L2-PlasmaMembrane.pdf

Nutritional Physiology
Unit 1
Unit of Life
The cell-Structure & Function
Dr. Sweta Patel

Plasma Membrane
◼ forms the cell’s flexible outer surface, separating the cell’s internal environment
(inside the cell) from the external environment (outside the cell).
◼ Selective barrier
◼ communication among cells and between cells and their external environment.
Cytoplasm
◼ consists of all the cellular contents between the plasma membrane and the nucleus
◼ Cytosol: The fluid portion: contains water, dissolved solutes, and suspended particles
Nucleus
◼ houses most of a cell’s DNA.
◼ Chromosome: a single molecule of DNA associated with several proteins, contains
thousands of hereditary units called genes that control most aspects of cellular
structure and function.

Plasma Membrane: structure
Lipid bilayer
3 types of lipid:
75% Phospholipid,
20% cholesterol,
5% glycolipids
Amphipathic
molecules: dual
nature
Lipid
Integral Protein
[Transmembrane
Protein]
Peripheral Protein
Modification of
Protein
1. Glycoprotein
2. Lipoprotein
Protein

Glycocalyx
◼ The carbohydrate portions of glycolipids and glycoproteins form
an extensive sugary coat.
◼ Cell to cell pattern varies
◼ The glycocalyx acts like a molecular “signature” that enables
cells to recognize one another.
◼ One of the basis of the immune response that helps us destroy
invading organisms.
◼ enables cells to adhere to one another in some tissues and
protects cells from being digested by enzymes in the extracellular
fluid.
◼ The hydrophilic properties of the glycocalyx attract a film of
fluid to the surface of many cells.

Function of Membrane Protein
Act as,
◼ Ion Channels
◼ Carriers
◼ Receptor
◼ Enzyme
◼ Linker
◼ Cell identity marker

1. Membrane Fluidity
◼ lipid molecules readily exchange places with
their neighbors within a monolayer (~107 times
a second).
◼ This gives rise to a rapid lateral diffusion,
10million times per second
◼ It depends on number of cholesterol and double-
bond [kink] present in fatty acids
◼ "flip-flop" : occurs less than once a month for
any individual molecule.
Membrane Physiology

Importance of membrane Fluidity
◼ Rigid membrane would lack mobility
◼ A completely fluid membrane would lack the
structural organization and mechanical support
required by the cell
◼ Allows interactions to occur within the plasma
membrane enables the movement of the membrane
components responsible for cellular processes such as
cell movement, growth, division, and secretion.
◼ self-seal if torn or punctured.

2. Membrane Permeability
◼ Selectively permeable
◼ permeable to nonpolar, uncharged molecules, such as oxygen,
carbon dioxide, and steroids
◼ Impermeable to ions and large, uncharged polar molecules
such as glucose
◼ Slightly permeable to small, uncharged polar molecules such
as water and urea, a waste product from the breakdown of
amino acids.
◼ Importance: allows a living cell to maintain different
concentrations of certain substances on either side of the
plasma membrane.

3. Electrochemical Gradient
◼ The selective permeability of the plasma membrane allows a living cell to
maintain different concentrations of certain substances on either side of the
plasma membrane.
A concentration gradient is a difference in the concentration of a
chemical from the inside to the outside of the plasma membrane.
◼ Typically, the inner surface of the plasma membrane is more negatively
charged and the outer surface is more positively charged. A difference in
electrical charges between two regions constitutes an electrical gradient.
◼ Because it occurs across the plasma membrane, this charge difference is
termed the membrane potential
Oxygen molecules and sodium ions (Na) are more concentrated in the
extracellular fluid than in the cytosol; the opposite is true of carbon dioxide
molecules and potassium ions (K).
◼ The combined influence of the concentration gradient and the electrical
gradient on movement of a particular ion is referred to as its
electrochemical gradient.

