Chapter 5b Respiration and cellular activities

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Chapter 5b Respiration and cellular activities

• I- The Krebs Cycle in LIPID (Fats) and PROTEIN
  metabolism (So far we have discussed only
  carbohydrates)
• II- Respiration and Heat production (Through
  regulation, some plants and animals can release
  larger quantities of heat, and produce less ATP)
• III- Control of respiration (Cell respiration and
  energy production are closely regulated)

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FAT and PROTEIN metabolism

• Carbohydrates (sugars) are the main source of
  energy for cell respiration, and account for the
  largest of cell respiration. When most
  organisms need energy, glucose is broken down
  thru cell respiration. Thus, glucose is the
  central energy-liberating pathway in all
  organims.
• Lipids (fats) are important energy storage
  molecules and are used upon certain
  circumstances. The energy yield of fats is higher
  than the energy yield of carbohydrates.
• Under some special conditions, proteins can also
  be used as substrates for cell respiration and
  ATP generation, but they are not the choice
  molecules to break down, as usually they are
  important structurally (skeletal system), or as
  functional molecules in other vital process
  (enzymes, hormones, etc.)
• The energy yield of carbohydrates and lipids (and
  proteins if needed) is liberated gradually thru a
  series of reactions, each catalyzed by specific
  enzymes.
• Fats and proteins are metabolized by being
  converted into molecules found into some step of
  the regular aerobic respiration process that we
  have already learned.

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Fat cells their number doesnt change much thru
time
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Breaking down fats and proteins

• Hydrolysis (breaking with water) is the process
  used in the initial breaking down of
  carbohydrates, fats and proteins. H2O is
  required.
• Macromolecules must be broken down initially
• Lipids (Fats and oils) ? to Glycerol and Fatty
  Acids.
• Proteins ? to individual Aminoacids, then
  remove amino groups
• Carbohydrates ? to glucose

Proteins to aminoacids
Carbohydrates to glucose
Lipids to Glycerol fatty acids
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(Remember how they are formed ?)
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Fats, Proteins Carbohydrates in cell respiration

• Cells can extract E from all 3 macromolecules.
• Fats and proteins are broken down into products
  that can enter the Krebs Cycle. They can enter at
  various places.
• Once these raw products enter the Krebs Cycle,
  the processes continue just as we have learned.
• Under normal conditions most energy requirements
  are met with carbohydrate and some fat
  metabolism. Structural proteins (muscles, etc)
  will be used as substrate for cell respiration
  only under very special circumstances
  (starvation).

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Metabolism of Fats (Fatty acids and glycerol)

• Fat metabolism requires O2 Most of it bypasses
  glycolysis enters the Krebs cycle. Without O2,
  most of the energy from fats cannot be
  transferred into ATP.
• From a fat only the glycerol portion (the
  smallest part) can be metabolized in the absence
  of O2.
• Glycerol (3-C molecule) is brought as PGAL into
  glycolysis, and will then continue to pyruvate
  and enter the Krebs Cycle, or to fermentation.
• Fatty acids are first broken down to acetate by
  enzymes from mitochondria then Coenzyme A (CoA)
  brings the acetate into the regular Krebs Cycle.
• Fatty acids can not go the anaerobic way
  (fermentation).
• Since fats are more completely reduced compounds
  than Carbohydrates (higher of H), they yield
  more E per unit weight 1 gram of fat yields over
  twice the amount of E than a carbohydrate.

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Metabolism of Proteins - aminoacids

• Proteins are first broken to aminoacids in
  digestion (Note that this is not part of cell
  respiration). Only in special conditions
  (starvation), structural proteins (muscle, etc)
  would be used in cell respiration.
• Other enzymes remove the amino groups and
  convert them first to ammonia (NH3) and then to
  urea or uric acid for excretion (waste removal,
  involving kidneys).
• Carbon skeletons are then brought into Krebs
  cycle as oxalacetate or ketoglutarate.
• Thus, protein metabolism bypasses completely both
  glycolysis and the possibility of fermentation
  after pyruvate.

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Using building blocks in synthesis

• In autotrophs, the Calvin Cycle (from
  photosynthesis) and the Krebs Cycle (from
  respiration) lead to the synthesis of every
  organic compound needed.
• In heterotrophs this is partly true, except for
  vitamins, some aminoacids and certain fatty acids
  we cannot synthesize.
• By investing energy (ATP), the cells can reverse
  the direction that some of these molecules move
  along the metabolic partways, and use the system
  in biosynthesis reactions to make fats or
  proteins, or carbohydrates. But additional ATP is
  needed (from food).
• Not all the pathways are 2-ways (double arrows),
  and even those that are, reversing implies
  different enzymes.

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Respiration and HEAT PRODUCTION

• Mammals have a tissue called brown fat.
• Rich in mitochondria, brown fat cells produce a
  lot of heat very fast, but produce little ATP.
• Brown fat cells are not the same as the lipocytes
  that we discussed above, where excess E from food
  is stored. Instead, brown fat cells have a
  specific function.
• Brown fat cells are useful in winter for
  hibernating animals, and in infants of all
  mammals, because they tend to loose heat very
  fast.
• Some plants are also able to divert E from cell
  respiration towards heat production, with less
  ATP generation. In an extreme case (skunk
  cabbage), the heat from the plant melts the snow
  allowing the emerging flowers to come out.

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Control of Respiration

• The organisms must decide whether glucose is
  going to be used in cell respiration, or whether
  it will be stored as starch, glycogen or fat.
• The controls operate in a supply-and demand
  principle.
• Under conditions of low E demand, cells use
  excess glucose to synthetize glycogen, and then
  fat (protein building has already been met).
• Under high E demand, glucose from blood (or plant
  tissues) is catabolized rapidly, and replenished
  with new glucose from glycogen (liver), or starch
  (plants).