Chapter 25: Metabolism and Nutrition Flashcards
define metabolism.
A. Metabolism – all of the biochemical reactions that occur within an organism, including synthetic and decomposition reactions
Catabolism – chemical reactions that break down complex organic compounds into simple ones, with a net release of energy.
- Ex. Glycolysis, the Krebs cycle, the electron transport chain
Anabolism – synthetic, energy-requiring reactions whereby small molecules are built up into larger ones.
- Ex. Formation of peptide bonds between amino acids during protein synthesis, the building of fatty acids into phospholipids that form the plasma membrane bilayer, and the linkage of glucose monomers to form glycogen
explain the role of ATP in anabolism and catabolism
ATP (adenosine triphosphate) – the main energy currency in living cells
- Used to transfer the chemical energy needed for metabolic reactions
- Consists of the purine base adenine and the 5-carbon sugar ribose, to which are added three phosphate groups in a line
coupling of catabolism and anabolism by ATP – chemical reactions of living systems depend on the efficient transfer of manageable amounts of energy from one molecule to another.
- ATP usually performs this task.
- A typical cell has about 1 billion ATP molecules, each of which lasts for less than a minute before being used. ATP is a convenient “cash” for moment-to-moment transactions, it is not like “gold in a bank” for long term.
ADP - when terminal phospate cut off of ATP
describe oxidation–reduction reactions.
Oxidation – the removal of electrons from an atom or molecule
- The result is a decrease in the potential energy of the atom or molecule
- Usually an exergonic reaction (energy releasing)
- Most biological oxidation reactions involve the loss of hydrogen atoms, so they are called dehydrogenation reactions.
- Example, conversion of lactic acid into pyruvic acid by the removal of 2 neutral hydrogen atoms (2H). They are actually removed as one hydrogen ion (H+) and one hydride ion (H-)
- The net resule of the complete oxidation of glucose does NOT include oxygen (but does include water, CO2, ATP, and waste heat)
Reduction – the addition of electrons to a molecule.
- Results in an increase in the potential energy of the molecule
- Usually an endergonic reaction (energy requiring)
- Example, conversion of pyruvic acid into lactic acid by adding back 2 H atoms.
Important to note – the hydrogen atoms do not remain free in the cell but are transferred immediately by coenzymes to another compound.
nicotinamide adenosine dinucleaotide – NAD – a derivative of the B vitamin niacin
- a coenzyme commonly used by animal cells to carry hydrogen atoms
flavin adenin dinucleotide (FAD) – a derivative of vitamin B2 (riboflavin)
- another coenzyme commonly used by animal cells to carry hydrogen atoms
Oxidation-reduction reactions – AKA redox reactions
- Whenever one substance is oxidized, another is simultaneously reduced. They are always coupled.
- Example: when lactic acid is oxidized to form pyruvic acid, the two hydrogen atoms removed in the reaction are used to reduce NAD+
Phosphorylation – the addition of a phosphate group to a molecule
- Increases potential energy
- Organisms use 3 mechanisms of phosphorylation to generate ATP:
- Substrate-level phosphorylation – generates ATP by transferring a high-energy phosphate group from an intermediate phosphorylated metabolic compound, a substrate, directly to ADP. In human cells, this process occurs in the cytosol (2 ATPs can come from this during glycolysis)
- Oxidative phosphorylation – removes electrons from organic compounds and passes them through a series of electron acceptors, called the electron transport chain, to molecules of oxygen O2. This process occurs in the inner mitochondrial membrane of cells.
- Photophosphorylation – occurs only in chlorophyll-containing plant cells or in certain bacteria that contain other light- absorbing pigments
Chemiosmosis - accumulation of large amounts of H+ b/w the inner and outer mitochondrial membranes
describe the fate, metabolism, and functions of carbohydrates.
carbohydrate metabolism – carbohydrates are hydrolyzed into glucose, fructose, and galactose, but most fructose and galactose is then converted to glucose by the liver.
