2.2: Energy - Biology

2.2: Energy - Biology

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Virtually every task performed by living organisms requires energy. For example, energy is required for the synthesis and breakdown of molecules, as well as the transport of molecules into and out of cells. In addition, processes such as ingesting and breaking down food, exporting wastes and toxins, and movement of the cell all require energy.

Scientists use the term bioenergetics to describe the concept of energy flow through living systems, such as cells. Cellular processes such as the building and breaking down of complex molecules occur through step-wise chemical reactions. Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed. Together, all of the chemical reactions that take place inside cells, including those that consume or generate energy, are referred to as the cell’s metabolism.

From where, and in what form, does this energy come? How do living cells obtain energy, and how do they use it? This section will discuss different forms of energy and the physical laws that govern energy transfer.


Thermodynamics refers to the study of energy and energy transfer involving physical matter. The matter relevant to a particular case of energy transfer is called a system, and everything outside of that matter is called the surroundings. For instance, when heating a pot of water on the stove, the system includes the stove, the pot, and the water. Energy is transferred within the system (between the stove, pot, and water). There are two types of systems: open and closed. In an open system, energy can be exchanged with its surroundings. The stovetop system is open because heat can be lost to the air. A closed system cannot exchange energy with its surroundings.

Biological organisms are open systems. Energy is exchanged between them and their surroundings as they use energy from the sun to perform photosynthesis or consume energy-storing molecules and release energy to the environment by doing work and releasing heat. Like all things in the physical world, energy is subject to physical laws. The laws of thermodynamics govern the transfer of energy in and among all systems in the universe. In general, energy is defined as the ability to do work, or to create some kind of change. Energy exists in different forms: electrical energy, light energy, mechanical energy, and heat energy are all different types of energy. To appreciate the way energy flows into and out of biological systems, it is important to understand two of the physical laws that govern energy.

The first law of thermodynamics states that the total amount of energy in the universe is constant and conserved. In other words, there has always been, and always will be, exactly the same amount of energy in the universe. Energy exists in many different forms. According to the first law of thermodynamics, energy may be transferred from place to place or transformed into different forms, but it cannot be created or destroyed. The transfers and transformations of energy take place around us all the time. Light bulbs transform electrical energy into light and heat energy. Gas stoves transform chemical energy from natural gas into heat energy. Plants perform one of the most biologically useful energy transformations on earth: that of converting the energy of sunlight to chemical energy stored within organic molecules (Figure (PageIndex{2}) below).

The challenge for all living organisms is to obtain energy from their surroundings in forms that are usable to perform cellular work. Cells have evolved to meet this challenge. Chemical energy stored within organic molecules such as sugars and fats is transferred and transformed through a series of cellular chemical reactions into energy within molecules of ATP (adenosine triphosphate). Energy in ATP molecules is easily accessible to do work. Examples of the types of work that cells need to do include building complex molecules, transporting materials, powering the motion of cilia or flagella, and contracting muscles to create movement.

A living cell’s primary tasks of obtaining, transforming, and using energy to do work may seem simple. However, the second law of thermodynamics explains why these tasks are harder than they appear. All energy transfers and transformations are never completely efficient. In every energy transfer, some amount of energy is lost in a form that is unusable. In most cases, this form is heat energy.

Thermodynamically, heat energy is defined as the energy transferred from one system to another that is not work. For example, when a light bulb is turned on, some of the energy being converted from electrical energy into light energy is lost as heat energy. Likewise, some energy is lost as heat energy during cellular metabolic reactions.

An important concept in physical systems is that of order and disorder. The more energy that is lost by a system to its surroundings, the less ordered and more random the system is. Scientists refer to the measure of randomness or disorder within a system as entropy. High entropy means high disorder and low energy. Molecules and chemical reactions have varying entropy as well. For example, entropy increases as molecules at a high concentration in one place diffuse and spread out. The second law of thermodynamics says that energy will always be lost as heat in energy transfers or transformations. Living things are highly ordered, requiring constant energy input to be maintained in a state of low entropy.

