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AS BIOLOGY UNITS
Unit F211 Cells, Exchange and Transport
Module 1 Cells
Cells are the basic units of all living things. Organisms function because of communication
and co-operation between specialised cells.
Cell division is a fundamental process, necessary for reproduction, growth and repair.
1.1.1 Cell structure
The cell is the basic unit of all living things.
An understanding of how to use a light microscope is developed along with an
understanding of why electron microscopes are so important in biology.
Careful observation using microscopes reveals details of cell structure and ultrastructure and
provides evidence to support hypotheses regarding the roles of cells and organelles.
Students should be able to:
(a) state the resolution and magnification that can be achieved by a light microscope, a
transmission electron microscope and a scanning electron microscope:
AS Biology p. 4-5; 8-9
MICROSCOPE TYPE
LIGHT
TRANSMISSION ELECTRON
MICROSCOPE (TEM)
SCANNING ELECTRON
MICROSCOPE (SEM)
MAXIMUM
MAGNIFICATION
1500
MAXIMUM
RESOLUTION
200nm*
500,000
0.1nm
100,000
0.1nm
*1nm = 0.000001 mm
(b) explain the difference between magnification and resolution:
AS Biology p.4
Magnification: the degree to which the image of an object is larger than the
object itself.
Resolution : smallest distance between two points which can be seen using an
optical instrument.
Limits to the resolution of optical microscopes are due to the long wavelength of
light. Resolution is inversely proportional to wavelength.
Wavelength of electrons c. 0.01 nm; wavelength of light c. 500 nm.
(c) explain the need for staining samples for use in light microscopy and electron
microscopy:
AS Biology p.5,9
Staining is required to ensure that the details of the biological material are
visible.
Light microscopes: the material is stained with coloured chemicals that binds to
chemicals in or on the specimen.
Electron microscopes: the biological material is coated with, or impregnated
with, electron-dense metals.
(d) calculate the linear magnification of an image:
AS Biology p.6-7
Actual size =
image size/magnification
Magnification = image size/actual size
(e) describe and interpret drawings and photographs of eukaryotic cells as seen under an
electron microscope and be able to recognise the following structures: nucleus, nucleolus,
nuclear envelope, rough and smooth endoplasmic reticulum (ER), Golgi apparatus,
ribosomes, mitochondria, lysosomes, chloroplasts, plasma (cell surface) membrane,
centrioles, flagella and cilia:
AS Biology p. 10-11
(f) outline the functions of the structures listed in (e):
AS Biology p. 11-13
ORGANELLE
NUCLEUS
NUCLEOLUS
NUCLEAR ENVELOPE
ROUGH ENDOPLASMIC
RETICULUM
SMOOTH ENDOPLASMIC
RETICULUM
GOLGI APPARATUS
MITOCHONDRIA
CHLOROPLASTS
LYSOSOMES
RIBOSOMES
CENTRIOLES
CELL SURFACE
MEMBRANE
CILIA
FLAGELLA
FUNCTION
Contains the cell’s genetic material, which codes for the
synthesis of proteins
The site of the synthesis of ribosomes
The membrane enclosing the nucleus
Synthesis of proteins by attached ribosomes; transport of
proteins for modification or secretion
Synthesis and transport of lipids
Modification and packaging of proteins into vesicles for
storage or secretion
The site of synthesis of ATP
The site of photosynthesis, which manufactures of
carbohydrates
Contain LYSOZYME, which break down materials inside the
cell
Large RNA/protein complexes that synthesise polypeptides
from amino acids, using the base sequence code of
messenger RNA (mRNA)
Organise the spindle during cell division
(not found in plant cells)
Selectively permeable, regulating the transport of materials
into and out of the cell
Cell recognition and signalling
Movement of liquids outside the cell
Locomotion of the cell
(g) outline the interrelationship between the organelles involved in the production and
secretion of proteins (no detail of protein synthesis is required):
AS Biology p.14
•
•
•
•
•
•
•
The coded information for making a protein is the base sequence of a specific gene
This DNA base sequence is transcribed into a base sequence in messenger RNA
(mRNA)
The mRNA leaves the nucleus via nuclear pores
The mRNA binds to ribosomes, most of which are attached to RER
The ribosome translates the mRNA base sequence, assembling the protein from its
constituent amino acids
The protein molecules are enclosed in vesicles, which bud off the RER, and are
transported to the Golgi apparatus
In the Golgi apparatus, the protein may be modified; it is then packaged into vesicles
for storage or secretion
(h) explain the importance of the cytoskeleton in providing mechanical strength to cells,
aiding transport within cells and enabling cell movement:
AS Biology p.10-11
•
•
•
•
The cytoskeleton is the internal framework of a cell.
It is made of microtubules of the protein tubulin.
Some microtubules move chromosome during mitosis (the spindle)
Some microtubules have other proteins associated, which move organelles
and other cell contents along the fibres
(i) compare and contrast, with the aid of diagrams and electron micrographs, the structure of
prokaryotic cells and eukaryotic cells:
AS Biology p.14-15
Prokaryotes
Eukaryotes
Smaller: 0.5-3 µm
Larger : c. 20µm (mean)
Cell walls not cellulose or chitin;
peptidoglycan etc.
Cell wall cellulose (plants) or chitin
(fungi).
No membrane-bound organelles.
Membrane-bound organelles.
Mesosomes etc.
No mesosomes
Circular DNA - no histones
Linear DNA with histones
Cilia / flagella made of flagellin, no
9+2 structure
Cilia / flagella made of tubulin, 9+2
structure.
Store glycogen and volutin
Store lipids, starch (plants),glycogen
(animals).
Ribosomes 70S
Ribosomes 80S
Very seldom multicellular
Many multicellular.
