Download Unit 1.1.3b - Cell Specialisation

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OK. So that’s great. Very interesting to know that stem cells differentiate. But what does the
word “differentiate” mean?!!?
“
Differentiation is the process by which a less specialised cell becomes a more
specialised cell.
A good example of this in our body is the way in which our red blood cells are formed.
”
Red blood cells are also known as erythrocytes. We already know a little bit about these red
blood cells from our past GCSE work, but may look into it again sometime!
When we cut ourselves, we lose blood. If I took a knife….stabbed you a few times…and then
left you on the side of the road for a bit….you’d lose a lot of blood. OK OK….so that doesn’t
happen THAT often! But if you go and DONATE blood….you’re losing blood. Is this ok?
How does our body compensate?
Our body makes the erythrocytes by certain stem cells differentiating into them. These cells are
called haematopoietic stem cells.
Haematopoietic
stem cells
→
Proerythroblast
→
Erythroblast
→
Erythrocyte
Haematopoietic cells are found in the bone marrow of adults. The bone marrow is found within
certain long bones such as the femur, hip and sternum. If someone has a bone marrow
transplant, bone marrow is often taken from the hip bone with a large needle.
In plants (yes yes…not as interesting as humans…deal with it!) there is a similar setup. The
phloem and xylem (hopefully you remember this from GCSE…) are also created from stem
cells:
a. Xylem vessels: these are used to transport water and dissolved minerals from the roots
to the leaves
b. Phloem vessels: these are used to transport sucrose from the leaves, where it is made in
photosynthesis, around the plant to where it is needed.
Both the Xylem and the Phloem are formed from the vascular cambium. This is a type of stem
called meristem which is an incompletely differentiated cell type.
So how is specialisation useful?
Our body is special.
Yes. It is. We are special. I’m sure your mummy has told you this lots of times
before
Specialisation allows a cell to carry out a specific function in an efficient manner
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Erythrocytes
-
no nucleus
o more space to carry haemoglobin
biconcave shape
o larger surface area to absorb O2
malleable
o can squeeze into small capillaries
Neutrophil
highly motile
o move to site of infection
cell surface receptors
o detect chemical gradients to direct them to site
secrete enzymes
o to break up ingested (phagocytosed) material
Epithelial cells
Various shapes and sizes depending on their function.
Functions of epithelial cells include secretion, absorption, protection, transcellular transport,
sensation detection, and selective permeability.
• Keratinized cells contain keratin (a cytoskeletal protein). While keratinized epithelium
occurs mainly in the skin, it is also found in the mouth and nose, providing a tough,
impermeable barrier.
• Ciliated cells have apical plasma membrane extensions composed of microtubules
capable of beating rhythmically to move mucus or other substances through a duct. Cilia
are common in the respiratory system and the lining of the oviduct
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Sperm Cell
-
3 sections
tail to allow cell to move
acrosome with enzymes
it is a haploid – it has half the number of chromosomes
0.01mm
contains enzymes
tail
Palisade cell
nucleus containing
chromosomes
-
found in upper part of leaf
absorbs sunlight
large number of chloroplasts
cells tightly packed together
μm = micrometer
1 millimetre = 1000μm
approximately 20 μm
cell wall
chromosomes inside nucleus
nucleus
mitochondria
chloroplast
large
central
vacuole
tonoplast
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Root hair cell
-
same as normal plant cells, but has no chloroplasts, as it is
underground.
root hair increases surface area for absorption of water and
minerals.
Water moving into cell
Soil particles
Guard cells
at bottom of leaf to stop excess loss of water
react to changes in sunlight, gases and humidity (for detail see below)
When conditions are conducive to stomatal opening (e.g., high light intensity and high humidity), a proton
pump drives protons (H+) from the guard cells. This means that the cells' electrical potential becomes
increasingly negative. The negative potential opens potassium voltage - gated channels and so an uptake of
potassium ions (K+) occurs.
To maintain this internal negative voltage so that entry of potassium ions does not stop, negative ions
balance the influx of potassium. in some cases chloride ions enter, while in other plants the organic ion
malate is produced in guard cells. This in turn increases the osmotic pressure inside the cell, drawing in
water through osmosis. This increases the cell's volume and turgor pressure. Then, because of rings of
cellulose microfibrils that prevent the width of the guard cells from swelling, and thus only allow the extra
turgor pressure to elongate the guard cells, whose ends are held firmly in place by surrounding epidermal
cells, the two guard cells lengthen by bowing apart from one another, creating an open pore through which
gas can move.
When the roots begin to sense a water shortage in the soil, abscisic acid (ABA) is released[2]. ABA binds to
receptor proteins in the guard cells' plasma membrane and cytosol, which first raises the pH of the cytosol of
the cells and cause the concentration of free Ca2+ to increase in the cytosol due to influx from outside the
cell and release of Ca2+ from internal stores such as the endoplasmic reticulum and vacuoles. This causes
the chloride (Cl-) and inorganic ions to exit the cells. Secondly, this stops the uptake of any further K+ into
the cells and subsequentally the loss of K+. The loss of these solutes causes a reduction in osmotic pressure,
thus making the cell flaccid and so closing the stomatal pores.
Organisms:
Cells → Tissues → Organs → Organ system → Organism
Many specialised cells make up a tissue. Many specialised tissues make up an organ. Many
specialised organs make up an organ system and many organ systems make up an organism.