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CHAPTER
CHAPTER 58
THE STRUCTURE
MEMBRANE AND
STUCTURE
FUNCTION
OF MACROMOLECULES
AND FUNCTION
How things get into and out of the cell
Section A: Membrane Structure
Introduction
 The plasma membrane separates the living cell from its nonliving
surroundings.
 This thin barrier, 8 nm thick, controls traffic into and out of the cell.
 Like other membranes, the plasma membrane is selectively permeable,
allowing some substances to cross more easily than others.
 The main macromolecules in membranes are lipids and proteins, but
include some carbohydrates.
 The most abundant lipids are phospholipids.
 Phospholipids and most other membrane constituents are amphipathic
molecules.
 Amphipathic molecules have both hydrophobic regions and hydrophilic
regions.
 The phospholipids and proteins in membranes create a unique physical
environment, described by the fluid mosaic model.
 A membrane is a fluid structure with proteins embedded or attached to a
double layer of phospholipids.
 The molecules in the bilayer ‫ طبقة مزدوجة‬are arranged as hydrophobic
fatty acid tails are sheltered ‫ محمية‬from water while the hydrophilic
phosphate groups interact with water.
 Membrane proteins are amphipathic, with
hydrophobic and hydrophilic regions.
Fig. 8.1b
 If at the surface, the hydrophilic regions
would be in contact with water.
 In this fluid mosaic
model, the hydrophilic
regions of proteins
and phospholipids are
in contact with water and
the hydrophobic regions
are in a nonaqueous ‫ال مائى‬
environment.
Fig. 8.2b
Cell membrane is fluid
•
Most of the lipids and some proteins can drift
laterally in the plane of the membrane, but
rarely flip-flop from one layer to the other.
•
The lateral movements of phospholipids are
rapid, about 2 µm/second.
•
Many larger membrane proteins move more
slowly but do drift.
•
As temperatures cool, membranes switch
from a fluid state to a solid state as the
phospholipids are more closely packed.
•
Membranes rich in unsaturated fatty acids
are more fluid than those dominated by
saturated fatty acids because the kinks
in the unsaturated fatty acid tails prevent
tight packing.
Fig. 8.4a & b
• To work properly with permeability, membrane must be fluid, about
as fluid as oil.
Membranes are mosaics of structure and function
• A membrane is a collage ‫ تـ َ َج ُمع‬of different proteins embedded
‫ ُمـ ْنغ َِمس‬in the fluid matrix of the lipid bilayer.
CD, Unit 2 ch. 8
Fig. 8.6, Page 142
A)- The plasma membrane has a unique collection ‫ تجمع مميَـز‬of proteins.
•
There are two populations of membrane proteins.
1.
Peripheral proteins ‫ طرفى‬are not embedded in the lipid bilayer at all.
Instead, they are loosely bounded to the surface of the protein, often
connected to the other population of membrane proteins.
2.
Integral proteins ( ‫تحد ) ُمندَمج‬
ِ ‫ ُم‬penetrate the hydrophobic core of the lipid
bilayer, often completely spanning the membrane (a transmembrane
protein).
Integral
proteins
•
Where they contact the core,
they have hydrophobic regions
with nonpolar amino acids, often
coiled into alpha helices.
•
Where they are in contact with the
aqueous environment, they have
hydrophilic regions of amino acids.
Peripheral
proteins
Peripheral
proteins
Fig. 8.7, Page 143
• The proteins in the plasma membrane may provide a variety of major
cell functions.
Fig. 8.9, Page 144
CD, Unit 1 ch. 5
B)- Membrane carbohydrates are important for cell-cell
recognition
• The membrane plays the key role in cell-cell recognition.
• Cell-cell recognition is the ability of a cell to distinguish one type
of neighboring ‫ المجاورة‬cell from another.
• It is the basis for rejection of foreign cells by the immune system.
• Cells recognize other cells by recognizing the surface molecules,
often carbohydrates, on the plasma membrane.
• Membrane carbohydrates may be covalently bonded either to
lipids, forming glycolipids, or, to proteins, forming glycoproteins.
• The four human blood groups (A, B, AB, and O) differ in the
external carbohydrates on red blood cells.
Functions of cell membrane (Plasma membrane)
1- Selective permeability ‫النفاذية اإلختيارية‬
 A steady traffic ‫ العبور المنتطم‬of small molecules and ions moves
across the plasma membrane in both directions.
