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Examples of aqa biology essays

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Anyone got aqa a level physics papers. Pdf examiners will discuss a range of specimen scripts at the standardising meeting that help them to make this decision. For example, in the cytoplasm, when triose phosphate molecules are oxidised to pyruvate as part of glycolysis, the coenzyme NAD is reduced forming reduced NAD. The coenzyme forms part of the active site of the dehydrogenase enzyme allowing it to function as a catalyst and be reformed.

The ATP formed as part of respiration is used in a wide variety of contexts in biology. For example in order for an animal to move and hunt for food within its environment, it has to contract its muscle tissue. The tissue is composed of cells containing actin and myosin filaments which move relative to each other to contract a sarcomere. For this to happen, actomyosin cross-bridges form between the actin and myosin.

Once activated by calcium ions, the enzyme ATPase then hydrolyses ATP to ADP and Pi releasing energy for the detachment and formation of more cross-bridges, giving rise to the sliding filament theory of muscle contraction. This enzyme also helps release energy from ATP in a wide variety of contexts, such as in the active transport of sodium ions out of an axon through sodium-potassium cation pump in the generation of a resting potential, or in the active transport of nitrate ions into a root hair cell to lower water potential to draw in water to generate a root pressure.

This essay has established that enzymes are fundamental biological molecules which offer a diverse range of functions to living organisms. Often the function of these organs requires the movements of materials across the membranes of the cells of which they are composed. This essay will describe the part played by the movement of specific substance in the functions of different organs and organ systems. The cell surface membrane is a plasma membrane composed of a phospholipid bilayer.

It acts as a hydrophobic barrier that prevents the passive diffusion of hydrophilic species such as glucose and amino acids into the cell. Hydrophilic channel proteins are embedded in the bilayer which provides a route by which polar substances can enter, either down a concentration gradient by facilitated diffusion or against a concentration gradient by active transport.

The relative movements of the lipid molecules together with the random arrangement proteins give rise to the term the fluid mosaic model of the cell surface membrane. Non-polar molecules such as fatty acids, oxygen and carbon dioxide are able to dissolve directly through the membrane and enter the cell by diffusion. This process is used in the lungs whose function is the gas exchange of carbon dioxide and oxygen across the epithelium of the alveoli.

Contraction of the intercostal muscles and the flattening of the diaphragm move the rib cage up and out, increasing the volume of the thorax. This decreases the pressure allowing air to be drawn into the lungs down a pressure gradient. This ventilates the epithelial cells of the alveoli allowing oxygen to diffuse through the membrane through the cells. The oxygen then continues to diffuse through the membrane of the red blood cells where it loads to haemoglobin forming oxyhaemoglobin.

The carbon dioxide follows the reverse route and is expelled from the lungs during expiration as the intercostal muscles relax. The oxygen helps cells to release energy as ATP during aerobic respiration. The oxygen helps to increase the permeability of the mitochondrial membrane allowing pyruvate formed in glycolysis to enter the matrix of the mitochondrion. The reduced NAD also formed passes its electrons down an electron transport chain in a series of redox reactions from one carrier molecule to the next.

In doing so it increases the permeability of channel protein the inner membrane to hydrogen ions, which then pass into the intermembrane space. All organisms use ATP as an immediate energy source for processes such as active transport. In plants, the roots are an organ system whose purpose is the uptake of mineral ions and water, and its movement though to the endodermis and xylem via the apoplast and symplast pathways. The root hair cells have specific channels for ions such as nitrate and potassium.

These channels have the enzyme ATPase which hydrolyses ATP and releases energy to absorb the ions against a concentration gradient into the cell. This movement into the cell from the soil lowers the water potential of the roots hair cells allowing water to enter by osmosis. Movement of this water then takes place via the symplast pathways through cell cytoplasm and apoplast pathways via gaps in the cell walls. Water crosses the junctions of adjacent cells through plasmodesmata, small.

Essay The part played by the movement of substances across cell membranes in the functioning of different organs and organ systems gaps that allow its smooth passage to the endodermis. Active transport of the mineral ions into the xylem allows the water to enter the xylem by osmosis generating a hydrostatic pressure called the root pressure. This creates a push, which together with the cohesion-tension pulls water up the xylem in a column through the hollow lignified xylem vessels.

Animals use an excretory system to remove any waste products such as urea. The role of one key organ, the kidney, is to form a more concentrated urine and reabsorb glucose, sodium ions and water while excluding the urea. The membranes of the kidney tubules are adapted to allow this function.

The narrowing of the afferent arteriole generates a hydrostatic pressure at the glomerulus which forces blood against the capillary network. Water and small molecules pass through the pores while proteins and cells are excluded by the process of ultrafiltration. These smaller molecules enter the Bowmans capsule and the proximal convoluted tubule, which has many sodium and glucose channels.

These allow the selective reabsorption of these materials into the surrounding tissues. This lowers the water potential so water moves out of the tubule by osmosis and is reabsorbed with the ions into the capillaries that surround the tubules. As the membrane does not have channels for urea, urea remains in the tubule increasing in concentration.

