Essay topics

  • Created by: Jmarie
  • Created on: 28-04-15 07:40


Energy is stored in glucose until the plant release it by respiration.

Animals obtain glucose by eating plants or other animals, then respire the glucose to relase energy.

Aerobic respiration produces CO2 & Water and releases energy using oxygen

Anaerobic respiration is without oxygen.

ATP is the immdeiate source of energy in a cell

A cell can't get energy directly from glucose, so in respiration the energy released from glucose is used to make ATP which carries energy around the cell where it's needed.  

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Light-dependent reaction: in the Thykaloids

Light is absorbed by chlorophyll and used to photolyse water into O2, H+ ions and electrons. The electrons replace excited electrons lost from chlorophyll in PSII. Excited electrons from PSII are passed along the electron transport chain, releasing energy which is used to pump protons from the stroma into the lumen which creates a proton gradient. Light energy absorbed by PSI excites the electrons, which are transferred to NADP with a proton from the stroma to form NADPH. The energy from the movement of protons back into the stroma used to synthesize ATP.


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Light-Independent aka Calvin cycle: stroma

This makes triosephosphate (TP) from CO2 and Ribulose bisphosphate, a (5C) compound. TP can be used to make glucose & other organic substances such as hexosugars and larger carbohydrates such as cellulose which is a form of stored energy. 5 out of 6 molecules of TP are used to regenerate RuBP by using ATP produced by the light independent reaction to allow the Calvin cycle to continue. Given that a hexosugar has 6 carbons, two molecules of TP are needed so the cycle must turn 6 times. Although, this seems inefficient this keeps the Calvin cycle continuing and ensures there's a sufficient supply of RuBP to combine with CO2.  

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  • Respiration: process that allows cells to use glucose to produce ATP which carries energy around the cell where its needed. It can be done areobically with oxygen or anaerobically without oxygen. Most reactions in respiration takes place at the mitochondria but glycolysis takes place in the cell cytoplasm due to the absence of glucose cariers on the mitochondrial membrane, given that it doesn't need oxygen to occur it's an anaerobic proccess. 
  • There's 2 stages in Glycolysis. First ATP is used to phosphorylate glucose to triosephosphate during phosphorylation. Then triose phosphate is oxidised, forming 2 pyruvate molecules, 2 NADH molecules and a net gain of 2 ATP molecules. 
  • The link reaction converts pyruvate to acetyl conenzyme A. Pyruvate is decarboxylated and reduced coenzyme NAD collects hydrogen changing pyruvate into acetate which is combine with coenzyme A to form acetyl coenzyme A. 
  • The Krebs cycle produces ATP and reduced coenzymes FAD and NAD after a series of oxidation-reduction reactions which take place in the matrix of the mitochondria.
  • These earlier stages is used to make coenzymes reduced NAD and reduced FAD,which transfers electrons for the ETC in final stage Oxidative phosphorlation. This creates a proton gradient which enables the synthesise of ATP by ATP synthase through Chemiosis.
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Anaerobic Respiration

Anaerobic respiration occurs when there's no oxygen & the process stops at glycolysis. Pyruvate is converted to ethanol during alcoholic fermentation in plants and yeast or lactate during lactate fermentation in animal cells and some bacteria. This regenerates NAD which enables glycolysis to continue when there's insufficient oxygen, therefore a small amount of ATP can still be produced to keep some biological process going. But is less efficient compared to aerobic respiration because only 2 ATP molecules is produced per glucose molecule. 

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Homeostasis is the maintenance of a constant internal environment within certain limits and a great deal of the hormone system and autonomic nervous system is dedicated to homeostasis. It's particularly important to maintain body temperature at 37oC, blood pH, blood glucose conc. and water potential.

 All homeostatic mechanisms use negative feedback to maintain a constant set point. Negative feedback means that whenever a change occurs this automatically causes a corrective mechanism to start, to bring levels back to normal set point. Receptors detect when a level is too high or too low and this information is sent to the nervous or hormonal system before information is sent to effectors. These cause a responses bringing the level back to normal.

Positive feedback occurs when the change stimulates the effectors to further increase the level away from the normal set point. This is potentially dangerous and is usually associated with a breakdown in the normal control mechanism EG Hyperthermia. 

