Externally, they provide a large surface area to maximise the sunlight absorbed and are thin to ensure there is a small distance for carbon dioxide to diffuse. Also, there is a thin cuticle of wax on the surface of the leaf to protect it without blocking the essential sunlight.
Internally, the leaf is carefully structured to make sure the plant can photosynthesize as much as possible; found in the upper portion of a leaf, is the upper epidermis which is thin and transparent to allow more light to reach the palisade mesophyll beneath it. The palisade mesophyll contains many chloroplasts inside each cell so light absorption is at a high capacity. Beneath this palisade mesophyll is the spongy mesophyll; which consists of mainly air to allow the carbon dioxide to diffuse easily. This spongy layer also creates a large internal surface area to volume ratio so there is more chance of diffusing the carbon dioxide.
The lower epidermis of a leaf’s anatomy is also thin and transparent like it’s upper opposite but contains guard cells which restrict or open the stomata depending on the conditions to minimise water loss and increase gas exchange.
These guard cells also contain chloroplasts, to once again maximise photosynthesis.
- Explain the factors which can limit the rate of photosynthesis in plants
Photosynthesis depends mainly on three factors; light intensity, carbon dioxide levels and temperature.
The more light a plant receives, the higher the rate of photosynthesis because plants require the light energy to transform the carbon dioxide and water into the glucose and oxygen, this is to an extent however, because other limiting factors will restrict any further photosynthesizing, as there are no substances to photosynthesize.
This can be carbon dioxide levels, which similarly, when increased increases the rate of photosynthesis until a certain point.
The other factor which can limit the rate of photosynthesis is Temperature; and once again, increasing the temperature will increase the photosynthesis rate because the more thermal energy transferred to kinetic energy means more collisions between the enzyme and substrate and more reactions, however after a certain temperature (usually between 37- 40 degrees Celsius), temperature can actually decrease the rate of photosynthesis, this is because the enzymes involved in catalysing photosynthesis become denatured and can no longer fit with their specific substrate and therefore, no longer speed up photosynthesis.
- Explain the effect of ph and temperature on enzyme activity
All enzymes work at an optimum ph and temperature.
The optimum ph varies depending on the particular enzyme and where it works in the body, for example, enzymes in the stomach will want to work in acidic conditions so hydrochloric acid is produced, and enzymes in the small intestine will want to work in alkaline conditions, so bile is produced. If these enzymes did not work at their appropriate ph conditions, the rate at which they catalysed could decrease or the enzyme itself could become denatured and would not catalyse at all.
Temperature affects enzyme activity because the higher the temperature, the more thermal energy transferred to kinetic energy, resulting in the increased movement of the enzyme and their substrates, more collisions meaning more reactions and at a quicker time frame.
However after a certain temperature (usually 37- 40 degrees Celsius/ Body temperature), temperature can actually decrease enzyme activity because the enzymes become denatured from the extreme heat, meaning their active site has been altered so much that the substrate specific to the original enzyme cannot fit and subsequently the rate of activity of all enzymes in the area decreases.
- Evaluate the uses of enzymes in industrial processes
Enzymes are frequently used in industry as they enable reactions to be carried out at lower temperatures, without the need for expensive equipment and lots of energy.
An example of enzymes being used in industrial processes, would be the pre-digestion of proteins in the manufacture of baby foods; protease, an enzyme which catalyses the breakdown of proteins is used to ensure that the baby consumer can easily digest the proteins as they are incapable of consuming whole proteins.
Carbohydrase, or amylase, is also used within industrial processes; they convert the relatively cheap starch syrup into the more valuable sugar syrup, which can in turn be used in sports drinks etc.
Another enzyme, named Isomerase is widely used in the production of fat slimming foods;
Isomerase catalyses the breakdown of glucose into fructose. Fructose, compared to glucose, is much sweeter and so can be used in small doses within fat slimming foods.
Natural Fructose is expensive and difficult to obtain, so Isomerase is greatly beneficial to this process.
Whilst there is the issue of enzymes sometimes being difficult to obtain (they need to be kept at high temperatures in fermenters) and the rarity of a consumer being allergic to the enzyme, the work of the enzyme is greatly beneficial in industrial processes.
- Outline the uses of enzymes at Home
Enzymes are used frequently at home; an example would be inside biological detergents, where you can find the enzymes Protease and Lipase.
