CBL 2

INTRODUCTION/BACKGROUND

DEFINITIONS (Tier 3 words, subject-specific terminology):

• Solvent – the medium in which solutes are dissolved (eg water)

• Solute – the substances that are dissolved in the solvent (eg sugar)When you throw a teaspoon of sugar in a cup of water, the sugar is the solute and the water is the solvent. Together, the sugar water is called a solution.

• Concentration gradient – The process whereby solutes move from an area of low solute concentration to high solute concentration (more solute for a fixed amount of solvent)

• Diffusion – Molecules will move from area of high solute concentration to low solute concentration (moving along the concentration gradient)

• Osmosis – Water will move from an area of low solute concentration to high solute concentration (due to the attraction to solute charges as water is polar. This means it is attracted to both positive and negative ions of the solute)

• Active transport – Moving molecules against the concentration gradient. Moving substances from low solute concentration to high solute concentration (against the concentration gradient)

• Selectively permeable membranes - also called partially permeable membranes or semi-permeable membranes. They let some substances pass through them, but not others.

BACKGROUND CONCEPTS:

What is diffusion?

Particles in liquids and gases are constantly moving. As a result of this movement, particles will spread out in all directions. Diffusion happens when solute particles are free to move:  in gases, and for particles dissolved in solutions. (Diffusion does not occur in solids because the particles are not mobile in the solid state.)

As a net result of random movement, particles move from a region where they are in high concentration to a region where they are in low concentration. 

TRANSPORT ACROSS THE CELL MEMBRANE - OVERVIEW

The cell is not isolated from ite environment - some substances need to enter the cell -  the raw materials for cell respiration (glucose, oxygen) as well as other substances such as amino acids for protein synthesis (making proteins). Some products of metabolism (carbon dioxide from respiration) need to leave the cell.  Water movement in and out is also essential for the cell. 

It is inportant for the cell to be able to control this movement, to some degree, and that is one of the roles of the cell  membrane.

The movement of substances in and out of the cell membranes can occur in two energy formats:

1. PASSIVE TRANSPORT  - NO ENERGY (ATP) required to be used. 

A. Diffusion

i) Simple Diffusion

ii) Facilitated Diffusion

B. Osmosis

2. ACTIVE TRANSPORT - ENERGY (ATP) required to be used

A. The Sodium-Potassium Pump

B. Bulk Transport

i) Exocytosis

ii) Endocytosis

1. PASSIVE TRANSPORT

The cell is not required to use any energy in any form of passive transport, i.e. simple diffusion, facilitated diffusion or osmosis. 

A. Diffusion

What is diffusion?

Dissolved or gaseous substances have to pass through the cell membrane to get into or out of a cell. Diffusion is one of the processes that allows this to happen.

Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. This is how the smell of cooking travels around the house from the kitchen, for example. This is shown in the left side of Diagrams 1 and 2 below,

Oxygen and carbon dioxide, dissolved in water, are exchanged by diffusion in the lungs:

• oxygen moves down a concentration gradient from the air in the alveoli to the blood

• carbon dioxide moves down a concentration gradient from the blood to the air in the alveoli

The dissolved substances will only continue to diffuse while there is a concentration gradient (high concentration hypertonic  to low concentration  hypotonic until the concentrations are the same  isotonic.

i) Simple diffusion

A few types of molecules can pass through the cell membrane without requiring assistance. They move from areas of high concentration (hypertonic) to low (hypertonic). This movement, called diffusion or simple diffusion is shown in the left of Diagrams 1 and 2. 

ii) Facilitated diffusion: The role of Transport Proteins in diffusion

Most molecules that need to enter or leave cells, or cross organelle membranes, need the help of proteins. Passive transport of materials across the plasma membrane from an area of their higher concentration to area of their lower concentration with the help of transport proteins is called facilitated diffusion. 

Transport proteins involved in passive transport - facilitated diffusion are: 

channel proteins -  

• “channel” proteins form open holes in the membrane of a cell and are open to both the outside and inside of the cell. Each channel protein has a size and shape that will accept only specific molecules. Water channel proteins (aquaporins) allow water to diffuse in single file across the membrane at a very fast rate - up to 100 million per second. Ion channel proteins allow ions to very rapidly diffuse across the membrane. Channel protein actions are shown in the right side of Diagram 1, the middle of Diagram 2,  in Diagram 3 and in the far left of Diagram 4 below.  The movement is with the concentration gradient, and requires NO ENERGY input from the cell.  

