Provide anatomy and physiology advice to clients

The notion of energy

Most energy comes from the sun Most of the energy in our world has come

from the sun. This excludes magnetic, radioactive and wind energy that arises from rotation of the earth. All the energy that we use in our bodies came from the sun.

Photosynthesis We’ve just heard that all energy comes from the

sun. How? It starts with photosynthesis. The process of photosynthesis is a remarkable process. Carbon dioxide and water are converted to glucose and oxygen, using the energy from the sun. A plant’s chloroplasts trap the sun’s light and use this energy to drive the synthesis of glucose from carbon dioxide and water.

Food and energy The glucose that is synthesised in photosynthesis

can then be converted to other molecules such as starch. The energy that was in sunlight is now stored in the plant. So when we eat plant material such as potatoes and corn, we are eating substances that contain stored energy. Similarly, when a cow or sheep for instance eats grass then they convert the substances and energy in grass to animal products. When we eat animal products we can use or store the energy contained in them.

Energy and seeds You may have wondered why seeds and nuts

contain fat. When you plant a seed such as a pumpkin seed under the ground, in due course it germinates and produces a shoot and roots. Now whilst the seed is underground it cannot use the sun’s energy to drive its germination. And so it uses its reservoir of stored energy, which in this case is fat. Once the shoot comes above the ground then the plant can harness the sun’s energy to drive its growth.

Law of Conservation of Energy You need to be aware that energy can not be

created nor destroyed but it can be converted from one form to another. The energy stored in the carbon (C-C) bonds in wood can be converted to heat energy in a fire. We cannot use the energy stored in wood because we cannot digest cellulose, but we can use the energy stored in a range of other plant and animal materials.

Energy conversion in us Our bodies have an ability to extract

most of the energy stored in the C-C bonds in meat and vegetables and convert it to energy that we can use or store for later use.

Energy in use Most of the energy that we use comes when a molecule

called Adenosine tri phosphate (ATP) is converted to Adenosine di phosphate (ADP). When this happens a certain amount of heat energy is generated. Naturally, it takes energy to make ATP from ADP and the energy that we extract from our food drives this reaction. Don’t forget that this energy originally came from the sun.

Why do we need energy? We need a constant supply of energy to maintain us

at 37 degrees Celsius (heat). We also require energy to drive active transport mechanisms in our body (the pumps). We require energy to power mechanical movement such as walking, running and all physical activity. And we need energy to drive other reactions where external energy is required. These reactions are generally enzyme catalysed reactions in our cells.

How do we define energy for our use? Energy is the capacity to do work. One old fashioned

measurement of energy is the calorie. One calorie is the energy needed to raise 1gram of water one degree Centigrade. These days we use Joules as a measurement of energy and 1 calorie is equal to 4.185 joules. Very often you will see kilo joules (Kj) as a measurement. This is 1000 joules.

Energy to boil water So to boil 1litre of water (say 1000g)

from room temperature (say 20 degrees) it takes 1000*80 cal of energy (80Kcal) or 334.5Kj of energy.

Energy sources We can extract energy from our foodstuffs from

carbohydrates, proteins, lipids and alcohol. This involves our cells breaking the C-C bonds and trapping the energy released as a sequence of special reactions. By the way: if we don’t use the energy that we eat then we store it for use later on as fat. Unfortunately, our capacity to store fat seems unlimited.

Basal Metabolic Rate (BMR) The basal metabolic rate is the amount of energy that a

person needs each day to survive. It depends on their level of activity, body surface area, weight and age. The basal metabolic rate is controlled by our thyroid gland. It is generally given as Kilo joules per day. For instance the BMR of a young female fitness student might be around 5000 kilo joules per day. This mans that her body uses 5000 kilo joules each day just to survive. Now that student might actually use 8000 kilo joules each day because she is an active sporting person. This extra bit of energy that she uses is needed because of her physical activity.

Other Factors influencing BMR Tall and thin people lose more heat than

short and stout people and tend to have a higher BMR. Our BMR slows down with age and this is one reason why people tend to put on weight as they get older. They eat the same amount but their energy requirement is reduced. If we eat less then our BMR slows down in anticipation. We can increase our BMR through regular exercise.

