Lipids - Online Tutor, Practice Problems & Exam Prep

Lipids are diverse macromolecules that are hydrophobic, meaning they do not mix well with water. Key types include phospholipids, fats, oils, waxes, and steroids. Phospholipids are amphipathic, containing both hydrophilic heads and hydrophobic tails, allowing them to form lipid bilayers essential for cell membranes. The fluid mosaic model describes these membranes as dynamic structures with embedded proteins that facilitate transport and signaling. Small nonpolar molecules can cross membranes freely, highlighting the selective permeability crucial for cellular function.

Lipids

In this video, we're going to do a quick refresher on lipids. So recall that lipids are a major class of macromolecule that all living cells contain, and lipids are incredibly diverse in both their structures and their functions. That makes them a little bit challenging to group together and to talk about. But one thing that all lipids have in common is that they are hydrophobic. Recall from your previous biology courses that the root "hydro" means water, and that "phobic" or "phobia" means fears. Therefore, hydrophobic means water-fearing, and, of course, this means that they don't dissolve very well with water or mix well with water. Now, examples of lipids include phospholipids, fats, oils, waxes, and steroids.

In our example below, what we're going to do is match each of these lipids with their corresponding description and function in the blocks below. The first block says, "major component of cell or plasma membrane." And recall from your previous biology courses that this is the phospholipid. Now the next block says, "long term energy storage, but specifically in plants." So these are our oils. As seen in the images below, vegetable oils and olive oils are extracted and produced by plants and they tend to be liquids at room temperature.

If we compare that directly to the next block, which is long term energy storage, but in animals, these are going to be our fats. For instance, fats are mentioned in this example. In our previous videos, we talked about how carbohydrates are used as short-term energy sources. So, cells typically use their short-term energy sources first, such as their carbohydrate energy sources. Then once those are depleted, they'll turn to use their long-term energy sources such as fats and oils.

The next block says sex hormones, and this includes estrogen and testosterone. It also includes components of the plasma membrane such as cholesterol. From this image, you can get a quick glimpse at how these are a structurally diverse group of lipids. These are, of course, the steroids. The last block says protection and prevention of water loss, and this includes beeswax and earwax. And, of course, this must be the waxes.

In our next video, we're going to talk about a very biologically important lipid, and this is the phospholipid. So I'll see you guys in that video.

Phospholipids

So phospholipids are biologically relevant lipids that are amphipathic. Recall from your previous biology courses, all this means is that they contain both a hydrophilic and a hydrophobic part. We covered hydrophobic in our previous videos, which means "water fearing." So hydrophilic means the complete opposite, "water loving." Phospholipids contain a polar hydrophilic head and non-polar hydrophobic tails, where the tails are hydrocarbon chains because they're long chains of just hydrogen and carbon atoms. The amphipathic nature of a phospholipid allows them to form membranes and compartmentalize the cell. We'll talk about the hydrophobic effect later on in our course.

However, what I want you to know in this video is that the hydrophobic effect explains how phospholipids form membranes when they are in aqueous solutions. Here in our example, we have two different representations of the head, which is polar and hydrophilic. This head consists of a phosphate group as well as a glycerol molecule. Below, you'll see that there are tails that are non-polar hydrophobic tails, which are hydrocarbons. Over here, you'll see that we have a phospholipid and they are able to form membranes when they are in aqueous solutions. They will continuously clump together and can form specific membranes as you can see in this diagram.

This transitions us perfectly into our next video where we're going to talk about cell membranes. So, I'll see you in that video.