• Extension: Channel proteins are special arrangements of amino acids which embed in the cell membrane, providing hydrophilic  (water-loving) passages for water and small, polar ions down their concentration or electric potential gradients, an energetically favourable reaction. These proteins are characterized by being open to both the intracellular and extracellular spaces at the same time. Channels are typically designed so that only  specific substances can pass through.

carrier proteins -

• carrier proteins are transport proteins specific for an ion, molecule, or group of substances. Carrier proteins "carry" the ion or molecule across the membrane by changing shape after  binding with the ion or molecule. They are only open to one side of the membrane at once.  When carrier proteins are involved in passive transport - facilitated diffusion, the movement is with the concentration gradient and requires NO ENERGY input from the cell. Carrier proteins are seen below in the far right of Diagram 2 and in Diagram 4.  

B. Osmosis

Osmosis is is a form of passive transport that’s similar to diffusion, but it involves movement of solvent (usually water) molecules, from a region of higher concentration of water on one side of the selectively permeable membrane to a region of lower concentration of water on the other side of the membrane. (A  high concentration of water molecules is found in a dilute solution, while a a low concentration of water molecules is found in a concentrated solution.)  Water is a polar molecule that will not pass through the lipid bilayer; however, it’s small enough to move through the protein channels of most cell membranes. The membrane allows the solvent (water) to move through, but keeps out the solute (the particles dissolved in the water). 

Typically, a cell contains about a 1% saline solution —1% salt (solute) and 99% water (solvent).  Transport by osmosis is affected by the concentration of solute (the number of particles) in the water. 

• A hypotonic solution has less solute and higher water concentration than inside the cell. An example is 100% distilled water, which has less solute than what is inside the cell. Therefore, if a human cell is placed in a hypotonic solution, molecules diffuse down the concentration gradient into the cell until the cell’s membrane bursts. See left box of the first diagram  below.

• An isotonic solution has the same concentration of solute and solvent as found inside a cell, so a cell placed in an isotonic solution — typically 1% saline solution for humans — experiences equal flow of water into and out of the cell, maintaining equilibrium, and no overall change is seen. See middle box of the first diagram  below.

• A hypertonic solution has more solute and lower water concentration than inside the cell. So if the membrane of a human cell was placed in a 10% saline solution (10% salt and 90%  water) water would flow out of the cell (from the higher concentration inside to the lower concentration outside), therefore shrinking it. See right box of the first diagram  below.

2. ACTIVE TRANSPORT

While passive transport is a great strategy for moving molecules into or out of a cell - it's cheap, easy, and all the cell has to do is sit there and let the molecules diffuse in - it doesn't work in every situation. Suppose glucose is more concentrated inside a cell than outside. If the cell needs more glucose to meet its metabolic needs, how can it get that  in?

The cell can't import glucose for free using diffusion, because the higher concentration inside the cell means the glucose will diffuse out rather than flowing in. Instead, the cell must bring in more glucose molecules via active transport. In active transport the cell expends energy (in the form of ATP) to move a substance against its concentration gradient.

Active transport occurs across a semipermeable membrane against the normal concentration gradient, moving from the area of lower concentration to the area of higher concentration and requiring an expenditure of the energy released from an ATP molecule.

Embedded with the hydrophilic heads in the outer layer of the membrane are transmembrane protein molecules able to detect and move compounds through the membrane. These carrier or transport proteins interact with the passenger molecules and use the ATP-supplied energy to move them against the gradient. The carrier molecules combine with the transport molecules — most importantly amino acids and ions — to pump them against their concentration gradients.

Active transport lets cells obtain nutrients that can’t pass through the membrane by other means. 

The main types of ActiveTransport are 

A. Pumps eg the Sodium-Potassium Pump

B. Bulk Transport involving Exocytosis and Endocytosis. 

A. PUMPS

Active Transport often involves carrier proteins that act as pumps that USE ENERY (supplied by ATP) to move ions and molecules across the membrane. They are often called cell membrane pumps. Active Transport  is especially important in keeping a constant ion concentration within the cell and between cells.

• The carrier protein first binds with a particle of the substance to be transported. (The carrier protein must be a similar shape that fits the molecule or ion to which it is binding .)

