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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?

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    $\begingroup$ What research have you done before asking it here? $\endgroup$ – another 'Homo sapien' Jan 26 '17 at 14:19
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    $\begingroup$ Asked my professor :P He suggested it might have been because plants don't have an inconsistent source of energy in the same way that plants do, but that didn't make much sense to me. $\endgroup$ – Sean_J Jan 26 '17 at 14:21
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    $\begingroup$ Not a homework problem! Although I guess there's not much I can do to convince people of that... Just a question I asked in class that my prof. didn't have a satisfying answer for. $\endgroup$ – Sean_J Jan 26 '17 at 14:29
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    $\begingroup$ I'd imagine since plants are already making carbohydrates and it would waste energy turning sugars into fats, there is just no benefit for them. Keep in mind that for plants and animals the majority of the calories we burn are carbohydrates, but plants will make more everyday while animals have to find it, and thus could go several days without. This is just supposition however. $\endgroup$ – John Jan 26 '17 at 15:09
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    $\begingroup$ Note that plants do commonly use fats for storage in at least one context, that of seeds (which humans exploit for edible oils). Seeds need to be compact for dispersal, so the high energy density is an advantage. The stored fat is used by a small plant (the seedling), so transport issues are less severe than in larger plants. $\endgroup$ – mgkrebbs Jan 26 '17 at 18:52
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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.

    types of plastidssource

  • 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.

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    $\begingroup$ A good effort, but I'm afraid I don't find the arguments very convincing. (1) "Fat hates water" -- but lipoproteins and fatty acid binding proteins solve this problem. (2) "Plants don't want to store" -- possible, but sidesteps rather than answers the question I think, and probably plants do need some energy stores. (3) No, membrane impermeability is due to hydrophobicity, not size; and see nr 1. (4) This is pure speculation, and the description of cardiovascular disease is completely off -- lipoproteins are nanometer scale particles and they do not "get stuck" in blood vessels. $\endgroup$ – Roland Mar 10 '17 at 20:53
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    $\begingroup$ (1) Still, molecular mechanisms have evolved for handling lipid solubility, and plant cells do in fact use these to store fat, see for example sciencedirect.com/science/article/pii/S0968000498013498. Therefore, solubility is likely not the answer. (3) No, I mean lipids are not that large (triglycierides ~1000Da) and size is not the important factor. (4) I think you have got the wrong impression from that web site. Atherosclerotic plaques result from inflammation, not cholesterol "sticking" to the vessel wall. I suggest you delete/revise this part of your answer. $\endgroup$ – Roland Mar 11 '17 at 9:05
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    $\begingroup$ The reasons stated are equally true for animals (specially first, third and fourth), but animals they do store fats. $\endgroup$ – Tushar Mar 11 '17 at 13:54
  • $\begingroup$ My point is not to criticize the poster for accepting your answer — I should have expressed myself better. Clearly he doesn't know the answer to the question, so one cannot expect his judgement to be perfect. However, when an accepted answer is incorrect, as in this case, it is important to comment on the incorrect answer, pointing this out. Otherwise other readers may not realize why it is wrong. No hard feelings. We all have our areas of experties and we all make mistakes. In your place I'd delete your answer, but that's entirely up to you. $\endgroup$ – David Mar 11 '17 at 15:49
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    $\begingroup$ Actually, hydrophobic means "lacking affinity for water", and literally means "fearing water", not hating it, from the Greek phobos meaning "fear" $\endgroup$ – Bohemian Mar 11 '17 at 16:41
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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).

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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.

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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.

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