5: Photosynthesis - Biology

5: Photosynthesis - Biology

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The energy that is harnessed from photosynthesis enters the ecosystems of our planet continuously and is transferred from one organism to another. Therefore, directly or indirectly, the process of photosynthesis provides most of the energy required by living things on earth. Photosynthesis also results in the release of oxygen into the atmosphere. In short, to eat and breathe, humans depend almost entirely on the organisms that carry out photosynthesis.

  • 5.1: Overview of Photosynthesis
    All living organisms on earth consist of one or more cells. Each cell runs on the chemical energy found mainly in carbohydrate molecules (food), and the majority of these molecules are produced by one process: photosynthesis. Through photosynthesis, certain organisms convert solar energy (sunlight) into chemical energy, which is then used to build carbohydrate molecules. The energy used to hold these molecules together is released when an organism breaks down food.
  • 5.2: The Light-Dependent Reactions of Photosynthesis
    How can light be used to make food? It is easy to think of light as something that exists and allows living organisms, such as humans, to see, but light is a form of energy. Like all energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is transformed into chemical energy, which autotrophs use to build carbohydrate molecules. However, autotrophs only use a specific component of sunlight .
  • 5.3: The Calvin Cycle
    Carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? The carbon atoms used to build carbohydrate molecules comes from carbon dioxide, the gas that animals exhale with each breath. The Calvin cycle is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules.
  • 5.E: Photosynthesis (Exercises)

Thumbnail: Plant cells (bounded by purple walls) filled with chloroplasts (green), which are the site of photosynthesis. Image used iwth permission (CC BY-SA 3.0; Kristian Peters)


You should have a working knowledge of the following terms:

  • algae
  • carotenoid
  • chlorophyll
  • chloroplast
  • electromagnetic radiation
  • granum (pl. grana)
  • NADP+ and NADPH
  • photochemistry
  • photon
  • photophosphorylation
  • photosynthesis
  • photosystems I and II
  • pigment
  • thylakoid
  • wavelength
  • Z-scheme

Some organisms can carry out photosynthesis, whereas others cannot. An autotroph is an organism that can produce its own food. The Greek roots of the word autotroph mean “self” (auto) “feeder” (troph). Plants are the best-known autotrophs, but others exist, including certain types of bacteria and algae (Figure 5.2). Oceanic algae contribute enormous quantities of food and oxygen to global food chains. Plants are also photoautotrophs, a type of autotroph that uses sunlight and carbon from carbon dioxide to synthesize chemical energy in the form of carbohydrates. All organisms carrying out photosynthesis require sunlight.

Figure 5.2 (a) Plants, (b) algae, and (c) certain bacteria, called cyanobacteria, are photoautotrophs that can carry out photosynthesis. Algae can grow over enormous areas in water, at times completely covering the surface. (credit a: Steve Hillebrand, U.S. Fish and Wildlife Service credit b: “eutrophication&hypoxia”/Flickr credit c: NASA scale-bar data from Matt Russell)

Heterotrophs are organisms incapable of photosynthesis that must therefore obtain energy and carbon from food by consuming other organisms. The Greek roots of the word heterotroph mean “other” (hetero) “feeder” (troph), meaning that their food comes from other organisms. Even if the food organism is another animal, this food traces its origins back to autotrophs and the process of photosynthesis. Humans are heterotrophs, as are all animals. Heterotrophs depend on autotrophs, either directly or indirectly. Deer and wolves are heterotrophs. A deer obtains energy by eating plants. A wolf eating a deer obtains energy that originally came from the plants eaten by that deer. The energy in the plant came from photosynthesis, and therefore it is the only autotroph in this example (Figure 5.3). Using this reasoning, all food eaten by humans also links back to autotrophs that carry out photosynthesis.