fate of glucose – glucose is the body’s preferred source for synthesizing ATP. Glucose is used in several ways:
- 1) immediate oxidation for ATP production
- 2) synthesis of amino acids for protein synthesis
- 3) synthesis of glycogen for storage in liver and skeletal muscle
- 4) formation of triglycerides via lipogenesis for long term storage after glycogen stores are full
- 5) excretion in urine if blood glucose is very high
- ATP production – in body cells that require immediate energy, glucose is oxidized to produce ATP. Glucose not needed for immediate ATP production can enter one of several other metabolic pathways
- Amino acid synthesis – cells throughout the body can used glucose to form several amino acids, which can then be incorporated into protein excess amino acids in the body are converted into glucose
c. Glycogen synthesis – hepatocytes and muscle fibers can perform glycogenesis, in which hundreds of glucose monomers are combined to form the polysaccharide glycogen. Total storage capacity of glycogen is about 125g in the liver and 375g in skeletal muscles.
d. Triglyceride synthesis – when the glycogen storage areas are filled up, hepatocytes can transform glucose to glycerol and fatty acids that can be used for lipogenesis, the synthesis of triglycerides. Triglycerides are then deposited in adipose tissue, which has virtually unlimited storage capacity.
glucose movement into cells
a. before it can be used by body cells, glucose must first pass through the plasma membrane and enter the cytosol.
- Glucose absorption in the GI tract and kidney tubules is accomplished via secondary active transport (Na+ - glucose symporters)
- Glucose entry into most other body cells is via GluT molecules, a family of transporters that bring glucose into cells via facilitated diffusion.
- A high level of insulin increases the insertion of GluT4 into the plasma membrane of most body cells, thereby increasing the rate of facilitated diffusion of glucose into cells.
- In neurons and hepatocytes, another type of GluT is always present in the plasma membrane, so glucose entry is always “turned on.”
- On entering a cell, glucose becomes phosphorylated. This reaction traps glucose within the cell because GluT cannot transport phosphorylated glucose.
glucose catabolism—general about:
a. cellular respiration – the oxidation of glucose to produce ATP. Involves 4 sets of reactions: Glycolysis (doesnt require o2 so aerobic or anearobic), Formation of acetyl coenzyme A, Krebs cycle (Requires O2), Electron transport reactions (requires O2)
1. glycolysis – Phosphofructokinase is key regulator a set of reactions in which one glucose molecule is oxidized and two molecules of pyruvic acid are produced. (no O2 = lactic acid)The reactions also produce two molecules of ATP and two energy- containing NADH + H+
Takes place in the cytosol
Promoted by Thyroid Hormone
- does not require oxygen, it can occur under aerobic or anaerobic conditions.
- When glycolysis occurs by itself under anaerobic conditions, it is referred to as anaerobic glycolysis.
formation of acetyl coenzyme A – a transition step that prepares the pyruvic acid for entrance into the Krebs cycle. This step also produces energy containing NADH + H+ plus carbon dioxide
Krebs cycle – these reactions oxidize acetyl coenzyme A and produce carbon dioxide, ATP, NADH + H+, and FADH2 1. Requires oxygen, referred to as aerobic respiration most abundant product is reduced coenzyme
TAKES PLACE IN THE MITOCHONDRIA
electron transport chain – these reactions oxidize NADH + H+ and FADH2 and transfer their electrons through a series of electron carriers. 1. Also requires oxygen and referred to as aerobic respiration
glucose anabolism – most glucose in the body is catabolized to generate ATP but some may take part in or be formed via several anabolic reactions
a. glycogenesis – the synthesis of glycogen stimulated by insulin
- if glucose is not needed immediately for ATP production, it combines with many of molecules of glucose to form the polysaccharide glycogen, the only stored form of carbohydrate in the body
- First glycose is phosphorylated to glucose 6-phosphate by hexokinase
- Then glucose 6-phosphate is converted to glucose 1-phosphate, then to uridine diphosphate glucose, and finally to glycogen.
b. Glycogenolysis – the process of splitting glycogen into its glucose subunits.
- When body activities require ATP, glycogen stored in hepatocytes is broken down into glucose and released into the blood to be transported to cells.
- Not simply a reversal of glycogenesis. Begins with phosphorylation splitting off glucose molecules from the branched glycogen molecule to form glucose 1-phosphate.
- Phosphorylase – the enzyme that catalyzes this reaction, is activated by glucagon from pancreatic alpha cells and epinephrine from the adrenal medullae.
- Glucose 1-phosphate is then converted to glucose 6-phosphate and finally glucose, which leaves cells via GluT transporters.
