Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Digestion questions

  1. Jun 21, 2008 #1

    1) Humans cannot digest cellulose. Plant cell walls are made of cellulose. How can we get
    any nutrition fro plants if we can't break it's outermost structure?

    2) When we take medicine by mouth, what prevents it from being broken down in the acidic stomach, or the enzymes on the small intestine?

  2. jcsd
  3. Jun 21, 2008 #2
    Good Question actually, I'm not sure of the answer myself however I had a little look and found a bit of information regarding your first question.

    Although cellulose is indigestible by humans, it does form a part of the human diet in the form of plant foods. Small amounts of cellulose found in vegetables and fruits pass through the human digestive system intact. Cellulose is part of the material called "fiber" that dieticians and nutritionists have identified as useful in moving food through the digestive tract quickly and efficiently.

    Last edited by a moderator: Apr 23, 2017
  4. Jun 21, 2008 #3
    For your first question; even though we can't fully digest the cellulose (note that complete digestion is breaking things down to almost the individual molecular level to be absorbed), we can still mash it up/break it apart -> to get to the sweet sweet innards of plant material. Similarly, alot of the good stuff (vitamins / minerals / sugars) get extracted as a (~)liquid from the cellular matrices.
    2)Oral medications only work because they start to be broken down - and then absorbed somewhere. Often the capsules / coatings give the actual medicine a protection period before it starts to get broken down.
    I assume that one of the factors that goes into what medications can be taken orally - is if they will be broken down before they will be absorbed/effective.
  5. Jun 21, 2008 #4
    1.) The above post already hit upon the answer - the combination of mechanical and chemical digestion, as well as extraction of nutrients in liquid form - lets us use the delicious planty goodness for our nutritional needs, and the cellulose goes through as dietary fiber. It's also interesting to note that the starchier plant-based foods do tend to get absorbed a bit better, as I recall, as there's less cellulose.

    2.) It should be noted that some drugs actually need to be metabolized in the gastrointestinal fluids before they're active, as they're technically a "prodrug."
  6. Jun 30, 2008 #5
    The part that we actually eat of many plants is not the green, cellulose part, but the fruit part (example: strawberry compared to a leaf). The fruit part is nutritious as it contains vitamins and minerals. The plants, like lettuce, that we actually do eat the green part is not very nutritious at all. On a salad, for instance, it is from all of the extra stuff that we put on the salad is where we get the nutrition from. The lettuce doesn't give us much, but it can help clean out the digestive system.
  7. Jun 30, 2008 #6


    User Avatar
    Gold Member

    http://www.nutritiondata.com/facts/vegetables-and-vegetable-products/2476/2" [Broken]
    Nearly 100% DV of Vitamins A, B6, C, K, Folate, Manganese, Calcium, Iron, Magnesium, Thiamine, etc.

    And just to explode another myth - that celery is a zero-sum food, I checked celery once - it is at least as nutritious, and in some cases, moreso, than an apple.
    Last edited by a moderator: May 3, 2017
  8. Jul 1, 2008 #7
    According to the page you attached, it is not nearly 100% DV for any of the vitamins listed, but the point is clear that it actually is more nutritious than I gave it credit for. I remember my biology teacher saying otherwise, so thanks for clarifying that.

    About celery, is it true that vitamin-wise celery is good for people but calorie-wise it does pretty much nothing?
  9. Jul 1, 2008 #8

    jim mcnamara

    User Avatar

    Staff: Mentor

  10. Jul 1, 2008 #9


    User Avatar
    Gold Member

    It's pretty low in calories, yes.
  11. Jul 2, 2008 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    We boil and then chew our food, this breaks up the internal structure and releases the nutritional components. The cellulose bit passes right through our digestive system (eat some boiled corn, the outer yellow part will pass right through, but the inner part will have been digested).
  12. Jul 2, 2008 #11


    User Avatar
    Gold Member

    As with nickels.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook