2.9: Alternative Text Descriptions

Figure 2.1.1: Elements and Compounds

This figure combines a simplified body diagram and a table to show the elemental composition of the human body. The body diagram highlights the four most abundant elements:

• Oxygen: 65 percent
• Carbon: 18 percent
• Hydrogen: 10 percent
• Nitrogen: 3 percent

A small remaining category labeled "Others" represents the rest of the body’s elements.

The table lists the elements, their chemical symbols, and their percentage in the body:

• Oxygen, O, 65.0
• Carbon, C, 18.5
• Hydrogen, H, 9.5
• Nitrogen, N, 3.2
• Calcium, Ca, 1.5
• Phosphorus, P, 1.0
• Potassium, K, 0.4
• Sulfur, S, 0.3
• Sodium, Na, 0.2
• Chlorine, Cl, 0.2
• Magnesium, Mg, 0.1

The table also notes that trace elements, each present in amounts less than 1.0 percent, include: boron, chromium, cobalt, copper, fluorine, iodine, iron, manganese, molybdenum, selenium, silicon, tin, vanadium, and zinc.

Figure 2.1.2: Two Models of Atomic Structure

This image contains two vertically arranged diagrams comparing two models of a helium atom: the planetary model at the top and the electron cloud model at the bottom. Labels identify key components in each model.

Figure 2.1.3: Isotopes of Hydrogen

The image compares the three isotopes of hydrogen: protium, deuterium, and tritium. The image is arranged in three side-by-side atom diagrams.

• On the left, protium, labeled 1H, shows a nucleus with one proton and no neutrons, with one electron shown outside the nucleus.
• In the center, deuterium, labeled 2H, shows a nucleus with one proton and one neutron, with one electron outside the nucleus.
• On the right, tritium, labeled 3H, shows a nucleus with one proton and two neutrons, with one electron outside the nucleus.

Each diagram uses a large circular orbit line around the nucleus to represent the electron shell. In all three isotopes, the atom has one electron, but the number of neutrons increases from left to right. The main concept illustrated is that all hydrogen isotopes have one proton and one electron, but they differ in the number of neutrons:

• protium: 0 neutrons
• deuterium: 1 neutron
• tritium: 2 neutrons

Figure 2.1.4: PET Scan

This image shows a medical imaging interface with multiple panels of PET/CT scans. The left column includes axial slices, while the center and right columns display coronal and sagittal views. Bright white and orange areas indicate regions of high metabolic activity, likely corresponding to tracer uptake. The scans reveal anatomical structures such as the spine, chest, and abdomen, with highlighted spots suggesting possible abnormal activity. The interface includes patient information fields, slice indicators, and imaging options.

Figure 2.1.5: Electron Shells

This image displays four Bohr models of atoms. Top left: hydrogen with one electron orbiting a nucleus labeled H. Top right: helium with two electrons around a nucleus labeled He. Bottom left: carbon with six electrons in two shells around a nucleus labeled C. Bottom right: neon with ten electrons in two shells around a nucleus labeled Ne. Each electron is shown as a blue sphere, and shells are represented by gray circles.

Figure 2.2.1: Sodium Chloride Formation

This image explains how ionic bonding forms between sodium and chlorine and how those ions arrange into a crystal structure. The image has three parts labeled (a), (b), and (c).

Part (a) - At the top, two atom diagrams are shown:

• on the left is a sodium atom, Na
• on the right is a chlorine atom, Cl

A single outer electron is shown moving from sodium to chlorine. This transfer is indicated by an arrow from the outer shell of sodium to the outer shell of chlorine.

Part (b) - In the middle, the result of the electron transfer is shown:

• sodium becomes Na+, meaning it has lost one electron and now has a net positive charge
• chlorine becomes Cl-, meaning it has gained one electron and now has a net negative charge

This part illustrates that ionic bonding forms when one atom loses an electron and another atom gains it, producing oppositely charged ions that attract each other.

Part (c) - At the bottom, a three-dimensional crystal lattice is shown.

• The lattice contains alternating sodium ions and chloride ions arranged in a repeating pattern. This represents the structure of an ionic solid, such as sodium chloride.
• The main idea of the image is that sodium transfers one electron to chlorine, forming Na+ and Cl-, and these oppositely charged ions then arrange in a repeating crystal lattice.

Figure 2.2.2: Covalent Bond

This image illustrates examples of covalent bonding, where atoms become stable by sharing electrons. The diagram has three labeled parts: (a), (b), and (c).

