8.1C: The Two Parts of Photosynthesis - Biology

8.1C: The Two Parts of Photosynthesis - Biology

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Light-dependent and light-independent reactions are two successive reactions that occur during photosynthesis.

Learning Objectives

  • Distinguish between the two parts of photosynthesis

Key Points

  • In light-dependent reactions, the energy from sunlight is absorbed by chlorophyll and converted into chemical energy in the form of electron carrier molecules like ATP and NADPH.
  • Light energy is harnessed in Photosystems I and II, both of which are present in the thylakoid membranes of chloroplasts.
  • In light-independent reactions (the Calvin cycle), carbohydrate molecules are assembled from carbon dioxide using the chemical energy harvested during the light-dependent reactions.

Key Terms

  • photosystem: Either of two biochemical systems active in chloroplasts that are part of photosynthesis.

Photosynthesis takes place in two sequential stages:

  1. The light-dependent reactions;
  2. The light-independent reactions, or Calvin Cycle.

Light-Dependent Reactions

Just as the name implies, light-dependent reactions require sunlight. In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and converted into stored chemical energy, in the form of the electron carrier molecule NADPH (nicotinamide adenine dinucleotide phosphate) and the energy currency molecule ATP (adenosine triphosphate). The light-dependent reactions take place in the thylakoid membranes in the granum (stack of thylakoids), within the chloroplast.


The process that converts light energy into chemical energy takes place in a multi-protein complex called a photosystem. Two types of photosystems are embedded in the thylakoid membrane: photosystem II ( PSII) and photosystem I (PSI). Each photosystem plays a key role in capturing the energy from sunlight by exciting electrons. These energized electrons are transported by “energy carrier” molecules, which power the light-independent reactions.

Photosystems consist of a light-harvesting complex and a reaction center. Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. In photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product. In photosystem I, the electron comes from the chloroplast electron transport chain.

The two photosystems oxidize different sources of the low-energy electron supply, deliver their energized electrons to different places, and respond to different wavelengths of light.

Light-Independent Reactions

In the light-independent reactions or Calvin cycle, the energized electrons from the light-dependent reactions provide the energy to form carbohydrates from carbon dioxide molecules. The light-independent reactions are sometimes called the Calvin cycle because of the cyclical nature of the process.

Although the light-independent reactions do not use light as a reactant (and as a result can take place at day or night), they require the products of the light-dependent reactions to function. The light-independent molecules depend on the energy carrier molecules, ATP and NADPH, to drive the construction of new carbohydrate molecules. After the energy is transferred, the energy carrier molecules return to the light-dependent reactions to obtain more energized electrons. In addition, several enzymes of the light-independent reactions are activated by light.


Photosynthesis sustains virtually all life on planet Earth providing the oxygen we breathe and the food we eat it forms the basis of global food chains and meets the majority of humankind's current energy needs through fossilized photosynthetic fuels. The process of photosynthesis in plants is based on two reactions that are carried out by separate parts of the chloroplast. The light reactions occur in the chloroplast thylakoid membrane and involve the splitting of water into oxygen, protons and electrons. The protons and electrons are then transferred through the thylakoid membrane to create the energy storage molecules adenosine triphosphate (ATP) and nicotinomide-adenine dinucleotide phosphate (NADPH). The ATP and NADPH are then utilized by the enzymes of the Calvin-Benson cycle (the dark reactions), which converts CO2 into carbohydrate in the chloroplast stroma. The basic principles of solar energy capture, energy, electron and proton transfer and the biochemical basis of carbon fixation are explained and their significance is discussed.

Keywords: membrane photosynthesis thylakoid.


