HUMAN AND ANIMAL BIOLOGY 2
LECTURE L14 – ENERGY METABOLISM AND THERMOREGULATION
Energy metabolism is all the chemical reactions in the body involved in energy storage and use. It is largely
influenced by hormones such as insulin and glucagon.
- Catabolism consists in breaking down nutrients to produce energy. It consists mainly in the
oxidation of glucose, needed for energy production (and heat is a by-product).
- Anabolism consists in building macronutrients from their building blocks (e.g. amino acids are
used to produce proteins) to store energy in them.
ATP production
ATP is the molecule where energy is stored. The reaction for the formation of ATP is: ADP (adenosine
diphosphate) + Pi (inorganic phosphate) + energy → ATP + H2O. The formation of ATP is a phosphorylation
reaction because it involves the addition of a phosphate (P) to ADP. ATP can be formed during glycolysis or
during oxidative phosphorylation (ADP + Pi → ATP). It can be formed using oxygen (aerobic production, so
produced by mitochondria) or without oxygen (anaerobic, no mitochondria).
When cells need energy to work, they obtain it by hydrolysing ATP in the process of ATP hydrolysis (l’inverso
della prima reazione sopra).
Cells can produce ATP using the energy that derives from the oxidation of glucose (see fig. 3.13): 1) glucose
is oxidised, and this reaction produces energy; 2) this energy is used to produce ATP; 3) ATP is broken down
and liberates additional energy, which can be used in different body processes.
Steps of glucose oxidation (See fig. 3.22):
1. Glycolysis → glucose is broken down and results into 2 molecules of pyruvate, 2 molecules of NADH
(a molecule that carries electrons), and 2 molecules of ATP. Glycolysis occurs in the cytosol.
2. Pyruvate is converted into Acetyl-CoA (beta-oxidation).
3. Acetyl-CoA is used into the Krebs Cycle (TCA), during which are produced NADH, FADH (carries
electrons like NADH), CO2 and ATP.
4. NADH and FADH transfer electrons through the electron transport chain, which releases energy. The
electron transport chain consists in a series of compounds that are able to bind electrons and release
them. During oxidative phosphorylation, electrons are accepted by oxygen.
5. Oxidative phosphorylation → the energy released by the electron transport chain is used to produce
large quantities of ATP, which is catalysed by the enzyme ATP synthase.
What happens if there is no/low oxygen during glucose oxidation (anaerobic situation)? → the electrons
released by NADH and FADH cannot bind to oxygen (perchè non c’è), so there is less production of ATP. In
addition, pyruvate accumulates in the cell and is converted into lactate (lactic acid, see fig. 3.23), which is in
turn converted into glucose, but ATP production is still very inefficient.
Why using anaerobic ATP production if it is inefficient? → it is used when you have to work fast (e.g. per fare
uno scatto durante la corsa) because it is faster to produce ATP with anaerobic production.
Metabolism of carbohydrates, fats and proteins (see fig. 3.24)
All these molecules contribute to our energy needs. Fats and proteins are used as energy sources when
glucose is low.
- Carbohydrates metabolism → when glucose is in high concentrations (e.g. after eating) it is
usually stored in the form of glycogen instead of being oxidised. The process of converting
glucose into glycogen to store energy is called glycogenesis. When glucose is needed, glycogen
, is broken down into glucose in the process of
glycogenolysis. At this point, some glucose is
used in glycolysis to produce ATP, while other
is transported to tissues through the blood.
Our body is also able to produce new glucose
from 1) glycerol (from tryglicerides), 2)
lactate, and 3) amino acids. This process is
called gluconeogenesis (è una sorta di
glycolysis al contrario), which is especially
important for the nervous system because it
always needs glucose as energy source
(perchè non è in grado di usare bene anche grassi e proteine come fonti di energia). See fig. 3.26
- Fat metabolism → fats are stored in adipose tissue, and are mainly stored as triglycerides. The
breakdown of triglycerides, called lipolysis, results in fatty acids and glycerol. Glycerol is then
involved in glycolysis, Krebs cycle and oxidative phosphorylation, while fatty acids are converted
into acetyl CoA in a process called beta oxidation. During this process, ketones can be produced
as a by-product that can be used by the nervous system instead of glucose. See fig. 3.27
Our body is also able to produce fats from other nutrients (such as proteins or carbohydrates) in
a process called lipogenesis.