Part (a): Single covalent bond in hydrogen gas - Two hydrogen atoms are shown forming hydrogen gas, H2. Each hydrogen atom contributes one electron, and the two atoms share this pair of electrons. This creates one shared pair, which is a single covalent bond.

Part (b): Double covalent bond in oxygen gas - Two oxygen atoms are shown forming oxygen gas, O2. Each oxygen atom has six electrons in its valence shells and needs two more to become stable. The two oxygen atoms share two pairs of electrons, creating a double covalent bond.

Part (c): Two double covalent bonds in carbon dioxide - One carbon atom is shown between two oxygen atoms, forming carbon dioxide, CO2. Carbon has four valence electrons and needs four more to become stable. Each oxygen atom shares two pairs of electrons with carbon. This creates two double covalent bonds, one between carbon and each oxygen, often written as O=C=O.

Figure 2.2.3: Polar Covalent Bond

This image illustrates three representations of a water molecule, H2O, and highlights its uneven charge distribution.The image has three labeled parts:

Part (a): Planetary model of a water molecule

The top diagram shows one oxygen atom bonded to two hydrogen atoms. The oxygen atom is in the center, and the two hydrogen atoms are positioned at an angle rather than in a straight line.

The oxygen region is labeled weakly negative, marked with delta minus, δ−. Each hydrogen region is labeled weakly positive, marked with delta plus, δ+.

This part shows that electrons are drawn more strongly toward oxygen, making the oxygen end of the molecule partially negative and the hydrogen ends partially positive.

Part (b): Three-dimensional model

The lower left diagram shows a three-dimensional view of the same water molecule. The oxygen atom is larger and centered above the two hydrogen atoms, creating a bent shape. The oxygen side is labeled δ−, and the hydrogen sides are labeled δ+. This part emphasizes the molecule’s shape and polarity.

Part (c): Structural formula

The lower right diagram shows a simplified structural formula with oxygen in the center connected to two hydrogen atoms. This is the standard structural representation of water.

Figure 2.2.4: Hydrogen Bonds

This image shows hydrogen bonding between water molecules. Solid lines represent covalent bonds within each water molecule between oxygen and hydrogen. Dashed lines represent hydrogen bonds between the hydrogen of one water molecule and the oxygen of another water molecule. The image’s main idea is that water molecules are linked to one another by hydrogen bonds, while the atoms within each water molecule are held together by covalent bonds.

Figure 2.3.1: The Three Fundamental Chemical Reactions

This figure is a three-part diagram labeled a, b, and c. It uses word examples to illustrate three types of chemical reactions: synthesis, decomposition, and exchange. Each part includes a short sentence explanation and a word “equation” with an arrow showing what changes.

(a) Synthesis reaction

Text states: In a synthesis reaction, two components bond to make a larger molecule. Energy is required and is stored in the bond.
Below the text is the example:
Note + Book → Notebook
This shows two smaller parts combining into one larger product.

(b) Decomposition reaction

Text states: In a decomposition reaction, bonds between components of a larger molecule are broken, resulting in smaller products.
Below the text is the example:
Bookwork → Book + Worm
This shows one larger item breaking into two smaller parts.

(c) Exchange reaction

Text states: In an exchange reaction, bonds are both formed and broken so that the components of the reactants are rearranged.
Below the text is the example:
Notebook + Worm → Note + Bookworm
This shows parts being rearranged between two starting items to form two new products.

Figure 2.3.2: Enzymes

This figure compares the energy changes in the same chemical reaction without an enzyme and with an enzyme.

The image has two side-by-side graphs. In both graphs, the vertical axis is energy and the horizontal axis is progress of reaction. In each case, the reaction starts with reactants and ends with products, and the products are shown at a lower energy level than the reactants.

Left graph, labeled (a) - This graph shows the reaction without an enzyme. The energy curve rises to a high peak before dropping to the products. A label indicates that activation energy is required. This means the reaction needs a larger energy input to begin.

Right graph, labeled (b) - This graph shows the reaction with an enzyme. The energy curve still rises and then falls, but the peak is lower than in the left graph. A label indicates that less activation energy is required. This means the enzyme lowers the energy barrier needed to start the reaction.

