Cracking the AP Biology Exam

5

Photosynthesis

Plants and algae are producers. All they do is bask in the sun, churning out the glucose necessary for life.

As we saw in Chapter 4, photosynthesis is the process by which light energy is converted to chemical energy. Here’s an overview of photosynthesis:

You’ll notice from this equation that carbon dioxide and water are the raw materials plants use in manufacturing simple sugars. But remember, there’s much more to photosynthesis than what’s shown in the simple reaction above. We’ll soon see that this beautifully orchestrated process occurs thanks to a whole host of special enzymes and pigments. But before we turn to the stages in photosynthesis, let’s talk about where photosynthesis occurs.

THE ANATOMY OF A LEAF

Photosynthesis occurs in the leaves of plants. Here’s a cross-sectional view of a typical leaf:

If you look closely at the leaf of any plant, the first thing you’ll notice is the waxy covering called the cuticle. The cuticle is produced by the upper epidermis to protect the leaf from water loss through evaporation. Just below the upper epidermis is the palisade parenchyma. These cells contain lots of chloroplasts, which are the primary sites of photosynthesis.

Now let’s look at an individual chloroplast. If you split the membrane of a chloroplast, you’ll find a fluid-filled region called the stroma. Inside the stroma are structures that look like stacks of coins. These structures are the grana.

The many disk-like structures that make up the grana are called the thylakoids. They contain chlorophyll, the light-absorbing pigment that drives photosynthesis, as well as enzymes involved in the process.

Now we’ll discuss some structures that are not involved in photosynthesis but are important for the AP test. Just below the palisade parenchyma, you’ll find irregular-shaped cells called spongy parenchyma. It allows for diffusion of gases, especially CO2, within the leaf. The vascular bundles are found in this layer of the leaf. The vascular bundles include xylem and phloem, “vessels” that transport materials throughout the plant. At the lower epidermis are tiny openings called stomates, which allow for gas exchange and transpiration. Surrounding each stomate areguard cells, which control the opening and closing of the stomates.