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BUILDING BLOCKS
Building blocks of life: organic
compounds
• Why organic?
• Because they all contain CARBON
• The main organic compounds are:
1) Carbohydrates –sugars, starches,
glycogen
2) Lipids (fats)
3) Proteins
1) Carbohydrates:
• Main function is to provide a quick source of
energy for all the functions cells have to perform
ie metabolic energy
• Metabolism = sum of all the chemical reactions in
your body
• Energy in a cell is ‘stored’ in the form of ATP
• What do the initials ATP stand for?
• A few carbohydrates have a structural function
2) Lipids (fats)
• Most lipids are hydrophobic which
means they are insoluble in water
• Fatty acids =smaller molecules and can
travel in blood
• Larger fatty molecules need help to
travel around the body; so they are
attached to protein – called lipoproteins
Functions of lipids:
• form part of cell membrane;
• steroids (eg cholesterol and the
hormones cortisol and aldosterone);
• vitamins A,D,K & E;
• sex hormones
• And can be converted in to ENERGY to
drive metabolism
3) Proteins:
• More complex in structure than carbohydrates
and lipids
• Comprised of amino acids
• Some have a structural role
– Collagen and keratin
• Others a physiological role eg
– enzymes (…….ase, eg salivary amylase)
– muscle contraction (actin & myosin)
– antibodies
– some hormones eg insulin
– haemoglobin
Other important organic compounds:
• Nucleic acids:
• DNA & RNA
– Comprised of nucleotides
– Found in the cell nucleus
• ATP
– Stores and transfers energy eg in muscle contraction,
moving substances across the cell membrane,
synthesising larger molecules from smaller ones
Inorganic substances are also very
important:
• Other elements including trace elements eg
Iron, calcium, potassium, iodine; Zinc,
Manganese…..
• Acids: H+ plus an anion (ie a -ve charge)
• Bases: OH- plus a cation (ie a +ve charge)
Atoms and Ions
What ions are we interested in?
•
•
•
•
Na +
Cl –
K+
Ca 2+
NB If you do not know what these symbols mean, then find out!
Question: What do the + and – attached to the
atom names above mean?
• These particles and their movement across
the cell membrane will be very important in
our understanding of how neurons function,
and how nerves ‘fire’ muscles into action.
So…now test yourself:
• Amino acids are the building blocks of………
• Lipids are soluble/insoluble in water
• True or false:
– carbohydrates have primarily a structural
function.
– Glucose has 5 carbon atoms
– Lipids are found in the cell membrane
• ATP stands for…………………….and its role in the
cell is to……………………………….
A typical animal cell:
• A typical cell:
The cell membrane:
a phospholipid bilayer
The cell membrane is bilaminar ie 2 layered
FLUID MOSAIC MODEL
• Mosaic=pattern of small pieces fitted together
• Membrane likened to “a mosaic of proteins
floating like icebergs in a sea of lipids” ( T & G,
1996, p55)
Functions of membrane proteins:
• Integral proteins:
• Some proteins act as channels; they have a pore
through which certain substances can pass into or
out of the cell
• Others act as transporters or carriers to move a
substance from one side of the membrane to the
other
• Others as receptors or recognition sites: attach to
a nutrient, hormone or neurotransmitter
• Other functions: enzymes, cytoskeleton anchors,
cell identity markers
How molecules and ions get into and out of
cells: diffusion, osmosis and active transport
• The composition of the cell membrane is
crucial to the transport of substances into and
out of the cell
• The plasma membrane that surrounds the cell
is impermeable to MOST dissolved substances
ie these cannot get in or out easily
• BUT the cell membrane is permeable to some
substances
Selective permeability: what can and
cannot pass into the cell
• Substances that can dissolve in lipids CAN pass
• Most large molecules CANNOT pass
• Molecules and ions which carry a charge MAY
pass but only through special channels or by
being carried by a transporter molecule (see
later)
• So, the presence of channels and transporters
in the membrane determine what can pass;
they are usually very selective
How substances move into and
out of the cell:
by both PASSIVE PROCESSES and
ACTIVE PROCESSES
Movement across the membrane:
• Passive processes: diffusion (simple and
facilitated), osmosis, bulk flow
• Active processes: active transport (primary
and secondary), vesicular transport (endoand exo-cytosis)
• Let us first look at some of the PASSIVE
processes
Diffusion
• Substances will move from an area of high
concentration to an area of low concentration –down
(or with) their concentration gradient
• Two of these substances are oxygen and carbon
dioxide, which move into and out of cells by a process
called diffusion.
• For example, in the alveolus of the lung, oxygen is in a
higher concentration in the alveolar sac than in the
capillary supplying the alveolus, and so oxygen will
move into the blood stream; carbon dioxide is in a
higher concentration in the blood stream and so will
diffuse out into the sac.
Gas exchange in the alveolar sac:
See also Fig 14.3
Page 216 in A & McH (2006)
Total width of air-blood exchange region is 0.2-0.6 µm
Simple diffusion: watch the animation and
answer the quick quiz questions at the end
• http://highered.mheducation.com/sites/0072
943696/student_view0/chapter3/animation__
how_diffusion_works.html
• What is meant by a ‘concentration gradient’?