Figure 5.3 The energy stored in carbohydrate molecules from photosynthesis passes through the food chain. The predator that eats these deer is getting energy that originated in the photosynthetic vegetation that the deer consumed. (credit: Steve VanRiper, U.S. Fish and Wildlife Service)

5: Photosynthesis - Biology

Have you ever noticed that plants need sunlight to live? It seems sort of strange doesn't it? How can sunlight be a type of food? Well, sunlight is energy and photosynthesis is the process plants use to take the energy from sunlight and use it to convert carbon dioxide and water into food.

Three things plants need to live

Plants need three basic things to live: water, sunlight, and carbon dioxide. Plants breathe carbon dioxide just like we breathe oxygen. When plants breathe carbon dioxide in, they breathe out oxygen. Plants are the major source of oxygen on planet Earth and help keep us alive.

We know now that plants use sunlight as energy, they get water from rain, and they get carbon dioxide from breathing. The process of taking these three key ingredients and making them into food is called photosynthesis.

How do plants capture sunlight?

Plants capture sunlight using a compound called chlorophyll. Chlorophyll is green, which is why so many plants appear green. You might think at first that it's green because it wants to absorb and use green light. However, from our study of light, we know that the color we see is actually the color of light that is reflected. So chlorophyll actually reflects green light and absorbs blue and red light.

More details on Photosynthesis

Inside a plant's cells are structures called chloroplasts. It's in these structures where the chlorophyll resides.

There are two main phases to the process of photosynthesis. In the first phase, sunlight is captured by the chloroplasts and the energy is stored in a chemical called ATP. In the second phase, the ATP is used to create sugar and organic compounds. These are the foods plants use to live and grow.

The first phase of the process must have sunlight, but the second phase can happen without sunlight and even at night. The second phase is called the Calvin Cycle because it was discovered and described by scientist Melvin Calvin.

Even though plants need sunlight and water to live, different plants need different amounts of each. Some plants need just a little water while others need a lot. Some plants like to be in the direct sunlight all day, while others prefer the shade. Learning about the needs of plants can help you learn where to plant them in your yard and how best to water them so they will flourish.

Now we know that plants need sunlight, water, and carbon dioxide to live. They take these three components and use chlorophyll to help convert them into food, which they use for energy, and oxygen, which they breathe out and we use to live. All plants use photosynthesis, so they all need some sunlight.

Spurthi's AP Biology Notebook

Photosynthesis is the process by which plants convert light energy into chemical energy. Plants need sunlight, CO2 and H2O to make sugar, which is the site of energy storage. This process takes place in the chloroplasts. The by-product of photosynthesis is oxygen. In aquatic plants, the photosynthetic rate, which is the rate of photosynthesis, can be measured by bubble production.

Observing the bubble production of the leaves of Elodea, an aquatic plant, is a simple way to measure the photosynthetic rate of a plant that is the purpose of this experiement. We will vary the light intensity by observing the plants at different distances from a constant light source. Then, since CO2 is necessary for photosynthesis, we will be varying the amounts of CO2 in the solution by adding sodium bicarbonate to see the effect of this gas on the photosynthetic rate. We hypothesize that if we add sodium bicarbonate and maximize light intensity, the photosynthetic rate will be higher than low light intensity and no sodium bicarbonate.

By observing bubble production of Elodea, we are able to measure the photosynthetic rate of the plant. First we need to make sure we have four test tubes, a test tube rack, and tap water, sprigs of Elodea, scissors, Bromothymol, beaker, light source, and measuring tool like a meter stick. Set up the four test tubes in the test tube rack and fill them with tap water and label each of the test tube with a number (1, 2, 3 and 4). Next take four sprigs of Elodea and cut equal-sized pieces (a few cm long) from the growing tips of each. Then place each of the four pieces in each of the four test tubes with the cut end up. Next fill up the beaker with cool tap water and place it right in front of the light source (preferably a photo flood lamp).Then measure 25 cm from the light and place the test tube rack there. The beaker will insulate the tubes from excess heat.

