Q1. - Campbell Biology

Question

How do the CO₂ molecules used in photosynthesis reach and enter the chloroplasts inside the leaf cell? (See Concept 7.2)

Step-by-Step Solution

Answer

Carbon dioxide (CO₂), being a non-polar molecule, cross the cell membrane of the leaf and chloroplast to reach the chloroplast's stroma, where photosynthesis occurs.

Step 1: Meaning of photosynthesis

Photosynthesis is a process by which plants, algae, and some prokaryotes synthesize their food. In the process, these organisms directly use light energy to synthesize organic compounds in the cell.

                

Plants utilize carbon and water during photosynthesis and transform them to glucose and oxygen, respectively, in the presence of sunlight. Thus, the process of photosynthesis requires three main components, including carbon dioxide, water, and sunlight.

 

Step 2: Phospholipid bilayer of the plasma membrane

The plasma membrane has a bimolecular lipid layer, the surface of which is disrupted by proteins. The phospholipid molecule is responsible for the membrane bilayer. 

 

The hydrophilic heads of the molecules are on the outside of the plasma membrane, while the hydrophobic tails point away from the water and towards the center of the bilayer.

 

Thus, the membranes are self-sealing, flexible, and selectively permeable to polar solutes and permeable to non-polar solutes.

Step 3: Entry of carbon dioxide into the chloroplasts

Plants require carbon dioxide to perform photosynthesis. Carbon dioxide enters the leaf cells through small holes present on the lower side of the leaf called stomata and then reaches the cell's chloroplast.

 

CO₂ moves across the cell membrane through the process of simple diffusion. This is possible despite the lipid bilayer of the plasma membrane because CO₂ is a non-polar molecule like lipids. 

 

Thus, these molecules quickly dissolve in the plasma membrane's chloroplast and transverse it without the help of any membrane proteins to reach the stroma of the chloroplast.