactions in cellular respiration that generates energy through the oxidation of acetyl CoA derived from carbohydrates, fats, and proteins.

The Krebs cycle occurs in the mitochondrial matrix and consists of a series of reactions that produce NADH, FADH2, and ATP.

Example: In one complete turn of the Krebs cycle, the following molecules are produced: 2 ATP, 6 NADH, 2 FADH2, and 4 CO2.

The Krebs cycle stages and NADH generation play a crucial role in cellular energy production, providing the electron carriers (NADH and FADH2) necessary for the next stage of cellular respiration.

Photosynthesis: Light Reactions and Calvin Cycle

Photosynthesis is the process by which plants and other organisms convert light energy into chemical energy. This process occurs in two main stages: the light reactions and the Calvin cycle.

The light reactions take place in the thylakoid membrane of chloroplasts. During this stage, chlorophyll and other pigments absorb light energy, which is used to split water molecules into oxygen, hydrogen ions, and electrons.

Definition: Light reactions are the first stage of photosynthesis where light energy is captured and converted into chemical energy in the form of ATP and NADPH.

The electrons released from water splitting are transported through a series of protein complexes, known as the photosynthetic electron transport chain. As the electrons move through this chain, they lose energy, which is used to generate a proton gradient across the thylakoid membrane.

Highlight: The photosynthesis light reactions and Calvin cycle work in tandem, with the light reactions providing the energy-rich molecules ATP and NADPH needed for the Calvin cycle.

This proton gradient is then used by ATP synthase to produce ATP, similar to the process in cellular respiration. Additionally, NADPH is produced during the light reactions, which will be used in the subsequent Calvin cycle.

The Calvin cycle, also known as the light-independent reactions or dark reactions, occurs in the stroma of the chloroplast. This cycle uses the energy from ATP and NADPH, along with carbon dioxide from the atmosphere, to produce glucose.

Vocabulary: The Calvin cycle is the second stage of photosynthesis, where carbon dioxide is fixed into organic compounds using the energy-rich molecules produced in the light reactions.

The cycle begins with the fixation of carbon dioxide into an organic molecule called RuBP (ribulose bisphosphate), which is then converted into PGA (3-phosphoglycerate). PGA is subsequently reduced to G3P (glyceraldehyde 3-phosphate) using the energy from ATP and NADPH.

Example: In the Calvin cycle, for every three turns of the cycle, one G3P molecule is produced that can be used to synthesize glucose, while the remaining G3P molecules are used to regenerate RuBP to keep the cycle going.

The intricate processes of cellular respiration and photosynthesis demonstrate the remarkable efficiency of living organisms in capturing, converting, and utilizing energy. These fundamental biochemical pathways form the basis of energy flow in the biosphere, enabling the diverse forms of life we see on Earth.