Leave the test tube rack at its position for five minutes. After five minutes, watch for bubbles at the cut end of the Elodea pieces especially at the bottom of the leaves. Then at 5-minute interval count the number of bubbles that formed in each test tube. As you tally the bubbles that form, record your data. After the counting interval, move the test tube rack to a 50 cm spot from the light source and repeat the same steps as you did after you placed the test tube 25 cm from the light source. After you are finished repeating the process, repeat it once more except this time move the test tubes 75 cm from the light source.

After you record your data for the bubble formation of test tubes 75 cm away from the light source, replace the water in test tubes 1 and 2 with fresh water. Clean out the water in test tubes 3 and 4 and replace it with 0.5% of sodium bicarbonate. Either in test tube 1 or 2, place one drop of Bromothymol to test for carbonic acid. If that solution in that test tube turned yellow/green we know it’s positive. After putting these four test tubes back into the test tube rack, place the rack 25 cm from the light source once again. Let the rack sit in its position for 5 minutes before you start counting the bubbles that occur at a 5 minute interval time. After you record your results for bubble count, clean up and dispose the Elodea pieces.

5: Photosynthesis - Biology

This lab gives students the opportunity to investigate what factors affect the rate of photosynthesis in floating leaf disks. They will design and conduct their own experiment to explore the effect of certain factors, such as different environmental variables, on the rate of photosynthesis. They will choose these factors or variables and hypothesize the effect they have on photosynthesis rates. To do this, they will use the floating leaf disk technique.

The floating leaf disk technique is a system that measures the accumulation of oxygen and therefore measures the rate of photosynthesis. When the leaf disks are submerged in water, oxygen bubbles become trapped in the air spaces between spongy mesophyll in the leaf. By drawing out these air bubbles from the spongy mesophyll, the spaces will be filled with the solution they’re in, allowing the leaf disks to sink. Only if the solution has enough bicarbonate ions (carbon dioxide) and enough light, will the leaf disks photosynthesize, collect oxygen, and float back to the surface again. The length of time it takes for those leaf disks to rise back to the top is a measure of the net weight of photosynthesis.

Using the floating leaf disk technique, students will be able to investigate the effect the factors of their choice will have on the rate of photosynthesis. They will set a control group that has no factors or variables and they will set an experimental group(s) that include the variables of their choice. To compare the rates of photosynthesis between these groups, they will compare the amount of floating leaf disks in each group. The larger the amount of floating leaf disks at a given time means the higher rate of photosynthesis. Their results should allow them to either accept or reject their hypothesis.

YouTube Video

  • Photosynthesis converts light energy into the chemical energy of sugars and other organic compounds.
  • The following equation summarizes photosynthesis: 6 CO2 + 6 H2O → 6(CH2O) + 6 O2 (sugar)
  • Leaf anatomy

Oxygen is a byproduct of photosynthesis.

A high rate of photosynthesis means a high rate of oxygen production.

The production of oxygen in the leaf disks causes them to rise to the surface.

The rate of photosynthesis increases when there is a lot of light present.

There are several properties/variables that affect the rate of photosynthesis including light intensity, color and direction, temperature, leaf color, size, and type. The effect these properties/variables have on photosynthesis varies.

Where does carbon dioxide enter through a plant?

Which of the following is an incorrect statement about the light-dependent reactions of photosynthesis?

ATP is the only product of the light-dependent reactions.

The light-dependent reactions occur in the thylakoid membrane.

Photosystem I and Photosystem II are involved in the light-dependent reactions.

Two separate light-dependent pathways occur in plants.

Where does the Calvin cycle occur in C4 plants?

In the Bundle sheath cells

The process by which ATP is made during light reactions is known as

Where does the first stage of photosynthesis occur?

Thylakoid membrane system

Which of the following reactions does cellular respiration and photosynthesis have in common?

The light-dependent and light-independent stages of photosynthesis are both linked together. Which of the following is generated by the light-independent reactions and is used in the light-dependent reactions?

Which of the following is not a type of pigment?

What is the source of the oxygen molecules that are produced through photosynthesis?

