Cells require nutrients and a means to transport waste. There are two main types of transport mechanisms - passive and active. Passive mechanisms like diffusion, facilitated diffusion, and osmosis move substances according to concentration gradients without energy usage. Active transport mechanisms like pumps require ATP to move substances against gradients. Other active mechanisms include endocytosis which transports particles into cells, and exocytosis which transports molecules out of cells.
2. Cells are bathed in extracellular
fluid that is rich in nutrients such as
oxygen, glucose and amino acids
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3. These nutrients are needed within
the cell and must be able to cross
the cell membrane
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4. The cell’s waste which accumulates
within the cell must also be able to
cross the cell membrane
Wastes are eventually eliminated
from the body
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5. There are a number of mechanisms that
help in the movement of water and
dissolved substances across the cell
membrane
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6. These transport mechanisms can be
divided into two (2) groups
• Passive Transport
• Active Transport
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7. Passive transport mechanisms do not
require any additional energy in the
form of ATP
Like a ball rolling down a hill
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8. Active transport mechanisms require
an input of energy in the form of ATP
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9. The passive mechanisms that move substances
across the membrane include:
• Diffusion
• Facilitated diffusion
• Osmosis
• Filtration
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10. Diffusion
This is the most common transport mechanism
And involves the movement of a substance from
an area of higher concentration to an area of
lower concentration
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11. For example …
A tablet of red dye is placed in a glass of water
The tablet dissolves
The dye moves from an area where it is most
concentrated to an area where it is less
concentrated
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12. Diffusion continues until the dye is evenly
distributed throughout the glass.
The point where no further net diffusion occurs is
called eqilibrium
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13. Some examples of Diffusion in the
human body ...
Diffusion causes oxygen to move across the
membrane of an alveolus of the lung and into the
blood
Oxygen diffuses from the alveolus and into the
blood because the concentration of oxygen is
greater within the alveolus than within the blood
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14. Carbon dioxide (a waste product) that
accumulates in the blood diffuses in the
opposite direction
Carbon dioxide moves from the blood into
the alveolus
The lungs then exhale the carbon dioxide,
eliminating it as a waste product
Diffusion moves oxygen into the blood and
carbon dioxide out of the blood
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15. Facilitated Diffusion
This form of diffusion is responsible for the
transport of many substances
Like diffusion, the substances move from a higher
concentration to a lower concentration
In facilitated diffusion the substance is helped
across the membrane within the membrane
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16. The helper molecule increases the rate
of diffusion
See Herlihy (2007) Fig. 3-8, p. 38
In this illustration a glucose molecule is being
carried ‘down hill’ (passive transport)
A glucose molecule is too large to pass
through the cell membrane without help!
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17. Osmosis
Osmosis is a special case of diffusion and
involves the diffusion of water through a
selectively permeable membrane
A selectively permeable membrane allows the
passage of some substances while restricting
the passage of others
The dissolved substances do not move
See Herlihy (2007) Fig 3-9 p. 38
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18. Whenever dissolved substances like glucose and
protein are confined in a space by a selectively
permeable membrane they can pull water into the
compartment by osmosis
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19. The strength of the osmotic pull is related directly
to the concentration of the solution
The greater the concentration – the greater the
pulling power
The more concentrated solution has more
osmotically active particles
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20. Examples of Osmosis in the Human Body
Because osmotic pressure pulls water into a
compartment it can cause swelling
For example, when tissue is injured this can
cause proteins to leak and accumulate in the
tissue space.
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21. The confined proteins act osmotically
pulling water toward them
This process causes an accumulation of
water in the tissue spaces. This
accumulation of water is called oedema
boostphysio.com
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22. TONICITY
Tonicity is the ability of a solution to affect the
volume and pressure within a cell
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23. This can be observed when a cell is placed in
solutions of different concentrations
The following three terms are used to illustrate
tonicity
• Isotonic solution
• Hypotonic solution
• Hypertonic solution
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24. ISOTONIC SOLUTION
An Isotonic solution has the same concentration as
intracellular fluid
If a red blood cell is placed in an isotonic solution
(0.9% NaCl) for example, no net movement of water
occurs so the cell neither loses or gains water
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25. HYPOTONIC SOLUTION
If a red blood cell is placed in pure water (a
solution containing no solute) water moves into
the cell by osmosis (from where there is more
water to where there is less
Pure water is more dilute than the inside of the
cell so it is said to be hypotonic
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26. Putting a red blood cell into pure water
(hypotonic) will cause water to rush into the
cell and make it to burst.
Hypotonic solutions then cause red blood cells
to burst or lyse
This process is called haemolysis and it is for
this reason that pure water is not administered
intravenously
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28. HYPERTONIC SOLUTIONS
If a red blood cell is placed within a very
concentrated salt solution water diffuses out
of the red blood cell and into the bathing
solution causing the red blood cell to shrink
(crenate)
The salt solution is referred to as a hypertonic
solution
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29. Crenated Red Blood Cell
sciencephoto.com
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30. Filtration
Water and dissolved substances move across the
semipermeable membrane in response to
difference in pressures
In diffusion and osmosis movement of these
substances occurred because of concentration
differences
In filtration pressure pushes substances across the
cell membranes
See Herlihy (2007) Fig 3 -11 p.39
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31. Where does filtration happen in the body?
The capillaries are small blood vessels that have
only one layer of cells with many little pores.
The pressure in the capillary pushes water and
dissolved substances out of the blood and
through the pores in the capillary wall into the
tissue space.
This process is called filtration
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32. The active transport mechanisms include:
• Active transport pumps
• Endocytosis
• Exocytosis
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33. Active Transport Pumps
This kind of transport system requires an input
of energy (ATP) to achieve its goal
Energy is required when the cell already
contains a great amount of a substance and the
only way to move additional substances into
the cell is to pump them in!
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34. For example, the cell normally contains
a large amount of potassium (K+), so
the only way to move more in is to
pump it in
To move K+ from an area of low
concentration to an area of high
concentration (up hill) requires energy.
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36. Endocytosis
This involves the intake of food or drink
by the cell
clker.com
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37. In Endocytosis the particle is too large to move
across the cell membrane by diffusion
The particle is surrounded by the cell membrane
which engulfs it and takes it into the membrane
There are two forms of Endocytosis:
• Phagocytosis
• Pinocytosis
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38. Phagocytosis
This is ‘cell eating’
This happens when white blood cells ‘eat’
bacteria and help the body to defend itself
against infection
student.ccbcmd.edu
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41. An example of exocytosis is when the pancreas
makes proteins for use outside of the cell.
The pancreatic cells synthesise the protein and
wrap it up in a membrane
This membrane-bound vesicle toward and fuses
with the cell membrane.
The protein is then expelled into the surrounding
space
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43. References
• Herlihy, B. ( 2014) The human body in health and illness. 5th (ed)
China. Elsevier.
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