B2.3 Cell specialization Flashcards
B2.3.1—Production of unspecialized cells following fertilization and their development into specialized
cells by differentiation
Students should understand the impact of gradients on gene expression within an early-stage embryo.
Fertilization is fusion of a male and female gamete
to produce a zygote (single cell). In multicellular
organisms this cell divides repeatedly to form an
embryo with many cells. Mitosis ensures that any cell in
the embryo has all the genes in the genome, so it could
develop in any way. In an early-stage embryo the cells
functions. This is called differentiation.
In early-stage embryos, gradients of signalling
chemicals known as morphogens become established.
Concentrations of morphogens indicate to a cell its
position in the embryo and therefore which pathway
are all unspecialized.
As an embryo grows, the cells develop along different
pathways and become specialized for specific
8-cell human embryo-all
the cells are unspecialized
at this stage
of differentiation it should follow. For example, there
are gradients of morphogens between the anterior
(front) and posterior (back) of early-stage embryos.
Morphogens are regulators of gene expression-they
determine which genes are transcribed to produce
mRNA and therefore which proteins are made in
each cell.
B2.3.2—Properties of stem cells
Limit to the capacity of cells to divide endlessly and differentiate along different pathways
Stem cells have two key properties:
2. Stem cells
* Self-replicating—a stem cell can divide endlessly to
produce more stem cells
* Undifferentiated—a stem cell has not developed
specialized features that would commit it to one
particular role. It retains the capacity to differentiate
along different pathways.
B2.3.3—Location and function of stem cell niches in adult humans
Limit to two example locations and the understanding that the stem cell niche can maintain the cells or
promote their proliferation and differentiation. Bone marrow and hair follicles are suitable examples.
All cells in an embryo are stem cells. Most cells
produced by division of stem cells become
differentiated. Only a small percentage of cells in an
3. Stem cell niches
adult’s body are stem cells, but they are present in
many human tissues, giving them considerable powers
of regeneration and repair.
over long periods of time or for them to proliferate
s p o n g y bone
The precise location of stem cells within a tissue
is called the stem cell niche. It must provide a
microenvironment with conditions n e e d e d either for
the stem cells to remain inactive and undifferentiated
rapidly and differentiate.
Examples of stem cell
niches:
1. Bone marrow-the
soft, spongy tissue
in the middle of the
femur, sternum, pelvis
and other large bones.
Bone marrow contains
haematopoietic stem
c o m p a c t b o n e
cells that produce huge
numbers of red and
white blood cells and
platelets each day. This
is made possible by
bone
m a r r o w
a generous supply of
blood carrying oxygen,
amino acids and other
nutrients. Bones receive
up to 15% of cardiac
output, much of which
goes to the marrow.
blood
supply to b o n e
m a r r o w
2. Hair follicles-pores
in the skin that hold
hairs. Stem cells at
the base of each hair
epidermis
divide repeatedly to
generate the many
cells n e e d e d for hair
growth. Human hairs
hair
typically grow at a
rate of 0.35 mm per
day. Blood capillaries
supply the necessary
nutrients.
s t e m
cells
B2.3.4—Differences between totipotent, pluripotent and multipotent stem cells
Students should appreciate that cells in early-stage animal embryos are totipotent but soon become
pluripotent, whereas stem cells in adult tissue such as bone marrow are multipotent.
Stem cells can be totipotent, pluripotent or multipotent.
* Stem cells in early-stage animal embryos are totipotent-they can differentiate into any cell type.
These cells are therefore very useful for use in stem cell therapies.
* During the development of embryos, stem cells soon become pluripotent-able to differentiate into many, but not all cell types.
* Stem cells in adult tissue such as bone marrow are multipotent-able to differentiate into several cell types.
B2.3.5—Cell size as an aspect of specialization
Consider the range of cell size in humans including male and female gametes, red and white blood cells,
neurons and striated muscle fibres.