• When the molecule binds with the carrier protein, chemical energy allows the cell to change the shape of the carrier protein so that the particle to be moved is released on the other side of the membrane (like an open door).  Once the particle is released, the carrier protein returns to its original shape.

Sodium-Potassium Pump

• The main function of the N+/K+ ATPase pump is to keep cells in a state of a low concentration of sodium ions and higher levels of potassium ions. 

• The sodium-potassium pump transports sodium out of and potassium into the cell in a repeating cycle of shape changes. In each cycle, three sodium ions leave the cell, while two potassium ions enter. This process takes place in the following steps:

1. To begin, the pump is open to the inside of the cell. In this form, the pump binds sodium ions, and will take up three of them.

2. When the sodium ions bind, they trigger the pump to break down ATP. 

3. The ATP breakdown makes the pump change shape and it now opens towards the space outside the cell. In this conformation, the pump releases the three sodium ions outside the cell.

4. In its outward-facing form, the pump switches binds with potassium ions. It will bind two of them, and this triggers athe pump to change back to its original form, opening towards the interior of the cell.

5. In its inward-facing shape, the two potassium ions will be released into the cytoplasm. The pump is now back to where it was in step 1, and the cycle can begin again.

The pump involves the carrier protein going back and forth between two forms: an inward-facing form that binds sodium and an outward-facing form that binds potassium. 

 Example of active transport in plants

In plants, active transport  enables roots to absorb nutrients from the soil. Plant nutrients are more concentrated inside the roots than in the surrounding soil. Active transport in the root cell membrane enables the plant to absorb the nutrients against the concentration gradient. Without active transport, nutrients would diffuse out of the roots.

B. BULK TRANSPORT

Bulk transport is a form of active transport where processes take in or release molecules from the cell that are too large to pass directly through the plasma membrane. Two process are involved in taking in or releasing large molecules from the cell - ENDOCYTOSIS and EXOCYTOSIS.

Endocytosis

Large molecules are engulfed and enclosed by a portion of the cell plasma membrane (packaged in membrane-bound sacs called a vesicle) and moved across the membrane. 

• During ENDOCYTOSIS the cell membrane  folds into a pouch that encloses the particles. 

• This pocket grows until it is pinched off,  forming a vesicle (membrane-wrapped bubble) trapping the pouch and its contents inside the cell.

• The vesicle can then fuse with other organelles (eg lysosomes) or release its contents into the cytoplasm.

Exocytosis

EXOCYTOSIS is the opposite of endocytosis.

• In exocytosis, the cell creates a vesicle to enclose something that is inside itself, for the purpose of moving it outside. This most commonly occurs to “export” an important product, such as in cells which make enzymes, hormones, and antibodies that are needed throughout the body.

• In eukaryotic cells, protein products are made in the endoplasmic reticulum. They are often packaged by the endoplasmic reticulum into vesicles, and sent to the Golgi apparatus.

• The Golgi apparatus can be thought of like a “post office.” It receives packages from the endoplasmic reticulum, processes them, and “addresses” them by adding molecules that will be recognised by receptors on the membrane of the cell that will receive the product.

• The Golgi apparatus then packages the finished “addressed” products into vesicles of its own, which move towards the cell membrane where they dock and join with it.

• In the joining process, the vesicle membrane becomes part of the cell membrane, and the vesicle’s contents are spilled into extracellular space.

• Mucus and waste products are other materials excreted by exocytosis.

Note: concentraion gradients and nature of materials have been covered in treatment of active and passive transport. 

In order to survive, cells must constantly interact with their surrounding environment.

Gases and food molecules dissolved in water must be absorbed, and waste products must be eliminated.  Each internal part of the cell has to be served by a part of the cell surface - the cell membrane. As a cell grows bigger, its internal volume gets bigger and the cell membrane expands. The volume increases more rapidly than the surface area, as we will see in an experiment, so the amount of surface area available to pass materials to a unit volume of the cell  decreases.

At some point, there is just enough surface available to service all the interior; to survive, the cell must stop growing.

The important point is that the surface area to the volume ratio gets smaller as the cell gets larger.

Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cell volume. When this happens, the cell must divide into smaller cells with favourable surface area/volume ratios, or cease to function.

That is why cells have  limiting size.

