3.10: Alternative Text Descriptions

Figure 3.1.1: Structure of a Phospholipid

This figure shows the structure of a phospholipid molecule. The molecule has two main parts: a hydrophilic head, which is attracted to water, and two hydrophobic fatty acid tails, which repel water and extend downward from the head.

Figure 3.1.2: Phospholipid Bilayer

This image illustrates the structure of a phospholipid bilayer, which forms the basic structure of cell membranes. The diagram shows two layers of phospholipids arranged horizontally. The top region is labeled “Extracellular” (outside the cell). The bottom region is labeled “Intracellular” (inside the cell). A bracket label on the right identifies the entire structure as the “Phospholipid bilayer.”

1. Phospholipid Arrangement - Each phospholipid molecule consists of:
   • A round head (colored red or orange) labeled “Hydrophilic head”
   • Two elongated tails (yellow, wavy lines) labeled “Hydrophobic tail”
2. Top Layer (Outer Leaflet)
   • The hydrophilic heads are positioned facing upward toward the extracellular space.
   • The hydrophobic tails extend downward toward the center of the bilayer.
3. Bottom Layer (Inner Leaflet)
   • The hydrophilic heads are positioned facing downward toward the intracellular space.
   • The hydrophobic tails extend upward toward the center.
4. Center of the Bilayer
   • The hydrophobic tails from both layers meet in the middle, forming a nonpolar interior.
   • This central region prevents water and many polar molecules from easily passing through.

Figure 3.1.3: Lipid Bilayer With Various Components

This image illustrates the fluid mosaic model of the cell membrane, showing the arrangement of lipids, proteins, and carbohydrates within a phospholipid bilayer. The diagram shows a horizontal cross-section of a cell membrane. The membrane separates two regions (top and bottom), representing the outside and inside of the cell. A bracket on the right labels the entire structure as the “Phospholipid bilayer.”

1. Phospholipid Bilayer Structure
   • The membrane consists of two layers of phospholipids:
     • Round red/orange spheres form the outer surfaces (hydrophilic heads)
     • Long, thin yellow tails extend inward from both sides (hydrophobic tails)
   • The hydrophobic tails face each other in the center, creating a nonpolar interior layer.
2. Membrane Proteins - Several types of proteins are embedded in or associated with the bilayer:
   1. Integral Membrane Proteins
   2. Large blue structures that span the entire membrane from top to bottom, labeled as “Integral membrane protein.”
   3. These proteins are embedded within the lipid bilayer.
3. Channel Proteins
   • A specific type of integral protein labeled “Channel protein.”
   • Appears as a tube-like structure forming a passage through the membrane.
   • Allows certain substances to move across the membrane.
4. Peripheral Membrane Proteins - Smaller blue structures attached to the surface of the membrane, not spanning it, labeled as “Peripheral membrane protein.”
5. Cholesterol
   • Small yellowish or tan molecules located between the phospholipid tails, labeled “Cholesterol.”
   • Positioned within the hydrophobic interior of the membrane.
6. Glycoproteins - Proteins with attached carbohydrate chains extending outward from the membrane surface, labeled as “Glycoprotein: protein with carbohydrate attached.”
7. Glycolipids
   • Lipids with attached carbohydrate chains extending outward, labeled as “Glycolipid: lipid with carbohydrate attached.”
   • The carbohydrate chains appear as branching structures projecting outward, likely on the extracellular side.

Figure 3.1.4: Diffusion Through Lipid Layer

This image illustrates the process of simple diffusion across a lipid bilayer (plasma membrane), showing how small uncharged molecules move from an area of high concentration to an area of lower concentration over time. The diagram is arranged horizontally and divided into three sections representing progression over time. At the top center is the label “Extracellular fluid.” At the bottom center is the label “Cytoplasm.” A horizontal bar at the bottom labeled “Time” includes an arrow pointing to the right, indicating the direction of time progression from left to right.

