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 forphotosynthesis

All of the other chloroplast pigments pass the energy they have absorbedon 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 blueand red parts of the visible spectrum. So doeschlorophyll b, but the peaks are not the same.The “valley” in this absorption curve is in thegreen-yellow part of the spectrum, which is the color of leaves.

Other pigments, called accessorypigments, absorb other wavelengths,making chloroplasts more efficient.That makes the overall absorptioncurve 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

1First, lightenergy iscaptured bythe chloroplast pigments.

2Some of the captured energy Is used to make sugarfrom carbondioxide.

*

* The CO2 in this position is a mistake. Ignore it.

Making a sugar molecule is a two-step process

The energy-capturing step is calledthe light reactions, because light energyis required: no light = no light reactions.

The sugar-making step iscalled the darkreactions,because it doesnot requirelight. It canoccur in thedark or in light.

(This CO2 is a mistake;ignore/erase it!)

The whole photosynthetic processoccurs in chloroplasts

This is a diagram of a chloroplast.

The thylakoids arespecialized membranesthat contain thephotosynthetic pigments.They are where the lightenergy is captured. Thestacks of thylakoids looka little like stacks ofcookies. The stacks ofthylakoids are calledgrana.

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 pigmentmolecules

3 The energized electrons are used to make ATP and energized electron carrier molecules

ATP and energized electronsare transferred to the chloroplast stroma

DARK REACTIONS

The dark reactions are a cyclesometimes called the Calvincycle. This cycle requiresseveral enzymes.

The most important enzymeis rubisco. Rubisco picks upcarbon dioxide (CO2) and adds it to 5-carbon sugars. Thosesugars now have 6 carbons.

As the cycle continues to turn,it generates a 3-carbon sugarthat is not needed to keep thecycle turning.

Two of those surplus 3-carbonsugars are combined to formglucose, a 6 carbon sugar.

There it is – the sugar, the keyto life!

More details, continued

Let’s revisit the photosynthesis picture

Overall process of photosynthesis

Water + carbon dioxide sugar + oxygen

light

chlorophyll aminerals from soil

Let’s revisit the photosynthesis picture

Overall process of photosynthesis

water + carbon dioxide sugar +

oxygen

light

chlorophyll aminerals 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 diagramwe looked at earlier

Let’s take a closer look at the diagramwe looked at earlier

The very first thing that happens is that light energy is absorbed byelectrons in the pigmentmolecules.

Some of those electrons arecarried to the dark reactions andnever return to the pigmentmolecules.

What happens when other raysof light zap the pigmentmolecules that lost their electrons?

How can a pigment capture lightenergy if it has lost its energy-capturing electrons?

Let’s take a closer look at the diagramwe looked at earlier

The very first thing that happens is that light is absorbed byelectrons in the pigmentmolecules.

Some of those electrons arecarried to the dark reactions andnever return to the pigmentmolecules.

What happens when other raysof light zap the pigmentmolecule that lost their electrons?

The answer is, those electronsmust be replaced. But HOW?

Let’s take a closer look at the diagramwe looked at earlier

The very first thing that happens is that light energy is absorbed byelectrons in the pigmentmolecules.

Some of those electrons arecarried to the dark reactions andnever return to the pigmentmolecules.

What happens when other raysof light zap the pigmentmolecule that lost their electrons?

The answer is, those electronsmust be replaced. But HOW?

The answer to “how” is thatwater (H2O) molecules are split into hydrogen ions (H+), oxygen ions (O=), and electrons. The electrons replace the electronsof the pigment molecules, and the oxygen ions combine to form O2.

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 aminerals 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 aminerals 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 aminerals 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.