Figure 1. The global carbon cycle

Figure 1. The global carbon cycle

The relationship between respiration, photosynthesis and global CO 2…

Figure 2. Location of the photosynthetic machinery

Figure 2. Location of the photosynthetic machinery

( A ) The model plant Arabidopsis thaliana…

Figure 3. Division of labour within the…

Figure 3. Division of labour within the chloroplast

The light reactions of photosynthesis take place…

Figure 4. The photosynthetic electron and proton…

Figure 4. The photosynthetic electron and proton transfer chain

The linear electron transfer pathway from…

Figure 5. Z-scheme of photosynthetic electron transfer

Figure 5. Z-scheme of photosynthetic electron transfer

The main components of the linear electron transfer…

Figure 6. Major photosynthetic pigments in plants

Figure 6. Major photosynthetic pigments in plants

The chemical structures of the chlorophyll and carotenoid…

Figure 7. Basic absorption spectra of the…

Figure 7. Basic absorption spectra of the major chlorophyll and carotenoid pigments found in plants

Figure 8. Jablonski diagram of chlorophyll showing…

Figure 8. Jablonski diagram of chlorophyll showing the possible fates of the S 1 and…

Figure 9. Basic mechanism of excitation energy…

Figure 9. Basic mechanism of excitation energy transfer between chlorophyll molecules

Two chlorophyll molecules with…

Figure 10. Basic structure of a photosystem

Figure 10. Basic structure of a photosystem

Light energy is captured by the antenna pigments…

Figure 11. Basic structure of the PSII–LHCII…

Figure 11. Basic structure of the PSII–LHCII supercomplex from spinach

The organization of PSII and…

Figure 12. S-state cycle of water oxidation…

Figure 12. S-state cycle of water oxidation by the manganese cluster (shown as circles with…

Figure 13. Basic structure of the PSI–LHCI…

Figure 13. Basic structure of the PSI–LHCI supercomplex from pea

The organization of PSI and…

Figure 14. Cytochrome b 6 f complex

Figure 14. Cytochrome b 6 f complex

( A ) Structure drawn from PDB code 1Q90. (…

Figure 15. Lateral heterogeneity in thylakoid membrane…

Figure 15. Lateral heterogeneity in thylakoid membrane organization

( A ) Electron micrograph of the…

Figure 16. The Calvin–Benson cycle

Figure 16. The Calvin–Benson cycle

Overview of the biochemical pathway for the fixation of CO…

( A ) Structure of the Rubisco enzyme (the large subunits are…

Figure 18. Diagram of a C 4…

Figure 18. Diagram of a C 4 plant leaf showing Kranz anatomy

Figure 19. The C 4 pathway (NADP…

Figure 19. The C 4 pathway (NADP + –malic enzyme type) for fixation of CO…

Process of Photosynthesis

Photosynthesis is the main process which drives life on Earth. Through photosynthesis, energy from the sun is captured in the bonds of organic molecules. These molecules, glucose molecules, are the basis of all life on Earth. Glucose will be used by the process of cellular respiration to harness chemical energy stored within the covalent bonds of the sugar.

Photosynthesis occurs in the leaves and green parts of plants. Organelles within plant cells, known as chloroplasts, contain specialized proteins capable of interacting with light. Cytochromes are these specialized proteins, which are attached to a heme group. Heme groups are also seen bound to hemoglobin, in blood cells. Instead of iron, these heme cells bind magnesium. The complex structure of the heme interacts with the photons of light passing through them.

The chloroplast uses the energy harnessed from these photons and their interaction with the cytochromes and other proteins to drive the formation of glucose. To do this, the chloroplasts will combine units of carbon dioxide into chains of 6 carbons, 12 hydrogens, and 6 oxygens. This is glucose, which can then be modified and combined with other glucose molecules to be stored as starches and complex sugars like fructose.

Photosynthesis Reaction

The photosynthesis reaction has two parts, commonly referred to as the Light reactions and the Calvin Cycle. The entire process of photosynthesis can be seen below.

ATP and NADPH are then used within the Calvin Cycle, a series of reactions which recycles these electron-carriers and produces glucose. The energy within and the hydrogen molecules are used to energize reactions throughout the cycle. The Calvin Cycle has three phases, carbon fixation, reduction, and regeneration of ribose. These reactions can be seen in the image below. Notice that the addition of one carbon dioxide in one turn of the reaction produces the 3-carbon molecule 3-phoshphoglycerate. Two of these molecules are then combined to produce a glucose, among other things.