1. Lipid Bilayer (Plasma Membrane)
   • A horizontal structure across the center of the image represents the lipid bilayer (plasma membrane).
   • It is made of two layers:
     • Round red or orange heads (hydrophilic) facing outward
     • Thin, yellow tails (hydrophobic) facing inward toward each other
2. Small Uncharged Molecules
   • Numerous small blue shapes, labeled “Small uncharged molecules,” are shown above and below the membrane.
   • These represent molecules such as oxygen or carbon dioxide that can diffuse directly through the membrane.
3. Left Section – Initial State
   • Many blue molecules are clustered in the extracellular fluid above the membrane.
   • Few or no molecules are present in the cytoplasm below.
   • This represents a high concentration outside the cell and a low concentration inside.
4. Middle Section – Diffusion in Progress
   • Some molecules have crossed the membrane and now appear in the cytoplasm.
   • The distribution begins to even out between the outside and inside.
   • This shows molecules moving down their concentration gradient.
5. Right Section – Equilibrium State
   • Molecules are more evenly distributed on both sides of the membrane.
   • The number of molecules outside and inside is similar.
   • Although molecules may still move, there is no net change in concentration.

Figure 3.1.5: Facilitated Diffusion

This image illustrates facilitated diffusion across a plasma membrane, showing two mechanisms: channel proteins and carrier proteins. The diagram is divided into two labeled sections: (a) and (b).

1. Channel Protein Transport - The top region is labeled “Extracellular fluid.” The bottom region is labeled “Cytoplasm.” A horizontal structure labeled “Plasma membrane” separates the two regions.
   • The membrane is shown as a phospholipid bilayer with round heads forming the outer surfaces and tails forming the inner region.
   • Channel Protein is a large structure labeled “Channel” spans the membrane. It forms a continuous opening (pore) from the extracellular fluid to the cytoplasm.
   • Molecules are small shapes represent molecules moving through the membrane. The arrows indicate movement from outside to inside through the channel.
2. Carrier Protein Transport has a similar membrane structure separates extracellular fluid (top) and cytoplasm (bottom). Multiple structures embedded in the membrane are labeled “Carrier proteins.”
   • Carrier Protein Function binds a molecule on one side of the membrane, changes shape to move the molecule across and releases the molecule on the opposite side.
3. Molecules are oval shapes that represent different types of molecules. The Arrows show molecules moving into the cell via carrier proteins.

Figure 3.1.6: Osmosis

This image shows osmosis in two stages. A container is divided by a semipermeable membrane, shown as a vertical dashed line. One side has a lower solute concentration and the other side has a higher solute concentration. The arrow between the two containers indicates the change over time. In the second stage, water has moved across the membrane toward the side with more solute particles, causing the liquid level on that side to rise and the level on the other side to fall. The image emphasizes that osmosis is the movement of water across a selectively permeable membrane from lower solute concentration to higher solute concentration.

Figure 3.1.7: Hypertonic Solution

compares how red blood cells respond to three different solution types: hypertonic, isotonic, and hypotonic. The diagram is arranged in three columns from left to right, each labeled at the top.

Figure 3.1.8: Sodium-Potassium Pump

This image illustrates the sodium–potassium pump, a form of active transport across the plasma membrane. It shows how sodium (Na⁺) and potassium (K⁺) ions move in opposite directions using energy from ATP. The diagram is a horizontal cross-section of a plasma membrane. The top region is labeled “Extracellular space.” The bottom region is labeled “Cytoplasm.” A phospholipid bilayer runs horizontally across the center. Several embedded protein structures represent transport proteins (pumps).

