Question 1
Brandt’s voles (Lasiopodomys brandtii) are small herbivorous rodents native to the steppe grasslands of Inner Mongolia. These voles live in large family groups and feed on the roots, stems, and leaves of various plants near their burrows. Their feeding behavior significantly affects the surrounding vegetation. To study this impact, researchers marked plots of land and tracked changes in the biomass of two types of grasses—rhizome grasses and bunch grasses—over a period from 2013 to 2017, comparing areas with and without voles.
(a) Identify the year in which the highest biomass of bunch grasses was observed in plots with voles present.
(b) Compare the mean biomass of the two types of grass in 2013, both in the presence and absence of voles.
(c) The voles obtain their nutrition from only one type of grass. Based on this, infer which type it is and explain why.
In a follow-up study conducted from 2019 to 2020, researchers examined how the presence of voles affected the plant volume of a specific bunch grass species, Achnatherum splendens , in designated plots. They measured the changes in the plant volume of A. splendens within these plots to assess the impact of voles.
(d) Calculate the mean difference in plant volume between the plots with voles present and those without voles throughout the study period.
(e) Summarize how these results seem to contradict the findings of the previous study.
(f) Propose one advantage of measuring plant volume instead of plant biomass.
Shrikes (Lanius spp.) are carnivorous, predatory birds that hunt small animals, including voles. During the second study on the plant volume of Achnatherum splendens, shrikes were observed visiting the marked plots, but they were not seen in the first study, which focused on the biomass of two types of grass. The graph illustrates the number of visits made by shrikes to 15 different marked plots, with the data categorized by the percentage of coverage by A. splendens in each plot.
(g) Calculate the percentage of plots that received two visits from shrikes.
(h) Describe the relationship between the number of shrike visits to the plots and the percentage of each plot covered by A. splendens.
The researchers prepared plots, some with and some without voles. Shrikes were present in all plots. At the end of one month, the mean changes in root biomass and plant volume of A. splendens in the plots were measured.
(i) Propose a possible explanation for the variation in A. splendens plant volume across the plots during the study period.
In the final experiment, the researchers created plots with varying levels of Achnatherum splendens cover, along with the plants typically consumed by voles. Shrikes were allowed access to the plots. Over a 3-week period, the researchers recorded the amount of time voles spent feeding and monitored the vole mortality rate.
(j) Explain the condition that promotes vole survival, providing a reason for it.
(k) The researchers concluded that voles are able to alter their ecosystem to ensure their survival. Discuss this conclusion, referring to all the data provided in the question.
Topic-A4.1.3
▶️Answer/Explanation
(a) 2015;
(b) When comparing the biomass of the two types of grasses in the absence of voles, the biomass of both bunch grasses and rhizome grasses was found to be similar, with no significant difference between the two types.
However, in the presence of voles, the biomass of bunch grasses was higher than that of rhizome grasses. This suggests that voles may have a greater impact on the rhizome grasses than on the bunch grasses.
Looking at the changes in biomass due to the presence of voles, it was observed that the biomass of rhizome grasses decreased, while the biomass of bunch grasses showed only a slight decrease or remained relatively unchanged. This indicates that voles have a more pronounced negative effect on rhizome grasses, while bunch grasses are less affected by vole feeding.
(c) The voles obtain their nutrition from only one type of grass – rhizome grass
because as seen in the graph the rhizome grass is as lower biomass/less when voles are present
(d) 1.1 (m3) (accept answers from 1 or 1.0 to 1.2);
the mean difference in plant volume between the plots with voles present and those without voles throughout the study period
=( plant volume with the plots with voles present) – ( plant volume with the plots with voles absent)
= (0.5) – (-0.5)
= 0.5+0.5
= 1.0
(e) A. splendens /bunch grass was reduced by presence of voles in this study , but bunch grass was not reduced by the presence of voles in the previous study .
Only rhizome grass was seen to be reduced by presence of voles in the previous study
(f) Measuring plant volume instead of plant biomass offers several advantages, including being non-destructive and quicker:
Non-destructive: Plant volume can be measured without the need to dry or kill the plants, preserving the integrity of the plants for future study.
Minimal environmental impact: Since plant volume can be measured without harming or altering the habitat, ecosystem, or environment as much, the process is less disruptive.
Volume as an indicator of size: Measuring plant volume provides an estimate of how much space, area, or height the plant occupies, offering an overall sense of the plant’s size and growth.
Allows repeated measurements: Because plant volume can be measured without harming the plant, it allows for repeated measurements of the same plants over time, providing a continuous record of growth.
-Tracks growth: By monitoring changes in volume, you can track increases in size and measure the plant’s growth over the course of the study.
(g) 20; cross check from the given graph
(h) As the number of visits of shrike increase ; more percentage of plat was covered by A. splendens . This type of relationship is known as positive correlation.
(i) a. increase (without voles) due to photosynthesis/growth / due to roots not being eaten;
b. decrease (with voles) due to voles feeding on roots/shoots/leaves/grass/A. splendens;
(j) a. low area/cover as this has lower mortality;
b. low area/cover due to less shrikes/predation;
c. low area/cover as this has higher feeding frequency;
(k) The researcher’s conclusion that voles are able to modify their ecosystem to ensure their survival can be discussed in light of several key data points.
First, voles seem to influence the A. splendens plant community in multiple ways. The data indicate that voles reduce the volume or area of A. splendens (as seen in the second and fourth datasets), likely through their feeding habits. Voles also reduce root biomass and even kill the roots of A. splendens (fourth dataset), further limiting the growth of this plant. Interestingly, while voles do not consume bunchgrass directly (first dataset), they still manage to reduce its abundance, as seen in the second dataset. This suggests that voles may be altering the environment in ways that negatively impact the growth of A. splendens, possibly through indirect effects such as soil disturbance or changes in microhabitats.
This reduction in A. splendens coverage is significant for vole survival. The fifth dataset shows that voles experience lower mortality when there is less A. splendens coverage, indicating that a less-dense plant community may be more favorable for vole survival. This is supported by data showing that the feeding frequency of voles is higher when the area covered by A. splendens is smaller (fifth dataset), suggesting that reduced A. splendens density allows voles to access more resources, possibly by increasing the availability of alternative plants or improving the overall quality of their foraging environment.
Furthermore, the relationship between A. splendens coverage and predation pressure is noteworthy. Fewer shrikes (or fewer shrike visits) are recorded in areas with less A. splendens(third dataset), which is important because it suggests that a reduced abundance of this plant may lower the risk of predation for voles. The fifth dataset supports this idea, showing that lower A. splendens coverage correlates with reduced predation and mortality due to shrikes. This reduced risk of predation would further contribute to the voles’ survival.
In summary, the data collectively suggest that voles are able to modify their ecosystem—primarily through their impact on A. splendens—in ways that enhance their own survival. By reducing the volume, root biomass, and coverage of A. splendens, voles seem to create a more favorable environment for themselves, with less predation from shrikes and improved foraging opportunities. These changes ultimately contribute to a lower mortality rate and higher feeding frequency, supporting the conclusion that voles can indeed alter their ecosystem to their advantage.
Question 2
The diagram displays the original depiction of the cell membrane created by Singer and Nicolson in 1972.
(a) Label the hydrophilic part of the phospholipid molecule with the letter H.
(b) Compare this model of the cell membrane with the Davson–Danielli model, highlighting the differences.
(c) Explain the function of molecules like M in sodium–potassium pumps.
Topic- B2.2.1
▶️Answer/Explanation
(a)
(b) The fluid mosaic model (FMM) of the cell membrane differs from the Davson–Danielli model in several key ways:
Protein Location
In the Davson–Danielli model, proteins are globular and positioned on the outer surfaces of the phospholipid bilayer, forming a sandwich-like structure with proteins on top and bottom of the membrane, surrounding the phospholipid layers. In contrast, the FMM model proposes that proteins are embedded within the phospholipid bilayer itself, distributed throughout the membrane in a “fluid mosaic,” with both lipids and proteins able to move laterally within the membrane.
Membrane Fluidity
The FMM model accounts for the fluid nature of the membrane, explaining how lipids and proteins can move laterally and how some proteins are restricted by anchoring to the cytoskeleton. This fluidity is a critical aspect of membrane function. The Davson–Danielli model, however, did not include any explanation for membrane fluidity and depicted the membrane as a more rigid, static structure.
Membrane Uniformity
The Davson–Danielli model assumed that all membranes had a uniform thickness and consistent lipid-to-protein ratio. In contrast, the FMM model recognizes that different membranes can vary in protein content, and that the membrane structure is not uniform across all types of cells or organelles.
The Davson–Danielli model, also known as the “sandwich model,” was proposed in 1935 by Hugh Davson and James Danielli. This model was based on electron microscopy images that suggested a three-layer structure within the membrane. However, the model faced several issues:
1. Permeability: It failed to explain the selective permeability of the membrane, as certain substances could pass through the membrane more easily than predicted by the model.
2. Membrane Solidification: The temperatures at which membranes solidified did not align with the predictions of the model, suggesting that the membrane was more fluid than the sandwich-like structure could account for.
(c)
The sodium-potassium pump is an enzyme found in the membrane of all animal cells that uses adenosine triphosphate (ATP) to move sodium (Na⁺) and potassium (K⁺) ions across the cell membrane.
Function
The sodium-potassium pump is an example of active transport, meaning it requires energy to function. This energy comes from the hydrolysis of ATP, which provides the necessary energy for the pump to undergo a conformational change that allows it to transport ions. The pump actively moves three sodium ions (Na⁺) out of the cell for every two potassium ions (K⁺) it moves into the cell. This process occurs against the concentration gradient for both ions, meaning the pump moves them from areas of lower concentration to areas of higher concentration. By doing so, the pump creates and maintains concentration gradients for sodium and potassium across the cell membrane.
Role in the Body
The sodium-potassium pump plays a crucial role in several physiological processes:
Cell Potential: The pump is essential for maintaining the resting membrane potential of the cell, a type of membrane potential. By keeping sodium concentrations high outside the cell and potassium concentrations high inside, it creates the electrochemical gradients necessary for generating action potentials in nerve cells.
Cellular Volume: The pump helps regulate the cell’s volume by controlling the osmotic balance between the inside and outside of the cell, thus preventing excessive water influx or efflux.
Nerve Signals: In axons, the sodium-potassium pump is critical for maintaining the ion gradients that allow for the propagation of action potentials. The pump helps establish the resting membrane potential so that neurons are prepared to transmit nerve signals when needed.
Filtering Waste: In the kidneys, the sodium-potassium pump helps in the reabsorption of sodium and the excretion of waste products, contributing to the filtration process.
Sperm Motility: The pump also plays a role in sperm motility by maintaining the ionic gradients necessary for sperm movement, which is essential for fertilization.
Question 3
The image depicts a cell in the anther of a lily (Lilium sp.) plant during the initial division of meiosis.
(a) (i) Determine the stage of meiosis displayed in the image.
(ii) Indicate where pollen is produced in the lily flower.
(b) Explain the difference between plant pollination and fertilization.
(c) The diagram illustrates a cross-section of a broad bean (Vicia faba) seed.
(i) Label the cotyledon on the diagram.
(ii) Summarize the role of the cotyledon.
Topic-D2.1.2 / A3.1.2
▶️Answer/Explanation
(a) (i) anaphase / anaphase I; Anaphase 1 is a stage of cell division that occurs during meiosis, when homologous chromosomes separate and move to opposite ends of the cell. It’s the third step of meiosis 1, and it’s characterized by two phases: anaphase A and anaphase B.
(ii) anther/stamen;
(b) The main difference between plant pollination and fertilization is that pollination is the physical act of transferring pollen grains to a flower, while fertilization is the process of fusing the male and female gametes:
Pollination- The transfer of pollen grains from the male part of a flower (anther) to the female part (stigma). Pollen can be transferred by wind or animals, such as bees, butterflies, moths, flies, and beetles.
Fertilization -The fusion of the male gamete (sperm) with the female gamete (ovum or egg). This process occurs inside the ovary after successful pollination. Fertilization is a cellular, genetic, and biochemical process.
After fertilization, the fertilized flower will produce fruit and seeds.
(i) 
(ii) Cotyledons, also known as seed leaves, have several roles in the development of a plant:
Provide nutrients: Cotyledons provide nutrients to the developing plant embryo until it germinates and becomes established as a photosynthetic organism.
Store food: Cotyledons contain reserve food material that nourishes the seedling.
Become photosynthetic: In some plants, cotyledons become photosynthetic.
Synthesize plant hormones: Cotyledons synthesize gibberellic acid, a plant hormone.
Absorb nutrients: Cotyledons absorb nutrients from the endosperm.
Maintain metabolism: Cotyledons maintain the stable metabolism of the plant embryo.
The number of cotyledons in a seed determines its classification:
Monocot: Seeds with one cotyledon, such as rice, wheat, and maize
Dicot: Seeds with two cotyledons, such as mango and neem seeds
Multicotyledonous: Seeds with more than two cotyledons, such as angiosperms
Question 4
The diagram illustrates the tandem repeats of the AGAT sequence on chromosome 5 (Chr 5) and the AGAA sequence on chromosome 18 (Chr 18) for a couple and their children. The number of repeats for each sequence is shown, with one child being adopted.
(a) Describe the role of tandem repeats in DNA profiling.
(b) Identify the adopted child and provide a reason for your choice.
(c) Describe the role of a promoter in DNA.
Topic-D3.2.1
▶️Answer/Explanation
(a)Tandem repeats are DNA sequences that are repeated multiple times in a head-to-tail manner on a chromosome, and they play a significant role in DNA profiling due to their variability and ability to provide unique genetic information. Here’s how they are used in various applications:
Genetic Markers
Tandem repeats can be used as genetic markers to track inheritance within families. The number of repeats at specific loci may differ between individuals and is inherited from parents to children. This variability in the number of repeats makes them useful for distinguishing between individuals and analyzing family relationships.
DNA Fingerprinting
Short tandem repeats (STRs) are often used in DNA fingerprinting to identify individuals based on the number of repeats at specific loci. In forensic analysis, STRs are analyzed using gel electrophoresis, a technique that separates DNA fragments based on size. This separation produces distinct bands that correspond to the different lengths of STR sequences. By comparing the pattern of bands produced on the gel, scientists can generate a unique genetic profile or DNA fingerprint for each individual.
Forensic DNA Typing
Multiple sites, or tandem repeat loci, are analyzed to generate a complete DNA profile. Since a combination of bands from various repeat regions is unique to each person, it allows for highly accurate identification. These profiles are used in forensic investigations, such as criminal cases and missing persons searches, as well as in paternity testing. In forensic settings, the combination of STRs can link a suspect to a crime scene or confirm familial relationships.
Tracing the Origin of an Outbreak
Polymorphic tandem repeats, such as variable number tandem repeats (VNTRs), can also be used to track the origin of an outbreak in microorganisms. By comparing the repeat patterns found in microbial genomes, scientists can trace the spread of infections, helping to identify the source of an outbreak and its transmission route.
Determining Parentage
Tandem repeats, particularly STRs, are often used in paternity testing and other genealogical DNA tests. The unique combination of tandem repeats in the DNA of the child and potential parents is compared to confirm biological relationships. This makes DNA profiling a reliable tool for determining parentage.
(b) Adopted child = child 1/the boy as neither parent has 12 tandem repeats on chromosome 5;
(c) A promoter is a non-coding (non-transcribed) section of DNA that signals the start of transcription for a gene. It plays a critical role in the regulation of gene expression. Here’s a detailed breakdown:
Function
The promoter serves as the **region where transcription is initiated** on a DNA strand. It is where RNA polymerase and other transcription factors bind to begin the transcription process. Transcription **starts at the promoter**, and the promoter also helps determine which strand of the DNA will be transcribed, known as the sense strand.
Location
Promoters are typically **located upstream of the gene**, or coding sequence, usually at the 5′ end. This upstream location ensures that the transcription machinery assembles in the correct position to begin transcribing the gene.
Binding and Separation
The promoter region is crucial for the **binding of RNA polymerase** and other transcription factors to the DNA. It provides the necessary site for **RNA polymerase to separate the DNA strands** so that it can start synthesizing RNA. This separation allows the enzyme to “read” the DNA template and produce the corresponding RNA molecule.
Control of Gene Expression
The promoter region is key in **controlling and regulating gene expression**. It helps determine when and where a gene is transcribed in an organism, allowing for precise regulation of gene activity depending on the cell type, developmental stage, or environmental conditions.
TATA Box
In many eukaryotic genes, the promoter contains a conserved sequence known as the TATA box, located 25 to 35 base pairs upstream of the transcription start site. The TATA box is important because it helps position RNA polymerase and the transcription factors correctly at the start of transcription.
Question 5
The diagram shows the bones and muscles of the human elbow joint.
(a) Identify structures X and Y.
(b) Summarize the antagonistic action of the elbow muscles..
(c) Give one reason why striated muscle cells are considered atypical.
(d) Explain how specialized cardiac muscle cells trigger each heartbeat.
Topic-B3.3.2
▶️Answer/Explanation
(a) X: ulna;
Y: biceps (muscle);
(b) The biceps and triceps muscles in the upper arm are antagonistic muscles that work in opposition to each other to bend and straighten the elbow:
Biceps
When the biceps contracts, it bends the elbow, flexes the arm, and pulls the forearm upwards. The biceps acts as the flexor. Meanwhile, the triceps relaxes to allow this movement to occur.
Triceps
When the triceps contracts, it straightens the arm, extending the elbow. The triceps acts as the extensor, and the biceps relaxes to enable this action. Antagonistic muscles work in opposition to each other, causing opposite movements. When one muscle contracts, the other relaxes. This coordinated action allows for smooth, controlled movements and helps maintain body or limb position.
(c) Striated muscle cells are considered atypical because they have multiple nuclei and are formed from the fusion of multiple cells:
Multiple nuclei
Striated muscle cells have multiple nuclei because they are formed when multiple cells fuse together. This is different from other cells, where the nucleus is usually located in the center.
Syncytium
Striated muscle cells are a syncytium, which means they are a multinucleate mass of cytoplasm surrounded by a single cell membrane.
Challenges cell theory
The features of striated muscle cells challenge the idea that cells work independently of each other, even in multicellular organisms.
Striated muscle cells are also large, long, and cylindrical. They are attached to the skeleton and are under the control of the will.
(d) The heart’s cardiac conduction system, a network of specialized muscle cells, triggers each heartbeat by sending electrical signals throughout the heart. Notably, cardiac muscle is myogenic, meaning it can contract without nerve stimulation:
1. Sinoatrial (SA) Node
The SA node, located in the right atrium, acts as the heart’s natural pacemaker and initiates each heartbeat. It generates an action potential, which triggers the rest of the heart’s electrical activity. This signal begins the contraction process of the atria.
2.Atrioventricular (AV) Node
The electrical signal from the SA node travels to the AV node, where it slows slightly, allowing the ventricles time to fill with blood before contraction. This delay ensures efficient heart function.
3. Bundle of His and Bundle Branches
The action potential then moves from the AV node to the Bundle of His and along the right and left bundle branches, directing the electrical impulse toward the ventricles.
4. Purkinje Fibers
The signal travels through the Purkinje fibers, which spread the impulse across the ventricles. This stimulates the ventricular muscle cells to contract, pumping blood out of the heart.
5. Relaxation
After the ventricles contract, they relax, and the cycle begins again at the SA node. This electrical activity is captured on an **electrocardiogram (EKG)**, which doctors use to monitor the heart’s conduction system.
The contraction of cardiac muscle cells is regulated by nerve signals and **epinephrine** (adrenaline). The **sympathetic nervous system** and epinephrine increase heart rate by making the SA node fire more rapidly, while the **parasympathetic nervous system** slows down the heart rate by reducing the firing rate of the SA node. This coordinated regulation ensures that the heart beats efficiently, with signals from the SA node propagated across the atria and ventricles to stimulate contraction.
Question 6
6. Water’s chemical and physical properties make it a vital medium for life.
(a) Explain how water functions as a coolant during sweating.
(b) Explain how the kidneys regulate water balance when the body is dehydrated.
(c) Describe the process by which water moves from the soil to the atmosphere in flowering plants.
Topic-C3.1.3
▶️Answer/Explanation
(a) Water cools the body through sweating via a process known as evaporative cooling:
1.Evaporation of Water
When sweat evaporates from the skin, water molecules separate and change from a liquid to a vapor (gas). This process is driven by the absorption of thermal energy (heat) from the body.
2.Breaking of Hydrogen Bonds
To transition from liquid to gas, the hydrogen bonds between water molecules must be broken. This requires energy, which is provided by heat from the skin. As these bonds are broken, heat is removed from the body.
3.Heat Removal from the Body
The energy required to break the hydrogen bonds is taken from the skin, which leads to cooling as the heat is removed during evaporation.
4.High Heat of Evaporation
Water has a high latent heat of evaporation, meaning it requires a large amount of energy to change from a liquid to a gas. This makes water an efficient mechanism for removing heat from the body, helping to regulate temperature during activities or in hot environments.
(b) When the body is dehydrated, the kidneys regulate water balance through a series of processes, primarily involving the hormone ADH (antidiuretic hormone), also known as vasopressin:
1. Hormonal Regulation
When the body is dehydrated, or the blood becomes hypertonic (i.e., the solute concentration is too high), the pituitary gland secretes ADH into the bloodstream. ADH signals the kidneys to conserve water.
2.Aquaporins and the Collecting Duct
ADH promotes the insertion of aquaporins (water channels) into the walls of the collecting duct in the kidneys. These aquaporins increase the permeability of the collecting duct to water, allowing more water to be reabsorbed from the filtrate back into the blood.