The ascending limb of the loop of Henle is impermeable to water. Sodium and chloride ions are actively transported out onto the surrounding tissues through a specific channel using ATP. This lowers water potential creating a water potential gradient that draws water from the descending limb by osmosis. This counter current multiplier further contributes to the reabsorption of water, one of the key functions of the kidney. A protein hormone, ADH is released by the pituitary gland and binds to specific receptors on the collecting ducts of the kidney in situations when the blood water potential is too low.

This increases the membranes permeability to water effectively increasing the volume reabsorbed at the same time decreasing the volume of urine produced One example of the consequences of uncontrolled ions movements is when the bacterium, Vibrio cholerae releases its toxin in the large intestine. The protein binds to and opens a chloride ion channels on the epithelium surface.

Chloride ions flood out into the lumen lowering water potential causing rapid loss of water, chronic diarrhoea and severe dehydration. In the absence of the toxin these ions would have remained inside the epithelial cells. Water alone cannot be used to rehydrate the sufferer as it cannot easily be absorbed through the intestinal epithelium. The reabsorption of water requires sodium and glucose, two key components of oral rehydration solutions. These species are taken up by co-transport in the small intestine region of the digestive system which lowers water potential sufficiently to allow the absorption of water and the rehydration of the sufferer.

This essay has highlighted how the movements of substances across cell membranes contributes to the functions of the root systems in plants, and the digestive and excretory systems of animals. They are bounded by a phospholipid bilayer through which materials must pass, and many organelles such as mitochondria which produce energy in the form of ATP and ribosomes the site of protein synthesis.

The cells functions rely on the efficient movement of substances into, through and out of, the cell. This essay will detail some of these movements emphasising how the movement is brought about. For a cell to function, glucose must enter to supply substrate for respiration. Glucose is a hydrophilic monosaccharide and cannot therefore diffuse directly through the hydrophobic membrane.

Specific extrinsic glucose channel proteins have a complementary shape that allows glucose to enter the cytoplasm by facilitated diffusion down a concentration gradient. In contrast, hydrophobic substances such as fatty acids are able to diffuse directly through the bilayer. Plant cells such as root hair cells use active transport to take up mineral ions such as nitrate and potassium ions from the soil against a concentration gradient.

The energy for this process is supplied by the hydrolysis of ATP using the enzyme ATPase and a specific protein channel in the membrane. Respiration of the glucose to form ATP involves the movements of many substances. Glucose diffuses through the cytoplasm and is converted by oxidation in a metabolic pathway to pyruvate, a substance which then diffuses through the mitochondrial membrane into its matrix.

These then diffuse to the cristae where they pass their electrons down an electron transport chain in a series of redox reactions. Part of this process involves the movement of hydrogen ions into the intermembrane space to generate an electrochemical gradient to provide energy for the production of ATP. ATP plays another role in the movements of actin filaments in a mammals muscle cells. For a sarcomere to contract, an actin fibre must move into a myosin filament the sliding filament theory.

Calcium ions bind to troponin causing the removal of tropomyosin from the myosin head binding site on the filament. The myosin head can attach to form an actomyosin cross-bridge that nods to the left drawing actin into myosin.

Calcium ions activate ATPase to hydrolyse ATP to ADP and Pi to provide energy for the further detachment and reformation of cross-bridges, in this way contracting the length of the sarcomere and muscle fibre allowing movement of the animal. All cells functions are controlled in the nucleus by DNA. The DNA is attached to structures called chromosomes which are too large to pass through the nuclear pores into the cytoplasm where the ribosomes can translate the genetic code and synthesise proteins.

Through the process of transcription, a molecule of messenger RNA mRNA is created that is a complementary copy of a gene. This is small enough to diffuse out through the pores and bind to the ribosomes on the rough endoplasmic reticulum situated outside the nucleus. An enzyme then joins adjacent amino acids forming a peptide bond by a condensation reaction. In this way a proteins primary structure is built up. Once completed, the finished. The finished protein is packaged into a vesicle that pinches off from the golgi and awaits secretion from the cell by exocytosis.

Before a cell can replicate, the DNA must be replicated by semi-conservative replication, in the phase of the cell cycle referred to as late interphase. Helicase binds to DNA breaking the hydrogen bonds between the polynucleotide strands revealing two template strands. DNA nucleotides line up on each strand adenine with thymine and cytosine with guanine and DNA polymerase joins adjacent nucleotides by a condensation reaction.

For cell replication to occur by mitosis, the replicated chromosomes must pass out of the nucleus and attach to spindle fibres at the cells equator. During prophase, the nuclear membrane disintegrates and the chromosomes coil up and become visible. Since there is now nothing to stop the DNAs movement, the chromosomes move to the equator and attach to the spindle fibres using their centromeres.

In metaphase, all the chromosomes are aligned at the equator. During anaphase, the centromeres then split and the spindles contract, pulling the sister chromatids to opposite poles of the cell. In this way each pole of the cell now has a full set of chromosomes and so telophase can occur to reform the nuclear envelopes. Following the splitting of the cytoplasm by cytokinesis, two new cells have been formed, each with a full complement of chromosomes. In plants water is moved up the xylem through xylem vessels that are hollow.

