3rd lecture MBBS-2024-Membrane Structure and Function.pdf

3rd lecture

1. 2024
3
rd
Lecture
The Cell
Physiology
Welcome to the fascinating study of the human body!
d
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2. Major TopicsMajor Topics
❖Membrane Structure and Function
❖Lipid Bilayer
❖Membrane Proteins & Carbohydrates
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3. Learning Objectives
After This lecture, you should be able to:
❖Describe the Membrane Structure
❖Explain the physiological roles of
Membrane Proteins & Carbohydrates
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4. 1. Intra Cellular Fluid (ICF) ⅔ (two-thirds) volume of water:
Potassium, magnesium and phosphate
2. Extra Cellular Fluid (ECF) 1/3 (one-third) Volume of water:
Sodium, Chlorine, Bicarbonate, Oxygen,
Glucose, Fatty Acids and Amino Acids
BODY FLUID COMPARTMENTS
ISF = interstitial fluid
Review
60%
In Adult Human, About 60% Of Their Bodies Are Water.
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5. ❑Internal environment (concept of a relatively stable internal environment) →
Claude Bernard (French physician)
❑Homeostasis (themaintenanceof astable internal environment) →
Walter Cannon (American physiologist)
Review
5

6. →Men's water content is higher than women
(due to less fat)
→The amount of water in lean individuals is
higher than that of obese people
→As the age increases, the amount of water in
the body decreases
Review
60%55%
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7. Cytoplasm: is all the
contents inside the cell
membrane, excluding the
nucleus.
Protoplasm: includes
cytoplasm and the nucleus of
the cell.
Review
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8. Protoplasm:
1.Water. 70 to 85 percent
2.Ions. Potassium, Magnesium, Phosphate, Sulfate, Bicarbonate,
and smaller quantities of Sodium, Chloride, Calcium.
3.Proteins. 10 to 20 percent of the cell mass
1.Structural proteins
2.Functional proteins
4.Lipids. 2 percent of the total cell mass.
1.Phospholipids, cholesterol
2.Triglycerides
5.Carbohydrates.
Review
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9. MEMBRANE STRUCTURE AND FUNCTION
➢Membranes are fluid (The membrane has a liquid state)
➢Membranes are mosaics of structure and function
➢Membrane carbohydrates are important for cell-cell recognition
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10. MEMBRANE STRUCTURE AND
FUNCTION
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11. 11

12. What are the general functions of the cell membrane?
Regulation of
exchange
with the
environment
Physical
isolation
Communication
between the cell
and its
environment
Structural
support
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13. What exactly do membranes do?
1.Acts as a barrier
2.Receives information from outside the cell
3.Involves the entry and exit of materials
4.Ability to move and expansion
Membranes are Active Dynamic !
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14. How can the cell membrane carry out such
diverse functions?
❖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.
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15. FLUID MOSAIC MODEL
Proposed that Membrane is a “Mosaic” of Proteins and Phospholipids that
are constantly moving and changing
Animation from: http://www.sp.uconn.edu/~terry/images/anim/fluidmem.gif
In 1972, Singer and Nicolson:
They proposed that membrane proteins are dispersed and individually
inserted into the phospholipid bilayer 15