3.Water Reabsorption by Osmosis
As water moves through the collecting duct, it is reabsorbed by osmosis. The surrounding medulla is hypertonic (has a higher solute concentration), which creates a concentration gradient that drives the movement of water from the filtrate into the bloodstream.
4.Blood Solute Concentration
This process helps to reduce the solute concentration in the blood, as more water is reabsorbed, diluting the blood’s solute content. As a result, less water is lost in the urine, leading to a smaller volume of more concentrated urine.
5.Negative Feedback
Once the blood’s solute concentration returns to normal levels (i.e., it becomes isotonic), the release of ADH is inhibited through negative feedback. This ensures that no further water is reabsorbed and that normal water balance is maintained.
(c)
Water is transported from the soil to the atmosphere in flowering plants through a process involving the roots, xylem, and leaves:
1. Roots
The plant roots take up water from the soil primarily by osmosis, where water moves from the soil (high concentration) into the root hair cells (lower concentration). Root hair cells are specialized structures that increase the surface area for water absorption.
In addition to osmosis, active transport of salts, ions, and minerals into the root increases the uptake of water, as the movement of these solutes into the root lowers the water potential, encouraging more water to enter.
2.Xylem
Once water enters the root hair cells, it moves into the xylem vessels in the root, which are narrow, hollow tubes responsible for transporting water up through the plant. As water is absorbed into the xylem, it is transported upward through the plant, from the roots to the leaves.
3.Leaves
Water reaches the leaves, where some is used in photosynthesis and the rest is lost through transpiration (evaporation) via small openings called stomata. This loss of water from the leaves creates a negative hydrostatic pressure within the xylem vessels, generating a transpiration pull, which pulls more water up from the roots.
4.Mechanisms of Water Movement
Transpiration pull: The loss of water from the leaves generates tension or suction that draws water upwards through the xylem.
Cohesion: Hydrogen bonding between water molecules allows them to stick together, maintaining a continuous column of water in the xylem vessels, preventing it from breaking.
Root pressure: The active transport of ions into the root xylem generates a positive pressure that can help push water upwards in the xylem, especially during periods of high water availability.
Capillary action: Water molecules adhere to the walls of the xylem vessels due to adhesion, allowing water to move upward through the narrow xylem vessels, even in the absence of active pressure. This process helps refill the xylem when needed.
5.Structural Adaptations
The xylem vessels are reinforced with lignin, a tough, woody substance, which strengthens the xylem walls and prevents the vessels from collapsing under the pressure of water movement.
Question 7
The identification of DNA as the genetic material, coupled with the discovery of its structure by Crick and Watson, had a revolutionary impact on modern biology.
(a) Explain why DNA replication is referred to as semi-conservative.
(b) Explain how sexual reproduction generates genetic variation.
(c) Describe how isolation contributes to the process of speciation.
Topic- D1.1.2
▶️Answer/Explanation
(a) DNA replication is described as semi-conservative because, during the process, each of the two resulting DNA molecules consists of one original strand (the conserved parental strand) and one newly synthesized strand. This method of replication ensures the preservation of genetic information while also creating new DNA.
1. Replication involves forming new strands from existing DNA: In DNA replication, the existing DNA strands serve as templates to form new complementary strands. This process occurs during cell division to ensure that the daughter cells receive identical copies of the DNA.
2. Nucleotides are added to exposed bases: As the two strands of the parental DNA molecule are separated by helicase, they expose their nucleotide bases. Free nucleotides in the cell are then added to these exposed bases, matching each base on the template strand with its complementary nucleotide (A pairs with T, and C pairs with G).
3. Complementary base pairing : The newly added nucleotides form complementary base pairs with the bases on the template strand. This is a key step in ensuring the accuracy of replication, as the base-pairing rule guarantees that the correct nucleotides are added to each strand. For example, if a template strand has an adenine (A), a thymine (T) will be added to the new strand.
4. Both strands (leading and lagging) act as templates: DNA has two strands that are oriented in opposite directions. Both strands — the leading strand (which is synthesized continuously) and the lagging strand (which is synthesized in fragments) — act as templates for the synthesis of new complementary strands. The replication machinery works on both strands simultaneously, ensuring that both new strands are synthesized from the existing template strands.
5. One original strand and one newly synthesized strand : After the replication process is complete, each of the two resulting DNA molecules consists of one original (conserved) strand from the parent molecule and one newly synthesized strand. This is why the replication process is called semi-conservative —each daughter DNA molecule conserves one of the original strands, ensuring accurate genetic transmission.
