MOVEMENT OF SUBSTANCES
ACROSS THE PLASMA
MEMBRANE
LEARNING OBJECTIVE
• Analysing the movement of substances across
the plasma membrane
• Understanding the movement of substances
across the plasma membrane in everyday life
• Appreciating the movement of substances
acrss the plasma membrane
Table content
• Movement across the plasma membrane
• Movement of substances across the plasma
membrane in everyday life
• Appreciating the movement of substances
across the plasama membrane
Movement across the plasma
membrane
Structure of plasma membrane
The permeability of plasma
membrane
The movement of
substances:passive transport
The movement of substances
:active transport
Structure of plasma membrane
The most accepted theory for plasma membrane structure
is the fluid mosaic model. According to this model, the
plasma membrane comprises of two lipid layers and
associated protein molecules, which attach to the lipid
layers directly or indirectly.
The plasma membrane does the major function of
regulating transportation of substances from inside the cell
to the outside and vice versa. The specificity of plasma
membrane structure plays a crucial role in the overall
functioning of the cell. In simple terms, it acts in a similar
manner to the skin of animals. Various scientific hypotheses
have been proposed to explain the structure of the plasma
membrane, out of which the most popularly accepted
theory is the fluid mosaic model.
Plasma Membrane Structure: The
Fluid Mosaic Model
• The fluid mosaic model was developed by S.J. Singer
and Garth Nicolson in 1972. As per this theory, the
plasma membrane consists of carbohydrates and
different types of lipids and proteins. And the model
is named in accordance to the structure of the
plasma membrane; the membrane is not rigid, but
more of a fluid type, containing various molecules
like a mosaic pattern. These molecules are arranged
in a proper manner, which helps in selective
movement of the substances. Discussed below is a
brief illustration of the fluid mosaic model:
Fluid-mosaic model
Lipid bilayer
• Lipid Bilayer: The fundamental part of the plasma membrane
structure is the lipid bilayer. Types of lipids present in the
plasma membrane are phospholipids, cholesterol and
glycolipids. However, as majority of the molecules are of
phospholipid type (containing a phosphate group), the two
lipid layers are better known as phospholipid layers.
The lipid tails are water repelling (hydrophobic), while
phosphate heads are water-attracted (hydrophilic). The
phospholipid bilayer is arranged in a specific fashion, with the
hydrophobic tails orienting towards the inside (facing each
other) and the hydrophilic head aligning to the outside. Thus,
both sides of the plasma membrane, one that faces the
cytosol and the other facing the outside environment, are
hydrophilic in nature.
The lipid tails are water repelling (hydrophobic),
while phosphate heads are water-attracted
(hydrophilic). The phospholipid bilayer is arranged
in a specific fashion, with the hydrophobic tails
orienting towards the inside (facing each other)
and the hydrophilic head aligning to the outside.
Thus, both sides of the plasma membrane, one that
faces the cytosol and the other facing the outside
environment, are hydrophilic in nature.
Membrane protein
• Membrane Proteins: Another key component of the
plasma membrane is proteins, which help in selective
transport of the macromolecules like sucrose, amino
acids and ions. Based on the location of proteins with
reference to the phospholipid bilayers, there are two
types of proteins:Integral membrane proteins attach
to the lipids of the bilayered structure. And integral
proteins that traverse the phospholipid bilayer are
called transmembrane proteins.
Peripheral membrane proteins are indirectly or
loosely attached to the membrane. They are noncovalently connected with the lipids or ends of the
integral proteins.
carbohydrates
• Carbohydrates: In addition to phospholipids
and proteins, the cell membrane also consists
of carbohydrates, basically glycoproteins and
glycolipids. These molecules are exclusively
arranged in the outer side of the cell
membrane, wherein the carbohydrate
portions are exposed to the external surface
of the cell.
Function of plasma membrane
• to act as an attachment to the non-living matter that
is found outside the cell membrane.
• is the transportation of materials, that is crucial for
the proper functioning of various cell organelles. This
semi-permeable membrane of the cells helps in the
transferring of those nutrients and chemicals that are
required for the cell functioning. The other foreign
materials are obstructed on their path thereby
preventing the invasion of the plasma membrane.
• it maintains a suitable 'cell potential'.
The permeability of
plasma membrane
Plasma membrane
• Plasma membrane is selectively permeable or semipermeables
• Its means that only some substances can move
across the plasma membrane freely while the others
cannot
• Two general factors that determine :
I. size
II. Polarity
The movement of
substances
• A passive process also
but involves only
water molecules
• Defined as the net
movement of freely
moving water
molecules from a low
to high solute
concentration.
• Its important to cell
because cell need
adequate water to
function well.
Facilitated diffusion
• The simplest type of
passive transport
• No energy need
• Small molecules can
pass through
:Oxygen,carbon
dioxide,lipid soluble
molecules.
• Simple diffusion is the
net movement of
molecules or ions from
higher to lower
concentration until
equilibruim achieved.
osmosis
Simple diffusion
Passive transport
• No energy need
• Water soluble which
are not soluble in
lipids cannot
• Pass throught the
phospholipids bilayer
but with the aids of
carrier protein and
pores that is called
facilitated diffusion
Active transport
• Is the movement of molecules or ions against the
concentration gradient across plasma membranes.
• Requires both carrier proteins and energy in it
process.
• Energy is from ATP
Tq,,,,