Topic 2 - Cells and membranes
Prokaryote cells
•The general size of a prokaryotic cell is about
1-2 um.
•Note the absence of membrane bound
organelles
•There is no true nucleus with a nuclear
membrane
•The ribosome's are smaller than eukaryotic
cells
•The slime capsule is used as a means of
attachment to a surface
•Only flagellate bacteria have the flagellum
•Plasmids are very small circular pieces of DNA
that maybe transferred from one bacteria to
another.
Cell Wall: Made of a murein (not cellulose). Provides structure to the cell, prevents explosion by osmotic
pressure.
Plasma membrane: Controls the entry and exit of substances, pumping some of them in by active
transport.
Cytoplasm: Contains all the enzymes needed for all metabolic reactions, since there are no organelles.
Ribosome: The smaller (70 S) type are all free in the cytoplasm, not attached to membranes (like RER).
They are used in protein synthesis which is part of gene expression.
Nucleoid: Is the region of the cytoplasm that contains DNA. It is not surrounded by a nuclear membrane.
DNA is always a closed loop (i.e. a circular), and not associated with any proteins to form chromatin.
Flagella: These long thread like attachments are generally considered to be for movement. They have an
internal protein structure that allows the flagella to be actively moved as a form of propulsion.
Pilli: These thread like projections are usually more numerous than the flagella. They are associated with
different types of attachment. In some cases they are involved in the transfer of DNA in a process
called conjugation or alternatively as a means of preventing phagocytosis.
Slime Capsule: A thick polysaccharide layer outside of the cell wall. Used for sticking cells together, as a
food reserve, as protection against drying out and chemicals, and as protection against phagocytosis.
In some species the capsules of many cells in a colony fuse together forming a mass of sticky cells
called a biofilm. Dental plaque is an example of a biofilm.
Plasmids: Extra-nucleoid DNA of up to 400 kilobase pairs. Plasmids can self-replicate particularly before
binary fission. They are associated with conjunction which is horizontal gene transfer. It is normal to
find at least one anti-biotic resistance gene within a plasmid. This should not be confused with
medical phenomena but rather is an ecological response to other antibacterial compounds produced
by other microbes. Commonly fungi will produce anti-bacterial compounds which will prevent the
bacteria replicating and competing with the bacteria for a resource.
• Prokaryotic cells divide by binary fission. This is an asexual
method of reproduction in which a 'parental' cell divides into
two smaller but equally sized cells. The cells are genetically
identical and form the basis of a reproductive clone.
This is an electron-micrograph of a bacteria E.coli
• 1. Note the double membrane of
this E. coli.
• 2. There is some evidence in the
image of pilli which are the
surrounding light grey masses.
• 3. In the cytoplasm of the bacterium
there are no visible organelles
which is consistent with how we
expect a prokaryote cell to appear.
• 4. The nucleoid region is not seen
well in this particular image but is
clearer in the next image.
Eukaryote cells - Animal
http://kirewyyhyv.blogspot.com/2011/05/animal-cell-labeled-with-functions.html
Eukaryote cells - Plant
http://www.lacs-ny.org/webpages/nmatuszczak/files/plant%20cell%20labeled.gif
Nucleus: This is the largest of the organelles. The nucleus contains the chromosomes which during
interphase are to be found the nucleolus. The nucleus has a double membrane with pores. The
nucleus controls the cells functions through the expression of genes. Some cells are multi nucleated
such as the muscle fibre.
Plasma membrane: controls which substances can enter and exit a cell. It is a fluid structure that can
radically change shape. The membrane is a double layer of water repellant molecules. Receptors in
the outer surface detect signals to the cell and relay these to the interior. The membrane has pores
that run through the water repellant layer called channel proteins.
Mitochondria: location of aerobic respiration. It is a Double membrane organelle. Inner membrane has
folds called cristae. This is the site of oxidative phosphorylation. Centre of the structure is called the
matrix and is the location of the Krebs cycle. Oxygen is consumed in the synthesis of ATP on the inner
membrane The more active a cell the greater the number of mitochondria.
Rough endoplasmic reticulum (rER): protein synthesis and packaging into vesicles. rER form a network of
tubules with a maze like structure. In general these run away from the nucleus The 'rough' on the
reticulum is caused by the presence of ribosomes. Proteins made here are secreted out of the cell.
Ribosomes: the free ribosome produces proteins for internal use within the cell.
Golgi apparatus: modification of proteins prior to secretion. Modified proteins are then packaged in
membrane sacs (vesicles) so they can be excreted through the plasma membrane.
Lysozyme: Formed on the golgi apparatus. Are membranes sacs containing hydrolytic (breaking things up
using water) enzymes. Functions include the digestion of old organelles, engulfed bacteria and
viruses and large molecules. Hydrolytic enzymes are retained within the vesicle membrane (sac) to
prevent autodigestion of the cell.
Comparison of animal and plant cells
Comparison of Pro- and Eukaryotes
2 Roles of extra-cellular components
Plant cell wall.
Composed of cellulose and Found around all
plant cells. Maintains the shape of the cell.
Provides structural support against the force of
gravity. Prevents excessive uptake of water by the
cell
Bone Interstitial matrix:
Bone has a matrix between its cells which includes
collagen with a calcium phosphate.
Other tissues are surrounded by a matrix
composed of a kind of gel that provides support for
the tissue.
Membrane structure
Hydrophilic end –
loves water
----------------------Hydrophobic end –
hates water
http://www.takdangaralin.com/wp-content/uploads/2009/10/image42.png
Movement through a membrane
http://kentsimmons.uwinnipeg.ca/cm1504/membranefunction.htm
Diffusion
• Diffusion is the movement of molecules from
a high concentration to a low concentration. It
is a Passive movement of molecules (requires
no energy).
Diffusion animation->
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html
Osmosis
• Osmosis is the passage of water from a region
of high water concentration through a semipermeable membrane to a region of low
water concentration.
• or because “high water concentration” is the same as “dilute solution:”
• Osmosis is the passage of water from a dilute
solution through a semi-permeable
membrane to a more concentrated solution
• for example the movement of water from the soil to the roots.
Osmosis animation -> http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
Consequences of Osmosis
•
•
•
•
Semi permeable = Cell
membranes will allow
small molecules like
Oxygen, water, Carbon
Dioxide, Ammonia,
Glucose, amino-acids,
etc. to pass through.
Cell membranes will not
allow larger molecules
like Sucrose, Starch,
protein, etc. to pass
through.
Isotonic – same
concentration both
sides of a membrane.
Hypertonic – the more
concentrated solution
Hypotonic – the less
concentrated solution
• Facilitated diffusion
Facilitated diffusion involves the use of a protein to
facilitate the movement of molecules across the
membrane. In some cases, molecules pass through
channels within the protein.
Animation ->
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html
• Active Transport.
Active Transport is a type of transport where
substances move against a concentration gradient,
in the opposite direction to diffusion. (From an area
of lower concentration to an area of higher
concentration.) Active transport needs energy and
a protein pump (found in a membrane) to work.
Animation -> http://highered.mcgraw-
hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
What happens to big food particles that
won’t fit through the membrane?
http://www.biosulf.org/1/images/endocytosis.gif