Chapter 5

Energy and Work in the Cell

Energy is "the capacity to do work" and is the focus of chapter 5 as it relates to the ability of cells to utilize energy to accomplish all of the tasks required to maintain life.

There are two forms of energy, kinetic and potential. Kinetic energy is energy of movement and also includes heat energy and light energy.

Potential energy is the capacity to do work based on arrangement (e.g. arrangement of atoms in molecules) or location. Chemical energy is a type of potential energy and is the energy that living cells use.

Thermodynamics is the study devoted to understanding energy conversions from kinetic to potential (or vice versa). Energy conversions are governed by two Laws of Thermodynamics and these laws must be understood before one can understand how a cell can utilize potential energy.

The 1st Law of Thermodynamics is the law of energy conservation. This means that the total amount of energy in the universe is constant. In an isolated system, energy can be changed from one form to another and transferred from one molecule to another, but energy cannot be created or destroyed.

The 2nd Law of Thermodynamics states that energy conversions reduce the order in the universe. That reduced order is called entropy. The effect of the 2nd Law is that when energy is converted from one form to another, it is never done with 100% efficiency; some energy escapes in the form of heat. Heat is one form of disorder or entropy.

 How do living organisms obtain energy? They regulate chemical reactions, through metabolism. Metabolism is the sum total of all chemical reactions in a cell. Catabolic reactions – are "break-down"reactions and the products of the reaction have lower potential energy than the reactants. These reactions have a net release of energy so they are called exergonic reactions. Examples of catabolic reactions would be hydrolysis and the reactions involved in cellular respiration.

Anabolic reactions are "build-up" reactions so the products have higher potential energy than the reactants. There is a net input of energy so these reactions are called endergonic. Products of endergonic reactions have higher potential energy. Examples would be biosynthetic reactions and dehydration synthesis.

All chemical reactions require an initial input of energy called the energy of activation.

 Exergonic Endergonic

Often, reactions are coupled, so the energy released from an exergonic reaction is used to drive an endergonic one. For example, the sun releases energy needed to drive photosynthesis.

*More energy is released from the exergonic reaction than can be captured in an endergonic reaction, and the ‘extra’ energy is lost as heat………remember the 2nd Law of Thermodynamics.

Energy given off from food catabolism is used to drive endergonic reactions such as synthesis of complex molecules.

Sometimes, coupled reactions occur in different places, so energy from the exergonic reaction must be transferred to the location of the endergonic reaction. To do this, cells must have energy carrying molecules……the primary one being adenosine triphosphate, ATP.

Most chemical reactions are reversible…they will proceed in either direction under a certain set of conditions. (Remember dehydration synthesis and hydrolysis?) At some point, reactions will reach equilibrium.

 How do cells regulate reaction rates? By using catalysts which speed up reactions by reducing the activation energy. In general, these are the attributes of catalysts.
    1. They speed up the rate of a reaction
    2. They cannot cause an unfavorable reaction to occur.
    3. They won’t change the equilibrium
    4. Catalysts are not consumed.


Energy

Time

Enzymes

**Enzymes are biological catalysts.

**Enzymes are proteins

**Enzymes are very specific. They have a 3-D structure with an active site that binds to only one substrate (reactant molecule).

**Enzymatic activity can be regulated in various ways.

Negative Feedback

Competitive Inhibition

Non-competitive Inhibition

**Enzymes are very efficient; a small amount of enzyme can catalyze a reaction for a lot of substrate. The turnover number is the number of molecules of substrate that 1 enzyme molecule can work on in 1 second. Frequently, enzymes are attached to membranes, ensuring that they will be in the proper location to perform their function. This brings us back to the ‘big picture’—the cell structure and where important molecules are located. Now we can revisit the structure of the cell membrane and get a better understanding of where these reactions are taking place.

Cell membrane components include: Phospholipid (bilayer), glycolipids, glycoproteins, proteins (including enzymes), cholesterol. And on the cytoplasmic side, the membrane is attached to cytoskeletal components (microtubules, etc.). On the extracellular side, the membrane is attached to matrix proteins which help hold adjacent cells together.

Cell membranes are selectively permeable. This means that only certain molecules can cross the membrane. There are several factors which effect membrane permeability (what gets in or out and how it occurs), but one major factor effecting these processes is the concentration gradient.

Passive Transport – no energy is expended and molecules move down the concentration gradient.

Diffusion

Osmosis

Hypotonic

Hypertonic

Isotonic

Facilitated Diffusion

Active Transport – energy (in the form of ATP) must be used to move certain molecules across the membrane.

Exocytosis

Endocytosis

Phagocytosis

Pinocytosis

Receptor mediated endocytosis