Overview of Metabolism

Introduction to Metabolism

energy metabolism- the way the body stores & utilizes energy
influenced by 4 things
- eating patterns
- growth
- stress
- metabolic rate
endocrine signals control whether the body stores or uses energy
hormones have coordinated regulation of metabolic pathways to maintain adequate energy supplies for all body cells
Two Critical Concepts Driving the Control of Energy Metabolism
1. The body has to store nutrients during intake periods & break down these stores between intake periods because food intake is intermittent
2. Blood glucose levels need to be maintained at all times because the brain depends on glucose as its main energy source

Energy input- sourced from carbs, fat, protein
energy output in six forms:
- mechanical work
- muscle contraction
- movement of cells, organelles, & appendages
- synthetic reactions
- fuel storage
- tissue building
- creation of essential functional molecules
- membrane transport
- minerals
- organic anions/cations
- amino acids
- signal generation/conduction
- electrical
- chemical
- mechanical
- heat production
- temperature regulation
- inefficient chemical reactions
- detoxification & degradation
- urea formation
- conjugation
- oxidation
- reduction
Main formulas
- input + production = utilization + output
- energy input = energy utilization + energy output
- energy input = work performed + heat released (energy output)
endocrine system is responsible for making sure that a steady supply of small nutrients is always available to cells for their energy demands
- replenishes the supply in 2 main ways
- absorption of more nutrients into the bloodstream
- mobilization of energy stores

important organ/tissue contributors
- liver
- adipose tissue
- muscle
blood glucose level- should be between 90-120 mg/dL
- important to maintain for brain, nervous tissue, & RBCs
metabolic processes
- glycogenesis- formation of glycogen from glucose
- glycogenolysis- breakdown of glycogen to make glucose
- gluconeogenesis- formation of glucose from other compounds like fatty acids or proteins
- lipogenesis- formation of lipids from FFAs (free fatty acids)
- lipolysis- breakdown of lipids to FFAs
- protein synthesis- synthesis of proteins from amino acids
- proteolysis- breakdown of protein to amino acids

absorptive/fed state- period immediately after eating when nutrients absorbed through intestinal wall into the circulatory & lymphatic systems (about 4 hours after each meal)
- focused on energy storage
- glycogenesis occurs
- increased lipogenesis, & protein synthesis
- decreased gluconeogenesis & lipolysis
- insulin high
postabsorptive/fasting state- period after the absorptive state has finished where blood glucose levels are maintained by converting other molecules to glucose
- usually in late morning, afternoon, and night if person is eating on a regular 3-meal schedule
- lower blood glucose and glucose production
- insulin low
- hepatic glycogenolysis & gluconeogenesis
- muscle glycogenolysis
- blood glucose maintained by liver
- important for uptake/utilization in non-insulin sensitive tissues (brain & central nervous system)

metabolic changes in the transition between absorptive & postabsorptive states triggered by endocrine signals
4 important hormones
- insulin
- made by beta cells in pancreas
- associated with energy storage
- glucagon
- made by alpha cells in pancreas
- associated with maintaining blood glucose levels
- epinephrine
- made in adrenal medulla
- associated with maintaining blood glucose levels
- cortisol
- made in adrenal cortex
- associated with maintaining blood glucose levels
mechanism of hormone action
- hormones bind to receptors that trigger a cascade that brings about the final response
- receptors are large proteins specific to hormones
- protein hormones receptors in PM
- steroid hormone receptors in cytoplasm
- thyroid hormone receptors in nucleus
- receptor number at target tissue determines the response of the target cell to a hormone

only 1-2 % of pancreas is actually used for endocrine functions
- rest is used for producing digestive enzymes

insulin lowers blood glucose
- increases glucose uptake by activating the GLUT4 Transporter
- increases glycogenesis
- decreases glycogenolysis
lowers blood amino acids
- increased amino acid uptake
- increased protein synthesis
lowers blood free fatty acids
- increased FFA uptake & lipogenesis
- inhibit lipase activity
mechanism of action for insulin
- insulin molecule binds to receptor in the cell membrane
- activates insulin signal pathway
- a vesicle containing the GLUT4 transporter is sent to the PM
- once the vesicle is fused with the PM, glucose is transported inside the cell
insulin secretion increases as blood glucose increases & vice versa

cortex is 80-90%
- androgens (50% of androgens in women) made in reticularis
- cortisol made in fasiculata
- aldosterone made in glomerulosa
medulla is 10-20%
- catecholamines (norepinephrine & epinephrine) are made here

stress, hypoglycemia, or morning time positively regulates the release of corticotropin releasing hormone (CRH) from the hypothalamus
- nighttime negatively regulates the release of corticotropin
CRH from the hypothalamus causes the pituitary gland to release adrenocorticotropin hormone (ACTH)
ACTH stimulates the adrenal glands to produce cortisol

Note

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