• Outside boundary
• Give shape
• Surround and protect the cell
• Passage way of materials into and out of the cell
• Also called “plasma membrane”
• Semipermeable membrane (a permeable membrane
allows all materials through, while an impermeable
membrane does not allow anything through)
• The fluid-mosaic model describes the plasma membrane of
animal cells. The plasma membrane that surrounds these cells
has two layers (a bilayer) of phospholipids (fats with
phosphorous attached), which at body temperature are like
vegetable oil (fluid). And the structure of the plasma membrane
supports the old saying, “Oil and water don’t mix.”
• Each phospholipid molecule has a head that is attracted to
water (hydrophilic: hydro = water; philic = loving) and a tail that
repels water (hydrophobic: hydro = water; phobic = fearing).
Both layers of the plasma membrane have the hydrophilic
heads pointing toward the outside; the hydrophobic tails form
the inside of the bilayer.
Fluid Mosaic Model
According to the fluid mosaic model of
S.J. Singer and G.L. Nicolson
(1972), which replaced the earlier
model of Davson and
Danielli, biological membranes can be
considered as a two-dimensional liquid
in which lipid and protein molecules
diffuse more or less easily. Although
the lipid bilayers that form the basis of
the membranes do indeed form two-
dimensional liquids by themselves, the
plasma membrane also contains a
large quantity of proteins, which
provide more structure. Examples of
such structures are protein-protein
complexes, pickets and fences formed
by the actin-based cytoskeleton, and
potentially lipid rafts.
Adolf Eugen Fick (3 September
1829 – 21 August 1901) was a
German-born physician and
In 1855, he introduced Fick's law of
diffusion, which governs the diffusion
of a gas across a fluid membrane. In
1870, he was the first to measure
cardiac output, called the Fick
Diffusion is one of several transport phenomena that occur in
nature. A distinguishing feature of diffusion is that it results in
mixing or mass transport, without requiring bulk motion. In Latin,
"diffundere" means "to spread out".
the process whereby particles of liquids, gases, or solids
intermingle as the result of their spontaneous movement
caused by thermal agitation and in dissolved substances move
from a region of higher to one of lower concentration
a passive process which means that no energy is needed
due to the random movement of particles
plays a part in moving substances in and out of the cell
The concept of diffusion is widely used in:
• physics (particle diffusion)
• finance (diffusion of people, ideas and of price
The concept of diffusion is typically applied to any
subject matter involving random walks in ensembles of
The diffusion of water through a selectively permeable
membrane is called osmosis. Water helps to
dissolve many of the substances involved in cell
processes. When water is lost (moves out of the cell) it
leaves behind a high concentration of the dissolved
substances- when water moves back into the cell, the
substances become more diluted and can be used by
the cell for its life functions.
Movement of molecules from an area of lower
concentration to an area of greater concentration.
Solution is a type of homogenous mixture in which the particles
of one or more substances (the solute) are distributed uniformly
throughout another substance (the solvent).
A solution typically consists of the dissolved material called the
solute and the dissolving agent called the solvent. A common
example is sugar (the solute) dissolved in water (the solvent).
Isotonic Solution is a solution having the same osmotic pressure
as blood; the concentration of the water molecules is the same as
that in the cell.
Isotonic solutions are commonly used as intravenously infused
fluids in hospitalized patients.
Hypotonic solution refers to any solution which has a lower
osmotic pressure than another solution (that is, has a lower
concentration of solutes than another solution).
In biology, a hypotonic solution refers to a solution that contains
less solute (more water) compared to the cytoplasm of the cell. If
the solution surrounding the cell is hypotonic osmosis causes
water to have a net flow into the cell, thus, resulting in the swelling
and expansion of the cell. When an animal cell is set to a
hypotonic environment the cell will eventually lyse (rupture) due to
the osmotic pressure. In a plant, the cell will not lyse but become
turgid because of its cell wall that prevents it from bursting.
In fact, it is the osmotic pressure (or turgor pressure) that helps
keep the plant from wilting and losing its shape.
Hypertonic solution is a solution that has higher
osmotic pressure (or has more solutes) than another
solution to which it is compared.
any solution with a higher salt concentration than
normal body cells so that the water is drawn out of
the cells by osmosis; contains higher concentration
of solutes than the cell.
Example: Plasmolysis is the process in plant cells
where the cytoplasm pulls away from the cell wall due
to the loss of water through osmosis. This occurs in a
Mediated transport refers to transport mediated by
a membrane transport protein. There are three types
of mediated transport: uniport, symport, and antiport.