The process by which plants lose water via evaporation through their leaves is known as

The rate of photosynthesis may vary with changes that occur in environmental temperature, wavelength of light, and light intensity. Using a photosynthetic organism of your choice, choose only ONE of the three variables (temperature, wavelength of light, or light intensity) and for this variable

Design a scientific experiment to determine the effect of the variable on the rate of photosynthesis for the organism.

Explain how you would measure the rate of photosynthesis in your experiment.

Describe the results you would expect. Explain why you would expect these results.

Verbal or Graphic description (a,b)

Enzyme kinetics or metabolic changes

Stomatal closing w/ high temp, limits CO2 & lowers rate

Excessive water loss, less reactant available for reaction

Absorption/reflection of light by chlorophyll

Accessory pigments absorbing green light

Relation between wavelength & energy

More photons hit photosystems

More e- flow in the electron transport system/time

Plateau caused by limiting factors

Photosynthesis is often described as two series of reactions, the light-dependent and the light-independent or Calvin cycle reactions.

Describe all steps that occur in the light-dependent reactions, including raw materials, energy transfer, and products.

Explain how the products of the light-dependent reactions are necessary to drive the reactions of the Calvin cycle.

Describe and discuss the C-4 pathway that can occur in some photosynthetic plants. Include in your discussion:

AP Biology Blog - Mark Ingram P.7

How does the oxygen that we breathe enter the air? And how does the carbon dioxide that we exhale get taken out of the air so that we do not suffocate on it? The answer to both of these questions is photosynthesis, a chemical process for creating energy and sugars within a plant. Photosynthesis takes place in the leaves of a plant, and within the leaves' cells, in the chloroplast. In the chloroplast are specialized molecules called chlorophyll that absorb light and this absorption excites an electron that then goes through a complex series of reactions to form NADPH, an electron carrier, and ATP, an energy source for the cell.

An Overview of Photosynthesis

However, the chlorophyll only absorb a certain frequency of light, and it only absorbs light from the visible light spectrum, so colored light. The chlorophyll do not even absorb the entire spectrum of visible light, usually only absorbing one or two colors of light between a certain wavelength very well, and reflecting the rest. The plant has developed two different types of chlorophyll to account for that, chlorophyll a and chlorophyll b, as well separate compound called carotenoids.

Absorption Rates of Chlorophyll and Carotenoids

On Friday, November 6th, Vikram, Vinay, Shreyan and I entered the lab again to learn more about photosynthesis. We decided to test the rate of the reactions of photosynthesis under different colored light so that we could see to what affect the absorption of light by the chlorophyll and carotenoids affected the overall reaction. We hypothesized that the rate of reaction would slow when under green light because the chlorophyll and the carotenoids have very low absorption rates in green light as seen above. Also, leaves reflect green light, as seen in their green color.

Our first problem is finding a method by which we could measure the rate of photosynthesis in a plant. Because it is a minuscule process, we couldn't observe it directly through a microscope, but instead would have to find evidence that it is occurring. Here is the chemical equation for photosynthesis:

To find the rate of photosynthesis, we could track the production of one of the products above, or the diminishing of one of the products. Carbon dioxide is a possibility, but would be difficult to measure in an open environment. Water is likewise hard to track, as well as glucose, because we cannot directly see the number of molecules. This leaves us with oxygen, which is also difficult to track individual molecules. Instead of doing that, however, we decided to use the buoyant properties of oxygen in water to measure the rate of reaction. Because as oxygen is produced in leaves during photosynthesis it is kept in the center of the leaf before being expelled through the stomata, the leave actually becomes quite airy. So we could put the leaves in water and watch them float to the top. But to photosynthesize, there must be a source of light, a source of H2O and a source of CO2. If the leaves were submerged, they would not have access to CO2 and would therefore not photosynthesize. Therefore we submerged the leaves in a bicarbonate solution, which would put CO2 into the water for the plant to grab and use for photosynthesis.

