hi this is Mr W from sciencemusicvideos and this video is
about carbohydrates.
Carbohydrates include foods like sugars and starches and also plant fiber like
cotton. All carbohydrates contain carbon, hydrogen, and oxygen. This formula says
that the ratio of hydrogen to oxygen is 2 to 1 like water that makes
carbohydrates "hydrates of carbon:" water attached to some number of carbon atoms.
the monomers of carbohydrates their building blocks or monosaccharides and
the monosaccharide that you learn most about is glucose that's because glucose
is the fuel of life it's the input for cellular respiration and it's the
product of photosynthesis glucose is chemical formula is c6h12o6 here's its
full structural formula an abbreviated structural formula you have to imagine
the carbon and hydrogen atoms at each angle vertex and a chain form in
solution which means in cells in life glucose is almost always in a ring
fructose is another monosaccharides should know it's a structural isomer of
glucose which means it has the same empirical formula but a different
structure fructose is naturally found in fruit where it attracts fruit eating
animals which eat the fruit swallow the seeds and then defecate those seeds far
away from the parent plant that's an important adaptation for plant dispersal
we humans love fructose and other sugars that's because our distant ancestors
were tree living primates who ate fruit and passed that preference on to us
today most of our fructose comes from corn and up to 10 percent of the
calories in a North Americans diet can come from this monosaccharides
monosaccharides are the monomers of carbohydrates in my video about monomers
and polymers I compared monomers to Legos that can be put together to create
larger structures let's see what happens when we start combining monosaccharides
when enzymes linked to glucose monomers together through a dehydration synthesis
we get maltose a disaccharide let's focus on the bond between the two
glucose monomers to do that we're going to use a system that biochemists have
devised where they sign a number to each of the carbons in
a monosaccharide here you can see the numbers for each carbon in glucose when
enzymes link the two glucose monomers together to form maltose they connect
the number one carbon of one glucose with the number four carbon of the
second glucose that's called an alpha 1-4 glycosidic linkage and the prefix
glyco is widely connected with the carbohydrates and we'll see it again
throughout this unit what do you need to know about that bond well the main thing
is that it's covalent gand and it has a specific shape and we have enzymes that
can break that bond changing maltose into two glucose monomers that we can
absorb into ourselves for energy here's the disaccharide sucrose it's table
sugar and it's what we put into our coffee and tea sucrose consists of a
glucose connected to a fructose and the bond between them is an alpha one two
glycosidic linkage what do you need to know about that it's another covalent
bond it has a different shape we have enzymes that can break it apart
releasing glucose and fructose but our cells can absorb and use for
energy sucrose in other words is a digestible sugar the last disaccharide
that we'll meet is lactose it's the sugar in milk and it consists of a
glucose linked to another mountain saccharide galactose all mammals drink
milk is infants that's what makes us mammals in order to digest the lactose
in milk we need to be able to break this beta 1-4 glycosidic linkage between the
glucose and the galactose and all mammals are born with an enzyme that
enables them to do that it's called lactase and it takes lactose and breaks
it apart into glucose and galactose now a lot of adult humans including me are
lactose intolerant that means that as adults we don't synthesize the lactase
enzyme and therefore we can't digest lactose and ingestion of lactose leads
to all kinds of indigestion cramping flatulence diarrhea nasty stuff lactose
intolerance might seem like some unfortunate disease but when you think
about it lactose intolerance is the norm for mammals
you only need to digest lactose when you're a suckling infant those adult
humans who can digest lactose are the inheritors of a genetic mutation
that arose thousands of years ago in a certain population and that mutation
keeps the lactase enzyme active throughout adulthood and those
populations where that mutation arose and spread a whole wide array of dairy
food sources was opened up and that became quite a beneficial adaptation for
those of us who haven't inherited that mutation don't despair there's an entire
industry producing lactose free dairy products and also lactase supplements so
that you can eat lactose and digest it without a problem if you want to learn
more about lactose intolerance and the evolution of lactose tolerance there's
more about that at my website now let's turn our attention to carbohydrate
polymers which are known as polysaccharides
we'll start with starch starch is a polymer of glucose and it's used by
plants for long term energy storage that's why you find starch in plant
parts like potatoes and also in seeds like beans and corn the simplest form of
starts his amylose it's just a straight chain of glucose monomers held together
by an alpha 1-4 glycosidic linkage the same one that we saw when we were
discussing maltose another form of starch is analog pectin in addition to
chains there are also branches between the number one carbon of one glucose and
the number six carbon of another glucose that bond is called a beta 1-6
glycosidic linkage to digest amylose or analog pectin we have enzymes that can
break that alpha 1-4 beta one six bond releasing the glucose monomers
that's why starchy foods like potatoes are loaded with food energy and that's
why potatoes were the key to astronaut Mark Watney survival in the science
fiction novel and movie the Martian here on earth starchy foods from plants like
wheat rice beans and corn along with potatoes are the staple foods that power
human civilization glycogen is an energy storage molecule that's made by animals
that's structurally very similar to have a low pectin in fact it's sometimes
called animal starch that subscript indicates the number of glucose monomers
that are chained together in that part of the molecule glycogen gets
huge molecular complexes like this in our liver tissue and our muscles when
our blood sugar level gets low our bodies will convert stored glycogen into
glucose on the other hand after a carbohydrate rich meal our bodies will
take excess blood glucose and converted into glycogen this inter conversion is
controlled by hormones like insulin and glucagon and breakdown of blood sugar
homeostasis can cause diseases like diabetes there's a lot more information
about this on my website cellulose is another polysaccharide that's built from
glucose monomers it's the primary component of plant cell walls and
because of that cellulose is the most common biological polymer on earth
cellulose is incredibly useful cotton fiber wood paper they're all composed to
varying degrees of cellulose like starch cellulose it's a polymer of glucose but
the glucose monomers are held together by a beta one four linkage
unlike starch we and many other animals don't have enzymes that can break that
bond that explains why a cup of potatoes
mostly starch has about ten times the food energy as a cup of celery which is
mostly cellulose we just don't have the enzymes that can break these glucose
monomers off the cellulose chain and feed them into cellular respiration you
know that cellulose is strong that's because the cellulose strands form
hydrogen bonds with one another and the collective effect of those hydrogen
bonds creates a strength that we see in things like rope and wooden boards
mammals like this deer have evolved mutualistic relationships with bacteria
that have enzymes that can break the glucose monomers off of cellulose making
its chemical energy available other mammals like goats and sheep and cows
have these say mutualistic bacteria and they live in a digestive chamber that's
called a rumen collectively these animals are called luminance among the
insects termites have evolved a mutualistic relationship with a
protozoan that enables them to digest the cellulose in wood that's
for the termites but it's disastrous for the billions of dollars of damaged
structures in the United States alone each year that's it for carbohydrates
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