CHAPTER 10: PHOTOSYNTHESIS

Autotrophs are organisms that create their own food, and do not need to eat or decompose other organisms for nutrition.

They make their organic molecules by using inorganic molecules in the environment and combining them. This is why they are called producers.

Plants are autotrophs. They are specifically called photoautotrophs, because they use light as energy to synthesize carbohydrates, lipids, and proteins.

Photosynthesis occurs in plants, algae, and sometimes in protists and some prokaryotes.

Some bacteria are chemoautotrophic, meaning that they use molecules like sulfur or ammonia as power when making organic molecules.

Heterotrophs are organisms that eat other organisms for energy.

Almost all heterotrophs depend of plants and other photoautotrophs for food and oxygen.

Chloroplasts

All green parts on a plant have chloroplasts. For example, leaves are green and they have lots of chloroplasts

A leaf is green because of a pigment called chlorophyll. Chlorophyll is green and absorbs light energy that begins photosynthesis and makes organic molecules in a plant.

Chloroplasts are found mostly in the mesophyll, which is the tissue in the inside of a leaf.

Carbon dioxide needed for photosynthesis enters the leaf through microscopic openings called stomata. Oxygen also leaves through these openings.

Remember the chloroplast’s structure?

Chlorophyll resides in thylakoids.

Photosynthesis

We can summarize the formula for photosynthesis with this equation

So in this equation, carbon dioxide, water, and light energy are used to create glucose and oxygen. This is how a plant provides its own food.

Oxygen released as a product of photosynthesis is not from CO₂.

It has been proven that plants split a water molecule, and that is where the product oxygen comes from.

Sadly, photosynthesis is not as easy as the equation makes it.

An Overview of the Steps of Photosynthesis

The two stages of photosynthesis are known as the light reactions and the Calvin cycle.

The light reactions are when the chloroplast converts solar energy to chemical energy.

Light absorbed by chlorophyll transfers electrons from water to an electron acceptor called NADP+, which hold electrons for a while.

So water is split in this process, and so oxygen is given off as a product during the light reactions of photosynthesis.

Light reactions use solar energy to reduce (add electrons) NADP+

The light reactions also make ATP by adding a phosphate group to ADP. This type of phosphorylation is called photophosphorylation.

So the two things formed by light reactions are NADPH and ATP.

The next step is called the Calvin cycle

The cycle is named after a guy named Melvin Calvin, who helped figure out the Calvin cycle.

The Calvin cycle takes carbon from CO₂ and uses it to make organic compounds. This process is called carbon fixation.

The carbon cycle then turns the organic compounds from carbon fixation into carbohydrates by adding electrons to them. Also, the cycle requires energy to turn the carbon compounds into carbohydrates.

This is where NADPH and ATP from the light reactions is used.

So the steps of the Calvin cycle are called “light-independent reactions” or “dark reactions” because they do not directly depend on light to take place.

The Calvin cycle occurs in the stroma of the chloroplast.

A Closer Look at Light Reactions

Light is a form of energy called electromagnetic energy. There are different electromagnetic waves, and each type has a different wavelength.

Wavelength is the distance between the crests of waves.

The range of electromagnetic waves is known as the electromagnetic spectrum.

The segment of the electromagnetic spectrum that is important to this chapter is visible light.

Light is said to have photons, particles with a quantity of energy.

The shorter the wavelength of a type of light, the greater the energy that wave has.

Substances that absorb light are called pigments.

Chlorophyll appears green because it reflects green light while absorbing red and blue light.

The type of light that a pigment absorbs and reflects is found out by placing a pigment in a spectrophotometer. A graph plotting the pigment’s light absorption versus its wavelength is called an absorption spectrum.

An action spectrum is a graph showing the performance of a pigment along several different types of light.

There is one type of chlorophyll called chlorophyll a, and it looks blue-green. Therefore, it reflects blue and green light.

Chlorophyll b looks yellow-green, and reflects green and yellow light

Cartenoids are pigments inside chloroplasts that absorb and get rid of excessive light energy that would damage the chloroplast. This is called photoprotection.

When a pigment is excited, one of its electrons gains energy and is said to be in an “excited state”.

Inside a chloroplast, chlorophyll is organized along with proteins and other kinds of organic molecules into photosystems.

A photosystem has a region of chlorophylls and cartenoids called the antenna complex.

When an antenna molecule absorbs a photon, this molecule passes the energy from the photon from pigment molecule to pigment molecule.

Finally, energy will come to a specific chlorophyll a that is located in a region called the reaction center.

A molecule called the primary electron acceptor is also in the reaction center. The chlorophyll a in the reaction center looses an electron to the primary electron acceptor

There are two different photosystems: Photosystem I and Photosystem II.

Photosystem I’s reaction center absorbs light best at a wavelength of 700 nanometers, so its reaction center is known as P700.

Photosystem II’s reaction center absorbs light that has a wavelength of 680 nanometers. It’s reaction center is called P680.

Light drives the synthesis of NADPH and ATP by energizing these two photosystems. This depends on the flow of electrons.

