Lactation and especially milk, which is the product of that unique mammalian process, are routinely encountered within our daily lives. Nevertheless, they often are poorly understood by many, even including many who are engaged in the business of producing milk. The overall course goal is to introduce fundamental concepts that form the basis for understanding the biology of lactation, the biology of the mammary gland, and the products of that important physiological process. As a learner in this course, you will be provided with a series of easily understood presentations that collectively will help you build a foundation for greater understanding of lactation. You will be able to engage with other learners so that you can extend your learning beyond the video presentations. Ultimately, you will be able to construct your own mental model for understanding the wide range of topics that relate to the biology of lactation. Upon completion of the course, you will be prepared to expand your knowledge and understanding of lactation from other sources and experiences as you pursue your individual interests. Before you start the course, I suggest that you identify a question or several questions about lactation that you already have on your mind. This could be from your own experiences, something you read about or saw, or something you have wondered about. Write down your question(s) and use that to help you decide how to engage with the content of this course. You might engage with the modules in the order they are presented, or start with a module that is of particular interest to you, or pick and choose modules in any order. I encourage you to engage in all of the types of learning activities that this course has to offer, including but not limited to, the discussion forums, quizzes, peer-review assignments, and concept maps and other learning aids.
Lactose
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来自 University of Illinois at Urbana-Champaign 的课程
Lactation Biology
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从本节课中
Module 3: Milk Composition
Milk is the product of the lactation process. Such a simple statement does not come close to doing justice to the complex nature of this biological fluid. In this module, we explore the major components of milk, as well as provide a basis for understanding milk as a product that we purchase in a store. Mechanisms of synthesis of milk components are introduced. And, several videos include discussions of the nature of differences in milk composition among mammalian species, including comparisons of cow milk with human milk.
与讲师见面
Dr. Walter Hurley, PhDProfessor Emeritus
Department of Animal Sciences
[MUSIC]
So we talked about water, we're going to talk a little bit about lactose now.
Lactose is the major carbohydrate in the milk of most species.
It is a disaccharide.
So we have alpha-lactose here, we have a galactose molecule.
A glucose molecule, this guy right here.
And they're held together by what's called a beta-1-4 linkage.
So the number 4 carbon on the glucose, and
that's attached to the number 1 carbon on the galactose.
So how is this produced?
And so this really starts to get into a little bit of a milk synthesis and
the importance of synthesis of lactose in terms of milk.
It also is going to link us back up to water as well.
Each of these guys, independently, the galactose and the glucose,
there are mechanisms by which those can transport back and
forth across cell membranes.
For example, the cell membrane of a mammary epithelial cell, or
the Golgi apparatus where lactose is produced.
It's actually produced inside the Golgi apparatus and continues
to be produced inside the secretory vesicles of the mammary epithelial cell.
So these guys can go across this barrier, this membrane.
Lactose, this disaccharide, two sugars put together, cannot.
And what that means is inside that Golgi apparatus is those
disaccharides being made, water's being drawn in to try to dilute it.
That's where your water comes from.
The bulk of why milk has water, this what's called the primary osmole.
It's the primary osmole in milk, there are other osmoles,
but they don't contribute as much as lactose does.
And so this is why these two are linked together.
We can go one step further on that, and that is,
there is a particular milk protein that is produced in the mammary epithelial
cell that's responsible for lactose being produced.
So all of a sudden, we can link three things together.
Particular proteins called alpha-Lactalbumin,
we'll talk about that later in another video, is responsible for
making lactose, which is the major milk sugar,
which is to a great extent responsible for the water that's in there.
So there's a linkage between some of these different components in terms of synthesis
and so on in the mammary gland.
This is a study done a number of years ago where they went in to mice and
they knocked out the functionality of the alpha-Lactalbumin gene.
So all of a sudden, those animals could lactate, the females could lactate, but
they could not produce lactose.
And that kind of gives you an idea here.
So, we have over here the wild type.
That's the ones that the gene was not knocked out.
And we have those where the gene was knocked out.
You can see, no lactose or very, very little lactose is being made over here.
So this is, this one indicates the lactose,
whereas normal level of lactose in the wild type.
Normal level of fat, normal level of total solids in the wild type.
A significantly higher level of total solids,
significant higher level of milk fat.
Again, can more concentrated milk is what it amounts to.
And at this level, with roughly 45% total solids,
that's not unlike that cream cheese I showed you a few moments ago.
Very, very viscous.
In fact, the young of these knock-out animals did not thrive very well, because
they just simply couldn't get the milk out of the mammary gland of those animals.
So again, milk is very, excuse me, lactose is very important for
[COUGH] bringing in the water, actually getting the milk out of the mammary gland.
Let's take a look at the concentration of this across different species.
And you can almost see this is a little bit inverse
to what we looked at in terms of total solids.
Whereas the total solids, same set of species, you remember,
the curve kind of went like that.
This is really going another direction.
The species at the bottom, including humans, humans in fact,
of the species that we know of,
have one of the highest levels of concentrations of lactose in the milk.
Again, needing all that energy, rapid energy for brain development and so on.
And then as we go up the line here, certainly these aquatic mammals at the top
have relatively little lactose in their milk.
Some other ones have less lactose, but there's actually some other reasons for
that in some of these other cases.
That's comparing between species, and
certainly there's a lot of variation within a species.
And one way we can think about that is strains or breeds of animals,
in this case our example is the breeds of dairy cattle.
Cattle that we often find in the United States and in Northern Europe, Ayrshires,
Brown Swiss, Guernseys, Holsteins, Jerseys.
And the red bars here are the lactose, green is fat, and the blue is protein.
But if you look across this,
we start to see another kind of theme arise with regard to lactose.
And that is, they're not very different.
And in fact, if we went back to that graph of species, you realize, well,
there's differences between them, but
they're not enormous differences like we saw on total solids.
And so lactose, it turns out, of the variables that are in milk,
whether it's water, fat, protein, lactose.
Lactose is in fact the least variable component.
Let me go ahead and write that down.
Doesn't mean it's not variable, between species, between breeds,
between strains, and so on.
But it is, It is the least variable component of the components in milk.
So again, this idea of lactose as a disaccharide, galactose, and glucose.
And as I said, most species, this is the major osmole.
It's the major carbohydrate in milk.
[COUGH] But there are some exceptions.
And what you find in most of those exceptions is, they have this, but
in addition to that, they have other sugars tacked on to the other end of this.
And so they might be trisaccharides, they might be four-sugars,
they might be branched sugars, and they get to be very complex.
So some of those are called oligosaccharides.
Marsupials, for example, do not apparently have very much lactose.
But they have these larger sugars, larger sugar chains.
This is typically what's called a reducing end here.
They typically do have that.
So again, these additional sugars are built on to the galactose side of this.
Bears, apparently, also have some of these.
Other species have these oligosaccharides, even if they have lactose
as the major sugar, they will have a lot of oligosaccharides.
In fact, human milk has a lot of oligosaccharides.
And some of those have some other functionalities to them in terms of
the impact of milk on the human infant.