- Phosphatase is the enzyme that converts glucose 6-phosphate. 1. It is not present in skeletal muscle cells, therefor glucose 1-phosphate is then catabolized for ATP production via glycolysis and the Krebs cycle and the remaining lactic acid produced by glycolysis in muscle cells can be converted to glucose in the liver.
c. Gluconeogenesis – the process by which glucose is formed from non- carbohydrate sources. stimulated by epinephrine
- The glycerol part of triglycerides, lactic acid, and certain amino acids can be converted in the liver to glucose.
- Large scale triglyceride and protein catabolism does not happen unless starving, eating very few carbs, or suffering from an endocrine disorder.
- In this case, glucose is not converted, it is newly formed.
- Amino acids and lactic acid can be converted to pyruvic acid, which can then be synthesized into glucose or enter the Krebs cycle.
- Glycerol may be converted into glyceraldehyde 3-phosphate, which may form pyruvic acid or be used to synthesize glucose.
- Gluconeogenesis is stimulated by cortisol, the main glucocorticoid hormone of the adrenal cortex, and by glucagon from the pancreas.
- In addition, cortisol stimulates the breakdown of proteins into amino acids, thus expanding the pool of amino acids available for gluconeogenesis.
- Thyroid hormones also mobilize proteins and may mobilize triglycerides from adipose tissue, thereby making glycerol available for gluconeogenesis.
describe the fate, metabolism, and functions of lipids and lipoproteins
lipid metabolism
lipoprotein – spherical particles with an outer shell of proteins, phospholipids, and cholesterol molecules surrounding an inner core of triglycerides and other lipids.
- Make lipids become water soluble.
- Several types of lipoproteins with different functions, but all are essentially transport vehicles.
- Provide delivery and pick up services so that lipids can be available when cells need them or removed from circulation when not needed.
Categorized and named mainly according to density. 4 classes low to high density, large to smallest size:
- Chylomicron (LARGE) - transport dietary lipids in the lymph and blood
- Very-low-density lipoproteins - transport endogenous triglycerides from hepatocytes to adipocytes for storage
- Low-density lipoproteins - transport cholesterol through the body for use in repair of membranes and synthesis of steroid hormones and bile salts
- High-density lipoproteins (SMALL) - transport excess cholesterol to the liver for elimination
cholesterol (sources and significance) – two sources of cholesterol in the body:
- some is present in foods – meat products
- most synthesized by hepatocytes
- fatty foods without cholesterol can still increase blood cholesterol level by stimulating reabsorption of cholesterol- containing bile salts and by liver breakdown of saturated fats in the body to make cholesterol
c. significance – increase in total cholesterol level increases risk of coronary artery disease. - Therapy to reduce blood cholesterol level – exercise, diet, drugs.
- Regular physical activity at aerobic and nearly aerobic levels raises HDL level.
- Dietary changes aim at reducing the intake of total fat, saturated fats, and cholesterol.
- Drugs used to treat high blood cholesterol levels include cholestyramine (Questran) and colestipol (Colestid), which promote excretion of bile in the feces, nicotinic acid (Liponicin), and the statin drugs which block the key enzyme needed for cholesterol synthesis.
fate of lipids – lipids may be oxidized to produce ATP.
- If no immediate need for lipids in this way, they are stored in adipose tissue throughout the body and in the liver.
- A few lipids are used as structural molecules or to synthesize other essential substances.
- Ex. Phospholipids which are components of the plasma membrane, lipoproteins which are used to transport cholesterol throughout the body, thromboplastin, which is needed for blood clotting, and myelin sheaths, which speed up nerve impulse conduction.
Essential fatty acids – that the body cannot synthesize are linoleic acid and linolenic acid. Dietary sources include vegetable oils and leafy vegetables.
triglyceride storage – triglycerides stored in adipose tissue constitute about 98% of all body energy reserves.
- Stored more readily than glycogen, in part because triglycerides are hydrophobic and do not exert osmotic pressure on cell membranes.
- Continually broken down, released, transported, redeposited, and resynthesized in other adipose tissue cells.
- Adipose tissue insulates and protects various parts of the body.
- Adipocytes in the subcut layer contain about 50% of the stored triglycerides
- The other 50% are split up:
- 12% around the kidneys
- 10-15% in the omenta
- 15% in genital areas
- 5-8% between muscles
- 5% behind the eyes, in the sulci of the heart, and attached to the outside of the large intestine
Lipid Catabolism - Lipolysis – the splitting of triglycerides into glycerol and fatty acids
- Essential for muscle, liver, and adipose tissue to oxidize the fatty acids derived from triglycerides to produce ATP.
