Starch: Videos & Practice Problems

Starch is a homopolysaccharide composed of D-glucose units linked by alpha 1,4-glycosidic linkages, serving as an energy storage molecule in plants. It exists in two forms: amylose, which is unbranched, and amylopectin, which is branched with branch points linked by alpha 1,6-glycosidic linkages. Amylose forms helical structures, while amylopectin has branches occurring every 24 to 30 residues. Both forms are digestible, contributing to their role in energy storage for plant cells.

Starch

In this video, we're going to begin our lesson on the polysaccharide starch. So, it turns out that the polysaccharide starch actually has 2 different forms, and those two different forms of the polysaccharide starch are the amylose form and the amylopectin form. And so notice here in our table for the polysaccharide column, we're showing you starch, but we're also telling you that starch has 2 different forms, the amylose form and the amylopectin form. And what you'll notice here in this table is that these first columns here that are boxed in apply to all forms of starch, amylose and amylopectin included. And so they are going to be exactly the same when it comes to all of these boxes that we have boxed in. And so really the only way that amylose and amylopectin differ from each other is in this last box over here. And so we'll talk about that when we get there. But let's go ahead and get started.

So, all forms of starch, amylose and amylopectin included, are going to be homopolysaccharides, which, of course, we know from our previous lesson videos just means that it's going to be made up of just one single type of repeating sugar unit. And that repeating sugar unit is going to be D-glucose for both the amylose and amylopectin form. Now all of these D-glucose residues, in the starch molecules are going to be linked to each other via alpha-1,4-glycosidic linkages, which is a little bit different than the glycosidic linkages of our previous polysaccharides that we talked about. And so in our course, the alpha configuration in the glycosidic linkage of polysaccharides is going to indicate an energy storage function. And so starch, both amylose and amylopectin, is going to serve as an energy storage molecule in plant cells. And so the organism is, of course, going to be plants. Now again, notice that for all of these boxes here, they're going to be identical for both forms of starch, amylose and amylopectin. And again, the only way that they differ is through this last box over here, the branched box. And so, this is what distinguishes amylose from amylopectin. And so, amylose, in terms of being branched, it's actually not going to be branched. So it has absolutely no branching in the amylose form of starch. However, the amylopectin form of starch is kind of pectin. It's like pecking all over the place. So it definitely is going to be branched. So in terms of branching, we can say yes. And all of the branch points in amylopectin are going to be associated with alpha-1,6-glycosidic linkages, which we'll be able to visualize a little bit better in our next couple of videos when we talk about amylopectin specifically. But for now, this here concludes our table here for the two forms of starch, amylose and amylopectin. And in our next lesson video, we'll be able to talk a little bit more about amylose specifically. So I'll see you guys in that video.

Starch

So, in this video, we can visualize how Amylose is an unbranched form of starch, just like what we said up above in our chart in our last lesson video. And so, when you take a look at this image down below, notice that it's representing the structure of amylose. And notice that amylose's structure is indeed unbranched. It's just a long linear chain here. Now, notice over here towards the end, we're showing you that amylose does actually form these helical loops, but still, the chains themselves are unbranched and there are no side chains coming off of these amylose structures. And so if we take a closer look at the individual sugar units here in amylose's structure, you'll notice that all of these are just straight D-glucose molecules, and they're all linked via these alpha-1-4 glycosidic linkages.

Now if we take a look at the disaccharide here that's found within amylose's structure, you'll note that this is a disaccharide that we talked about in our previous lesson videos. And so we have 2 D-glucose molecules that are linked via an alpha-1-4 glycosidic linkage. This is, of course, going to be the disaccharide maltose. And so recall from our previous lesson videos, we said that maltose is digestible to most animals. And so if maltose is digestible to most animals, that's going to translate to starch also being digestible to most animals. And so this here concludes our lesson on how amylose is an unbranched form of starch, and in our next lesson video, we'll be able to take a closer look at amylopectin. So I'll see you guys there.