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Controling blood pH, enzymes, glucose

  • Enzyme activity is highest at their optimum pH which increases the rate of metabolic reactions. If blood pH is too low or too high enzymes become denatured. The hydrogen bonds holding their 3D shape can be broken which alters the enzyme's active site shape. Therefore, slowing the rate of metabolic reactions.  
  • Control of blood glucose is important because glucose is the transport carbohydrate in animals, and its concentration in the blood affects every cell in the body. The hormonal system controls blood glucose concentration using hormones insulin and glucagon which are secreted from the islets of Langerhans. When glucose levels are too high, B cells secrete Insulin in to the blood. Insulin lowers blood glucose levels by binding to receptors on the cell surface membrance of liver cells and muscle cells. This increases the permiability of cell membranes to glucose which increases the uptake of glucose and activates glycogenesis. If blood glucose concentration is too high blood water potential is reduced, allowing water to diffuse out of cells into the blood by osmosis which cause cells to shrivel up and die. If blood glucose is too low, there's insufficinet glucose for respiration to provide energy for cells to carry out normal activities.
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Body temperature needs to be regulated in a process (Thermoregualtion) to keep enzymes working close to their optimum temperature and to prevent them from denaturing.Mammals and birds which can generate their own heat and are called endotherms; while other animals EG Lizards relying on gaining heat from their surroundings, are called ectotherms. If body tempertaure's too high, this can be detected by thermoreceptors in the skin or the hypothalamus, which sends to nerve impulses to the heat loss centre in the hypothalamus. This then sends impulses to the effectors such as the smooth muscles in peripheral arterioles in the skin relax causing vasodilation, heat carried to the core is lost by convection &radiation. Redness. Sweat glands secrete sweat & erector pili muscles in the skin cause hairs to lie flat which increases heat lost from the skin by convection.

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Menstrual cycle

Lasts about 28 days. It's a cycle of physiological changes in which the female body prepares for reproduction. The pituitary gland, below the hypothalamus secretes Follicle Stimulating Hormone (FSH) and Lutenising Hormone (LH). FSH stimulates the growth and development of the follicle which secretes oestrogen, a hormone which inhibits FSH release through negative feedback. But when oestrogen conc. increases to a high level osetrogen stimulates the anterior pituitary gland to release LH and FSH through positive feedback, during ovulation. LH will stimulate the development of the corpus luteum which secretes progesterone, a hormone which inhibits FSH and LH release and maintains the uterus lining. If no embryos implants, the corpus luteum breaks down and stops releasing progesterone. Therefore, FSH & LH is no longer inhibited and their conc. increases. Given that the uterus lining isn't maintained a new cycle begins.   

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Protein synthesis

Proteins are synthesised according to the specific base sequence in DNA called a gene found at the nucleus.

The first stage of proetin synthesis is transcription: RNA Polymerase enzyme attaches to the DNA at the begining of a gene before breaking the hydrogen bonds between DNA base pairs seperating the strands. RNA Polymerase lines up free RNA nucleotides alongside the template strand before joing these by complementary base pairing to form a pre-mRNA strand. Splicing removes introns from pre-mRNA producing a smaller mRNA strand consisting of only exons which can move out through a nuclear pore into the cytoplasm for translation.

Translation is the second process of protein systhesis which occurs at the ribosomes in the cytoplasm. mRNA attaches to a ribosome and the anticodon on transfer RNA (tRNA) molecules binds to a codon on mRNA by complementary base pairing, carrying a amino acid to the ribosome. Two amino acids attached to tRNA molecules are joined by a peptide bond. As the ribososme moves along the mRNA, tRNA molecules detach and collect another amino acid forming a polypeptide chain, until a stop codon is reached and the polypeptide chain is complete. 

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Energy transfer & energy loss

  • An ecosystem includes all the organisms living in a particular area and all the non-living abiotic conditions. The main route by which energy enters an ecosystem is photosynthesis. During photosythesis, plants convert sunlight energy into chemical energy that cen be used used by other organisms. Energy moves up through the food chain as it passes between trophic levels when primary, secondary and tertiary consumers eat other organisms. Energy locked up in bones, faeces gets recycled back into the ecosystem by microorganisms called decomposers.
  • Energy that'sn used for growth and reproduction isn't lost, it becomes biomass in an organism.
  • Energy transfer is less efficient in endotherms organisms than in ectotherms because endotherms use alot of energy from respiration to keep their body tempertaure constant, whereas ectotherms such as insects don't need to do this, instead they alter their behaviour.
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1. Interaction between organisms