The problem with this plan was that the leaves already contained oxygen, so they would float on their own. The only way around this was to evacuate the oxygen from the leave by sucking it out using pressure. At a lower pressure, the oxygen will expand and fill the space, leaving the leaf for the most part and entering the surrounding space around the leaf. Doing this evacuation on a leaf would require larger syringes and tools than we have available so instead we decided to cut the leaves into small dots so they'd fit in the syringe and it would even make the submersion in water easier because then we would not have to find a large tank to submerge full leaves in.

As you can see, a leaf consists of cells but also quite a bit of empty space to hold gases like CO2 and O2

For our experimental variable, which is the color of the light that the plant would be exposed to, we figured we could change this using different colored cellophane. The cellophane would absorb all of the light that wasn't its own color, and let that color through to hit the leaf-dots below. For our controls we performed an experiment with clear cellophane that would supposedly let all light through, but we also did an experiment without cellophane to see if the clear stuff did in fact absorb even a tiny amount of light.

Because of the time consuming nature of the reaction and its effects on the leaf-dots, and the limitations of our equipment, the lab group joined forces with a lab group consisting of Christos, Jorgos and Callen. One group would do three cups with different variables, and the other would do three different cups with different variables, and the data would be shared among us all. Vikram, Vinay, Shreyan and I did the experiments with blue cellophane, yellow cellophane and no cellophane at all.

With this plan of attack, we began our experiment.

First, the lab group went outside to pick two fresh ivy leaves to perform our experiment with. We needed to get still living leaves so that they would perform photosynthesis. Next, using the apparatus shown below, I hole-punched 30 dots of leaf out of the leaves and split them into 3 piles. These would be later used for our experiment.

Next, the group created 300mL of a 0.2% bicarbonate solution to submerge the leaves in. Then we evenly separated the solution into three cups and cut three pieces of cellophane for the cups one blue, one yellow and one clear. Then a drop of dilute soap was added to each cup. The soap acts as a wetting agent to force the leaf dots to allow water from the solution into the cell and sink. This also helps the leaves get their source of CO2 from the solution.

Next we evacuated the leaf dots of all air. To do this, we put 10 dots at a time into a syringe with a stopper and sucked up 10 mL of the bicarbonate solution into the syringe. Then we upended the syringe and squeezed the stopper so all of the air in the solution was evacuated. Once this was done, I put my finger on the opening of the syringe and pulled back on the stopper, effectively creating a vacuum in the syringe chamber, for 10 seconds. I then released the stopper and poured out the 10 leaf dots into one of the cups. This was repeated for each separate group of leaf dots.

Once the first set of leaf dots was evacuated, we immediately started our experiment so as to avoid sunlight causing the leaves to begin their photosynthesis while we evacuated the rest of the dots. This would've caused a skew in our data, but luckily we removed that confounding variable. The experiment consisted of a piece of colored (or clear) cellophane covering the cup and the cup was then placed under a lamp. The light from the lamp then caused the cells in the leaf dots to photosynthesize. We also started a separate timer for each experiment group so we could accurately keep time.

After all of the dots were in their cups and the cups were placed under the lamp, we observed the number of leaf dots that had risen completely to the top and recorded this data. After all the dots had risen, or 30 minutes, the experiment was stopped, and the dots were poured out.

Here are the results of trials with the blue, yellow and lack of cellophane.

LS1-5 to LS1-7 – Photosynthesis

Rate of Photosynthesis – this lab asks students to place elodea in a test tube and count bubbles under different conditions this lab is simpler than the AP Lab and requires a lab report.

Do Plants Consume or Release CO2 – using phenol red as an indicator, students observe changes in color in plants that are stored in the light and those stored in the dark

Photosynthesis Simulation – a virtual lab that measures plant growth and rate of photosynthesis under different colors and intensity of light

Waterweed simulator – another virtual lab that counts bubbles (oxygen) produced during photosynthesis.