Comparing the Rate of Photosynthesis (With Diagram)

Wilmott’s bubbler consists of a wide mouthed bottle fitted with a cork through which is inserted a glass tube. The lower end of this tube is fitted with a cork with hole through which a twig of Hydrilla plant is inserted its other end terminates in a narrow bent nozzle.

The upper half of this tube is surrounded by another glass tube which acts as water reservoir. Whole of the apparatus is filled with water (preferably of the pond from where the Hydrilla plant has been collected). Care is taken so that the level of water in the reservoir remains above the bent nozzle (Fig. 11.35). The apparatus is placed in sun light.

After sometimes as a result of the photosynthesis, O, bubbles come out from the cut end of the Hydrilla twig which gradually find their exit through the nozzle and can easily be counted. The rate of photosynthesis under different conditions can be studied by plotting the number of bubbles against specified period of time (e.g., 0.5 or 1 min.) or by noting the time in which a specified number or bubbles (e.g., 5 or 10) are released.

For comparing the effects of different wavelengths of light the bubbler is wrapped in cellophane papers of different colours. Effect of different intensities of light can be compared by placing the bubbler under shade, less intense and more intense light. The intensity of the light can be measured by a lux meter.

The effect of varying concentrations of CO, on the rate of photosynthesis can be observed by adding increasing quantities of sodium bicarbonate (50-100 mg at one time) in the bottle of the bubbler. Similarly, effect of temperature on the rate of photosynthesis may be compared by warming the apparatus at different temperatures.

Asexual Reproduction

Flowering plants can self-propagate through asexual reproduction. This is accomplished through the process of vegetative propagation. Unlike in sexual reproduction, gamete production and fertilization do not occur in vegetative propagation. Instead, a new plant develops from parts of a single mature plant. Reproduction occurs through vegetative plant structures derived from roots, stems, and leaves. Vegetative structures include rhizomes, runners, bulbs, tubers, corms, and buds. Vegetative propagation produces genetically identical plants from a single parent plant. These plants mature faster than and are sturdier than plants that develop from seeds.

CAM Photosynthesis

CAM is an abbreviation of crassulacean acid metabolism. In this type of photosynthesis, organisms absorb sunlight energy during the day then use the energy to fix carbon dioxide molecules during the night. During the day, the organism's stomata close up to resist dehydration while the carbon dioxide from the previous night undergoes the Calvin cycle. CAM photosynthesis allows plants to survive in arid climates and therefore is the type of photosynthesis used by cacti and other desert plants. However, non-desert plants like pineapples and epiphyte plants such as orchids also use CAM photosynthesis.


Learn about the process that plants, algae, and some bacteria use to make their own food and the oxygen we breathe.

(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

(adenosine triphosphate) chemical found in most living cells and used for energy.

(singular: bacterium) single-celled organisms found in every ecosystem on Earth.

series of reactions that take place during photosynthesis, where carbon dioxide and water from the atmosphere are converted into sugar.

greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.

smallest working part of a living organism.

part of the cell in plants and other autotrophs that carries out the process of photosynthesis.

negatively charged subatomic particle.

"simple sugar" chemical produced by many plants during photosynthesis.

smallest physical unit of a substance, consisting of two or more atoms linked together.

chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.

process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.

organism that produces its own food through photosynthesis and whose cells have walls.

Media Credits

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Tyson Brown, National Geographic Society


National Geographic Society

Production Managers

Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society

Program Specialists

Sarah Appleton, National Geographic Society
Margot Willis, National Geographic Society

Specialist, Content Production


André Gabrielli, National Geographic Society

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What, there are three different types of photosynthesis?

As if Photosynthesis was not complicated enough, there are actually different variations of how plants convert CO2 (Carbon dioxide) to C6H12O6 (Carbohydrates). Plants have various physiologies to adapt to various environments on earth. Alfalfa for instance can remain persistent and prolific during certain drought episodes due to its deep taproot that can help the plant utilize deep water sources. In term this causes the Alfalfa legume to be sensitive to poorly drained soils that are not very permeable to surface water. So the question someone could ask is do all desert plants have long roots? The answer is no, but one way desert plants conserve water and grow in a hot and arid climate is by the way they photosynthesize.