1. Ions and Their Distribution
   • Sodium ions (Na⁺) are illustrated as hexagonal shapes, mostly concentrated in the extracellular space (above the membrane).
   • Potassium ions (K⁺) are illustrated as oval shapes, primarily concentrated in the cytoplasm (below the membrane).
   • A vertical scale on the right labeled “Concentration” shows:
     1. Higher sodium concentration outside
     2. Higher potassium concentration inside
2. Plasma Membrane Structure
   • The membrane consists of a phospholipid bilayer with circular heads on both sides (hydrophilic) and tails facing inward (hydrophobic)
   • Embedded within the membrane are large brown protein structures, representing the sodium–potassium pump.
3. Step-by-Step Process:
   1. Sodium Binding and Transport Out
      • Three sodium ions bind to the pump on the cytoplasmic side.
      • An arrow indicates that these sodium ions are transported outward across the membrane to the extracellular space.
   2. ATP Energy Input
      • A molecule labeled ATP is shown near the pump.
      • ATP is broken down into ADP (adenosine diphosphate) and a phosphate group.
      • The phosphate attaches to the pump, causing a conformational (shape) change.
   3. Potassium Binding and Transport In
      • Two potassium ions bind to the pump on the extracellular side.
      • After another shape change, arrows show potassium ions being transported into the cytoplasm.
   4. Release and Reset
      • The phosphate group is released.
      • The pump returns to its original shape, ready to repeat the cycle

Figure 3.1.9: Process of Endocytosis

This image illustrates the process of endocytosis, where a cell takes material into the cell by forming vesicles from the plasma membrane. The diagram is divided into three labeled sections from left to right: Phagocytosis, Pinocytosis, and Receptor-mediated endocytosis. The image is arranged horizontally in three panels separated by vertical lines. A boundary represents the plasma membrane, enclosing the cytoplasm below. The region above the membrane represents the extracellular environment. Each panel shows a different mechanism of endocytosis using arrows and structural changes in the membrane.

1. Left Panel – Phagocytosis (“Cell Eating”) involves the engulfing of large particles such as food or debris.
   • A large particle is located outside the cell. The membrane engulfs the particle completely.
   • A vesicle forms inside the cell labeled “vacuole”.
2. Middle Panel – Pinocytosis (“Cell Drinking”) involves the uptake of extracellular fluid and dissolved molecules in small quantities.
   • The extracellular space contains fluid and small dissolved substances.
   • The plasma membrane folds inward to form small pockets.
   • These pockets pinch off to form small vesicles inside the cytoplasm, labeled “Vesicle.”
3. Right Panel – Receptor-Mediated Endocytosis is highly specific, allowing the cell to take in particular substances using receptors.
   • small particles bind to specific receptor proteins embedded in the membrane.
   • The membrane forms an indentation labeled “Coated vesicle”
   • The bound molecules cluster at receptor sites.
   • The membrane folds inward and pinches off to form a coated vesicle containing selected molecules.

Figure 3.1.10: The process of exocytosis

This image illustrates the process of exocytosis, where materials inside a cell are transported to the cell membrane and released into the extracellular space. The diagram shows a cross-section of a cell boundary, with the extracellular fluid labeled at the top, the cytoplasm labeled at the bottom, a horizontal line represents the plasma membrane, separating the two regions and arrows indicate the direction of movement from inside the cell to outside.

1. Vesicle Inside the Cell
   • A circular structure labeled “Vesicle” is located within the cytoplasm.
   • The vesicle contains several small particles, representing molecules to be transported.
   • An arrow points from the vesicle toward the plasma membrane.
2. Fusion with the Membrane
   • The vesicle moves upward and fuses with the plasma membrane.
   • The membrane bulges outward where the vesicle joins it.
   • This fusion creates an opening between the vesicle interior and the extracellular space.
3. Release of Contents
   • The particles move out of the vesicle and into the extracellular fluid, indicated by arrows.
   • The vesicle membrane becomes part of the plasma membrane after fusion.

Figure 3.1.11: Pancreatic Acinar Cell

This image is an electron micrograph of pancreatic acinar cells, showing numerous dark, round secretory vesicles clustered within the cell. The labeled acinar cell occupies the center of the image, and the labeled secretory vesicles appear as dense dots packed throughout the cytoplasm.

