Download PowerPoint

CHAPTER 5
THE STRUCTURE AND
FUNCTION OF
MEMBRANE
STRUCTURE
MACROMOLECULES
AND FUNCTION
How things get into and out of the cell
Cell membrane
The plasma membrane functions as a selective
barrier that allows passage of oxygen, nutrients,
and wastes for the whole volume of the cell.
Prevents the entry of unwanted matter and the
escape of needed materials.
Maintain a steady environment: Homeostasis.
Membrane Structure
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.
 Membrane is made of special kind of lipid;
phospholipids.

Phospholipids
and
most
other
membrane constituents are amphipathic
molecules.
Amphipathic
molecules
have
both
hydrophobic regions and hydrophilic regions.
“attracted to water”
hydrophilic
phosphate
“repelled by water”
hydrophobic
Hydrophilic head
- water loving
Hydrophobic tail
- water hating
4.6
The Fluid-Mosaic
Membrane Model
• The phospholipids and proteins in membranes create a
fluid mosaic model.
 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
environment
Fig. 8.2b
• A- Membrane is a collage ‫ مجموعة مختلفة من‬of different
proteins embedded in the fluid matrix of the lipid bilayer.
 Membranes are not static ‫ ;ساكن‬they have a fluid
consistency ‫اتساق‬.
• Most membrane lipids and proteins can drift
‫ تنجرف‬about laterally in the plane ‫ مستوى‬of the
membrane.
 Cholesterol enhances membrane fluidity, allows
animal membranes to function in a wide range of
temperatures and also makes the membrane less
permeable to biological molecules.

There are two populations of membrane proteins.
Integral proteins ‫ ُمندَمج‬penetrate the hydrophobic core of
the lipid bilayer (transmembrane protein).
The integral proteins has a middle area, hydrophobic regions
with surface area, in contact with the nonpolar amino acids.
And aqueous environment, they have hydrophilic regions of
amino acids
1.

2-Peripheral proteins
are not
Integral
proteins
Peripheral
proteins
embedded in the lipid bilayer at all.
Instead, they are loosely bounded
to the surface of the integral
proteins
Peripheral
proteins
Function of membrane protein
The proteins in the plasma membrane may
provide a variety of major cell functions:-
•
•
•
•
•
Transportation
Enzymes
Receptor sites
Cell adhesion
Attachment to the
cytoskeleton



Most of the lipids and
some proteins can drift
laterally in the plane of the
membrane, but rarely flipflop 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.

Membranes rich in unsaturated fatty acids
are more fluid than those dominated by
saturated fatty acids because the kinks
‫منعطفات‬
in the unsaturated fatty acid tails prevent
tight packing.
B- Membrane carbohydrates are important for cell-cell
recognition
 Usually branched molecules of 15 or less sugar units.
 Some are covalently bonded to lipids: Glycolipids.
 Most are covalently bonded to proteins:Glycoproteins.
 Function: Cell-cell recognition.


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.
The four human blood groups (A, B, AB, and O) differ in the
external carbohydrates on red blood cells.
Carbohydrate chains
Hydrophilic ‫ُمحب للماء‬
Phospholipid
Hydrophobic ‫كاره للماء‬
Proteins