- Catalyzed by enzymes called lipases.
- Enhanced by epinephrine and norepinephrine. 1. These hormones are released when sympathetic tone increases, as during exercise.
- Other lipolytic hormones include cortisol, thyroid hormones, and insulinlike growth factors.
- By contrast, insulin inhibits lipolysis.
Lipid Anabolism - Lipogenesis – synthesis of lipids from glucose or amino acids
- Stimulated by insulin
- Occurs when individuals consume more calories than are needed to satisfy their ATP needs.
- Excess dietary carbs, proteins, and fats all have the same fate, they are converted into triglycerides.
describe the fate, metabolism, and functions of proteins.
protein metabolism – proteins are broken down into amino acids. Not stored, they are either oxidized to produce ATP or used to synthesize new proteins for body growth and repair. Excess dietary amino acids are not excreted, they are converted into glucose or triglycerides.
fate of proteins – almost immediately after digestion, amino acids are reassembled into proteins.
- Active transport of amino acids into body cells is stimulated by insulinlike growth factors (IGFs) and insulin.
- Many proteins function as enzymes, others are involved in transportation (hemoglobin), or serve as antibodies, clotting chemicals (fibrinogen), hormones (insulin), or contractile elements in muscle fibers (actin and myosin).
- Several proteins serve as structural components of the body (collagen, elastin, keratin)
protein catabolism – a certain amount occurs in the body each day, stimulated mainly by cortisol from the adrenal cortex.
- Proteins from worn out cells are broken down into amino acids.
- Some new proteins are synthesized as part of the recycling process.
- Hepatocytes convert some amino acids to fatty acids, ketone bodies, or glucose
- Cells throughout the body oxidize a small amount of amino acids to generate ATP via the Krebs cycle and the electron transport chain.
- Before amino acids can be oxidized, they must first be converted to molecules that are part of the Krebs cycle or can enter the Krebs cycle (ex. Acetyl CoA)
Deamination – removal of the amino group (NH2) from an amino acid.
- Occurs in hepatocytes
- Produces ammonia (NH3) which is converted to urea by liver cells and excreted in the urine.
protein anabolism – the formation of peptide bonds between amino acids to produce new proteins
- carried out on the ribosomes of almost every cell in the body
- directed by the cell’s DNA and RNA.
- Hormones that stimulate protein synthesis: IGFs, thyroid hormones, insulin, estrogen, and testosterone
- Adequate dietary intake is required but excess dietary intake will not increase muscle or bone mass, only regular forceful, weight-bearing muscular activity can.
essential amino acids – must be present in the diet because they cannot be synthesized in the body in adequate amounts.
Humans are unable to synthesize 8 amino acids:
-
Isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
b. Synthesize two others in inadequate amounts: -
Arginine and histidine
c. Complete protein – contains sufficient amounts of all essential amino acids - Beef, fish, poultry, eggs, and milk are examples of complete proteins
d. Incomplete proteins – does not contain all the essential amino acids. - Leafy green vegetables, legumes, grains are examples
nonessential amino acids – can by synthesized by body cells.
- Formed by transamination – the transfer of an amino group from an amino acid to pyruvic acid or to an acid in the Krebs cycle.
- Once appropriate essential and nonessential amino acids are present in cells, protein synthesis occurs rapidly.
describe the reactions of key molecules and the products formed during metabolism.
Although there are thousands of different chemicals in cells, three molecules
- glucose 6-phosphate
- pyruvic acid
- acetyl coenzyme A
play pivotal roles in metabolism. These molecules stand at “metabolic crossroads”; as you will learn shortly, the reactions that occur (or do not occur) depend on the nutritional or activity status of the individual.
glucose 6-phosphate
- Synthesis of glycogen.
- Release of glucose into the bloodstream.
- Synthesis of nucleic acids.
- Glycolysis.
- a) Can be used to make ribose-5-phosphate
- b) Can be dephosphorylated to glucose
- c) Can be used to synthesize glycogen
- d) Can be converted to pyruvic acid
pyruvic acid
- Production of lactic acid
- Production of alanine.
- Gluconeogenesis.
fate of pyruvic acid Pyruvic acid in the presence of low oxygen is reduced to lactic acid, which is converted to either glycogen or carbon dioxide. In the presence of high oxygen levels, pyruvic acid is converted to an acetyl unit, which may be carried into the Krebs cycle by coenzyme A or converted into fatty acids, ketone bodies, or cholesterol.
acetyl coenzyme A
- Entry into the Krebs cycle.