ENERGY

Some organisms such as plants make the energy they need by converting energy from sunlight and storing it in the chemical bonds of glucose. This process is called photosynthesis. The flow of energy through living organisms begins with photosynthesis. 

Cells in living organisms like plants and animals release the stored energy  by breaking the chemical bonds in glucose to produce ATP. The process in which glucose is broken down and ATP is made is called cellular respiration.

Photosynthesis and cellular respiration are the reverse of each other: the products of one process are the reactants of the other. Together, the two processes store and release energy in living organisms. The two processes also work together to cycle energy, as well as oxygen and carbon dioxide in Earth’s atmosphere.

The energy molecule - ATP

ATP (adenosine triphosphate) molecules store smaller quantities of energy, but each releases just the right amount to actually do work within a cell. Muscle cell proteins, for example, work by using the energy released when bonds in ATP break . The process of photosynthesis also makes and uses ATP - for energy to build glucose. ATP, then, is the usable form of energy for cells. 

A molecule of glucose, C6H12O6, carries a packet of chemical energy just the right size for transport and uptake by cells. Glucose is the "deliverable" form of energy, carried in the blood through capillaries to each of a person's 100 trillion cells. Glucose is also the carbohydrate produced by photosynthesis, and as such is the near-universal food for life – much easier to carry around, but too large for some things. Just like we find several sizes of money useful, organisms need several sizes of energy – a smaller quantity for work within cells,  a larger quantity for  storage, transport and delivery to cells. 

An ATP molecule is like a rechargeable battery: its energy can be used by the cell when it breaks apart into ADP (adenosine diphosphate) and P (phosphate), and then the "worn-out battery" ADP can be recharged using new energy to attach a new phosphate and rebuild ATP. The materials are recyclable, but energy is not.

A single cell uses about 10 million ATP molecules per second, and recycles all of its ATP molecules about every 20-30 seconds.

Biochemical Processes

Autotrophs (self-feeding) and Heterotrophs (other-feeding)

Living organisms obtain chemical energy in one of two ways.

Autotrophs, shown in the diagram below, store chemical energy in carbohydrate food molecules they build themselves. Food is chemical energy stored in organic molecules. Food provides both the energy to do work and the carbon to build bodies. Because most autotrophs transform sunlight to make food, the process they use is called photosynthesis. Only three groups of organisms - plants, algae, and some bacteria - are capable of this life-giving energy transformation. Autotrophs make food for their own use, but they make enough to support other life as well. Almost all other organisms depend absolutely on these three groups for the food they produce. The producers, as autotrophs are also known, begin food chains which feed all life.

Heterotrophs cannot make their own food, so they must eat or absorb it. For this reason, heterotrophs are also known as consumers. Consumers include all animals and fungi, and many protists and bacteria. They may consume autotrophs or other heterotrophs or organic molecules from other organisms. Heterotrophs are limited by total dependence on those autotrophs that originally made the food. If plants, algae, and autotrophic bacteria vanished from earth, animals, fungi, and other heterotrophs would soon disappear as well. All life requires a constant input of energy. Only autotrophs can transform that ultimate, solar source into the chemical energy in food that powers life.

Photosynthesis provides over 99% of the energy for life on earth by transforming light energy to chemical energy stored in food.

While different types of cells might need different specific nutrients, all living things require the same basic elements. The four most abundant elements in living things are:

• carbon

• hydrogen

• oxygen

• nitrogen

Other elements, including phosphorus, calcium, and iron, are also important, but are needed in much smaller quantities.

Every living cell is made of some combination of the same 20 elements. Cells get these elements by taking them in from their environment. They can come from a variety of different food sources, some of which are small molecules, like salts and water. Other elements come from larger macromolecules, like starches, proteins, and fats.

Gases

Cells require oxygen to carry out cell respiration. Some plant cells require carbon dioxide to carry out photosynthesis. These will be covered in Section 2.3. 

Matter

The macromolecules (big molecules) are the basic, energy-providing molecules. They mostly provide the elements used by the cell. The four major macromolecules are:

• carbohydrates

• lipids

• proteins

• nucleic acids

Carbohydrates and lipids are both made of carbon, hydrogen, and water, which are all found in the major macromolecules of a cell.