Figure 3.2.1: Prototypical Human Cell

Figure 3.2.2: Animal Cell Structure

This image has three labeled panels comparing rough endoplasmic reticulum and smooth endoplasmic reticulum.

• In panel (a), a cell illustration shows the endoplasmic reticulum near the nucleus. Rough ER appears as folded membrane sacs covered with small dots representing ribosomes. Smooth ER appears as connected tubular membranes without ribosomes.
• In panel (b), an electron micrograph shows rough ER as stacked, wavy membrane layers with many small dark particles attached to the surface. These particles are labeled ribosomes.
• In panel (c), an electron micrograph shows smooth ER as membrane structures without attached ribosomes. The smooth ER is labeled directly in the image.

Figure 3.2.3: Golgi apparatus

This image has two panels illustrating the structure and function of the Golgi apparatus.

In panel (a), a cell diagram shows the Golgi apparatus positioned near the rough endoplasmic reticulum and the nucleus. Transport vesicles bud from the rough ER and move to the cis face of the Golgi apparatus. The Golgi is shown as a stack of flattened membrane sacs called cisternae. Material moves through the Golgi from the cis face to the trans face. Secretory vesicles then bud from the trans face and move toward the plasma membrane.

The labeled structures in panel (a) include:

• nucleus
• rough ER
• transport vesicle
• cis face
• cisternae
• trans face
• Golgi apparatus
• secretory vesicle
• plasma membrane

In panel (b), an electron micrograph shows the Golgi apparatus as a curved stack of flattened membranes. The cis face and trans face are labeled to show the two sides of the organelle.

Figure 3.2.4: Mitochondrion

This image has two panels showing the structure of a mitochondrion.

In panel (a), a diagram shows an oval-shaped mitochondrion cut open to reveal its internal structure. The outer membrane forms the smooth outer boundary. Inside it is the inner membrane, which folds inward repeatedly to form cristae. The narrow region between the outer membrane and inner membrane is labeled the intermembrane space.

In panel (b), an electron micrograph shows a mitochondrion with the same major structures labeled. The outer membrane outlines the organelle. The inner membrane appears as folded layers inside the mitochondrion. These folds correspond to the cristae seen in the diagram. The space between the two membranes is labeled as the intermembrane space.

Figure 3.2.5: Peroxisome

This image shows a circular cross-section with an outer boundary labeled plasma membrane. The membrane is identified as a lipid bilayer. Inside the membrane is a large central structure labeled crystalline core. The image highlights the relationship between the outer membrane and the internal core.

Figure 3.2.6: Cytoskeletal Filaments

This image compares three types of cytoskeletal structures found in cells: microtubules, microfilaments (actin filaments), and intermediate filaments. Each structure is shown at two levels: a cellular view and a molecular subunit view.

1. Microtubules - Microtubules provide internal support for the cell. They serve as tracks for movement of materials inside the cell. Microtubules help organize cell shape and assist in processes such as cell division.
   • Microtubules are shown as hollow cylindrical structures.
   • They are made from repeating units labeled tubulin dimers.
   • These dimers are arranged in long columns to form a tube-like structure labeled Column of tubulin dimers.
   • The diameter is labeled 25 nanometers (nm).
2. Microfilaments (Actin Filaments) - Microfilaments support the cell’s shape and surface structure. These enable cell movement and contraction. Microfilaments are involved in processes such as muscle contraction and cell motility.
   • Microfilaments are shown as thin, flexible strands.
   • They are made of repeating units labeled actin subunits.
   • These subunits form a twisted, chain-like structure.
   • The diameter is labeled 7 nanometers (nm).
3. Intermediate Filaments - Intermediate filaments provide tensile strength, helping cells resist stretching forces. These stabilize cell structure and help maintain mechanical integrity.
   • Intermediate filaments are rope-like fibers made of coiled strands.
   • The subunits are described as fibrous proteins (keratins) twisted together.
   • The diameter is labeled 8 to 12 nanometers (nm).

Figure 3.3.1: The Nucleus

This image shows a eukaryotic cell nucleus. It includes the nuclear envelope, which is a double membrane dotted with nuclear pores. Inside the nucleus is condensed chromatin and a dense, spherical nucleolus. Surrounding the nucleus are membrane folds labeled as cisternae. The image highlights the structure and organization of the nucleus and its components. Labels included:

• Nucleus
• Nuclear pores
• Nuclear envelope
• Chromatin(condensed)
• Nucleolus
• Cisternae

Figure 3.3.2: Comparison of Skeletal, Cardiac, and Smooth Muscle Tissue

This image compares the three types of muscle tissue: skeletal muscle (A), cardiac muscle (B), and smooth muscle (C). Each row includes a longitudinal view on the left and a cross-sectional view on the right.

Figure 3.3.3: Red Blood Cell Extruding its Nucleus

This image contains two panels. In both panels, there are several microscopic cells visible in gray scale, which are false-colored for clarity. The dominant cells are highlighted in a reddish-orange color, while internal structures, primarily the nuclei, are a darker gray.

• Left Panel: Depicts a developing red blood cell (erythroblast) actively extruding its large, dark, and irregularly shaped nucleus. A small portion of cytoplasm is visible around the edge of the nucleus as it is pushed out of the cell.
• Right Panel: Shows the result of this process. The cell on the right is a mature red blood cell, which is biconcave (disk-shaped with a depression in the center) and lacks a nucleus, appearing as a uniform reddish-orange color. To its left is a discarded nucleus after being expelled, appearing as a dense, dark gray mass.

Figure 3.3.4: DNA Macrostructure

This image shows a diagram illustrating how DNA is packaged inside a cell. A chromosome and its component chromatin are shown to expand into nucleosomes with histones, which further unravel into a DNA helix and finally into a DNA ladder.

• On the left, there is a X‑shaped chromosome.
• A thin fiber labeled chromatin extends from the chromosome.
• The chromatin fiber appears coiled, and a zoomed‑in section shows groups of small spherical proteins labeled histones.
• DNA wraps around these histone proteins to form bead‑like structures called nucleosomes.
• To the right, the nucleosomes unwind into a long, twisted ladder‑shaped DNA helix.
• At the bottom of the image, the DNA is shown in even more detail as a sequence of base pairs (A, T, C, and G) connected by colored lines, representing the genetic code.

Figure 3.3.5: Molecular Structure of DNA

This image explains the structure of DNA, including its overall shape, the arrangement of nucleotides, and how individual components connect to form the molecule.

• DNA is shown as a double helix, meaning two strands twist around each other. Each strand has ends labeled 5-prime and 3-prime, and the two strands run in opposite directions. This double-stranded structure helps DNA store and protect genetic information and supports accurate copying during replication.
• Each strand is made of a repeating sugar-phosphate backbone. The backbone is labeled as alternating phosphate groups and five-carbon sugar molecules that form a long chain. This backbone provides structural support and stability and forms the outer framework of the DNA molecule.
• Attached to each sugar is a nitrogenous base. The four bases are labeled adenine, thymine, guanine, and cytosine. These bases extend inward from each strand toward the center of the double helix. The sequence of the bases stores genetic information and provides the instructions used to build proteins.
• Bases from one strand pair with bases on the opposite strand to form base pairs. The base pairing is labeled to show that adenine pairs with thymine and guanine pairs with cytosine. This complementary base pairing supports accurate DNA replication because one strand can serve as a template for building the other.
• The paired bases are connected by hydrogen bonds, which are labeled in the diagram. These bonds help hold the two strands together while still allowing the strands to separate during replication and transcription.
• A single nucleotide is also shown and labeled as the building block of DNA. Each nucleotide consists of a phosphate group, a sugar, and a nitrogenous base. Nucleotides link together to form DNA strands, and their sequence stores biological information.

