In this explainer, we will learn how to describe the structure of the chloroplast and explain how the chloroplast is adapted for its function.
Plants are multicellular organisms that, like humans, require nutrition to keep them alive and functioning. Unlike humans, plants do not possess a specialized digestive system to take in and absorb nutrients from food sources. Amazingly, plants have evolved to solve this problem by making their own food! They carry out this process, known as photosynthesis, in specialized cell structures called chloroplasts. You can see chloroplasts within plant cells in the micrograph below.
Photosynthesis is the process by which green plants convert carbon dioxide and water into sugars, such as glucose, and oxygen in the presence of sunlight.
Let’s look at photosynthesis in more detail.
When plants photosynthesize, they take in carbon dioxide and water and convert them into sugars, primarily glucose, and oxygen using light energy. This light energy can be from natural sunlight or artificial light, like a lamp. This process primarily takes place in the leaves, as the majority of chloroplasts are found here, but the parenchyma tissues of plant stems may also contain chloroplasts to carry out a small amount of photosynthesis.
Photosynthesis provides major benefits for the plant. The glucose produced is a sugar and acts as a source of nutrition for the plant, allowing it to grow, reproduce, and carry out other essential life processes. One of these essential life processes is cellular respiration, which breaks down glucose to release energy. Cellular respiration uses oxygen, so some of the oxygen produced by photosynthesis can be reused in this process.
Definition: Cellular Respiration
Cellular respiration is a process in living organisms through which carbon-containing compounds (such as glucose) are broken down to release energy in the form of ATP.
Figure 2 shows a basic outline of the major organelles contained within a plant cell. A chloroplast is highlighted. A plant cell will usually contain multiple chloroplasts, potentially up to hundreds of them!
Figure 3 shows the structure of a single chloroplast in more detail. The chloroplasts are examples of organelles surrounded by a double membrane, with an intermembrane space between the two layers. This double membrane is only around 10 nanometres, or 0.0000001 centimetres, thick.
Inside the chloroplast is a fluid matrix called the stroma. The stroma contains many enzymes, along with the DNA of the chloroplast and ribosomes. It also contains starch grains, which act as insoluble stores of glucose. When needed by the plant, these grains are converted back into the soluble sugar glucose and translocated around the plant to be used in cellular respiration. Suspended in the stroma are disklike structures called thylakoids. These thylakoids form stacks of around 15 or more disks. A stack of thylakoids is a granum, the plural of which is grana. These grana are interconnected by thin membranous folds called lamellae, which help increase the available surface area to capture light energy. Within each thylakoid is an aqueous space called the lumen.
Chloroplasts are fairly unique organelles. We have already mentioned that, unlike most other organelles, they possess their own DNA and ribosomes. They also are surrounded by a double membrane. These features are similar to certain kinds of bacteria. This has led scientists to believe that chloroplasts were once separate prokaryotic organisms that formed a symbiotic relationship with the cells of plants. They were then incorporated into the cells of these ancestral plants and eventually evolved into plant cells as we know them today! This theory also applies to mitochondria and their presence in plant and animal cells, as they always contain their own DNA and ribosomes and are surrounded by a double membrane.
Example 1: Recalling the Structures Contained within A Chloroplast
Which of the following statements correctly links the grana to the thylakoids?
- Disklike structures called thylakoids form stacks called grana.
- Circular grana come together in stacks to form a single thylakoid.
- A single thylakoid cell is formed of multiple grana components.
Chloroplasts are the organelles within plant cells that act as the site of photosynthesis. There are multiple, specialized structures within a chloroplast that allow it to carry out this function. Let’s have a look at them, using the diagram below:
As we can see from the diagram, the thylakoids are disklike structures suspended in the matrix of the stroma. These thylakoids have a large surface area, which aids them in their role of capturing sunlight for photosynthesis. Multiple thylakoids are arranged in stacks called grana (singular: granum).
Therefore, the statement that correctly links the grana to the thylakoids is that disklike structures called thylakoids form stacks called grana.
Let’s look in more detail at the functions of each of the specialized structures within a chloroplast.
The primary role of the chloroplast is to act as the site of photosynthesis within a plant cell. To do this, it must be able to absorb light energy. Pigments are molecules that absorb some specific wavelengths of light and reflect others. The thylakoid membranes within the chloroplast contain specialized pigments, called chlorophyll, that are able to absorb this energy and utilize it in photosynthesis. Chlorophyll is commonly found in plants in two forms, chlorophyll A and chlorophyll B. Figure 4 shows how the pigment chlorophyll A will absorb the red, orange, blue, and most yellow wavelengths of light but is much worse at absorbing green wavelengths. It is because of this that many leaves that contain chlorophyll will look green to the human eye.
Key Term: Pigment
Pigments are molecules that absorb specific wavelengths of light and reflect others.
Chlorophyll is a class of green pigments found in the chloroplasts of plants that absorbs the light energy required for photosynthesis.
Pigment-containing organelles are called plastids. Due to its appearance, chloroplasts are also known as green plastids.