The functions of these cells include transporting water and mineral ions up the plant from the roots and provide support. They are dead cells that have lignified walls providing strength and support. The water is pushed upwards via a root pressure. The active transport of mineral ions from the endodermis into the xylem lowers the water potential, allowing water to enter by osmosis.

This creates the hydrostatic pressure, called root pressure, which pushes water up the plant through the hollow xylem vessels. Water is also pulled up by the cohesion-tension mechanism. Evaporation of water from mesophyll cells creates a water potential gradient a tension that draws water from the xylem to replace that which was lost.

Adjacent water molecules have a weak inter molecular force called a hydrogen bond a cohesion between them, so as one molecule is drawn through the cells, the next follows pulling water through the xylem vessels in a column. All living organisms such as plants and animals require energy to power their cellular processes.

This energy may be in the form of the molecule ATP, or as heat. This essay will detail the processes by which energy is transferred inside organisms and discuss how these transfers are utilised. In biological processes, the immediate energy source is often in the form adenosine triphosphate ATP.

This hydrolysis releases a packet of energy of approximately 31 kJmol-1 which can be used to provide energy for processes such as muscle contraction, the generation of nerve impulses or active transport. Plants are able to produce ATP during the light dependent reaction of photosynthesis in the thylakoid of the chloroplast.

Red and blue wavelengths of light are absorbed by chlorophyll on Photosystem II. The energy absorbed is transferred to electrons and excites them to a higher energy level. This stimulates photolysis of water which results in the formation of hydrogen ions, electrons and oxygen gas.

The electrons formed during photolysis then replace those excited by the light. The excited electrons are then passed along an electron transport chain in a series of redox reactions from one carrier to the next. In this way one can consider the energy in light as being transferred to the ATP. The hydrogen ions released by photolysis are used to reduce NADP.

The ATP and reduced NADP are then used for the reduction of glycerate-3phosphate to triose phosphate in the light independent reaction in the stroma of the chloroplast. This triose phosphate can then be used to form glucose which can be used as a substrate for respiration , stored as starch an insoluble energy store in leaves or tubers or incorporated into the other plant tissues. The glucose or starch can be used as an energy source directly by the plants, or indirectly by animals which consume, digest, absorb and assimilate the sugars.

Glucose, a six carbon monosaccharide, is a commonly used respiratory substrate in, for example by intestinal epithelial cells in animals to provide energy for sodium-glucose cotransport. The glucose enters the cell through a specific protein channel by facilitated diffusion, down a concentration gradient using only kinetic energy provided by heat. In the cytoplasm each glucose is phosphorylated by two molecules of ATP effectively transferring energy to make it more reactive.

The fructose bisphosphate formed is then cleaved and oxidised to release 4 ATP giving a net gain of 2 molecules. Each molecule of glucose forms two molecules of pyruvate and two of reduced NAD, a coenzyme used to transfer hydrogen and electrons. Under anaerobic conditions the pyruvate is reduced to lactate in muscle or ethanol and carbon dioxide in yeast. In the presence of oxygen the pyruvate enters the matrix of mitochondrion where it undergoes oxidative decarboxylation forming acetyl coenzyme A.

The energy from the original glucose fuel is now stored in the electrons in the bonds of these reduced coenzymes. Upon diffusion to the cristae, these electrons are passed along an electron transport chain in a series of redox reactions. As the electrons pass down, protein channels in the. This in turn opens a channel that allows the protons to flood back across the membrane generating an electrochemical gradient which activates ATPase in the stalk particles leading to the production of ATP.

It is important to note that the respiration process also produces a lot of heat that is used to provide kinetic energy of processes such as osmosis and diffusion of molecules such as oxygen across exchange surfaces. As previously mentioned the ATP is used to provide energy to a host of reactions in all organisms. In animals, hydrolysis of ATP provides energy for the detachment and formation of actomyosin cross bridges during muscle contraction.

The ratchet process allows actin filaments to be drawn into myosin reducing the length of the sarcomere. In this way an animal, such as an antelope may run and avoid predation by cheetahs. Transfer of energy from ATP also plays a role in the transfer if nervous impulses along neurones to the muscles. ATP supplies energy for the pumping of three sodium ions out, and two potassium ions into the axon of a nerve against a concentration gradient. This resting potential of mV is then maintained by reduction of the membrane's permeability to sodium ions.

Once an action potential arrives at a synapse or neuromuscular junction, ATP again provides the energy to move and fuse vesicles containing the neurotransmitter, acetylcholine which the pre-synaptic membrane. Also in animals, kidneys use ATP for the active transport of sodium chloride out of the ascending limb of the loop of Henle in order to lower water potential in the surrounding tissues.

This generates a water potential gradient which causes the reabsorption of water through the permeable descending limb leading to the production of more concentrated urine. Plants require the translocation of glucose as sucrose from source cells to sinks roots, fruits and shoots to provide an energy source for non-photosynthetic tissues. This is achieved through active transport of sucrose using energy from ATP into the phloem which lowers water potential.

