What is the Difference Between Thylakoid and Stroma

The main difference between thylakoid and stroma is that thylakoid is the flattened sac inside the chloroplast where photosynthesis occurs whereas stroma is the fluid-filled, internal space of the chloroplast.

Thylakoid and stroma are two important structures of the chloroplast. The chloroplast is the membrane-bound organelle that conducts photosynthesis.

Key Areas Covered

1. What is Thylakoid
   • Definition, Characteristics, Importance
2. What is Stroma
   • Definition, Characteristics, Importance
3. Similarities Between Thylakoid and Stroma
   • Outline of Common Features
4. Difference Between Thylakoid and Stroma
   • Comparison with Key Differences               

Key Terms

Stroma, Thylakoid

What is Thylakoid

The thylakoid refers to the little, round, flat, pillow-shaped sacs inside the chloroplast. It is a membrane-bound structure and its space inside the thylakoid membrane is the thylakoid lumen. However, the functional parts of the thylakoid are its membrane and the lumen. In addition, the light-trapping green pigment, chlorophyll occurs in the thylakoid membrane. These chlorophylls are organized into photosystem 1 and photosystem 2 on the thylakoid membrane.

Figure 1: Chloroplast

Furthermore, the main function of the thylakoid is to undergo the light reaction of photosynthesis. Here, the light energy of the sunlight is converted into electrical energy by chlorophyll. This electrical energy in the form of high-energy electrons is passed through membrane proteins from one to another, providing the power to pump protons from the stroma into the thylakoid lumen. When these pumped proteins are rushed back into the stroma, energy is released. However, this energy is readily used by the enzyme, ATP synthase by synthesizing ATP. On the other hand, NADP+ reductase is the enzyme that uses electrons released from photosystem 2 for the production of NADPH.

What is Stroma

Stroma is the colorless fluid, surrounding the grana, the stacks of thylakoids. Generally, the main function of the stroma is to undergo the dark reaction of photosynthesis. However, photosynthesis is the process of converting light energy into chemical energy through cellular respiration. Furthermore, the two steps of photosynthesis are the light reaction and the dark reaction.

Figure 2: Photosynthesis

Moreover, the dark reaction of photosynthesis requires enzymes. These enzymes occur in the stroma of the chloroplast. The dark reaction uses carbon dioxide and water to produce simple carbohydrates. Additionally, it uses NADPH and ATP produced in the light reaction on the thylakoid membranes.

Similarities Between Thylakoid and Stroma

• Thylakoid and stroma are two structural components of the chloroplast.
• Their main function is to undergo the light reaction and the dark reaction of photosynthesis.

Difference Between Thylakoid and Stroma

Definition

Thylakoid refers to each of a number of flattened sacs inside a chloroplast, bounded by pigmented membranes on which the light reactions of photosynthesis take place, and arranged in stacks or grana. Meanwhile, stroma refers to the fluid-filled internal space of the chloroplasts which encircle the grana and the thylakoids.

Significance

Usually, thylakoid is the membrane-bound compartments of the chloroplast while stroma is the fluid-filled internal space of the chloroplast.

Function

The main function of the thylakoid is to undergo the light reaction of photosynthesis while the main function of the stroma is to undergo the dark reaction of photosynthesis.  

Produces

Thylakoid produces ATP and NADPH while stroma produces carbohydrates.

Conclusion

In brief, thylakoid is the membrane-bound sacs that occur in the lumen of the chloroplast. Besides, the main function of chloroplast is to undergo photosynthesis to fix carbon dioxide into simple sugars. Importantly, the thylakoid membrane undergoes the light reaction of photosynthesis in two photosystems, photosystems 1 and 2. Also, the light reaction produces ATPs and NADPHs. In contrast, the stroma is the fluid-filled lumen of the chloroplast, containing the enzymes that the dark reaction requires. Therefore, the dark reaction of photosynthesis occurs in the stroma. Hence, the main difference between the thylakoid and stroma is their function.

References:

1. Wikimedia Foundation. (2022, September 7). Thylakoid. Wikipedia. Retrieved September 12, 2022.
2. (2022, March 23). What is stroma? – definition, structure and functions. BYJUS. Retrieved September 12, 2022.

Image Courtesy:

1. “Chloroplast II ” By Kelvinsong – Own Work (CC BY-SA 4.0) via Commons Wikimedia
2. “Simple photosynthesis overview” By Daniel Mayer – Own Work (CC BY-SA 4.0) via Commons Wikimedia