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2020 IB Cell Structure

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Cell structure and organelles

A cell is the smallest part of an organism all cells consist of a membrane, cytoplasm and a nucleus.
Types of typical cells :
Feature
Eukaryote ( true nucleus )
Plant Cell
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Prokaryote ( no nucleus )
Animal Cell
Bacteria
 Present but not made of cellulose
1- Cell wall
 Present and made
of cellulose
 Absent
2- Nucleus
 Present
 Present
 Absent ,
 Genetic material DNA spreads in
cytoplasm . Plus small circular
DNA called plasmids
3- Shape
 Regular shape
because of cell
wall
 Irregular shape due to
no cell wall
 Different shapes :
 Spherical – Rod – spiral
4- Size
 Large
 Small
 Very small
5- Organelles
 Present
 Present
 Absent , except ribosomes which
needed for protein synthesis
a.
Chloroplast
 Present
 Absent
b.
Mitochondria
 Present
 Present
c.
Vacuole
 Mainly one
 Large
 Permanent
 Many
 Small
 Temporary
d.
Carbohydrate
storage
 Starch
 Glycogen
e.
Rough
endoplasmic
reticulum ( RER)
 Present
 Present
Dr.Adel Guirguis Master degree in Genetic engineering and tissue culture
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Dr.Adel Guirguis Master degree in Genetic engineering and tissue culture
Part
Function
Cytoplasm
– Contains cell organelles
– Chemical reactions take place
e.g. respiration and making proteins
Cell Membrane
 Forms a barrier between the cell and its surroundings.
 Controls what enters and exits the cell
e.g. glucose
Cell wall
 keeps contents of cell inside ;
Stops cells
from bursting
when
they filltowith
– Allows
movement
of simple
substances
enterwater
and leave the cell e.g. O2 ,
– CO
Gives
andshape
waterto cells
2
– Allows water and dissolved substances to pass through freely
Nucleus
– Nucleus :
contains DNA in the form of chromosomes
Functions : Controls:
1- Cell division
2- Cell development
3- Cell activities
4- The type and quantity of enzymes
produced by the cytoplasm
5- Regulates the chemical changes which take place in the cell
Mitochondria
 Release energy from food substances during aerobic respiration
 Folded inner membrane to allow large surface area for enzymes needed for
aerobic respiration
 Active cells ( muscle and liver cells ) contain more mitochondria than less
active one .
Rough Endoplasmic
Reticulum ( RER )
Chloroplast
Carry ribosomes which needed for protein synthesis
( ex. Enzymes – Antibodies-some hormones like insulin )
1- Photosynthesis
2- Starch storage
Sap vacuole
1- Full of water to maintain shape and firmness of cell
2- Stores , pigments , salts and sugars
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Dr.Adel Guirguis Master degree in Genetic engineering and tissue culture
Stem cells :


Are unspecialized cells ( 2n ) chromosomes , only divide by mitosis to produce daughter identical cells
(2n ) chromosomes .
Daughter cells then become specialized to do certain functions
Examples :
1- Bone marrow that divides all the time by mitosis to produce different types of blood cells
To replace old ones.
 Red blood cells ( live for 120 days )
 White blood cells
 Platelets
2- Stem cells lie in lower part of skin epidermis divide to replace worn out skin cells
3- Cambium cells in between xylem and phloem in vascular plants .
It divide by mitosis to form new and extra xylem and phloem tissues
Specialized Cells:
It have :
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



Particular job
Distinct shape
Loses ability to divide any further
Special chemical reactions

The specialization of cells to carry out a particular function in an organism is sometimes referred to as
division of labor
Dr.Adel Guirguis Master degree in Genetic engineering and tissue culture
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Dr.Adel Guirguis Master degree in Genetic engineering and tissue culture
Some specialized cells in human :
Cell
Ciliated cells
Adaptations
1- The air passages of breathing system ( trachea and
bronchi ) cilia move mucus that trap dust and pathogens
up the nose and throat .
2- The oviducts in the female reproductive system , cilia
move the egg from the ovary to the uterus .
Red blood cells :
 It has no nucleus , so more volume to carry
 More haemoglobin to transport more oxygen
 Haemoglobin picks up oxygen to form oxyhaemoglobin
 Biconcave shape : gives it a high surface area
 Small enough to pass through capillaries
Transport oxygen from lungs to tissues
Muscle cells :
– Muscle cells make up fibres that are able to contract to
move the body
– They contain a lot of mitochondria to provide the energy
needed.
Contract and relax
– Skeletal muscles are attached to the skeleton when they
contact they move bones at joints .
– Smooth muscle :
o in the gut for peristalsis
o in arteries and veins to control diameter of lumen
– Cardiac muscles in heart
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Dr.Adel Guirguis Master degree in Genetic engineering and tissue culture
Cell
Adaptations