The movement of a solute across a membrane
with the assistance of a transport agent, such as a
protein, that is specific for certain solutes.
Movement of a solution across a membrane with
the aid of a transport agent (e.g., protein).
Passive transport is a movement of biochemicals
and other atomic or molecular substances across
membranes. Unlike active transport, it does not
require an input of chemical energy, being driven by
the growth of entropy of the system. The rate of
passive transport depends on the (semi-)permeability
of the cell membrane, which, in turn, depends on the
organization and characteristics of the membrane
lipids and proteins. The four main kinds of passive
transport are diffusion, facilitated diffusion, filtration
There is no energy required.
Active transport is the movement of all types of molecules across
a cell membrane against its concentration gradient (from low to
high concentration). In all cells, this is usually concerned with
accumulating high concentrations of molecules that the cell
needs, such as ions, glucose and amino acids. If the process uses
chemical energy, such as from adenosine triphosphate (ATP), it is
termed primary active transport. Secondary active transport
involves the use of an electrochemical gradient. Active transport
uses cellular energy, unlike passive transport, which does not use
cellular energy. Active transport is a good example of a process for
which cells require energy.
Examples of active transport include the uptake of glucose in
the intestines in humans and the uptake of mineral ions into
root hair cells of plants.
The process of moving sodium and potassium ions across the cell
membrance is an active transport process involving the hydrolysis
of ATP to provide the necessary energy. It involves an enzyme
referred to as Na+/K+-ATPase. This process is responsible for
maintaining the large excessof Na+ outside the cell and the large
excess of K+ ions on the inside. A cycle of the transport process is
sketched below. It accomplishes the transport of three Na+ to the
outside of the cell and the transport of two K+ ions to the inside.
This unbalanced charge transfer contributes to the separation of
charge across the membrane. The sodium-potassium pump is an
important contributer to action potential produced by nerve cells.
This pump is called a P-type ion pump because the ATP
interactions phosphorylates the transport protein and causes a
change in its conformation.
Endocytosis is an energy-using process by which
cells absorb molecules (such as proteins) by
engulfing them. It is used by all cells of the body
because most substances important to them are
large polar molecules that cannot pass through the
hydrophobic plasma or cell membrane. The process
which is the opposite to endocytosis is exocytosis.
Entry into the cell of materials that are too large
to get in by mere diffusion.
• In phagocytosis, or "cell eating," the cell engulfs
debris, bacteria, or other sizable objects. Phagocytosis occurs
in specialized cells called phagocytes, which include
macrophages, neutrophils, and other white blood cells.
Invagination produces a vesicle called a phagosome, which
usually fuses with one or more lysosomes containing hydrolytic
enzymes. Materials in the phagosome are broken down by
these enzymes and degraded.
• Bacteria, dead tissue cells, and small mineral particles are all
examples of objects that may be phagocytosed.
• Phagocytosis occurs in our body when some foreign particle
enters inside it, our white blood cells engulf it, and then digest
it. It is also how some microorganisms like amoeba get their
• Example: white blood cell engulfing a large, liquid protein
In pinocytosis, or "cell drinking," the cell engulfs extracellular
fluid, including molecules such as sugars and proteins. These
materials enter the cell inside a vesicle, although they do not mix
with cytoplasm. Epithelial cells in capillaries use pinocytosis to
engulf the liquid portion of blood at the capillary surface. The
resulting vesicles travel across the capillary cells and release
their contents to surrounding tissues, while blood cells remain in
The cell takes in surrounding fluids, including all solutes present.
Pinocytosis also works as phagocytosis; the only difference is
that phagocytosis is specific in the substances it transports.
Phagocytosis engulfs whole particles, which are later broken
down by enzymes, such as cathepsins, and absorbed into the
Pinocytosis, on the other hand, is when the cell
engulfs already-dissolved or broken-down food.
Pinocytosis is non-specific and non-absorptive.
Molecule-specific endocytosis is called receptor-
Pinocytosis is otherwise known as cell-drinking, fluid
endocytosis, and bulk-phase pinocytosis.
Example: The uptake of extracellular fluids such as
hormones and enzymes by the body cells
• A process of cellular secretion or excretion in which
substances contained in vesicles are discharged
from the cell by fusion of the vesicular membrane
with the outer cell membrane.
• Cells expels large molecules.
• Golgi bodies
• Example: hormone insulin is spilled out into the