In addition to carbon, hydrogen, and oxygen, proteins can provide a few other elements. All amino acids contain nitrogen, which is an important element in the structure of proteins and nucleic acids. Some proteins also contain sulfur. In addition to being a part of protein structure, sulfur can act as a cofactor for some enzymes.

Nucleic acids provide carbon, hydrogen, and oxygen, but also contain phosphorus. Phosphorus is found in DNA, as well as the molecules used by the cell to store energy. 

Photosynthesis

Photosynthesis is often considered to be the single most important life process on Earth. It changes light energy into chemical energy and also releases oxygen. Without photosynthesis, there would be no oxygen in the atmosphere. Photosynthesis involves many chemical reactions, but they can be summed up in a single chemical equation:

6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2 

Photosynthetic autotrophs capture light energy from the sun and absorb carbon dioxide and water from their environment. Using the light energy, they combine the reactants to produce glucose and oxygen, which is a waste product. They store the glucose, usually as starch, and they release the oxygen into the atmosphere.

The process of photosynthesis, which usually begins the flow of energy through life, uses many different kinds of energy-carrying molecules to transform sunlight energy into chemical energy and build food. Some carrier molecules hold energy briefly, quickly shifting it like a hot potato to other molecules. This strategy allows energy to be released in small, controlled amounts. An example starts in chlorophyll, the green pigment present in most plants, which helps convert solar energy to chemical energy. When a chlorophyll molecule absorbs light energy, electrons are excited and "jump" to a higher energy level. The excited electrons then bounce to a series of carrier molecules, losing a little energy at each step. Most of the "lost" energy powers some small cellular task, such as moving ions across a membrane or building up another molecule. Another short-term energy carrier important to photosynthesis, NADPH, holds chemical energy a bit longer but soon "spends" it to help to build sugar.

Two of the most important energy-carrying molecules are glucose and adenosine triphosphate, commonly referred to as ATP. These are nearly universal fuels throughout the living world and are both key players in photosynthesis, as shown below.

Glucose

A molecule of glucose, which has the chemical formula C6H12O6, carries a packet of chemical energy just the right size for transport and uptake by cells. In your body, glucose is the "deliverable" form of energy, carried in your blood through capillaries to each of your 100 trillion cells. Glucose is also the carbohydrate produced by photosynthesis, and as such is the near-universal food for life.

The first diagram shows how we summarise the chemical reactions that make up photosynthesis. 

The second set of two give an idea of the complexity of the biochemical pathways for photosynthesis (not needed to learn in detail). 

CELL RESPIRATION

Cellular (or cell) respiration is the biochemical pathway by which cells release energy from the chemical bonds of food molecules, and provide that energy for the essential processes of life. Cell respiration is as a production process for the energy molecule ATP. 

Food consists of organic (carbon-containing) molecules which store energy in the chemical bonds between their atoms. Organisms use the atoms of food molecules to build larger organic molecules including proteins, DNA, and fats (lipids) and use the energy in food to power life processes. By breaking the bonds in food molecules, cells release energy to build new compounds. Although some energy dissipates as heat at each energy transfer, much of it is stored in the newly-made molecules. Chemical bonds in organic molecules are a reservoir of the energy used to make them. Fuelled by the energy from food molecules, cells can combine and recombine the elements of life to form thousands of different molecules. Both the energy (despite some loss) and the materials (despite being reorganised) pass from producer to consumer.

All living cells must carry out cellular respiration:  aerobic respiration in the presence of oxygen and/or anaerobic respiration. Prokaryotic cells carry out anaerobic cellular respiration within the cytoplasm or on the inner surfaces of the cells; it is anaerobic respiration because they lack mitochondria. Eukaryotic cells contain mitochondria, the site of most of the reactions involved in aerobic respiration.

While respiration “burns” glucose for energy, it doesn’t produce light or intense heat as the reactions we think of as burning do. This is because cell respiration releases the energy in glucose slowly, in many small steps. It uses the energy that is released to form molecules of ATP. 

Cellular respiration in eukaryotes involves many complex chemical reactions, which are summed up with this chemical equation:

C6H12O6 + 6O2 → 6CO2 + 6H2O + Chemical Energy (in ATP)

REMOVAL OF WASTES

Cellular respiration produces H2O and CO2 as metabolic wastes. 

CO2 normally binds with water H2O to form carbonic acid H2CO3, helping to maintain the blood's acid/base balance - pH. Since too much CO2 would lower the blood's pH too much, making it too acid, excess CO2 must be constantly removed.