- Synthesis of lipids.
compare metabolism during the absorptive and postabsorptive states.
A. metabolic adaptations
absorptive state – ingested nutrients are entering the bloodstream, glucose is readily available for ATP production. Insulin dominates absorptive state
a. Roughly 4 hours required for complete absorption, so roughly 12 hours per day spent in absorptive state assuming no snacks between meals. absorptive state reactions – two metabolic hallmarks of the absorptive state are the oxidation of glucose for ATP production and the storage of excess fuel molecules for future between-meal use.
a. 8 reactions dominate during the absorptive state:
- about 50% of the glucose absorbed from a typical meal is oxidized by cells throughout the body to produce ATP via glycolysis, the Krebs cycle, and the electron transport chain
- Most glucose that enters hepatocytes is converted to glycogen. Small amounts may be used for synthesis of fatty acids and glyceraldehyde 3-phosphate
- Some fatty acids and triglycerides synthesized in the liver remain there, but hepatocytes package most into VLDLs which carry lipids to adipose tissue for storage
- Adipocytes also take up glucose not picked up by the liver and convert it to triglycerides for storage. Overall, about 40% of the glucose absorbed from a meal is converted to triglycerides, and about 10% is stored as glycogen in skeletal muscles and hepatocytes.
- Most dietary lipids are stored in adipose tissue; only a small portion is used for synthesis reactions. Adipocytes obtain the lipids from chylomicrons, from VLDLs, and from their own synthesis reactions.
- Mandy absorbed amino acids that enter hepatocytes are deaminated to keto acids, which can either enter the Krebs cycle for ATP production or be used to synthesize glucose or fatty acids
- Some amino acids that enter hepatocytes are used to synthesize proteins (ex. Plasma proteins)
- Amino acids not taken up by hepatocytes are used in other body cells (ex. Muscle cells) for synthesis of proteins or regulatory chemicals such as hormones or enzymes.
postabsorptive state – absorption of nutrients from the GI tract is complete, and energy needs must be met by fuels already in the body.
Cortisol primary hormone involved in protein breakdown in the postabsorptive state
Epinephrine and Glucagon main hormonones stimulate glycogenolysis in postabsorptive state.
- The main metabolic challenge during the postabsorptive state is to maintain the normal blood glucose level of 3.9-6.1 mmol/liter.
- Homeostasis of blood glucose concentration is especially important for the nervous system and for RBCs because:
- The dominant fuel for ATP production in the nervous system is glucose, because fatty acids cannot pass the blood-brain barrier
- RBCs derive all their ATP from glycolysis of glucose because they have no mitochondria, so the Krebs cycle and the electron transport chain are unavailable to them.
c. Usually late morning, late afternoon, and most of the night assuming no snacks between meals.
postabsorptive state reactions – during postabsorptive state, both glucose production and glucose conservation help maintain blood glucose level. Hepatocytes produce glucose molecules and export them into the blood, and other body cells switch from glucose to alternative fuels for ATP production to conserve scarce glucose.
a. 4 major reactions that produce glucose during the postabsorptive state:
- Breakdown of liver glycogen – during fasting, a major source of blood glucose is liver glycogen, which can provide a 4-hour supply of glucose. Liver glycogen is continually being formed and broken down as needed. Increase in lipolysis.
- Lipolysis – Glycerol, produced by breakdown of triglycerides in adipose tissue, is also used to form glucose
- Gluconeogenesis using lactic acid – during exercise, skeletal muscle breaks down stored glycogen and produces some ATP anaerobically via glycolysis. Some of the pyruvic acid that results is converted to acetyl CoA, and some is converted to lactic acid, which diffused into the blood. In the liver, lactic acid can be used for gluconeogenesis, and the resulting glucose is released into the blood.
- Gluconeogenesis using amino acids – modest breakdown of proteins in skeletal muscle and other tissues releases large amounts of amino acids, which then can be converted to glucose by gluconeogenesis in the liver.
b. 5 major reactions produce ATP without using glucose during the postabsorptive state:
- Oxidation of fatty acids – the fatty acids released by lipolysis of triglycerides cannot be used for glucose production because acetyl CoA cannot be readily converted to pyruvic acid. But most cells can oxidize the fatty acids directly, feed them into the Krebs cycle as acetyl CoA, and produce ATP through the electron transport chain.