Figure 2.4.1: Dehydration Synthesis and Hydrolysis Monomers

This image is a two-part diagram labeled (a) and (b) that compares dehydration synthesis and hydrolysis. Both parts use simplified shapes to represent two monomers and show how a covalent bond forms or breaks, along with the role of water

(a) Dehydration synthesis - Monomers are joined by removal of OH from one monomer and removal of H from the other at the site of bond formation. What the diagram shows:

• On the left are two separate ovals labeled Monomer 1 and Monomer 2, separated by a plus sign.
• Near the end of Monomer 1 is the label OH.
• Near the end of Monomer 2 is the label H (shown as part of an HO/H notation near the bonding site).
• An arrow points to the right, showing the reaction proceeding.
• Above the arrow is a water droplet labeled H2O, indicating that water is produced.
• On the right, the two monomers are now connected by a line with an O in the middle, representing a covalent bond linking the monomers.
• Text beneath the O reads: “Monomers linked by covalent bond.”

(b) Hydrolysis - Monomers are released by the addition of a water molecule, adding OH to one monomer and H to the other. What the diagram shows:

• On the left, two monomers are already connected by a line with an O in the middle, labeled “Monomers linked by covalent bond.”
• An arrow points to the right, showing the reaction proceeding.
• Above the arrow is a water droplet labeled H2O, indicating that water is added as a reactant.
• On the right, the bond is broken and the diagram shows two separate ovals labeled Monomer 1 and Monomer 2, separated by a plus sign.
• The end of Monomer 1 is labeled OH, and the end of Monomer 2 is labeled H, showing that water has been split and its parts added to the two monomers.

Figure 2.4.2: Dissociation of Sodium Chloride in Water

This image illustrates the process by which water dissolves sodium chloride (NaCl) into ions and how water molecules surround those ions. The diagram is divided into three main visual sections: a single water molecule, a crystal lattice of sodium chloride, and hydrated ions in solution.

1. Top Left – Water Molecule: A single water molecule (H₂O) is shown. This indicates that water is a polar molecule, with uneven charge distribution.
   • One oxygen atom (O) is at the center.
   • Two hydrogen atoms (H) are attached at an angle.
   • The hydrogens are labeled with partial positive charges (δ⁺).
   • The oxygen is labeled with a partial negative charge (δ⁻).
2. Bottom Left – Sodium Chloride Crystal: A cubic structure labeled “Crystal of NaCl” shows a repeating arrangement of ions. This structure represents the solid form before dissolution.
   • Sodium ions (Na⁺) and chloride ions (Cl⁻) are arranged in an alternating pattern.
   • The ions form a regular lattice structure, representing solid table salt.
   • Labels indicate:
     • Na⁺ (positively charged sodium ions)
     • Cl⁻ (negatively charged chloride ions)
3. Right Side – Hydrated Ions in Solution
   • Two separate clusters show how water molecules surround ions after dissolution:
4. Hydrated Sodium Ion (Na⁺)
   • A central Na⁺ ion is surrounded by several water molecules.
   • The oxygen ends (δ⁻) of water molecules point toward the sodium ion.
   • This orientation occurs because the oxygen is negatively polarized and is attracted to the positive sodium ion.
   • The structure is labeled “Hydrated sodium ion.”
5. Hydrated Chloride Ion (Cl⁻)
   • A central Cl⁻ ion is surrounded by multiple water molecules.
   • The hydrogen ends (δ⁺) of water molecules point toward the chloride ion.
   • This orientation occurs because the hydrogens are positively polarized and are attracted to the negatively charged chloride ion.
   • The structure is labeled “Hydrated chloride ion.”
6. Arrows and Process
   • Arrows connect the NaCl crystal to the hydrated ions, indicating the dissolution process:
   • Solid NaCl separates into individual Na⁺ and Cl⁻ ions.
   • Water molecules reorganize around each ion to stabilize them in solution.

Figure 2.4.3: Acids and Bases

This image shows two pairs of beakers demonstrating how acids and bases behave in water. In the top pair, labeled (a), the first beaker contains acid layered above water. After mixing, the second beaker shows water with hydrogen ions (H⁺) dispersed throughout, indicating acid dissociation. In the bottom pair, labeled (b), the first beaker contains base above water. After mixing, the second beaker shows water with hydroxide ions (OH⁻), indicating base dissociation. Arrows between the beakers represent the process of mixing and ion formation.