During the cleaning of clothes using biological detergents, protease and lipase efficiently catalyse the breakdown of certain stains that may appear on your clothes; Protease will catalyse the breakdown of Proteins whereas Lipase will catalyse the breakdown of fats, or lipids, into fatty acids and glycerol, subsequently removing the possible stain.
At home, it is also possible, that a product you purchased was made possible through enzyme activity; for example baby foods, where protease pre-digests proteins ready for the baby to consume or fat slimming foods, where isomerase catalyses the breakdown of glucose into the sweeter fructose.
- Evaluate the use of biological Detergents
Biological Detergents contain two enzymes- Protease and Lipase. They work to remove stains that may be found on clothes; Protease catalyses the breakdown of Proteins, whilst Lipase catalyses the breakdown of fats, or lipids, into fatty acids and glycerol.
The use of enzymes in these detergents means that effective washing can be carried out in warm water, so there is no need to use more energy in cleaning your clothes, and so, is more energy efficient. The breakdown is much quicker with the addition of enzymes and so benefits time-wise, particularly in today’s impatient society.
However, there are some disadvantages to using enzymes in biological detergents; enzymes will only work at temperatures at maximum 40 degrees Celsius, otherwise they become denatured and will not catalyse and perform its function in the detergent. The enzymes are very sensitive and so the ph and temperature must be controlled well for a good outcome.
Furthermore, some people may be allergic to the enzymes if they are left on the clothes, and so this is a flaw in its production, but it can be argued that some people could still be allergic to substances found in natural detergents, and they do not work as effectively.
- Explain what happens in an athlete’s muscles when running
Whilst an athlete is running at a small pace, their muscles are contracting and require oxygen from the heart in the form of blood to keep on moving. To meet this demand, the heart begins to pump blood at a quicker pace compared to the resting rate, and in doing so can supply the muscles with the oxygen they require to respire. This is respiration and occurs normally, even when sitting still, but occurs at a higher rate when running slowly. The equation for respiration is:
Oxygen + Glucose Water + Energy + Carbon Dioxide.
However, if the athlete begins to speed up, or has been running for a long duration, it is likely they will anaerobically respire; this is when little amounts of oxygen and glucose create lactic acid and small doses of energy. Anaerobic Respiration cannot be kept up for a long amount of time; this is because the lactic acid is toxic and causes muscle fatigue, so the muscles cannot keep working. Also, the energy supplied in anaerobic respiration is too little to keep going on.
- Describe the changes that take place in the human body during exercise to ensure that the muscles receive enough oxygen and what happens if the oxygen is in short supply.
During exercise of the Human Body, the heart beats at a quicker pace to meet with the demands of respiration in muscle cells, so they are able to keep contracting and ensuring movement. This requires Glucose and Oxygen, and Water, Energy and Carbon Dioxide are released as a result.
However, when the oxygen is in short supply, your body still needs to continue respiring, and so anaerobically respires. Anaerobic respiration consists of little amounts of oxygen combined with glucose to release lactic acid and small doses of energy.
The lactic acid, however, is toxic and causes muscle fatigue and pain. The small doses of energy are not enough for the human body to continue exercising at a high rate, and your body must pay back and Oxygen Debt.
An Oxygen Debt consists of repaying back the oxygen that did not enter the body during anaerobic respiration, it also neutralises and oxidises the build-up of lactic acid and removes it from the body so it can no longer cause pain.
- Compare and contrast anaerobic and aerobic respiration
Both anaerobic and aerobic respiration provide energy for the body to move, however within anaerobic respiration the energy doses are much smaller in comparison and an additional waste product is lactic acid which can cause muscle fatigue and pain. Anaerobic respiration cannot last for long as the lactic acid is toxic and an oxygen debt must be repaid to oxidise the lactic acid. However, aerobic respiration can last all the time, as long as oxygen and glucose are available. Anaerobic respiration only occurs when there is a little supply of oxygen, for example, when running vigorously.
The equation for aerobic respiration is:
Oxygen + Glucose Water + Energy + Carbon Dioxide.
Whereas, the equation for anaerobic respiration is:
Glucose (+Little Oxygen) Lactic Acid (+Little Energy)
Most Cells will aerobically respire, however only muscle cells will anaerobically respire with the exception of yeast or bacterial cells which live in little-oxygen conditions.