This causes water to enter by osmosis generating a hydrostatic pressure which forces the sugar solution up and down the phloem towards the sink regions. In order to provide support, plants need a flow of water to enter the roots from the soil. This is achieved again by active transport of mineral ions such as potassium and nitrate from the soil, against a concentration gradient, using energy supplied by ATP.

The lower water potential in the root hair cells now draws in water which moves towards the endodermis through the apoplast gaps in the cell walls and symplast through cell cytoplasm pathways. In summary, this essay has shown how energy in light is transferred firstly to glucose, released as heat during respiration and stored as ATP, and how this molecule's hydrolysis releases energy that can be used by plants and animals for many important and diverse life-processes.

The combination of any of the twenty plus amino acids in any length and sequence allows an almost infinite number of possible structures and functions. This essay will detail how the variation in structure of the protein is related to specific functions. The sequence of amino acids in the polypeptide chain is termed the primary structure.

The primary structure is unique to a given protein. The primary structure can fold regularly to form either an -helix or -pleated sheet. The secondary structure is held together by hydrogen bonds between adjacent peptide bonds. The primary structure can further fold in an irregular but not random manner to form an overall three dimensional shape that more specifically determines the biological functions of the individual protein.

This 3D structure is held together by bonds formed between the R-groups of amino acids. These bonds give a greater strength to the molecule allowing it to withstand some variations in temperature or pH. Some proteins adopt a structural role. For example, keratin, a protein in skin, is formed from coils that twist together to form rope-like structures that are both flexible and strong. This strength is utilised in animals as claws or horns for predation or protection, or hair as camouflage or insulation.

Collagen, another important structural protein that comprises connective tissue in animals, is composed of coils that are more tightly bound giving a more rigid structure. For movement, animals use muscle contraction. Muscle fibres are composed of two protein filaments, myosin a thicker filament and actin a thinner one. Actomyosin cross-bridges can form between the two which move relative to one another on hydrolysis of ATP drawing actin into myosin.

This sliding filament theory shows how a sarcomere contracts. This contraction is used in a variety of applications including constriction or dilation of blood vessel to modify blood flow through tissues, pupil diameter to control light entry into eyes or the generation of a force at a joint to move a hand away from a hot object. Some proteins adopt a transport role. Channel proteins in cell membranes offer a hydrophilic passage through the hydrophobic lipid bilayer.

They have a specific three dimensional shape that is complementary to the given species they transport. For example, sodium gated channels in membranes of sensory neurones allow the passage of sodium into the axon during the generation of an action potential. Similar transport proteins are carrier proteins that can change shape on binding of their transporter molecule, e. Proteins form a key role in the infectivity of pathogens and the immunity of the host. Proteins on the surface of pathogenic bacteria act as antigens which identify a cell as non-host.

Some of these antigens can break away and act as toxins. For example, the bacteria Vibrium cholerae releases a protein toxin that opens chloride ion channels in the large intestine causing loss of chloride from epithelial cells, and loss of large volumes of water as diarrhoea and chronic dehydration. Variation in the antigenic structure brought about by mutation of the pathogens DNA can increase the infectivity of the pathogen as.

Phagocytosis of pathogens eventually leads to activation of B-cells which divide by mitosis forming clones that differentiate to form plasma cells. These cells release antibodies that are globular proteins which have variable regions that have a complementary shape to a specific antigen, allowing it to agglutinate many pathogenic particles. A key role for proteins is to act as enzymes; biological catalysts that lower activation energy of specific reactions, allowing them to take place under controlled conditions at body temperature.

They have an active site that has a specific three-dimensional shape that is complementary shape to a given substrate. This provides specificity to reactions. On binding, the enzyme and substrate form an enzyme-substrate complex which places strain on the bonds allowing them to break more easily.

For example, the enzyme sucrase has an active site that is complementary to the disaccharide sucrose. Lactose, another disaccharide that has a similar but subtly different shape to sucrose, will not fit into this site, and is therefore not hydrolysed by the enzyme. DNA polymerase condenses adjacent DNA-nucleotides together during the formation of the phosphatesugar backbone of DNA during semi-conservative replication.

These reactions highlight the high degree of specificity elicited by the flexible nature of the primary and tertiary structures of proteins. Chemical coordination in animals is largely brought about using protein hormones. These hormones have a tertiary structure that is complementary to that of a receptor molecule often another protein or glycoprotein positioned on the cell-surface membrane of the target cell.

Examples are insulin, a protein released by -cells of the islets of Langerhan in the pancreas during conditions of high blood sugar concentrations. The insulin travels in the blood to hepatocytes in the liver and binds to a specific membrane receptor that causes activation of phosphorylase enzyme that condense glucose into glycogen in the process of glycogenesis.

Glucagon is released from -cells in the pancreas and stimulates the hydrolysis of glycogen into glucose when blood sugar is low. Other examples of endocrine hormones include follicle stimulating hormone that matures the ova in a follicle during the follicular phase of the menstrual cycle, and luteinising hormone, that causes rupture of the follicle and the release of the ova, once it has matured.

When these features interact, chemical elements, materials and energy are transferred and flow through the various components. This essay will describe these transfers with an emphasis on compounds of carbon, nitrogen, pesticides, water and energy.

Energy transfer through ecosystems takes place primarily by photosynthesis and respiration. Light energy is absorbed by chlorophyll in palisade cells of green plants. The energy is used to absorb electrons to higher energy levels. Once the electrons are passed down an electron transfer chain in a series of redox reactions, the energy released is used to activate the enzyme ATPase resulting in the production of ATP.

Reduced NADP is also formed, in the light dependent reaction and these two products are then used to reduce glyceratephosphate to triose phosphate in the Calvin cycle. The carbohydrate formed can then be used to synthesise glucose, lipids, starch, cellulose and other useful molecules which make up parts of the productivity of energy for the ecosystem. Thus the plants act as producers through transduction of the energy in sunlight and making it available in a chemical form.

The chemical potential energy in the form of the carbon containing compounds of the producers is then available for consumption by primary consumers, such as chickens, sheep and horses. These animals then eat the plants and digest some of the food molecules which they absorb and assimilate and use for growth, repair and respiration. The remaining biomass they cannot digest due to a lack of specific enzymes is egested and is made available for respiration by decomposer bacteria and fungi which decompose the faecal material by saprobiotic nutrition.

The carbohydrates absorbed by the animal are respired aerobically in the cells of the animal to release energy in the form of heat and ATP. Glucose enters the cells by facilitated diffusion and undergoes glycolysis in the cytoplasm. This involves the phosphorylation, and subsequent oxidation of the molecule to form pyruvate, and a net gain of two molecules of ATP.

The pyruvate then diffuses into the mitochondrion where it undergoes decarboxylation in the link reaction forming acetyl co-enzyme A, a twocarbon molecule that combines with a C4 to form a C6, that is decarboxylated and oxidised systematically to release CO2, some ATP and reduced co-enzymes such as reduced NAD and reduced FAD. These coenzymes release their energy during oxidative phosphorylation in the electron transport chain where it is used to form large amounts of ATP 38 molecules per respired glucose.

This energy is used by the animal for processes such as active transport of sodium and potassium ions into and out of the axon of a nerve cell in the generation of a resting potential, or in the detachment and reformation of actomyosin cross bridges as part of muscle contraction for movement. In this way, some more of the energy consumed by the animal is lost as to the environment as heat and so less is available to secondary consumers such as foxes when they consume the chickens.

It is these losses as the food chain is transversed that create the pyramids of biomass and energy at each trophic level. Less and less energy is available to the next trophic level as more and more is lost through movement, heat and excretion. One key chemical element that is recycled and transferred around ecosystems is.

All organisms have DNA as their genetic code and DNA is composed of nucleotides such as adenine and thymine that have nitrogenous bases. The DNA codes for proteins which in turn contain nitrogen in the amino acid monomers of which they are composed.

Atmospheric nitrogen, an unreactive element, is fixed by nitrogen-fixing bacteria such as Rhizobium in the root nodules of leguminous plants to form ammonium ions that are released into the soil. Nitrifying bacteria then oxidise the ammonium to nitrite and then nitrate, which can either be denitrified to nitrogen or absorbed by plants through the root hair cells by active transport.

The plant can then assimilate the nitrogen into its biomass, and on death, decomposer fungi can hydrolyse the proteins into amino acids and de-aminate them to ammonium which is release back in the soil for nitrification.

Since the plants biomass can also be consumed by the consumers as previously discussed, the nitrogen can be cycled and recycled around the ecosystem being made available for assimilation into biologically important molecules as it is transferred. Another important material that is transferred is water.

Water evaporates from the sea and condenses as a cloud. The water then falls back to the earth as rain. Plants can absorb the water from the soil by actively transporting mineral ions into their root hair cells to lower water potential, allowing water to enter by osmosis. The water is forced up the xylem through the generation of a root pressure by the pumping of ions into the xylem to lower water potential.

Evaporation of water through the gas exchange surface, the stomata on the underside of the leaf, draws water up the xylem by cohesion-tension creating a transpiration stream that returns the water to the atmosphere.

Animals also use the water they drink to transport hydrophilic substances such as glucose through their blood. The water is lost back to the environment in three ways, i through loss as sweat when used to regulate body temperature through the evaporation from the skin, ii excretion of water from the bladder as a concentrated urine, that was produced by the kidneys to remove the metabolic waste product, urea, or iii loss through the exchange surface of the lungs when exhaling.

Finally, other materials can be transferred through ecosystems. These include genetic material when the pollen grains of plants fertilise or the eggs and sperm fuse during sexual reproduction or man-made substances such as pesticides that can undergo bioaccumulation in organisms as they move up the food chain due to their incorporation into fatty tissues.

This essay has highlighted the transfers of a diverse range of materials and energy through the biotic and abiotic components of an ecosystem. Despite this, they may have different appearances or phenotypes as they are known. This essay will explore some of the reasons behind these differences. The characteristics of an organism are encoded within its genes.