This is an important concept in understand
movement of substances across the cell
membrane
Facilitated diffusion: watch the animation
and answer the quick quiz questions at the end
• http://highered.mheducation.com/sites/0072
943696/student_view0/chapter3/animation__
how_facilitated_diffusion_works.html
Osmosis
• Diffusion of a solvent (in this case, water)
through a selectively permeable membrane; ie
the cell membrane)
• Water moves from an area of high
concentration to an area of low concentration
• What determines the concentration of water
molecules is the inclusion of other substances
called the solute
Demonstration of osmosis:
watch the animation and answer the quick quiz
questions at the end
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/ch
apter2/animation__how_osmosis_works.html
Remember: The cell membrane is a
phospholipid bilayer
• Substances that are soluble in lipid will be able to
pass through
• the cell membrane is permeable to oxygen,
carbon dioxide, steroids, fat soluble vitamins and
many other substances including water
• Water is also able to easily diffuse through the
cell membrane through pores in the channels
• Ions (Na+, Cl-, K+, Ca2+ , HCO3-, and urea) diffuse
in and out through protein channel pores down
their concentration gradients
Now test yourself:
• What is meant by polarity?
• True or false:
– Osmosis requires energy in the form of ATP
– Diffusion is a special case of osmosis
– Substances will move by diffusion up their
concentration gradient
– Facilitated diffusion requires a special carrier
protein in the cell membrane
How substances move into and
out of the cell:
PASSIVE PROCESSES
ACTIVE PROCESSES
The active processes:
1) Primary active transport
2) Secondary active transport
3) Vesicular processes (endocytosis and
exocytosis)
The structure of the cell membrane:
• The cell membrane is composed of a
…………………… bilayer
• Cells are surrounded by fluid…and have fluid
inside them
• The concentration of substances inside and
outside the cell is usually different
• To enter a cell, a substance may have to move
AGAINST its concentration gradient
Cell membrane/plasma membrane
potential or resting potential
• A tiny electrochemical potential difference exists
between the inside and outside of the cell
• This membrane potential is a means of storing
potential energy (like a battery) and is important
in excitable cells (such as neurons and muscle
fibres –see next lecture)
• Have a look at the following slide: what is the
charge just outside the cell: positive or negative?
And inside the cell, it is……………..?
Cell membrane potential
Humanbiologylab.pbworks.com
Another factor that contributes to the
resting potential:
• Membrane permeability :
• In a resting nerve or muscle cell, the
permeability of the plasma membrane is 50100 GREATER to K + than to Na+
• For more detail, see Tortora & Grabowski p343
(8th Ed)
Active processes:
• Active transport:
• Used when substances have to get into cells
against the concentration gradient
• ENERGY is required for this process, in the
form of ATP
• Integral membrane proteins act as ‘pumps’ to
push specific ions and molecules across the
membrane
Actively transported substances
include:
•
•
•
•
•
Na +
Cl –
K+
Ca 2+
Plus others eg H+, I-
• NB if you are unfamiliar with these symbols, then ensure you
understand what they mean
1) Primary active transport:
• The cell uses energy from ATP to change the shape of
transport proteins in the plasma membrane
• This change of shape is accompanied by movement of
substances across the cell membrane
• The sodium pump is an example of primary active
transport. Exports Na+ from the cell and imports K+
• Hundreds of sodium pumps exist in the cell membrane
per square micrometer (µm3) NB1µm = diameter of a
smoke particle
Sodium Pump in action:
• http://highered.mheducation.com/sites/0072495855/stude
nt_view0/chapter3/animation__sodiumpotassium_exchange_pump__quiz_1_.html
• sodium pump animation
• NB There are 3 animations in this series, the first 2 seem
similar
• Don’t forget the short quiz to test yourself after watching
the animation
2) Secondary active transport:
• secondary active transport animation
• The sodium pump maintains a large concentration
gradient of Na+ across the plasma membrane; this
electrochemical gradient can be used as a source of
energy.
• A sodium ion and another substance (eg glucose) bind
to a symporter protein which then changes its shape
and allows both substances to cross the membrane;
one with its gradient (Na+) and one against its gradient
(glucose)
• The Na+ is then pumped back out using the sodium
pump; ATP is required for this
3) Vesicular transport: Endocytosis
and Exocytosis
• Molecules which are too large to be moved by
primary and secondary active transport are
moved by mechanisms which involve being
engulfed in folds of the plasma membrane
• vesicular transport animation
Sample exam questions:
1) Describe 3 functions for proteins in the
human body.
2) What is meant by the fluid mosaic model in
relation to the cell membrane?
3) What is the function of the sodium pump?
4) Explain how the resting potential of a cell
membrane is achieved.
Quiz: Match the type of cell transport, with its
description
Type of transport
description
Osmosis
Cell uses energy from ATP to change the
shape of transport proteins in the plasma
membrane
Primary active transport
A sodium ion and another substance cross
the membrane with the help of a
symporter membrane protein
Endocytosis
Large molecules are engulfed in folds of
the plasma membrane
Secondary active transport
Movement of water through a selectively
permeable membrane down its
concentration gradient
Next week:
We will look at these processes in relation to specialised cells:
neurons and muscle cells
WHAT TO DO NEXT:
• Read:
• Chapters 3 (pp 40-46 esp) and 6 (pp69-78) in
Atkinson & McHanwell (2002)
• Or
• Chapter 11 in Seikel, King & Drumwright (2005)
• Other reading:
• Chapter 4 in Love & Webb (mainly nerve
physiology)
• Chapter 2 in Tortora et al (the chemical level of
organisation)