Section b questions and answers

Reproduction - this function of life is needed for living organisms to make new living organisms. Whether it is asexual or not, all living beings have a function which allows for the production of the organism’s offspring. Nutrition - the consumption of food which a living organism needs to gain energy. Energy is needed to carry out other living functions and other actions within the organism. Excretion - The creation of waste products within living organisms, which mainly comes from consumed food. However it can also be caused by other functions of life, for example growth, movement. This process is needed, because living beings need to excrete waste otherwise it may cause illness or even the death of the living being. Movement - the movement of living organisms without the need for outside help. For example the flow of material within the organism or external movement of the organism or parts of the organism.

Coughing to clear the airways is accomplished by muscle contractions. Explain muscle contraction.[9] (2017 HL 6c)

A muscle is made up of cells called muscle fibres. Each muscle fibre is a long, multinucleated cell, made up of long fibres called myofibrils. Myofibrils in turn are made up of repeating units called sarcomeres. Sarcomeres are units of overlapping protein filaments: thick myosin and thin actin. They are arranged end to end, and shorten during muscle contraction. A nerve impulse triggers the release of calcium ions from the sarcoplasmic reticulum. Calcium ions bind to troponin. The troponin causes movement of tropomyosin, which exposes myosin binding sites on the actin filaments. The hydrolysis of an ATP molecule causes the myosin heads to attach to the binding sites on the actin to form cross-bridges. The myosin heads shift positions, causing a power stroke which pulls the actin filaments towards the centre of the sarcomere. This causes the muscle to contract. (There should also be a drawing of a contracted and a not contracted sarcomere).

The structure of organisms is based on organic molecules containing carbon. Explain the cycling of carbon in an ecosystem.[7] (2018 SL 5c)

Animals and plants performing respiration release carbon dioxide into the atmosphere. Autotrophs then absorb the released carbon dioxide during the process of photosynthesis in order to create organic compounds/carbohydrates. The carbon compounds are transferred between trophic levels as consumers feed, only to be released again from dead matter during decomposition. If the respiration by organic matter is anaerobic, by cattle for example, then methane will be released with the help of methanogens. When methane is released into the atmosphere it is oxidized into carbon dioxide. An example of how large quantities of carbon dioxide can be released into the atmosphere in a short amount of time is on the occasion of a volcanic eruption.

Alternative answer

The cycle starts with autotrophs/producers absorbing carbon dioxide from the atmosphere/air/water. autotrophs then produce carbohydrates/organic compounds or in other words they perform photosynthesis. Carbon compounds then pass along food chains/trophic levels as food for the consumers in the ecosystem. Cellular respiration then releases carbon dioxide into the atmosphere/water. In other words, the carbon dioxide is realesed from dead matter by decomposition or as earlier mentioned respiration. Methane is being produced during anaerobic respiration of organic matter. Methane is then oxidized into carbon dioxide in the atmosphere. Fossil fuels were the made from partially decomposed organic matter. The combustion of fossil fuels releases carbon dioxide into the atmosphere.

Explain how a nerve impulse travels down a non-myelinated neuron.[9]

Sodium is found in greater concentrations outside of the cell while potassium is found in greater concentrations inside the cell. Sodium-potassium pumps exist in the plasma membrane to maintain the the concentration gradients and the membrane potential. Nerve impulses have a domino effect. An action potential in one part of the neuron causes another action potential in the adjacent part and so on. This is due to the diffusion of sodium ions between the region of the action potential and the resting potential. It is the movement of sodium and potassium that reduce the resting potential. If the resting potential rises above the threshold level, voltage gated channels open. Voltage gated sodium channels open very fast so that sodium can diffuse into the cell down its concentration gradient. This reduces the membrane potential and results in more sodium channels opening. Sodium ions are positively charged and so the inside of the cell develops a net positive charge compared to the outside of the cell. This results in depolarization as the potential across the membrane is reversed. A short while after this, voltage gated potassium channels open and potassium ions flow out of the cell down the concentration gradient. Since potassium ions are positively charged, their diffusion out of the cell causes a net negative charge to develop again inside the cell compared to the outside. The potential across the membrane is restored. This is called repolarization. Finally, the concentration gradients of both ions are restored by the sodium-potassium pump. Sodium is pumped out of the cell while potassium is pumped in. The resting potential is restored and the neuron is ready to conduct another nerve impulse.

Explain how the structure of the nephron and its associated blood vessels enable the kidney to carry out its functions. [8] (2016 HL 6c)

Redacted answer
Osmoregulation/excretion of nitrogenous waste is a function of the kidney. In the kidney, we have a nephron and this is responsible for the filtration of removing waste from the blood. The first part of the filtration takes place in the Bowman’s capsule more specifically the glomerulus. Smaller molecules are filtered out in the glomerulus as the glomerulus is permeable to smaller molecules. The second part of this process takes place in the proximal convoluted tubule. This pumps proteins to reabsorb specific solutes. The next process of this takes place in the loop of henle which is located in the medulla. The medulla is hypertonic which means that the concentration of salt is high. First, water is reabsorbed and permeable in the descending limb of the loop of henle and however, the ascending loop is impermeable to water and is instead is responsible for active transport of sodium ions out from the filtrate to medulla thus why it is hypertonic. The next part of the nephron is the distal convoluted tubule and this adjusts pH as well as the concentration of Na+/K+/H+. The last step is the collecting duct and the collecting duct permeability to water varies due to the number of aquaporins/ADH. Finally, the duct gathers the filtrate for preparation for excretion.

Markscheme answer:
Osmoregulation/excretion of nitrogenous waste/urea is a function of the kidney
Ultrafiltration in the glomerulus/smaller molecules filtered out in the glomerulus/ capillary walls/glomerulus permeable to smaller molecules
Basement membrane/filtration slits/podocytes act as filter OR prevent loss of large proteins/prevent loss of blood cells
high blood pressure in glomerulus due to larger afferent than efferent arteriole
Microvilli/coiling/convolutions give large surface area OR pump proteins to reabsorb specific solutes in proximal convoluted tubules
Water reabsorbed in descending limb of loop of Henle OR descending limb permeable to water
Active transport/active pumping of sodium ions/Na+ out of ascending limb from filtrate to Medulla
Ascending limb is impermeable to water
Loop of Henle creates solute gradient/high solute concentration/hypertonic conditions in medulla
Distal convoluted tubule adjusts pH/adjusts concentration of Na+/K+/H+
Water reabsorbed in collecting duct
Collecting duct permeability to water varies due to number of aquaporins/ADH
Osmoregulation by varying the amount of water reabsorbed in the collecting duct

Discuss the control of blood glucose levels and the consequences if they are not maintained.[8] (2019 SL 6c)

The pancreas produces hormones that control levels of glucose and if glucose levels in blood are high, beta-cells of the pancreas produce insulin which causes the cells to absorb glucose. The liver stores excess glucose as glycogen, if glucose levels in the blood are low, alpha-cells of the pancreas produce glucagon causing the liver to break down glycogen into glucose. Glucagon increases levels of glucose in the blood. Negative feedback controls the glucose levels and these are the consequences:
- If the pancreas produces little/no insulin a person can develop type I diabetes
- A person with type I diabetes usually is dependent on injections of insulin
- Type II diabetes occurs when the body becomes resistant to insulin
- Type II diabetes can sometimes be controlled by diet and exercise
- Thus the consequence is diabetes which usually lead to a shorter life

Outline the energy flow between trophic levels in a food chain.[6] (2014 HL 7b)

The original source of energy is the sun. The primary producers take up the sunlight, plants do so through photosynthesis, and use part of the energy obtained for their own energy requirements which is lost through cell respiration.
The energy travels between the trophic levels from primary producers to primary consumers to secondary consumers, and it goes on.
Though trough every trophic level the energy passes on to approximately 80-90% is lost through heat and feeces. Leaving only around 10-20% of the energy for the consumer to utilize. This is repeated and less and less energy is transferred to the higher trophic levels.

Alternative answer
The source of energy within a food chain comes from the sun(light). Autotrophs/producers on first trophic level “capture” the energy from sunlight and through processes such as photosynthesis the energy is converted to organic molecules. Part of that energy, plants also use for cellular respiration. In the second trophic level, consumers use energy from consumption of previous trophic level. Approximately only 10% of the energy from one trophic level is transferred to another tophic level, the rest is lost. Lost energy is in the form of heat. Within an ecosystem, energy isn’t recycled unlike nutrients.
MISSED POINTS:
Energy travels from producer to 1st consumer, from 1st consumer to 2nd consumer, etc.
Any decent diagram which shows energy travel between trophic levels
Not all material is consumed/assimilated/digested and lost in faeces.

Water is essential to life on Earth. Outline two properties of water that are important for living organisms. [4] (2013 HL 5a)

1-Water is a good solvent. Due to the polarity of water molecules, it can dissolve many substances (which are often referred to as water-soluble substances). The majority of chemical reactions in living organisms happen in different kinds of solutions. This is also one reason why water and solutions made of water are such good transport mediums.
2-Water has high heat capacity.
Large amounts of energy is needed to change the temperature and break the hydrogen bonds of water. This is important because it increases the stability of water-based habitats and because substances such as blood can effectively disperse heat throughout the body.

Describe how water is transported in angiospermophytes. [6] (2013 HL 5b)

Water moves from the soil up into the roots to the stem of the plant, after which it is transported to the leaves and other parts of the plant up through the transpiration stream via the xylem. The xylem is essentially plant vascular tissue and is made out of dead cells with highly lignified cell walls. As water from the cell walls of the leaves evaporates, evaporated water is replaced by water from the xylem. The reason why water can be transported upwards in this manner is due to several reasons. Firstly, the reason why the water collumn doesn't break is due to the very high cohesion of water molecules as well as the adhesion of water molecules to the xylem walls. In addition, the low pressure created at the top of the xylem makes it possible for the water to be transported upwards due to negative pressure.

Outline the process of in vitro fertilization. [8] (2011 HL)

So in IV fertilization drugs are used to stop the menstrual cycle, so that later FSH can be injected to stimulate many follicles to develop. To make the follicles mature HCG is injected then the eggs are extracted from the ovaries. Then sperm samples are collected and given a swim-up test. Then semen is mixed with eggs in a dish - fertilization occurs. The embryos are then placed in the uterus using a catheter (long plastic tube). It can be up to 4 embryos implanted. Pregnancy scan is used to see if the procedure was successful to prevent things like low sperm count or blocked oviduct. In some cases there is a need donor embryos in female infertility and the same goes for males- who need donor sperm .

Explain the process of genetically modifying bacteria (8) (2018 SL 6c)

Genetic modification carried out by a gene transfer between species, which is possible because genetic code is universal. When modifying a gene, mRNA for the required gene is extracted. Then, DNA copies from mRNA made from reverse transcriptase. PCR is used to amplify the DNA, and cut plasmids are mixed with the DNA copies. Next, bacteria that take up plasmids are identified. The bacteria will reproduce carrying the transferred genes, as seen in, for example, the production of human insulin using E.coli

Growth in living organisms includes replication of DNA. Explain DNA replication. [8] (2016 HL 7c)

Gyrase starts by untwisting the double helix. Helicase breaks the hydrogen bonds between the nitrogenous bases and therefore separates the two strands. The process of DNA replication is semi-conservative and acts as a template for the new strand. DNA primase adds RNA primer. DNA polymerase III adds nucleotides in a 5’ - 3’ adding guanine to cytosine and adenine to thymine . The leading strand that is 5’ - 3 runs continuously while the lagging strand that runs 3’- 5’, synthesises from 5’-3’ and then does backstitching, synthesizing the DNA in fragments. DNA Polymerase I removes the RNA primer and replaces it with the belonging nucleotide. Ligase unites fragments and seals the strand.

Explain how nerve impulses pass from one neuron to another neuron. [8] (2014 HL 5b)

When the impulse potential reaches the presynaptic membrane, calcium channels open and calcium diffuses into the neuron. Acetylcholine is then released into the synaptic cleft, it is then released into a receptor via exocytosis, causing ion channels to open and sodium enters the receptor. When depolarization occurs in the postsynaptic neuron, acetylcholine gets broken down by enzymes.

Explain the roles of specific hormones in the menstrual cycle, including positive and negative feedback mechanisms. [8] (2017 HL 7c)

The pituitary gland secrets the follicle-stimulating hormone which will stimulate the ovary so follicles can develop. These follicles will secrete oestrogen, another hormone, which in turn stimulates more FSH receptors to develop more follicles. A positive feedback loop. The oestrogen also encourages the development of the endometrium lining. Then the luteinizing hormone levels rise which results in ovulation, and the LH causes a negative feedback loop on the follicle cells and oestrogen production. The LH then encourage the follicle to develop into a corpus luteum, which starts producing progesterone. The progesterone thickens the uterus lining. High progesterone levels result in a negative feedback on the pituitary gland, preventing FSH and LH secretion. When the progesterone levels drop the FSH secretion is allowed, and the falling progesterone levels lead to menstruation and the break down of the lining in the uterus.

Explain the production of antibodies. [7] (2017 HL 6c)

Antibody production being the main function of the immune system makes antibodies necessary for the body’s resistance to infectious disease. The antibody production is carried out by B cells. Each antibody corresponds to an antigen, that stimulates the production of antibodies. When a pathogen enters the body it is detected as foreign to the immune system by the molecules on their surface. When the pathogen is detected Macrophages will digest the pathogen. The macrophage will then become an antigen-presenting cell. Helper T cells will recognize the antigen that is presented and activities. As the T helper cells have been able to recognize the antigen it can activate the b cell type that can produce antibodies specific to the antigen. The B cells will start to clone by mitosis to form plasma cells. These plasma cells will secrete antibodies that can fight the antigens.

In hot, dry conditions plants lose water rapidly due to transpiration. Explain how the structures and processes of the plant allow this water to be replaced. [8] (2016 HL 7c)

The water evaporated from the leaf via transpiration creates low pressure of water in the leaf. This causes the water to be drawn up the xylem to the leaf via adhesion. The cohesive property of water and the structure of the xylem vessels allow transport under tension. The adhesive property of water and evaporation generate tension forces in leaf cell walls. The water then enters the xylem through the roots via osmosis. Active uptake of mineral ions in the roots causes absorption of water via osmosis. Xerophytes are plants that grow in these conditions. They can preserve water by having a thick, waxy cuticle that reduces transpiration. Also, they can have small or reduced leaves that therefore reduce exposure to stomata and reduce the rate of transpiration. The leaves can also have small hairs that reflect sunlight and reduce transpiration. Also, these plants can have CAM physiology that explains when stomata open at night.

Explain how abiotic factors affect the rate of transpiration in terrestrial plants. [8] (2015 HL 7b)

Transpiration is the process in which gas exchange occurs in the leaves of a plant, the loss of water vapour.
Terrestrial plants are plants that grow on land.
Abiotic factors that can affect the rate of transpiration:

Wind: wind causes the movement of humid air surrounding the plant, this causes a greater difference in water concentration outside the plant, causing the increase of transpiration in an attempt to even out the concentration of H2O in the atmosphere and leaf.
Light: As light is available the stomata tend to open (they close during dark) causing the transpiration rate to increase.
Humidity: Higher humidity levels in the atmosphere are closer to the concentration of water in the leaf. Therefore the transpiration rate wouldn’t be as high if the atmosphere was “dry”.
Temperature: Heat causes evaporation of water vapour. Higher temperature increase the transpiration rate

Outline how ventilation in humans ensures a supply of oxygen. [4] (2018 HL 6b)

First off ventilation is the process of exchanging gases between the lungs and the air. To allow for this process to happen first the diaphragm contracts and lowers, which leads into the contraction of muscles. This then leads to the “new” air being brought into the alveoli with a “new” set of oxygen. The oxygen concentration within the alveoli is higher than the blood within the blood capillaries. This difference in concentration allows for the oxygen being brought in by the alveoli to diffuse into the red blood cells within the capillaries.

Outline the processes that occur during the first division of meiosis [6] (2015 HL 5a)

The first division of meiosis can be divided into 4 different steps named prophase, metaphase, anaphase and finally telophase. After the DNA replicates to prepare the cell for division, the homologous chromosome pair up to form a bivalent and crossing over between non-sister chromatids occurs. While the nuclear envelope breaks down, the homologous pair move to align to the equator of the cell. The microtubules attach to the centromeres causing the chromosomes to be pulled to the opposite poles and the nuclear envelops reform because of Meiosis II. The result of this first division is two haploid cells which will divide again before completing meiosis.

Discuss the role of genes and chromosomes in determining individual and shared character features of the members of a species. [7] (2017 HL 7a)

A mutation in a gene is a change in the nucleotide sequence of a DNA segment that codes for a specific trait. Alleles are alternative forms of a gene that code for the different variations of a specific trait. The gene for eye colour, for example, has alleles that encode various shades / pigments. The alleles vary only by one or a few bases from one another. Mutation forms new alleles. Dominant alleles determine traits even though there are recessive alleles present. Both alleles influence the characteristic with codominance. Since alleles are alternate variants of a single gene, they have gene sequences that are very similar. Not all genes in an organism are expressed.The number of genes found in an organism can vary between organisms and is not a valid biological complexity indicator. Genes from parents are inherited and passed onto offspring.
The position of a gene on a particular chromosome is called the locus. DNA is packed and organized into discrete structures called chromosomes. The number of chromosomes is a characteristic feature of members of a particular species. Nondisjunction refers to the inability of chromosomes to properly differentiate, resulting in gametes with one additional or one missing chromosome (aneuploidy). The resulting offspring will have additional or missing chromosomes in each cell of their body if a zygote is produced from a gamete that has undergone a nondisjunction event. Down syndrome is one of the disorders resulting from nondisjunction cases. Human sex determination occurs according to the X - Y system. There are two copies of the larger X chromosome in females. Males have chromosomes of one X and one Y (and hence determine gender in offspring). DNA can be shared between non-sister homologous chromatids when chiasmata are formed between bivalents in prophase I. This genetic material exchange is called crossing over and creates fresh variations of alleles on the chromosomes. These chromosomes that consist of genetic material from both homologues are called recombinant chromosomes. In haploid cells, crossing over results in new allele combinations and thereby increases the genetic diversity of possible offspring.

Draw an annotated diagram of the fluid mosaic model of membrane structure. [4] (2018 SL 5a)

Answer.

Explain chemiosmosis as it occurs in photophosphorylation [8] (2015 HL 7a)

Photophosphorylation is the production of ATP by utilizing light absorbed by the photosystems.
By means of photolysis, which is done by photosystem II, water is separated and an electron and a hydrogen ion are obtained (among other products). The electron is transferred through the thylakoid membrane through a series of electron carriers that also increase its energy using light. Part of this energy is used to transport protons into the inside of the thylakoid, buiding up a proton gradient. By diffusion, the hydrogen ions move through the ATP synthase, a protein complex located in the thylakoid membrane. The (electrochemical) energy of the movement of these hydrogen ions, in combination with Inorganic phosphate, generates ATP. We call this process of ATP synthesis through electron transport "chemiosmosis."

Cells go through a repeating cycle of events in growth regions such as plant root tips and animal embryos. Outline this cell cycle [4] (2017 HL 8a)

Mitosis is when a division if the cell that creates two identical daughter cells which consist of the stages: prophase, metaphase, anaphase and telophase. After the stages are complete the cell goes into cytokinesis and following that, interphase. Interphase is the phase where the cell goes through S, G1 and G2 which are growth phases in preparation for mitosis.

Alternative answer
The cell cycle can be divided into four parts: G₁, in which the cell grows and organelles duplicate, S, in which DNA replicates, G₂, in which the cell grows and prepares for mitosis, and M, which is the mitosis itself.
Mitosis is the process of the nucleus dividing into two identical daughter nuclei, and it consists of prophase, metaphase, anaphase and telophase. The actual division of the cytoplasm goes right after mitosis and is called cytokinesis.
G₁, S and G₂ are called interphase, so basically everything in between cell divisions when the cell is metabolically active.

Describe the processes involved in absorbing different nutrients across the cell membrane of villus epithelium cells lining the small intestine. [4] (2018 SL 5b)

The process of absorbing different nutrients can come in all sorts of ways such as diffusion, facilitated diffusion, and active transport. Fatty acids are absorbed by diffusion, nutrients move down a concentration gradient. Certain monosaccharides, such as fructose, are absorbed by facilitated diffusion, the nutrients enter the villi from protein channels. Amino acids are absorbed by active transport as the nutrients are absorbed against concentration gradients involving protein pumps.

Describe the evolution of antibiotic resistance in bacteria. [4] (2018 SL 6b)

The resistance towards antibiotics within bacteria is often due to a genetic variation within that bacteria population. This genetic variation can be the result of a genetic mutation and allows for a population of bacteria to be resistant to a specific antibiotic. This resistance means that the bacteria, which don’t have it, are vulnerable to the antibiotic and can’t commit to the living function which is reproduction. Obviously the bacteria with resistance outlive the one’s without when in presence of the antibiotic. This means that the resistant bacteria overtake the population and replace all the bacteria without the mutation.

a. antibiotic resistance exists as a genetic variation (within the population) - DONE
b. (antibiotic resistance) may occur from transfer of genetic material - MISSED
OR
(antibiotic resistance) may occur through mutation - DONE
c. resistance is specific to one antibiotic - DONE
d. only bacteria with resistance gene reproduce in the presence of antibiotic - DONE
e. frequency of resistant bacteria increases in population - DONE
f. resistant population replaces non-resistant over time - DONE

Explain the processes involved in the Krebs cycle [8] (2014 8c)

Krebs cycle only occurs during aerobic respiration in the mitochondria. Decarboxylation is the one of the primary processes in the krebs cycle, it involves the removal of CO2 and results in the release of energy in the form of ATP. For each instance it takes place, NADH(H+) and CO2 are formed. Acetyl CoA from the link reaction releases an acetyl group. This acetyl group is joined to a 4-carbon molecule to form a 6-carbon molecule, citrate. Afterwards, decarboxylation changes citrate to a 5-carbon molecule, thereafter to glutamate: a 4-carbon molecule. then it’s converted into the original 4-carbon molecule oxaloacetate and the cycle repeats itself.

Distinguish between aerobic and anaerobic respiration [5] (2014 8a)

The basic gist of the two are that they are near opposites of each other, hence “aerobic” and “not aerobic”. Thus: Aerobic respiration requires O2, uses the krebs cycle, uses mostly fats and proteins, has a high yield of ATP and its end products are CO2 and H2O. On the other hand anaerobic respiration doesn’t require O2, doesn’t utilise the krebs cycle, uses mostly sugars, has a low yield of ATP and produces CO2 and ethanol in plants/fungi and lactate in animals as end products.

Outline the control of metabolism by end-product inhibition. [5] 2019 HL 5b)

The metabolic pathway is a chain of enzyme-catalyzed reactions, which can be controlled by the end product inhibition, a form of negative feedback. In the end-product inhibition, the final product of a chain of reactions inhibits an earlier enzyme in the metabolic pathway. The product binds to an allosteric site and temporarily inactivates the enzyme (changes the shape of active site) through non-competitive inhibition. The enzyme can no longer function, the rate of product formation will decrease consequently leading to less enzyme inhibition. The metabolic pathway restarts when the end-product detaches.

Outline the roles bacteria play in the carbon cycle. [3] (2018 SL 6a)

Within the carbon cycle bacteria carry out quite a few roles. One of the more obvious roles of bacteria within the cycle, is their decomposition of dead organic material. This decomposing process is done by saprotrophic bacteria, which turn dead organic material into peat to form more aqueous soils. A different type of bacteria is also what allows for the stabilizing of CO2 into photosynthesis.

Mark Scheme:
a. decomposition of dead organic material «by saprotrophic bacteria» - DONE
b. «decomposition» leads to CO2 formation/regeneration due to respiration - MISSED
c. «saprotrophic bacteria only» partially decompose dead organic matter in acidic/anaerobic conditions in waterlogged soil - DONE
d. results in peat formation in bogs/swamps - DONE (didn’t mention swamps)
e. photosynthetic bacteria/cyanobacteria fix CO2 in photosynthesis - DONE (didn’t mention the name of the bacteria)

Discuss alternative models of membrane structure including evidence for or against each model. [8] (2018 HL 5a)

Answer.

Outline the role of condensation and hydrolysis in the relationship between amino acids and polypeptides [4] (2013 HL)

Condensation is two molecules joining together via a covalent bond, releasing a water molecule in the process. Through condensation, many amino acids can be joined together to form polypeptides. Hydrolysis is the addition of water to break large molecules into smaller molecules. Via hydrolysis, polypeptides can be broken down into dipeptides or amino acids.

Explain how evolution may happen in response to environmental change with evidence from examples. [8] (2015 5c)

Natural selection can only occur if there is variation among members of the same species. Mutation, meiosis and sexual reproduction cause variation between individuals in a species. For example, a person's skin colour, hair colour, dimples, freckles, and blood type are all examples of genetic variations that can occur in a human population. Species tend to produce more offspring than the environment can sustainably support. For example, cheetahs and lions both feed on the same prey; they compete for this resource. Therefore, if they live in the same area, one or both species will have less food leading to an increase in the mortality rate. Adaptations are features of organisms that aid their survival by allowing them to be better suited to their environment. Organisms with beneficial adaptations will be more likely to survive long enough to reproduce and pass on these genes. If environmental conditions change, what constitutes a beneficial or detrimental trait may change, and thus the allele frequencies in a population are constantly evolving. Evolution is the cumulative change in the heritable characteristics of a population.
Markscheme answer
A. Variation in population;
B. (variation is) due to mutation/sexual reproduction;
C. valid example of variation in a specific population;
D. more offspring are produced than can survive / populations over-populate;
E. competition / struggle for resources/survival;
F. example of competition/struggle for resources;
G. survival of fittest/best adapted (to the changed environment)/those with beneficial adaptations/converse;
H. example of changed environment and adaptation to it;
I. favourable genes/alleles passed on / best adapted reproduce (more) /converse;
J. example of reproduction of individuals better adapted to changed environment;
K. alleles for adaptations to the changed environment increase in the population;
L. example of genes/alleles for adaptations increasing in a population;
M. evolution by natural selection;
N. evolution is a (cumulative) change in population/species overtime /change in allele frequency;

Outline the process used to load organic compounds into phloem sieve tubes. [3] (2018 HL 5c)

Compounds located at the source are actively loaded into the phloem sieve tubes through companion cells. Materials can be pumped across the interviening cell wall by memrane proteins called apoplastic loading, a process which requires ATP. Protons are transported out of the phloem through protein pumps, creating a high hydrogen ion concentration gradient. Protons diffuse passively into the phloem via co-transport protein, requiering sucrose movement. Sucrose is then built up in the phloem sieve tubes.

Explain the stages of aerobic respiration that occur in the mitochondria of eukaryotes. [8] (2018 HL 6a)

Firstly, in the link reaction, pyruvate is converted into acetyl CoA via decarboxylation. For each pyruvate, 2 CO2 molecules are released. Afterwards in the Krebs cycle itself, a 4-carbon molecule is combined with acetyl CoA . Each decarboxylation also, among other things, generates ATP. Across the whole Krebs cycle, 3 reduced NAD molecules and 1 reduced FAD molecule are produced. These molecules are then carried to the inner membrane of the mitochondria. The transfer of electrons between these molecules in the electron transport chain in the membrane of the cristae is coupled to proton pumping. The protons accumulate in intermembrane space and an electrochemical gradient between intermembrane space and matrix is established, where the protons diffuse through ATP synthase to generate ATP.

Outline the role of the parts of an alveolus in a human lung. [4] (2018 SL 6a)

Alveoli are the small sacks of air that can be found in the lungs of mammals. An individual alveolus is made up of two different types of cells, type I pneumocytes that work to optimize the gas exchange, and type 2 pneumocytes that produce and secrete a surfactant in order to reduce the surface tension in the lungs, hence, preventing the alveolus from collapsing. Another type of cell found in the alveoli is alveolar macrophages that help the immune system by removing debris and cleaning up particles not caught by the mucus and cilia in the respiratory tract. The very thin spherical walls of an alveolus allow a maximized gas exchange, and in order to make the gas exchange to the blood efficient, each alveolus is enclosed in capillaries.

Many diseases are caused by bacteria and other pathogens. Explain, using examples, how other factors can lead to disease in humans. [7] (2018 SL 6c)

According to the markscheme this is an open-ended question, so it's a bit tricky:
For any example of non-pathogenic disease, two of these factors must be described:
- Name of the disease
- Type (genetic, lifestyle/habits, environmental, psychological...)
- Symptoms/description
- Cause
Examples to do: (two full examples would give max points +1)
Sickle cell anemia
COPD
Asbestosis
Lupus
Asthma
Coronary artery disease

Outline how light intensity and concentration of carbon dioxide affect photosynthesis (6) (2014 HL 6b)

Low light intensity affects light-dependent reactions, this causes the photosystems to work slower and therefore the rate of photolysis, NADPH production and ATP production are reduced. Low carbon dioxide concentration affects the Calvin cycle; RUBISCO will have less carbon dioxide available and its reaction (of the latter) with ribulose biphosphate form glycerate 3 phosphate will slow down, making the overall cycle take place at a lower rate.

Draw a molecular diagram of an amino acid to show its general structure. [3] (2017 SL 5a)

Answer.

Outline the role of ribosomes in translation. [4] (2017 SL 5b)

ribosomes are composed of two subunits, large and small. During translation ribosomal subunits assemble together on the strand of mRNA, where they proceed to attract tRNA hence both trna and mRna attach to the ribosome. the ribosome moves along the mRNA A chain of amino acids emerges as the ribosome decodes the mRNA sequence into a polypeptide or protein.

Explain the roles of named types of blood proteins in different defence mechanisms. [8](2017 SL 5c)

Answer.

Describe the structure and function of starch in plants. [3] (2017 SL 6a)

Starch is a polysaccharide as it is composed of many sugar molecules linked together (a-glucose subunits). It has a helical shape as the orientation of its glucose subunits is the same. This polysaccharide functions as an energy storage found in plants.

Outline the production of carbohydrates in photosynthesis. [4] (2017 SL 6b)

In the light phase (light-dependent reactions), chlorophyll absorbs light that is converted to chemical energy in the form of ATP and NADPH. In this process, water is split and oxygen is produced as a byproduct.
In the light-independent phase energy from ATP and NADPH is invested in a cycle that fixates CO₂ from air or water and produces carbohydrates

Describe the role of oxygen in aerobic cell respiration [5] (2018 HL 6b)

Oxygen is the final electron acceptor at the end of the electron transport chain, allowing further electrons to enter the chain. Oxygen accepts electrons from the final carrier protein, as it is highly electronegative, and produces water, as shown in the following chemical equation:
½O2 + 2e- + 2H+→ H2O
This helps to maintain a proton gradient across the inner membrane of the mitochondria by the removal of protons from the stroma. Through this gradient electrons can flow. Therefore, as long as oxygen is present, there will be a constant movement of electrons through the electron transport chain.

Discuss the processes in the carbon cycle that affect concentrations of carbon dioxide and methane in the atmosphere and the consequences for climate change. [8] (2017 SL 6c) (Warning: this one looks quite likely)

Within the carbon cycle CO2 is produced within the respiration carried out by organisms, as well as the combustion of fossil fuels. Fossil fuels, like peat, are obtained from biomass, which is dead organic matter. Before CO2 is oxidized it is CH4 which is produced by the respiration of biomass. The CO2 is then converted into organic compounds by producers through photosynthesis. When greenhouse gases such as CO2 and CH4 are released into the atmosphere at a rate higher than the one in which photosynthesis turns them into organic compounds, the greenhouse effect occurs. This means that the gases, which are now in the atmosphere, absorb the radiation reflected from the sun by Earth, and thus raise the heat of the planet. This would mean that the more fossil fuels combusted, the more CO2 is moved into the atmosphere, and thus the higher the temperature on Earth. CH4 is also involved in a feedback loop, which involves the defrosting of the tundra. When the tundra is defrosted, CH4 is released into the atmosphere and that leads to even more defrosting occurring due to the raised heat. Both these gases and their raising of temperature leads to more extreme and outlying climates throughout the world.
Mark Scheme:
a. CO2 is produced from respiration in organisms/combustion of biomass/fossil fuels - DONE
b. CH4 is produced by anaerobic respiration of biomass/«methanogenic» bacteria - DONE
c. CH4 is oxidized to CO2 and water - DONE
d. CO2 is converted into carbohydrates/organic compounds by autotrophs/producers/photosynthesis - DONE
e. CO2 can be converted to calcium carbonate/fossilized into limestone - MISSED
f. «partially» decomposed organic matter/biomass can be converted into peat/coal/oil/gas/fossil fuels - DONE (missed “partially”)
g. CO2 and CH4 are both greenhouse gases/increase greenhouse effect - DONE
h. both absorb long-wave radiation from the earth and retain the heat in the atmosphere - DONE (missed “long-waved”)
i. increased CO2 concentrations in the atmosphere correlate with increased combustion of fossil fuels - DONE
j. rising average global temperatures correlate with more greenhouse gases in the atmosphere - DONE
k. cattle production/rice paddy/defrosting of tundra increase CH4 in the atmosphere - DONE (missed “cattle production/rice paddy”)
OR
increasing CO2 leads to acidification of marine/aquatic environments - MISSED
l. the global temperature increase influences/disrupts climate patterns - DONE

Explain how chemical energy for use in the cell is generated by electron transport and chemiosmosis. [8] (2017 HL 5a)

Answer.

Outline four different functions of membrane proteins. [4] (2017 HL 5b)

Some functions that membrane proteins carry out are; the transport of electrons, they attach to the cytoskeleton and extracellular matrix for stability,and also allow a cell to interact with another cell. Cell to cell recognition, is another function of membrane proteins, which is possible due to cellular adhesion. Cellular adhesion is another function of membrane proteins, and is the process through which cells attach to neighbouring cells. Membrane proteins also allow for channels to conduct “passive transport” more specifically facilitated diffusion. Put simply, this means that membrane proteins allow for the movement of different substances across the membrane, without the use of energy needed. Membrane proteins also allow the movement of molecules across the membrane depending on the concentration of different sections of the membrane.
Mark Scheme:
a. receptor/binding site for hormone/neurotransmitter MISSED
b. cell-to-cell communication / cell recognition DONE
c. channels «for passive transport» / facilitated diffusion DONE
d. pumps / active transport DONE
e. cell adhesion DONE
f. «immobilized» enzymes/enzymes embedded in the membrane MISSED
g. electron transport / electron carriers DONE

Distinguish between anabolism, catabolism and metabolism. [3] (2017 HL 5c)

Metabolism includes both anabolism and catabolism within it, and is used as a broad term to refer to all the reactions that take place in a cell/organism. Anabolism is the building and growth of larger and more complex molecules using smaller molecules. Catabolism is the breaking down of larger and more complex molecules into smaller molecules.
Mark Scheme:
a. metabolism is all enzyme-catalyzed reactions in a cell/organism/is anabolism plus catabolism DONE (but missed “enzyme-catalyzed”)
b. anabolism is synthesis of polymers/complex/larger molecules/larger substances «from smaller molecules/monomers» DONE
c. catabolism is breaking down «complex» molecules/substances «into simpler/smaller ones/into monomers» DONE

Alternative answer:
Metabolism is a set of all enzyme-catalyze reactions in a living organism which include anabolism and catabolism. Anabolism is the synthesis of large molecules from smaller molecules whereas catabolism is breaking down complex molecules into smaller ones

Describe the functioning of immunoglobulins. [3] (2017 HL 6a)

Immunoglobulins function as antibodies. Immunoglobulins attract phagocytes/macrophages to engulf pathogens. There are a variety of binding sites, immunoglobulins bind specifically to one or a few closely related antigens on bacteria/viruses and pathogens. This aids the destruction of the bacteria/viruses and pathogens.

Outline how antibiotics offer protection from certain forms of infectious disease. [4] (2017 HL 6b)

Answer.

