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ESSAY
A cycle is a biological pathway or process, in which the end product of one cycle
becomes the starting point of the next cycle
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Calvin cycle is a part of the light-independent stage of photosynthesis but it relies on the
products of the light-dependent reactions
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Its
starting product is carbon dioxide which combines with 5C RuBP
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It produces 3C TP
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TP is
first converted into 5C RuB, and then phosphorylated (using ATP) into RuBP, which can then
combine with another molecule of carbon dioxide
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Cyclic photophosphorylation is one of the light-dependent reactions that take place during
photosynthesis and occurs in the thylakoid membrane, specifically in the PS1 (P700)
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This energy excites electrons, some of which will enter the next stages
of non-cyclic photophosphorylation and be transferred to NADP
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This reaction produces small amounts of ATP
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Krebs cycle is the third stage of respiration and takes place in the matrix of the mitochondria
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The aim of this cycle is to produce ATP, reduced NAD and
reduced FAD which will then be used in oxidative phosphorylation to generate more ATP
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5C compound undergoes a series of reactions which turn it back into oxaloacetate, and
produce CO (5C is decarboxylated to become 4C oxaloacetate), reduced NAD, reduced
FAD and ATP
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2

2

Nitrogen is an essential component of amino acids and DNA and is needed for growth of all
living organisms
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The nitrogen cycle starts off with the inorganic
nitrogen gas from the atmosphere, which cannot be absorbed by plants or animals
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Plants use nitrogen for growth and their nitrogen compounds might be eaten by
primary consumers (which then are eaten by secondary consumers etc)
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Ammonium compounds
from the soil are nitrified by nitrifying bacteria into nitrites, and then nitrates
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Carbon is another element used to make essential compounds, e
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plants use CO to make
glucose, which enables animals to respire
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Carbon dioxide from air and water is absorbed by plants, which convert it
into carbon compounds during photosynthesis and store it in plant tissues
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Carbon compounds in the dead organisms are digested by decomposers (saprobiontic
microorganisms)
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If dead organic matter is not decomposed, over time it is turned into fossil fuels
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burning of
the fossil fuels
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The last biological cycle takes place in the body, specifically in female humans and primates
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Its aim is to prepare the body for pregnancy and it is
repeated if fertilisation does not occur
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Follicular stage
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The follicle releases oestrogen, which stimulates the
uterus lining to thicken (so that a fertilised egg can implant) and inhibits FSH
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When oestrogen level is high enough, it boosts the secretion of LH and
FSH from the pituitary gland and ovulation occurs (around day 14)
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Luteal stage
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Progesterone maintains the thick lining of the uterus; it also inhibits
FSH and LH
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The drop in the level of progesterone means the uterus lining is no longer maintained,
breaks down and leaves the body via the vagina
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The cycle starts again, to give the body another ‘chance’
to become impregnated and reproduce
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Photosynthesis (light-independent)
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6xCO enters
2

the cycle
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Combines with 6xC5 RuBP
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Produces
12xC3 TP
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g
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10xC3 TP → 6xC5 RuB →
regenerate 6xC5 RuBP using ATP
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2

2 Cyclic photophosphorylation
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Thylakoid membrane
(PS1/P700)
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Electrons are excited
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Small amounts of ATP (no reduced NADP or O )
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3 Krebs cycle
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Matrix of the mitochondria
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Decarboxylation and dehydrogenation
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Decarboxylation and dehydrogenation, produces reduced FAD, reduced NAD and more CO
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2

2

4 Nitrogen cycle
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Nitrogen gas from the atmosphere, Rhizobium, ammonia → plants
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Nitrification: nitrites → nitrates
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Nitrates used in anaerobic respiration by
denitrifying bacteria
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Carbon is used to make essential compounds, e
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plants use CO to make
glucose
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2

Eaten by primary consumers, secondary, tertiary
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Respiration returns carbon dioxide to the air and water
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6 Oestrous cycle
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Hormone levels fluctuate and the lining of the
uterus prepares for pregnancy
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Thickening of uterus lining - oestrogen
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Luteal stage → Follicle develops into corpus luteum - LH
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No fertilisation → breakdown of corpus luteum,
progesterone falls, FSH rises
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Back to day 1
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Uses rubisco, ATP and
reduced NADP
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PS1
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Uses NAD and FAD,
produces CO and ATP
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Photosynthesis,
respiration,
decomposition,
combustion
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Follicular, ovulatory, luteal
stages
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by PS1
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Electrons recycled
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4C oxaloacetate
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2

Nitrogen cycle

Carbon cycle

Oestrous cycle

Cardiac cycle

Inorganic nitrogen gas Nitrates in the soil are used
in the atmosphere
for respiration to produce
atmospheric nitrogen gas
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Hormone levels
Menstruation
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SAN initiates a wave Cardiac cycle repeats,
of excitation at regular therefore is called
intervals
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(No products)

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