Video Transcript
Which of the following statements about the structure of biological membranes is correct? Cell membranes are formed from a single layer of phospholipids and a single layer of glycolipids connected by hydrophobic tails. Cell membranes are formed from a single layer of phospholipids connected by the hydrophobic heads. Cell membranes are formed from two layers of phospholipids, where the hydrophilic phospholipid tails linked together in a matrix. Or cell membranes are formed from two layers of phospholipids, where the hydrophilic phospholipid heads face outward.
This question is asking us about the structure of biological membranes. So what is a biological membrane anyway? To answer this question, let′s first remove the answer choices so we have more room to work with. Cells have a lot of internal components that have specific functions. For example, the nucleus is a place to store DNA, the mitochondria release energy, and the endoplasmic reticulum, or ER for short, is a place where proteins can be produced. Each of these components is contained using a biological membrane. And if it were to suddenly disappear, then everything might start to get mixed up and drift apart. But if we have a biological membrane, then each of these components can stay in place and be separated from its external environment.
Since this question is asking us about the structure of a biological membrane, let′s zoom in so we can take a closer look. The membrane is made up of a structure called the phospholipid bilayer. This separates the cytoplasm on one side from the extracellular space on the other side. The phospholipid bilayer is mostly made up of numerous phospholipids. Let′s look a little closer at its structure. A phospholipid is made up of two distinct parts: the hydrophobic tail and the hydrophilic head.
The hydrophilic head contains a phosphate group. This phosphate group carries an oxygen with a negative charge. And this will interact with the slight positive charge on a water molecule. This is why the head component is said to be hydrophilic; -philic is a suffix that describes the property of being attracted to something like the water molecule here. Attached to the hydrophilic head is glycerol, attached to glycerol is the tail component that contains chains of hydrocarbons. These don′t have an electric charge, so these are said to be hydrophobic. This means that the hydrophobic tail is repelled by water.
So to put this together, the hydrophilic heads have an affinity for the water in the cytoplasm and in the extracellular space, while the hydrophobic tails are repelled by the water in these spaces and are attracted to each other. So when phospholipids are placed into water, they naturally organize into two layers, or a bilayer. Here, the hydrophilic heads face outward and the hydrophobic tails face inward.
We now have enough information to answer this question. The correct statement regarding the structure of biological membranes is that “Cell membranes are formed from two layers of phospholipids, where the hydrophilic phospholipid heads face outward.”