- Evaluate the following statement:
`It is safer to use adult stem cells, using embryonic stem cells is not safe`
Stem Cells have the ability to differentiate into many types of cells and subsequently have the ability to save many lives which suffer from disorders, such as Paralysis.
Embryonic Stem Cells are found in the Embryo, whereas adult stem Cells are found in the bone marrow. The benefits of using embryonic Stem Cells are that they have the ability to develop into any type of animal cell, whilst Adult Stem Cells have the ability to differentiate, the possibility of cells is limited.
It can be argued that using Embryonic Stem Cells is not safe, even though they produce the wider outcomes. This is because by taking the stem cells from the embryo, the embryo is at risk of being destroyed, some religious views see this as murder and completely not safe, because at the point of fertilisation is the beginning of a new life, however it can be argued that the embryo is not yet a life form until it goes on to be born.
By using adult Stem cells, the adult is not at risk of death by this procedure and it can be argued that this process is safer, however on a wide scale, this could be said to be more dangerous as adult stem cells cannot produce all the necessary cells in the human body and is limited to a certain amount.
- Outline the Stages of Mitosis
Mitosis is a form of cell division which occurs when cells need repairing or for growth.
Before the cell is able to split, copies of the genetic information and different organelles are created to ensure that each of the two new daughter cells are identical to the mother cell.
These organelles and genetic information are then pulled and moved to opposite poles of the cell, where the cell then separates in the centre and the products are two new identical daughter cells. It is now possible for the two new daughter cells to also divide through the same process of Meiosis.
- Outline the Stages of Meiosis
Meiosis is a form of cell division which occurs in either the testes for males, or ovaries for females when creating gametes.
Initially, copies of the chromosomes are created and these move to opposite sides of the pole, the copies do not yet separate like Mitosis; instead the chromosomes exchange combinations of DNA so there is variation between them.
The Cell then separates; each current daughter cell with variation between them. These two cells then separate once again and the result is 4 new sex cells, each containing different genetic information in 23 chromosomes (for humans). Compared to the original cell, the amount of chromosomes has halved, so this type of cell division is sometimes called Reduction Division. The complete amount of necessary chromosomes is created at fertilisation.
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Compare Mitosis and Meiosis by referring to what takes place and the kind of products that are made.
Mitosis occurs all around the body in our body cells, whereas Meiosis occurs only in the testes or ovaries respectively.
During Mitosis, there is one single division which produces two identical daughter cells with a full set of chromosomes (Diploid), however, during Meiosis, there are two divisions and four daughter cells which have half the chromosomes of the original (Haploid). What are produced are gametes, which will become a diploid at fertilisation.
The purpose of Mitosis is to repair damaged cells and for growth, unlike Meiosis which is only used in the production of gametes.
- Evaluate the use of Embryonic Stem Cells
Embryonic Stem Cells have the ability to differentiate into absolutely any type of animal cell, and since their discovery, scientists have tried applying their use to cure disorders such as paralysis.
However there are strong ethical and religious views surrounding their use; taking stem cells from the embryo would destroy the embryo and subsequently their possible life, it is argued at what point a life begins, some say it is at fertilisation whereas others say it is when they are born. On the other hand, scientists are suggesting that these stem cells should be taken from the leftover embryos from in-vitro fertilisation because their fate would be to be discarded in the future, so it makes sense to make use of their benefits beforehand.
Being able to harness the use of Embryonic Stem Cells could greatly increase the quality of lives of the affected, however, a large source of Embryonic Stem Cells would be difficult to obtain considering the ethical issues regarding it.
- Explain what Mendel’s Experiments taught us about inheritance
Before Mendel’s Experiments, it was believed that the characteristics from each parent would be merged in their offspring; for example, a combination of a tall and a short parent would result in the offspring being a medium height.
Mendel disproved this theory by conducting a series of experiments with tall and short peas.
He breeded some tall pea plants with some short pea plants and found that all the offspring were tall, contrary to the original theory. He then breeded this 1st generation together, to find that although most of the offspring were tall, a small amount were short.
He concluded, that from each parent, the offspring received an alternative form of the same gene, or an allele, the characteristic which showed up would be dominant, as shown by the 1st generation of pea plants; the allele for tall height was dominant. He also concluded, that one particular allele would only show its characteristic if it was homozygous for recessive and without presence of the dominant allele. As proved in the 2nd generation of Pea Plants; a minority were of a short-height.