Figure 3.3.6: DNA Replication

This image illustrates the process of DNA replication, showing how a double-stranded DNA molecule is copied to form two new strands.

• The DNA molecule is shown partially unwound, forming a replication fork, which is a Y-shaped region where replication occurs. One part of the DNA remains in its original double-stranded form, while another region is being actively copied. The replication fork is the site where DNA strands separate and new strands are synthesized.
• Two strands of DNA serve as templates for building new complementary strands. Each template strand contains a sequence of bases. Each original strand provides the pattern for forming a new strand, ensuring accurate duplication of genetic information.
• A protein labeled helicase is positioned at the replication fork. It separates the two strands of the DNA molecule. Helicase breaks the bonds between paired bases, allowing the strands to unwind and be copied.
• Individual units called free nucleotides are shown near the replication area. These nucleotides match with exposed bases on the template strands. Free nucleotides are the building blocks used to assemble new DNA strands.
• Enzymes labeled DNA polymerase are attached to the strands being formed. These enzymes move along the template strands. DNA polymerase adds nucleotides to the growing strand, forming a new complementary DNA strand. It ensures correct base pairing during replication.
• One new strand is labeled the leading strand. This strand is synthesized in a continuous direction along the template. The leading strand is built smoothly and continuously, allowing rapid replication.
• The second new strand is labeled the lagging strand. This strand is formed in short segments, not continuously. Because of strand direction, the lagging strand is assembled in pieces and later joined together.
• Each strand has labeled ends called 5-prime (5′) and 3-prime (3′). The strands run in opposite directions. DNA polymerase can only add nucleotides in one direction, which explains why one strand is continuous and the other is built in segments.
• Specific pairing occurs between bases: adenine pairs with thymine and guanine pairs with cytosine. Complementary pairing ensures accurate copying of genetic information.

Figure 3.4.1: The Genetic Code

This image shows the process of gene expression from DNA to protein. A DNA template strand is transcribed into an RNA strand, and the RNA strand is then translated into a protein.

• At the top is DNA, labeled template strand, shown as a sequence of nucleotide bases.
• An arrow points downward to transcription, where the DNA sequence is copied into RNA.
• The RNA strand contains the bases A, U, C, and G, with uracil replacing thymine.
• Below the RNA strand is translation, where the RNA is read in groups of three bases called codons.
• Each codon corresponds to a specific amino acid.
• At the bottom, the resulting protein is shown as a sequence of amino acids: methionine, proline, glutamine, serine, valine, histidine, alanine, leucine, methionine, and cysteine.

Figure 3.4.2: Transcription from DNA to mRNA

This image shows transcription, the process of making RNA from DNA.

• At the top left is a chromosome, which unwinds into a long DNA double helix.
• A section of the DNA opens, exposing the bases.
• A yellow enzyme labeled RNA polymerase sits on the DNA and moves along it in one direction.
• As RNA polymerase moves, it builds an RNA transcript by matching complementary bases.
• The RNA strand forms below the DNA and contains the bases A, U, C, and G, with uracil replacing thymine.
• The newly made RNA strand separates from the DNA as the DNA helix closes behind the enzyme.

Figure 3.4.3: RNA Splicing

This image is a biological process diagram illustrating RNA splicing, the mechanism by which introns are removed from a pre-messenger RNA (pre-mRNA) transcript and exons are joined together to form a mature RNA molecule.

At the top of the diagram is a horizontal bar labeled “pre-mRNA transcript.” The transcript is divided into alternating colored segments representing exons and introns. From left to right, the segments are labeled Exon 1, Intron, Exon 2, Intron, and Exon 3. Exons are shown in shades of yellow, blue, orange, and tan, while introns are shown in green.