Figure 2.4.4: pH scale

The figure shows a vertical pH scale ranging from 0 to 14. The lower end of the scale is labeled Acid, pH 7 is labeled Neutral, and the upper end is labeled Base. An arrow points downward toward increasing acidity and upward toward increasing basicity. Examples are placed along the scale as follows:

• pH 14: Liquid drain cleaner
• pH 13.5: Bleaches, oven cleaner, lye
• pH 10.5-11.5: Ammonia solution
• pH 9.5: Baking soda
• pH 8: Sea water
• pH 7.4: Blood
• pH 7: Neutral
• pH 6.3-6.6: Milk, urine, saliva
• pH 5: Black coffee
• pH 2.5-3.5: Grapefruit juice, soda, tomato juice
• pH 2: Lemon juice, vinegar
• pH 0: Battery acid, hydrochloric acid

The figure illustrates that substances with lower pH values are more acidic, substances with higher pH values are more basic (alkaline), and neutral substances are near pH 7.

Figure 2.5.1: Comparison of Monosaccharides

This image presents structural diagrams of five monosaccharides (simple sugars) arranged in two groups: hexoses (six-carbon sugars) at the top and pentoses (five-carbon sugars) at the bottom. Each structure is shown as a ring with labeled atoms and functional groups.

Figure 2.5.2: Three Important Disaccharides

This image presents three labeled diagrams demonstrating how monosaccharides (single sugars) bond together to form disaccharides (two-sugar molecules). Each section is labeled (a), (b), and (c), and shows the individual sugar structures connected by an oxygen bridge.

1. Formation of Sucrose
   • The diagram title states: “The monosaccharides glucose and fructose bond to form sucrose.”
   • On the left is glucose, shown as a six-membered ring with hydroxyl (–OH) groups and a CH₂OH group.
   • On the right is fructose, shown as a five-membered ring with hydroxyl groups and two CH₂OH groups.
   • The two molecules are connected by a single oxygen atom (O) forming a glycosidic bond between them.
   • The resulting molecule is labeled “Sucrose.”
2. Formation of Lactose
   • The diagram title states: “The monosaccharides galactose and glucose bond to form lactose.”
   • On the left is galactose, shown as a six-membered ring with hydroxyl groups.
   • On the right is glucose, also a six-membered ring.
   • The two monosaccharides are linked by an oxygen bridge, forming a glycosidic bond.
   • The combined molecule is labeled “Lactose.”
3. Formation of Maltose
   • The diagram title states: “Two glucose monosaccharides bond to form maltose.”
   • Two glucose molecules, each shown as six-membered rings, are displayed side by side.
   • The two glucose units are connected by a central oxygen atom, forming a glycosidic bond.
   • The resulting disaccharide is labeled “Maltose.”

Figure 2.5.3: Three Important Polysaccharides

This image shows four polysaccharide structures. On the left, starch is represented by two forms: amylose, a straight chain of glucose units, and amylopectin, a branched chain. In the center, glycogen is shown as a highly branched structure, more complex than amylopectin. On the right, cellulose is depicted as multiple straight chains aligned in parallel, forming a rigid fiber structure. These arrangements highlight the differences in branching and organization among starch, glycogen, and cellulose, which affect their biological roles and digestibility.

Figure 2.5.4: Triglycerides

This image illustrates the process of triglyceride formation through dehydration synthesis, showing how glycerol reacts with three fatty acid chains to form a triglyceride (neutral fat) and three water molecules. The diagram is arranged from left to right:

• Reactants on the left (glycerol and fatty acids)
• A right-pointing arrow indicating a chemical reaction
• Products on the right (triglyceride and water molecules)

Headings at the top label each section are labeled as “Glycerol”, “3 fatty acid chains”, “Triglyceride, or neutral fat” and “3 water molecules”

1. Left Side – Reactants
   1. Glycerol - This structure provides three bonding sites for fatty acids.
      • A vertical three-carbon molecule is shown.
      • Each carbon is bonded to:
        • A hydrogen atom (H)
        • A hydroxyl group (–OH)
2. Three Fatty Acid Chains
   • Three long hydrocarbon chains are shown side by side.
   • Each chain includes:
     • A carboxyl group (–COOH) at one end
     • A long chain of repeating CH₂ units ending in CH₃
3. The hydroxyl group (–OH) on each fatty acid is positioned to react with glycerol.
4. Reaction – Dehydration Synthesis
   • Small circles highlight hydrogen (H) from glycerol and hydroxyl (OH) from fatty acids.
   • These combine to form water (H₂O).
   • This removal of water is called dehydration synthesis.
   • The diagram shows this occurring three times, once for each fatty acid.
5. Right Side – Products
   • Triglyceride (Neutral Fat)
     • The resulting molecule shows glycerol bonded to three fatty acid chains.
     • Each fatty acid is attached to glycerol through an ester linkage (–O–C=O–).
     • The structure resembles a vertical glycerol backbone with three long chains extending outward.
6. Water Molecules
   • A label indicates “3 H₂O” (three water molecules).
   • These are produced as byproducts of the reaction.