Genes are sequences of bases on DNA which code for the sequence of amino acids in a polypeptide chain. Since proteins, as enzymes and gene expression factors, determine the structure and functions of cells, they determine the development of an organism. While organisms of the same species carry the same genes at given loci, the structures of the genes vary.

These alternate forms of genes or alleles can give rise to the intraspecific variations seen within populations of the same species. Variation in alleles can be due to factors such as mutation or through crossing over or independent segregation in meiosis.

Mutations, such as substitution mutations, alter the base sequence of DNA, which in turn alters the complementary mRNA formed at transcription and so the sequence of amino acids in the protein that was coded for. They may occur randomly during the semi-conservative replication of DNA during later interphase of mitosis, or as a result of exposure to an environmental mutagen.

X-rays and many estrogenic chemicals found in food packaging, for example can cause subtle alterations in the base sequence. If, for example the mutated base sequence now codes for a non-functional form of an enzyme that causes the darkening of a yellow pigment to brown in the coats of a Labrador, then one offspring of two chocolate Labradors may appear pale yellow due to the presence of the mutation, whereas another puppy may be darker due to normal amounts of pigment being deposited in the fur.

Mutations can be relatively rare, but meiosis is adapted to bring variation into a sexually reproducing population. Meiosis is the process of cell division that halves the chromosome number to produce haploid cells that are genetically different. This is an important process as it not only introduces genetic variations into a population, but also allows the restoration of the diploid number of chromosomes on fertilisation.

During the first division of meiosis, homologous chromosomes line up with their partners and crossing over may occur. The homologous chromosomes join at points called chiasmata and sections of the chromatids are exchanged resulting in new recombinations of maternal and paternal alleles. Each of the four new daughter cells formed will carry one copy of each chromosome. Before these chromosomes are separated into the cells, they align randomly along the equator of the cell.

One of each of the four versions of the twenty three homologous pairs in the case of humans is randomly and independently segregated into a daughter cell, resulting in new combinations of maternal and paternal chromosomes. In this way, each gamete formed by both parent is genetically unique, and since the process of sexual reproduction is a random event in that any sperm may fertilise the particular ovum released during that particular menstrual cycle, then no two offspring from the same parents can carry exactly the same alleles.

Since it is these the combined effects of alleles and the environment which ultimately determine the nature and. The genotype of an organism is its genetic constitution, in other words the alleles that make up each gene. For example, the gene for eye colour may have two alleles B brown being dominant to b blue.

Two heterozygous parents each carrying Bb will each form gametes B and b, which could combine by random fusion during sexual reproduction forming a variety of possible combinations, e. BB or Bb which would give a brown eyed offspring, of bb which would give blue eyed. Since every gene has two alleles, there are a multitude of possible combinations of subtle differences in phenotype, each of which could give rise to slightly different offspring.

Such examples of discontinuous variation give rise to differences in the offspring of a set of parents, but also the multitude of other polygenetic inherited characteristics combine to give wide variations in the siblings produced. For example, one sibling may have darker hair than another, be taller and have blue eyes compared to a shorter, blonder, brown eyed sibling.

An organism inherits its alleles from its parents, but offspring from the same parents are rarely similar in appearance. Even identical twins with the same genotype, can vary due to subtle differences in environmental factors that influence them in different ways. For example, each twin may be influenced by peer pressure to eat different foods and or take part in different sports. Thus one twin, the one who took part in sports may appear leaner and have a more defined musculature than the less athletic one.

The effects of childhood illness or conditions such as acne may also create differences in the appearance of the siblings later in life. One sibling who may have developed severe acne may be more facially scarred that one who didnt. Environmental influences are present in all species. One key factor in plants for example is the availability of key nutrients or energy.

Two dandelion plants from the same parents may differ in appearance if one has been grown in a shaded and dry field poor in magnesium. This plant would appear more yellow compared to another grown in magnesium rich soil. While it is difficult to ascribe exact causes to subtle intraspecific variations, even those obvious in siblings of the same parents, some of the genetic and environmental reasons behind those differences are understood. The variation it creates is of key importance to the population as it creates a large gene pool that has a greater ability to adapt and survive under more adverse or hostile conditions.

The genetic and environmental causes of variation described here contribute to this diversity. One key molecule that is of fundamental importance to these interactions is carbon dioxide, as it transfers carbon atoms between organisms and organisms with their environment. This essay will explore some of the important roles of CO2 in a variety of organisms such as plants and animals. Todays atmosphere contains low levels of CO2, a gas that is a limiting factor for productivity in an ecosystem.

Plants can absorb it through their gas exchange surface, the stomata, on the underside of the leaves. If diffuses through the air spaces and diffuses into the stroma of chloroplasts inside palisade cells. This, simple monosaccharide is then combined into larger molecules such as glucose, and ultimately the polysaccharides starch and cellulose. In this way, the carbon dioxide acted as a source of carbon atoms which were incorporated into the biomass of the plants, and therefore the available biomass of an ecosystem.

It follows that the higher the CO 2 levels in the atmosphere, the higher the productivity, but other limiting factors such as temperature, pollution and availability of water and nutrients in the soil will also play a role. This biomass of the producers may then be consumed by herbivores such as sheep to provide energy and the building blocks for growth and repair.

The food, e. The glucose can then enter cells where it undergoes aerobic respiration, a process that provides energy in the form of ATP but also releases the CO 2 back out into the atmosphere. The glucose is oxidised to pyruvate, which is then systematically decarboxylated in the link and Krebs cycle in the matrix of the mitochondria.

The carbon dioxide is released as a waste gas and accumulates in the tissue fluid of the respiring cells. Here, it plays a key role in oxygen transport and delivery to respiring cells. Oxygen is bound to haemoglobin in the form of oxyhaemoglobin. When the partial pressure of oxygen is low, for example when CO2 is formed by respiring muscle cells, then the oxygen is unloaded from the haemoglobin in the red blood cells and is made available for respiration.

The carbon dioxide then replaces oxygen on the haemoglobin and can be transported to the lungs where it is returned to the atmosphere when expiration takes place. The CO2 also has some important physiological roles. An increased rate of respiration, for example as a person runs, produces more waste CO2 at a time when the oxygen demand is high.

The CO2 dissolves in the blood plasma releasing hydrogen ions which lower pH. This reduced pH causes a shift in the oxygen dissociation curve to the right further reducing the affinity of haemoglobin for oxygen. At this lower partial pressure, and with a shifted curve, even greater amounts of oxygen are unloaded and made available for respiration.

In order to satisfy the need to inspire more oxygen and expire more carbon dioxide to accommodate this increased rate of respiration, the CO 2 also acts on chemoreceptors in the walls of the carotid arteries and aorta. The reduced pH caused by the dissociation of. CO2 in the blood plasma activates these chemoreceptors which send an increased frequency of impulses to the cardio-acceleratory centre in the medulla of the brain.

This in turn sends an increased frequency of sympathetic nervous impulses to the sinoatrial node, the pacemaker in the wall of the right atrium, causing it to send an increased frequency of waves of excitation, increasing the heart rate. This increased heart rate increases the cardiac output resulting in a faster delivery of blood to the gas exchange surfaces, the alveoli in the lungs.

Thus, more oxygen can be transported around the body, and more CO2 returned to the atmosphere. Since all living organisms respire, the carbon dioxide removed by plants for photosynthesis is constantly replaced. This constant recycling of carbon is crucial to the existence of all life on this planet, as changing carbon dioxide concentration could have disastrous consequences for all life.

Each organism is adapted by natural selection to exist optimally within its environment to increase its chances of survival. Human activities such as deforestation and the burning of fossil fuels to provide energy have resulted in an increase in atmospheric carbon dioxide concentrations.

This extra trapped kinetic energy causes a warming effect referred to as global warming. The global consequences of this phenomenon are wide and diverse in that polar ice caps are melting, changing the environment for organisms that are adapted for arctic conditions. The increased sea levels may cause flooding of low-lying coastal land, increasing the salination of soil, but decreasing the concentration of salt in sea. These changes will act as selection pressures on organisms, forcing the process of natural selection.

Those members of a given species that are best adapted to survive the changes are more likely to survive and pass on their beneficial alleles to their offspring. In this way, the allele frequencies may change, ultimately altering the phenotypes. For example, salination of soil causes by coastal flooding would favour xerophytically adapted plants, as the reduced water potential in the soil would make it hard for the plant to take up enough water for its needs.

Xerophytic adaptions such as sunken stomata and rolled leaves would help reduce water loss by evaporation, reducing transpiration rates. Such huge changes in the environment would destabilise the complex food webs and reduce diversity. While carbon dioxide levels are only one factor in environmental change, the molecules key importance to life makes it essential that its levels are closely monitored and action taken to minimise the various impacts of its change.

In multicellular organisms these cells differentiate and become specialised to perform a given function. These specialisations then impact on the shapes and structures of the cells. This essay will describe how the shapes of some examples of animal and plant cells relate to the cells function. Species in the kingdom Plantae, are anchored in the soil by their roots. The functions of these roots include the absorption of water and mineral ions from the soil which they deliver to the xylem for transport up and around the plant.

The roots have an outer layer of cells called root hair cells that are adapted for this purpose. The cells have a long thin extension of the cell surface membrane that extends into the soil. This dramatically increases the surface area of the membrane and hence the number of channel proteins. Minerals such as nitrate and potassium ions are transported across this membrane by active transport using energy supplied by ATP.

The ions pass through a specific hydrophilic channel protein against a concentration gradient. A higher surface area of membrane means a greater concentration of these channels and so a greater efficiency in the absorption from the soil. These ions then lower the water potential inside the root hair cell allowing water to enter by osmosis.

Here again, the presence of a greater number of hydrophilic channels offer the small water molecules an easy passage through the membrane. The water moves via the apoplast and symplast pathways to the endodermis, where it enters the xylem tissue, the vascular route by which water passes up the stem of the plant. The xylem tissue is composed of xylem vessels that are tube-shaped hollow cells with pits in the side. They are hollow to allow an unhindered passage of water up through the plant.

