Unit 3A
Human Form & Function
Cells, metabolism & regulation
Cellular Activity
Some useful websites for Cell
Biology
• http://www.bscb.org/?url=softcell/centriole
s
• http://biology.tutorvista.com/animal-andplant-cells/centrioles.html
The cell membrane
Drawing by Mariana Ruiz Villarreal
The Fluid Mosaic Model
What is the fluid mosaic
model??
• The cell membrane is said to be fluid
because the molecules it is made of
are constantly changing position.
• It is said to be mosaic because it is
composed of many different kinds of
molecules.
http://www.youtube.com/watch?v=Rl5E
mUQdkuI
The cell membrane
• The cell membrane (or plasma membrane)
is the outer boundary of a cell.
• It separates the intercellular components
of the cell from the extracellular
components.
• The cell membrane is selectively
permeable (differentially)– i.e. it allows
some substances to pass through more
easily than others.
Structure of the cell membrane
• The membrane is a phospholipid bilayer (i.e. two layers of phospholipid
molecules.
• The phosphate ‘head’ of the phospholipid
molecules are hydrophilic (attracted to
water), and form the outer and inner
boundaries of the membrane.
• The lipid ‘tail’ of the phospholipid
molecules are hydrophobic (repelled by
water), and form the inner layer of the
membrane.
• The phospholipid bi-layer also contains the
steroid compound cholesterol
• It helps to maintain the stability of the
membrane by being wedged between the
phospholipids.
• A variety of protein molecules are embedded
in the membrane to allow the movement of
substances into and out of the cell either
passively(no energy required) or
actively(using energy in the form of ATP).
Functions of the cell membrane
The cell membrane:
• Isolates the cell from the surrounding
tissue fluid – physical barrier
• Determines what substances enter and
leave the cell – regulation of the
passage of materials (the entry of ions
and nutrients and, the removal of wastes
and the release of secretions).
Functions of the cell membrane
• Sensitivity – contains a variety of
receptors that respond to substances in
the cell’s environment e.g. hormones,
antigens
• Support – the internal part of the cell
membrane is attached to the
microfilaments of the cells cytoskeleton.
Membrane Proteins
• Are very large molecules and make up
55% of the membrane weight.
• They include:- Channel Proteins
Carrier Proteins
Receptor Proteins
Cell Identity Markers
Membrane transport proteins
• Membrane proteins that aid the movement of
substances in and out of the cell include:
• Channel proteins (ion channels) –
open channels that allow simple diffusion.
• Carrier proteins that allow facilitated
diffusion (e.g. glucose) and active transport
(specific membrane pumps).
Membrane transport proteins
• Receptor Proteins – sensitive to molecules
outside the cell.
• When present they bind with the receptor
protein to trigger changes within the cell.
• Some hormones work in this way e.g Insulin
• The hormone Insulin binds to a specific
receptor protein which leads to an increase
in glucose absorption by the cells
Membrane transport proteins
• Receptor proteins are specific e.g. only
bind with specific molecules.
• There are a limited number of receptor
proteins in the membrane of each cell.
• Once each receptor is bound to a
molecule there can be no further increase
in the rate of the cell’s activity.
Membrane transport proteins
• E.g When each insulin receptor in the
membrane is bound to insulin, the rate of
glucose uptake cannot increase further
even if the amount of insulin increases.
• Different cells have different types and
numbers of receptor proteins which is why
there is a variation in the sensitivity of cells
to hormones and to other substances.
Membrane transport proteins
• Receptor proteins are important in
communication between cells.
• Substances produced in one cell can
stimulate and initiate changes in adjacent
cells e.g. nerve cells ( one nerve cell
releases a neurotransmitter that is
detected by receptor proteins of an
adjacent cell which responds by producing
a nerve impulse)
Cell Identity markers
• These are various protein and carbohydrate
molecules exposed on the surface of the cell
membrane.
• They act as markers and indicate to cells of the
immune system whether a cell is normal or
abnormal.
• Major histocompatibility complex (MHC) is a
group of genes that are unique in every
individual they code for small protein molecules
that act as ‘self’ markers on all body cells.
• MHC molecules initiate the immune response.
• http://www.youtube.com/watch?v=OlHez8
gwMgw
• http://www.youtube.com/watch?v=o4Srx4
mUmaI
Membrane transport
• Transport processes are either
passive or active.
• Passive processes require no
cellular energy and include
diffusion, osmosis & facilitated
diffusion.
• Active processes require ATP and
include specific membrane pumps
and vesicular transport.
• http://www.youtube.com/watch?v=2UPqL
m-uDnI
• http://www.youtube.com/watch?v=RPAZvs
4hvGA
Membrane Transport
• Diffusion -2 ways in cell membranes
1. Substances soluble in lipids i.e.
alcohol, steroids and fatty acids can
diffuse through the lipid part of the
membrane.
So can carbon dioxide and oxygen
molecules
Membrane Transport
Diffusion
2. Substances can pass through membrane
channels formed by channel proteins.
Water soluble molecules can enter or leave
through the membrane channels but only if they
are small enough.
If the channels are
only water and sodium,
calcium and chloride ions will be able to pass
through. Glucose would be too BIG even though
they are relatively small.
really small
Membrane Transport
• Osmosis (diffusion of water)
• Passive process
• Water can travel across the membrane
through carrier proteins when there is a
concentration gradient.
• Movement from high concentration to low
concentration
Membrane Transport
• Carrier Mediated – proteins in the cell bind to molecules
to be transported and help their passage across the
membrane.
• Carrier proteins are specific; they will only bind to a
particular molecule.
• They become saturated; once all the carriers are
occupied, any increase in concentration is ineffectual.
The rate of transportation does not increase.
• Carrier activity is regulated by substances like
hormones.
Membrane Transport
• Facilitated Diffusion - passive
• Molecules such as glucose attaches to a binding
site on the carrier.
• The carrier changes shape and the molecule is
released on the other side of the membrane.
• Happens across a concentration gradient!
http://highered.mcgrawhill.com/sites/0072507470/s
tudent_view0/chapter3/animation__how_facilitated
_diffusion_works.html
Simple diffusion (left) &
facilitated diffusion
Diagram created by LadyofHats
Membrane Transport
• Active transport – requires energy from
ATP.
• Substances are transported across the
membrane against the concentration
gradient from a region of low concentration
to a region of high concentration.
Active transport (sodium pump)
Diagram created by LadyofHats
Membrane Transport
• Vesicular transport – Movement across the cell
membrane in structures called vesicles.
• Active process – energy is required to form the
vesicle.
• Endocytosis – taking liquids and solids into the cell.
• Pinocytosis – taking liquids into the cell.
• Phagocytosis – taking solids into the cell
• Exocytosis – taking solids and liquids from the cell.
The vesicle fuses with the cell membrane then
pushes the contents out into the extracellular fluid.
Endocytosis
Diagram created by LadyofHats
Exocytosis
Diagram created by LadyofHats
Other Web Links
• http://prezi.com/fztruy_c7jfa/biology-mindmapcell-membrane/