Download Metabolism Chp 7

Biol 221 metab s’08 1
Biol 221: Metabolism (Chap 7)
Biol 221 Students: Please find on the Biology department website:
• Energy Expenditure Extra Credit Assignment (instead of a lecture) for Thursday, March 13th
 it also includes a list of things to know from Chapter 8 for the test on March 18th
• An updated version of the Personal Nutrition Project with Parts 1, 2, & 3, all of it due March 27th
Reminder: March 13th is the last “drop without penalty” day. If you need to know what your grade is:
Ask Melanie, the department secretary (657-2031) OR email me: [email protected]
Metabolism:
total of all the chemical reactions in living cells
• Regulated by hormones
• Conducted by enzymes & coenzymes (see p. 327, figure 10-1)
(coenzymes: small organic molecules that assist enzymes)
Energy metabolism:
all the reactions the body uses to get and spend energy (from food or body stores)
Where does the energy come from?
Sun  plants (photosynthesis)  animals
Humans!
Plants convert light energy into chemical energy via photosynthesis:
CO2 + H2O Carbohydrate
What are the basic energy units derived from food?
• CHO glucose
(after absorption & transport, the liver converts fructose and galactose to glucose)
• Lipids  glycerol and fatty acids
• Pro  amino acids
The ups and downs of metabolism: (fig 7-2)
• anabolism: building up-- requires energy
usually done with condensation rxtns
• catabolism: breaking down--usually releases energy
usually done with hydrolysis rxtns
coupled reactions: when energy released from a catabolic rxtn is used to fuel an anabolic rxtn,
i.e., energy transfer (fig. 7-4)
“High-energy storage compound”:
Energy from catabolism is used to attach phosphate group onto ADP to make ATP  then when
energy is needed: breaking apart ATP releases energy that can be used to build another
compound
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Breaking Down Nutrients for Energy (p. 217-230)
Please note: This is a really simplified version! Most of these reactions are actually several steps
with many enzymes, but for this class, this level of information is enough.
Through all of these processes, watch how many carbons are in the molecules!!!
I. From CHO: fig 7-5
A. Glycolysis: Glucose Splitting
one glucose (6C) and 2 ATP  two pyruvate (3C) + 4 ATP & Coenzymes take 2 H
(so you net 2 ATPs & can get energy from H/electrons in Electron Transport Chain-ETC)
• This process is anaerobic (doesn’t require oxygen)
• This process is also reversible
So, anything that can be converted into pyruvate can become glucose!
(fig 7-8)
B. What do you do with pyruvate?
1. If there’s no oxygen or not enough mitochondria (Fig 7-6), then the muscles can take:
2 pyruvates + those 2 Hydrogens 2 lactic acids
Then in the liver, lactic acid can be converted back into glucose via the Cori cycle
OR
2. if oxygen is present (aerobic) (Fig 7-7)
2 pyruvate + 2 CoA + oxygen --> acetyl CoA (2C) + two CO2 and 2 H go to ETC
THIS IS IRREVERSIBLE!
Once a compound becomes acetyl CoA, it CANNOT be used to make glucose
C. Okay, so what can you do with acetyl CoA?
1. If there’s excess energy, you can make fatty acids (and stick that in your adipose tissue).
And, because you can make acetyl CoA from so many, many things (see fig 7-8), you can make
fat-based energy storage from almost anything you eat! Isn’t that great?!?!
Or, if you prefer:
2. Convert it into energy via the TCA (aka Krebs, Citric Acid) cycle and ETC
II. From lipids (Fig 7-11)
• Glycerol (3C) is easily converted into pyruvate (3C)
So, it can be used for energy via the steps mentioned above
OR
Say you need some glucose. Pyruvate can become glucose, right?
As you may recall:
Gluconeogenesis: using non-carbohydrates (e.g., protein or glycerol) to make glucose
Very inefficient! Uses only 3 of the carbons out of that huge triglyceride!
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So, let’s find some use for those fatty acids, shall we?