There are two different forms of electron flow: Cyclic and noncyclic.

Here is the steps of noncylic electron flow:

1. When photosystem II absorbs lights, an electron excited to a higher energy level in the reaction center chlorophyll is captured by the primary electron acceptor. The oxidized (lost an electron) chlorophyll is now missing an oxygen.

2. An electron takes electrons from water and replaces the missing electron in the chlorophyll. This reaction splits a water molecule, and then the split oxygen binds with another oxygen to form O₂.

3. Each electron excited by light passes from the primary electron acceptor of photosystem II to photosystem I. The electron is transferred by a type of electron transport chain.

4. As these electrons go down the electron transport chain, the thylakoid membrane uses the energy from the transfers of electrons to make ATP. This type of ATP production is called noncyclic phosphorylation.

5. Now, when the electron reaches the end of the electron transport chain, it binds to P700 after an electron from the chlorophyll is captured by the primary electron acceptor.

6. The primary electron acceptor of photosystem I is sends the electron down another electron transport chain. Then an enzyme called NADP+ reductase reduces NADP+ to NADPH.

Cyclic Electron Flow

Cyclic electron flow only uses photosystem 1. It makes only ATP, but no NADPH and releases no oxygen. ATP generation in cyclic energy flow is called cyclic photophosphorylation.

Cyclic electron flow is used to make extra ATP, because the Calvin cycle requires more ATP than NADPH, so cyclic electron flow makes up the difference.

Mitochondria and chloroplasts both make ATP by chemiosmosis

However, the difference is that mitochondria use electrons taken from food, while chloroplasts turn light energy into chemical energy

Difference in the spatial organization of chemiosmosis of mitochondria and chloroplasts

- mitochondria: the inner membrane pumps protons from the matrix to the innermembrane space

- chloroplasts: thylakoid pumps protons from the stroma into the thylakoid.

A Closer Look at the Calvin Cycle

Carbon enters the Calvin cycle, and then leaves in the form of sugar

NADPH and ATP are used to add electrons to make the sugar.

The carbohydrate made by the Calvin Cycle is called glyceradehyde 3-phosphate (G3P).

In order to make G3P, the Calvin cycle must occur three times in order to fix three carbons.

Here are the steps for the Calvin Cycle:

1. Carbon fixation: The Calvin cycle incorporates carbon from CO₂ by attaching it so a five-carbon sugar called ribulose biphosphate (RuBP). The enzyme that puts these two together is called RuBP carboyxylase, and it is called rubisco. The product of the reaction is two molecules of 3-phosphoglycerate.

2. Reduction: ATP and NADPH are used to transform the molecule into G3P. For every three molecules of CO2, there are 6 molecules of G3P; only 1 is a net gain. The other five are recycled.

3. Regeneration: the remaining 5 molecule of G3P are reconverted back into RuBP to continue the Calvin cycle.

So, in order to make one G3P molecule, the Calvin Cycle used nine molecules of ATP and six molecules of NADPH.

In hot, dry environments, plants have adapted to maximize photosynthesis, while limiting water loss, which is called transpiration in cells.

Most plants will close the stomata to prevent water loss, but this limits CO2 intake, which slows down the rate of photosynthesis.

C₃ plants have a first organic product of 3-phosphoglycerate during the Calvin Cycle. When their stomata close, they do not produce as much food because they do not get much CO₂.

If CO₂ isn’t present, then the Calvin cycle will use O₂ instead. Rubisco will add O₂ instead of CO₂ to RuBP.

This whole process is called photorespiration. Photorespiration creates no ATP and produces no food, so it is pretty wasteful.

Back in ancient times, photorespiration was probably not a big deal because there was more CO₂ in the environment than O₂.

Plants have adapted by developing C4 photosynthesis and CAM

C₄ plants are called this because carbon fixation in the Calvin cycle makes a product with four carbon atoms.

In a C₄ plant, there are two cells that engage in photosynthesis, bundle sheath cells and mesophyll cells.

The Calvin Cycle only occurs in the bundle sheath cells

Carbon fixation occurs in the mesophyll cells. This happens when a molecule of CO₂ is attached to a molecule of PEP to form a 4 carbon product. The enzyme that does this is called PEP carboxylase.

PEP carboxylase has a higher affinity for CO₂, so it is effective when there is not much CO₂ in the cell.

Once the mesophyll ships its 4 carbon product to the bundle sheath cells, the products release CO₂ and then the Calvin Cycle uses this CO₂.

Another adaption has evolved in plants called CAM plants. These are succulent plants, like cacti

These plants open their stomata at night and close them during the day. Closing stomata during the day keeps CO₂ and water inside the cell.

At night when stomata are open, CO2 is incorporated into other organic acids; this mode of carbon fixation is called crassulacean acid metabolism (CAM)

CAM plants store these organic acids in the mesophyll until their stomata close in the morning, and light reactions occur and create ATP and NADPH.

Then during the Calvin cycle, the organic acids give off CO for the plant to use.

DONE!!!!!!