The absence of organelles further facilitates the passage of water. The pits in the sides allow lateral passage of ions from the xylem out into the surrounding tissues to lower water potential, allowing water to pass out and be used for support, and, amongst other things, photosynthesis. Plants are a kingdom of species that are autotrophic. They derive their energy through photosynthesis by absorbing specific wavelengths of sunlight and converting it into chemical potential energy, driving the combination of carbon dioxide and water to form glucose and oxygen.

The palisade cells are situated mainly on the upper surface of the leaf and have adaptations for the absorption of specific wavelengths of red and blue light. The palisade cells have a block-like shape that allows them to stack together with little gaps between them like bricks in a wall.

This presents a high surface area for light to strike which increases the efficiency of the process. Each palisade cell has a large number of chloroplasts which contain the green pigment chlorophyll which absorbs the energy and uses it to excite electrons in the light-dependant reaction of photosynthesis. When their energy is released, in the electron transport chain, ATP and reduced NADP are formed which are used to reduce glyceratephosphate to triose phosphate and ultimately glucose.

Animals are a kingdom of species that are heterotrophic. They derive their energy by consuming, digesting and absorbing plant and animal matter which they use to provide nutrients such as carbohydrate and lipid for their growth and repair. The polysaccharide. Essay Cells are easy to distinguish by their shape.

How are the shapes of cells related to their function starch, formed from the glucose in plants, is consumed by herbivores and digested in the gut by amylase to maltose. This disaccharide is then further hydrolysed by maltase in the intestinal epithelium to glucose which is absorbed by the epithelial cells located there. These epithelial cells have a brush border composed of microvilli, close-packed folded extensions of the membrane that massively increase the surface area and the concentration of channels.

The glucose is absorbed through a sodium-glucose cotransporter protein on the cell surface. The many mitochondria inside these cells provide ATP by respiration for the active transport of a potassium ion from the blood into the cell at the same time as transporting a sodium ion out.

This lowers the concentration of sodium ions inside creating a gradient that draws in the sodium and the glucose through the co-transporter protein. Glucose channels on the basement membrane then allow the glucose to enter the blood by facilitated diffusion. One of the roles of glucose in the animal is to supply a source of energy for aerobic respiration. This process requires a delivery of oxygen from the lungs to the respiring tissues. The oxygen is carried bound to haemoglobin inside red blood cells, small cells that have a biconcave shape that can flex easily and offer a high surface area of contact with the capillary walls for efficient gas exchange.

This reduces the diffusion pathway of the oxygen and carbon dioxide increasing gas exchange rates. An absence of organelles also increases the room available for the haemoglobin, each molecule of which can load four oxygen molecules. The respiration of this glucose provides energy in the form of ATP. This is used to power a multitude of process including the generation of a resting potential in a nerve cell.

A nerve cell, or neurone, is adapted for the passage of electrical impulses from the central nervous system to remote effectors or from receptors. Thread-like extensions of the membranes or dendrites allow the synapsing of information with other neurones allowing the propagation of the action potentials to more effectors. A long thin myelinated axon allows a direct connection from the central nervous system, along which the waves of depolarisation can pass.

The rate of conductance of these impulses is further increased by the presence of myelination which effectively insulates the membrane allowing impulses to pass only at the nodes of Ranvier. The cell body of sensory neurones is located off to the side of the axon so as not to interfere with the passage of the impulses. This essay will examine some of these functions and look at how they contribute to health and disease. Triglycerides are lipids that are composed of a glycerol head group linked by an ester linkage to three fatty acid tails formed by condensation reactions.

The fatty acid tails can be saturated, in that they contain only carbon to carbon single bonds, or be unsaturated with at least one double bond. The tails can be up to approximately seventeen carbon atoms long giving rise to the molecules very hydrophobic water hating properties. This hydrophobic nature makes them insoluble and so good candidates for storage functions.

Lipids have a lower density than water which also makes them good thermal insulators. Many animals have a thin layer of saturated fats under the skin which helps to minimise heat loss through radiation. This means that they contribute to an animals ability to maintain its body temperature by homeostasis.

This is more pronounced in animals that are adapted to arctic conditions. Often these animals are much larger and have a thicker layer of fat beneath the skin. The increased size this gives lowers the surface area to volume ratio resulting in a lower effective surface from which heat would be lost.

Coupled with more insulation, this means the animal would need a lower respiration rate in order to survive, thus helping to preserve fuel sources. The dense number of carbon to carbon and carbon to hydrogen bonds in lipids makes these molecules energy rich and so they can act as useful fuels. A fuel is any substance that releases energy in a useful form when reacted with oxygen. Hydrolysis of triglycerides by lipase enzymes in adipose tissue releases fatty acids which can be converted to respiratory substrates such as glucose in the process of gluconeogenesis.

In healthy individuals, carbohydrates are the usual energy source unless exercise reduces the blood glucose concentrations resulting in hypoglycaemia.

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AQA Biology Essay 1 2018

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