Starch

Alright. So in this video, we can visualize how amylopectin is really just a branched form of amylose. And so the amylopectin branch points are going to occur on average every 24 to 30 residues or so, and they're going to be linked via alpha 1-6 glycosidic linkages instead of alpha 1-4 glycosidic linkages. But again, this is only going to occur at the branch points. And so, if we take a look at our image down below, notice it's supposed to be representing amylopectin structure. And so notice here we have our main chain of amylopectin, and so it goes through like this and then coming off of the main chain, we have these branches that I'm highlighting here in blue And so all of these blue are branches off of the main chain. So this is really the main difference between amylopectin, which is branched, and amylose, which is up above and is not branched. And so if we zoom in here on one of the branch points, which you'll notice if we, get rid of these colors here, that the branch point right here is connected to the main chain via an alpha 1-6 glycosidic linkage. And, really, it's only these branch points that are going to contain the alpha 1-6 linkages. And so, these circled, branch points in red down below will also have, alpha 1-6 glycosidic linkages. But all of the other linkages that we see are going to be alpha 1-4 glycosidic linkages throughout the entire thing. Again, the only ones that are going to have alpha 1-6 glycosidic linkages are, the branch points. And so, this here concludes our video on amylopectin structure and how it's a branch form of amylose, and we'll be able to get some practice applying the concepts that we've learned in our next couple of videos. So, I'll see you guys there.

Which of the following is a structural polysaccharide in plant cells?

• A. Glycogen.
• B. Amylose.
• C. Chitin.
• D. Starch.
• E. Cellulose.

Answer: (E)

Cellulose fibers resemble ___________ in proteins; whereas amylose's structure is similar to _____________.

• A. α-helices; β-sheets.
• B. β-sheets; α-helices.
• C. β-sheets; the hydrophobic core.
• D. α-helices; β-turns.
• E. β-turns; coiled-coils.

Answer: (B)

Cellulose, an unbranched β(1-4)-linked homopolysaccharide of D-glucose, differs from starch in that starch is:

• A. A β-1,6-linked D-mannose homopolysaccharide that is branched.
• B. A β-1,6-linked D-glucose homopolysaccharide that is unbranched.
• C. An α-1,6-linked D-glucose homopolysaccharide that can be branched or unbranched.
• D. An α-1,4-linked D-glucose homopolysaccharide that can be branched or unbranched.
• E. An α-1,4-linked D-mannose homopolysaccharide that is branched.

Answer: (D)

Here’s what students ask on this topic:

What are the two forms of starch and how do they differ?

Starch exists in two forms: amylose and amylopectin. Amylose is an unbranched, linear polymer of D-glucose units linked by α(1→4) glycosidic bonds. It forms helical structures. Amylopectin, on the other hand, is a branched polymer. It has a main chain of D-glucose units linked by α(1→4) glycosidic bonds, with branch points occurring every 24 to 30 residues, linked by α(1→6) glycosidic bonds. The branching in amylopectin allows for more rapid mobilization of glucose units, making it more readily accessible for energy release compared to amylose.

What is the primary function of starch in plants?

Starch serves as an energy storage molecule in plants. It is composed of D-glucose units, which can be broken down to release glucose when the plant needs energy. This energy storage function is facilitated by the α(1→4) glycosidic linkages in both amylose and amylopectin, and the α(1→6) linkages at the branch points in amylopectin. The stored glucose can be mobilized during periods of low photosynthetic activity or when the plant requires additional energy for growth and development.

How are the glucose units in amylose and amylopectin linked?

In amylose, the glucose units are linked by α(1→4) glycosidic bonds, forming a linear, unbranched chain. In amylopectin, the glucose units are also primarily linked by α(1→4) glycosidic bonds, but it has additional branch points where the glucose units are linked by α(1→6) glycosidic bonds. These branch points occur approximately every 24 to 30 glucose residues, creating a branched structure.

Why is starch digestible to most animals?

Starch is digestible to most animals because it is composed of D-glucose units linked by α(1→4) and α(1→6) glycosidic bonds, which can be broken down by enzymes such as amylase. Amylase hydrolyzes these glycosidic bonds, converting starch into maltose and eventually into glucose, which can be readily absorbed and utilized by the body for energy. The digestibility of starch is crucial for its role as an energy source in the diet of many animals, including humans.

What is the structural difference between amylose and amylopectin?

The primary structural difference between amylose and amylopectin is that amylose is a linear, unbranched polymer of D-glucose units linked by α(1→4) glycosidic bonds, while amylopectin is a branched polymer. Amylopectin has a main chain of D-glucose units linked by α(1→4) glycosidic bonds, with branch points linked by α(1→6) glycosidic bonds occurring every 24 to 30 residues. This branching allows amylopectin to be more compact and more readily mobilized for energy release compared to the linear structure of amylose.