A2.2 Cell structure Flashcards
A2.2.1—Cells as the basic structural unit of all living organisms
The ancient Greeks debated whether living organisms were composed of an endlessly divisible fluid or of indivisible
subunits. The invention of the microscope settled this debate- organisms are made of cells. A cell is the smallest unit of self-sustaining life. Unicellular organisms only have one cell. Multicellular organisms have more than one cell and usually have many
A2.2.2—Microscopy skills
Application of skills: Students should have experience of making temporary mounts of cells and tissues,
staining, measuring sizes using an eyepiece graticule, focusing with coarse and fine adjustments,
calculating actual size and magnification, producing a scale bar and taking photographs.
NOS: Students should appreciate that measurement using instruments is a form of quantitative
observation.
Multiple skills are needed to study cells using a
m i c r o s c o p e :
Skill 1: Making temporary mounts
Put the cells or tissue onto a microscope slide in a drop
of water. Lower a cover slip onto the sample carefully
to avoid creating air bubbles. Ensure there is only a thin
sample on the slide by squeezing out any excess fluid.
cover slip
(thin glass) m i c r o s c o p e
actual diameter = 12 × 1.5 um = 18 um
Skill 4: Focusing with coarse and fine adjustments
Focusing knobs change the distance between the
specimen on the microscope slide and the objective lens,
cells in a drop of
w a t e r o r stain
which allows the specimen to be brought into focus.
Example
Look at the photograph. Root cells are visible in the
microscope field of view. The cell on the left has a diameter
of 12 graticule units.
Calibration:
1 graticule unit = 1.5 um
* Start with the specimen and lens as far apart as possible.
* While looking down the microscope, use the coarse
focusing knob to move the specimen and objective lens
Colourless or white structures in cells are very hard to see closer together until the specimen comes into focus.
unless they are stained. A stain is a pigment that binds to * The fine focusing knob can then be used to get the sharpest
specific chemicals. For example, methylene blue binds possible focus, or to focus on a particular level in the
to DNA, so is useful for revealing nuclei in cells. Stains are specimen.
usually added to cells or tissues on the microscope slide - eyepiece
before the cover slip is added.
nose piece
graticule
objective lens
slide (glass)
Skill 2: Staining Skill 3: Measuring sizes using an eyepiece
A graticule is a graduated scale that is placed inside
the eyepiece of a microscope. It is used like a ruler
to measure the lengths of structures seen with the
microscope. The graticule must be calibrated for each
objective lens, so the eyepiece units can be converted
into micrometres.
stage —
c o a r s e
focusing
knob
condenser
lens and
diaphragm- fine
focusing knob
3 0 4 0 50
lamp
Skill 5: Taking photographs
Microscopes used for research usually have an inbuilt
camera that can take photomicrographs of images.
High quality photos can now be taken with any
microscope, simply by holding the camera lens of a
smartphone close to the eyepiece of the microscope. It
may be necessary to adjust the brightness of lighting and
the focus to produce the best possible photo.
Skill 6: Calculating actual size and magnification Skill 7: Adding a scale bar
1. Choose an obvious length, for example the maximum
diameter of a cell. Measure it in the image.
2. Measure the same length on the actual specimen.
3. If the units used for the two measurements are different,
convert them to the s a m e units. (1 mm = 1,000 um) |
4. Divide the length on the image by the length on the
A2.2 Cells
Only one equation needs to be memorized for calculations: magnification = size of image
a c t u a l size Calculating the magnification of an image (which
could be a drawing, diagram or photomicrograph): actual specimen. The result is the magnification.
Example
The thickness of the leaf in the electron micrograph below
is 32 mm = 32,000 um. The actual thickness of the leaf in is
80 pm.
