The Cell Membrane
Purpose of the membrane
1) Transport raw materials into the cell.
2) Transport manufactured products and
wastes out of the cell.
3) Prevent the entry of unwanted matter
into the cell.
4) Prevent the escape of matter needed
for cellular functions.
The cell membrane is composed of
A Phosphate group and two fatty acids
bonded to a glycerol “backbone”.
The phosphate group (head) is
„hydrophillic‟ (likes water) the two fatty acid
“tails” are „hydrophobic‟ (water hating).
(Pg 51 Fig 2.22)
Importance of their dual nature
When phospholipids are spread throughout
water, a curious thing happens. The attractions
and repulsions of the heads and tails cause
them to form a cage-like spherical layer. (Pg
51, fig 2.23).
-The water attracting heads face in and out of
-The non-polar tails face each other.
-This phospholipid bilayer forms the basis of
the cell membrane.
There are numerous proteins,
carbohydrates and other lipids spread and
moving throughout the membrane
creating a fluid-like consistency.
Benefits of the Fluid Mosaic Model
Cholesterols found in animal cell membranes.
These keep the membrane fluid at lower
temperatures. They also prevent some other
molecules from passing through.
Protein and carbohydrate arrangements in the
cell membrane that allow the cell to be
“recognized” by other cells. (Pg 52, fig 2.24)
Outside the cell
In multicellular organisms, the cells are
bathed in a thin layer of extracellular
fluid, which is made of water, mineral
and some cell wastes that have been
Transport Through the
Passive and Active
Crossing the Membrane
The membrane is a cell‟s primary means
for it to maintain homeostasis (the
constant steady state of conditions
achieved to remain healthy).
The plasma membrane is selectively
permeable. It allows some materials to
• Diffusion- a passive way that molecules can
move across the membrane from an area of
[high] to [low] ([ ] means concentration).
- materials that pass easily are:
gases, small uncharged molecules, and fat
Any atom above absolute zero (-273 oC) is in
motion. The random motion within a liquid is
called Brownian motion. (Think of dye
spreading through a liquid).
The difference in concentrations of a substance
across a membrane is called the concentration
The diffusion of water is called Osmosis.
Where water moves from a [high] to [low].
There are different types of environments that
will lead to a net direction of movement:
Isotonic condition – when the concentrations
of solutes on both sides of the membrane are
the same causing no net movement of water.
Hypotonic condition – when the [water] is higher
([lower solute]) outside the cell. Water will move
into the cell to balance out concentrations, causing
the cell to swell and possibly burst.
Hypertonic condition – when the [water] is higher
([lower solute]) inside the cell. Water will move out
of the cell, causing the cell to shrivel.
No energy is required to diffuse water, runs on
Facilitated Diffusion – molecules that do not
readily pass, due to size or charge etc.., may
need assistance to cross the membrane.
Specialized transport proteins help substances
move in this case.
Transport proteins are selective and will only
move the molecule/ion it was designed to
Numerous carrier proteins lie within the membrane to
facilitate the movement of glucose (sugar). These
carriers still operate by concentration gradients and
do not require energy.
Channel Proteins – are transport proteins that carry
charged particles. They have a tunnel-like shape and
have their own charge. They attract the ion like
attracting a magnet. Size is a limiting factor for using
Certain cells require materials that may not be
present in great amounts. In this case, a cell
may need to pump these materials in or out of
the cell against the normal concentration
This moving materials from [low] to [high]
requires the cell to spend energy, and is
therefore called Active transport.
4 Important examples of Active Transport
- kidney cells pump glucose and Amino
acids out of the urine and back into the blood.
- Intestinal cells pump in nutrients from the
- Root cells pump in nutrients from the soil
- Gill cells in fish pump out sodium ions.
Active transport pumps are protein pumps in the
membrane that grab ions and other materials and
pump them across the membrane against the
- one of the best understood is the Na+/K+ pump.
In some cases the unequal distribution of Na+ (or
other molecule) can allow another molecule to
„piggyback‟ into the cell when regular diffusion
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Material too big or material that is
charged can not freely cross the cell
The cell must accommodate these
substances by folding in on itself
creating a vesicle to take in or expel
these large molecules.
2 Types of Bulk transport
Endocytosis (Endo = into; cyto = cell) – the
membrane folds inward, trapping matter from the
extracellular fluid and enters the cell as a vesicle.
3 Types of Endocytosis
Phagocytosis (phago = eating) – the membrane folds
inward, trapping solid matter from the extracellular
Pinocytosis (pino = drinking) – the membrane folds
inward trapping fluids from the extracellular fluid.
Receptor-mediated Endocytosis – the membrane
takes in particles after they bind to specialized
receptor proteins embedded in the membrane.
Cholesterol is brought in this way.
Exocytosis (exo= exit) – the reverse of endocytosis.
A vesicle moves out towards the plasma membrane,
fuses with it and opens up its’ contents into the
- This is important in the body secreting manufactured
products from within the cells.
Endocytosis and Exocytosis