Photosynthesis supplement Dr. Ross Clark

Where photosynthesis happens Photosynthesis occurs in all green parts of plants.

Plants are green because they have chloroplasts Chloroplasts are green because they contain chlorophyll

Chlorophyll Chlorophyll a is the only pigment that is absolutely essential for photosynthesis. Chlorophyll b is another chlorophyll. But it is not essential for photosynthesis All of the other chloroplast pigments pass the energy they have absorbed on to chlorophyll a molecules.

Chlorophyll is green because it reflects green Compare this graph with the one to the right. Chlorophyll a has absorption peaks in the blue and red parts of the visible spectrum. So does chlorophyll b, but the peaks are not the same. The “valley” in this absorption curve is in the green-yellow part of the spectrum, which is the color of leaves. Other pigments, called accessory pigments, absorb other wavelengths, making chloroplasts more efficient. That makes the overall absorption curve smoother. The accessory pigments carotene (which is orange) and xanthophyll (yellow) reflect the wavelengths of light that they do not absorb.

Making a sugar molecule is a two-step process 1 First, light energy is captured by the chloroplast pigments. 2 Some of the captured energy Is used to make sugar from carbon dioxide. * * The CO 2 in this position is a mistake. Ignore it.

Making a sugar molecule is a two-step process The energy-capturing step is called the light reactions, because light energy is required: no light = no light reactions. The sugar- making step is called the dark reactions, because it does not require light. It can occur in the dark or in light. (This CO 2 is a mistake; ignore/erase it!)

The whole photosynthetic process occurs in chloroplasts This is a diagram of a chloroplast. The thylakoids are specialized membranes that contain the photosynthetic pigments. They are where the light energy is captured. The stacks of thylakoids look a little like stacks of cookies. The stacks of thylakoids are called grana.

Here are a few more details cutaway view of chloroplast LIGHT REACTIONS Light penetrates chloroplast Light strikes pigment molecules, and is absorbed by electrons in the pigment molecules 3The energized electrons are used to make ATP and energized electron carrier molecules ATP and energized electrons are transferred to the chloroplast stroma

DARK REACTIONS The dark reactions are a cycle sometimes called the Calvin cycle. This cycle requires several enzymes. The most important enzyme is rubisco. Rubisco picks up carbon dioxide (CO 2 ) and adds it to 5-carbon sugars. Those sugars now have 6 carbons. As the cycle continues to turn, it generates a 3-carbon sugar that is not needed to keep the cycle turning. Two of those surplus 3-carbon sugars are combined to form glucose, a 6 carbon sugar. There it is – the sugar, the key to life! More details, continued

Let’s revisit the photosynthesis picture Overall process of photosynthesis Water + carbon dioxide sugar + oxygen light chlorophyll a minerals from soil

Let’s revisit the photosynthesis picture Overall process of photosynthesis water + carbon dioxide sugar + oxygen light chlorophyll a minerals from soil Let’s account for the things in this process – Carbon dioxide – diffuses into plants from atmosphere Sugar – produced by dark reactions (Calvin cycle) in chloroplast stroma

Where do water and oxygen fit into all this?

Let’s take a closer look at the diagram we looked at earlier

The very first thing that happens is that light energy is absorbed by electrons in the pigment molecules. Some of those electrons are carried to the dark reactions and never return to the pigment molecules. What happens when other rays of light zap the pigment molecules that lost their electrons? How can a pigment capture light energy if it has lost its energy- capturing electrons?

Let’s take a closer look at the diagram we looked at earlier The very first thing that happens is that light is absorbed by electrons in the pigment molecules. Some of those electrons are carried to the dark reactions and never return to the pigment molecules. What happens when other rays of light zap the pigment molecule that lost their electrons? The answer is, those electrons must be replaced. But HOW?

Let’s take a closer look at the diagram we looked at earlier The very first thing that happens is that light energy is absorbed by electrons in the pigment molecules. Some of those electrons are carried to the dark reactions and never return to the pigment molecules. What happens when other rays of light zap the pigment molecule that lost their electrons? The answer is, those electrons must be replaced. But HOW? The answer to “how” is that water (H 2 O) molecules are split into hydrogen ions (H+), oxygen ions (O=), and electrons. The electrons replace the electrons of the pigment molecules, and the oxygen ions combine to form O 2.

The splitting of water during photosynthesis is called photolysis. Photo means, “with light” Lysis means, “splitting” Manganese ions are necessary for the photolysis of water.

Let’s revisit the overall picture again Overall process of photosynthesis Water + carbon dioxide sugar + oxygen light chlorophyll a minerals from soil Let’s account for the things in this process – water – is used to replace electrons lost by pigment molecules carbon dioxide – diffuses into plants from atmosphere sugar – produced by dark reactions (Calvin cycle) in chloroplast stroma oxygen – is a left-over produced by splitting water to replace the pigment electrons = a left-over that changed the atmosphere

Let’s revisit the photosynthesis picture Overall process of photosynthesis Water + carbon dioxide sugar + oxygen light chlorophyll a minerals from soil Let’s account for the things in this process – water – is used to replace electrons lost by pigment molecules Carbon dioxide – diffuses into plants from atmosphere Sugar – produced by dark reactions (Calvin cycle) in chloroplast stroma Oxygen – is a left-over produced by splitting water to replace the pigment electrons = a left-over that changed the atmosphere a left-over that changed the planet

Let’s revisit the photosynthesis picture Overall process of photosynthesis Water + carbon dioxide sugar + oxygen light chlorophyll a minerals from soil Let’s account for the things in this process – water – is used to replace electrons lost by pigment molecules Carbon dioxide – diffuses into plants from atmosphere Sugar – produced by dark reactions (Calvin cycle) in chloroplast stroma Oxygen – is a left-over produced by splitting water to replace the pigment electrons = a left-over that changed the atmosphere a left-over that changed the planet a left-over that made humans possible

Just think about that.

Take care of your plants. They are taking care of you.