Potential and Kinetic Energy

When an object is in motion, there is energy associated with that object. Think of a wrecking ball. Even a slow-moving wrecking ball can do a great deal of damage to other objects. Energy associated with objects in motion is called kinetic energy. A speeding bullet, a walking person, and the rapid movement of molecules in the air all have kinetic energy. Now what if that same motionless wrecking ball is lifted two stories above ground with a crane? If the suspended wrecking ball is not moving, is there energy associated with it? The answer is yes. The energy that was required to lift the wrecking ball did not disappear, but is now stored in the wrecking ball by virtue of its position and the force of gravity acting on it. This type of energy is called potential energy (Figure (PageIndex{3}) below). If the ball were to fall, the potential energy would be transformed into kinetic energy until all of the potential energy was exhausted when the ball rested on the ground. Wrecking balls also swing like a pendulum; through the swing, there is a constant change of potential energy (highest at the top of the swing) to kinetic energy (highest at the bottom of the swing). Other examples of potential energy include the energy of water held behind a dam or a person about to skydive out of an airplane.

Potential energy is not only associated with the location of matter, but also with the structure of matter. Even a spring on the ground has potential energy if it is compressed; so does a rubber band that is pulled taut. On a molecular level, the bonds that hold the atoms of molecules together exist in a particular structure that has potential energy. The fact that energy can be released by the breakdown of certain chemical bonds implies that those bonds have potential energy. In fact, there is potential energy stored within the bonds of all the food molecules we eat, which is harnessed for use. The type of potential energy that exists within chemical bonds, and is released when those bonds are broken, is called chemical energy. Chemical energy is responsible for providing living cells with energy from food. The release of energy occurs when the molecular bonds within food molecules are broken.

CH 2-1,2-2 Energy

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All living things are able to maintain a more-or-less constant internal environment. Regardless of the conditions around them, they can keep things relatively stable on the inside. The condition in which a system is maintained in a more-or-less steady state is called homeostasis . Human beings, for example, maintain a stable internal body temperature. If you go outside when the air temperature is below freezing, your body doesn’t freeze. Instead, by shivering and other means, it maintains a stable internal temperature.

Figure 2.2.2 Homeostasis of body temperature.

2.2) Levels of organisation

Most cells, when they have finished dividing and growing, become specialised.

  • They do one particular job
  • They develop a distinct shape
  • Special kinds of chemical change take place in their cytoplasm.

‘Division of labour’- the specialisation of cells to carry out particular functions in an organism.

Palisade mesophyll cells – photosynthesis

Nerve cells – conduction of impulses

Sperm and egg cells – reproduction

Tissue is a group of cells with similar structures, working together to perform a shared function.

Eg. Bone, nerve, muscle, epidermis, xylem

Organ is a structure made up of a group of tissues, working together to perform a specific function.

Eg. Stomach, heart, lungs, intestines, brain, eyes

Organ system is a group of organs with related functions, working together to perform a body function.

Physical Activity: Beneficial Effects


The total energy cost of maintaining constant conditions in the body plus the energy cost of physical activities.

Physical activity that is regular, planned, and structured with the aim of improving or maintaining one or more aspects of physical fitness.

A state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.

Any bodily movement produced by skeletal muscles that results in energy expenditure.

A measure of the ability of the body to cope with physical activity or exercise.

Structure of Carbohydrates

All carbohydrates are formed from the elements carbon (C), hydrogen (H) and oxygen (O). The formula of a carbohydrate is always (CH2O)n. The n represents the number of times the basic CH2O unit is repeated, e.g. where n = 6 the molecular formula is C6H12O6. This is the formula shared by glucose and other simple sugars like fructose. These simple sugars are known as monosaccharides.

The molecular formula, C6H12O6, does not indicate how the atoms bond together. Bonded to the carbon atoms are a number of – H and – OH groups. Different positions of these groups on the carbon chain are responsible for different properties of the molecules. The structural formulae of α and β glucose are shown below.