CO2 is excreted from the cell via diffusion into the blood stream, where it is transported in three ways:

• Up to 7% is dissolved in its molecular form (CO2) in blood plasma

• About 70-80% is converted into hydrocarbonate ions

• The remainder binds with haemoglobin in red blood cells, is carried to the lungs, and exhaled.

H2O diffuses out of the cell. In humans, it diffuses into the blood stream, from where it is excreted in the form of perspiration, water vapour in breath, or urine from the kidneys. 

ENZYMES

Enzymes are proteins and occur naturally in living biological systems, acting in many metabolic pathways. They are biological catalysts.  They work in both anabolic (building molecules) and catabolic (breaking down molecules) reactions in cells.

[A catalyst is a substance which speeds up a reaction, taking less time for it to occur,  but isn't itself changed in the reaction.  Catalysts lower the amount of energy the cell needs to provide for the reaction to take place - called the activation energy.  See Diagram 1.]

Enzymes are highly specific, acting on a single substrate (simply means the substance on which an enzyme acts) or small group of related substrates.  Enzymes have an active site - a small portion of the molecule which matches the shape of part  of the substrate. Here, the enzyme changes shape slightly, fitting tightly with the substrate and forming the enzyme/substrate complex. See Diagram 2.

The substrate binds to the active site of the enzyme  forming the enzyme-substrate complex. Strain is induced in the bonds causes them to break and the products leave the active site, making it available for further reactions. By binding to a substrate and holding it, the reaction happens more efficiently. See Diagram 3.

Models of Enzyme Activity

There are two main models of enzyme activity:

1. Lock and Key Model

The specific action of an enzyme with a single substrate can be explained using a Lock and Key analogy first postulated in 1894 by Emil Fischer. In this analogy, the lock is the enzyme and the key is the substrate. Only the correctly sized key (substrate) fits into the key hole (active site) of the lock (enzyme). Smaller keys, larger keys, or incorrectly positioned teeth on keys (incorrectly shaped or sized substrate molecules) do not fit into the lock (enzyme). Only the correctly shaped key opens a particular lock. 

2. Induced Fit Model

Not all experimental evidence can be adequately explained by using the enzyme model assumed by the lock and key theory. A modification, called the induced-fit theory, has been proposed. The induced-fit theory assumes that the substrate plays a role in determining the final shape of the enzyme and that the enzyme is partially flexible. This explains why certain compounds can bind to the enzyme but do not react because the enzyme has been distorted too much. Other molecules may be too small to induce the proper alignment and therefore cannot react. Only the proper substrate is capable of inducing the proper alignment of the active site.

Factors affecting enzyme activity

The activity of enzymes is affected by pH, temperature,  enzyme concentration and  substrate concentration. Enzymes work best within specific temperature and pH ranges, at which they experience maximal activity.  Sub-optimal conditions can cause an enzyme to lose its ability to bind to a substrate.

• Temperature: Raising temperature generally speeds up a reaction, and lowering temperature slows down a reaction. However, high temperatures can cause an enzyme to lose its shape (denature) and stop working.

• pH: Each enzyme has an optimum pH range. Changing the pH outside of this range will slow enzyme activity. Extreme pH values can cause enzymes to denature.

• Enzyme concentration: Increasing enzyme concentration will speed up the reaction, as long as there is substrate available to bind to. Once all of the substrate is bound, the reaction will no longer speed up, since there will be nothing for additional enzymes to bind to.

• Substrate concentration: Increasing substrate concentration also increases the rate of reaction to a certain point. Once all of the enzymes have bound, any substrate increase will have no effect on the rate of reaction, as the available enzymes will be saturated and working at their maximum rate.

• Enzymes are "specific." Each type of enzyme typically only reacts with one, or a couple, of substrates. Some enzymes are more specific than others and will only accept one particular substrate. Other enzymes can act on a range of molecules, as long as they contain the type of bond or chemical group that the enzyme targets.

• Enzymes are reusable. Enzymes are not reactants and are not used up during the reaction. Once an enzyme binds to a substrate and catalyzes the reaction, the enzyme is released, unchanged, and can be used for another reaction. This means that for each reaction, there does not need to be a 1:1 ratio between enzyme and substrate molecules.