The easiest Way to learn the core principles of Cell membrane. Its semi permeable nature; the fluid mosaic model and the Different membrane proteins. How substances are transferred across the membrane. The importance of simple diffusion, facilitated diffusion, active transport, endocytosis , and exocytosis.

1. Histology Made Absolutely Easy:
The Cell Membrane:

2. The Cell:
• Is the basic functional unit of all organisms.
• Cells that are similar or function similarly are
grouped together to form Tissues.
• Tissues are grouped together to form Organs.
• Organs grouped together form the Organ
system.

3. A Typical Human Cell:

4. Components of a Cell:
The cell is a mass of Protoplasm separated from the external environment by a Plasma
Membrane.
The Protoplasm is made up of two components:
1. Cytoplasm: that contains numerous organelles:
• Mitochondria
• Endoplasmic Reticulum
• Golgi Apparatus
• Ribosomes
• Lysosomes
• Peroxisomes
• The cytoskeleton of the Cell: (a) Microfilaments
(b) Intermediate filaments
(c) Microtubules
• Centrosome and centrioles
• Cytoplasmic Inclusions
2. Nucleus: that houses the genome of the cell.

5. The Cell Membrane/Plasma Membrane/
Plasmalemma
Functions are:
• Separates the contents of the cell from the external environment
• Maintains the shape of the cell
• Controls the transport of molecules in and out of the cell (selective
permeability)
• Regulates cell–cell interactions
• It bears receptors that aid in recognizing antigens and foreign cells
• Transduces extracellular physical or chemical signals into intracellular
events.

6. Structure of The Cell Membrane:
• Thickness of cell membrane: approximately 7.5nm (nanometers)
• It is made up of: (1) Lipid Bilayer
(2) Associated Proteins
Phospholipids
Cholesterol
Glycolipids
Integral Proteins
Peripheral Proteins

7. Lipid Bilayer:
(1) Phospholipids:
• The Cell Membrane is a Lipid Bilayer that is, it is made up of Two leaflets of phospholipids
in which are studded proteins.
• The proteins and phospholipids exit in a ratio of 1:1, except in myelin sheaths where
phospholipids outnumber proteins in a ratio of 4:1.
• Each phospholipid is made up of: (a) a single Polar head (that either projects out side the
cell OR into the cytoplasm)
(b) Two Nonpolar fatty acyl tails that project in the
center of the Cell membrane facing one another.
• Phospholipid molecule is amphipathic because it posses both a “hydrophilic end” (which
is the polar head) and a “hydrophobic end” (which are the 2 fatty acids chains).
• The tails of the fatty acids face each other and form weak bonds that hold the two leaflets
together.
Polar Head
NonPolar region made
up of two fatty acyl tails
Phospholipid projecting
towards the out side of
the cell
Phospholipid projecting
into the cytoplasm of the
cell

8. Membrane Faces:
Is the part of the outer leaflet
facing the external environment
Is the part of the outer leaflet
facing the hydrophobic part of cell
membrane
Is the part of the inner leaflet
facing the hydrophobic part of cell
membrane
Is the part of the inner leaflet
facing the Cytoplasm
Copyright: Junqueira’s basic
histology
Copyright: Histology and
cell biology

9. (2) Cholesterol
• Constitutes 2% of the Plasma membrane lipids.
• It is amphipathic in nature.
• It is present at both the leaflets of the cell membrane.
• Maintains the structural integrity of the cell membrane.
• Cell membrane has to maintain its fluidity in order to carry out processes like exocytosis and
endocytosis.
Fluidity increases due to: (i) increase in temperature
(ii) decreased saturation of fatty acyl tails.
Fluidity decreases due to: (i) Increase in membrane cholesterol content.
(3) Glycolipids:
• Constitute 5% of the Plasma membrane lipids.
• Are Amphipathic in nature.
• Located on the extracellular part of the outer leaflet.
• Carbohydrate side chains from glycolipids and glycoproteins form the “fuzzy” material outside
the cell membrane called “glycocalyx”
Copyright: Junqueira’s basic
histology

10. Review the Different components of the Lipid Bilayer by using the Diagram
below:
(1) Note the Phospholipid bilayer with is hydrophobic and hydrophilic portions.
(2) Note that cholesterol is present in both the leaflets and extends through
hydrophobic and hydrophilic potions of the cell membrane.
(3) Note the extracellular location of glycolipids where they form the
glycocalyx with there polysaccharide side chains.