For any cell type, there is an ideal size at which the cell’s
function can be performed most efficiently. These are
examples of the wide range of cell size in humans:
male g a m e t e s only 3 um wide (but 50 um long),
which makes it easier for a sperm to swim to the egg
red blood cells- small in size (7 um in diameter and
1-2 um thick) allowing passage through narrow
capillaries and giving a large surface area-to-volume
ratio so entry and exit of oxygen is rapid
white blood cels— B lymphocytes are 10-12 um in
diameter when inactive, but when activated they grow into
30 um plasma cells that can produce antibodies in bulk
female gametes—110 um in diameter, with a very large
volume of cytoplasm that contains enough food to sustain
the embryo during the early stages of development
neurons-motor neurons have a cell body with diameter
20 um, but the axon extending out from this can be a
metre or more long, allowing signals to be carried this far
striated muscle fibres-very large cells with diameter
20 to 100 um and lengths that can exceed 100 mm,
allowing large and powerful muscle contractions.
B2.3.6—Surface area-to-volume ratios and constraints on cell size
Students should understand the mathematical ratio between volume and surface area and that exchange
of materials across a cell surface depends on its area whereas the need for exchange depends on cell
volume.
NOS: Students should recognize that models are simplified versions of complex systems. In this case,
surface-area-to-volume relationship can be modelled using cubes of different side lengths. Although the
cubes have a simpler shape than real organisms, scale factors operate in the same way.
As the size of any object is increased, the ratio between
the surface area and the volume decreases.
Consider the surface area-to-volume ratio (SA/V) of
cubes of varying size:
Length of sides 1mm 10 mm 100 mm
Surface area (mm?) 6 600 60,000
Volume (mm?) 1,000 1,000,000
Surfacearea /mm? =mm-
volume 6 0.6 0.06
mm’
unfolded
* The surface area-to-volume ratio of cells also decreases as
a cell grows larger.
* The rate at which materials enter or leave a cell depends
on the surface area of the cell.
* However, the rate at which materials are used or produced
depends on the volume.
* A cell that becomes too large may not be able to take in
essential materials or excrete waste substances quickly
enough. Surface area-to-volume ratios therefore place
limits on how large cells can grow before they must divide.
B2.3.7—Adaptations to increase surface area-to-volume ratios of cells
Include flattening of cells, microvilli and invagination. Use erythrocytes and proximal convoluted tubule
cells in the nephron as examples.
For any volume, a sphere has the smallest surface area-to-
volume ratio. Any c h a n g e in shape to a sphere increases
the ratio. Many cell types specialized for an exchange
process (absorption or secretion/ excretion) have
shapes that provide a particularly large area of plasma
membrane, across which substances are transferred:
* flattening-to make the cell very wide and thin.
Red blood cells are flattened discs and type |
pneumocytes are an extreme example of flattening
* microvilli-finger-like projections on the exposed
surface of epithelial cells. Proximal convoluted
tubule cells in kidney nephrons (described in
Section D3.3.8) have many microvilli in their outer
membrane which reabsorb glucose and other useful
substances from the filtrate flowing past them
* invagination-infoldings of the plasma membrane
to form tubules, folds or sacs. Proximal convoluted
t u b u l e cells h a v e basal c h a n n e l s that increase t h e
surface for pumping of sodium ions out of the cell,
to generate the Nat concentration gradient that is
used for cotransport of glucose.
B2.3.8—Adaptations of type I and type II pneumocytes in alveoli
Limit to extreme thinness to reduce distances for diffusion in type I pneumocytes and the presence of
many secretory vesicles (lamellar bodies) in the cytoplasm that discharge surfactant to the alveolar lumen
in type II pneumocytes. Alveolar epithelium is an example of a tissue where more than one cell type is
present, because different adaptations are required for the overall function of the tissue
The alveoli of the lungs (see
Section B3.1.4) are the air sacs
where gas exchange happens.
The wall of the alveolus is an epithelium
that is o n e cell thick, but contains two cell
types with differences in structure which
are adaptations for different functions.
