Lecture 2: Energy Metabolism

Question 1

What is ‘metabolism’?

Answer 1

The sum total of all chemical reactions occurring in an organism.

Question 2

Define ‘anabolic pathways’

Answer 2

• Assembling simple molecules into complex ones.
• Requires energy.
• e.g. building proteins and enzymes etc.

Question 3

Define ‘catabolic pathways’

Answer 3

• Breakdown of complex molecules into simple ones.
• Releases energy.

Question 4

Anabolic (requires energy) and catabolic (releases energy) pathways

Answer 4

• Leads to a continual energy flow in a system.
• Energy is ‘lost’ as heat.
• But energy isn’t really ‘lost’. 1st law of thermodynamics: total energy input must equal total energy output. (Energy can’t be created or destroyed).

Question 5

How does energy balance work?

Answer 5

1. Energy input = food energy
2. Broken down into smaller pathways
3. A lot of energy is stored (long term energy storage is in the form of ATP, glucose or glycogen)
4. Energy output through (a) External work which involves all processes related to movement or (b) Internal work which is the sum total of all reactions for normal functioning e.g. respiration, all these processes release thermal energy (heat)

• the balance between input & output determines body condition & performance.

Question 6

Explain ‘Dynamic energy budgets’

Answer 6

• framework to predict how well an animal will do in different environments.
• it changes in response to different life stages & environments.

1. Energy input through ingestion of food
2. Assimilation of energy (make it usable by cells) during digestion in gut.
3. Some energy will not be fully broken down, it’s egested in faeces.
4. Energy that is up-taken is assimilated into energy reserves.
5. It can be mobilised (used) into processes an organism requires: (a) Somatic maintenance: general processes which allow an organism to function including growth (so it’s higher in juvenile organisms). (b) Energy to reproduce: (higher in sexually mature adults).

Question 7

Example of ‘Dynamic energy budgets’ : Impact of size

Answer 7

• Larger organisms require more total energy.
• But they require less energy per kilo of body weight.
• Smaller, endothermic animals require more energy per kilo of body weight.
• And require more energy for thermo-regulation (maintain a stable internal body temp)

Question 8

Example of ‘Dynamic energy budgets’ : Sea urchins exposed to ocean acidification stressor

Answer 8

• in optimal conditions just over half the sea urchins energy is used for general maintenance, a quarter for growth, a quarter for reproduction.
• in the acidification condition the sea urchins have to use more energy just to survive (general maintenance) and it uses less on growth and reproduction.

Question 9

Explain regulation of energy balance

Answer 9

1. In the hypothalamus, theres a sensor called the ‘Arcuate nucleus’.
2. In the ‘Arc’ is 2 integrators (takes info about a set point & signals to effectors) that regulate appetite and satiety (fullness).
3. These are controlled by hormones. (a) ‘NPY’ = stimulates appetite. (b) MSH’s (comes from POMC) = appetite suppressor.
4. After integrators sense appetite & satiety they exert the effects by hormones.
5. This pathway works by negative feedback.
6. In the gastrointestinal tract is series of ‘mechano & chemoreceptors’ (e.g. stretch receptors in the stomach wall) that send signals when the stomach is full or empty via the ‘vagus nerve’ directly to the brain.
7. Then there is a series of hormonal feedbacks:
   (a) mid and long term energy balance: 2 main hormones. ‘Leptin’ which is produced by adipose (fat) tissues. If greater supply of fat in body, leptin produced. This has negative effect on appetite. ‘Glucose’ at high levels has negative impact on appetite. Both of these inhibit ‘NPY’ release & reduce appetite.
   (b) short term energy balance regulators (in gastrointestinal tract, secreted from intestine): 2 main hormones: ‘CCK’ and ‘PPY’ both inhibit ‘NPY’, so suppresses appetite.
   (c) ‘Ghrelin’ (hunger hormone) (secreted from stomach): stimulates ‘NPY’, increases appetite.

Question 10

Example of the regulation of energy balance: Hibernating squirrel experiment.

Answer 10

Check book

Question 11

What does ‘Basal metabolic rate (BMR)’ mean?

Answer 11

• A stable rate of energy metabolism in endothermic animals under minimal environmental and physiological stress.
• (the resting period (but not sleeping))
• optimal temperature (doesn’t expend energy to regulate temperature)

Question 12

What does ‘Standard metabolic rate (SMR)’ mean?

