The principle of limiting factors as applied to the effects of temperature, carbon dioxide concentration and light intensity on the rate of photosynthesis.

There are certain things that are needed for photosynthesis to take place. The availability of these things affects the rate of reaction.

If there is a lack of something that is needed then the rate of reaction will decrease, making it a limiting factor.

Temperature can be a limiting factor because heat energy increases kinetic energy and therefore there are more collisions and so more enzyme-substrate complexes formed.

Temperature directly correlates to rate of photosynthesis until it gets over a heat where it will denature enzymes; then the rate will decrease as there are fewer enzymes to aid the reactions.

Carbon dioxide is a chemical used so the more of it there is the more reactions can be done and visa versa.

Light intensity has an effect because the protons from light are what power the light-dependent reaction, so if there is not enough light the process will happen less frequently.

The light-independent reaction in such detail as to show that • carbon dioxide is accepted by ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP) • ATP and reduced NADP are required for the reduction of GP to triose phosphate • RuBP is regenerated in the Calvin cycle • Triose phosphate is converted to useful organic substances.

In the stroma of a chloroplast the light-independent stage of photosynthesis happens. It involves taking up carbon to make glucose using the products from the light-dependent stage.

The molecule RuBP (ribulose biphosphate) is made up of five carbons, a molecule of carbon from CO2 is added to make a six carbon compound. This is very unstable and quickly breaks down into two bits of GP (glycerate 3-phosphate) each with three carbons.

GP is then reduced by NADPH, which gives over its H to become NADP, and restructured by ATP, giving energy as it breaks bonds. The molecule made from this is triose phosphate and because there were two GP we have two triose phosphate molecules too.

One carbon is removed, and this carbon is used to make glucose. There are 6 carbons in a glucose so it is made every sixth time the process is repeated.

The remaining five carbons (two from one molecule of triose phosphate and three from the other) are the rearranged using the energy from ATP to create RuBP (the origional 5 carbon molecule).

This is known as the Calvin cycle.

The light-dependent reaction in such detail as to show that • light energy excites electrons in chlorophyll • energy from these excited electrons generates ATP and reduced NADP • the production of ATP involves electron transfer associated with the electron transfer chain in chloroplast membranes • photolysis of water produces protons, electrons and oxygen.

Inside a plant cell there are chlorophyll organelles and these contain grana, stacks of thylakoids.

The thylakoids have phospho-lipid membranes which have proteins studded throughout. There are intrinsic proteins which have the pigment chlorophyll attached and these are called photosystems (PS). There are two different types of photosystems because there are two different pigments of chlorophyll which absorb different wavelengths of light, they are PSII and PSI.

When light hits PSII the chlorophyll absorbs protons (light) which energise two electrons. These electrons have so much energy they leave the outer shell (highest energy level) of their atom and become free.

They move to a protein which can take them on- an electron carrier- but are then moved from this to another one and so forth in what is know as an electron transfer chain (ETC). (When every the electrons join a molecule we say the molecule is reduced and when it leaves the molecule is oxidised so you often hear 'the electrons move through the ETC in a series of oxidation reduction reactions).

Each time the electrons move they loose some energy to the proteins this means that 1) the electron will end up attached to PSI at a normal energy level and 2) the ETC will have gained some energy from them; it uses this to change a ADP molecule and a inorganic phosphate into ATP, which moves into the stroma.

The second photosystem absorbs protons and the energy excites the electrons again. The energy is also used to split water into hydrogen and oxygen (and electrons which go to PSII to replace the ones that were energised out). The oxygen is a waste product, but the hydrogen joins up with the excited electrons and a molecule of NADP to form NADPH (reduced NADP) which is carrys the two electrons and hydrogen out of the thylakoids and into the stroma.

The light-independent and light-dependent reactions in a typical C3 plant.

The basic equation for photosynthesis is:

       6CO2        + 6H2O > C6H12O6 +  6O2

Carbon dioxide +  water >    glucose    + oxygen

However, this is a massive over simplification, and there are actually many things happening in-between the products and reactants.

Photosynthesis is a series of reactions that take place in the chloroplast of leaf cells, these reactions are split into two stages: the light dependent and the light independent.

Leaf structure:

• Wax cuticle lets light in but stops too much water from being lost
• Palisade cells are long to increase the chances of sunlight hitting the chlorophyll that they contain
• Spongy mesophyll has air gaps for quick gas exchange
• Vascular bundle (xylem (and phloem) brings water to the reaction
• Stomata allows CO2 in but doesn't let too much water out
• Leaves are thin so the diffusion distance is small and the SA for catching light is big

Chloroplasts:

• The chloroplast envelope- double plasma membrane to control the movement of substances in and out
• The grana- stacks of disks (thylakoids) which contain chlorophyll and have a large surface area for the first stage of photosynthesis.
• Interlamellae link gramun so that ions can be transported between them.
• The stoma- matrix with the enzymes needed for the second stage of photosynthesis
• DNA and ribosomes to manufacture proteins for photosynthesis

The synthesis of ATP from ADP and phosphate and its role as the immediate source of energy for biological processes.

Synthesis

ATP and ADP + an inorganic phosphate

ATP is made up of adenine, ribose and three phosphates. When energy needs to be released, the last phosphate group is broken off by hydrolysis (bonding the molecule with water), releasing energy stored in the bond. This leaves ADP and an inorganic phosphate, which can be turned back into ATP by condensation (removal of water).