Type I > 0
Type I pneumocytes: make up 95%
of the alveolus wall; adapted to carry
out gas exchange; very little cytoplasm
so are extremely thin and permeable-gases
Type l I
5% of the area of alveolus wall but are more numerous;
have a dense cytoplasm with many vesicles (lamellar
bodies)-contain a fluid that is produced and then
secreted by exocytosis; fluid keeps the inner surface
contains a natural detergent (surfactant), which
reduces surface tension, so preventing the sides of the
alveoli from sticking together.
only have to diffuse a
very short distance to
pass through them.
Type Il pneumocytes:
- air in alveolus
Type I pneumocyte
basement membrane
- wall of adjacent
capillary (one cell thick)
of the alveolus moist and allows gases to dissolve;
-blood plasma
in capillary
B2.3.9—Adaptations of cardiac muscle cells and striated muscle fibres
Include the presence of contractile myofibrils in both muscle types and hypotheses for these differences:
branching (branched or unbranched), and length and numbers of nuclei. Also include a discussion of
whether a striated muscle fibre is a cell.
For any volume, a sphere has the smallest surface area-to-
volume ratio. Any c h a n g e in shape to a sphere increases
the ratio. Many cell types specialized for an exchange
process (absorption or secretion/ excretion) have
shapes that provide a particularly large area of plasma
membrane, across which substances are transferred:
* flattening-to make the cell very wide and thin.
Red blood cells are flattened discs and type |
pneumocytes are an extreme example of flattening
* microvilli-finger-like projections on the exposed
surface of epithelial cells. Proximal convoluted
tubule cells in kidney nephrons (described in
Section D3.3.8) have many microvilli in their outer
membrane which reabsorb glucose and other useful
substances from the filtrate flowing past them
* invagination-infoldings of the plasma membrane
to form tubules, folds or sacs. Proximal convoluted
t u b u l e cells h a v e basal c h a n n e l s that increase t h e
surface for pumping of sodium ions out of the cell,
to generate the Nat concentration gradient that is
used for cotransport of glucose.
B2.3.10—Adaptations of sperm and egg cells
Limit to gametes in humans.
Male gametes travel to female gametes and because of
this they have very different adaptations. Male gametes
much larger as they contain nearly all the food reserves
for the early development of the embryo. Because of
the large investment of resources, smaller numbers
in all animals and some plants are motile-they can swim.
The faster they swim, the more chance of reaching the
egg first and fertilizing it, so small size and an efficient
propulsion system are needed. Male gametes are
produced in larger numbers than female, to increase the
chance of one of them fertilizing an egg.
In humans (and other species) female gametes are
plasma
membrane
layer of follicle cells
(corona radiata)
of female than male gametes are produced. Female
gametes have a mechanism for allowing one sperm to
penetrate but not more.
head (3 um wide and 4 um long)
acrosome-contains enzymes
that digest the zona pellucida
around the egg
haploid nucleus-contains the
- 23 chromosomes that are
passed from father to offspring
two
centrioles
first polar
c e l l - n o t
needed
so breaks
down
haploid nucleus-contains the 23 chromosomes
that are passed from mother to offspring
cytoplasm (or yolk)—
containing droplets
of fat and other
nutrients needed
during early stages
of embryo
development
cortical granules—
harden the zona
zona pellucida-protects
pellucida to prevent
multiple fertilization
the egg cell and restricts
entry of sperm
diameter of egg cell = 110 um
tail-provides the propulsion that allows
the sperm to swim up the vagina,
uterus and oviduct until it reaches the egg
mid-piece (7 um long)
helical mitochondria- produce
c e n t r i o l e ATP by aerobic respiration to
plasma
membrane
supply energy for swimming and
other processes in the sperm
microtubules in a 9 + 2 array—
protein fibres-
make the sperm tail beat from
strengthen the tail
side to side and generate the
forces that propel the sperm
