Question

Glyceraldehyde 3 -phosphate dehydrogenase in chloroplasts uses NADPH to participate in the synthesis of glucose. In gluconeogenesis in the cytoplasm, the isozyme of the dehydrogenase uses NADH. Why is the use of NADPH by the chloroplast enzyme advantageous?

Short answer

Using NADPH in chloroplast enzymes efficiently harnesses energy from photosynthesis for glucose synthesis.

Step-by-step solution

Understanding the Roles of NADPH and NADH

In cellular metabolism, NADPH and NADH are used as electron carriers. NADPH is primarily involved in anabolic (biosynthetic) reactions, providing reducing power and being used in photosynthesis and other biosynthetic pathways. NADH is generally used in catabolic (energy-yielding) processes and is involved in ATP production during cellular respiration.

Role of Chloroplasts in Photosynthesis

Chloroplasts perform photosynthesis, converting light energy into chemical energy. This process produces NADPH in the light-dependent reactions, which supplies the energy and reducing power needed in the Calvin cycle to convert carbon dioxide into glucose.

Why NADPH is Advantageous for Chloroplast Enzymes

Using NADPH in chloroplast enzymes, such as glyceraldehyde 3-phosphate dehydrogenase, is advantageous because it directly utilizes the reducing power generated by the chloroplast during photosynthesis. This integration ensures efficient energy use, as the reducing power from the light reactions is channeled directly into the Calvin cycle without the need for conversion to another form.

NADH’s Role Outside Chloroplasts

In contrast, NADH used in cytoplasmic reactions, like gluconeogenesis, is linked mainly to energy extraction and ATP production, complementing the energetic demands of the cell when generating glucose from non-carbohydrate precursors.

Conclusion on Electron Carrier Utilization

In summary, the use of NADPH in chloroplasts ensures that the energy harnessed from sunlight is effectively used in the biosynthetic processes of photosynthesis, while NADH accounts for energy usage in respiration and related pathways.

Key concepts

NADPH

NADPH stands for nicotinamide adenine dinucleotide phosphate. It's a crucial electron carrier involved in anabolic reactions. This molecule is essential in photosynthesis, where it provides the needed electrons to synthesize sugar. Think of NADPH as a battery of the cell, carrying energy and electrons to power reactions.
NADPH is primarily produced during the light-dependent reactions of photosynthesis in plants. In this phase, sunlight is captured and transformed into chemical energy. This energy is stored in the form of ATP and NADPH.
What's interesting is how NADPH differs from its cousin, NADH. While NADH is involved in catabolic processes, breaking down molecules, NADPH is used for building them up, crucially supporting the formation of glucose in plants.

Chloroplast

Chloroplasts are the powerhouses of plant cells, similar to how mitochondria function in animal cells. These little green organelles are where photosynthesis happens. They contain chlorophyll, which is vital for capturing light energy.
Inside chloroplasts, two main phases of photosynthesis occur: the light-dependent reactions and the Calvin cycle.

• Light-dependent reactions: Here, sunlight is converted into chemical energy, producing ATP and NADPH.
• Calvin cycle: This is where the produced ATP and NADPH come into play, helping convert carbon dioxide into glucose.

Chloroplasts make it possible for plants to generate their food and release oxygen, which is necessary for most life on Earth.

Calvin Cycle

The Calvin cycle, also known as the light-independent reactions or dark reactions, is a series of biochemical processes that occur in the chloroplast stroma. It is here that carbon dioxide is fixed and converted into glucose, a type of sugar.
The Calvin cycle does not require light directly, but it uses the ATP and NADPH generated in the light-dependent reactions.

• Carbon fixation: This is the first step, where carbon dioxide is attached to a five-carbon sugar called ribulose bisphosphate (RuBP).
• Reduction phase: ATP and NADPH are used to convert the fixed carbon into glyceraldehyde-3-phosphate (G3P).
• Regeneration: RuBP is regenerated, allowing the cycle to continue.

The Calvin cycle is essential for producing the glucose that plants use for energy and as a building block for growth.

Glyceraldehyde 3-Phosphate Dehydrogenase

Glyceraldehyde 3-phosphate dehydrogenase (G3PDH) is an enzyme crucial in the Calvin cycle. It facilitates the conversion of 1,3-bisphosphoglycerate into glyceraldehyde-3-phosphate (G3P).
This enzyme is unique in chloroplasts because it uses NADPH as the electron donor to complete the reaction. By contrast, in cytoplasmic processes like gluconeogenesis, a similar enzyme uses NADH instead.
The use of NADPH by G3PDH in the chloroplast is efficient. It uses NADPH directly from photosynthesis, which highlights the plant's strategic use of available resources to produce glucose efficiently.

Electron Carriers

Electron carriers are molecules that transfer electrons from one place to another within the cell. They play a pivotal role in both photosynthesis and cellular respiration.
In photosynthesis, NADPH acts as a significant electron carrier, transporting electrons from the light-dependent reactions to the Calvin cycle.

• During light-dependent reactions, electron transport chains move electrons through various proteins, culminating in the reduction of NADP^+ to form NADPH.
• These movements and transformations facilitate the storage and transfer of energy, which ultimately ends up in glucose.

The elegant dance of electrons via carriers like NADPH ensures that plants can capture solar energy and utilize it for survival.