- Protein metabolism → proteins are broken down into amino acids with proteolysis. Then, the
amino acids are deaminated = an NH2 is removed. This process produces ammonia NH3, which
is toxic, so it is converted into urea by the liver and then expelled with urine. After deamination,
the amino acids are converted into pyruvate, acetyl CoA or other substances that can enter the
Krebs cycle. See fig. 3.28
Energy metabolism
Energy metabolism refers to the way our body stores and uses energy; it is influenced by eating patterns (e.g.
how often we eat) and other factors such as growth, stress and metabolic rate. Energy metabolism is mainly
controlled by the endocrine system (hormones), and this control depends on two factors:
1. Since food intake is intermittent, the body must store nutrients when there is intake and break down
these stores when we are not eating.
2. Since the brain depends on glucose as a nutrient, levels of glucose in the blood must always be
maintained.
Energy intake, use and storage
During digestion, food is broken down into nutrients that are then absorbed into the blood. These nutrients
are then taken up by cells that use them for different purposes. See fig. 21.1.
- Carbohydrates: they are found in the blood mainly as glucose, transported inside cells by glucose
transporters. In cells, glucose can 1) be oxidised to produce energy, 2) be a substrate to produce
other molecules or 3) can be converted into glycogen for storage. If glucose levels in cells
decrease, glycogen will be broken down into glucose via glycogenolysis.
- Proteins: they are found in the blood as amino acids. In cells, they can be used to 1) synthesize
new proteins or 2) produce energy during catabolism (they are broken down).
- Fats: triglycerides are transported in the blood in lipoproteins (proteins that bind to fats). Before
entering cells, triglycerides must leave the lipoproteins and are broken down into fatty acids, that
go into cells, and monoglycerides by the enzyme lipoprotein lipase. Monoglycerides remain in
, the blood and are broken down by the liver. Fatty acids in cells can be 1) oxidised for energy or
2) used to produce new triglycerides that will be stored in adipose tissue. These stores can be
broken down again to produce energy or to release fatty acids and glycerol in the blood to be
used by other cells.
Energy balance
Energy balance means that the amount of energy that you take in from nutrients should be = to the amount
of energy that you use.
- If you have more energy coming in than you are using, the balance is positive and you store
energy as fat.
- If you are using more energy than you are taking in, the balance is negative and you liberate
energy (wight loss).
The processes that require energy in cells can be classified as: mechanical work, used to generate movement;
chemical work, to form bonds in chemical reactions; transport work, to move molecules inside cells.
When nutrients are broken down, the body releases energy in form of heat or uses that energy to perform
work. The amount of energy used per unit of time in this way is called metabolic rate, which depends on age,
gender, muscular activity etc. The basal metabolic rate BMR is the rate of energy needed to perform all body
activity when the body is relaxed.
NB: the body is generally never in energy balance, because the intake of energy is discontinuous because you
eat only some times during the day; the use of energy is continuous, because energy is used in all body
processes (so even when you are resting). Absorption of nutrients:
- Absorptive phase: nutrients are absorbed from the digestive tract for 3-4 hours after eating
(positive energy balance, because you are taking in more energy than you are using). Nutrients
and energy are stored in this phase (corresponds to anabolism), and glycogenesis and lipogenesis
occur.
- Postabsorptive phase: no absorption of nutrients (between meals) (negative energy balance,
because you are not absorbing nutrients but you are using energy). During this phase, the body
uses its stores to liberate the energy it needs.
Metabolism during the absorptive phase → it is mainly anabolism, because most reactions in this phase
involve the synthesis of macromolecules. Each nutrient is used differently in each type of cell: see fig. 21.3
- Glucose: it is used in most body cells to produce energy, but also in the liver and in muscle cells
where it is converted into glycogen (storage).
- Fatty acids: they are used in the liver and in adipose tissue, where they can be stored or used to
produce triglycerides with glycerol.
- Amino acids: they are used in the liver, where they are converted into fatty acids, and in muscles
to constitute proteins.
Metabolism during the postabsorptive phase → it is mainly catabolism, so the stores of molecules are broken
down to liberate energy.