Magnification of micrograph = 32,000 = 400
Calculating the actual size of a specimen:
1. Rearrange the equation to this:
actual size = .
size of image
magnification
2. Divide the size in the image by the magnification.
Example
The diameter of a nucleus in a drawing is 12 mm and the
magnification of the drawing is x 1,000.
actual diameter of nucleus =12 mm
1,000
= 0.012 mm = 12 um
Scale bars allow sizes of structures in images to be
d e d u c e d .
1. Rearrange the equation to this:
size of image = actual size x magnification
length of scale bar = length it indicates x magnification
2. Decide what length of scale bar is appropriate,
e.g. 10 um.
3. Multiply this length by the magnification to obtain the
length of scale bar that should be added to the image.
(Divide by 1,000 to convert um to mm.)
Example
A scale bar on the drawing from the previous example has
a length of 10 um:
length of scale bar = 10 um x 1,000
= 10,000 um = 10 mm
8 0 p m
Scanning electron micrograph of a leaf, prepared using
the freeze-fracture technique
A2.2.3—Developments in microscopy
Include the advantages of electron microscopy, freeze fracture, cryogenic electron microscopy, and the
use of fluorescent stains and immunofluorescence in light microscopy
Microscopes were first invented in the 17th century.
This led to the discovery of cells. Improved light
microscopes in the 19th century allowed the discovery
of bacteria, chromosomes, mitosis, meiosis, gametes
within cells. A sample is plunged into liquefied propane
1. Fluorescent stains and immunofluorescence
Fluorescence is absorbance of light and re-emission
at a longer wavelength. Fluorescent stains have been
used in microscopy for over 100 years and fluorescence
microscopes have been developed with intense single-
wavelength light sources such as high-power LEDs or
lasers. Light re-emitted by a stained sample generates
Immunofluorescence is a development of fluorescent
staining. Antibodies that bind to a specific chemical in
the cell are produced. A fluorescent marker is linked to
r e - e m i s s i o n
and fertilization. Many developments in microscopy have
since been made.
particularly bright images.
the antibodies. Images produced of cells treated with
these antibodies show the cell structure overlain with the
bright colour of the fluorescent marker where the specific
chemical occurs in the cell. Multicoloured fluorescent
images can be produced using multiple types of
antibodies with fluorescent markers of different colours.
fluorescent
marker a n t i b o d y
protein
(antigen)
2. Electron microscopes
Magnification can be increased with a microscope
until a point beyond which the image can no longer be
focused sharply. This is because the resolution of the
microscope has been exceeded. Resolution is the ability
of a microscope to show two close objects separately
in the image. Electron microscopes have better
resolution than light microscopes, so they can give
much higher magnification and smaller structures can be
seen. Electron microscopes have allowed scientists to
investigate the detailed structure (ultrastructure) of cells.
Microscope
Light
Electron
Resolution
0.25 m
0.25 nm
Magnification
× 500
× 5 0 0 , 0 0 0
at - 190°C so it rapidly freezes. A steel blade is then
used to fracture the frozen sample. The fracture goes
through the weakest points of the cells. A vapour of
platinum or carbon is fired onto the fracture surface at
an angle of about 35° to form a coating. This creates a
replica of the fracture surface. The replica is removed
from the frozen sample and can be examined using an
electron microscope. It is 2 nanometres thick on average
3. Freeze-fracture electron microscopy
This technique is used to produce images of surfaces
coating is applied. This gives the impression of a 3D
image with shadowing.
The weakest point in cells is usually the middle of
membranes, between the two layers of phospholipid.
The freeze-fracture process gives a unique image of this
vacuole
but thickness varies because of the angle at which the
part of cells. The image below shows part of a yeast cell.
p l a s m a
m e m b r a n e
cell
wall
illumination
2 0 0 nm
4. Cryogenic electron microscopy
Cryo-EM is used for researching the structure of proteins.
A thin layer of a protein solution is applied to a grid and
then flash-frozen with liquid ethane at - 183°C to create
smooth vitreous ice and prevent the formation of water
crystals. The grid is placed in an electron microscope
and detectors record patterns of electrons transmitted by
individual protein molecules. Computer algorithms are
used to produce a 3D image of the protein molecules.