Figure 2.5.5: Fatty Acid Shapes

This image shows two molecular diagrams comparing saturated and unsaturated fatty acids. Each structure includes a carboxyl group on the left and a long hydrocarbon chain extending to the right. The image contains two labeled sections labeled as (a) Saturated at the top and (b) Unsaturated at the bottom.

• Both structures are displayed horizontally from left (functional group) to right (carbon chain).
• Atoms are labeled using chemical symbols (C for carbon, H for hydrogen, O for oxygen).

1. Saturated Fatty Acid:
   • The structure begins on the left with a carboxyl group (–COOH):
     • A carbon atom double-bonded to one oxygen (C=O)
     • The same carbon bonded to a hydroxyl group (–OH)
   • To the right of this group is a long chain of carbon atoms.
   • Each carbon atom is connected by single bonds only.
   • Every carbon in the chain is bonded to the maximum number of hydrogen atoms.
   • The chain appears straight and linear, with no bends.

Key feature:

• No carbon–carbon double bonds
• Fully “saturated” with hydrogen atoms

2. Unsaturated Fatty Acid
   • The structure also starts with a carboxyl group (–COOH) on the left.
   • A carbon chain extends to the right, but includes a key difference:
   • One or more carbon–carbon double bonds (C=C) are present.
   • The location of a double bond is highlighted in the diagram.
   • The presence of the double bond causes the chain to bend or kink, changing its shape.
   • Some hydrogen atoms are missing compared to the saturated structure because of the double bond.

Key features:

• At least one carbon–carbon double bond
• The chain is not straight; it has a visible bend

Figure 2.5.6: Other Important Lipids

This image presents three categories of lipids labeled (a), (b), and (c): phospholipids, sterols, and prostaglandins. Each section includes both structural diagrams and descriptive labels. The image is divided into three horizontal sections (a) Phospholipids at the top, (b) Sterols in the middle and (c) Prostaglandins at the bottom Each section contains labeled molecular structures and explanatory text.

1. Phospholipids: “Two fatty acid chains and a phosphorus-containing group are attached to the glycerol backbone. Example: Phosphatidylcholine.” Structure Components:
   1. A glycerol backbone (central vertical component)
   2. A phosphorus-containing group attached to one end:
      • Labeled as the polar “head”
      • Includes a phosphate group with additional chemical groups
   3. Two fatty acid chains attached to the glycerol:
      • Labeled as nonpolar “tails”
      • One chain is straight (saturated-like)
      • One chain contains a bend (unsaturated-like with a double bond)

Schematic Representation is a simplified diagram shows a round polar head and a two wavy nonpolar tails. Phospholipids are amphipathic molecules with both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

2. Sterols: “Four interlocking hydrocarbon rings from a steroid. Example: Cholesterol (cholesterol is the basis for all steroids formed in the body).” Structure Components:
   • A molecule composed of four fused rings:
     • Three hexagon-shaped rings
     • One pentagon-shaped ring

• Several attached groups, including:
  • A hydroxyl group (–OH) on one end
• Multiple CH₃ (methyl): Sterols have a rigid ring structure and play structural and signaling roles in biological systems.

3. Prostaglandins: Two example molecules are shown: PGF₂α and PGE₂. Structure Components:
   • Each molecule contains:
     • A five-membered ring
     • Two extended hydrocarbon chains branching from the ring
     • Functional groups such as:
       • Hydroxyl (–OH) groups
       • Carbonyl (C=O) group
       • Carboxyl (–COOH) group

Figure 2.5.7: General structure of an amino acid

This image shows the general chemical structure of an amino acid, highlighting its key functional groups and how they are arranged around a central carbon atom. Overall layout is:

• The diagram is divided into two labeled regions across the top:
  • “Amino group” on the left
  • “Carboxyl group” on the right
• At the center is a carbon atom labeled as the alpha (α) carbon, which connects all parts of the molecule.