Body Mass Index (BMI) The BMI is used to give people an idea

if they weigh too much. The formula is:

BMI = (Weight in Kilograms / (Height in Metres) x (Height in Metres))

It’s relevance is widely disputed but it is used as a measure to help people know if they have to reduce energy intake or increase energy output.

Some BMI classifications18.5 or less is underweight 

18.5 to 24.99 is normal weight

25 to 29.99 is overweight

30 to 34.99 is Obesity (Class 1)

35 to 39.99 is Obesity (Class 2)

40 or greater is Morbid Obesity

How much energy does food provide? Carbohydrates

about 16Kjoules/g Protein about

16Kjoules/g Fats about

37Kjoules/g Alcohol about

21Kjoules/g

How is energy in foods extracted by us? In this session we will look at glucose oxidation

and in the next session look at other food material. Glucose circulates in our blood after a carbohydrate meal. Glucose is also released from the liver if we have not eaten carbohydrates for more than 4 hours. Our blood glucose levels must not drop below about 80mg of glucose per 100ml of blood, because glucose is needed constantly by our brain cells.

The fate of glucose So the carbohydrates that we eat are mostly

converted to glucose and absorbed into our blood. And if we haven’t eaten for a while then our liver releases glucose into our blood to keep our brain working. Glucose cannot enter our cells and needs insulin to facilitate its entry. Once into the cell it undergoes a series of reactions called the oxidation of glucose.

Oxidation of glucose- anaerobic glycolysis Firstly glucose (which is a 6 carbon

compound) is converted to 2 molecules of pyruvate. These are 3 carbon compounds. This occurs in the cytoplasm of cells and occurs without oxygen. This means that it is anaerobic. There are actually 10 enzyme catalysed reactions in this process so it is very complex. But more importantly, 2 molecules of ATP are produced.

Oxidation of glucose-aerobic The 2 molecules of pyruvate are then converted

to 2 molecules of acetyl coenzyme A, which then enters the mitochondria in the cell. Remember that the mitochondria are the powerhouses of the cell. In a complex sequence of reactions called the Kreb’s or Citric acid cycle the energy that was in glucose is transferred to a series of molecules called NAD and they become NADH. A representation of the cycle is on this slide.

Oxidation of glucose- oxidative phosphorylation These NADH molecules now carry the electrons to a very

complex series of reactions on the mitochondrial membrane. The electrons are shuffled down a pathway and along the way energy is trapped. Finally, they are given to oxygen and that oxygen is converted to water. The whole process is called respiration and yes, this is the reason why you breathe, so that oxygen can be involved in energy trapping. As a result of all of this complex process glucose is converted to carbon dioxide and water and 38 molecules of ATP are formed. In this slide you can see these 3 components of glucose oxidation.

Energy used in muscle contraction We are going to look at muscle contraction in more detail in another

session but it is worth putting the oxidation of glucose into the context of muscle contraction. ATP can be stored in muscles in the form of creatine phosphate and we have enough stored for muscles to contract for about 15 seconds. Our muscles are also saturated with a compound called glycogen. Glycogen is made from lots of glucose molecules joined together and it is easily broken down to glucose. We have enough glycogen in our muscles for the anaerobic production of energy for about 60 seconds of activity. These are generally used as energy sources for power. They are quick to start. On the other hand, aerobic energy takes longer to start up but it can last significant lengths of time, because it draws on a variety of substrates. We will look at these in our next session.

Formation of lactate During glycolysis NAD is converted to NADH. If

oxygen is limited then NADH builds up and pyruvate cannot be converted to acetyl CoA. The whole system would come to a halt but fortunately our body has a solution. When oxygen supply is limited and energy is required the pyruvate is converted to lactate and at the same time NADH is converted back to NAD. Lactate then diffuses out of the muscles and travels by the blood back to the liver where it is converted to glucose and the process can start up again.

Activity Describe in simple terms how we extract

energy from glucose.