This format of dominant and recessive alleles is used today and can detect the probability of an offspring having a particular characteristic.
- Discuss the ethics associated with embryo screening for genetic disorders
Genetic Disorders can severely damage a person’s quality of life, whether you are the person affected by the disease or somebody close to the affected.
The creation of embryo screening however, enables parents to detect if their embryo has a certain disorder and subsequently, continue or stop with the process of keeping the embryo.
The advantages include being able to make an informed decision on abortion, or being able to mentally and financially prepare for a child with a disease. However many regard discarding an embryo as murder, and it can be argued when a life begins; some say at fertilisation whilst others say at birth.
Religious Views regarding the issue include that we have no right to decide the features of a child and in doing so, we are playing God. It can also be argued that embryo screening is a step towards `Designer Babies`, where babies can be personally designed to having wanted features that we deem pretty.
On the other hand, through Embryo screening, it is possible to eradicate life-destroying genetic disorders like Cystic Fibrosis, because the allele will no longer be present in people, and so cannot be inherited by offspring. This would then save any costs that would be necessary in treating these diseases, like medical equipment.
- Compare the issues involved in the use of embryo screening for Cystic Fibrosis and for Polydactyl
Embryo Screening can lead to the destruction of embryos and their possible life. However, scientists and people alike are more inclined to screen embryos with possibility of a disease like cystic fibrosis over a disease like Polydactyl, even though screening for them both could result in the destruction of embryos.
It can be argued that Cystic Fibrosis is a more worthy disease to eradicate over Polydactyl because it can frequently disturb everyday activities through the production of thick, heavy mucus and difficult breathing, whilst Polydactyl simply gives extra digits on hands and toes, which will not significantly impact their quality of life.
It is also more likely that if an embryo has polydactyl, it will be kept compared to an embryo with cystic fibrosis, because polydactyl is easy to manage and can be cured through surgery whereas a child with cystic fibrosis will be financially and mentally distressing.
- Describe how a species may arise by isolation
One large population of a particular species may become isolated, or separated, by a barrier like mountains or lakes and form two different populations.
The environments in which these two populations encounter may be different. Naturally, there is variation between the animals; they each have a wide range of alleles coding for certain characteristics. This is called Genetic Variation.
We then have Natural Selection happening in both or one of the populations; particular alleles are randomly selected which enable certain members of a population to survive in the particular conditions of their new habitat and subsequently breed with other successful members to create offspring with beneficial features enabling them to survive easier.
It is then that we have Speciation; both populations of the original ancestor of species are now so different, that if these populations were to meet again, they could not successfully breed and so cannot be classified as the same species.
- Describe how a fossil is formed
Fossils are the remains of organisms. They can be formed in various ways; one of them being during their decay- softer tissue will easily decay but the harder parts, such as bones will not easily decay and may remain for long periods of time.
Another way for an organism to become fossilised is when one or more conditions for decay are absent, or that when the organism is decaying, parts of it are replaced by other materials so we still have an outline or idea of the original organism.
The traces of an organism can also be preserved, for example, footprints or burrows.
It is also possible that whole animals, even with their soft tissue, can be preserved; this is done as they can become fossilised in amber or ice, which rapidly preserves them and does not allow conditions for decay to occur.
- Outline the possible causes of extinction
Extinction is when all organisms of a species have died out and there is no possibility of future generations.
There are many factors which can be the source of extinction, sometimes there are a whole variety of factors which contribute to the extinction of a species.
One possible cause of extinction could be the arrival of a new competitor, whom has a more successful adaptation to enable them to capture more prey and subsequently wipe out the original species’ food source. Without a food source, the organisms will not be able to survive which could lead to their eventual extinction.
Another cause could be the arrival of a new predator, which could eliminate all members of a species, when combined with all predators.
The Dodo Bird encountered both the arrival of new competitors and predators, when humans inhabited their islands and bought along with them threatening pets.
Another possible cause of extinction would be if a disease was introduced to a species, and cause extinction before the organisms develop immunity against it.
Finally, another possible cause of extinction could arise from one single catastrophic event, a famous example being the extinction of the dinosaurs in which it is thought that a large meteorite struck the Earth and eliminated almost all life.
Other single catastrophic events could include Super Volcanic Eruptions or Deforestation.