Answer 12

• a resting MR at a given body temperature for ectotherms.
• for ectotherms MR is dependent on external temperature (MR increases as external temperature increases). Therefore we need to define the external temperature.

Question 13

What does ‘Aerobic metabolic scope’ mean?

Answer 13

• the metabolic range of which an animal is capable.
• occurs between BMR or SMR to max sustainable MR (MMR).

Question 14

How to measure metabolic rate?

Answer 14

1. Direct calorimetry
   - measures the heat given off (all metabolic reactions produce heat).
   - the heat produced will tell how much energy the animal is using.
   - e.g. treadmill inside a calorimeter. Cold H2O goes in, heat from human inside heats up H2O, measure the difference in H2O temp.
   - difficult
2. Respirometry
   - measures O2 used and CO2 released.
   - e.g. fish in sealed tank. Inflow of H2O has O2 & CO2 measured, and H2O coming out has O2 and CO2 measured, then estimate MR.

Question 15

Evaluate Respirometry to measure metabolic rate.

Answer 15

• assumes metabolism is totally aerobic.
• assumes energy used is proportional to O2 used. But, depending on the type of fuel the animal is using, different amounts of energy are being liberated e.g. 38.9 KJ of energy produced per gram of fat, 17.6 KJ per gram of protein and 17.1 KJ per gram of carbohydrates.
• when oxidising/ breaking down fuels as part of metabolism we are using and releasing different amounts of O2 & CO2. The standard oxidisation of glucose = takes in O2 and releases the same volume of CO2. This is not the case for fats and proteins. (SEE RESPIRATORY QUOTIENTS CARD)

Question 16

Respiratory quotients

Answer 16

RQ = rate of CO2 production / rate of O2 consumption

1. Carbohydrates/glucose: C6 H12 O6 + 6O2 > 6CO2 + 6H2O > RQ = 1
   - Carbohydrates/glucose: 1 glucose + 6 Oxygens releases 6 carbon dioxide + 6 water molecules + 2 ATP. RQ = 6/6 = 1
2. Fats (tripalmitin): RQ = 102/145 = 0.7
3. Proteins: (complicated as they don’t fully brake down & varies). RQ = 0.8

• the RQ value allows us to estimate what fuel the animal was using and then we know how much energy that type of fuel will release. (this allows us to estimate the MR (how much energy being used))

Question 17

How does size and mode of thermal-regulation affect metabolic rate (energetic demands)?

Answer 17

• Kleiber’s law (1932) = when double mass get an increase of MR only to 3/4 of expected.
• (mass specific MR = total MR / unit of mass of organism)
• e.g. mass specific MR of a mouse is over 20 times that of an elephant.
• smaller animals have a higher mass specific MR.
• endotherms have increased metabolic demands when it comes to thermoregulation.

Question 18

How does locomotion affect metabolic rate (energetic demands)?

Answer 18

• Larger animals have smaller overall costs of transport. Due to inertia, smaller animals take more energy to get started/ take off, they have to take more steps/wing flaps.
• Although it takes more energy overall for larger animals to move their mass, they are more efficient and its takes less energy per unit of body mass
• Running is the most energy expensive and swimming is the least.
• Energy use is about proportional to speed/ velocity when running.
• Energy use in faster swimmers gets exponentially more expensive.
• Small birds have higher total energy use (less efficient flyers) and big birds have lower energy use
• For small birds when they first take off it takes a lot of energy to get going. Then they enter a state where energy use is declining (their most efficient state), this is when they are able to glide. But then energy use gets more intensive towards the end when they get faster because they are encountering more air resistance.
• There is clearly optimum speeds for particular birds to be flying.
• Larger animals less affected by inertia.

Question 19

How does development affect metabolic rate (energetic demands)?

Answer 19

• Energy uses in different stages of development are different.
• E.g. energy use in zebra fish (ectotherm). Their energetic rate is strongly influenced by temperature. Their metabolism is higher at a higher temperature.
• There is a peak in energetic demand + MR during early stages of development because the animal is undergoing rapid organogenesis (developing organs).
• For many experiments in adult animals, heart rate will be used as a proxy for MR. The faster the heart beats the more O2 is delivered around the body to respiring tissue.
• Heart rate is faster at higher temperatures according to higher MR.
• Embryos are permeable to O2 (not reliant on heart pumping O2 around body to deliver O2 to respiring tissues). So don’t have a peak in heart rate during early stages like seen in MR^.