(b) Sexual reproduction generates genetic variation in offspring through several mechanisms that shuffle and recombine genetic material. The following processes contribute to this variation:
1. Meiosis Produces Haploid Gametes-
Meiosis is the process of cell division that results in the formation of haploid cells, called gametes (sperm and egg), each containing half the number of chromosomes as the original parent cell. In humans, this means that each gamete contains 23 chromosomes, while the parent cell contains 46 chromosomes. Meiosis also breaks up combinations of genes and alleles present in the parent cell, leading to genetic variation in the offspring.
2. Crossing Over and Chiasmata Formation-
During prophase I of meiosis I, homologous chromosomes (chromosomes that have the same genes but possibly different alleles) pair up in a process called synapsis. While they are aligned, crossing over (also known as recombination) occurs, where sections of chromatids are exchanged between homologous chromosomes. This exchange occurs at points called chiasmata, where the chromosomes physically exchange segments of genetic material. This process results in recombination, creating chromosomes with new combinations of alleles that were not present in the parents.
3. Independent Assortment of Chromosomes-
During metaphase I of meiosis I, the homologous chromosomes align randomly along the equator of the cell. The pairs of chromosomes are called bivalents. The way each pair of homologous chromosomes aligns and separates during anaphase I is random, which is known as independent assortment. This random orientation means that the maternal and paternal chromosomes of each pair are distributed independently into the daughter cells. As a result, different combinations of chromosomes can be inherited by the offspring, further increasing genetic diversity.
4. Random Fusion of Gametes during Fertilization-
After meiosis, when gametes are formed, fertilization occurs when a male gamete (sperm) fuses with a female gamete (egg). The specific sperm that fertilizes the egg is random, meaning that any sperm could potentially combine with the egg. This randomness in fertilization adds another layer of genetic variation, as it brings together alleles, genes, and genetic material from two different parents, resulting in offspring with a unique combination of genetic traits.
5. Random Mating and Genetic Diversity-
In many species, random mating further contributes to genetic variation. Any female can mate with any male , and during fertilization, the combination of gametes from two different individuals results in offspring with new genetic combinations. This further increases the diversity of traits in the population.
(c) Speciation- It is the process by which a new species forms from an existing one, often through the splitting of a population. This process depends on reproductive isolation, meaning populations can no longer interbreed, and their gene pools become separate. Over time, genetic divergence occurs due to mutations, genetic drift, and natural selection, leading to the populations becoming distinct species.
Types of Isolation Leading to Speciation:
1.Geographic Isolation: A physical barrier (e.g., a river or mountain) separates populations, preventing interbreeding. Over time, mutations and selection pressures in different environments cause genetic divergence. For example, Darwin’s finches on different islands evolved into distinct species due to geographic isolation.
2. Temporal Isolation: Populations breed at different times (e.g., different seasons or times of day), preventing mating. For example, two plant species may flower at different times of the year, preventing cross-pollination.
3. Behavioral Isolation: Differences in mating behaviors (e.g., mating calls or rituals) prevent populations from interbreeding. For instance, cricket species with different mating songs won’t recognize each other as mates.
Genetic Divergence and Adaptation:
When populations are isolated, allele frequencies diverge due to different selection pressures and mutations. This leads to evolutionary divergence, where populations adapt to their unique environments. Over time, this divergence can make populations so genetically distinct that they can no longer interbreed.
Polyploidy and Instant Speciation:
In some cases, polyploidy (having extra sets of chromosomes) can cause instant reproductive isolation, leading to new species. This is common in plants and can result in rapid speciation.
Question 8
Carbon is a fundamental element in living organisms, making up around 50% of all dry biomass, and is found in a diverse array of compounds.
(a) Describe how cellulose is formed from monosaccharides.
(b) Outline the reasons that climate change is a threat to coral reefs.
(c) Explain the process by which carbon dioxide is transformed into organic molecules during the light-independent reactions of photosynthesis.
Topic-C1.3.1
▶️Answer/Explanation
(a) Cellulose is a polysaccharide formed from β-glucose monosaccharides linked by β-1,4 glycosidic bonds through condensation reactions, which involve the removal of water molecules.
Explanation:
Cellulose consists of many β-glucose units joined by glycosidic bonds, specifically the β-1,4 type. These bonds are formed through condensation reactions, where water molecules are removed. The resulting polymer is unbranched, with only 1-4 glycosidic linkages between glucose units, which contributes to its linear structure.
