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Lesson Video: Structure of the Chloroplast Biology

In this video, we will learn how to describe the structure of the chloroplast and to explain how the chloroplast is adapted for its function.

14:48

Video Transcript

In this video, we will learn how to describe the structure of the chloroplast. By discussing the different structures of the chloroplast, we will explain how the chloroplast is adapted for its function of storing light energy in the form of sugar molecules.

Plants are multicellular organisms that like humans require nutrition to keep them alive and functioning. Unlike humans, however, 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, which is known as photosynthesis, in specialized cell organelles called chloroplasts. If you were to look at some typical plant cells under a microscope, you’ll probably be able to see some chloroplasts. Let’s recap the process of photosynthesis before we talk about chloroplasts in more detail.

Photosynthesis is the process by which organisms like green plants convert carbon dioxide and water into sugars like glucose and oxygen. It must take place in the presence of light energy. This light energy can be from natural sunlight or from artificial light like from a lamp. The process of photosynthesis primarily takes place in the leaves, as this is where most of the chloroplasts will be found. But the parenchyma tissues of plant stems may also contain chloroplasts, so some photosynthesis can occur here too. Most plants cannot carry out photosynthesis in their roots. This is because most plant roots tend to grow below ground and they don’t receive the light required for photosynthesis.

Let’s take a closer look at a typical photosynthetic cell. This diagram is a basic outline of all the major organelles contained within a photosynthetic plant cell. Like all plant cells, it has a cell wall, supporting the cell and helping it to keep its structure. It also has a vacuole, which helps the cell maintain water balance. This cell is called a photosynthetic cell because it contains the light-sensitive pigment chlorophyll stored in a double–membrane bound organelle called a chloroplast. A plant cell will usually contain multiple chloroplasts and potentially up to hundreds of them.

Photosynthesis provides major benefits for the plant. The glucose that it produces is a sugar and it 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. Cellular respiration occurs in organelles called mitochondria, and it breaks down the glucose that’s produced in photosynthesis to release energy. The most common form of cellular respiration uses oxygen. So some of the oxygen that’s produced by photosynthesis can be reused in this process. Now, let’s take a closer look at the organelle in which photosynthesis occurs, the chloroplast.

As we mentioned briefly earlier, chloroplasts are examples of organelles that are surrounded by a double membrane. Between these two membranes is an intermembrane space. This double membrane is only around 10 nanometers thick, which is 0.000001 centimeters. 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 called a granum or plural grana. These grana are interconnected by thin membranous folds called lamellae, or a singular lamella. Within each thylakoid is an aqueous space called the lumen. We already mentioned that 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. Let’s look in more detail at the functions of each of the specialized structures within the chloroplast to explain how this happens.

Pigments are molecules that absorb some specific wavelengths of light and are believed to reflect others. The thylakoid membranes within the chloroplast contain specialized pigments called chlorophyll. Chlorophyll is able to absorb light energy and utilize it in photosynthesis. It is commonly found in plants in two forms: chlorophyll A and chlorophyll B.

This image shows an approximate representation of the visible light spectrum. Chlorophyll A and B can absorb light energy with different wavelengths. The pigment chlorophyll A will absorb the blue, most yellow, orange, and red wavelengths of light, but it is much worse at absorbing green wavelengths. When light energy is used, the light vanishes. As the blue, yellow, and red light energy is used, only the green light remains. It has been proposed that this is why many leaves that contain chlorophyll look green to the human eye.

Chlorophyll has a very complex structure. Chlorophyll A, for example, has the chemical formula C55H72O5N4Mg, and a part of it looks like this. The central magnesium ion is thought to be crucial for the ability of this pigment to absorb light. Interestingly, chlorophyll has a very similar structure to the heme group that’s found in hemoglobin in animals’ red blood cells, with the exception that the magnesium ion is replaced with an ion of iron and hemoglobin.

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. It is advantageous for a plant to have different photosynthetic pigments because they can use the light energy of different light wavelengths. These other pigments may also have functions of protecting chlorophyll from photooxidation. However, these other 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. Photosystems are functional and structural units of protein complexes involved in photosynthesis. 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 and oxygen as a byproduct. The second stage of photosynthesis, the light-independent stage or Calvin cycle, which uses the chemical energy from the light-dependent stage to transfer carbon dioxide into glucose, 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.

Now that we’ve discussed the different structures and functions of those structures of a chloroplast, let’s test our newly acquired knowledge on a couple of practice questions.

