Video Transcript
Which of the following statements
about lipids is not true? (A) Functions of lipids in animal
cells include being a major component of the cell membrane and being used as a
respiratory substrate. (B) Lipids are insoluble in
water. (C) Functions of lipids in plant
cells include being broken down to provide energy in photosynthesis and being a
major component of the cell wall. Or (D) the elements that comprise
the basic structure of lipids are carbon, hydrogen, and oxygen.
Lipids are macromolecules
characterized by hydrocarbon chains. Lipids are an important part of our
diet and an essential component in our bodies. Let’s discuss some of the
properties of these molecules.
Lipids are biological
macromolecules that consist mainly of hydrocarbons. There are a variety of types of
lipids with a range of different functions. Some examples of lipids are waxes,
oils, steroids, including steroid hormones, phospholipids, the fat soluble vitamins
like A, D, E, and K, and triglycerides. The fact that lipids are largely
insoluble in water allows them to form semipermeable membranes and waterproof
coatings. So we can eliminate answer (B) as
it is true that lipids are insoluble in water.
The most basic lipid is a fatty
acid. Fatty acids are composed of a
carboxyl head with a hydrocarbon chain. Our example is stearic acid, which
has 17 carbons. Lipids generally have a similar,
albeit more complex, structure to fatty acids. They are all composed mainly of
hydrogen, carbon, and oxygen. So we can eliminate answer (D) as
it is true that the elements that comprise the basic structure of lipids are carbon,
hydrogen, and oxygen.
In cells, lipids form cell
membranes by creating a phospholipid bilayer. The hydrophilic phosphate heads of
the phospholipid molecules face outwards so are in contact with the watery contents
of the cell or tissue fluid. The hydrophobic fatty acid tails
face inwards, towards one another. This arrangement provides a
selectively permeable barrier, allowing some substances to move freely across the
membrane, while the movement of many others is facilitated by various transport
proteins embedded in the bilayer.
We have already seen how fatty
acids are primarily made up of hydrocarbons. These molecules store energy and
can therefore be used as a substrate in cellular respiration. Therefore, we can eliminate answer
(A) as we have just seen how lipids are a major component of the cell membrane and
can be used as a respiratory substrate. So this statement is true.
Now let’s discuss answer (C). Plant cell walls are mostly
composed of cellulose, not lipids. In addition, light is used as the
source of energy in photosynthesis, not lipids. We have now established that the
statement about lipids which is not true is statement (C). Functions of lipids in plant cells
include being broken down to provide energy in photosynthesis and being a major
component of the cell wall.
