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By the end of this section, you will be able to:
  • Explain the processes of glycolysis
  • Describe the pathway of a pyruvate molecule through the Krebs cycle
  • Explain the transport of electrons through the electron transport chain
  • Describe the process of ATP production through oxidative phosphorylation
  • Summarize the process of gluconeogenesis

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms. The family of carbohydrates includes both simple and complex sugars. Glucose and fructose are examples of simple sugars, and starch, glycogen, and cellulose are all examples of complex sugars. The complex sugars are also called polysaccharides    and are made of multiple monosaccharide    molecules. Polysaccharides serve as energy storage (e.g., starch and glycogen) and as structural components (e.g., chitin in insects and cellulose in plants).

During digestion, carbohydrates are broken down into simple, soluble sugars that can be transported across the intestinal wall into the circulatory system to be transported throughout the body. Carbohydrate digestion begins in the mouth with the action of salivary amylase    on starches and ends with monosaccharides being absorbed across the epithelium of the small intestine. Once the absorbed monosaccharides are transported to the tissues, the process of cellular respiration    begins ( [link] ). This section will focus first on glycolysis, a process where the monosaccharide glucose is oxidized, releasing the energy stored in its bonds to produce ATP.

Cellular respiration

This figure shows the different pathways of cellular respiration. The pathways shown are glycolysis, the pyruvic acid cycle, the Krebs cycle, and oxidative phosphorylation.
Cellular respiration oxidizes glucose molecules through glycolysis, the Krebs cycle, and oxidative phosphorylation to produce ATP.


Glucose is the body’s most readily available source of energy. After digestive processes break polysaccharides down into monosaccharides, including glucose, the monosaccharides are transported across the wall of the small intestine and into the circulatory system, which transports them to the liver. In the liver, hepatocytes either pass the glucose on through the circulatory system or store excess glucose as glycogen. Cells in the body take up the circulating glucose in response to insulin and, through a series of reactions called glycolysis    , transfer some of the energy in glucose to ADP to form ATP ( [link] ). The last step in glycolysis produces the product pyruvate    .

Glycolysis begins with the phosphorylation of glucose by hexokinase to form glucose-6-phosphate. This step uses one ATP, which is the donor of the phosphate group. Under the action of phosphofructokinase, glucose-6-phosphate is converted into fructose-6-phosphate. At this point, a second ATP donates its phosphate group, forming fructose-1,6-bisphosphate. This six-carbon sugar is split to form two phosphorylated three-carbon molecules, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, which are both converted into glyceraldehyde-3-phosphate. The glyceraldehyde-3-phosphate is further phosphorylated with groups donated by dihydrogen phosphate present in the cell to form the three-carbon molecule 1,3-bisphosphoglycerate. The energy of this reaction comes from the oxidation of (removal of electrons from) glyceraldehyde-3-phosphate. In a series of reactions leading to pyruvate, the two phosphate groups are then transferred to two ADPs to form two ATPs. Thus, glycolysis uses two ATPs but generates four ATPs, yielding a net gain of two ATPs and two molecules of pyruvate. In the presence of oxygen, pyruvate continues on to the Krebs cycle (also called the citric acid cycle    or tricarboxylic acid cycle (TCA)    , where additional energy is extracted and passed on.

Questions & Answers

why sickle cell carrier people don't get malaria
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What is the amniotic fluid
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structure of heart and it's function (10 mark )
Priyanka Reply
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cesar Reply
describe varicocele
what do you mean by peritoneum
Siba Reply
It is thick covering surrounding the abdomen
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no it is in kidney
structure of heart and it's function
Serous membrane lining the cavity of the abdomen
it is four lobs structure and it is triangular in shaped. it 's function pumping the blood
explain root of lungs
Glomerular pressure -capsule pressure -colloid osmosis pressure
malulu Reply
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spleen is important?
helpful in destruction of rbc
It is imp in storing blood and destruction of microbes and harmful particles
ty sir
what are the sources of glucose in the body
describe mechanism of micturition
Pancreatic hormones with function
mami Reply
Insulin, which helps to regulate our blood sugar levels.
glucagon which is antagonistic to insulin increase the blood glucose level,. Somatostatin help to regulate the levels of both insulin and glucagon
thnks for helping
what is a lymph node.?
what is life
Yar Reply
life is the existence of an individual human being animal or plant
how I join this
to day I am new person and I can't participate questions so to morow I shall participate question sopleas excuse me
I had a debate earlier about nutrition and it didn't get a clear answer on that,can one tell me what the definition of nutrition.?
the nutrition is nourish person is feeling an nutrition
I think nutrition is the process of taking food and using it for growth, metabolism and repair.
life is full of happy and sorrow
life is achievement
life is the nothing but god gave us 1 body. and we all service k in this body. The things which we do for the Survivation for this body I felt that this is called as the life
Yes God gave us life but not god who gave us the life. Hope u understood what i meant by God n not god who gave life..... ?
What's the question?
Sherman Reply
once you lose pigmentation can you ever get it back
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females are lesser prone to acne
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why please expain
described the skin
Yayra Reply
skin is outer covery of human body and it is largest organ of human body. it do three fauntion protection regulation and sensation of human body that is men fauntion of human skin it has seven part.
how thick is the epidermis?
function of the endocrine system
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produces hormones that plays specific functions
has endocrine gland calld as ductless gland so as produces hormones
why is it important to view the body a as whole
Ileana Reply
mind body and soul to be in a whole of the spirit

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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