- Oxidation of lactic acid – cardiac muscle can produce ATP anaerobically from lactic acid
- Oxidation of amino acids – in hepatocytes, amino acids may be oxidized directly to produce ATP
- Oxidation of ketone bodies – hepatocytes also convert fatty acids to ketone bodies, which can be used by the heart, kidneys, and other tissues for ATP production
- Breakdown of muscle glycogen – skeletal muscle cells break down glycogen to glucose 6-phosphate, which undergoes glycolysis and provides ATP for muscle contraction.
explain what is meant by the term, energy balance.
Energy balance refers to the precise matching of energy intake (in food) to energy expenditure over time. When the energy content of food balances the energy used by all cells of the body, body weight remains constant (unless there is a gain or loss of water). In many people, weight stability persists despite large day-to-day variations in activity and food intake. In the more affluent nations, however, a large fraction of the population is overweight. Easy access to tasty, highcalorie foods and a “couch-potato” lifestyle both promote weight gain. Being overweight increases the risk of dying from a variety of cardiovascular and metabolic disorders, including hypertension, varicose veins, diabetes mellitus, arthritis, and certain cancers.
discuss the various factors that affect metabolic rate.
Hormones - Thyroid hormones *thyroxine and triiodothyronine) are the main regulators of BMR (basal metabolic rate), slow response - a few days calorigenic effect
- Exercise
- Nervous System - sympathetic stimulation stress/excercise
- Body temperature
- Ingestion of food - raises rate 10-20% due to energy cost of digesting.
- Age
- Other: gender, climate, sleep, malnutrition
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- Define the basal metabolic rate (BMR).
A. heat and energy balance
metabolic rate - the overall rate at which metabolic reactions use energy.
Some energy is used to produce ATP, some is released as heat
Many factors affect metabolic rate so it is measured under standard
conditions called the basal state
basal state - standard conditions for measuring metabolic rate – body is in a quiet, resting, and fasting condition
basal metabolic rate (BMR) - the measurement obtained under the basal state conditions.
The most common way to determine BMR is by measuring the amount of oxygen used per kilocalorie of food metabolized.
BMR is about 1200-1800 Cal/day in adults, with an additional 500-3000 Cal needed based on daily activities, ranging from sedentary to Olympic athlete level of activity.
- Describe the factors that influence body heat production.
A. body temperature homeostasis - maintained near 37 degrees Celsius.
Core temp – temp of body structures deep to skin and SQ layer
Too high kills by denaturing body proteins
Too low causes cardiac arrhythmias that result in death
Shell temp – temp near body surface, in skin and SQ layer.
a. Shell temp may be 1-6 degrees C lower than core temp.
B. heat production - production of body heat is proportional to metabolic rate
Factors that affect metabolic rate and thus rate of heat production:
Exercise – during strenuous exercise, the metabolic rate may increase to as much as 15x the basal rate. In well trained athletes, may increase up to 20x
Hormones – thyroid hormones are the main regulators of BMR. BMR increases as blood levels of thyroid hormones rise.
Response to changing levels of thyroid hormones is slow, taking several days to appear.
Thyroid hormones increase BMR in part by stimulating aerobic cellular respiration. As cells use more oxygen to produce ATP, more heat is given off, and body temp rises.
Other hormones have minor effects on MBMR: Testosterone, insulin, and human growth hormone can increase metabolic rate by 5-15%
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c. Nervous system – sympathetic division of ANS is stimulated during exercise or stress. It’s postganglionic neurons release NE and it also stimulates release of epinephrine and NE by the adrenal medullae.
1. Both epi and norepi increase the metabolic rate of body cells.
d. Body temp – the higher the body temp, the higher the metabolic rate.
1. Each 1 degree C rise in core temp increases the rate of biochemical reactions by about 10%. Thus metabolic rate may be increased substantially during a fever.
e. Ingestion of food – raises the metabolic rate 10-20% due to the energy costs of digesting, absorbing, and storing nutrients.
1. Food-induced thermogenesis – the effect of raised metabolic rate due to digestion, absorption, and storing of nutrients. Is greatest after eating a high-protein meal and less after eating carbs and lipids.
Age – metabolic rate of a child, in relation to its size, is about double that of an elderly person due to the high rates of reactions related to growth.