Nerve cells
They conduct electrical impulses which travel to and
from the brain.
– They are very long and their chemical reactions cause
impulses to travel through their fibers.
– They also have a layer of fat ( myelin sheath ) for
insulation and speed up nerve impulse.
Function : transmit nerve impulse
– Has an acrosome containing digestive enzymes to
Sperm cell
dissolve the ovum’s cell membrane
– Has a haploid nucleus (1n) chromosome to fuse with
egg nucleus (1n) to form zygote (2n)
– Contains either X or y chromosome to determine gender
of baby.
– Has a tail to swim towards the ovum.
Function :
contains 1n chromosomes = 23 chromosome
– Has many mitochondria to provide energy for
swimming.
– Has a small streamlined shape for easy swimming .
– Haploid cell (1n)
Egg cell
– Large in size to store food reserve inside
– Immobile to settle in oviduct
Function :
contains 1n chromosomes from mother = 23
chromosomes
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Dr.Adel Guirguis Master degree in Genetic engineering and tissue culture
Some specialized cells in plants
Cell
Adaptations
– Long extensions that give them a large surface area to
Root hair cells :
absorb water and ions from the soil.
– Large surface area for cell membrane for more carrier
proteins needed for active transport of ions
– Vacuole with high concentration of, salts, sugars and
solutes , to give, lower water potential than soil , this
promotes osmosis.
– Thin cell wall to allow short distance for diffusion .
Function :
– More mitochondria to provide, energy ( ATP ) for active
Absorption of water and mineral salts
Xylem vessels:
transport
– The xylem cells are dead , with no cytoplasm , no nucleus
to form :
– long hollow tubes running throughout the root, stem, and
leaves.
– It has thick cell walls to allow support
– The cell walls are thickened and bands or spirals of a water
proof material called lignin
– They help to support the plant
Function :
– These cells allow water and ions to move from roots to the
Transports water and minerals to the
plant
Palisade mesophyll cells
rest of the plant .
– Have many chloroplasts to hold a lot of chlorophyll.
To absorb as much light as possible
– palisade cells arranged lengthways ;
 allowing less cell walls to scatter light .
 to allow free movement of chloroplast up or down
according to light intensity , also to avoid harm to
chlorophyll during intense light .
Function : Absorption of light energy .
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– palisade cells close together ;
to absorb as much light as possible ;
Dr.Adel Guirguis Master degree in Genetic engineering and tissue culture
2.3 Size of cells and specimens
Microscopes and magnification :
Magnification means how many times an object enlarged .
o X10 means the object is magnified 10 times .
o 1 cm = 10 mm
o 1mm = 1000 micrometers =103 µm
o So , 1cm = 10 mm X 103 µm = 104 µm
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Dr.Adel Guirguis Master degree in Genetic engineering and tissue culture
Practice :
1- The following figure shows one pollen grain, as seen with the aid of an electron microscope.
This pollen grain has been magnified 200 times.
(s05/6/Q2c )
 Calculate the actual size of this grain.
Show your working.
......................................................................................................................................................................
......................................................................................................................................................................
......................................................................................................................................................................
......................................................................................................................................................................
actual size………………[2]
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Dr.Adel Guirguis Master degree in Genetic engineering and tissue culture
We can also use a scale line to work out magnification .
2- The following figure is a photomicrograph of some frog blood cells. ( w15/53/ Q3b)
o Measure the length of the scale bar in millimetres.
o length of scale bar ........................................ mm
o Use the scale bar to calculate the magnification of the photomicrographs.
Show your working.
o Give your answer to the nearest whole number.
o ......................................................................................................................................................................
......................................................................................................................................................................
......................................................................................................................................................................
......................................................................................................................................................................
magnification × ......................
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Dr.Adel Guirguis Master degree in Genetic engineering and tissue culture
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