Cryo-EM can now give resolutions of 0.12 nm so
individual atoms in a protein can be located.
Cryo-EM analyses proteins at the instant in time when
the water around them froze. This allows scientists to
research proteins that change from one form to another
as they carry out their function.
A2.2.4—Structures common to cells in all living organisms
Typical cells have DNA as genetic material and a cytoplasm composed mainly of water, which is enclosed
by a plasma membrane composed of lipids. Students should understand the reasons for these structures
Cell structure is very varied, but there are some key features that all typical cells have:
* Cytoplasm composed mainly of water-contains enzymes which catalyse many chemical reactions.
* DNA as genetic material-needed for producing mRNA by transcription, so proteins can be synthesized.
* Plasma membrane composed of lipids- controls the movement of substances in and out of the cell and allows different chemical conditions to be maintained inside the cell from those outside, such as pH.
A2.2.5—Prokaryote cell structure
Include these cell components: cell wall, plasma membrane, cytoplasm, naked DNA in a loop and 70S
ribosomes. The type of prokaryotic cell structure required is that of Gram-positive eubacteria such as
Bacillus and Staphylococcus. Students should appreciate that prokaryote cell structure varies. However,
students are not required to know details of the variations such as the lack of cell walls in phytoplasmas
and mycoplasmas
In prokaryote cells, there is no nucleus and instead
the DNA is in the cytoplasm. There is usually a single
chromosome D N A molecule) which is circular and naked
(not associated with proteins). Bacteria are prokaryotes.
* The cell is bounded by both a cell wall and a plasma
membrane. The principal component of the cell wall is
peptidoglycan.
* The cytoplasm has a high protein content, mostly
enzymes. This makes it appear dark in electron
micrographs, whereas the region of cytoplasm that holds
the DNA is usually paler. There are many ribosomes in the
cytoplasm. They are “7OS” which indicates a relatively
small size. S = svedberg (used to measure how fast
something sediments when centrifuged)
ell wall plasma
m e m b r a n e
naked DNA
in a loop
* 70S ribosomes
nucleoid
(region of cytoplasm
containing the DNA)
cytoplasm with
enzymes
A2.2.6—Eukaryote cell structure
Students should be familiar with features common to eukaryote cells: a plasma membrane enclosing a
compartmentalized cytoplasm with 80S ribosomes; a nucleus with chromosomes made of DNA bound to
histones, contained in a double membrane with pores; membrane-bound cytoplasmic organelles
including mitochondria, endoplasmic reticulum, Golgi apparatus and a variety of vesicles or vacuoles
including lysosomes; and a cytoskeleton of microtubules and microfilaments.
In eukaryotic cells there is usually a nucleus and many
other organelles. An organelle is a discrete structure
that is adapted to perform one or more vital functions
within a cell. Organelles in eukaryotes can be classified
according to how many membranes surround them:
0
Number of
m e m b r a n e s
80S ribosomes, microtubules, microfilaments
Rough ER, smooth ER, Golgi apparatus,
lysosomes, vesicles, vacuoles
Nucleus, mitochondria, chloroplasts
The diagram shows the types of organelles that occur
in most animal cells. There would be one nucleus only,
but many of each of the other organelles, which would
b e densely p a c k e d in the cytoplasm.
Membrane-bound organelles divide the cytoplasm of
eukaryotic cells into many small compartments.