1. Central Structure – Alpha Carbon
   • A central carbon atom (C) is shown in the middle of the diagram.
   • This carbon is labeled as the α (alpha) carbon, indicated by an arrow and text.
   • The alpha carbon forms four bonds, connecting to:
     • An amino group on the left
     • A carboxyl group on the right
     • A hydrogen atom above
     • A variable side chain (R group) below
2. Left Side – Amino Group: The amino group is responsible for the basic (proton-accepting) properties of amino acids.
   • The amino group consists of:
     • One nitrogen atom (N)
     • Two hydrogen atoms (H) attached to the nitrogen
     • This group is commonly written as –NH₂.
3. Right Side – Carboxyl Group: The carboxyl group provides the acidic (proton-donating) properties.
   • The carboxyl group contains:
     • One carbon atom double-bonded to an oxygen (C=O)
     • The same carbon bonded to a hydroxyl group (–OH)
     • This group is commonly written as –COOH.
4. Bottom – Side Chain (R Group)
   • Below the alpha carbon is a box labeled “R”, representing the side chain.
   • The side chain can vary in:
     • Size
     • Structure
     • Chemical properties
   • This variation determines the identity and function of each amino acid.

Figure 2.5.8: Peptide Bond

This image illustrates how two amino acids join to form a peptide bond through a dehydration (condensation) reaction. The diagram is arranged in two parts: reactants at the top and the resulting bonded structure at the bottom.

Figure 2.5.9: Four levels of protein structure

This image illustrates the four levels of protein structure: primary, secondary, tertiary, and quaternary. The diagram progresses from a simple chain of amino acids to a complex, folded protein composed of multiple subunits. Arrows indicate progression from simpler to more complex levels of organization. Amino acids are represented as labeled spheres (A1 through A11).

1. Primary Structure: The primary structure is the linear sequence of amino acids linked by peptide bonds.
   • A straight horizontal chain of connected spheres labeled A1 to A11.
   • Each sphere represents an amino acid.
   • The label “Chain of amino acids” appears near this section.
2. Secondary Structure: Secondary structure refers to local folding patterns stabilized by hydrogen bonds. Two alternative folding patterns are illustrated.
   • Alpha-helix (left side)
     • The amino acid chain coils into a spiral shape.
     • Dotted lines connect amino acids that are not adjacent, representing hydrogen bonds that stabilize the helix.
   • Beta-pleated sheet (right side)
     • The chain folds into a zigzag, sheet-like structure.
     • Multiple strands lie side by side.
     • Dotted vertical lines labeled “bonds” indicate hydrogen bonding between strands.
3. Tertiary Structure: The tertiary structure represents the overall three-dimensional shape of a single protein molecule, formed by interactions among side chains.
   • A single polypeptide chain is shown folded into a complex three-dimensional shape.
   • The structure appears compact and globular.
   • A labeled heme unit (a circular structure with a central component) is embedded within the folded protein.
4. Quaternary Structure
   • Multiple folded protein units are grouped together to form a larger structure.
   • The example shown is labeled “Hemoglobin (globular protein)”.
   • Several subunits, each containing a heme group, are clustered together.

Figure 2.5.10: Enzyme action sequence

This image shows a step-by-step process of enzyme activity involving two substrates combining to form a product. The sequence is divided into four labeled panels: (a), (b), (c), and (d), arranged from left to right. Overall layout:

• A large green shape represents the enzyme in each panel.
• The enzyme contains a recessed area labeled “Active sites.”
• Two smaller shapes labeled S₁ and S₂ represent substrates.
• Arrows and labels describe the progression from substrates to product.

1. Substrates Bind to the Enzyme: Substrates bind to the enzyme’s active site based on shape compatibility.
   • Two separate substrate molecules, labeled S₁ and S₂, are shown above the enzyme.
   • Arrows point downward from each substrate to the enzyme.
   • The enzyme has specifically shaped active sites that match the substrates.
2. Substrates Fit into Active Site: The enzyme brings substrates together and facilitates their interaction.
   • The two substrates are now positioned within the enzyme’s active site.
   • A label states: “Substrates form a product.”
   • The enzyme holds both substrates in close proximity.
3. Product Formation: The enzyme catalyzes the reaction, converting substrates into a product.
   • A single combined shape labeled “Product” is shown within the enzyme’s active site.
   • The substrates have chemically reacted to form one product.
4. Product Release: Enzymes are reusable and can catalyze the same reaction multiple times.
   • The product is shown moving away from the enzyme.
   • A label states: “Product detaches and process can repeat.”
   • The enzyme remains unchanged and ready to bind new substrates.