What can diffuse through membrane
◼ Protein - free Lipid Bilayers Are Highly
Impermeable to Ions
◼ The smaller the molecule and the more soluble it is
in oil – can pass easily
◼ Small nonpolar molecules, such as O2 (32 daltons)
and CO2 (44 daltons), readily dissolve in lipid
bilayers and therefore rapidly diffuse across them
◼ Uncharged polar molecules also diffuse rapidly
across a bilayer if they are small enough. Water (18
daltons), ethanol (46 daltons), and urea (60 daltons),
glycerol (92 daltons) diffuses less rapidly; glucose
(180 daltons), can not
◼ lipid bilayers are highly impermeable to charged
molecules (ions)
Transportation Across the membrane

Types Transportation
carrier proteins channel proteins
carriers, permeases, or transporters
bind the specific solute to be transported
and undergo a series of conformational
changes in order to transfer the bound
solute across the membrane
need not bind the solute
they form hydrophilic pores that
extend across the lipid bilayer
concentration
gradient
Against concentration
gradient
passive Diffusion Active Diffusion

Passive Diffusion
◼ Is a passive process in which the random mixing of particles in a solution occurs
because of the particles’ kinetic energy.
◼ They move down their concentration gradient.

Several factors influence the diffusion rate
of substances across plasma membranes
◼ Steepness of the concentration gradient : The greater the difference in concentration
between the two sides of the membrane, the higher the rate of diffusion.
❑ charged particles
◼ Temperature: The higher the temperature, the faster the rate of diffusion
❑ diffusion processes occur more rapidly in a person with a fever
◼ Mass of the diffusing substance: The larger the mass - the slower its diffusion rate.
❑ Smaller molecules diffuse more rapidly than larger ones.
◼ Surface area: The larger the membrane surface area -the faster the diffusion rate.
❑ the air sacs of the lungs have a large surface area
◼ Diffusion distance: The greater the distance -the longer it takes.
❑ Diffusion across a plasma membrane takes only a fraction of a second because it
is so thin.

Simple Diffusion
◼ is a passive process in which substances move freely through the lipid
bilayer of the plasma membranes of cells without the help of membrane
transport proteins
◼ Nonpolar, hydrophobic molecules including oxygen, carbon dioxide, and
nitrogen gases; fatty acids; steroids; and fat-soluble vitamins (A, D, E, and
K).
◼ Small, uncharged polar molecules such as water, urea, and small alcohols
also pass through the lipid bilayer by simple diffusion.

Facilitated Diffusion
◼ Solutes that are too polar or highly charged to move through the lipid bilayer
by simple diffusion can cross the plasma membrane by a passive process
called facilitated diffusion.
◼ CHANNEL-MEDIATED FACILITATED DIFFUSION
Through a membrane channel [ion channels] - integral transmembrane
proteins that allow passage of small, inorganic ions that are too hydrophilic to
penetrate the nonpolar interior of the lipid bilayer.
❑ Eg. For Na, K, Ca ions
◼ CARRIER-MEDIATED FACILITATED DIFFUSION
◼ a carrier (also called a transporter) is used to move a solute down its
concentration gradient across the plasma membrane.
◼ The solute binds to a specific carrier on one side of the membrane and is
released on the other side after the carrier undergoes a change in shape.
❑ E.g Glucose transporter

Osmosis
◼ Type of passive diffusion
◼ is a type of diffusion in which there is net movement of a solvent through a
selectively permeable membrane
◼ the solvent is water, which moves by osmosis across plasma membranes
from an area of higher water concentration to an area of lower water
concentration.
◼ During osmosis, water molecules pass through a plasma membrane in two
ways:
◼ (1) by moving through the lipid bilayer via simple diffusion, as previously
described, and
◼ (2) by moving through aquaporins- integral membrane proteins that
function as water channels.
◼ the solution with the impermeable solute also exerts a force, called the
osmotic pressure.

Tonicity
◼ Isotonic solution: The concentrations of solutes that cannot
cross the plasma membrane are the same on both sides of the
membrane in this solution.
❑ E.g NaCl: 0.9%
◼ hypotonic solution: a solution that has a lower concentration
of solutes than the cytosol.
◼ hypertonic solution: higher concentration of solutes than
does the cytosol

Active Diffusion
◼ Uphill [against concentration gradient]
◼ Some polar or charged solutesTwo
sources of cellular
◼ energy can be used to drive active
transport:
◼ (1) Energy obtained from hydrolysis
of adenosine triphosphate (ATP) is the
source in primary active transport;
◼ (2) energy stored in an ionic
concentration gradient is the source in
secondary active transport..

Primary Active Transport
◼ Carrier proteins that mediate primary active transport are often called
pumps.
◼ hydrolysis of ATP changes the shape of a carrier protein
◼ E.g Sodium potassium pump
❑ Three Na + in the cytosol bind to the pump protein
❑ Binding of Na triggers the hydrolysis of ATP into ADP
❑ This chemical reaction changes the shape of the pump protein, expelling
the three Na into the extracellular fluid
❑ Two K + in the extracellular fluid to the pump protein
❑ The binding of K + triggers release of the phosphate group from the
pump protein
❑ As the pump protein reverts to its original shape, it releases K + into the
cytosol.