- Proteins: they are broken down into amino acids from muscle cells, to supply energy to non-
nervous tissue.
- Triglycerides: they are broken down into fatty acids and glycerol to be used both in non-nervous
and nervous tissue. Nervous tissue is able to use fatty acids as nutrients because they are first
converted into ketones by the liver (ketogenesis), which can substitute glucose.
- Glycogen: it is broken down into glucose to be used mainly in nervous tissue.
,Role of insulin in metabolism
Insulin is a hormone secreted by beta cells in the pancreas. Its secretion is high during the absorptive phase
of metabolism when glucose levels in the blood are high, because beta cells are sensitive to the concentration
of glucose. Glucose enters beta cells and undergoes oxidative phosphorylation; after a series of steps (see
fig. 21.6) calcium enters beta cells and this stimulates insulin release in the blood to regulate glucose
concentration.
Functions of insulin: main function is to reduce glucose levels in blood when they are too high (e.g.
hyperglycemia/diabetes. The opposite is hypoglycemia, when glucose levels are too low); it can be made in
3 ways: 1) insulin promotes the uptake of glucose by cells, 2) converts glucose into glycogen or 3) inhibits
gluconeogenesis.
Role of glucagon in metabolism
Glucagon is a hormone produced by alpha cells in the pancreas. Its secretion is high during the post-
absorptive phase, when glucose levels in blood decrease. In general, glucagon has opposite effects of insulin.
Functions glucagon: in liver, it promotes glycogenolysis and gluconeogenesis (both processes that increase
glucose levels) and inhibits synthesis of glycogen and proteins (because they make glucose levels even lower).
In adipose tissue, glucagon stimulates lipolysis and inhibits triglycerides synthesis.
NB: insulin and glucagon control together the glucose concentration via negative feedback: when glucose is
high, insulin secretion increases and glucagon secretion decreases, and vice versa.
Thermoregulation
Thermoregulation is the ability of the body to regulate and maintain its temperature into certain limits.
Organisms with this ability are called homeothermic; those that do not have this ability are called
poikilothermic, and their temperature depends on the temperature of the environment.
Animals can also be divided into 1) ectotherms, if the source of heat they use to increase body temperature
comes from the environment (e.g. they choose to stay somewhere with a favourable temperature); these
animals lose heat as soon as they produce it, or 2) endotherms (birds and mammals), if their source of heat
is in their body, because they are able to retain heat released during metabolism.
Heat is a by-product of metabolism (process of thermogenesis). If heat production is higher than heat loss,
body temperature rises and hyperthermia occurs (around 43°, can lead to unconsciousness, convulsions,
death); if heat production is lower than heat loss, body temperature drops and there is hypothermia.
Ways to lose heat:
- Radiation: heat is transferred from the body to the environment in form of electromagnetic
waves.
- Evaporation: through respiration or sweating, liquids are converted to gas and heat is lost. Sweat
is produced by eccrine glands (skin) and apocrine glands (in hair follicles, e.g. in armpits).
- Convection: heat is transferred from a place (body) to another (environment) by a moving gas
(air) or liquid.
- Conduction: heat is exchanged via contact with other objects that have a different temperature
from your body (e.g. se tocchi il ghiaccio la tua temperatura si abbassa).
- Blood supply: blood vessels are dilated (vasodilation) when your body temperature is high and
you want to lose heat, because blood can move more freely. Blood vessels are constricted
(vasoconstriction) when your body temperature is low and you want to maintain heat, because
, blood cannot move as freely. This mechanism is regulated by the sympathetic nervous system,
and it work well when environmental temperature is in the thermoneutral zone at 25-30°C.
Normal body temperature in humans is around 37°. The hypothalamus is the thermoregulatory centre of the
body, and receives information about body temperature from thermoreceptors in different parts of the
body. Then, a thermoregulatory response is initiated to return body temperature to normal.
Generation of heat in a cold environment: the primary mechanism to produce heat when the environment
is cold is shivering → muscles contract and generate heat. This mechanism is used by mammals that
hibernate. Another way they have to generate heat is having brown adipose tissue (a non-shivering
thermogenesis method), or creating a better insulation in the body e.g. by having a thick fur.
