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Movement in cells and organisms is essential for basic, life-sustaining functions. In some
cases, such as in the heart, the control of the movement of the heart is essential for that
organism’s survival, if it stops, tissues become starved of oxygen and die. In plants, the
movement of protons in the light-dependent reaction, as well as the movement of gases in
the plant, is essential for photosynthesis. In this way, the control of movement is perhaps
more important than the movement itself.
In humans, the control of the heart is essential in maintaining life, responsible for providing
respiring tissues with the oxygen they need via the blood. The cardiac cycle begins with a
wave of electrical excitation which spreads from the sinoatrial node (SAN) across both atria,
causing them to contract. The atrioventricular septum, however, is not myogenic, so the
non-conductive atrioventricular septum prevents the wave from reaching the ventricles.
However, the wave enters a second group of cells called the atrioventricular node (AVN).
The AVN, after a short delay, conveys a wave of electrical excitation between the ventricles,
along a series of specialised muscle fibres called Purkinje fibres, which make up the bundle
of His. This bundle conducts the wave through the atrioventricular septum to the base of
the ventricles, where the bundle branches into smaller fibres of Purkinje tissue. This causes
the ventricles to contract at the same time, form the bottom up to force blood up and out of
the heart. In this way, the SAN and AVN control the heart rate. Their importance, therefore,
is paramount. For example, if too much carbon dioxide builds up in the blood, it results in an
acidic pH. Chemoreceptors in the wall of the carotid arteries and the aorta detect this and
increase the frequency of nervous impulses to the medulla oblongata. The medulla
oblongata increases the frequency of impulses via the sympathetic nervous system to the
SAN. This then increases the rate of production of electrical waves of the SAN, resulting in
an increased heart rate. This leads to an increased amount of blood flow, resulting in more
carbon dioxide being removed by the lungs. In this way, the medulla oblongata controls the
SAN which controls the heart rate. Without this form of control, the blood can become too
acidic, leading to a condition called acidosis. This signals that the body does not have
enough oxygen to provide respiring cells, which could lead to tissue and cell death. In this
way, the medulla oblongata, SAN and AVN are responsible for ensuring adequate
oxygenation of tissues to ensure their survival.
In humans, the heart is responsible for pumping blood around the body, such as to the lungs
for gas exchange. In plants, however, gas exchange does not require a mass transport
system. For example, the movement of carbon dioxide and oxygen in and out of the plant is
controlled by the guard cells, which open and close the stomata. If the water content of the
plant is too low, the guard cells become flaccid, resulting in the stomata closing. This then
prevents carbon dioxide from entering the leaf, resulting in the reduction of photosynthesis,
as carbon dioxide is needed for the light-dependent reaction. This also allows water to be
conserved until more can be found, as water is also a starting reactant of photosynthesis,
needed for photolysis to provide electrons for the electron transport change. This means
the guard cells and stomata become important in the regulation of photosynthesis.
Furthermore, the control of protons in the electron transport chain is also important in
photosynthesis. As electrons move down the chain, after being excited at photosystem II by
light energy, they lose energy. This energy is then used by proton pumps to pump protons
from the stroma into the thylakoid via active transport. This leads to a high concentration of
protons inside the thylakoid. Therefore, they leave the thylakoid by facilitated diffusion
1
Movement in cells and organisms is essential for basic, life-sustaining functions. In some
cases, such as in the heart, the control of the movement of the heart is essential for that
organism’s survival, if it stops, tissues become starved of oxygen and die. In plants, the
movement of protons in the light-dependent reaction, as well as the movement of gases in
the plant, is essential for photosynthesis. In this way, the control of movement is perhaps
more important than the movement itself.
In humans, the control of the heart is essential in maintaining life, responsible for providing
respiring tissues with the oxygen they need via the blood. The cardiac cycle begins with a
wave of electrical excitation which spreads from the sinoatrial node (SAN) across both atria,
causing them to contract. The atrioventricular septum, however, is not myogenic, so the
non-conductive atrioventricular septum prevents the wave from reaching the ventricles.
However, the wave enters a second group of cells called the atrioventricular node (AVN).
The AVN, after a short delay, conveys a wave of electrical excitation between the ventricles,
along a series of specialised muscle fibres called Purkinje fibres, which make up the bundle
of His. This bundle conducts the wave through the atrioventricular septum to the base of
the ventricles, where the bundle branches into smaller fibres of Purkinje tissue. This causes
the ventricles to contract at the same time, form the bottom up to force blood up and out of
the heart. In this way, the SAN and AVN control the heart rate. Their importance, therefore,
is paramount. For example, if too much carbon dioxide builds up in the blood, it results in an
acidic pH. Chemoreceptors in the wall of the carotid arteries and the aorta detect this and
increase the frequency of nervous impulses to the medulla oblongata. The medulla
oblongata increases the frequency of impulses via the sympathetic nervous system to the
SAN. This then increases the rate of production of electrical waves of the SAN, resulting in
an increased heart rate. This leads to an increased amount of blood flow, resulting in more
carbon dioxide being removed by the lungs. In this way, the medulla oblongata controls the
SAN which controls the heart rate. Without this form of control, the blood can become too
acidic, leading to a condition called acidosis. This signals that the body does not have
enough oxygen to provide respiring cells, which could lead to tissue and cell death. In this
way, the medulla oblongata, SAN and AVN are responsible for ensuring adequate
oxygenation of tissues to ensure their survival.
In humans, the heart is responsible for pumping blood around the body, such as to the lungs
for gas exchange. In plants, however, gas exchange does not require a mass transport
system. For example, the movement of carbon dioxide and oxygen in and out of the plant is
controlled by the guard cells, which open and close the stomata. If the water content of the
plant is too low, the guard cells become flaccid, resulting in the stomata closing. This then
prevents carbon dioxide from entering the leaf, resulting in the reduction of photosynthesis,
as carbon dioxide is needed for the light-dependent reaction. This also allows water to be
conserved until more can be found, as water is also a starting reactant of photosynthesis,
needed for photolysis to provide electrons for the electron transport change. This means
the guard cells and stomata become important in the regulation of photosynthesis.
Furthermore, the control of protons in the electron transport chain is also important in
photosynthesis. As electrons move down the chain, after being excited at photosystem II by
light energy, they lose energy. This energy is then used by proton pumps to pump protons
from the stroma into the thylakoid via active transport. This leads to a high concentration of
protons inside the thylakoid. Therefore, they leave the thylakoid by facilitated diffusion
1