Glucose is so small that it can pass through the villi and capillaries into our bloodstream. The molecules subsequently release energy as a result of respiration. Simple glucose molecules are capable of so much more. They can combine with others to form bigger molecules.

Each glucose unit is known as a monomer and is capable of linking others. This diagram shows two molecules of β glucose forming a disaccharide.

In your examinations look for different monosaccharides being given, like fructose or α glucose. You may be asked to show how they bond together. The principle will be exactly the same.

A condensation reaction means that as two carbohydrate molecules bond together a water molecule is produced. The link formed between the two glucose molecules is known as a glycosidic bond.

A glycosidic bond can also be broken down to release separate monomer units. This is the opposite of the reaction shown above. Instead of water being given off, a water molecule is needed to break each glycosidic bond. This is called hydrolysis because water is needed to split up the bigger molecule.

Like disaccharides, they consist of monomer units linked by the glycosidic bond. However, instead of just two monomer units they can have many. Chains of these ‘sugar’ units are known as polymers. These larger molecules have important structural and storage roles.

Starch is a polymer of the sugar, glucose. The diagram below shows part of a starch molecule.

The table classifies carbohydrates

How useful are polysaccharides?

  • Starch is stored in organisms as a future energy source, e.g. potato has a high starch content to supply energy for the buds to grow at a later stage.
  • Glycogen is stored in the liver, which releases glucose for energy in times of low blood sugar.

Both starch and glycogen are insoluble which enables them to remain inside cells.

  • Cellulose has long chains and branches which help form a tough protective layer around plant cells, the cell wall.
  • Pectins are used alongside cellulose in the cell wall. They are polysaccharides which are bound together by calcium pectate. Pectins help cells to bind together.

Together the cellulose and pectins give exceptional mechanical strength. The cell wall is also permeable to a wide range of substances.

2.2.U2) Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and solvent properties of water.

Property Explanation in terms of hydrogen bonding and dipolarity Example of a benefit to living organisms
Cohesion Ability of like molecules to stick together water is strongly cohesive due to the many hydrogen bonds formed between them (tetrahedral arrangement). Surface tension : Cohesive hydrogen bonds resisting object trying to penetrate surface. Allows organisms, such as pond skaters, to move across the surface of the water.
Adhesion Ability of dissimilar molecules to stick together dipolarity of water molecules makes them stick to surfaces that are polar, and are therefore hydrophilic. Capillary action : Adhesive forces between water and cellulose (in xylem vessels) allows water to be transported up plant stems via the transpiration stream.
Thermal Due to the extensive hydrogen bonding between water molecules, hydrogen bonds need to be broken before they can change state, which requires the absorption of significant energy (heat). Water thus has high melting and boiling points, and a high specific heat capacity. The thermal properties of water causes it to be liquid in most habitats on Earth, making it suitable for living organisms. High specific heat capacity makes its temperature change relatively slowly, which makes it a stable habitat.
Solvent Many substances dissolve in water because of its polarity, including those composed of ions or polar molecules. Metabolic reactions almost always happen in water, as water in cells dissolves the reactants/substrates.

The Chemistry of Cellular Respiration

Shown below is the chemical equation for cellular respiration with symbols for the reactants and products of the reaction.
Label the diagram below and color it.

/>Glucose (purple) />Oxygen (red) />ATP (orange)
/>Carbon Dioxide (green) />Water (blue)

8. Products are what is created during a reaction. What are the three products of cellular respiration?

9. Reactants are what goes into the reaction, what are the two reactants needed for respiration to occur?

10. What would happen if oxygen were not available?

11. Refer to your notes or textbook to write down the equation for PHOTOSYNTHESIS.

12. How are photosynthesis and cellular respiration similar?

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When does the breaking of chemical bonds release energy?