ATP + H2O > ADP +P(i)
ADP + P(i) > ATP + H2O

Energy source

Energy is released in suitable, small amounts.
It is soluble.
There is a single, simple reaction to release it.

Energy is needed for the processes that living things carry out, for example active transport, muscle contraction, glycolysis.

Population size may vary as a result of • the effect of abiotic factors • interactions between organisms: interspecific and intraspecific competition and predation.

The size of a population changes over time and is effected by many different factors.

They key factors are:

• Birth and death rates
• Immigration and emigration- individuals moving in and out of the habitat
• Biotic potential- the maximum rate that a species can reproduce at
• Carrying capacity (K)- biotic and abiotic limiting factors on population size

Average population growth curve

A species moves into an environment, and begins to breed, birth rate is above death rate which means there is population growth. At first there are a small number of individuals so only a few individuals are born meaning the growth rate is slow, but as more as the population grows the birth rate increases meaning population growth increases.

The population grows so large that the environment can no longer support it, this means it has surpassed the carrying capacity, for example there are too many individuals for the amount of food, this causes the death rate to increase. An increase in death rate above birth rate means the population starts to decrease.

The population decreases below the carrying capacity meaning that it is no longer limited by the environment, e.g. there is enough food for all the individuals. This means that the death rate decreases below the birth rate so population beings to increase. The population will increase above the carrying capacity and the cycle will repeat its self so the population fluctuates around the carrying capacity.

Abiotic limiting factors

These are the non-living factors that stop a population from growing above a certain point:

• Water- involved in respiration
• Humidity- low humidity means water evaporates out of organisms
• PH- effects enzyme action which changes the speed of chemical reactions
• Temperature- ''
• Light- energy source
• Hummus (organic matter)- nutrients available for reactions

Biotic limiting factors

These are the ways in which plants and animals limit the size of a population:

• Food- plants and animals supply energy for survival
• Disease- viruses, fungi and bacteria can cause death and infertility
• Competition- for food, water, shelter
• Predation- animals and plants are often killed as a result of being prey

Interspecific competition: a niche is the needs of a species e.g. the types of food they eat. If two species have the same niche then one of the species will out compete the other and the other will die out (competitive exclusion principle).

Intraspecific competition: individuals in a species have the same niche, therefore if the population grows above carrying capacity and there is not enough of a resource there will be fierce competition.

In species that have a metamorphosis the first stage is much slower (as there is a higher density of individuals) which means only a few will make it to adulthood, this is beneficial as the mature and immature individuals eat different food so there is less competition.

Predation: predators eat prey, so there is less prey available, this means the predators have less to eat so their population decreases, this means that the prey are killed less often so the population increases, which leaves more food for the predators so their population in turn increases and the cycle will repeat.

A critical appreciation of some of the ways in which the numbers and distribution of organisms may be investigated. Random sampling with quadrats and counting along transects to obtain quantitative data. The use of percentage cover and frequency as measures of abundance. The use of mark–release–recapture for more mobile species.

There are many reasons that people would need to keep track of populations, like conservation or pest control, and there are many ways to do it.

Sampling involves using a quadrat to discover the number of a species in at a given point. There are two main types of quadrat: the frame quadrat is a square divided up into sections, like a grid; the point quadrat is a standing frame which allows for needles to be dropped through it so they touch the ground (which ever species the needle touches is recorded).

Sampling can be random to avoid bias, this involves dividing an area up on a grid and the using a random number generator to create coordinates which pick out an area to use a quadrat in. Alternatively sampling may need to be systematic for instance if you are looking for a pattern. In much the same way the given area is divided up but this time the quadrats are at regular intervals.

Transects are a form of systematic sampling where the data are taken at equal spaces along a single straight line; this is useful if you are looking for a change over distance e.g. distance from sea. There are two types of transect: a line transect is where you count everything touching the tape measure at a given point; belt transect is where you put a quadrat down at a given point.

The data gained from sampling can be used to calculate abundance in multiple ways:

Frequency: this is looks at how many times the species was present out of all the samples taken.
For example, if there are ten squares in your quadrat and there is grass in five of the squares then the frequency is 5/10.

Percentage cover: this focuses specifically on the percentage of space on the ground that a species covers. It is useful if the plant is very abundant or it is difficult to distinguish between different plants e.g. grass.
For example: there are ten squares in my frame quadrat, daisies cover 3 whole square and two half squares
3+0.5+0.5= 4
4/10= 0.4
0.4 *100= 40%

For more mobile species, so animals, the previously stated methods of data collection are not very effective: the same animal could walk around and be counted in every single quadrat you do, alternatively all the animals might leave an area you are sampling from at that moment. The way to get round this is by using a method called mark-release-recapture:

Mark: spend an allotted amount of time capturing as many of a species as you can, every time you mark one you must put a mark on it.

Release: put all the members of the species back where you found them.

Recapture: go back at a time with the exact same conditions (e.g. time of day/season/temperature) and spend the exact same amount of time capturing as many of the species as you can. Record the number of animals you caught and the number that had marks on then put the figures into the Lincoln index:

number in first sample * number in second sample
/number of marked individuals in second sample

The marks may cause a disadvantage to the animals, they may die which will affect your data, but, also, it is a moral issue. Other ethical issues include taking removing animals from their natural habitat and disturbing a habitat by entering it