Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It only takes a minute to sign up.
Sign up to join this community
In my introductory biology class, we are learning about biomolecules. The textbook says fats are a more efficient energy store than carbohydrates.
So my question is - why would plants store their energy as carbohydrates and not as fats, if fats are a more efficient energy store?
The question was:
Why would plants store their energy as carbohydrates and not as fats, if fats are a more efficient energy store?
But before trying to answer it you have to be quite clear what is meant by efficient. Without qualification this term is meaningless. A little reflection will tell you that whatever positive properties are implied by ‘efficiency’ may have different importance in relation to the very different modes of existence of animals and plants. One will also want to test any explanation on the exceptions — the instances where animals store carbohydrates (generally polysaccharides) and plants store fats. With this in mind I argue as follows:
1. Carbohydrate is the straightforward solution
Plants synthesize glucose from carbon dioxide, animals take in carbohydrates in their diet and break them down to monosaccherides. Hence, storing excess as polysaccheride (glycogen in animals, starch in plants) involves developing and using a relatively simple polymerization/depolymerization system. One would therefore assume it to be the default. The question is then, “in what circumstances is it advantageous to store surplus as fat?”.
2. Fat is a more concentrated energy store, so is used to minimize weight
Dull eh? But that’s the answer. The following extract from Berg et al. explains this clearly (triacylglycerol is the chemical name for triglyceride or fat):
Triacylglycerols are highly concentrated stores of metabolic energy because they are reduced and anhydrous. The yield from the complete oxidation of fatty acids is about 9 kcal g–1 (38 kJ g–1), in contrast with about 4 kcal g–1 (17 kJ g–1) for carbohydrates and proteins. The basis of this large difference in caloric yield is that fatty acids are much more reduced. Furthermore, triacylglycerols are nonpolar, and so they are stored in a nearly anhydrous form, whereas much more polar proteins and carbohydrates are more highly hydrated. In fact, 1 g of dry glycogen binds about 2 g of water. Consequently, a gram of nearly anhydrous fat stores more than six times as much energy as a gram of hydrated glycogen, which is likely the reason that triacylglycerols rather than glycogen were selected in evolution as the major energy reservoir. Consider a typical 70-kg man, who has fuel reserves of 100,000 kcal (420,000 kJ) in triacylglycerols, 25,000 kcal (100,000 kJ) in protein (mostly in muscle), 600 kcal (2500 kJ) in glycogen, and 40 kcal (170 kJ) in glucose. Triacylglycerols constitute about 11 kg of his total body weight. If this amount of energy were stored in glycogen, his total body weight would be 55 kg greater.
The animal exception — glycogen
A limited amount of fuel is stored as glycogen in animals. The increased weight load is offset by the advantage of rapid mobilization and the fact that glucose is obtained (animals cannot convert fatty acids to glucose).
The plant exception — oils in seeds
Plants do not walk around, so weight is not generally a consideration that would induce them to store their reserves as fat. They can develop large tubers under the ground, for example. However there is an advantage to minimizing weight in the energy reserves required for the development of seeds — it allows for easier spreading in the wind or through animal consumption. Thus one can rationalize why the energy reserves in seeds are oils (lower melting point triglycerides on account of their greater unsaturation).
There are quite some reasons for why plants prefer carbohydrates for energy storage rather than fats. I will reach some of them one at a time.
Fat hates water: By just applying some common sense, one would get to know that fats are hydrophobic, meaning they literally 'hate' water i.e. don't dissolve in water. So, they cannot be transported very easily. On the other hand, most carbohydrates are water-soluble and can be easily transported via phloem fibers. So, plants prefer to use carbohydrate instead of fat. But they can store fat, by converting them into lipoproteins, by binding them with fatty-acid binding protein, by storing them as fatty-acids (i.e. with terminal -COOH which makes them a bit water-soluble) or by storing them in special compartments called elaioplasts. Yet, as one might assume, this would require more energy intake than using carbohydrates, so plants don't prefer this process too much.
Plants don't want to store everything: Obviously, plants photosynthesize because they need energy, and because they need energy to survive. So, storing every bit of energy would not be very clever, they need some of it handy. Fats are storehouses of energy i.e. they store energy for extreme conditions, when there is no primary energy source left. Under normal conditions, and at night, carbohydrates are the primary energy source.
Cells don't let them in: Plasma membranes are semipermeable, quite a general fact. So, they do not allow large molecules to enter inside. And fats are one of the largest biomolecules (after, obviously, nucleic acids). So they cannot be transported inside or outside cells. You would say 'but cells are the ones which make fats, so they are already inside!' No, that is why there is phloem, because there are cells which don't synthesize them (say, root cells). On the other hand, carbohydrates, being much smaller, can be transported through small channels or even through simple diffusion through the plasma membrane. Also, as already stated, fats are quite hydrophobic, unlike carbohydrates. So, they are, in a sense, repelled by hydrophilic layer of cell membrane just like water itself hates it. So, they find it even more difficult to cross plasma membranes because of combined effect of size and hydrophobicity.
There can be lots of other reasons which I can't think of right now, but I hope even this much is helpful.
Here is my try.
Why do plants store energy as carbohydrates and not as fats?
Is it because they can't? That isn't the answer as we have corn oil, palm oil, coconut oil, olive oil, sunflower oil etc. So plants can and do store energy as lipids.
Perhaps the question is better rephrased as "Why isn't the main store of energy in plants lipids like mammals."
My guess is because plants do not move as actively as animals. A plant is rooted to a spot by its root system. Hence there isn't an advantage of a storing energy in a high density manner, particularly when lipid synthesis takes more energy compared to sugar synthesis.
So aside for specific examples, there is no advantage to store energy in lipids for a plant. There is no need to pay for a compact source of energy when you will never move as an adult.
As a seed... that may be useful... and this is why all our plant oils comes from seeds of one sort or another.
Quick answer: Animals need mobility while plants favour stability.
Explanation: As you mentioned fat is a more effective storage form of energy. Plants though, reserve energy through starch (carbohydrate) and not through fats as it would be expected. This doesn't mean they don't use fats at all (i.e. oil seeds).
An energy storing molecule must save energy (as the name indicates), but it shouldn't be too heavy and it should be stable enough so that it's functional within the organism. Fat is the most lightweight molecule storing energy. One gram/fat stores more energy than one gram/starch or protein. Thus, the weight of a moving organism would be less if it stored fat instead of starch. But plants don't move around so weight saving is not a real necessity.
So, a heavy starch molecule is more stable than a lighter fat molecule which is comparatively more important for plants in order to provide long-term stability. Another reason why they store mainly starch instead of fats is alternate flowering for example, where the plants save up some starch every year (depending on the plant) and then use all the saved energy at once while blooming. Fats wouldn't have lasted for that long because when fats or oils are exposed to air, they react with the oxygen or water vapor and form short-chain carboxylic acids.
