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Crash Course Ap Bio Photosynthesis Essay

The AP Biology course has so many different concepts and ideas within it that it may seems like there are way too many to understand.  We get that, and want to make sure that you know the most important concepts as well as possible.  We’ve picked out a fewof the most important AP Bio concepts for you which fall under the umbrella of two critical processes: photosynthesis and cellular respiration.


The process of photosynthesis is an essential AP Biology concept to understand.  Photosynthesis is the means by which most of the plant life on Earth gets its energy.  All animals receive nutrients either from plants or from animals that get their energy from plants.  This means that plants are the basis for all of the ecosystems on Earth,making photosynthesis a very important process.  So, in order to understand how photosynthesis happens, biologists have to break it down into two parts, the light and dark reactions.

1. Light Reaction (or the Light-Dependent Reactions)

This is the first step in the process of photosynthesis.  The light reaction is the process by which the leaf of a plant will absorb energy from the sun (in the form of photons) and will transfer it into electron carriers and energy (in the form of Adenosine Triphosphate, or ATP).

First, light will enter the leaf and will be absorbed by pigments within photosystem II, which is the first half of the light reaction.  After the light is absorbed by the pigments, the photon is bounced around inside the photosystem until it reaches the pigment that is correct for that wave of light.  Once it reaches that pigment, it will excite an electron and will move the electron up to the primary electron acceptor.

After reaching the primary electron acceptor, the electron will be moved to photosystem I.  It will be transported across what is known as the electron transport chain.  All you need to know about the ETC is that the electron will move down it and as it moves down it will transfer energy, which is used by the plant to combine ADP and a phosphate group into ATP.

Once the electron reaches photosystem I (just remember that photosystems go in reverse, II is first and I is second), it is excited by another photon and the electron again is bounced to a primary electron acceptor.  Only this time when the electron falls, it is used to combine two hydrogen ions and one NADP+ ion to create NADPH.  This will be used later on in the dark reactions to further the process.  Once the NADPH has been made, the ATP and the NADPH move into the dark reactions, the second part of the cycle.

2. Dark Reaction (or the Light Independent Reaction/Calvin Cycle)

The next and final process of photosynthesis is the Calvin cycle.  This is the process that takes the NADPH, ATP and CO2 and and converts them into G3P (which can be turned into glucose, the basic unit of energy).  This process is a cycle, so part of the product, G3P, will be used to start and end the cycle.

The cycle starts with three five-carbon chains that have a phosphorus attached to each of them using some of the ATP, creating three RuBPs.  After this, carbon dioxide is fixed (carbon will be added) to the RuBP and will create three unstable six-carbon compounds.  Then the compound will fall apart and will make six three-carbon chains.  Next, an electron will be donated to the three carbon chain and it will create the end product, 6 G3Ps.  One of these G3Ps will be removed from the cycle and the other five will be used to repeat the cycle and create the five-carbon chain.  Finally, after enough G3P is made, it will be turned into glucose and thus photosynthesis is complete.

The Different Parts of Cellular Respiration

Cellular respiration is what cells will do in order create energy from glucose.  The currency of energy that is used by cells is called ATP (which pops up often on the AP Biology exam), which is the product of this reaction.  Cell respiration can be broken down into five different steps: glycolysis, formation of acetyl COA, Krebs cycle, oxidative phosphorylation and chemiosmosis.

1. Glycolysis and Formation of Acetyl COA

The first step in the essential AP Biology concept of cellular respiration is glycolysis, which is a simple reaction that moves in a linear chain.  All you need to know for the AP Biology exam is that one glucose will be turned into two pyruvate, two ATP and two NADH (similar to NADPH, it is an electron carrier).  The only other thing you will need to know about glycolysis is that it takes place in the cytoplasm of the cell.

The next step is another simple process that takes place in the cytoplasm of the cell.  This step takes one pyruvate and will turn it into one acetyl COA using coenzyme A.  This process will also create two NADH during the process.  Now, onto the Krebs cycle.

2. The Krebs Cycle

This is again another cycle, which is common among AP Biology concepts, but this takes place in the matrix of the mitochondria.  It will take the acetyl COA and will use it to create electron carriers and energy for the next part of the reaction.  This process will create one ATP, three NADHs and one FADH2.  In this process acetyl COA will be combined with oxaloacetate, which will then turn into citric acid.  After this, this compound will go around the cycle, releasing two carbons along the cycle and creating two carbon dioxides in the process.  This process will then repeat itself and allow the electron carriers to move onto the next cycle.