Cell Membranes

So we already know that all cells contain a cell or plasma membrane that separates the inside of a cell from the outside or the exterior. And we'll be talking about membranes at different points throughout our entire biochemistry course. But in this video, we're just going to do a quick review. So recall from your previous biology courses that the fluid mosaic model describes a membrane's fluidity as well as the assortment of components that are embedded inside which means that they are partially penetrable. Some things can cross the membrane whereas other things cannot, and that contributes to its functions such as the transport of materials or biosignaling. And typically, only small nonpolar molecules, such as carbon dioxide or oxygen gas, are capable of freely crossing the membrane from one side to the other without requiring an input of energy. And so, down below, what we have is the typical structure of a cell membrane, and I don't want you to memorize all of these components. It's more so a review and a refresher. Notice that the major component of the plasma membrane is the phospholipid which is represented by all of these yellow balls and the brown tails, and they create the phospholipid bilayer which again is very fluid-like. These phospholipids can shift past one another and they can move from one side of the membrane to the other. The proteins that are embedded inside of it, again, create a mosaic-like look. All of these proteins can also shift in the membrane, so they are not stationary or stuck. They can move around and squeeze between the phospholipids. We have a bunch of different types of proteins; we have channel proteins that allow membrane. We've got cholesterol inside the whole membrane. We've got cholesterol inside of the membrane that helps to make it more rigid. We've talked about how carbohydrates can be attached to different molecules to create glycoproteins and glycolipids, and all of these components come together to create the cell membrane as we know it. This concludes our refresher on the cell membrane and I'll see you guys in the practice videos.

Which of the following is incorrectly matched?

• A. Oils; short-term energy storage in plants
• B. Fats; long-term energy storage in animals
• C. Phospholipids; major component of cell membranes
• D. Steroids; include sex hormones & cholesterol

What characteristic do all lipids have in common?

• A. Polymers of glycerols & hydrocarbon chains
• B. Hydrophilic
• C. Hydrophobic
• D. Polar

What types of molecules are able to freely cross a semi-permeable membrane without an energy input?

• A. Small, Polar molecules
• B. Small, Nonpolar molecules
• C. Large, Polar molecules
• D. Large, Nonpolar molecules

Here’s what students ask on this topic:

What are the main types of lipids and their functions?

Lipids are diverse macromolecules that include phospholipids, fats, oils, waxes, and steroids. Phospholipids are major components of cell membranes, forming lipid bilayers due to their amphipathic nature. Fats serve as long-term energy storage in animals, while oils perform a similar function in plants. Waxes provide protection and prevent water loss, as seen in beeswax and earwax. Steroids, such as cholesterol, are involved in cell membrane structure and function as hormones like estrogen and testosterone. All lipids share the common characteristic of being hydrophobic, meaning they do not mix well with water.

How do phospholipids contribute to the structure of cell membranes?

Phospholipids are crucial for cell membrane structure due to their amphipathic nature, containing both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails. This allows them to form a bilayer in aqueous environments, with hydrophobic tails facing inward and hydrophilic heads facing outward. This bilayer forms the basic structure of cell membranes, creating a semi-permeable barrier that regulates the entry and exit of substances. The fluid mosaic model describes this membrane as dynamic, with proteins and other molecules embedded within the phospholipid bilayer, contributing to its fluidity and functionality.

What is the fluid mosaic model of the cell membrane?

The fluid mosaic model describes the cell membrane as a dynamic and flexible structure composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. The phospholipids provide fluidity, allowing them to move laterally within the layer. Proteins embedded in the membrane serve various functions, such as transport, signaling, and structural support. Cholesterol molecules within the bilayer add rigidity and stability. Carbohydrates attached to proteins and lipids form glycoproteins and glycolipids, contributing to cell recognition and signaling. This model highlights the membrane's selective permeability, allowing certain molecules to cross while restricting others.

Why are lipids considered hydrophobic?

Lipids are considered hydrophobic because they do not mix well with water. This characteristic arises from their molecular structure, which typically includes long hydrocarbon chains or rings that are nonpolar. Water molecules are polar, meaning they have a partial positive charge on one end and a partial negative charge on the other. Nonpolar molecules, like lipids, do not interact favorably with polar water molecules, leading to their hydrophobic nature. This property is crucial for the formation of cell membranes, as it drives the assembly of lipid bilayers in aqueous environments, creating a barrier that separates the cell's interior from its exterior.

What role do steroids play in the body?

Steroids are a class of lipids that play several vital roles in the body. Cholesterol, a type of steroid, is an essential component of cell membranes, contributing to their fluidity and stability. Steroid hormones, such as estrogen, testosterone, and cortisol, are involved in regulating various physiological processes. Estrogen and testosterone are key in reproductive functions and secondary sexual characteristics, while cortisol helps manage stress and metabolism. Steroids also participate in signaling pathways and immune responses, highlighting their importance in maintaining overall health and homeostasis.