 For example, sugars, amino acids, and other nutrients enter a muscle
cell and metabolic waste products leave it.
 The cell absorbs O2 and expels CO2.
 It also regulates concentrations of inorganic ions, like Na+, K+, Ca2+,
and Cl-, by passing them across the membrane.
 However, substances do not move across the barrier
indiscriminately ‫ عشوائيا‬as membrane is selectively permeable.
 Hydrophobic molecules, like hydrocarbons, CO2, and O2, can
dissolve in the lipid bilayer and cross easily.
 Ions and polar molecules pass through with difficulty.
 This includes small molecules, like water, and larger critical molecules,
like glucose and other sugars.
 Proteins can assist and regulate ‫ يساعد و ينظم‬the transport of ions and
polar molecules.
Selective Permeability
CO2
CO2
Nucleus
O2
O2
The cell is able to take up particular
molecules and exclude others
2- Passive transport ‫ اإلنتقال السلبى‬is diffusion across a membrane
• Diffusion: ‫اإلنتشار‬
Is the tendency ‫ إستعداد‬of molecules of any substance to spread out ‫ لإلنتشار‬in the
available space randomly ‫عشوائيا‬.
•
For example, a permeable membrane ‫ غشاء منفز‬separating a solution with dye
molecules from pure water, dye molecules will cross the barrier randomly.
•
The dye will cross the membrane until both solutions have equal
concentrations of the dye (dynamic equilibrium ‫)التعادل الديناميكى‬.
Fig. 8.10a, Page 145
• A substance will diffuse from where it is more concentrated to where
it is less concentrated, down its concentration gradient ‫مـُنحدر التركيز‬.
Fig. 8.10b, Page 145
• Passive transport: ‫اإلنتقال السلبى‬
is the diffusion of a substance across a biological membrane which
requires no energy from the cell to make it happen.
• The concentration gradient represents potential energy and
drives
diffusion.
CD Unit 2, Chapter 8.
2. Osmosis is the passive transport of water
• Differences in concentration of dissolved materials in two solutions
can lead to the movement of ions from one to the other.
• The solution with the higher concentration of solutes is hypertonic.
• The solution with the lower concentration of solutes is hypotonic.
• Solutions with equal solute concentrations are isotonic.
• Osmosis:
Is a passive transport in which water
diffuses across a selectively permeable
membrane from the hypotonic solution
to the hypertonic solution until the
solutions become isotonic.
Fig. 8.11, page 146
Principal of water movement
Osmosis
Isotonic
Osmosis
Selectively
permeable
membrane
Low conc. of sugar
hypotonic
High conc. of sugar
hypertonic
Passive transport of water (Osmosis)
• Hypertonic solution: ‫عالى التركيز‬
contains high concentration of solute ‫ ُمذاب‬molecules.
• Hypotonic solution: ‫منخفض التركيز‬
contains low concentration of solute molecules.
• Isotonic solution: ‫متعادل‬
contains equal concentrations of solute molecules
Biological
Membrane
H2O
Hypertonic
Hypotonic
Osmoregulation ‫التوازن األسموزى‬
•
The cell in a hypertonic
environment will loose water,
shrivel ‫تنكمش‬, and die.
•
A cell in a hypotonic solution will
gain ‫ تسحب‬water, swell, and burst.
•
Organisms without rigid walls
have osmotic problems in either
a hypertonic or hypotonic
environment and must have
adaptations for osmoregulation
to maintain ‫ للحفاظ على‬their internal
environment.
•
Example, Paramecium have a
specialized organelle (the
contractile vacuole), that
functions as a pump to force ‫تقزف‬
water out of the cell.
Fig. 8.13 & 8.12, Page 147
Fig. 8.12, Page 147
Specific proteins facilitate ‫ تـ ُ َسهل‬passive transport
•
Many polar molecules and ions diffuse passively through the lipid bilayer
with the help of transport proteins (gated channels ‫)قنوات ُمبَوبة‬.
•
The passive movement of molecules down its concentration gradient via a
transport protein is called facilitated diffusion.
•
Many transport proteins simply provide
channels allowing a specific molecule or
ion to cross the membrane.
•
Fig. 8.14a & b, Page 148
Other transport proteins translocate
‫ ينقل‬the molecules across the
membrane as the protein changes
shape (active transport)
CD U 2 Ch. 8