Chemiosmosis Coloring – color the membrane of a chloroplast to to learn how water and electrons are shuffled to create ATP

Photosystems Labeling – practice labeling the photosystems

Photosynthesis and Respiration Model – students examine a model, focus on key details and answer an essential question about how the two processes are related

LS1-6 Construct and revise an explanation based on evidence for how carbon, hydrogen, and oxygen from sugar molecules may combine with other elements to form amino acids and/or other large carbon-based molecules.

  • Students construct an explanation that includes: a) The relationship between the carbon, hydrogen, and oxygen atoms from sugar molecules formed in or ingested by an organism and those same atoms found in amino acids and other large carbon-based molecules
  • Students identify and describe the evidence to construct the explanation, including:
    a) All organisms take in matter and rearrange the atoms in chemical reactions.
    b) Cellular respiration involves chemical reactions in which energy is released
    c) Sugar molecules are composed of carbon, oxygen, and hydrogen atoms.
    d) Amino acids and other carbon-based molecules are composed of carbon, oxygen, and hydrogen
    e) Chemical reactions can create products that are more complex than the reactants.
    f) Chemical reactions involve changes in the energies of the molecules involved in the reaction.

Concept Map – Organic Compounds – organizes the four main groups of organic compounds: nucleic acids, lipids, carbohydrates, and proteins details how these compounds are used by living systems

Elements found in living things – coloring shows the proportion of elements, C, H, N, P, K, O, S, and Ca

(Article) Wood Alcohol Poisonings – an article that helps students understand how a slight change in the chemical structure, ethyl to methyl can turn a substance into a poison.

LS1-7 Use a model to illustrate that cellular respiration is a chemical process whereby the bonds of food molecules and oxygen molecules are broken and the bonds in new compounds are formed resulting in a net transfer of energy

  • From a given model, students identify and describe the components of the model relevant for their illustration of cellular respiration, including: i. Matter in the form of food molecules, oxygen, and the products of their reaction (e.g., water and CO2) ii. The breaking and formation of chemical bonds and iii. Energy from the chemical reactions.
  • From the given model, students describe the relationships between components, including: a)Carbon dioxide and water are produced from sugar and oxygen b) The process of cellular respiration releases energy

(Lab) Respiration – using germinating and non-germinating seeds, measure the rate of oxygen consumption using respirometers
(Case Study) Mystery of the Seven Deaths – examine a case of poisoning, shows how cyanice interferes with the functioning of the mitochondria and cellular respiration

Cellular Respiration Concept Map – map of the steps involved in cellular respiration
Cellular Respiration Virtual Lab – AP Lab that can be performed online

The Carbon Cycle – simple diagram of the carbon cycle identify how respiration and photosynthesis are related

Classroom Leaf Disk Photosynthesis Kit | AP Biology

With this photosynthesis kit, students explore the impact of different variables on the photosynthetic rate. Includes a detailed teacher's manual, reproducible student worksheets & materials sufficient for 8 lab groups. Read More


AP® Biology Investigation #5: A Photosynthesis Kit from Innovating Science

This Classroom Leaf Disk Photosynthesis Kit contains the necessary components, and detailed instructions, to take students through the popular leaf disk photosynthesis demonstration.

In this lab, students will use leaf disks to measure the accumulation of oxygen and relate it to the rate of photosynthesis. They will use guided inquiry to explore the impact of different variables on the photosynthetic rate, and they will learn about the necessary aspects and conditions for photosynthesis to occur.

This photosynthesis lab kit includes a detailed teacher's manual, reproducible student worksheets, and materials sufficient for 8 lab groups. This lab meets AP Science Practices 1, 2, 3, 6, and 7, and Big Idea 2.


The Classroom Leaf Disk Photosynthesis Kit Includes:

  • 1 Hole Punch
  • 16 Dispensers, 10 ml
  • 16 Plastic Cups, 30 ml
  • Dilute Soap Solution
  • 50 g Sodium Bicarbonate

Materials Needed but not Supplied:

  • Living Plant Leaves
  • Light Source
  • Timers or Stopwatches
  • Distilled or Deionized Water


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