The reason why plants transport sucrose rather than glucose is due to the fact that it is more efficient and that nearby cells would not take up all the glucose too quickly.

Hence the plant convert glucose to sucrose before transporting them throughout the plant.

However, why is that humans and other animals all transport glucose in their bloodstreams? Why is it that they do not convert glucose to sucrose before transporting them since, as mentioned above, it would be more efficient?


Short answer digestion and absorption.

Animals have to digest sucrose into it's component sugars before it can be absorbed. Turning it back into sucrose afterwards would take energy and have no real benefit so it has had no reason to evolve.

The real question is why do plants make sucrose, and the answer is by living at a much slower pace,As you mentioned it is has slower uptake on the cellular level which is fine for plants but a detriment for more energy demanding animals. Stability is the other concern, sucrose is a lot more stable than glucose, animals are burning through glucose as fast as they can release it from storage so it doesn't matter how stable it is, whereas plants keep it around for longer and move it slower so it has a chance to breaks down on the way which would be a problem. Sucrose is more chemically stable than glucose and slightly more favorable osmotically (takes less water to dissolve the same energy value of sucrose than glucose) Both of these reasons make it more beneficial for plants to convert the sugars to sucrose, (if starch was water soluble they would likely only convert it to that.) They do this right at the end of photosynthesis then break it down just as it is being used in respiration so the energy investiture is minimal, while for animals they would be spending energy to break it down (digestion), then energy to put it back together, then energy to break it down again for respiration, its two extra steps for no benefit.

Just as a extra you can see why animals would evolve to break it down for absorption since as has been mentioned glucose is easier to absorb, whereas plants don't have to worry about extracting it from anything.


Short answer: Because sucrose is a pain.

Sucrose = glucose + fructose bound together. Obviously, a larger molecule has the disadvantage of inherent decreased cell membrane permeability except if you're using transporter constructs.

Glucose metabolism largely comes standard across human cells. Meanwhile, sucrose must first be split into glucose and fructose, then the fructose must largely be metabolized in the liver (https://en.wikipedia.org/wiki/Fructose#Fructose_metabolism).


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