Figure 2.5.11: Nucleotides

This image explains the structure of nucleotides and how they combine to form nucleic acids (DNA and RNA). Each base is shown with detailed chemical structures including carbon (C), nitrogen (N), oxygen (O), and hydrogen (H) atoms. The diagram is divided into three labeled sections: (a), (b), and (c), each focusing on a different component of nucleic acid structure.

1. Nucleotide Structure and Polynucleotide Chain - Nucleotides link through the sugar and phosphate groups to form long chains, with bases projecting from the backbone.A nucleotide is shown as consisting of three main components:
   • Phosphate group
     • Displayed as a circular group containing phosphorus (P) bonded to oxygen atoms.
     • Positioned to the left of the sugar.
   • Pentose sugar
     • Shown as a five-sided ring labeled “Pentose sugar.”
     • Located in the center of the nucleotide.
   • Nitrogenous base
     • Shown as a hexagonal or ring-like structure attached to the sugar.
     • Highlighted with a box labeled “Nitrogenous base.”
   • Polynucleotide Formation
     • Arrows indicate how nucleotides connect to form a chain.
     • The connection between nucleotides is labeled “Phosphodiester linkage.”
     • A vertical chain on the right shows repeating sugar–phosphate units forming a polynucleotide backbone, with bases extending outward.
2. Nitrogenous Bases- This section shows two categories of nitrogen-containing bases:
   • Pyrimidines (single-ring structures)
     • Cytosine (C)
     • Thymine (T) — labeled “in DNA”
     • Uracil (U) — labeled “in RNA”
   • Purines (double-ring structures)
     • Adenine (A)
     • Guanine (G)
3. Pentose Sugars - Two types of five-carbon sugars are shown:
   • Deoxyribose (in DNA): Contains one fewer oxygen atom (missing an –OH group compared to ribose)
   • Ribose (in RNA): Contains an additional hydroxyl (–OH) group
   • Both sugars are drawn as five-membered rings with labeled atoms.

Figure 2.5.12: DNA

This image depicts the DNA double helix structure. Two strands twist around each other forming a spiral, with a sugar-phosphate backbone on the outside and nitrogenous bases paired inside. The base pairs include adenine (red) paired with thymine (yellow) and guanine (blue) paired with cytosine (green). Labels indicate the sugar-phosphate backbone and base pairs. A color key identifies each base type. This arrangement illustrates the complementary pairing and the helical shape of DNA.

Figure 2.5.13: Structure of Adenosine Triphosphate (ATP)

This image illustrates the structure of adenosine triphosphate (ATP) and related molecules, showing how its components are organized and how energy is stored in its bonds. The diagram is arranged horizontally from left to right. It shows three main components: Adenine (nitrogenous base) on the far left, Ribose (five-carbon sugar) in the center and Phosphate groups on the right. Labels at the bottom identify related molecules: AMP, ADP, and ATP.

1. Left Section – Adenine
   • A complex double-ring nitrogenous base labeled “Adenine.”
   • The structure includes nitrogen (N) and carbon (C) atoms arranged in rings.
   • Adenine is part of a larger unit called adenosine when combined with ribose.
2. Middle Section – Ribose Sugar
   • A five-membered ring labeled “Ribose.”
   • Contains oxygen and carbon atoms with attached hydrogen (H) and hydroxyl (–OH) groups.
   • Ribose connects:
     • Adenine on the left
     • Phosphate groups on the right
     • The combination of adenine + ribose is labeled “Adenosine.”
3. Right Section – Phosphate Groups
   • Three circular groups labeled as phosphate are connected in a chain.
   • Each phosphate contains phosphorus (P) bonded to oxygen atoms (O).
   • Red lines between phosphates are labeled “High-energy bonds.”
4. Bottom Labels – Related Molecules: Three labeled lines show increasing numbers of phosphate groups:
   • Adenosine monophosphate (AMP) → one phosphate
   • Adenosine diphosphate (ADP) → two phosphates
   • Adenosine triphosphate (ATP) → three phosphates

Figure 2.5.13: ATP Structure

This figure shows the chemical structure of adenosine triphosphate (ATP) and identifies its major components. ATP is made of three parts arranged from left to right: adenine, ribose, and three phosphate groups.