Torpor → temporary sleep made by some small mammals during which their body temperature
drops, and this helps them save a lot of energy.
Hibernation → prolonged inactivation. Mammals prepare for it by storing fat and decreasing
metabolism, body temperature, heart rate and respiration. When they wake up, they shiver and use non-
shivering thermogenesis to generate heat.
NB: fever is a condition induced by chemicals called pyrogens, produced by certain white blood cells when
there is an infection. They do so because fever enhances the ability of the body to defend itself.
Hormones
Growth hormone: It promotes growth of tissues by 1) stimulating protein synthesis, so cells increase in size
(hypertrophy) and 2) stimulating cells division, so there are more cells (hyperplasia). This hormone is
secreted by the anterior pituitary gland; its secretion is regulated by two hormones of the hypothalamus:
growth hormone releasing hormone (GHRH) and growth hormone inhibiting hormone (GHIH, or
somatostatin).
Insulin-like growth factors (IGFs) are other substances produced by the liver that function as hormones to
promote growth and that also have the ability to reduce blood glucose levels (like insulin).
Thyroid hormones:
- Increase oxygen consumption and use of energy, so heat production also increases.
- When too much energy is stored in the body (in form of glycogen, protein or fat), TH promote
glycogenolysis, proteolysis and lipolysis to liberate this energy. When too little energy is stored
in the body, TH promote glycogenesis and protein synthesis.
- TH are also important for normal growth, development and functioning of organs, as they
stimulate GH secretion.
Glucocorticoids: main function is to maintain normal concentrations of enzymes necessary for the
breakdown of nutrients and for the conversion of amino acids into glucose. They are also required for GH
secretion, and have effects on immune system, nervous system and kidneys.
LECTURE L15 – DIGESTIVE SYSTEMS AND FORAGING BEHAVIOUR
Foraging behaviour
You can predict what is the diet of animals depending on the appearance of their teeth, shape of the skull
and of their body (e.g. if they are suited for running).
Essential nutrients that we need to take from food are fatty acids, proteins, minerals. These nutrients are
needed to obtain energy and build new cells.
NB: during life, animals eat a lot more than their body weight because they use much energy to produce ATP,
to carry out the body processes and some nutrient molecules are excreted and not used.
,In an ecosystem, the biomass and energy tend to be lost as you go up
in the pyramid made of producers → primary consumers → secondary
→ tertiary. It means that the biggest animals eat from the first trophic
level (producers, so they are herbivorous) because there is more
energy that can be obtained from it.
The optimal foraging strategy depends on the availability of preys, the
search cost to prey (è conveniente cacciare un determinato animale?) and the handling cost (il predatore
deve pulire la preda? È difficile da mangiare?). Example: birds tend to eat small worms because they are
easier to handle, but this means that they will obtain less energy from a small worm compared to a bigger
one.
Feeding mechanisms:
- Feeding on particulate matter: particulate matter = plankton (tiny organisms transported by
water currents). There are different methods animals use to feed on plankton:
Suspension feeding → animals that use this method move their cilia to produce currents that
bring food in their mouth, or they trap particles in a mucus that brings food into the GI tract.
Filter feeding → some animals (e.g. herrings, whales) have filtering devices that filter water and
let food particles pass through them.
Deposit feeding → some animals eat deposits of disintegrated organic matter that accumulates
on a substrate; these animals pass the substrate through their bodies and remove nutrients from
it.
- Feeding on food masses: fishes, reptiles and amphibians use their teeth to hold prey and prevent
it to escape and they often swallow it whole. True mastication/chewing is almost limited to
mammals, who have 4 different types of teeth: 1) incisors, used to bite/cut/strip (e.g. in rodents),
2) canines, to seize/pierce/tear (typical in carnivores, in herbivores are absent/useless), 3)
premolars and molars, to grind/crush (well developed in herbivores because they can destroy
cellulose for better digestion). Some animals also have modified teeth used for defence or attack
(e.g. zanne elefanti o dei cinghiali). Animals with these different kinds of teeth are called
heterodonts. Most mammals also grow a first set of deciduous teeth (denti da latte) which are
then replaced by definitive teeth.
