AP Biology 3.4 Photosynthesis Study Notes - New Syllabus Effective 2025
AP Biology 3.4 Photosynthesis Study Notes- New syllabus
AP Biology 3.4 Photosynthesis Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the photosynthetic processes and structural features of the chloroplast that allow organisms to capture and store energy.
Key Concepts:
- Photosynthesis
3.4.A.1 – Photosynthesis
Photosynthesis is the series of reactions that use carbon dioxide (CO2), water (H2O), and light energy to make carbohydrates and oxygen (O2).
i. Photosynthetic organisms capture energy from the sun and produce sugars that can be used in biological processes or stored.
ii. Photosynthesis first evolved in prokaryotic organisms.
iii. Scientific evidence supports the claim that prokaryotic (cyanobacterial) photosynthesis was responsible for the production of an oxygenated atmosphere.
iv. Prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis.
3.4.A.2 – Chloroplasts and Thylakoids
Chloroplasts are membrane bound organelles that are found in plant cells and green algae. They serve as the site of photosynthesis where the light energy is converted into chemical energy. The chloroplast houses the green pigment chlorophyll that absorbs sunlight for the production of organic compounds.
The chloroplasts have internally bound membranes called thylakoids that are arranged in stacks called grana. The thylakoids consist of the photosystems that serve as the site for absorption of sunlight. The surrounding space in the stacked grana is called stroma. The stroma contains dissolved enzymes, chloroplast genome and starch granules.
Stroma and thylakoids are found within the chloroplast.
i. The stroma is the fluid within the inner chloroplast membrane and outside the thylakoid. The carbon fixation (Calvin cycle) reactions of photosynthesis occur in the stroma.
ii. The thylakoid membranes contain chlorophyll pigments organized into two photosystems, as well as electron transport proteins.
iii. Thylakoids are organized in stacks called grana. The light reactions of photosynthesis occur in the grana.
3.4.A.3 – Light dependent reaction of photosynthesis
The light-dependent reactions occur in the thylakoid membranes of chloroplasts, whereas the Calvin cycle occurs in the stroma of chloroplasts. Embedded in the thylakoid membranes are two photosystems (PSI and PSII), which are complexes of pigments that capture solar energy. Chlorophylls a and b absorb violet, blue, and red wavelengths from the visible light spectrum and reflect green. The carotenoid pigments absorb violet-blue-green light and reflect yellow-to-orange light. Environmental factors such as day length and temperature influence which pigments predominant at certain times of the year. Although the two photosystems run simultaneously, it is easier to explore them separately. Let’s begin with photosystem II.
A photon of light strikes the antenna pigments of PSII to initiate photosynthesis. In the noncyclic pathway, PSII captures photons at a slightly higher energy level than PSI. (Remember that shorter wavelengths of light carry more energy.) The absorbed energy travels to the reaction center of the antenna pigment that contains chlorophyll a and boosts chlorophyll a electrons to a higher energy level. The electrons are accepted by a primary electron acceptor protein and then pass to the electron transport chain also embedded in the thylakoid membrane. The energy absorbed in PSII is enough to oxidize (split) water, releasing oxygen into the atmosphere; the electrons released from the oxidation of water replace the electrons that were boosted from the reaction center chlorophyll. As the electrons from the reaction center chlorophyll pass through the series of electron carrier proteins, hydrogen ions (H+) are pumped across the membrane via chemiosmosis into the interior of the thylakoid. If this sounds familiar, it should. We studied chemiosmosis in our exploration of cellular respiration in Cellular Respiration. This action builds up a high concentration of H+ ions, and as they flow through ATP synthase, molecules of ATP are formed. These molecules of ATP will be used to provide free energy for the synthesis of carbohydrate in the Calvin cycle, the second stage of photosynthesis. The electron transport chain connects PSII and PSI. Similar to the events occurring in PSII, this second photosystem absorbs a second photon of light, resulting in the formation of a molecule of NADPH from NADP+. The energy carried in NADPH also is used to power the chemical reactions of the Calvin cycle.
3.4.B.1 – Electron Transport Chain
The electron transport chain is a series of proteins and organic molecules found in the inner membrane of the mitochondria. Electrons are passed from one member of the transport chain to another in a series of redox reactions. Energy released in these reactions is captured as a proton gradient, which is then used to make ATP in a process called chemiosmosis. Together, the electron transport chain and chemiosmosis make up oxidative phosphorylation. The key steps of this process, shown in simplified form in the diagram above, include:
- Delivery of electrons by NADH and FADH2. Reduced electron carriers (NADH and FADH2) from other steps of cellular respiration transfer their electrons to molecules near the beginning of the transport chain. In the process, they turn back into NAD+ and FAD, which can be reused in other steps of cellular respiration.
