Drugs

Excitatory drugs:

-NICOTINE
Found in tobacco products. It is a stimulant which mimics the neurotransmitters acetlcholine (Ach) as they bind to the same receptors. It causes the calming effect. Unlike Ach the nicotine cannot be broken down by the enzme acetyl choline esterase. Therefore the effect of nicotine is longer lasting. The effect is pleasure because the nicotine excites the post synaptic neurone releasing a molecule called dopamine. And dopamine gives the feeling of pleasure the molecules is a 'reward' of our brains.

-COCAINE
Dopamine transporters are responsible for removing dopamine molecules from the synaptic cleft after they have done their job.
However, cocaine blocks and inhibits the activities of these transporters. Thus dopamine is trapped in the synaptic cleft. Therefore dopamine binds again and again to the receptors resulting in the overstimulation of the cell.

Outcome: concentrates the reward pathway of our brain. The part that also controls voluntary movements. This is why concaine users are unable to stay still.

-AMPHETAMINE
-stimulates tramission of adrenergic synapses and gives the user energy and increase alertness.
This drug moves into the cesciles of the presynaptic neurgone and cuases their release into the synaptic cleft. Normally these neurotransmitters would be broken down by enzymes but the amphetamine inhibits this from happening.

The high concentrations of dopamine cause euphoria as high concentraions of noradrenaline cause increased alertness and energy.

Inhibitory drugs

-BENZODIAZEPINE
medical use: reduces anxiety and against epileptic seizuers.

Benzodiapine increases the binding of GABA which is an inhibitory neurotransmitters. When the GABA binds to the post synaptic membrane it causes chloride ions to enter and potassium ions to exit . This hypoerpolarizes the neurone and inhibits an action potential from being generated.

The increased binding of GABA causing the post synaptic neurone to become more hyperpolarized.


-ALCOHOL

When alcohol enters the brain it interacts with GABA receptors to make them more inhibitory and it beinds to glutamate receptrs repventing the glutamate from exciting the cell.

As a result the areas invovling memory formation, decision-making and impulse control is slowed down.

-TETRAHYDROCANNABINOL [THC]

This is the main chemical in marijuana.
Usually inhibitory neurotransmitters are active in the synapse. These neurotransmitters inhibits dopamine from being released.

The active chemical in marijuana binds to the receptors that inhibit the release of dopamine: cannabinoid receptors. As a result the inhibition if turned off and dopamine is allowed to squirt into the synapse.
As a result the user feels increased pleasure due to the abnormally high concentrations of dopamine.

Questions

1a) explain how pre-synaptic neurons can affect post-synaptic transmission of impulses [7]

Presynaptic neurons can either be exicitory or inhibitory. Specific types of neurons such as chlorogenic neurons release a type of neurotransmitter called acetylcholine.
The neurotransmitters are released when the action potential reaches the terminal buttons of the pre-synaptic neuron. This causes the calcium ion channels to open and thus difuse into the neuron. This causes the vesicles containing the neurotransmitters to to fuse with the plasma membrane in a process called exocytosis releasing the neurotransmitters into the pre-synaptic cleft.
The neurotransmitters then bind with receptors on the post synaptic cleft.
This binding action causes sodium ion channels or gates to open causing sodium ions to diffuse into the cells causing depolarization and this generates an action potential and thus the impulse is passed on.
However, in an inhibitory situation. The binding action of the neurotransmitters cause chlorine channels and potassium channels to open thus resulting in an influx of chlorine ions into the post synaptic neuron and potassium ions to diffuse out thus causing hyperpolarization.
The neurotransmitters are then deactivated/destroyed by enzymes and is recycled.
Examlpes of inhibitory neurotransmitters: GABA. 

1b) Explain the process of synaptic transmission [7]

Presynaptic neurons can either be excitory or inhibitory depending on the type of membrane receptor on the post-synaptic neurone.
Pre-synaptic neurons releases neurotransmitters into the synaptic cleft this is done by a process called exocytosis which requires energy. The fusing of the vesicles containing the neurotransmitter is triggered by the action potential triggering the opening of calcium ion channels. The influx of calcium ions is what triggers the vesicles to move and fuse with the membrane. Thus releasing the neurotransmitters into the synaptic cleft.
The neurotransmitters bind with the receptors on the post synaptic cleft. In an excitory situation the binding action causes sodium ion channels to open and causes sodium ions to diffuse into the cell causing depolarization causing an action potential to be created and the impulse to be passed on.