The orientation of the glucose units alternates in an up-down-up pattern, giving the cellulose molecule a straight, rigid configuration. This alternating orientation is crucial for the molecule’s structural integrity. Moreover, multiple cellulose molecules can form cross-links between each other, creating microfibrils that further strengthen the overall structure. These microfibrils are what give cellulose its remarkable tensile strength and crystallinity.
Cellulose is a key structural component of plant and algal cell walls, providing strength and support for the plant. It also plays an important role in paper production. Humans cannot digest cellulose due to the absence of the enzyme that breaks down the β-1,4 glycosidic linkages. However, modified forms like oxidized cellulose can be hydrolyzed and broken down, which makes it useful in applications like tissue engineering.
(b) Climate change threatens coral reefs in several critical ways, driven primarily by increased greenhouse gases, particularly carbon dioxide (CO₂), which enhance the greenhouse effect and contribute to global warming. These effects disrupt the delicate balance of coral ecosystems, leading to long-term degradation.
1. Increased Greenhouse Gases and Global Warming
Enhanced Greenhouse Effect: The rising concentration of CO₂ and other greenhouse gases in the atmosphere intensifies the greenhouse effect, trapping more heat in the Earth’s atmosphere. This leads to global warming, causing ocean temperatures to rise.
Ocean Temperature Increase: As global temperatures increase, so do ocean temperatures. Coral reefs are highly sensitive to temperature changes, and even a slight rise in sea surface temperature can trigger severe ecological stress.
2. Coral Bleaching and Loss of Symbiotic Algae
Coral Bleaching: Higher ocean temperatures cause coral bleaching, a phenomenon in which corals expel their symbiotic algae called zooxanthellae. These algae are crucial to coral health, as they provide corals with nutrients through photosynthesis.
-Loss of Zooxanthellae: The expulsion of zooxanthellae leads to a reduced supply of nutrients for the coral, as the algae are no longer performing photosynthesis to feed the coral. Without these nutrients, corals become stressed and weakened.
-Bleaching Impact on Growth: The loss of zooxanthellae and the resulting nutrient shortage significantly reduce coral growth and reproductive rates. In severe cases, bleaching can kill corals if the stress persists for too long.
3. Ocean Acidification Due to Increased CO₂
CO₂ Dissolving in Oceans: Increased CO₂ in the atmosphere not only contributes to global warming but also dissolves into the oceans, where it reacts with water to form carbonic acid.
Lowered pH and Increased Acidity: The formation of carbonic acid lowers the pH of the ocean, making it more acidic. This process, known as ocean acidification, disrupts marine ecosystems, especially those that rely on calcium carbonate, like corals.
4. Impact on Coral Skeletons
Coral Skeletons Dissolve: The increased acidity in ocean water reduces the availability of carbonate ions, which are essential for corals to build their calcium carbonate skeletons. In more acidic conditions, coral skeletons dissolve, weakening the reef structure. Inability to Absorb Carbonate: As ocean acidification intensifies, corals are less able to absorb the necessary carbonate to form their skeletons, impeding their growth and making them more vulnerable to damage.
5. Compounding Stressors
Vulnerable to Storm Damage: Weakened corals, due to bleaching and acidification, are more vulnerable to damage from storms and physical disturbances, further accelerating reef degradation.
(c) The light-independent reactions of photosynthesis, also known as the Calvin cycle, occur in the stroma of the chloroplast and involve the conversion of CO₂ from the atmosphere into organic molecules. These reactions rely on the energy (ATP) and electrons (NADPH) produced in the light-dependent reactions. Here’s how the process works:
1. Carbon Fixation
The Calvin cycle begins with a 5-carbon compound, ribulose bisphosphate (RuBP), which is the starting point of the cycle.
CO₂ from the atmosphere is fixed by joining it to RuBP in a reaction called carboxylation. This step creates an unstable 6-carbon compound that immediately splits into two 3-carbon molecules of glycerate-3-phosphate (G3P).
This reaction is catalyzed by the enzyme RuBP carboxylase (commonly called Rubisco).
2. Reduction of G3P
The two G3P molecules are then reduced into triose phosphate (TP) using energy from ATP and electron from NADPH. During this process, NADPH is oxidized, providing the necessary hydrogen atoms (electrons and protons) for the reduction.
3. Formation of Organic Molecules
Some of the triose phosphate (TP) molecules are used to form glucose or other sugar molecules (e.g., fructose or starch) that serve as energy storage for the plant.
The rest of the TP molecules are used to regenerate RuBP, ensuring that the cycle can continue. The regeneration of RuBP requires additional ATP.