Prokaryotic cells are not compartmentalized in this way
and the whole cell is a single compartment.
uclear. m e m b r a n e c h r o m o s o m e s
consisting of
DNA and histones
nuclear pore-
lysosome rough
endoplasmic
r e t i c u l u m
cell wall plasma
m e m b r a n e
naked DNA
in a loop
* 70S ribosomes
nucleoid
(region of cytoplasm
containing the DNA)
cytoplasm with
enzymes
A2.2.7—Processes of life in unicellular organisms
Include these functions: homeostasis, metabolism, nutrition, movement, excretion, growth, response to
stimuli and reproduction.
uclear. m e m b r a n e c h r o m o s o m e s
consisting of
DNA and histones
nuclear pore-
lysosome rough
endoplasmic
r e t i c u l u m
cell wall plasma
m e m b r a n e
naked DNA
in a loop
* 70S ribosomes
nucleoid
(region of cytoplasm
containing the DNA)
cytoplasm with
enzymes
7. Life processes in unicellular organisms
Unicellular organisms consist of only one cell, which
carries out all the functions of life. Amoeba is an
example. It is 0.25-0.75 mm long.
Metabolism-Produces enzymes to catalyse chemical
reactions in the cytoplasm.
Nutrition-Feeds on smaller organisms which are
engulfed by endocytosis and digested in vesicles.
Growth-Increases in size and dry mass by assimilating
digested foods.
Excretion-Metabolic waste products diffuse out of the
cell, for example CO, from respiration.
Homeostasis-Regulates internal conditions, for example
by expelling excess water using contractile vacuoles.
Movement-Draws cytoplasm from one side of the cell
and uses it to extend the cell another side-known as
a m e b o i d movement.
Response-Reacts to stimuli, for example by moving
towards higher concentrations of peptides released by
bacteria.
Reproduction-Reproduces asexually using mitosis or
sexually using meiosis and gametes.
100 um
80S
ribosomes
smooth-
endoplasmic
reticulum Golgi
mitochondrion
apparatus
cytoplasm
vesicles plasma
membrane
A2.2.8—Differences in eukaryotic cell structure between animals, fungi and plants
Include presence and composition of cell walls, differences in size and function of vacuoles, presence of
chloroplasts and other plastids, and presence of centrioles, cilia and flagella.
Cell walls are tough layers, outside the plasma Cilia and flagella are whip-like structures with a 9+2
membrane. The main component in plant cell walls is cellulose and in fungal cell walls it is chitin. Animal cells arrangement of microtubules inside and plasma
membrane on the outside. They protrude from the cell
and generate movement by a beating action. Some
do not have walls, which allows them to take in food by endocytosis but makes them vulnerable to bursting if too much water enters by osmosis. Vacuoles are single-membrane sacs of fluid in the types of animal cell have many cilia, which are small and
move fluids adjacent to the cell. Male gametes (sperm)
cytoplasm. There is often a large permanent vacuole in cells of fungi and plants, used for storage of substances and pressurizing the cell. Two types of small temporary
vacuole occur in some animal cells but not plant or in animals have a single flagellum (tail), which is much
longer than cilia and causes the sperm to move. Plant
and fungus cells have no cilia. Some plants, including
ferns and mosses, have motile male gametes with a
fungus cells: contractile vacuoles that expel excess water
by exocytosis and food vacuoles that digest food or
pathogens taken in by endocytosis.
Plastids are a family of double-membraned organelles.
Plant cells have varied types such as chloroplasts (for
photosynthesis) and amyloplasts (to store starch). Animal
flagellum, but conifers, flowering plants and almost all
fungi do not.
Animals Fungi Plants
Cell wal
Vacuoles Small Large Large
Plastids
Centrioles
and fungus cells have no plastids.
Centrioles are organelles composed of a 9+2
Cilia Some
arrangement of microtubules. They are used in animal
Flagella Some Some Some
cells to organize assembly of a spindle of microtubules
during mitosis and meiosis
A2.2.9—Atypical cell structure in eukaryotes
Use numbers of nuclei to illustrate one type of atypical cell structure in aseptate fungal hyphae, skeletal
muscle, red blood cells and phloem sieve tube elements.
Eukaryote cells vary in structure in many ways. Typical cells have one nucleus, but there are atypical cells with a
different number.
Some cells are anucleate-they do not have a nucleus,
so cannot transcribe DNA to make mRNA and cannot
synthesize proteins.