However, in an inhibitory situation the binding action causes chlorine and potassium ion channels to open causing chlorine ions to diffuse in and potassium ions to diffuse out thus causing hyperpolarization which prevents an action potential from being generated and thus inhibits the passing on of the impulse. The neurotransmitters are then destroyed/degraded/broken down by enzymes and recycled. An example would be the neurotransmitters acetyl chloline being broken down by the enzyme acetyl chline esterase.

Whether it is a excitatory or inhibitory situation will depend on the type of receptor on the pre-synaptic neuron.

Question 7

7a) The carbon cycle invovles both the production and the fixation of carbon dioxide. Draw a labelled diagram to show the processes invovled in the carbon cycle. [5]

Reference [URL]: http://eo.ucar.edu/kids/green/cycles6.htm

7b) Explain the light-independent reactions of photosynthesis [8]

Light independent reactions occur in the stroma of the chloroplasts in a metabolic pathway called the calvin cycle.

Reference [URL]: http://www.bio.umass.edu/biology/conn.river/calvin.html



The cycle uses ATP and NADPH2 as an energy source. The first stage is carbon dioxide fixation. This is when carbon dioxide from the atmosphere joins with ribulose biphosphate (RuBp) this reaction is catayalsed by the enzyme Rubisco or RuBP carboxylase. It is the most abundant protein on earth. Then we get a 6 carbon compound which due to it’s instability then splits in half to form two molecules of phosphoglyerate (a  3 carbon molecule). Then this molecule is reduced in the second stage known as reduction using ATP and NADPH2 from the light dependent reaction to convert phosphoglycerate into glyceraldehyde 3 phosphate. This is the molecule that is a component of glucose and other sugars. The last phase is called the regeneration phase where ATP is used to convert some of the glyceraldehyde 3 phosphate back to RuBP thus completely the cycle. Note that for every 3 molecules of carbon dioxide (3 turns of the calvin cycle) one molecule of glyceraldehyde (G3P) is removed from the cycle. For each G3P produced the cycle uses 9 molecules of ATP and 6 molecules of NADPH2. One glucose molecule consists of two G3P molecules therefore 6 turns of the calvin cycle produced one molecule of glucose thus expending 18 molecules of ATP and 12 molecules of NADPH2.

Note that the light independent reaction although it doesn’t require light directly still needs light as the light dependent reactions provide the calvin cycle with the ATP and NADPH2.

7c) Outline the consequences of rising carbon dioxide concnetrations in the Earth's atmosphere. [5]

Gas produced by humans activity retain heat even more than the natural levels of greenhouse gases that retain some of the heat and trap it in the atmosphere. Gases such as carbon dioxide, methane and the oxides of nitrogen. Increased by increased activities such as burning fossil fuels and cattle ranching. A rise in human population resulting in the increase in these human activities and hence an increase in the concentration of these greenhouse gases. Since all 3 atmospheric gases have high potential for absorbing heat, these gases are responsible for intensifying the earth’s natural greenhouse effect.

Summary of impacts:

-increased photosynthetic rate

-extinction of certain species

-melting glaciers (land ice)

-a rise in sea level which would result in floods of coastal areas.

A case study for the impact would be human impact on artic ecosystems. Ecosystems are changing in other words intact ice which has the capacity to be the home for algae as they provide an area for the algae to attach. The surface gives the algae a place to attach and the transparency allows sunlight to pass through. As artic ice melt there is less and less surface for algae to stick to and there is a reduction in food for the next trophic level. As the algae is a producer which supports a lot of food chains and is one of the foundation for artic foodwebs.

Moreover, polar bears use ice for hunting. The technique is that they stand on the ice and swoop seals to eat when the seals jump. However, since there is less ice this technique becomes less productive and so there has been a reduction in successful catches resulting in the decrease in the number of polar bear cubs. Which the decrease in the polar bear population due to the rising carbon dioxide concentration has lead to changes in the population of other trophic levels as well such as the rise in the population of seals.