The breaking of chemical bonds never releases energy to the external environment. Energy is only released when chemical bonds are formed. In general, a chemical reaction involves two steps: 1) the original chemical bonds between the atoms are broken, and 2) new bonds are formed. These two steps are sometimes lumped into one event for simplicity, but they are really two separate events. For instance, when you burn methane (natural gas) in your stove, the methane is reacting with oxygen to form carbon dioxide and water. Chemists often write this as:

This balanced chemical equation summarizes the chemical reaction involved in burning methane. The reactants are on the left, the products are on the right, and the arrow represents the moment the reaction happens. But there are a lot of interesting things happening that are hidden behind that arrow. A more detailed equation would look something like this:

The first line of the equation contains the original reactants: methane molecules and oxygen molecules. The first arrow represents the breaking of the bonds, which requires energy. On the middle line are the atoms, now broken out of molecules and free to react. The second arrow represents the forming of new bonds. On the last line are the final products. It takes a little energy, such as the spark from the igniter in your stove, to get the reaction started. That is because bonds must be broken before the atoms can be formed into new bonds, and it always takes energy to break bonds. Once the reaction has started, the output energy from one burned methane molecule becomes the input energy for the next molecule. Some of the energy released by each bond that is formed in making carbon dioxide and water is used to break more bonds in the methane and oxygen molecules. In this way, the reaction becomes self-sustaining (as long as methane and oxygen continue to be supplied). The igniter can be turned off. If breaking bonds did not require energy, then fuels would not need an ignition device to start burning. They would just start burning on their own. The presence of spark plugs in your car attests to the fact that breaking chemical bonds requires energy. (Note that the combustion of methane actually involves many smaller steps, so the equation above could be expanded out into even more detail.)

The textbook Advanced Biology by Michael Roberts, Michael Jonathan Reiss, and Grace Monger states:

Biologists often talk about energy being made available by the breakdown of sugar, implying that the breaking of chemical bonds in the sugar molecules releases energy. And yet in chemistry we learn that energy is released, not when chemical bonds are broken, but when they are formed. In fact, respiration supplies energy, not by the breaking of bonds in the substrate, but by the formation of strong bonds in the products. However, the overall result of the process is to yield energy, and it is in this sense that biologists talk about the breakdown of sugar giving energy.

The total energy input or output of a reaction equals the energy released in forming new bonds minus the energy used in breaking the original bonds. If it takes more energy to break the original bonds than is released when the new bonds are formed, then the net energy of the reaction is negative. This means that energy must be pumped into the system to keep the reaction going. Such reactions are known as endothermic. If if takes less energy to break the original bonds than is released when new bonds are formed, then the net energy of the reaction is positive. This fact means that the energy will flow out of the system as the reaction proceeds. This fact also means that the reaction can proceed on its own without any external energy once started. Such reactions are known as exothermic. (Endothermic reactions can also proceed on their own if there is enough external energy in the form of ambient heat to be absorbed.) Exothermic reactions tend to heat up the surrounding environment while endothermic reactions tend to cool it down. The burning of fuels is exothermic because there is a net release of energy. Cooking an egg is endothermic because there is a net intake of energy to make the egg cooked. The bottom line is that both endothermic and exothermic reactions involve the breaking of bonds, and both therefore require energy to get started.

It makes sense that breaking bonds always takes energy. A chemical bond holds two atoms together. To break the bond, you have to fight against the bond, like stretching a rubber band until it snaps. Doing this takes energy. As an analogy, think of atoms as basketballs. Think of the energy landscape of chemical bonds as a hilly terrain that the basketballs are rolling over. When two balls are placed near a round hole, gravity pulls them down to the bottom where they meet and stop. The two balls now stay close together because of the shape of the hole and the pull of gravity. This is like the chemical bond uniting atoms. To get the balls away from each other (to break the bonds), you have to roll them up opposite sides of the hole. It takes the energy of your hand pushing the balls to get them up the sides of the hole and away from each other. The energy you put into the system in order to pull apart the balls is now stored as potential energy in the balls. Atoms don't literally roll up and down hills, but they act like they are moving in an energy landscape that is very similar to real hills.

Watch the video: Energie u0026 Energieumwandlung einfach erklärt 1 - Thermodynamik u0026 Umwandlung - Stoffwechselbiologie (February 2023).