SECONDARY ACTIVE TRANSPORT
◼ Secondary active transport indirectly uses energy obtained from the
hydrolysis of ATP.
◼ The energy stored in a Na+ or H + concentration gradient is used to drive
other substances across the membrane against their own concentration
gradients.
◼ Some of the stored energy can be converted to kinetic energy (energy of
motion) and used to transport other substances against their concentration
gradients.
◼ If these transporters move two substances in the same direction they are
called symporters
❑ Na + /glucose and Na/amino acid symporters
◼ If these transporters move two substances in the opposite direction they are
called antiporters
❑ Na+ /H + and Na+/Ca+ antiporters

Transport in Vesicles
◼ small, spherical sac.
◼ Vesicles also import materials from and release materials into
extracellular fluid
◼ Endocytosis: materials move into a cell in a vesicle formed
from the plasma membrane.
❑ Receptor mediated
❑ Phagocytosis
❑ Bulk phase endocytosis
◼ Exocytosis: materials move out of a cell by the fusion with the
plasma membrane of vesicles formed inside the cell.

Endocytosis
1. Receptor-mediated endocytosis
◼ highly selective type- cells take up specific ligands.
◼ A vesicle forms after a receptor protein in the plasma membrane recognizes and
binds to a particular particle in the extracellular fluid.
◼ E.g. For cells take up cholesterol containing low-density lipoproteins (LDLs),
transferrin (an iron-transporting protein in the blood), some vitamins, antibodies,
and certain hormones by receptor-mediated endocytosis.

Phagocytosis
◼ The cell engulfs large solid particles
◼ E.g worn-out cells, whole bacteria, or viruse
◼ Phagocytes, those cells, that are able to
carry out phagocytosis [Macrophage,
neutrophiles]
◼ Phagocytosis begins when the particle binds
to a plasma membrane receptor on the
phagocyte, causing it to extend pseudopods
◼ Pseudopods surround the particle outside
the cell, and the membranes fuse to form a
vesicle called a phagosome
◼ Phagocytosis is a vital defense mechanism
that helps protect the body from disease

bulk-phase endocytosis [Pinocytosis]
◼ tiny droplets of extracellular fluid are taken up.
◼ No receptor proteins are involved; all solutes.
◼ Dissolved in the extracellular fluid are brought into the cell the
plasma membrane folds inward and forms a vesicle containing a
droplet of extracellular fluid.
◼ The vesicle detaches or “pinches off” from the plasma
membrane and enters the cytosol.

EXOCYTOSIS
◼ exocytosis releases materials from a cell.
◼ All cells carry out exocytosis, but it is especially important in
two types of cells:
◼ (1) secretory cells that liberate digestive enzymes, hormones,
mucus, or other secretions;
◼ (2) nerve cells that release substances called neurotransmitters
◼ In some cases, wastes are also released by exocytosis.
◼ During exocytosis, membrane-enclosed vesicles called
secretory vesicles
Segments of the plasma membrane lost through endocytosis are
recovered or recycled by exocytosis. The balance between
endocytosis and exocytosis keeps the surface area of a cell’s
plasma membrane relatively constant.

TRANSCYTOSIS
◼ Transport in vesicles may also be used to successively move a
substance into, across, and out of a cell. In this active process,
called transcytosis.
◼ vesicles undergo endocytosis on one side of a cell, move
across the cell, and then undergo exocytosis on the opposite
side.
◼ E.g.
◼ Transcytosis occurs most often across the endothelial cells that
line blood vessels and is a means for materials to move
between blood plasma and interstitial fluid. For instance, when
a woman is pregnant, some of her antibodies cross the placenta
into the fetal circulation via transcytosis

Cytoplasm
◼ Cytosol(intracellular fluid) + organelles
◼ Cytosol:
❑ 75–90% water
❑ Various dissolved and suspended components: different types of ions,
glucose, amino acids, fatty acids, proteins, lipids, ATP, and waste
products.
◼ Function: The cytosol is the site of many chemical reactions required
for a cell’s existence. For example, enzymes in cytosol catalyze glycolysis.

L2-PlasmaMembrane.pdf

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