Absorption

Absorption

Carbs and Lipids

• Carbs are first broken down into disaccharides (2 sugar groupings) and then further into single sugars by these enzymes.
• The carbs need to be in single sugar states in order to be absorbed by the small intestine cells.
• This is a form of chemical digestion.
• Once they enter the cell of the small intestine using glucose uniporters, they are repackaged and shipped off to be used elsewhere in the body. The other cells will take in the glucose through a uniporter as well.

• "Elsewhere" may include
• being used in cellular respiration (producing ATP)
• The synthesis of amino acids needed for protein production
• it can be stored short term as glycogen in the liver and skeletal muscles
• its short term because it can be quickly broken down to use in cellular respiration
• it can be stored long term as triglyceride (fat)
• must join lipids that were also absorbed to form a triglyceride.
• used for energy storage and protection
• made by lipogenesis and degraded by lipolysis depending on if the body is in a fed or fasted state.

Ketones

• the brain can only use carbs and ketones for energy. If there are no carbs available, ketones will be made as a replacement.
• made from acetyl CoA in the liver through a process called ketogenesis

Proteins

• proteins will be made using absorbed amino acids if the body is in a fed state
• if the body is in a fasted state, proteins will be broken down and the amino acids will be converted to other molecules that can be used in cellular respiration.

Biosynthesis of macromolecules

• lipids, proteins, DNA, etc., are closely tied to the intermediates in cellular respiration
• if needed, we are able to pull products from the cellular respiration system to produce important things our bodies need.

• We are also able to do the reverse ––> we can take use proteins, fats and carbs to feed the cellular respiration pathway
• we don't have to start at the beginning.
• Ex. fats can break down and make G3P. This can then be used to make pyruvate and continue on to the krebs cycle
• Ex. amino groups can be used to create Acetyl CoA and take part in the krebs cycle, completely skipping glycolysis.
• In order to maintain order in the body, we have a hierarchy for what to use first
• Carbs - not so important, easily replaceable in diet
• Fats - storage that is used once carbs are all gone
• Proteins - important for muscle development, but will be used for energy if no carbs or fats are available (malnourished people = no muscle)
• FUN FACT: The new "keto" diet acts to put your body into a state of ketosis. This basically means it performs ketogenesis by breaking down fats and proteins. The diet removes essentially all carbs, resulting in only fats and proteins being burned for energy.