Other factors – gender (metabolic rate is lower in females, especially during pregnancy and lactation), climate (lower in tropical regions), sleeping (lower), malnutrition (lower).
describe the role of the hypothalamus in the regulation of food intake.
- The hypothalamus contains the neurons of the feeding center that stimulate eating and of the satiety center that signal fullness.
- Two nuclei in the hypothalamus that help regulate food intake are the arcuate and paraventricular nuclei.
- The hormone leptin, released by adipocytes, inhibits release of neuropeptide Y from the arcuate nucleus and thereby decreases food intake.
- Melanocortin also decreases food intake.
- It is thought that changes in blood chemistry (in terms of nutrients and hormone balance), as well as distention of the gastrointestinal tract, initiate appropriate hypothalamic activity.
energy homeostasis and regulation of food intake
Energy homeostasis – the precise matching of energy intake (in food) to energy expenditure over time.
a. When intake balances output, body weight remains constant.
Regulation of food intake – many factors affect, including neural and endocrine signals, levels of nutrients in blood, psychological elements such as stress or depression, signals from the GI tract and special senses, and neural connections between the hypothalamus and other parts of the brain.
a. Satiety – the feeling of fullness accompanied by a lack of desire to eat
1. Leptin and insulin – act on the hypothalamus to inhibit circuits that stimulate eating while also activating circuits that increase energy expenditure. Both pass the BBB
* Leptin helps decrease adiposity – total body-fat mass
- Neuropeptide Y – neurotransmitter that stimulates food intake when leptin and insulin levels are low.
- Melanocortin – a neurotransmitter that acts to inhibit food intake. 1. Stimulated by leptin, so when leptin level is normal, less desire to eat.
- Other hormones and neurotransmitters also contribute but are not discussed here.
describe the various mechanisms of heat transfer.
mechanisms of heat transfer
Conduction - heat exchange that occurs between molecules of two materials that are in direct contact with each other.
- At rest, about 3% of body heat is lost via conduction to solid materials in contact with the body: chair, clothing, jewelry.
- Heat can also be gained via conduction, ex. Soaking in a hot tub
- Heat loss or gain via conduction is much greater when the body is submerged in cold or hot water because water conducts heat 20x more effectively than air.
Convection - the transfer of heat by the movement of a fluid (a gas or a liquid) between areas of different temperatures.
- The contact of air or water with the body results in heat transfer by both conduction and convection.
- When cool air contacts the body, it becomes warmed and less dense and is carried away by convection currents created as the less dense air rises.
- The faster air moves, by a breeze or fan, the faster the rate of convection.
At rest, about 15% of body is heat is lost to the air via conduction and convection
Radiation - the transfer of heat in the form of infrared rays between a warmer object and a cooler one without physical contact.
- Body loses heat by radiating more infrared waves than it absorbs from cooler objects
- If surrounding objects are warmer than you, you absorb more heat than you lose by radiation
- At 21 degrees C, about 60% of heat loss occurs via radiation in a resting person
Evaporation - the conversion of a liquid to a vapor.
a. Under typical resting conditions, about 22% of heat loss occurs through evaporation of about 700mL of water per day, 300mL in exhaled air, and 400mL from the skin surface.
* Insensible water loss – because we are not normally aware of this water loss through the skin and mucous membranes of the mouth and resp system
- Relative humidity affects evaportion: higher humidity = lower rate of evaporation. At 100% humidity, heat is gained via condensation of water on the skin surface as fast as heat is lost via evaporation.
- Evaporation provides the main defense against overheating during exercise.
- Under extreme conditions, a max of about 3 liters of sweat can be produced each hour, removing more than 1700 Calories of heat if all of it evaporates. (Note, sweat that drips off the body rather than evaporating removes very little heat)
explain how normal body temperature is maintained by negative feedback loops involving the hypothalamic thermostat.
hypothalamic thermostat - the control center that functions as the body’s thermostat is a group of neurons in the anterior part of the hypothalamus, the preoptic area. preoptic area of the hypothalamus
Neurons of the preoptic area generate nerve impulses at a higher frequency when blood temp rises and at a lower frequency when blood temp decreases. Nerve impulses from the preoptic area propagate to two other parts of the hypothalamus: the heat-losing center and the heat-promoting center, which, when stimulated by the preoptic area, begin a series of responses that raise or lower temp.