Example 2: Explaining How Thylakoids Are Adapted for Their Function
How are the thylakoids adapted for their function?
- Thylakoids contain photosynthetic pigments within their membrane to absorb light.
- Thylakoids have a small surface-area-to-volume ratio, so reactions can occur more quickly.
- Thylakoids have a double membrane that allows them to control what enters and leaves the chloroplast.
- Thylakoids contain specialized enzymes that carry out the process of respiration.
Thylakoids are disklike structures contained within the chloroplasts of plant cells. The thylakoids will form stacks, called grana, and their function is to assist the chloroplast with carrying out photosynthesis.
The primary way thylakoids are adapted to help the chloroplast with this function is by containing photosynthetic pigments within their membranes. These photosynthetic pigments, which are usually chlorophyll in vascular or flowering land plants, absorb available sunlight and transfer it to reaction centers, where the energy is used to carry out photosynthetic reactions.
Looking at our options, it seems that option A is the correct answer. However, we can double-check this by eliminating the other choices.
Option B will not be correct, as the disklike structure of thylakoids actually provides them with a large surface-area-to-volume ratio. It also allows them to contain more pigments and so increase the rate of photosynthetic reactions. Option C is also incorrect, as the thylakoids do not have a double membrane and also do not control what enters or leaves the chloroplast. This is the function of the chloroplast membrane. Option D is also incorrect, as although thylakoids may contain specialized enzymes, it is to aid the process of photosynthesis, not respiration. Respiration is carried out by a different organelle, the mitochondria.
Therefore, thylakoids are adapted for their function as they contain photosynthetic pigments within their membrane to absorb light.
Chlorophyll has a very complex structure. Figure 5 shows the molecular structure of chlorophyll A, which has the formula . The central magnesium ion is thought to be crucial for the ability of this pigment to absorb light. Interestingly, as we can see in Figure 5, chlorophyll has a very similar structure to the heme group found in the hemoglobin of animal red blood cells!
Chlorophyll is not the only pigment that may be contained within a chloroplast. Plants may also contain other photosynthetic pigments, such as xanthophyll and beta carotene, which are responsible for giving plant structures a yellow, orange, or reddish color. These pigments are not as common or abundant as chlorophyll pigments, so this explains why the majority of plant leaves are green.
In the thylakoid membranes, pigments like chlorophyll are contained within photosystems. Figure 6 shows a simplified diagram of a photosystem. The primary function of these structures is to absorb and utilize sunlight for the first stage of photosynthesis, which is called the light-dependent stage. This stage is made up of a series of reactions that absorb light energy and use it to split water to generate chemical energy in the form of ATP.
The second stage of photosynthesis, the light-independent stage (or Calvin cycle), is carried out in the stroma of chloroplasts. The stroma is well adapted for this, as it contains the necessary enzymes for the chemical reactions involved in the light-independent stage.
Example 3: Recalling the Locations of the Light-Dependent and Light-Independent Reactions in the Chloroplast
Which of the following tables correctly matches structures within the chloroplast to the photosynthetic reaction that occurs in each?
Structure Stroma Lamellae Reaction Light-dependent reaction Light-independent reaction Structure Inner membrane Outer membrane Reaction Light-dependent reaction Light-independent reaction Structure Chloroplast envelope Stroma Reaction Light-dependent reaction Light-independent reaction Structure Thylakoid Stroma Reaction Light-dependent reaction Light-independent reaction
Chloroplasts are specialized organelles found in plant cells that have a primary function of carrying out photosynthesis. They have multiple adaptations that allow them to do this, such as containing specialized photosystems to capture light or possessing enzymes that catalyze photosynthetic reactions.
There are two major stages of photosynthesis: light dependent and light independent (also known as the Calvin cycle).
The light-dependent stage, as the name suggests, requires light energy to take place. The thylakoids of the chloroplasts contain specialized structures within their membranes that are adapted to absorb light, called photosystems. Therefore, the light-dependent stage is primarily located within the thylakoid, specifically the thylakoid membranes.
The light-independent stage occurs after the light-dependent stage, as it uses some of the products formed. This stage requires specialized enzymes and coenzymes, along with the carbon dioxide that is taken in by plant leaves. All of these substances are found within the stroma of the chloroplast, which is the fluid-filled medium in which the other structures are contained. Therefore, the light-independent stage is primarily located within the stroma.
Therefore, our answer to this question should be option D: The thylakoid is the site of the light-dependent reaction, and the stroma is the site of the light-independent reaction.
Let’s summarize what we have learned about the structure of the chloroplasts in this explainer.
- Chloroplasts are the organelles within a plant cell that act as the site of photosynthesis.
- Chloroplasts have a double membrane and contain thylakoids arranged into stacks called grana suspended in the stroma.
- Chloroplasts contain photosynthetic pigments, which are primarily chlorophyll A and B, but may also contain pigments like beta carotene and xanthophyll.
- The thylakoids are the site of the light-dependent reactions of photosynthesis and contain photosynthetic pigments for this function.
- The stroma is the site of the light-independent reactions of photosynthesis and contains the required enzymes, coenzymes, and carbon dioxide.