CHAPTER 8
MEMBRANE STUCTURE AND FUNCTION
How things get into and out of the cell
Active Transport ‫النقل النشط‬
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Active transport is the pumping ‫ضخ‬
َ of solutes against their
concentration gradients ‫اإلنحدار التركيزى‬
• Some facilitated transport
proteins can move solutes
against their concentration
gradient, from the side where
they are less concentrated to
the side where they are more
concentrated.
• This active transport requires
metabolic energy via ATP.
• Active transport is critical ‫ بالغ األهمية‬for a cell to maintain its internal
concentrations of small molecules.
• Active transport is performed by specific proteins embedded in
the membranes (transport protein).
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Fig. 8.14b, Page 148
1)- Small molecules (Ions )
• The sodium-potassium pump
actively maintains the gradient
of sodium (Na+) and potassium
ions (K+) across the membrane.
– The animal cell has higher
concentrations of K+ and lower
concentrations of Na+ inside
the cell.
– The sodium-potassium pump
(T. protein) uses the energy of
one ATP to pump 3 Na+ ions
out and 2 K+ ions in.
1ATP
T. protein
Low conc.
+ 2
of K
High conc.
+
of Na
High conc.
+
of K
Low conc.
3 of Na+
Outside the cell
Na
Na
Na
Protein
molecule
Na
ATP
Cellular
membrane
Inside the cell
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4
Fig. 8.15, Page 149
Two roles of
membrane protein
Fig. 8.16, Page 150 Both diffusion and facilitated diffusion are forms of passive transport of
molecules down their concentration gradient, while active transport requires an investment of
energy to move molecules against their concentration gradient.
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Some ion-pumps generate voltage ‫ُجهد كهربائى‬
across membranes
• All cells maintain a voltage across their plasma membranes.
– The cytoplasm of a cell is negative in charge compared to the
extracellular fluid because of an unequal distribution of cations and
anions on opposite sides of the membrane.
– This voltage, the membrane potential ‫الجهد الغشائى‬, ranges from -50 to -200
millivolts.
• The membrane potential acts like a battery.
• The membrane potential favors the passive transport of cations
into the cell and anions out of the cell.
• Two combined forces, collectively called the electrochemical
gradient ‫اإلنحدار الكهروكيميائى‬, drive the diffusion of ions across a
membrane.
• Ions diffuse not simply down its concentration gradient, but
diffuses down its electrochemical gradient.
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2)- Large molecules (macromolecules)
Large molecules are transported by Exocytosis and endocytosis


1.
Small molecules and water enter or leave the cell through the lipid
bilayer or by transport proteins.
Large molecules, such as polysaccharides and proteins, cross the
membrane by vesicles ‫أوعية‬.
Exocytosis ‫اإلخراج الخلوى‬:
a transport vesicle budded from ‫ ينشأ من‬the Golgi apparatus is moved
by the cytoskeleton to the plasma membrane.
When the two membranes come in contact ‫تالمس‬, the bilayers fuse
‫ يندمج‬and spill ‫ يُفرع‬the contents to the outside.
2-
Endocytosis ‫اإلدخال الخلوى‬,
a cell brings in macromolecules and particulate matter by forming
new vesicles from the plasma membrane
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A)- Phagocytosis:
Called “cellular eating”. The
cell engulfs ‫ تـَ ْبلَع‬a particle by
extending pseudopodia ‫أقدام‬
‫ كاذبة‬around it and packaging
it ‫ تـُغلفها‬in a large vacuole.
• The contents of the vacuole
are digested when the
vacuole fuses with a
lysosome.
Fig. 8.19a, Page 152
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B)- Pinocytosis, ‫الشرب الخلوى‬
“cellular drinking”,
a cell creates a vesicle
around droplets ‫ نقاط‬of
extracellular fluid ‫السائل‬
‫الموجود خارج الخلية‬.
– This is a non-specific
process ‫عملية غير متخصصة‬.
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Fig. 8.19b, Page 152
C)- Receptor-mediated endocytosis: ‫اإلدخال الخلوى عن طريق المستقبالت المتخصصة‬
It Is called (Selective eating) which very specific in what substances are
being transported.
• It is triggered ‫ تـُستـَحث‬when extracellular substances bind to special
receptors ‫ ُمستقبـِالت خاصة‬, on the membrane surface. This triggers the
formation of a vesicle ‫وعاء‬
• It enables a cell to take large quantities of specific materials that may
be in low concentrations in the environment.
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Fig. 8.19c, Page 152
Things Transport
Active
Passive
Large
molecules
Small
Molecules/ions
(Membrane)
(T. protein)
Exocytosis
Facilitated
diffusion
(T. protein)
Diffusion
(Membrane)
Endocytosis
Phagocytosis
Pinocytosis
Receptor-mediated
endocytosis
Cellular eating
Cellular drinking
Selective eating
Using specific
receptors (coated
vesicles) that bind to
specific legends and
engulf it. It is specific
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