Some cells are multinucleate-they have many nuclei,
Red blood cells
Skeletal muscle is made up of muscle fibres. Each fibre
in mammals do not
have a nucleus,
so there is more
space for
is enclosed inside a plasma membrane like a cell, but
allowing them to produce more mRNA and therefore
more protein.
of muscle fibre
haemoglobin.
Without a source
of proteins for
repair or other
functions they have a
limited lifespan-four months at most.
Phloem sieve
tube elements
are the subunits
of the tubes
that transport
sugar-containing
sap in plants.
They initially
plasma
membrane
cytoplasm containing
haemoglobin but no
nucleus, mitochondria
or ribosomes
companion cell with
a nucleus and many
organelles
sieve tube element
with flowing sap but
n o n u c l e u s
have a nucleus
but it breaks down, so sap can flow more easily. However,
they are supplied with proteins by adjacent companion
cells, which have a nucleus and rough ER.
is 300 or more mm long (so much larger) and contains
hundreds of nuclei.
plasma membrane
multiple nuclei within
one muscle fibre
striated appearance due to regular arrays of
protein filaments used in muscle contraction
Aseptate fungi consist of thread-like structures called
hyphae. These hyphae are not divided up into subunits
containing a single nucleus. Instead, there are long
undivided sections of hypha which contain many nuclei.
hypha with
branching but no
internal divisions
multiple
nuclei
cell wall large permanent vacuole
A2.2.10—Cell types and cell structures viewed in light and electron micrographs
Application of skills: Students should be able to identify cells in light and electron micrographs as
prokaryote, plant or animal. In electron micrographs, students should be able to identify these structures:
nucleoid region, prokaryotic cell wall, nucleus, mitochondrion, chloroplast, sap vacuole, Golgi apparatus,
rough and smooth endoplasmic reticulum, chromosomes, ribosomes, cell wall, plasma membrane and
microvilli.
The appearance of cell structures in light and electron micrographs is described in Section A2.2.11. Cell types can be identified by the structures that are present. Absence of a structure in one thin section of part of a cell seen in an
electron micrograph does not prove that it is absent in the cell.
- Is a cell prokaryotic or eukaryotic?
eukaryote because prokaryotes are unicellular or simple chains of cells.
If the cell is part of a multicellular tissue, it must be from a - Is it a plant or animal cell?
There are differences in structure between plant and animal cells but not all eukaryotes are plants or animals.
Prokaryotic cells
* Nucleoid region visible in the cytoplasm
* Cytoplasm without membrane-bound organelles
Eukaryotic cells
* Nucleus (or chromosomes
if the cell is dividing)
* Mitochondria or other membrane-bound organelles
Plant cells
* Cell wall always present
* Large permanent| vacuole often present
* Chloroplasts or other plastids present
Animal cells
* Cell wall never present
* Only small and temporary vacuoles
* Cilia present in some animal cells
A2.2.11—Drawing and annotation based on electron micrographs
Application of skills: Students should be able to draw and annotate diagrams of organelles (nucleus,
mitochondria, chloroplasts, sap vacuole, Golgi apparatus, rough and smooth endoplasmic reticulum and chromosomes) as well as other cell structures (cell wall, plasma membrane, secretory vesicles and
microvilli) shown in electron micrographs. Students are required to include the functions in their
annotations.
Drawings should be done with a sharp pencil. Structures should be labelled, but it is usually better to annotate
(add notes to) drawings with the functions of each structure.
Boundaries of the cell:
Show the plasma membrane as a single unbroken line.
If there is a cell wall outside the plasma membrane,
show it as a double line, as it is far thicker than the cell
membrane. Hatching can be used to indicate that the
wall is solid. In a healthy plant cell or bacterium, the
plasma membrane is pushed up against the cell wall by
turgor pressure.
cell wal—
protects
the cell and
allows high
pressures
p l a s m a
m e m b r a n e
separates
the cell from
the disorderly
exterior
to develop
Animal cells do not have a cell wall so their plasma
membrane can change shape, with invaginations and
protrusions. Some animal cells have microvilli-many
finger-like protrusions, which increase the surface area
for absorption.