16. In the Fluid Mosaic Model of membrane, some proteins called
Integral or Transmembrane Proteins are inserted into the lipid bilayer,
and some called Peripheral Proteins are bound to the cell membrane
directly by protein-protein interactions.
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17. Organization of plasma membrane lipids
oPhosphatidylethanolamine
oPhosphatidylserine
ophosphatidylinositol.
❖Outer Leaflet
•Phosphatidylcholine
•sphingomyelin
•glycolipids
❖Cholesterol is distributed in both
inner and outer leaflets
❖Inner Leaflet
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18. The Fluid Mosaic Model has been updated → Carbohydrates are only present on the
outer surface of the plasma membrane. There is also an interaction between membrane
proteins with cytoskeleton, and this plays an important physiological role. There also
exist the tiny, dynamic, and ordered domains of cholesterol and Sphingolipids named
lipid-enriched domains in the plasma membranes of eukaryotic cells
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19. Lipid rafts:
✓are formed by the interaction of sphingomyelin, glycolipids, and cholesterol
✓are involved in several physiological functions;
They are essential for maintaining cellular functions, including Spatial Plasma Membrane
Organization, Signal Transduction, and Receptor Activation, as well as newer functions
involving intracellular Trafficking of lipids and proteins from the ER and Golgi to the Plasma
Membrane.
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20. •Membrane molecules are held in place by relatively weak
hydrophobic interactions.
•Most of the lipids and some proteins can drift laterallyin the plane
of the membrane, but rarely flip-flop from one layer to the other.
1. Membranes are fluid
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21. Lateral movement Flip-flop
Membrane phospholipids can either move laterally in one bilayer or
rotationally from one side to the other side of the membrane, which is
called transverse diffusion or flip-flopping.
Lateral movement of phospholipids frequently occurs and is what
provides the membrane fluidity
flip flop occurs at a much more slower rate than the lateral diffusion
and is rare.
(~ once per month)(~10
7
times per second)
Membrane Phospholipids Movements
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22. Membrane fluidity is influenced by temperature and by its constituents.
1.Temperature:
•At High Temperature, phospholipids have enough kinetic energy to
overcome the intermolecular forces
•At Low Temperature, membranes switch from a fluid state to a solid
state as the phospholipids are more closely packed.
Factors that influence membrane fluidity
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23. 2.The Length of fatty acid tail: intermolecular interactions between
the phospholipid tails → add rigidity to the membrane.
the longer tails, the more interactions, the less fluidity.
Factors that influence membrane fluidity
23

24. ❖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.
Factors that influence membrane fluidity
3.The degree of saturated fatty acid content of membrane.
✓Phospholipid tails can be saturated or unsaturated.
✓The terms saturated and unsaturated refer to whether or not double
bonds are present between the carbons in the fatty acid tails.
oSaturatedtailshavenodoublebondsand,asaresult,have
straight,unkinkedtails.
oUnsaturatedtailshavedoublebondsand,asaresult,have
crooked,kinkedtails.
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25. ❖Some cells can alter the lipid composition
of membranes to compensate for changes
in fluidity caused by changing
temperatures
•For example, cold-adapted organisms, such as winter wheat,
increase the percentage of unsaturated phospholipids in the
Autumn →
This allows these organisms to prevent their membranes from
solidifying during winter.
Fluid Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
Carbon tails
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26. 1.Control of membrane fluidity, depending on the temperature
Factors that influence membrane fluidity
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27. ❖is fited between phospholipid molecules in the plasma membrane
of Animal Cells.
❖Has different effects on membrane fluidity at different
temperatures:
1.Lessfluid at warmer temperatures (by reducing phospholipids movement)
2.More fluid at lower temperatures (by preventing close packing of phospholipids)
Cholesterol
✓At warm temperatures, it limits the movement of phospholipids and reduces fluidity.
✓At cool temperatures, it maintains fluidity by preventing tight packing.
THE STEROID CHOLESTEROL:
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28. In Summary
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29. Membrane Proteins
➢Although the basic structure of biological membranes is provided
by the lipid bilayer,membraneproteins perform most of the
Specific FunctionsOf Membranes.
❖ Cell membrane & organelle membranes each have
unique collections of proteins
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30. Types of Cell Membrane Proteins
1.Integral or Intrinsic Proteins → bind to hydrophobic
center of the lipid bilayer
2.Peripheral or Extrinsic Proteins → bind to the hydrophilic
polar heads of the lipids or to the integral proteins
Amphipathic
Cytosolic
1
2
3
Differentmembrane proteins are associated with the membranes in different ways.
Transmembrane Proteins That Extend Through The Lipid Bilayer
Peripheral
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31. Membrane Proteins
❖ Membrane Proteins Uses:
➢ Enzymes
➢ Structural
➢ Transport
➢ Motor
➢ Storage
➢ Signaling
➢ Receptors
➢ Gene regulation
➢ Special functions
❖Have many functions in the cell:
Communicate
Maintain their shape
Carry out changes triggered by Chemical messengers
Transport and Share material
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32. •The proteins in the plasma membrane may provide
a variety of major cell functions.
Hydrophilic
channel
shuttle
Enzymes
Signal
Receptor
Glycoprotein
(identification tags)
Adjacent cells
various kinds of junctions
(gap junctions
or tight junctions)
oMaintain cell shape
oStabilizes the location
of certain membrane proteins
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33. Membrane Proteins have a variety of functional tasks
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34. Functions of Cell Membrane Integral Proteins
i) Transmembrane proteins → Span the entire bilayer, which act as:
a. Channels (pores) → for diffusion of small, water–soluble substances.
b. Carriers → actively transport materials across the bilayer.
c. Pumps → actively transport ions across the bilayer.
d. Receptors → initiate intracellular reactions when activated.
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35. Attachmentof Membrane Proteins
Cytoskeleton:the system of filaments
and fibers in the cytoplasm of a cell
extracellular matrix (ECM): the network
of macromolecules found outside of cells,
such as collagen, enzymes, and
glycoproteins, to membrane proteins.
Membrane proteins are important in the
cytoskeletonand extracellular matrix (ECM).
➢Attaching filaments or fibers in the cytoplasm can help the cell to maintain its
particular shape. It also keeps the location of membrane proteins stable.
➢Attaching membrane proteins to the extracellular matrix can
help the ECM to mediate changes that occur in extracellular
and intracellular environments.
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36. •One role of membrane proteins is to reinforce the shape of
a cell and provide a strong framework.
•On the cytoplasmic side, some membrane proteins connect to
the cytoskeleton.
•On the exterior side, some membrane proteins attach to the
fibers of the extracellular matrix, or to the neighbor cells.
Attachmentof Membrane Proteins
36