A large downward arrow points from the pre-mRNA transcript to the center of the figure, where a structure labeled “Spliceosome” is depicted. The spliceosome is illustrated as a large oval-shaped complex containing intertwined colored shapes that surround and interact with the RNA transcript. A label identifies one of the green looped RNA regions as an “Intron.”

Within the spliceosome, the RNA transcript is shown looping so that the intron region folds inward while Exon 1, Exon 2, and Exon 3 are positioned along a lower horizontal strand. This arrangement represents the spliceosome bringing exon sequences together while preparing the intron for removal.

Two diagonal arrows extend downward from the spliceosome, indicating the formation of two separate products after splicing.

On the lower left is the excised intron, shown as a green lariat-shaped loop with a tail. A label identifies this structure as “Intron.” This represents the intron sequence that has been removed from the transcript.

On the lower right is a horizontal RNA strand labeled “Spliced RNA.” The strand contains the exon segments joined together in sequence as Exon 1, Exon 2, and Exon 3, with no intron sequences present. The exons are shown as contiguous colored blocks, illustrating that the introns have been removed and the remaining coding sequences have been connected to form the mature RNA product.

Figure 3.4.4: Translation from RNA to Protein

This image shows the process of translation, where a ribosome reads genetic information from messenger RNA (mRNA) to build a protein.

• The image is arranged vertically in three stages, showing progression over time. A strand labeled mRNA runs horizontally and is marked with direction from 5-prime (5′) on one end to 3-prime (3′) on the other. A structure labeled ribosome surrounds part of the mRNA and consists of a large subunit and a small subunit. Transfer RNA, labeled tRNA, interacts with the mRNA inside the ribosome. Each tRNA carries an amino acid and contains an anticodon.
• In the first stage, the ribosome is assembled on the mRNA. A codon labeled AUG is identified on the mRNA, and a tRNA with anticodon UAC binds to this codon. This tRNA carries the amino acid methionine (Met). The AUG codon acts as the start signal for translation, and the binding of the first tRNA establishes the starting point for protein synthesis.
• In the second stage, a second tRNA binds to the next codon on the mRNA. Two tRNA molecules are now held inside the ribosome at the same time. The second tRNA carries another amino acid, such as phenylalanine (Phe). The ribosome positions the two amino acids close together, and a chemical bond forms between the amino acids, beginning the polypeptide chain. The ribosome then moves along the mRNA to read the next codon.
• In the third stage, multiple tRNA molecules interact with the mRNA one after another. Additional amino acids, such as arginine (Arg), are added. A chain labeled polypeptide chain extends from the ribosome. The ribosome continues linking amino acids in sequence, and the growing chain represents a new protein being synthesized. The order of codons determines the sequence of amino acids, which determines protein structure.
• The mRNA contains codons such as AUG, UUU, and CGA, and tRNA molecules have matching anticodons such as UAC, AAA, and GCU. Codon-anticodon pairing ensures that the correct amino acid is added at each step, which maintains accuracy in protein synthesis.
• The ribosome moves along the mRNA from the 5′ end toward the 3′ end. This directional movement ensures that the protein is built in the correct order, following the genetic sequence.

Figure 3.4.5: From DNA to Protein: Transcription through Translation

This image shows the central dogma of molecular biology, explaining how genetic information flows inside a cell. The figure shows a schematic of a cell where transcription from DNA to mRNA takes place inside the nucleus and translation from mRNA to protein takes place in the cytoplasm.

• The image shows a cell with a nucleus in the center. Inside the nucleus, DNA is shown at the top.
• An arrow labeled transcription points from DNA to mRNA, indicating that DNA is copied into messenger RNA. The mRNA then moves out of the nucleus into the surrounding area of the cell.
• In the cytoplasm, an arrow labeled translation points from mRNA to a protein. Translation occurs at a ribosome, shown as a rounded structure, where the mRNA is used to build a chain of amino acids.

Figure 3.5.1: Cell Cycle

This image shows the cell cycle as a circular process.