Gene storage in cells:
The nucleus of a eukaryotic cell has a double
membrane with pores in it. Show it with an unbroken
pencil line that doubles back at the pores. The interior
of the nucleus has a distinctive grainy appearance.
Much of it is lighter euchromatin, with patches of
darker heterochromatin often around the edge.
Heterochromatin is parts of chromosomes that have
remained condensed after mitosis. During mitosis
the chromosomes all become densely stained
heterochromatin. Then the nuclear membrane breaks
down, releasing chromosomes into the cytoplasm.
euchromatin-DNA with genes
that need to be accessible for
transcription
nuclear pore-for
r i b o s o m e s a n d mRNA to
exit the nucleus
double nuclear
membrane—to separate
chromosomes from
cytoplasm
heterochromatin
—DNA not in use
cell wall
plasma
membrane
microvilli—
increase the
surface area for
absorption
nucleoid— contains
the bacterial
c h r o m o s o m e
dense staining in the centre of the cell.
The nucleoid of a prokaryotic cell is a region of less
Single-membraned organelles:
Double-membraned organelles:
The large permanent sap vacuole of plant and
Inside the double membrane of a chloroplast is an
fungus cells appears white or very pale in electron
micrographs. The single membranes of vacuoles,
vesicles, endoplasmic reticulum and Golgi apparatus
extensive network of thylakoids, which are single-
membraned spaces. Most of these are disc-shaped
and arranged in stacks (grana). Grains of starch and
should be shown with a single line.
droplets of oil are sometimes visible.
v a c u o l e
vesicles for
thylakoids for light
absorption and
ATP synthesis
stroma with
enzymes for
synthesizing sugars
for storing sap
transport inside cells
The Golgi apparatus is a stack of flattened membrane-
bound sacs, which are usually curved with vesicles at
the ends.
vesicles-for transport of proteins
to and from the Golgi apparatus
flattened sacs
(cisternae)-for 1
processing proteins
The inner membrane of the mitochondrion is infolded,
which increases its surface area. Mitochondria appear
darker than surrounding cytoplasm due to proteins in
the fluid inside them, which stain densely in electron
micrographs.
Endoplasmic reticulum is a network of membrane-
bound spaces. In smooth endoplasmic reticulum they
are mostly tubular. In rough endoplasmic reticulum
they are lamellar (sheet-like). Ribosomes appear as
densely stained granules with a diameter two to three
times the thickness of a cell membrane. Free ribosomes
float in the cytoplasm. Rough endoplasmic reticulum
(RER) has ribosomes attached to its outer surface.
SER-
t u b u l e s
in which
lipids are
synthesized
ribosomes for protein synthesis
infoldings of the inner
membrane (cristae) which
increase the area for
ATP production
A2.2.12—Origin of eukaryotic cells by endosymbiosis
Evidence suggests that all eukaryotes evolved from a common unicellular ancestor that had a nucleus and
reproduced sexually. Mitochondria then evolved by endosymbiosis. In some eukaryotes, chloroplasts
subsequently also had an endosymbiotic origin. Evidence should include the presence in mitochondria
and chloroplasts of 70S ribosomes, naked circular DNA and the ability to replicate.
NOS: Students should recognize that the strength of a theory comes from the observations the theory
explains and the predictions it supports. A wide range of observations are accounted for by the theory of
endosymbiosis.