37. Pemphigus
Cell-Cell Adhesion Molecules
Autoantibodies against E-Cadherin
E cadherin
E cadherin is the cadherin in the epithelial
tissues, and performs as a key mediator of cell-
cell adhesion. Loss of E-cadherin can promote
invasiveness and metastatic behavior in many
epithelial tumors.
painful sores (wounds) and
blisters on skin and mouth
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38. Some Membrane Proteins act as Enzymes
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39. Bacterial Lectin
Carbohydrates of the plasma membrane are major recognition
and attaching sites forpathogensduring infection
influenza virus, pathogenic E. colibacteria, and some protozoa
These pathogens have proteins, known as lectins, that bind to specific
carbohydrates of particular cells.
Membrane Carbohydrates are important for cell-cell
recognition and adhesion.
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40. Membrane carbohydrates perform two main functions:
1.Cell Recognition;
2.Structural Role as Physical Barrier;
cell-cell signaling or cell-pathogen interactions
Blood Groups Are Determined By Cell
Surface Carbohydrates Of Erythrocytes
Carbohydrate Chains
The four human Blood Groups (A, B, AB, and O) differ in
the external carbohydrates on red blood cells (RBC).
O A B AB
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41. Membrane carbohydrates may be covalently bonded either to lipids, forming
glycolipids, or, more commonly, to proteins, forming glycoproteins.
→ →
41

42. Membrane Carbohydrates Functions
lymphocytes
Endothelial
cells
Selectin
Infection
Carbohydrate
42

43. Cell-Cell recognition is also the basis for the
rejection of foreign cells by the immune system.
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44. Carbohydrates as recognition molecules are also
important during embryonic development.
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45. Glycocalyx
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It is composed of Glycolipids and Glycoproteins
and covers the cell membrane.
(Pericellular Matrix)
➢Main Functions of Glycocalyx:
1.It binds to the extracellular
Ca
2+
to stabilize membrane
structures
2.It acts as an attachment matrix
for other cells (cell adhesion
molecules).

46. Transport of Substances Through
Cell Membranes
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