• Most of the circle is interphase, divided into three stages:
  • G₁, where the cell grows and carries out normal functions.
  • S, where DNA is copied.
  • G₂, where the cell prepares for division.
• A small wedge labeled M phase represents mitosis, when the cell divides.
• Arrows around the circle show the cycle moving in a continuous direction.
• A side branch labeled G₀ shows cells that exit the cycle and are not actively dividing.

Figure 3.5.2: The Structure of Chromosomes

This image shows two paired chromosomes and labels their major structural parts.

The two chromosomes are identified as homologous chromosomes. Each chromosome is made of two sister chromatids joined near the center. The central joining region is labeled the centromere. A structure at the centromere is labeled the kinetochore. The purpose of the diagram is to identify the relationship between homologous chromosomes, sister chromatids, the centromere, and the kinetochore.

Figure 3.5.3: Cell Division: Mitosis Followed by Cytokinesis

This image is a chart summarizes the stages of mitosis in six columns: prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. Each column includes a cell diagram, a bulleted description, and a microscope image.

• Prophase: Chromosomes condense and become visible. Spindle fibers emerge from the centrosomes. The nuclear envelope breaks down, and centrosomes move toward opposite poles.
• Prometaphase: Chromosomes continue to condense. Kinetochores appear at the centromeres. Mitotic spindle microtubules attach to the kinetochores.
• Metaphase: Chromosomes line up at the metaphase plate. Each sister chromatid is attached to a spindle fiber from opposite poles.
• Anaphase: Centromeres split in two. Sister chromatids, now called chromosomes, are pulled toward opposite poles. Some spindle fibers lengthen the cell.
• Telophase: Chromosomes arrive at opposite poles and begin to decondense. Nuclear envelope material surrounds each set of chromosomes. The mitotic spindle breaks down, and spindle fibers continue to push the poles apart.
• Cytokinesis: In animal cells, a cleavage furrow separates the daughter cells. In plant cells, a cell plate forms as a precursor to a new cell wall and separates the daughter cells

Figure 3.5.4: Control of the Cell Cycle

Figure 3.6.1: Hematopoiesis

This image shows hematopoiesis, the process by which all blood cells develop from stem cells.

• At the top is a multipotential hematopoietic stem cell.
• This stem cell divides into two main pathways:
  • Common myeloid progenitor
  • Common lymphoid progenitor
• The myeloid lineage produces:
  • Megakaryocytes, which form thrombocytes or platelets
  • Erythrocytes (red blood cells)
  • Mast cells
  • Myeloblasts, which develop into basophils, neutrophils, eosinophils, and monocytes
  • Monocytes further differentiate into macrophages and dendritic cells
• The lymphoid lineage produces:
  • Natural killer cells
  • Small lymphocytes, which become T lymphocytes and B lymphocytes
  • B lymphocytes can further develop into plasma cells

Figure 3.6.2: Transcription Factors

This image illustrates how transcription begins on a DNA strand.

• On the left, several transcription factors bind to a specific DNA region called the promoter.
• In the center, RNA polymerase binds to the transcription factors and the promoter, forming a transcription initiation complex.
• On the right, RNA polymerase is fully positioned on the DNA, and transcription begins, shown by an arrow indicating movement along the DNA.

Figure 3.6.3: Stem Cells

This image shows how stem cell potency becomes more limited as cells differentiate. At the top, a totipotent embryonic stem cell is shown as the most flexible cell type. Arrows point downward to pluripotent embryonic stem cells. Below that, pluripotent cells give rise to multiple cell lineages. These then lead to multipotent stem cells, which are more restricted and produce a narrower range of related cell types. At the bottom, the diagram shows examples of specialized cells produced through differentiation, including muscle cells, red blood cells, epithelial cells, and a neuron. The purpose of the diagram is to compare totipotent, pluripotent, and multipotent stem cells and show that differentiation leads to increasingly specialized cells.