  • Interspecific competition is competition for resources (such as food, space, water, light, etc.) between members of different species, which could lead to one species out-competing another one.
  • Intraspecific competition is competition for resources between members of the same species. This is more significant than interspecific competition, since member of the same species have the same niche and so compete for exactly the same resources. Intraspecific competition tends to have a stabilising influence on population size because it is density dependent. EG If the population gets too big, intraspecific population increases, so the population falls again.
  • Intraspecific competition is also the driving force behind natural selection, since the individuals with the “best” genes are more likely to survive and pass on their genes to their offspring which increases the frequency of this advantageous gene. Some species use aggressive behaviour to minimise real competition. Ritual fights, displays, threat postures are used to allow some individuals (the “best”) to reproduce and exclude others (the “weakest”). Similarly, the populations of predators and their prey depend on each other, so they tend to show cyclical changes. If the population of the prey increases, the predator will have more food, so its population will start to increase. As more prey will be eaten, the prey population will decrease causing a cycle in both populations.
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2. Interaction between organisms

Parasites feed on larger host organisms, harming them. Parasites and their hosts have a close symbiotic relationship, so their populations also oscillate. If the population of parasite increases, they kill their hosts, so their population decreases. This means there's fewer hosts for the parasite, so their population decreases. This allows the host population to recover, so the parasite population also recovers. 

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Bacterial cell

The cell membrane is a phospholipid bilayer that completely surrounds a bacterial cell which is important because any break in the bilayer will lead to the death of the bacteria. The phospholipids are arranged in a bilayer (i.e. a double layer), with their polar, hydrophilic phosphate heads facing out towards water in the external environment of the membrane, and their non-polar, hydrophobic fatty acid tails facing each other in the middle of the bilayer.

Likewise, the bacterial cell membrane is a highly selective barrier. This barrier prevents materials from simply diffusing into and out of the cell. Therefore, allowing the cell to take up chemicals and nutrients needed for survival while keeping the vital cell components separated from the environment.

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Bacteria and viruses

Pathogens- microorganisms that cause infectious disease are called pathogens.

Bacteria are very small single-celled organisms but not all cause disease. But once pathogenic bacteria are inside they then attach to the host cell by receptor binding protein molecules (ligands) found in the microbial wall or viral coat. Pathogens enter by endocytosis or by producing enzymes that opens the host cell membrane. Pathogens can produce poisons (toxins) which make us feel ill. EG Exotoxins are secreted by Escherichia Coli – which affect lining of intestines and cause diarrhoea.

Viruses are much smaller than bacteria and all viruses are pathogens. Viruses produce toxins and they damage the cells in which they reproduce, leading to illness. They do this by invading cells, reproducing inside them and bursting them. This causes damage to tissues, leading to illness. Example: HIV virus damages white blood cells, reducing immunity and leading to AIDS.

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The digestive system

Digestion- the process in which large molecules are hydrolysed by enzymes to produce smaller molecules that can be absorbed and assimilated.

The small intestine is the site of final digestion and absorption. These contain villi; large structures composed of hundreds of cells whilst on them are microvilli, small subcellular structures formed by the folding of the plasma membrane of individual epithelial cells.

To maximise the rate of absorption this has three features dictated by Fick’s law: a large surface area to volume ratio, short diffusion distance and a high concentration gradient which is maintained by mixing the fluids on either side of the exchange surface through the wave action of the microvilli. Combined this makes the rate of absorption of the microvilli on the cell surface membrane on epithelial cells more efficient.

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Life on Earth evolved in the water, and all life still depends on water. At least 80% of the total mass of living organisms is water. Water molecules "stick together" due to their hydrogen bonds, so water has high cohesion. This explains why long columns of water can be sucked up tall trees by transpiration without breaking. It also explains surface tension, which allows small animals to walk on water.

Water is important during the light dependent reaction of photosynthesis of plants. As the excited electrons from chlorophyll leave PSII to move along the ETC, they must be repalced. Light energy splits water into protons (H+ ions), electrons and oxygen. Therefore, providing the electrons required, but also produce (H+ ions) that can be used to produce reduced NADP.

Osmosis is the diffusion of water across a membrane and can be quantified using water potential. The rule is that water always "falls" from a high to a low water potential.The water potential of the blood plasma that surrounds a cell affects the state of the cell, due to osmosis. If the surrounding solution hypotonic, there's a net diffusion of water into cell causing it to swell and bursts (lysis)in a animal cell whereas in a plant cell this causes it to become turgid. If hypertonic there's a net diffusion of water out of cell causing the cell to shrink and crenate in an animal cell and this causes the cytoplasm to shrink from cell wall and cell plasmolyses.