Outline the process of speciation. [4] (2017 HL 7b)

Speciation is a process where one species diverge into two different species. This can occur due to three different reasons,
geographic isolation- two populations of two species cannot interact with each other
temporal isolation - They are not active at the same time, such as the times they reproduce.
Behavioural isolation - They don’t mate with each other.
If one population separates, the gene pool of each population will be different than the original gene pool of the original population. Fruits for example. If the seed of apples originating from a certain geographical location is transported by human means to another country, The apples will adapt their gene pool to the “new” environment. For example, the apples that grow bigger and lighter will be specifically chosen by humans, and their seeds will be grown. And the alleles that gives bigger and lighter apples will be a majority in the gene pool. Overtime the apples will have become a new species that are bigger and lighter than the apples that hadn’t been moved. This is a gradual process.
Markscheme answer:
a. speciation is the splitting of a species «into two species»
b. reproductive isolation/lack of interbreeding
c. isolation due to geography/«reproductive» behavior/«reproductive» timing
d. polyploidy can cause isolation
e. gene pools separated
f. differences in/disruptive selection cause traits/gene pools to change/diverge
g. gradualism / speciation/changes accumulating over long periods
h. punctuated equilibrium / speciation/changes over a short time period

Describe, using one example, how homologous structures provide evidence for evolution. [4] (2017 HL 7c)

Homologous structures have similar structures that evolved from a common ancestor. The process of a structure adapting to different function is called adaptive radiation, and one example of it is the pentadactyl limbs; limbs with five digits that nevertheless have very different functions. Lastly, similar bone structure like forelimbs with two bones and hindlimbs with only one are also an example.

Draw molecular diagrams to show the condensation reaction between two amino acids to form a dipeptide. [4] (2017 HL 6a)

Outline the roles of the different binding sites for tRNA on ribosomes during translation. [4] (2017 HL 6b)

The ribosome has three binding sites which are A (aminoacyl), P (peptidyl), and E (exit). The A binding site is the one which binds to the aminoacyl tRNA, which contains the new amino acid to be added to the polypeptide chain. The P binding site is the one that attaches to the tRNA holding the polypeptide chain. The polypeptide chain at the P site is then transferred to tRNA in the A site, and is later attached to the amino acids by a peptide bond. The tRNA, which doesn’t contain amino acids any longer, is let go from the ribosome through the E binding site.
Mark Scheme:
a. A, P and E binding sites are on the large subunit of the ribosome DONE
b. initiation of translation starts with binding of met-tRNA to the start codon MISSED
c. large sub-unit binds with «start» tRNA in the P site MISSED
d. A binding site holds the tRNA with the next amino acid to be added DONE
e. peptide bond is formed between the amino acids of the A site and the polypeptide at the P site DONE
f. polypeptide is transferred to the tRNA in the A site DONE
g. the tRNA «with polypeptide» in A site then moves to P site MISSED
OR
P binding site holds the tRNA attached to the growing polypeptide DONE
h. E binding site «exit» is where the tRNA «from P site without amino acid» leaves the ribosome DONE

Outline how reproductive isolation can occur in an animal population. [3] (2017 HL 7a)

Reproductive isolation results from three different type of isolations:
⁃ temporal isolation which refers to incompatible time frames that prevent populations from encountering each other
⁃ geographic isolation created by a population being separated by a barrier, such as a river or mountain, and not being able to contact
⁃ behavioural isolation when one population’s habits are not compatible with those of another population

Describe the different cell types in the seminiferous tubules that are involved in the process of spermatogenesis. [4] (2017 HL 7b)

Describe the different cell types in the seminiferous tubules that are involved in the process of spermatogenesis. [4] (2017 HL 7b). spermatogonia matures and divides by mitosis into primary spermatocytes
primary spermatocytes is divided by meiosis I into secondary spermatocytes
secondary spermatocytes is divided by meiosis II into spermatids
spermatids differentiate into spermatozoa
Sertoli cells provides nourishment to these developing cells . interstitial cells produce testosterone

Draw a labelled diagram of the formation of a chiasma by crossing over. [3] (2017 HL 8b)

Answer.

Outline the roles of helicase and ligase in DNA replication. [4] (2019 7a)

Helicase uncoils the two strands of the DNA by breaking hydrogen bonds between base pairs. Ligase joins the okazaki fragments by joining sugar-phosphate backbones to form a continuous brand.

Markscheme answer:
helicase:
a. unwinds/uncoils the DNA «double helix»
️b. breaks hydrogen bonds «between bases» ✔️
c. separates the «two» strands/unzips the DNA/creates replication fork ✔
️ ligase: d.
seals nicks/forms a continuous «sugar-phosphate» backbone/strand ✔️
e. makes sugar-phosphate bonds/covalent bonds between adjacent nucleotides ✔️
f. after «RNA» primers are removed/where an «RNA» primer was replaced by DNA ✔️
g. «helps to» join Okazaki fragments ✔️
️

Explain the control of gene expression in eukaryotes. [8] (2017 HL 8c)

In Eukaryotes, mRNA conveys genetic information from the DNA to the ribosomes, where the mRNA guides polypeptide production. Most genes are turned off, that is, not expressed, or are only expressed at certain times. Others are only expressed in certain cells. For genes to be expressed the production of specific mRNA is required, so anything that regulates this production (transcription) regulates the expression. This can be hormones or transcription factors (proteins that attach to the DNA and may enhance or prevent the binding of RNA polymerase). Nucleosomes also limit and control access of transcription factors, and through that control of production of the mRNA, and thereby gene expression. Gene expression can also be regulated during the post transcriptional processing (removing introns and such) since some introns may contain regulators that block the process.
Finallly, gene expression can be affected by DNA methylation. Through the addition of a methyl group to the DNA, the methylated genes are silenced. Some DNA methylation patterns are inherited.

Outline how translation depends on complementary base pairing. [3] (2017 SL 5a)

Answer.

Describe the polymerase chain reaction (PCR), including the role of Taq DNA polymerase. [4] (2017 SL 5b)

The polymerase chain reaction (PCR) is a reaction that allows for a small sample of DNA to be amplified/copied multiple times. The process consists of three stages (denaturing, annealing, extending). During the first step, the sample of DNA is heated up in order to separate the double helix into two single strands. Then, the temperature is decreased and primers attach to the strands, initiating polymerisation. Next, an enzyme called TAQ polymerase synthesizes two new strands of DNA, using the original strands as templates. TAQ polymerase is used because it has special properties which enable it to withstand high temperatures without denaturing. Therefore, it can work effectively despite the high temperatures used for the PCR method. Once TAQ polymerase has attached new nucleotides to the DNA strands, two double-stranded sequences of target DNA have been formed.