Evidence suggests that all eukaryotes evolved from a
unicellular ancestor that had a nucleus and r e p r o d u c e d
both sexually by meiosis and fertilization, and
asexually by mitosis. This common ancestor respired
anaerobically but then it ingested an aerobically
respiring bacterium. The bacterium remained alive
inside a vacuole in the cytoplasm, giving the common
ancestor a supply of ATP produced efficiently by
aerobic respiration. The bacterium was provided with
food, allowing it to grow and divide and therefore be
passed on to daughter cells when the host cell divided
This type of relationship is endosymbiosis because the
organisms live together (symbiosis) with one inside the
other (endo). It is a mutualistic relationship because
they both benefit. This relationship continued when
the host cell divided as long as both daughter cells
generations the relationship became so close that
neither the host cell nor the bacteria inside it could
survive without the other and the bacteria evolved into
the mitochondria now occurring in all typical eukaryotic
cells. Some eukaryotes also ingested photosynthetic
contained at least one aerobic bacterium.
The bacteria continued to grow, divide, pass on genes
and evolve inside the host cells. Gradually over many
bacteria, which developed into the chloroplasts of
plants and eukaryotic algae.
nucleus 1. Anaerobic eukaryote
ancestor engulfs an
a n a e r o b i c b a c t e r i u m
mitochondrion
2. Anaerobic eukaryote
ancestor evolves into
aerobic eukaryote with
m i t o c h o n d r i a
3. Aerobic eukaryote ancestor
engulfs a photosynthetic bacterium
chloroplast
4. Heterotrophic aerobic ancestor
evolves into a photosynthetic
eukaryote
This account of the evolution of eukaryotic cells is
known as the endosymbiotic theory. The structure of
mitochondria and chloroplasts provides evidence for
endosymbiosis. They:
* have a loop of naked DNA, as in bacteria; the DNA
contains genes, which are transcribed into RNA
* have 70S ribosomes and make some of their own
proteins-suggests they were once independent
cells; size of ribosome is the same as in bacteria
* reproduce by splitting in two, as in bacteria
* are double membraned —expected if a bacterium
with its own plasma membrane was ingested in a
by evolution.
vacuole formed by endocytosis. Originally there
would have been a bacterial cell wall between the
two membranes, but it had no function so was lost
A2.2.13—Cell differentiation as the process for developing specialized tissues in multicellular organisms
Students should be aware that the basis for differentiation is different patterns of gene expression often
triggered by changes in the environment.
Cells differentiate in a multicellular organism by
developing along different pathways, despite all having
the same genome. This is achieved by differences in
gene expression between cells. Housekeeping genes
are expressed in all living cells, as they are required
for basic functions such as respiration. Other genes
are only expressed in some cells as they cause the
development of specialized structures. For example,
genes for synthesis of haemoglobin are only expressed
during the development of red blood cells.
Chemical signals in a cell’s environment (surroundings)
determine which genes are expressed and therefore
how a cell differentiates. A tissue develops because a
group of cells receive the same signal, so they develop
the same structure and carry out the same function.
Cells in a tissue interact with each other and (except in
blood) they use membrane proteins for the cell-to-cell
adhesion that maintains the integrity of the tissue.
The advantage of cell differentiation is that form can
match function more specifically. A specialist usually
performs a function better than a generalist.
A2.2.14—Evolution of multicellularity
Students should be aware that multicellularity has evolved repeatedly. Many fungi and eukaryotic algae
and all plants and animals are multicellular. Multicellularity has the advantages of allowing larger body size
and cell specialization.
Al plants and animals are multicellular. Multicellularity
evolved independently more than once in the origins
of plants, and at least once in animals. Many fungi
and eukaryotic algae are multicellular. These are the
advantages of being multicellular:
* Lifespan can be longer, because the death of one cell
does not prevent the continued survival of an individual.
* Larger body size is possible-useful in animals that
are predators, or plants that compete for light.
* Cell differentiation- each cell carries out its function
more effectively and more complex body forms can
develop.
ancestrall heterotrophic
unicellular
eukaryotes
ancestral
photosynthetic
multicellular
animals
protista
e.g. Amoeba
unicellular eukaryotic algae
unicellular
eukaryotes
multicellular algae and
p l a n t s