• fatty acids
Fatty acid oxidation: Breakdown of fa into acetyl CoA
See Fig 7-10
Generally speaking:
fatty acid + CoA (one for every 2C) 
acetyl CoA (one from every 2C) + some Hydrogens
Remember, acetyl CoA can't be used to make glucose!
But, it can go to the TCA & ETC and generate lots (9 kcal/g!) of energy (see below)
III. From Protein: Amino acids: Not a preferred energy provider! (Fig 7-13)
Proteins have many important roles, we don’t want to squander our precious essential amino
acids on energy production!
But if you’re going to go ahead and use it for energy anyway, at least do it right:
Step A. Deamination: Liver takes the nitrogen (NH2) off
Amino acid ammonia NH3 + possibly a keto acid
(this can be reversed to make non-essential aa—transamination; see fig 7-14, 15)
If that nitrogen isn’t used to make other compounds (like non-essential aa), it is excreted
(liver turns 2 NH3 + CO2 urea + H2O, kidneys put urea into urine) fig 7-16, 17
Step B. Depending on its side chains aa can:
a) make pyruvate (and ∴ gluc) these are called glucogenic aa
b) make acetyl CoA from ketogenic aa
c) enter the TCA cycle at other points
Are we there yet? The much-talked-about
TCA cycle: Fig 7-18: Making energy from all that acetyl CoA
Oxaloacetate (4C) + acetyl CoA (2C) (and oxygen, so take a deep breath)
Oxaloacetate + two CO2 + CoA + lots of energy (heat or ATP)
FYI: Oxaloacetate can be made from pyruvate + CO2 (∴ Fat can’t make it!)
Each turn gives off 8 electrons, which then speed themselves to the ever-popular…
Electron transport chain (ETC): Figure 7-19
Basically a whole lot of proton shuffling occurs, and this collection of energy adds a P onto ADP
to make ATP, storing that energy in the phosphate bond. It also requires oxygen, so it’s
another pretty good reason to breathe.
Would you look at how all those B vitamins fit into these reactions!?! (p. 347, figure 10-13)
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Responses to Energy Availability: Feasting & Fasting
Under normal circumstances,
Energy intake should ≈ energy expenditure (BALANCE)
When we eat more than our bodies need at that time, we are in a state of:
“Feasting” – storing excess energy
• Excess fat:
easiest form for body to store into fat cells!
• Excess CHO:
First, glycogen stores are replenished
Any leftover CHO will be made into fat
• Excess pro:
We don’t have “storage units” for amino acids—so if they aren’t used to make proteins:
they get deaminated  pyruvate or acetyl CoA  fat
• Alcohol: excess energy from it is made into (you guessed it) fat
Also, alcohol slows body's usage of fat as fuel--favors fat storage
TO SUM: Excess energy in any form can be stored as fat
Dietary fat (as compared to CHO, pro) is especially easy to store as body fat
Between meals, or when food is unavailable (e.g., crash diets, famine, anorexia, poverty):
“Fasting”: releasing energy from storage
If it’s not in the diet, your body will get energy from its stores…and for some cells (brain, RBC)
that energy has to be from glucose!
FIRST:
• liver glycogen provides glucose (lasts only for a few hours)
• fatty acids also used during this time
NEXT:
as glucose levels dip, fatty acid utilization increases
BUT: brain & RBC uses glucose as its energy source
So, low BG--> break down protein (muscle) to get glucose
NEXT:
Body adapts and some brain cells use ketone bodies (made from acetyl CoA, from fatty acids)
By day 10 of fasting, ketones provide most of the fuel for the nervous system
CONSEQUENTLY…
Body adapts by conserving energy  metabolism slows down  fat loss is reduced
(which is good during a famine, but not so good for a dieter: causes the “Yo-Yo” Diet Effect)
Some results of extended fasting: wasting of lean tissues, reduced disease resistance, disturbed
fluid and electrolyte balances…and after about 2 months, organ failure and death.
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