Which of the following statements correctly links the grana to the thylakoids? (A) Circular grana come together in stacks to form a single thylakoid. (B) Disklike structures called thylakoids form stacks called grana. Or (C) a single thylakoid cell is formed of multiple grana components.

Let’s review the structures within a chloroplast to answer this question. Chloroplasts are cell organelles that are surrounded by two membranes. The inside of a chloroplast is filled with a fluid matrix called the stroma. The stroma contains many enzymes along with the DNA of the chloroplast and ribosomes. It also contains disklike structures called thylakoids. These thylakoids, which are one of the key terms used in our question, form stacks of around 15 or more disks. A stack of thylakoids is called a granum or plural grana. This is the other key term that’s been used in the question.

The grana are interconnected by thin membranous folds called lamellae or a singular lamella. Within each thylakoid is an aqueous space called the lumen. So now we know the relationship between thylakoids and the grana, which is (B). Disklike structures called thylakoids form stacks called grana.

Let’s try and solve a second practice question together.

Absorption spectra for the visible light spectrum are provided. Which line shows the correct absorption for the photosynthetic pigment chlorophyll a?

We know that plants can use light to make their own food through a process called photosynthesis. You might remember that photosynthesis is the process of turning carbon dioxide and water into glucose and oxygen using light energy. The glucose sugar provides energy for growth, reproduction, and metabolism, so light is absorbed for the process of photosynthesis. This absorption is performed by photosynthetic pigments, such as chlorophyll a, which reside in the chloroplasts of a photosynthetic plant cell.

Let’s take a closer look at one of these chloroplasts. The chloroplast interestingly has two membranes, and it contains its own DNA within a fluid-filled space called the stroma. Within the stroma, there are also grana or a singular granum. Each granum is made up of disklike structures called thylakoids, and they are linked by lamellae, which in the singular form is a lamella. It’s the thylakoids within the chloroplast that contain the pigments that absorb the light energy to initiate photosynthesis. They contain chlorophyll a and chlorophyll b. Chlorophyll a is one of the most abundant photosynthetic pigments in a plant’s leaf. It absorbs red, orange, yellow, and blue light but not green. In fact, since green light is not absorbed, most plant leaves look green to our eyes.

Now, let’s take a look at the absorption spectra provided by the question. Each of the three black traces on the graph shows the level of light of different wavelengths that are absorbed by different molecules. So which one of the three black traces shows an absorption of red, orange, yellow, and blue but not green wavelengths of light? The solid line, line one, shows absorption peaks in all colors except green. Therefore, the line that illustrates the absorption spectrum for chlorophyll a is line one.

Let’s solve one final practice question together.

How are the thylakoids adapted for their function? (A) Thylakoids contain photosynthetic pigments within their membrane to absorb light. (B) Thylakoids have a small surface-area-to-volume ratio, so reactions can occur more quickly. (C) Thylakoids contain specialized enzymes that carry out the process of respiration. Or (D) thylakoids have a double membrane that allows them to control what enters and leaves the chloroplast.

Thylakoids are disklike structures contained within the chloroplasts of some plant cells. The thylakoids form stacks called grana or a singular granum, and their function is to assist the chloroplast with carrying out photosynthesis. The primary way that thylakoids are adapted to help the chloroplast with this function is by containing photosynthetic pigments within their membranes. These photosynthetic pigments, which are most commonly chlorophyll A in most vascular 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. But let’s double-check this by discussing the other choices. Option (B) is not correct as the disklike structures of the 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 although thylakoids may contain specialized enzymes, it is to aid the process of photosynthesis and not to aid respiration. Respiration is carried out by different organelles, the mitochondria. Option (D) is also incorrect as the thylakoids do not have a double membrane, and they also do not control what enters or leaves the chloroplast. Instead, this is the role of the double membrane that surrounds the chloroplast itself and not the membrane of the thylakoids.

So the correct answer is option (A). Thylakoids contain photosynthetic pigments within their membrane to absorb light.

Let’s summarize the key points that we’ve learned in this video about the structure of the chloroplast. Chloroplasts are organelles within a plant cell that acts as a site of photosynthesis. They have a double membrane and contain thylakoids arranged into stacks called grana or a singular granum suspended in a fluid called the stroma. Chloroplasts contain photosynthetic pigments, which are primarily chlorophyll A and B, but they 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.

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