Chapter 2- Cellular activity Stage 3
Notes
Structure of the Lipid Bilayer (what makes up the structure)
The lipid bilayer (phospholipid bilayer/cell membrane) is a structural
component of the cell that isolates the cell components (organelles,
cytoplasm) from the extracellular environment.
1. Phospholipid molecules= contains a phosphate group and a lipid
molecule.
2. Each phospholipid molecule contains a hydrophilic head and a
hydrophobic tail. It is arranged in two layers.
3. The membrane has cholesterol and protein molecules scattered
within the actual bilayer membrane.
4. There are short polysaccharide (complex carbohydrate) chains that
can also be attached to the membrane.
The structure of the phospholipid bilayer molecule
1. Hydrophilic= water loving (attracted to water).
a. Hydro= water
b. Philic= loving.
2. Hydrophobic= water hating (not attracted to water).
a. Hydro=Water
b. Phobic= fearing
The phospholipid arrange themselves so the heads face a watery fluid on
both sides (extracellular fluid and intracellular fluid) and the tails point
towards each other forming a “water hating” area.
Why is the inside membrane made of lipids? Lipids are fats, like oil, that are insoluble in water.
 Hydrophobic nature of inner membrane allows lipid-soluble
substances to pass through membrane across the concentration
gradient.
 lipid-soluble molecules can cross the membrane directly through
the bilayer, this is not for large molecules such as glucose, water
and other polar molecules
Molecules that are hydrophilic, on the other hand, cannot pass through
the plasma membrane
Why is the outside part of the membrane (hydrophilic heads) water
soluble?
Channel proteins allow water-soluble molecules to pass through
the otherwise hydrophilic membrane .
Molecules that are hydrophilic can easily pass through the plasma
membrane
Membrane proteins
 Channel Proteins- allows small molecules to diffuse into the cell
membrane
 Carrier Proteins- larger molecules like glucose, potassium and
sodium to be carried into the cell via a large protein molecule. It
requires ATP to allow the glucose molecule into the cell. These
proteins are also specific and only bind to a particular moleculeEG- the carrier for glucose will only transport glucose and no other
molecule.
 Receptor Proteins-these are proteins which are sensitive to
particular molecules. They allow particular molecules to bind to the
protein which can change the cells activities. One particular protein
only binds to one particular molecule.
 Cell identity markers- various protein or carbohydrate molecules.
They are located on surface of the membrane and detect
molecules whether they are their own or invaders. (example=
MHC)
Membrane transport
1. Diffusion (or Simple Diffusion)
A few substances can diffuse directly through the lipid bilayer part of the
membrane. The only substances that can do this are lipid-soluble
molecules such as steroids, alcohol and fatty acids, CO2 and O2 can
pass easily through the lipid membrane. For these molecules the
membrane is no barrier at all. Since diffusion is (obviously) a passive
diffusion process, no energy is involved and substances can only move
down their concentration gradient (high to low).
a. Osmosis
Water soluble (able to be dissolved in water) molecules can enter or
leave the cell through the membrane protein channels.
These molecules can be water and ions like sodium, calcium and
chloride. Even glucose is too big to fit through the channels. They must
bind to a carrier protein.
2. Carrier Mediated Transport
a. Facilitated Diffusion
Passive transport is the transport of substances across a membrane by a
trans-membrane protein molecule.
 The transport proteins tend to be specific for one molecule (a bit
like enzymes), so substances can only cross a membrane if it
contains the appropriate protein.
 A passive diffusion process, so no energy is involved and
substances can only move down their concentration gradient (High
to low).
There are two kinds of transport protein:
a) Channel Proteins form a water-filled pore or channel in the
membrane. Most channels can be gated (opened or closed),
allowing the cell to control the entry and exit of ions. Water
molecules can move through the channels easily.
b) Carrier Proteins have a binding site for a specific solute. This
means if it is a glucose carrier protein, then no other solute can
bind to it.
a. Carrier proteins are specific. They will only bind to a
particular molecule.
b. Carrier proteins can become saturated where once all the
carrier proteins are occupied the concentration will not
change.
b. Active transport
The protein binds a molecule of the substance to be transported on one
side of the membrane, changes shape, and releases it on the other side.
The proteins are highly specific, so there is a different protein pump for
each molecule to be transported.
 These proteins are gated which means that they will only open and
close to allow certain molecules in or out. (Animation)
 This method of transport requires cellular energy (ATP)
 Substances move across the membrane against the concentration
gradient (Low to high)
3. Vesicular transport
Large molecules (such as proteins, polysaccharides and nucleotides) and
even whole cells are moved in and out of cells by using membrane
vesicles.
 This method of transport requires cellular energy (ATP)
There are two smaller processes that occur.
a) Endocytosis- the taking in of liquids/solids in membrane bound
vesicles.
a. Pinocytosis- Taking of liquids into the cell in vesicles.
Example= small protein molecules.
b. Phagocytosis- Taking in of solid particles. Example= such as
a white blood cell ingesting a bacterial cell.
b) Exocytosis- When contents such as liquids or solids inside the cell
are passed to the outside and into the extracellular fluid.
Cells rely on the body to maintain a constant internal environment to
function. This is called Homeostasis.
Antigens- are very large molecules (either proteins, carbohydrates etc).
Any substance capable of triggering an immune response is called an
antigen.
Antigen - a nonself marker that triggers the formation of lymphocyte
armies
Antibodies - molecules which bind to antigens and are recognized by
lymphocytes
Antibodies
1. White blood cells can engulf foreign cells and digest them.
2. Producing antibodies- when your WBC comes across a foreign cell
they will start to produce chemicals called antibodies to kill the
new invading cells. The antibodies are then produced rapidly and
flow all around the body to kill all similar bacteria or viruses.
3. Once your body had produced antibodies to tackle this new strain
of bacteria or virus, it is said you have built up a “natural
immunity”. This means if you are infected by the same bacteria or
virus, your antibodies will kill them and won’t become ill.
4. Antibodies, also called immunoglobulin’s. They are large Y-shaped
proteins which function to identify and help remove foreign
antigens or targets such as viruses and bacteria. Every different
antibody recognises a specific foreign antigen. This is because the
two tips of its “Y” are specific to each antigen, allowing different
antibodies to bind to different foreign antigens.
5. If circulating antibodies come in contact with the target or antigen
they were generated to fight, then the antibodies bind to the
target (like the lock and key model) An antibody only attaches to
the site of an antigen if it fits). Depending on the antigen, the
binding may impede the biological process causing the disease or
may recruit macrophages to destroy the foreign substance.
White blood cells are different to lymphocytes- within the lymphoid
tissue (lymphatic system within the lymph node) are two types of
lymphocytes
 B cells- which provide antibody-mediated immunity (these cells
circulate around the body and attack invading agents). They also
release antibodies into the blood and lymph. It provides resistance
to virsues or bacteria toxins. These B cells respond to specific
antigens
 T cells which provide cell mediated immunity (they form special
lymphocytes that destroy invading agents
The Major Histocompatibility Complex (MHC) is a set of molecules
displayed on cell surfaces that are responsible for lymphocyte
recognition and "antigen presentation". The MHC molecules control the
immune response through recognition of "self" (your own antigens) and
"non-self" (invaders) and, consequently, serve as targets in
transplantation rejection.
Antigen-presenting cell - a macrophage which digests a foreign cell, but
leaves the antigens intact. It then binds these antigens to MHC
molecules on its cell membrane. The antigen-MHC complexes are
noticed by certain lymphocytes (recognition) which promotes cell
division (repeated cell divisions)