Thursday, 20 November 2014

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
  • 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.


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.