B. Thermoregulation - if core temp declines, several mechanisms act via negative feedback loops to raise temp to normal.
Thermorreceptors in the skin and hypothalamus send nerve impulses to the preoptic area and the heat-promoting center in the hypothalamus and to hypothalamic neurosecretory cells that produce thyrotropin-releasing hormone. In response, the hypothalamus discharges nerve impulses and secretes TRH, which in turns stimulates thyrotrophs in the anterior pituitary to release thyroid- stimulating hormone (TSH)
Nerve impulses from the hypothalamus and TSH then activate several effectors. Each effector responds in a way that helps increase core temp to the normal value:
a. Vasoconstriction – the heat-promoting center sends nerve impulses that stimulate sympathetic nerves that cause blood vessels of the skin to constrict.
* Less blood flow = less heat loss
b. Epi and norepi – released by the adrenal medullae in response to nerve impulses in sympathetic nerves.
* These hormones increase cellular metabolism, which increases heat production.
c. Shivering – the heat-promoting center stimulates parts of the brain that increase muscle tone and hence heat production.
- Shivering is the repetitive cycle of the agonist muscle contracting, stretching muscle spindles in its antagonist, initiating a stretch reflex, and going back and forth.
- Body heat production can rise to about 4x the basal rate in just a few mins of maximal shivering
d. Thyroid hormone – the thyroid responds to TSH by releasing more thyroid hormones into the blood. As thyroid hormones slowly increase the metabolic rate, body temp rises.
describe how to select foods to maintain a healthy diet.
nutrition
nutrients – chemical substances in food that provide energy, forms new body components, or assists in various body functions.
a. 6 main types of nutrients are:
- Water – the nutrient needed in the largest amount, 2-3 liters per day.
- Carbohydrates
- Lipids
- Proteins
- Organic nutrients: carbs, lipids, and proteins provide the energy needed for metabolic reactions and serve as building blocks to make body structures.
- Minerals
- Vitamins
- Some minerals and many vitamins are components of the enzyme systems that catalyze metabolic reactions.
Essential nutrients – specific nutrient molecules that the body cannot make in sufficient quantity to meet its needs and thus must be obtained from the diet
- Ex. Some amino acids, fatty acids, vitamins, and minerals
minerals—general
- inorganic elements that occur naturally in the earth’s crust.
- In the body, they appear in combination with one another, in combination with organic compounds, or as ions in solution.
- Minerals constitute about 4% of total body mass and are concentrated most heavily in the skeleton.
- Typical dietary intake provides sufficient potassium, sodium, chloride, and magnesium, but some attention must be paid to ensure adequate calcium, phosphorus, iron, and iodide is taken in iodide is requireed for thyroid gland to synthesize thyroid hormones
- Excess amounts of most minerals are excreted in the urine and feces.
V. vitamins—general
- an organic molecule necessary in trace amounts that acts as a catalyst in normal metabolic processes in the body.
- Do not provide energy or serve as the body’s building materials
- Most vitamins with known functions are coenzymes.
- Most vitamins cannot be synthesized by the body and must be ingested in food.
- Others, such as vitamin K are produced by bacteria in the GI tract and then absorbed.
- Provitamins – raw material required for the body to assemble some vitamins. 1. Ex. Vitamin A is produced by the body from the provitamin beta- carotene.
- No single food contains all required vitamins, best to eat a varied diet.
- Two main groups of vitamins: fat-soluble and water soluble.
- Fat soluble – absorbed with other dietary lipids in the small intestine and packaged into chylomicrons
- Cannot be absorbed in adequate quantity unless ingested with other lipids.
- May be stored in cells, particularly hepatocytes.
- Water soluble – dissolved in body fluids.
- Excess quantities are excreted in the urine.
Antioxidant vitamins – inactivate oxygen free radicals
- Include vitamins C, E (fat-soluble), and beta-carotene.
Basal Metabolic Rate (BMR)
metabolic rate - the overall rate at which metabolic reactions use energy.
- Some energy is used to produce ATP, some is released as heat
- Many factors affect metabolic rate so it is measured under standard
conditions called the basal state
basal state - standard conditions for measuring metabolic rate – body is in a quiet, resting, and fasting condition
basal metabolic rate (BMR) - the measurement obtained under the basal state conditions.
- The most common way to determine BMR is by measuring the amount of oxygen used per kilocalorie of food metabolized.
- BMR is about 1200-1800 Cal/day in adults, with an additional 500-3000 Cal needed based on daily activities, ranging from sedentary to Olympic athlete level of activity.