11. The Glycocalyx:
• Is the “sugar coat” located on the outer surface of the outer leaflet of the Cell membrane.
• Is responsible for the “fuzziness” seen on Electron Microscope.
• Composition: Consists of polar oligosaccharide side chains covalently linked to proteins and
some lipids of the plasma membrane.
• Functions:
(a) The glycocalyx aids in attachment of some cells to extracellular matrix components.
(b) It binds antigens and enzymes to the cell surface.
(c) It facilitates cell-cell recognition and interaction.
(d) It protects cells from injury by preventing contact with inappropriate substances.
(e) It assists T cells and antigen-presenting cells in aligning with each other.

12. Membrane Proteins:
• Membrane proteins make up
50% of the Cell Membrane.
• These proteins are of 2 types:
(1) Integral Proteins
(2) Peripheral Proteins
Integral Proteins:
• Also called “Transmembrane Proteins” because they span the entire length of the Cell
membrane.
• Are amphipathic in nature and therefore they have hydrophobic and hydrophilic portions.
• Are also called Multipass proteins because of their extensive size they are long and folded and
make several passes through the cell membrane.
• They function as (1) Membrane Receptors
(2) Transport Proteins
• Because the same integral membrane proteins have the ability to float like icebergs in the sea
of phospholipids this model is referred to as the fluid mosaic model of membrane structure.
Peripheral Proteins:
• DO NOT extend through the entire length of the cell membrane. They are usually found at the
cytoplasmic end or the extracellular aspect.
• Usually function as (1) part of the cytoskeleton (2) intracellular secondary message system
Copyright: Color textbook of
histology

14. Transport System Across the Cell
Membrane:
Copyright: Junqueira’s Basic Histology

17. Copyright: Color
textbook of histology
Copyright: Color
textbook of
histology

18. Active Transport Passive Transport
Definition:
Types of Transport:
Types of Particles
transported:
Movement of molecules
DOWN the concentration
gradient. It goes from high
to low concentration, in
order to maintain
equilibrium in the cells.
Does not require cellular
energy.
Active Transport uses ATP
to pump molecules
AGAINST/UP the
concentration gradient.
Transport occurs from a
low concentration of
solute to high
concentration of solute.
Requires cellular energy.
Endocytosis, cell
membrane/sodium-
potassium pump &
exocytosis
Diffusion, facilitated
diffusion, and osmosis.
Proteins, ions, large cells,
complex sugars.
Anything soluble (meaning able
to dissolve) in lipids, small
monosaccharides, water,
oxygen, carbon dioxide, sex
hormones, etc.

19. Active Transport Passive Transport
Examples:
Importance:
Functions :
Diffusion, osmosis, and
facilitated diffusion.
Phagocytosis, pinocytosis,
sodium/potassium pump,
secretion of a substance
into the bloodstream
(process is opposite of
phagocytosis &
pinocytosis)
In eukaryotic cells, amino
acids, sugars and lipids need
to enter the cell by protein
pumps, which require active
transport. These items either
cannot diffuse or diffuse too
slowly for survival.
It maintains equilibrium in the
cell. Wastes (carbon dioxide,
water, etc.) diffuse out and are
excreted; nutrients and oxygen
diffuse in to be used by the cell.
Maintains dynamic equilibrium
of water, gases, nutrients,
wastes, etc. between cells and
extracellular fluid; allows for
small nutrients and gases to
enter/exit. No NET
diffusion/osmosis after
equilibrium is established.
Transports molecules through
the cell membrane against the
concentration gradient so more
of the substance is inside the
cell (i.e. a nutrient) or outside
the cell (i.e. a waste) than
normal. Disrupts equilibrium
established by diffusion.

20. Types of Endocytosis:

22. Exocytosis:

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