The three main types of photosynthesis are C3, C4, and CAM (crassulacean acid metabolism). In college I had to memorize some of their pathways and mechanisms, but I will highlight what gives one an advantage over another and what types of crops, forages, and weeds have specialized C3 and C4 photosynthesis. This will tell us why they can do well in certain climates and times of the year and when we can expect certain plants to be more abundant.

Rubisco is the name of the enzyme (protein) that “grabs” the CO2 molecule and puts it into the assembly line that will create the carbohydrates. It is known as the most abundant protein in the world. When we examine the quality of feed in our forages, it is rubisco that makes up most of the protein value in the forage analysis. That is one of the main reasons leaves are desired over stems in hay.

C3 photosynthesis is the predominant way plants will take in carbon dioxide and produce carbohydrates. In C3 photosynthesis Rubisco takes the CO2 and it is reduced into carbohydrates all in the same place and time. By that, I mean in the same cell chloroplast and during the day (sunshine) when the stomata are open and the CO2 is entering the cell and the water is leaving through the same opening. The issue with this is that it has the greatest water loss and during very high photosynthetic times (July) it becomes stressful for the plant. Another issue is that oxygen is generated during photosynthesis and the oxygen will inhibit rubisco and slow photosynthesis down when the system is running very fast. It seems counterintuitive, but the slow down allows the plant to deal with too much light that could cause damage. Ever notice that cool season grasses do not grow too fast in July and August? Cool season grasses have a C3 photosynthesis mechanism.

Now let us transition to some of the C4 grasses, also known as “warm season grasses” such as corn, sorghum, crab grass, sugarcane, bermuda grass, and foxtail. These plants have rubisco in one cell and they have a mechanism of pulling the CO2 in a different cell that is connected by openings between the cells called plasmodesmata connecting the two cells together. So what happens is that the plant can concentrate its CO2 where the rubisco is located and prevent that oxygen inhibition caused in the C3 mechanism. These plants don’t have that high sunlight, July inhibition. In addition to that, the specialization of the cells allows for approximately 40% less water usage per weight of CO2 reduction. This just means that it is 40% more efficient in water usage on average. There is always variation among species. C4 plants can also partially close their stomata to prevent water loss and because they concentrate the CO2 in a different area, the oxygen will not inhibit the rubisco enzyme. This is one of the major reasons why warm season paddocks are desired in a rotational grazing operation. It allows for growth during the July and August time period, when the cool season, C3 grasses are inhibited and not actively growing.

Here is the misconception many dicots (broadleaves) are also C4 plants, it is not just the grasses! Sedges and many of the Amaranthus species are C4 plants, they seem to be the largest plant families in this C4-broadleaf category. So Palmer Amaranth and Spiny Amaranth, along with the sedges do great in July and August. The fact that they are C4 plants could be contributing to this phenomenon. Knowing this allows a farmer to possibly tackle a weed before it takes over a field when a desirable cool season crop could be growing slowly or possibly dormant. Only 1% of all known plant species have C4 metabolism and even less have CAM metabolism.

Finally there is CAM photosynthesis. CAM is found in desert plants. What these plants do is open up their stomata at night to allow CO2 in to minimize the water loss during the hot days. The CO2 is stored in the plant vacuole as malic acid during the night. When the desert sun comes out, the stomatal openings are closed and the CO2 is “removed” from the malic acid to then be introduced to rubisco and make carbohydrates. By comparison, CAM is even more water efficient than C4 is. If C4 is 40% more water efficient, CAM is 83% more efficient as compared to most C3 photosynthetic processes. Cacti, many succulents, and the pineapple have CAM photosynthetic metabolism.

What two types of reactions occur in photosynthesis?

The process of photosynthesis is basically divided into two steps i.e Light reactions and dark reactions ( known as Calvin cycle or #C_3# pathway).

Breif concept to these is #2# processes is as follows:

Light reactions happens during daytime only. During light reactions, the sunlight energy is trapped and absorbed by photosynthetic pigments that are arranged to form photosystems. These photosystems are present in thylakoid membranes of chloroplast. This absorbed energy is converted into chemical energy of #ATP# and #NADPH# . Oxygen is released during non-cyclic photophosphorylation as a result of photolysis.

