Discuss the importance of cycles in biology
A significant stage of respiration is the Krebs cycle, which occurs in the mitochondrial matrix.
Pyruvate produced in glycolysis is oxidised and decarboxylated into acetate which combines
with a coenzyme to form acetyl coenzyme A in the link reaction. Acetylcoenzyme A then
reacts with a 4C molecule, releasing the coenzyme and forming a 6C molecule which enters
the krebs cycle. In a series of oxidation-reduction reactions, the 4C molecule is regenerated.
During this, 2 CO2 molecules are lost, coenzymes NAD and FAD are also reduced as well
as the production of ATP via substrate level phosphorylation. The krebs cycle produces
important products needed for oxidative phosphorylation, the final stage of respiration. The
reduced NAD and FAD produced are oxidised, releasing hydrogen atoms that are split into
protons and electrons. These electrons are transferred down the electron transport chain, by
redox reactions, releasing energy. This energy is used to actively pump protons from the
matrix into the intermembrane space, creating an electrochemical gradient used to transport
protons into the matrix via ATP synthase, releasing energy to synthesise ATP from ADP and
Pi. This production of ATP is relied on by many processes, including active transport where
molecules move against their concentration gradient. Particularly in the co-transport of
glucose in the ileum of the small intestine, sodium ions are actively transported from
epithelial cells to the blood using energy released from the hydrolysis of ATP produced by
the oxidation of reduced coenzymes in respiration. Without this, glucose wouldn't be able to
enter the blood and be circulated through the body to respiring cells to continue the cycle.
The cell cycle is another important cycle in biology. In the G1 phase of interphase, new
organelles are synthesised - followed by S phase where semi-conservative DNA replication
occurs and finally G2 phase of interphase where spindle fibre formation from centrioles
occurs and cell growth continues. After interphase, Mitosis begins in prophase where
chromosomes become condensed and visible before lining at the middle of the cell in
metaphase. Spindle fibres also attach to the centromeres of each chromosome in
metaphase, contracting to separate sister chromatids to opposite poles of the cell in
anaphase. Finally, in telophase the spindle fibres are broken down and nuclear envelopes
begin to form around each group of chromosomes followed by cytokinesis where the
cytoplasm divides forming two genetically identical daughter cells. This is important in our
primary immune response against pathogens, where the cell mediated and humoral
responses occur to produce specific B memory cells and antibodies. Specific helper-T cells
undergo mitosis to form clones which stimulate cytotoxic-T cells which kill pathogenic cells
by secreting perforin (a glycoprotein) which creates pores in the surface membrane,
increasing membrane permeability, and ultimately leading to cell death. T cells also stimulate
the clonal expansion of B cells in the humoral response. After these specific B cells are
activated, they divide by mitosis into B plasma and memory cells, specific to the pathogen,
which remain in the blood and recognise pathogens upon secondary exposure. This allows
the secondary response to produce specific antibodies quicker and at a faster concentration,
destroying pathogens before symptoms can occur.
The Calvin cycle occurs in the stoma as part of the light independent reaction of
photosynthesis. It begins with a reaction between RuBP, a 5 carbon molecule and a
molecule of CO2, catalysed by rubisco to form 2 molecules of glycerate- 3 phosphate. The
GP is then reduced into triose phosphate, by gaining a hydrogen from reduced NADP and
A significant stage of respiration is the Krebs cycle, which occurs in the mitochondrial matrix.
Pyruvate produced in glycolysis is oxidised and decarboxylated into acetate which combines
with a coenzyme to form acetyl coenzyme A in the link reaction. Acetylcoenzyme A then
reacts with a 4C molecule, releasing the coenzyme and forming a 6C molecule which enters
the krebs cycle. In a series of oxidation-reduction reactions, the 4C molecule is regenerated.
During this, 2 CO2 molecules are lost, coenzymes NAD and FAD are also reduced as well
as the production of ATP via substrate level phosphorylation. The krebs cycle produces
important products needed for oxidative phosphorylation, the final stage of respiration. The
reduced NAD and FAD produced are oxidised, releasing hydrogen atoms that are split into
protons and electrons. These electrons are transferred down the electron transport chain, by
redox reactions, releasing energy. This energy is used to actively pump protons from the
matrix into the intermembrane space, creating an electrochemical gradient used to transport
protons into the matrix via ATP synthase, releasing energy to synthesise ATP from ADP and
Pi. This production of ATP is relied on by many processes, including active transport where
molecules move against their concentration gradient. Particularly in the co-transport of
glucose in the ileum of the small intestine, sodium ions are actively transported from
epithelial cells to the blood using energy released from the hydrolysis of ATP produced by
the oxidation of reduced coenzymes in respiration. Without this, glucose wouldn't be able to
enter the blood and be circulated through the body to respiring cells to continue the cycle.
The cell cycle is another important cycle in biology. In the G1 phase of interphase, new
organelles are synthesised - followed by S phase where semi-conservative DNA replication
occurs and finally G2 phase of interphase where spindle fibre formation from centrioles
occurs and cell growth continues. After interphase, Mitosis begins in prophase where
chromosomes become condensed and visible before lining at the middle of the cell in
metaphase. Spindle fibres also attach to the centromeres of each chromosome in
metaphase, contracting to separate sister chromatids to opposite poles of the cell in
anaphase. Finally, in telophase the spindle fibres are broken down and nuclear envelopes
begin to form around each group of chromosomes followed by cytokinesis where the
cytoplasm divides forming two genetically identical daughter cells. This is important in our
primary immune response against pathogens, where the cell mediated and humoral
responses occur to produce specific B memory cells and antibodies. Specific helper-T cells
undergo mitosis to form clones which stimulate cytotoxic-T cells which kill pathogenic cells
by secreting perforin (a glycoprotein) which creates pores in the surface membrane,
increasing membrane permeability, and ultimately leading to cell death. T cells also stimulate
the clonal expansion of B cells in the humoral response. After these specific B cells are
activated, they divide by mitosis into B plasma and memory cells, specific to the pathogen,
which remain in the blood and recognise pathogens upon secondary exposure. This allows
the secondary response to produce specific antibodies quicker and at a faster concentration,
destroying pathogens before symptoms can occur.
The Calvin cycle occurs in the stoma as part of the light independent reaction of
photosynthesis. It begins with a reaction between RuBP, a 5 carbon molecule and a
molecule of CO2, catalysed by rubisco to form 2 molecules of glycerate- 3 phosphate. The
GP is then reduced into triose phosphate, by gaining a hydrogen from reduced NADP and