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1 Gene technology

Gene technology is the techniques that can be used to study and alter genes and their function. Scientists use PCR, In vivo cloning and DNA probes for many purposes as well as study genes. These include genetic engineering, DNA finger printing, diagnosing diseases and treating genetic disorders.

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2 Gene technology

There are three ways that DNA fragments can be produced. One method is by using the enzyme reverse transcriptase. Many cells only contain two copies of each gene, making it difficult to obtain a DNA fragment containing the target gene. Therefore, mRNA it is often easier to obtain. EG pancreatic cells produce the protein insulin and these have large amounts of mRNA coding for insulin. mRNA is isolated from cells then mixed with free nucleotides and reverse transcriptase which uses the mRNA as a template to synthesise new strands of cDNA. The cDNA is a complementary copy of the mRNA because of specific bas pairing except that it replaces Uracil with Thymine.

Another method is by using the Polymerase Chain Reaction which produces millions of copies of a fragment of DNA in just a few hours. The reaction mixture used consists of the DNA sample, free nucleotides, primers and DNA polymerase. Primers are short pieces of DNA that are complementary to the bases at the start of the fragment to be copied. DNA polymerase enzyme lines up free DNA nucleotides alongside each template strand and binds these by complementary base pairing.

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3 Gene Cloning

Gene cloning involves producing many identical copies of a gene either through In vitro cloning where the gene copies are made outside of living cells using PCR or In vivo cloning where the gene copies are made within a living organism. To produce recombinant DNA the vector DNA is first isolated.The vector DNA is then cut open using the same restriction endonuclease enzyme used to isolate the DNA fragment containing the target gene to form complementary sticky ends. This means the vector DNA and DNA fragment can be joined togther by DNA ligase to form a recombinant DNA containing DNA from two different sources. In genetic engineering a vector is needed to carry the transformed gene into a host cell since it's not part of the cell's normal genome it won't be replicated when the cell divides and therefore expressd. 

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4 Genetic engineering

A vector used such as bacteria is used to due its ability to readily take up the plasmid which makes them easy to handle in a test tube and pass on plasmid genes on to all daughter cells.

Host cells that take up the plasmid containing the gene of interest are called transformed organsims. As not all host cells  will have taken up the vector, marker genes can be used to identify the transformed cells using replica plating. Colonies that grow on the first plate treated by an anitbiotic such as tetracycline but not on the replica plate treated with ampicillin indicate that this the colony containing the desired gene.

Genetically modified crop plants can be be produced by inserting a gene that codes for a desireable charcteristic such as drought resistance into a plasmid which is then added to a bacterium. This is used as a vector to to get the gene into the plant cells. The transformed plant will have the genes that is expressed enabling these palnts to survive during drought conditions. Transformed agricultural animals can also be produced, where the desired gene such as resistance to disease, is inserted into an animal embryo in order to improve agricultural productivity.

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Pulmonary Tuberculosis (TB)

Is an infectious disease caused by the bacterium Mycobacterium  tuberculosis which is transmitted by droplets from coughs and sneezes from infected individuals. At the alveoli and bronchioles the bacterial cells multiply and remain alive but dormant. These stimulate an inflammatory response by white blood cells of the immune system resulting in the formation of fibrous scar tissue. This reduces the elasticity of the alveoli and increases the diffusion pathway. Therefore, reducing the rate of oxygen diffusion. Overtime, bacteria emerge and start reproducing inside the lung epithelial cells killing them. The alveoli have a smaller surface to volume ratio which further reduces rate of gas exchange.  As a result, the sufferer experiences the symptoms of persistent cough, tiredness and weight loss.

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Is an infectious disease caused by the bacterium vibrio cholera which is transmitted through drinking contaminated water. The bacterium binds to the epithelium of the small intestines and secretes a toxin which causes chloride ion protein channels to open. Chloride ions diffuse into the lumen of the small intestines, lowering the water potential of the lumen. Subsequently, water in the blood moves out of the blood capillaries by osmosis into the lumen which results in diarrhoea and dehydration.  In severe cases up to 20 litres of water can be lost per day and if untreated by oral rehydration therapy this can lead to death.     

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