3. Oxidative Phosphorylation and Chemiosmosis

These two steps will occur at the same time.  All of those electron acceptors that were made in the previous steps will be used to release the electrons that will go down the electron transport chain.  As these electrons move down the chain, protons will be pumped out of the matrix of the mitochondria.  This will cause a higher concentration of protons to exist, increasing the proton concentration  Because of this, the protons will want to re-enter the matrix, and they will do so using a channel known as ATP Synthase.  This will cause this protein to create more ATP as the protons travel across it.  Finally after the ATP is made, the electrons will be accepted by the final electron acceptor, oxygen.  This will combine with the protons (also called hydrogen ions) to make water.

How to Get Better?

If you have read this and you understand these AP Biology concepts, then you are that much closer to succeeding on your AP Biology exam!  If you want to test your knowledge of these topics, practice questions are a great way to do this.

Photo by Light-dependent reactions of photosynthesis at the thylakoid membrane” Licensed under Public Domain via Wikipedia.

By the way, you should check out for your AP Biology review. We have hundreds of AP Biology practice questions written just for you!

AP Biology 12 - Unit 3: Cellular Respiration and Photosynthesis
Learning Goal: How does glucose give you energy? What is the significance of cellular respiration and photosynthesis?
- Concept 1: Analyzing the Processes of Cellular Respiration
- Concept 2: Analyzing the Processes of Photosynthesis

Date                                         What are we Learning?
March 12                                               Introduction
                                                                       - The Big Picture!
                                                                      - Discussion
                                                                       - Review of Energy Transformations
                                                                       - Go over "Review of Energy Transformations" Handout with KEY 
                                                                      - Reading Guide and Key
                                                                        - HW: Bioflix video and questions on Cellular Respiration
                                                                      - (Prepare for AP Lab Inv 6- November 16th)
March 14 (and tutorial)                                          Concept 1: Analyzing the Processes of Cellular Respiration 
                                                                - Try Comparison Charts - compare with KEY
                                                                 Complete Mastering Biology online mini-assignment "Cellular Respiration"
March 16 (and tutorial)                          Concept 1: Analyzing the Processes of Cellular Respiration
                                                                    Lab: AP Inv 6: Cellular Respiration
                                                                    Day 1 - What is the rate of cellular respiration in germinating peas?
                                                                   - Project Criteria
April 4                         Concept 1: Analyzing the Processes of Cellular Respiration
                                                                     Lab: AP Inv 6: Cellular Respiration
                                                                     Day 2 - Your experimental design!
                                                                      Lab outline 1 and 2 
                                                                     See Lab Criteria and mini lab example
                                                                    Complete Mastering Biology online mini-assignment "Cellular Respiration"
                                                                   - Project Criteria
April 6 (and tutorial)                                                    Concept 1: Analyzing the Processes of Cellular Respiration
                                                                   - Try These ​ and answers
                                                                  - Time to work on project and discuss
April 10                                                   Concept 1: Analyzing the Processes of Cellular Respiration
                                                                      - Time to work on projects and discuss
                                                                          Mini Poster Due - gallery walk
​                                                                         Practice Free Response Question page 1 and page 2 and answer
April 12 (and tutorial)                                                   Concept 1: Analyzing the Processes of Cellular Respiration
                                                                 Checkpoint #1 - Cellular Respiration
Introduction to Photosynthesis   - Handout and KEY
April 16                                                      Concept 2: Analyzing the Processes of Photosynthesis
                                                                 - Cellular Respiration Projects Due
                                                                 - Refer to criteria and be ready to present to your classmates Project Criteria
                                                                 - student notes
                                                                 Complete Mastering Biology online mini-assignment "Photosynthesis"
April 18 (and tutorial)                                               Concept 2: Analyzing the Processes of Photosynthesis
                                                            Comparison chart of photosynthesis and  KEY 
                                                             - C4 and CAM questions and KEY
April 20                                    Concept 2: Analyzing the Processes of Photosynthesis
                                              Lab: AP Inv 5: Photosynthesis Lab Day 1 - Investigating the effect of light on photosynthesis
                                               - Key Elements to include in your experimental design
                                               - See Criteria for Lab
                                               - Analyzing CAM and C4 plants  Handout and key
April 24 (and tutorial)                                     Concept 2: Analyzing the Processes of Photosynthesis
                                                Lab: AP Inv 5: Photosynthesis Lab Day 2 - Your experimental design!
                                              - See Criteria for Lab
April 26                                 Concept 2: Analyzing the Processes of Photosynthesis
 Photosynthesis Free Response Practice Question & answer
                                            - AP Test Format: Read pg. 10-24 in Holtzclaw
                                            - Our Unit Test: 75 min: (40 min: 28 MC, 2 Grid-in; 3 min: Free Response Reading; 32 min: 1 long free, 2 short free)
April 30th                                     Checkpoint #2 - Concept 2   
                                                - Photosynthesis Labs Due
                                                   - See Criteria   
May 2                                   Unit Test