Question

Which statement about thylakoids in eukaryotes is not correct? a. Thylakoids are assembled into stacks. b. Thylakoids exist as a maze of folded membranes. c. The space surrounding thylakoids is called stroma. d. Thylakoids contain chlorophyll.

Short answer

All the given statements about thylakoids in eukaryotes (a, b, c, and d) are correct. However, the question asks for the statement that is not correct. Therefore, the question should be rephrased to ask for the correct statement about thylakoids in eukaryotes.

Step-by-step solution

Understanding Thylakoids

Thylakoids are critical components of eukaryotic cells' photosynthetic machinery, found within chloroplasts. Their structure and role in photosynthesis should be understood before examining each statement.

Statement (a): Thylakoids are assembled into stacks.

This statement is correct. Thylakoids are in fact organized in stacks called grana within the chloroplasts. Grana are connected by intergranal lamellae, which are essentially unstacked thylakoid membranes.

Statement (b): Thylakoids exist as a maze of folded membranes.

This statement is also correct. Thylakoids are membrane-bound structures that are folded and stacked, creating a large surface area for the light-dependent reactions of photosynthesis to take place.

Statement (c): The space surrounding thylakoids is called stroma.

This statement is correct. The stroma is the fluid-filled space surrounding thylakoids within chloroplasts. The stroma is the site of the light-independent reactions of photosynthesis (the Calvin cycle).

Statement (d): Thylakoids contain chlorophyll.

This statement is correct. Thylakoids contain numerous pigment molecules, including chlorophyll, which capture light energy for the process of photosynthesis. After examining each statement, we can conclude that all the statements are correct, meaning the exercise's phrasing is inconsistent with its aim. To correct the inconsistency, the exercise should be rephrased so the student can identify the correct statement about thylakoids in eukaryotes.

Key concepts

Photosynthesis

Photosynthesis is a process used by plants, algae, and some bacteria to convert light energy, usually from the sun, into chemical energy in the form of glucose. This process is crucial for life on Earth as it provides oxygen and organic compounds used by nearly all other organisms. Photosynthesis occurs mainly in the leaves of plants, within the chloroplasts of their cells.

The overall chemical equation for photosynthesis is usually summarized as:

• 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

This means carbon dioxide and water, with the help of light energy, are transformed into glucose and oxygen.
Photosynthesis consists of two main stages: the light-dependent reactions and the light-independent reactions, also known as the Calvin cycle. The light-dependent reactions convert solar energy into chemical energy in the form of ATP and NADPH, while the Calvin cycle uses that energy to synthesize glucose from carbon dioxide.

Understanding photosynthesis helps us appreciate the critical role it plays in the global carbon cycle and in sustaining life on our planet.

Chloroplasts

Chloroplasts are specialized organelles found in plant cells and in some algae. They are the sites where photosynthesis takes place. Each chloroplast contains a system of thylakoid membranes, where the light-dependent reactions of photosynthesis occur.

The structure of a chloroplast includes:

• Outer and Inner Membranes: These form a double envelope surrounding the organelle, with an intermembrane space in between.
• Stroma: The aqueous fluid that fills the interior of the chloroplast. It contains enzymes, chloroplast DNA, ribosomes, and is where the Calvin cycle takes place.
• Thylakoids: These are flattened, disk-shaped membranes stacked in some regions, forming grana, where chlorophyll and other pigments are located.

Chloroplasts capture light energy and convert it into chemical energy, supporting the plant's life processes. The green pigment chlorophyll, crucial for absorbing light, is housed within the thylakoid membranes, underscoring the significance of chloroplasts in photosynthesis.

Without chloroplasts, plants would not be able to perform photosynthesis and produce the energy necessary for their survival and growth.

Light-dependent reactions

The light-dependent reactions are the first phase of photosynthesis. They occur in the thylakoid membranes of the chloroplasts. These reactions require sunlight to produce energy carriers, namely ATP and NADPH, which are essential for the Calvin cycle.
During these reactions, chlorophyll absorbs sunlight which excites electrons to a higher energy state. These high-energy electrons are then transferred through a series of proteins known as the electron transport chain. As electrons move through the chain, their energy is used to pump hydrogen ions into the thylakoid lumen, creating a concentration gradient.

• This gradient drives ATP synthesis via an enzyme called ATP synthase as hydrogen ions flow back into the stroma.
• Meanwhile, electrons are eventually transferred to NADP⁺, reducing it to NADPH.

Both ATP and NADPH are then utilized in the next stage of photosynthesis, the Calvin cycle, to help convert carbon dioxide into glucose.
Light-dependent reactions are crucial for capturing the sun’s energy and setting the stage for the rest of the photosynthetic process, highlighting the brilliant adaptation of plants to harness and convert solar energy into a usable chemical form.