- Feeding on fluid: it is typical of parasites, that absorb nutrients provided by the host, suck blood
or other body fluid, or feed on content of the host’s intestine.
What factors affect foraging behaviour in animals? → internal/external constraints, learnt behaviour,
competition with others, availability of food, weather conditions...
External constraints → e.g. the risk of being pried by others when you are looking for food. To avoid
it, animals tend to reduce the time they spend looking for food, they choose safer places to do it even if there
is less food or look for food in groups or try to mimetize.
Internal constraints → e.g. morphology and physiology, for example a cat cannot hunt like cheetahs
do because it is too small.
Insectivores were one of the first animals that evoluted. They had a simple GI tract with an elongated pyloric
region of the stomach because they had to eat the exoskeleton of insects (harder to digest).
Coevolution is when predators develop a hunting strategy depending on the strategy of the prey to increase
its chance of catching it, or vice versa.
, Digestion
Digestion is the enzymatic breakdown of food particles into nutrients. Enzymes are produced in salivary
glands, stomach, pancreas, intestine and in many cells. There are also bacteria in the caecum and colon that
produce enzymes.
Digestion started in bacteria as intracellular digestion (fig. 32.7 zoo): food particles are taken up in the cell
by phagocytosis inside a food vacuole; nutrients are absorbed in the cytoplasm and wastes are expelled via
exocytosis. In extracellular digestion (e.g. in vertebrates), the cells lining the lumen (cavity) of the GI tract
secrete substances (e.g. enzymes) to digest food outside of the cell, and other cells absorb the nutrients. The
cells of the intestine perform both intracellular and extracellular digestion.
One of the most important parts of the GI tract are sphincters → they are circular muscles that close a certain
compartment (e.g. they separate the oesophagus from the stomach).
Food is moved in the GI tract by cilia or specialised musculature. In e.g. mammals, the gut is usually lined
with a longitudinal and a circular smooth muscle layer. These muscles perform two movements:
segmentation, used to mix food; and peristalsis, used to move food forward in the GI tract.
Glands associated with the GI tract → saliva glands, liver, pancreas, rectal glands.
GI tract:
1. Mouth, teeth and pharynx. It can be different depending on the species and its diet, e.g. mouth and
teeth can be specialised to grab, grind, chew, ruminate etc. Teeth can also have a secondary function
such as defence. The mouth contains saliva glands to lubricate the food and make swallowing easier;
these glands can be specialised to be also poison glands or have anticoagulant properties. Saliva
contains amylase, and enzyme that starts the breakdown of starch (a carbohydrate).
2. Oesophagus. Its function is to transport food from the mouth to the stomach (and back, in ruminants)
thanks to muscle contraction. In birds, the oesophagus expands into a crop used as temporary food
storage.
3. Stomach. Its function is storage, digestion of proteins and defence against pathogens. In
ruminants, digestion of cellulose starts in the pre-stomaches thanks to fermentation by
bacteria that produce the enzyme cellulase; in non-ruminant herbivores, cellulose
fermentation occurs in the intestine. The purple part is the cardia (sphincter), which
allows passage of food from oesophagus to stomach. The orange part is the corpus, which
contains cells that produce mucus and gastric juice, which contains HCl and pepsinogen
(the inactive form of pepsin) for the breakdown of proteins. The green part is the fundus,
which produces mucus. The yellow part is the pyloric region, a sphincter that connects Ruminant
stomach and intestine.
4. Small intestine. Its function is digestion of carbohydrates, proteins, fats thanks to enzymes. These
enzymes are produced by the intestinal cells (enterocytes), or are contained into bile (produced by
liver and stored in the gall-bladder) and pancreatic juice. The first tract of the intestine is the
duodenum, where pancreatic juice and bile enter the intestine, followed by jejenum and ileum.
Intestinal cells then absorb nutrients through villi and microvilli, which increase the intestinal surface
for better absorption.
5. Large intestine or colon. Its function is reabsorption of water for the formation of faeces, and volatile
fatty acids. In non-ruminant herbivores, digestion of cellulose
occurs here in a region called cecum.
6. Rectum. Its function is resorption of water, storage of faeces and
creation of pellets.
NB: the morphology of the GI tract depends on the diet of animals (fig.
28.11 zoo):