Also as temperature increased the habitat of organisms from ore temperature climate extend northwards. This is evident by mosquitoes and robins arriving in areas up north where they were never found before. New species arising is means that new pathogens may be introduced.

Also detritus which is frozen in the tundra will thaw and begin to decompose, releasing even more carbon dioxide gas and methane into the air.

Question 5

5a) Plants are classified together in a kingdom. Other organisms are classified in other kingdoms. Outline the value of classifying organisms. [4]

Classification allows us to arrange all the different organism into neat groups in order to allow us to easily identify different species easily if they are put into groups with other organisms who share similar characteristics. Moreover, classification allows us to predict which group or taxa the organisms is roughly in based on the fact that different taxa’s share common characteristics which becomes more and more specific as we move down the taxa system. Also, classification allows us to see the evolutionary links because if organisms share similar characteristics then it can be an indication that they share a common ancestor because classification emphasizes homologous structures as traits which are common using classification may be traced back to a common ancestors. Lastly, classification allows effective communication if all scientists use the same terminology allows scientists to have a common language so that they can easily share findings.

5b) Draw a labelled diagram to show the external parts of a NAMED dicotyledonous plant. [5]

Binomial name: Catharanthus roseus
common name: Madagascar Periwinkle

reference [URL]: http://www.google.co.th/imglanding?q=dicot%20diagram&imgurl=http://www.cic-caracas.org/departments/science/images/08Dicot.jpg&imgrefurl=http://www.cic-caracas.org/departments/science/Topic13.php&usg=__IJenLKCgDV2PKO3jwMZQlkqvAdY=&h=260&w=311&sz=13&hl=th&sig2=lrBOhm28kONUCuJ67xDWow&zoom=1&um=1&itbs=1&tbnid=GD-XoVF5YwW84M:&tbnh=98&tbnw=117&prev=/search%3Fq%3Ddicot%2Bdiagram%26um%3D1%26hl%3Dth%26biw%3D1259%26bih%3D654%26tbm%3Disch&ei=7unuTb-_DYuzrAe54KDJBQ&um=1&biw=1259&bih=654&tbm=isch&start=0#tbnid=GD-XoVF5YwW84M&start=0

 5c) Explain how roots absorb water and then transport it to the xylem, noting any special adaptations that help these processes to occur. [9]

Transport of water into the roots occurs by a process called osmosis. Osmosis is the movement of water from an area of high water concentration to an area of low water concentration across a partially permeable membrane. This process is only possible because the concentration of water in the soil is greater than inside the cells. Water then crosses the epidermal cells of the root hair and travels by 2 possibly routes.
The first route is called the apoplastic root. This is when water move into the root hair straight through the cell walls. It does not penetrate through the cell wall and through the plasma memebrane but instead seeps through the cell wall (analogy: like water seeping through blotting paper) and move within the extracellular spaces (spaces outside of the cell). However, the water cannot get into the endodermic because of the Casparian strip which is impermeable to water. Therefore the water has to pass through the endodermal cell (this time water goes through a plasma membrane) and then it enters the vascular tissue that is the xylem. This is an adaptive process which allows selective filtering of certain minerals in the water because the plasma membrane of the endodermal cells are permeable to certain minerals.
The second route is called the symplastic route whereby the water move into the roote hairs and they go straight through the plasma membrane and straight through the endodermal cells and into the vascular tissue as in the apoplastic route.
Specifical adaptations:
The roots have a large surface area in relation to their volume by having the branching of the roote and root hairs increase the surface area even more. Since water is absorbed into the roots by osmosis it is only possible because of the solute concentration difference inside the root being higher inside the root then in the soil. This solute concentration is created by the active transport of mineral ions into the root. Lastly, once water enters xylem it is transported UPWARDS due to the transpiration pull (analogy: tugging a rope) which is made possibly by the polar nature of water molecules which result in hydrogen bonding resulting in the cohesion between water molecules.

A diagram to show the two different routes that water can take to reach the xylem

Reference [URL]: http://www.bodine.phila.k12.pa.us/herbstritt/PlantScienceCurriculumSummary.pdf