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UNIT 1: CELLULAR FUNCTIONS
The
functional unit of any living organism is the cell. The functional units that make up any
cell are called organelles. All
cells, from bacteria to giant squid cells, contain the same basic parts or
organelles. The difference is that
some cells contain more or less of certain organelles, depending on their
function. For example, sperm cells
contain a lot of mitochondria so that they can produce large amounts of ATP
energy for locomotion, while pancreatic cells contain a lot of Golgi bodies
since they produce and package large amounts of protein – insulin. All cells contain the same basic parts,
yet each type possesses its own unique ratio of each organelle.
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following his initial
observations, Schleiden, Schwann, and Virchow (Figure 2.2, p. 38) each made
contributions to the development of the cell theory:
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all living things are
composed of cells
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cells are the basic
units of living organisms
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all cells come from
pre-existing cells
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Schleiden first observed
that all plant tissue was made of cells
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Schwann then extended
this observation to animal tissue, and eventually all living things
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Virchow furthered both
discoveries by claiming that cells could only arise from pre-existing cells,
which laid to rest the theory of spontaneous generation – living
organisms can be generated from non-living matter
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any cell will grow to a
particular size until its large volume, compared to its cell membrane surface
area, reduces both its ability to take in nutrients, water, and other materials,
and its ability to eliminate waste products effectively
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all of what enters and
exits a cell must go through the cell membrane
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a smaller cell has a
greater cell membrane surface area compared to its volume than a larger
cell
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if a cell gets too big,
its cell membrane area cannot absorb or eliminate enough material efficiently to
sustain its large size
·
Table 2.1, p. 40
demonstrates the effect that size has on surface area to volume ratio – not that
the smaller cube possesses the largest
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lack internal
compartments and membrane-bound organelles
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prokaryotic are only
comprised of unicellular organisms like all the bacteria, all the cells that
belong to the kingdoms Archaebacteria and Eubacteria
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“eu”, meaning “good” in
Greek, refers to the fact that eukaryotic cells have a good or true nucleus, as
well as other compartmentalized and membrane-bound structures
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these kinds of cells can
be found in both unicellular organisms – like Protists (Paramecium, Amoeba, and Euglena)
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the compartmentalized,
membrane-bound structures are called organelles, each having its own specific
function
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for a detailed
difference between these two types of cells, see Figure 2.5, p. 41, or see http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/proeu/proeu.html
2. Non-Cytoplasmic Organelles and Their
Function
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it has an extremely
crucial role in controlling what enters (nutrients, oxygen, etc.) and what exits
(cellular waste, carbon dioxide, etc.) a cell
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the actual membrane
itself, as well as the components that the membrane accommodates, both have a
large impact on the cell’s survival
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the plasma membrane’s
architecture is illustrated in Figure 2.6, pp. 42-43 – referred to as the fluid mosaic model
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the four main
constituents of the membrane that contribute to its proper functioning are
the:
o
phospholipids –
amphophilic molecules
o
cholesterols – maintain
membrane fluidity
o
proteins – integral (intrinsic)
proteins form channels to facilitate movement into and out of cells
-- extrinsic proteins have attachments that
help maintain the structure of the cell (i.e. cytoskeleton), or serve
as recognition sites for cell to cell communication
o
glycocalyx – binding
sites or recognition sites for messenger molecules, as well as lubrication and
adhesion to cells
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found in Archaebacteria,
Eubacteria, some protists, fungi, and all plant cells
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made of cellulose –
tough polysaccharide
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the cell membrane
presses up against the cell wall, giving plant tissue its rigidity and ability
to stand upright, against gravity’s forces
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since the cell wall is
resistant, it helps to structurally support the cell
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the membrane pushes on
the cell wall like a balloon would push up against pantyhose if it were placed
inside them and blown up
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a cell membrane will
never burst, despite being stretched by large amounts of water pressure, as long
as it is contained by its cell wall
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cell walls play a role
in both living and dead cells
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the strength of wood,
which is mostly dead cells, arises from the fact that it consists of lignified
cell walls
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when penicillin is added
to a bacteria culture, it kills it because it prevents the bacteria from growing
a cell wall
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therefore, when a
bacteria absorbs water, and its cell membrane expands, the entire cell will
explode, since no cell wall is there to resist the expansion
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the most obvious
organelle seen when viewing a cell
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in eukaryotes, it
contains the DNA in a form called chromatin – thickens during cell division to
become chromosomes
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the coating on the
nucleus – which protects and controls the passage of materials into and out of
the nucleus, is called the nuclear
envelope (see Figure 2.9, p. 46)
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the nuclear envelope has
nuclear pores (Figure 2.10, p. 48)
to allow substances like messenger proteins, mRNA, and other important materials
to enter and exit the nucleus
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the nucleus also
contains another structure called the nucleolus – a dark, dense structure
that is composed of DNA, granules, and protein fibres (see Figure 2.10, p.