- Electron transfer and proton pumping. As electrons are passed down the chain, they move from a higher to a lower energy level, releasing energy. Some of the energy is used to pump H+ ions, moving them out of the matrix and into the intermembrane space. This pumping establishes an electrochemical gradient.
- Splitting of oxygen to form water. At the end of the electron transport chain, electrons are transferred to molecular oxygen, which splits in half and takes up H+ to form water.
- Gradient-driven synthesis of ATP. As H+ ions flow down their gradient and back into the matrix, they pass through an enzyme called ATP synthase, which harnesses the flow of protons to synthesize ATP.
The electron transport chain is a collection of membrane-embedded proteins and organic molecules, most of them organized into four large complexes labeled I to IV. In eukaryotes, many copies of these molecules are found in the inner mitochondrial membrane. In prokaryotes, the electron transport chain components are found in the plasma membrane.
As the electrons travel through the chain, they go from a higher to a lower energy level, moving from less electron-hungry to more electron-hungry molecules. Energy is released in these “downhill” electron transfers, and several of the protein complexes use the released energy to pump protons from the mitochondrial matrix to the intermembrane space, forming a proton gradient.
3.4.B.2 – Photosynthesis
Photosynthesis is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabolism. Photosynthesis usually refers to oxygenic photosynthesis, a process that produces oxygen. Photosynthetic organisms store the chemical energy so produced within intracellular organic compounds (compounds containing carbon) like sugars, glycogen, cellulose and starches. To use this stored chemical energy, an organism’s cells metabolize the organic compounds through cellular respiration. Photosynthesis plays a critical role in producing and maintaining the oxygen content of the Earth’s atmosphere, and it supplies most of the biological energy necessary for complex life on Earth.
The first photosynthetic organisms probably evolved early in the evolutionary history of life using reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons. Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible. The average rate of energy captured by global photosynthesis is approximately 130 terawatts, which is about eight times the total power consumption of human civilization. Photosynthetic organisms also convert around 100–115 billion tons, of carbon into biomass per year. Photosynthesis was discovered in 1779 by Jan Ingenhousz who showed that plants need light, not just soil and water.
Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the hydrogen carrier NADPH and the energy-storage molecule ATP. During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide.
Most organisms that use oxygenic photosynthesis use visible light for the light-dependent reactions, although at least three use shortwave infrared or, more specifically, far-red radiation.
Some organisms employ even more radical variants of photosynthesis. Some archaea use a simpler method that employs a pigment similar to those used for vision in animals. The bacteriorhodopsin changes its configuration in response to sunlight, acting as a proton pump. This produces a proton gradient more directly, which is then converted to chemical energy. The process does not involve carbon dioxide fixation and does not release oxygen, and seems to have evolved separately from the more common types of photosynthesis.
3.4.B.3 – Photosystems
Photosystems, large complexes of proteins and pigments (light-absorbing molecules) that are optimized to harvest light, play a key role in the light reactions. There are two types of photosystems: photosystem I (PSI) and photosystem II (PSII).
Both photosystems contain many pigments that help collect light energy, as well as a special pair of chlorophyll molecules found at the core (reaction center) of the photosystem. The special pair of photosystem I is called P700, while the special pair of photosystem II is called P680.
In a process called non-cyclic photophosphorylation (the “standard” form of the light-dependent reactions), electrons are removed from water and passed through PSII and PSI before ending up in NADPH. This process requires light to be absorbed twice, once in each photosystem, and it makes ATP . In fact, it’s called photophosphorylation because it involves using light energy (photo) to make ATP from ADP (phosphorylation). Here are the basic steps:
Light absorption in PSII. – When light is absorbed by one of the many pigments in photosystem II, energy is passed inward from pigment to pigment until it reaches the reaction center. There, energy is transferred to P680, boosting an electron to a high energy level. The high-energy electron is passed to an acceptor molecule and replaced with an electron from water. This splitting of water releases the O2 we breathe.
ATP synthesis – The high-energy electron travels down an electron transport chain, losing energy as it goes. Some of the released energy drives pumping of H+ ions from the stroma into the thylakoid interior, building a gradient. H+ ions from the splitting of water also add to the gradient.) As H+ ions flow down their gradient and into the stroma, they pass through ATP synthase, driving ATP production in a process known as chemiosmosis.
Light absorption in PSI – The electron arrives at photosystem I and joins the P700 special pair of chlorophylls in the reaction center. When light energy is absorbed by pigments and passed inward to the reaction center, the electron in P700 is boosted to a very high energy level and transferred to an acceptor molecule. The special pair’s missing electron is replaced by a new electron from PSII (arriving via the electron transport chain).