Question 20

How does reproduction affect metabolic rate (energetic demands)?

Answer 20

• Reproductive strategy.
• r selected species produce many offspring. And K selected species produce fewer offspring.
• As we move closer to K selected species we have increased size of offspring, increased survival, increased parental care and increased energetic demand per offspring produced.
• When lots of offspring produced, offspring size is smaller.

Question 21

How does Circadian rhythms (24-hour internal clock in brain that regulates alertness & sleepiness by responding to light changes in environment) affect metabolic rate (energetic demands)?

Answer 21

• E.g. Southern rock lobster.
• MR varies based on circadian rhythms.
• During night MR is higher so lobster is nocturnal.
• Introduce stress, a kairomone produced by an octopus which is a predator of the lobster. Exposed at midnight (active hours). Those exposed to this stressor had a decrease in MR, so predator response was hiding.
• Think about the energetic trade offsof this, if they hiding they aren’t out gathering food so gonna negatively impact them.
• This was performed at a higher temperature, at a higher temperature the lobster has a higher total energetic demand, the lobsters changed their behaviour, they didnt hide when exposed to stressor because they couldn’t afford to, they needed to forage for food due to the higher temperature causing a higher metabolic demand.

Question 22

How does temperature affect metabolic rate (energetic demands)?

Answer 22

1. Ectotherms:
   - an increase in environmental temperature leads to an increase in MR.
   - Until a certain point, (the upper limit that organism can tolerate). Above this, proteins start to denature and there is a rapid decline in MR as the organism is not gonna survive much longer.
2. Endotherms (regulate body temperature to be stable):
   - Euthermic zone (the temperature zone endotherms can regulate their body temperature) is between upper and lower temperature limits. Within this limit endotherms can effectively regulate temperature.
   - Below these rates the animal becomes hypothermic (body temperature is declining) and above it becomes hyperthermic (body temperature is increasing).
   - At the euthermic zone is the most beneficial for the animal as it has to take minimal energy to thermoregulate, so energy use is low and stable. This is known as the thermal neutral zone (when no energy is required to regulate temperature).
   - as we move to the limits of the euthermic zone, MR changes.
   - getting colder the animal expends more energy in order to maintain body temperature above that
   - if external temperature is increasing, animal has to use more energy to bring body temperature back down
   - outside of these ranges the organism isn’t functioning, so MR drops considerably

Question 23

Effect of temperature on enzyme activity

Answer 23

• Rate enhancing: kinetic energy of molecules (enzymes & substrates)
• Destructive effects: molecules denatured
• Increase in temperature leads to an exponential increase in rate of enzyme activity (not linear).
• Reaches a maximum and optimum temperature where rate is fastest and enzyme is most efficient.
• Above that point we have destructive effects. Breakdown in tertiary structure of the enzymes and they are not functioning as they should.

Question 24

Assess the effects of temperature on biochemistry (temperature quotient)

Answer 24

During the rate-enhancing phase:
- Temperature quotient: (Q10) = change in biological function for a 10˚C change in temperature.
- we can measure what happens on the rate of that enzyme if we increase temperature.
- we measure the rate at a temperature and we will measure the rate of that temperature minus or plus 10˚C. Then looking at the ratio between the 2 gives us the Q10, which tells us the effect of temperature on that enzyme.
- but we often measure over different temperatures that are not 10˚C apart.

Question 25

Heat exchange

Answer 25

Heat is lost and gained to environment via:
- Environmental gain: radiation from the sun, conduction by touch
- Environmental loss: convection to the air, evaporative loss to H2O
- Internal generation (from enzymatic reactions)
- Internal retention: SA to volume ratio (larger animals have a smaller SA to volume ratio, allowing them to more effectively retain heat.

Question 26

Thermo-regulation strategies

Answer 26

Classification based on source of body heat:
Ectothermy: body temperature dependent on heat transfer from/to environment, (dependant on environment)
Endothermy: body temperature dependent on internal (metabolic) sources of heat, (generate own heat)

Classification based on stability of body temperature:
Poikilothermy: body temperature varies with environment (not stable), (most often ectotherms)
Homeothermy: body temperature maintained within a narrow range (most often endotherms)

E.g. an otter (mammal) is both an endotherm and a homeotherm.
E.g. a fish is both a poikilotherm and an ectotherm.
E.g. But many large flying insects and large reptiles are poikilotherms but do still generate some sources of internal heat (endotherm), so it’s not always the case.