Dark reactions to not require light and can take place in the presence or absence of light. In these reactions, sugars are produced in stroma of chloroplast. using the assimilatory power #("ATP & NADPH"# ) produced during light reactions and carbon dioxide. Dark reactions can be summarized as:

#3CO_2# + #6NADPH# + #9ATP to (CH_2O)_3# + #6NADPH# +
#9ADP# + # 9# Pi

Two rubrics will be used to assess student lab report (sample of report included below) and student participation.
A written exam will be used to determine if a student has met the goals of the activity.

Example of student report (some areas are condensed down here to save space)
Student name ____________
Rate of Photosynthesis Research

Background: Where in a leaf does photosynthesis mainly occur? How does carbon dioxide get into a leaf? Where / how does oxygen leave a leaf? How does water get into a leaf from the roots?

Obtain a prepared slide of a leaf cross section (x-section). Using 100x make a sketch of
what you see.

Use the text book or internet to label the following tissues
Upper epidermis, lower epidermis, palisade mesophyll, spongy mesophyll,
vein (label both xylem and phloem), guard cells, stoma

1) In which of the labeled structures, does most of the photosynthesis occur? (hint
there are more chloroplasts here)

2) Through what structure does carbon dioxide get into the leaves so photosynthesis can occur?

3) What is the function of the guard cells?

4) Of what purpose does the spongy mesophyll serve to the leaf and the process of photosynthesis?

5) Through which of the labeled structures does water get to the leaves from the roots?

6) Through which of the labeled structures are sugars, that are made during photosynthesis,
transported to other parts of the plant where they can be used for energy or stored?

7) discuss the variations / adaptations that desert plants, water plants, and plants that grow
well in shade have in their leaves that allow them to survive in their particular environments.

Write out a balanced equation for photosynthesis:

Experimental Design
Your table will design an experiment to test how a selected variable affects the rate of photosynthesis. Follow the information below to make "sinking plant disks". You will measure how long it takes (in seconds) for the disks to float as a way to measure the rate of photosynthesis.

Preparation of the leaf disks:
1) Use the cork borer (to cut out the number of disks needed for your experiment).
2) Put disks in a syringe and suck up 5 cc (5 ml) of .2% sodium bicarbonate (baking soda)
3) Put finger over end of syringe, pull back on plunger to about the 35 cc mark (on a 60 cc syringe) and hold this position for 30 seconds. You should see air coming out the sides of the disks. As this is done, the oxygen is being removed from the spongy layer of the leaf and the .2% sodium bicarbonate is entering the spongy layer. This is the source of carbon dioxide needed for the plant to carry out photosynthesis
4) Carefully squirt out the .2% sodium bicarbonate. Suck up about 10 cc's of water. Check to see if the plant disks sink in the water. If they don't, remove the water and try steps 2 and 3 again.
5) Choose the disks that sink. Make sure enough disks are available to properly complete a controlled experiment. They are now ready to be used in your experimental set up. The disks will float when they have produced a measured amount of oxygen through photosynthesis. The time needed for the disks to float is an indirect measure
of the rate of photosynthesis occurring in the leaf disks.

Descriptive title of experiment:

The effect of ________________________________________________ on
the rate of photosynthesis.

Hypothesis (use if/ then format)

Explain the logic of the stated hypothesis

Sketch of the experimental design used. (the sketch should be specific enough so that the experiment could be reproduced exactly as it was set up include all measurements, angles, label materials / solutions used, wattage and type of light bulbs, etc.)

Is the data collected qualitative or quantitative data? discuss

The independent variable (manipulated) variable in the experiment is

The dependent variable (responding) in the experiment is

How was the experiment controlled?

Data chart: (you design, label and fill in with data)(you must have enough data to make a graph)

Graph of data (obtain a piece of graph paper, make appropriate graph that has a title and is properly labeled, attach graph to this lab report)

Results / discussion / analysis of data:

Findings/Conclusion/ list possible sources of error

Application to World environmental issues: (list ideas generated during brainstorming session)