Explain benefits and risks of using genetically modified crops for the environment and also for human health. [8] (2017 SL 5c)

Genetically modified crops have been ‘improved’ by genetic engineering for different reasons which can have both positive and negative effects on the environment and human health. For example, some crops are genetically modified to have higher nutritional value and increase vitamin levels whilst staying safe for consumption. Also, crops can be produced to rid of allergens and toxins which is also a benefit for human health and can produce toxins which are only harmful to pests which can reduce land clearing and the carbon footprint. GMC’s can also increase crop yield which is a benefit for the environment. On the other hand, there are more risks when it comes to genetically modified crops than benefits, for instance, by introducing proteins and vitamins that were missing this could cause adverse health reactions for consumers, as well as having negative effects towards other organisms that already inhabit the ecosystem. Moreover, GM crops could reduce biodiversity as they compete with plants indigenous to a region. Another risk that could happen over time is GM crops with pest toxins could cause resistant pest species to evolve which is bad for the environment.

Explain how the small intestine moves, digests and absorbs food. [8] (2017 SL 6a)

The small intestine is where the final stages of digestion of macromolecules occur. The contraction of circular and longitudinal muscle of the small intestine mixes the food with enzyme and moves it along the gut. The first part of the small intestine is called the duodenum. Digestive enzymes secreted by the pancreas get mixed with the food from the stomach in the duodenum, encouraging the breakdown of large molecules such as starch and proteins. Bile, which is secreted by the liver, is also active in the duodenum by breaking down fats. The middle part of the small intestine is the jejunum, which is where most of the carbohydrates, fats, minerals, proteins and vitamins are absorbed. The last part of the small intestine is called the ileum, which is where the final stages of digestion take place. The ileum absorbs bile acids, fluid, and vitamin B-12. Finger-shaped structures called villi line the entire small intestine, helping the absorbtion of nutrients.

Distinguish between the structures of the different types of fatty acids in food. [4] (2017 SL 6b)

Fatty acids are saturated or unsaturated. Saturated fats have no double bonds, and mono/polyunsaturated have one or more double bonds. If a double bond has hydrogen on the same side of the bond, then it's a cis-fatty acid (this nomenclature is old and not the recommended by the IUPAC). The trans-form has hydrogen atoms on opposite sides of the carbon double bond. The hydrogen and carbon chain can vary in position/ the position of the carbon double bond can vary.
Alternative answer
There are different types of fatty acids, they can be saturated or unsaturated, unsaturated fatty acids can be distinguished further as monounsaturated or polyunsaturated. The different types can be told apart by looking for double bonds, the lack of double bonds lets you know that it is a saturated fatty acid, while the presence of at least one double C=C bond lets you know that it is an unsaturated fatty acid. If the fatty acid is bent at the double carbon bond, this indicates cis form, if there is no bend at the double carbon bond then this indicates trans form. The length of the hydrocarbon chain that makes up the fatty acid can vary between different fatty acids.

Outline how leptin controls appetite. [3] (2017 SL 6c)

Leptin is a hormone that suppresses appetite. It is secreted by adipose tissue. The target cells for leptin are in the hypothalamus. This causes the hypothalamus to control and inhibit appetite. The level of this hormone is controlled by the amount of adipose tissue, if the amount of adipose tissue increases, blood leptin concentration rises.

Explain how evolution occurs and which factors can cause the process to be rapid. [7] (2018 HL 7)

Natural selection causes a change in the heritable characteristics of a species. Natural selection “selects” the individuals from a population that are more likely to survive and produce offspring. For example, rabbits, If offsprings of a rabbit is a lot this is called overpopulation because the environment can’t support all the rabbits. So the rabbits that have inheritable advantages such as bigger ears are more likely to survive predators because they are better observers. The “weaker” rabbits will die and their offspring won’t be as many or none at all compared to the rabbits with larger ears that survives. The population of the rabbits are over time stronger and more likely to live in an environment with predators. Another example of surviving traits are peacocks, despite their happy colour that could attract predators the alleles for colourful feathers have survived. This is because the more colourful feathers are more attractive to the female peacocks and therefore they are more likely to produce offspring that carries the alleles for colourful feathers. In this way, the genepool will always have a majority of alleles for colourful feathers.
Markscheme answer:
a. evolution is a change in the heritable characteristics «of a species»
b. natural selection «causes evolution» ✔️
c. overpopulation/over-reproduction/more offspring «than the environment can support» ✔️
d. competition «for resources/mates» ✔️
e. variation in population/species ✔️
f. mutation/meiosis/sexual reproduction contributes to variation ✔️
g. adaptation increases chance of survival ✔️
h. reproduction/offspring produced «by the better adapted/by those that survive» ✔️
i. characteristics passed to offspring by reproduction/variation is heritable ✔️
j. allele frequencies/number of organisms carrying a gene changes/gene pool changes ✔️
k. environmental change stimulates/triggers/speeds up natural selection/evolution ✔️
l. increase in rainfall/introduction of antibiotic/pollution on tree trunks/other valid example of environmental change/new selection pressure ✔️
m. artificial selection/selective breeding can speed up evolution ✔️

Explain how natural selection can lead to speciation (7) (2019 HL 7b)

Variation is required for natural selection in species. Mutations/meiosis and sexual reproduction are all sources of variation. When this happens, competition can occur between the species as there is more offspring than the environment can support. Sometimes the environment or the other organisms will be too much for some organisms and so they will die, those that don’t due must adapt so the species are better suited to their environment, this leads to the survival of the fittest and more reproduction. If a population is divided and isolated from each other the population can’t interbreed. Each new population will face a different environment. With time, each population will adapt to their new environment and be genetically different so the 2 populations can't interbreed. This will lead to speciation, as it is the formation of a new species becoming separate species.

Explain how meiosis results in an effectively infinite genetic variety of gametes. [8] (2013 HL 5c)

Meiosis starts with a cell that contains one homologous chromosome from the mother and one from the father. In prophase I, they become homologous chromosomes pairs by crossing over and forming intersections called chiasma. When the chiasma is resolved each chromosome ends up with a chunk of the other meaning new alleles are added to the chromosome. Crossing over can occur at many points and at random positions which creates genetic variation as each chromosome that ends up in the gamete is not identical to any chromosome the parent initially had. During metaphase I, a random orientation of bivalents also occurs because the chromosomes move to opposite pole (breaking the chiasma) and sister chromatids are assorted randomly giving rise once again to additional genetic variation. By the end of Meiosis, there are four genetically different gametes.

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