NADPH formation – The high-energy electron travels down a short second leg of the electron transport chain. At the end of the chain, the electron is passed to NADP (along with a second electron from the same pathway) to make NADPH.
The net effect of these steps is to convert light energy into chemical energy in the form of ATP and NADPH. The ATP and NADPH from the light-dependent reactions are used to make sugars in the next stage of photosynthesis, the Calvin cycle. In another form of the light reactions, called cyclic photophosphorylation, electrons follow a different, circular path and only ATP (no NADPH) is produced.
3.4.B.4 – Thylakoid Membrane
Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as grana (singular: granum). Grana are connected by intergranal or stromal thylakoids, which join granum stacks together as a single functional compartment.
In thylakoid membranes, chlorophyll pigments are found in packets called quantasomes. Each quantasome contains 230 to 250 chlorophyll molecules.
The thylakoid membrane is the site of the light-dependent reactions of photosynthesis with the photosynthetic pigments embedded directly in the membrane. It is an alternating pattern of dark and light bands measuring one nanometer each. The thylakoid lipid bilayer shares characteristic features with prokaryotic membranes and the inner chloroplast membrane. For example, acidic lipids can be found in thylakoid membranes, cyanobacteria and other photosynthetic bacteria and are involved in the functional integrity of the photosystems. The thylakoid membranes of higher plants are composed primarily of phospholipids[5] and galactolipids that are asymmetrically arranged along and across the membranes. Thylakoid membranes are richer in galactolipids than phospholipids; they predominantly consist of hexagonal phase II forming monogalacotosyl diglyceride lipid. Despite this composition, plant thylakoid membranes have been shown to assume largely lipid-bilayer dynamic organization. Lipids forming the thylakoid membranes, rich in high-fluidity linolenic acid are synthesized in a complex pathway involving exchange of lipid precursors between the endoplasmic reticulum and inner membrane of the plastid envelope and transported from the inner membrane to the thylakoids via vesicles.
3.4.B.5 – Chemiosmosis
Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membrane during cellular respiration or photosynthesis.
In this process, ATP- Adenosine triphosphates are produced as the result of the proton gradient that exists across the thylakoid membrane. The fundamental components necessary for the chemiosmosis process are proton gradient, ATP synthase, and proton pump. The enzyme which is required for the synthesis of ATP molecules is called ATP synthase.
The enzyme ATP synthase consists of two subunits, namely: F0 and F1. The F0 subunit is involved in the proton transportation across the membrane, which causes changes in F1 configuration leading to the activation of enzymes. The enzyme phosphorylates ADP (meaning: to add a phosphate group) and converts ADP molecules to ATP molecules. The proton gradient that exists across the membrane is the primary driving force of ATP synthase.
In the light reaction phase of photosynthesis, chlorophyll absorbs light with the help of photosystems. This results in hydrolysis, where water molecules are split, releasing electrons and protons in the process. The released electrons get excited and move to a higher energy level and are carried by the electron transport system.
Meanwhile, the released protons from the stroma begin accumulating inside the membrane. This results in the creation of a proton gradient, a product of the electron transport chain. The small quantity of the resultant protons is used by photosystem I to reduce NADP+ to NADPH by electrons from the photolysis of water. Eventually, the proton gradient collapses, and it releases energy and protons that are carried out back to the stroma via F0 of ATP synthase. This resultant energy induces changes in F1 configuration and this, in turn, activates the ATP synthase, which converts ADP to ATP.
3.4.B.6 – Convert CO2 into Carbohydrate by energy captured during light dependent reactions
The energy captured during the light-dependent reactions of photosynthesis is stored in ATP and NADPH. This energy is then used to power the Calvin cycle, which occurs in the stroma of the chloroplast, to convert carbon dioxide into carbohydrates.
Elaboration:
Light-dependent reactions: These reactions occur in the thylakoid membranes of chloroplasts and use light energy to generate ATP and NADPH.
ATP and NADPH: These molecules are energy-carrying molecules that store the captured light energy.
Calvin cycle: This cycle is a series of chemical reactions that take place in the stroma of the chloroplast.
Carbon fixation: In the Calvin cycle, carbon dioxide from the atmosphere is incorporated into organic molecules.
Sugar production: The energy stored in ATP and NADPH is used to reduce the fixed carbon dioxide and ultimately produce carbohydrates.
Location: The entire Calvin cycle, including the carbon fixation, reduction, and regeneration steps, occurs within the stroma of the chloroplast.