48)
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the nucleolus is
responsible for making ribosomes – proteins that are crucial in the construction
of polypeptide chains during protein synthesis
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a highly organized area
of matter, between the nucleus and the cell membrane of a cell, that contains
all the organelles, as well as the fluid called cytosol – contains a concentrated mix
of ions and molecules such as enzymes, amino acids, ATP, carbohydrates, oxygen,
and carbon dioxide
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they are surrounded by a
single-layered membrane called a tonoplast, and they’re found mainly in
plant cells to store water (for photosynthesis and hydroskeleton support) and to
store starch molecules
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vessicles are used for
transporting materials from one area of the cell to another – kind of like the
cell’s “in-house” courier service moves materials to from one organelle to
another
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Figure 2.10, p. 48, V,
shows a vessicle approaching the nucleus
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they consist of a
combination of RNA and protein
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their function is to
assemble amino acids together in protein synthesis
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part of the ER may
contain ribosomes, which gives in a “rough” appearance – therefore, E.R. with
ribosomes is called rough endoplasmic
reticulum (RER)
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protein synthesis of
proteins that are most likely destined for use outside the cell, are synthesized
on the RER (see Figure 2.12 b, p. 49)
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smooth endoplasmic reticulum
(SER) does not contain
ribosomes (Figure 2.12 a, p. 49)
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the function of the SER
is to make lipids – typically phospholipids and steroids, and to store calcium
ions
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it is common to find a
lot of SER in the testes and the ovaries, both tissue cells where high levels of
steroid hormones production occur
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they appear to be
flattened stacks of membrane that function to receive, modify, and transport
proteins produced by the ER
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if the proteins are to
be used outside the cell, the Golgi complexes package them into vessicles and
send them to the cell membrane for export out of the cell (see Figure 2.13, p.
50)
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they are membrane-bound
sacs that form digestive compartments
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they possess hydrolitic enzymes – substances that
aid in catabolic reactions
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unicellular organisms
use them to break down food
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the white blood cells of
humans, namely the neutrophils and macrophages) contain many lysosomes to engulf
and destroy invading bacteria
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lysosomes are also
referred to as “suicide bags” – they digest damaged or old parts of the cell
that they are a part of, to help the cell recycle some of the molecules into
forming new organelles (see Figure
2.14, p. 51)
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research has linked the
accumulation of the undigested material in lysosomes over time to ageing
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when lysosomes lack the
enzymes they should have, it could lead to diseases known as lysosomal storage
diseases
o
if lysosomes in human
brain cells are missing necessary digestive enzymes that normally break down
excess fat in the brain, it can cause a lysosomal storage disease called
Tay-Sachs disease -- a hereditary condition that results in the deterioration of
the brain -- humans with Tay-Sachs disease don’t live past five years old!
o
some other lysosomal
diseases are gangliosidosis, Sly syndrome, and Hurler syndrome
o
there are approximately
30 human diseases linked to lysosomal dysfunction, which clearly indicates the
importance of this organelle to the cell
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three examples of
lysosomal tissue digestion are the loss of the tail in a tadpole, the loss of
unwanted tissue during insect metamorphosis, and the loss in tissue between
human fingers during embryo development
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they are the main site
of cellular respiration – an extremely important process that produces ATP (the
cell’s “gasoline”) – by extracting energy from glucose
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this process will be
studied in detail in SBI 4U1
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the structure of the
mitochondrion is illustrated in Figure 2.16, p. 53 --
YOU MUST KNOW IT!!
CHLOROPLAST
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found only in
photosynthetic organisms
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their structure is
illustrated in Figure 2.17, p. 54
-- YOU MUST KNOW IT!!
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they contain chlorophyll – a pigment that absorbs
the sun’s photonic energy and uses it to fix/incorporate CO2 into
glucose, in a process known as photosynthesis
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the theory suggests that
larger cells engulfed these smaller organisms, and instead of destroying them,
they harbored them for their beneficial properties
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the evidence to support
this theory is that…
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mitochondria and
chloroplasts have their own DNA and ribosomes, therefore they reproduce on their
own, separately from the cell
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both mitochondria and
chloroplasts are about the same size as bacteria
o
recent discoveries have
found that some amoebas that are infected with bacteria actually survive, while
still harboring over 40 000 of them within their bodies, and once the bacteria
are removed the amoebas die, proving that it is possible for an organism to
become dependent on an invading organism, and that it is possible for both
invader and host to co-exist
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consists of a supportive
network of fine protein fibres – microfilaments, intermediate filaments,
and microtubules that support the
cell, help anchor the organelles in place throughout the cell, and play a role
in relaying messages back and forth between the cell membrane and the interior
of the cell
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flagella are long and
less numerous, while cilia are short and more numerous
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cilia surround a Paramecium to help it swim in its
environment
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human sperm cells use a
flagellum in a whip-like manner to swim to the egg during fertilization (see
Figure 2.9, p. 56)