(j) compare and contrast, with the aid of diagrams and electron micrographs, the structure
and ultrastructure of plant cells and animal cells:
AS Biology p.10
1.1.2 Cell membranes
Membranes are a fundamental part of the cell.
The structure of the cell surface membrane allows cells to communicate with each other.
Understanding this ability to communicate is important as scientists increasingly make use of
membrane-bound receptors as sites for the action of medicinal drugs. Understanding how
different substances enter cells is also crucial to the development of mechanisms for the
administration of drugs.
Students should be able to:
(a) outline the roles of membranes within cells and at the surface of cells;
AS Biology p.16
As well as the cell surface membrane, membranes are also found in many
organelles.
Functions:
•
•
•
•
•
Separate cell contents from the outside environment
Separate cell contents from the cytoplasm
Cell recognition and signalling
Holding the components of some metabolic pathways in place
Regulating the transport of materials in and out of cells
(b) state that plasma (cell surface) membranes are partially permeable barriers;
AS Biology p.17
Cell membranes are partially permeable:
•
•
The phospholipid bilayer is a barrier to water and all water soluble
molecules, but some of these are able to ‘leak’ through, eg water, O2, CO2
Other chemicals have to pass through channels in order to penetrate the
membrane
(c) describe, with the aid of diagrams, the fluid mosaic model of membrane structure;
AS Biology p.18-19
The term fluid mosaic refers to the arrangement of the molecules within cell
membranes. It is fluid because the component molecules are able to move
within it, and mosaic because it is composed of more than one type of molecule.
The main features of the fluid mosaic model:
•
•
•
A phospholipid bilayer forms the basic structure
Protein molecules embedded in the phospholipid bilayer may be free, or
bound to other components within the structure
The phospholipids within the bilayer are able to move, and may flip
between the layers
(d) describe the roles of the components of the cell membrane; phospholipids, cholesterol,
glycolipids, proteins and glycoproteins;
AS Biology p.18-19
Phospholipids:
•
•
•
•
Comprised of glycerol, two fatty acids, and a phosphate
The phosphate heads are hydrophilic, therefore located on the outside of
the bilayer
The hydrophobic fatty acid chains are located on the inside of the bilayer
They form a barrier to water and water-soluble molecules
Cholesterol:
•
•
•
Molecules slot between fatty acid chains of phospholipids
Provides mechanical stability to membrane
Reduces permeability of membrane to water and water-soluble molecules
Proteins:
•
•
•
•
Channel proteins allow the movement of larger, hydrophilic
molecules/ions eg glucose, Na+, through the membrane
Carrier proteins actively transport substances across the membrane,
using energy from ATP, eg the uptake of magnesium ions by roots of
plants
Enzymes and coenzymes are embedded in both cell surface and
organelle membranes, many of which have an increased surface area eg
adenyl cyclase in cell surface membranes, ATP synthase in mitochondrial
membranes
Receptors for hormones eg adrenaline
Glycoproteins:
•
•
•
•
Proteins with attached carbohydrate groups
Adhesive filaments that bind cells together
Hormone receptors
Antigens in cell surface membranes
Glycolipids:
•
Lipids with attached carbohydrate groups
•
•
Hormone receptors
Antigens in cell surface membranes
(e) outline the effect of changing temperature on membrane structure and permeability;
AS Biology p.19
As the temperature increases, the molecules in the membrane move faster, and
the membranes become leaky, allowing the passage of chemicals through more
easily.
Organisms living in environments with temperature extremes have adapted the
cholesterol content of their membranes to compensate.
(f) explain the term cell signaling;
AS Biology p.20-21
Cell signalling: communication between cells, or within cells, involving
chemicals.
Examples:
•
•
Hormones
Cytokines
Both of these types of signalling chemicals are known as ligands.
(g) explain the role of membrane-bound receptors as sites where hormones and drugs can
bind;
•
•
Signal molecules fit into complementary receptors in the membranes or
cytoplasm of target cells.
This triggers a response in the target cells, usually the result of a complex
series of chemical reactions within the cell, eg the binding of insulin to its
receptor increases the permeability of the cell membrane to glucose, and
the rate of conversion of glucose to glycogen
•
Some medicinal drugs fit into receptors, blocking the effects of the
ligand eg β-blockers prevent the sino-atrial note from increasing heart
rate, so lowering blood pressure
•
Viruses bind to receptors on the cell surface membrane in order to
penetrate into the cell
•
Some toxins also bind to receptors eg the Clostridium botulinum toxin
binds to receptors in the membranes of muscle cells, preventing them
contracting
(h) explain what is meant by passive transport (diffusion and facilitated diffusion including the
role of membrane proteins), active transport, endocytosis and exocytosis;
AS Biology p.22-27
Passive transport:
Diffusion – the movement of molecules from a region of high concentration to a
region of low concentration, down a concentration gradient, eg the exchange
of respiratory gases.
Facilitated diffusion – the movement of molecules down a concentration
gradient, using channel or carrier proteins eg the uptake of glucose by liver cells.
Active transport:
The movement of molecules from a region of low concentration to a region of
high concentration, against a concentration gradient, using the energy from ATP
to drive protein pumps eg the Na+/K+ pump in the membranes of neurones.
Bulk transport:
Endocytosis:
•
•
•
The import into the cell of material from the outside, by the formation of
vesicles budding in from the cell surface membrane.
Phagocytosis – the uptake of large lumps of material eg bacteria by
phagocytes.
Pinocytosis – the uptake of droplets of liquid eg the products of digestion
by the epithelial cells of villi.
Exocytosis – the fusion of vesicles with the cell surface membrane to excrete or
secrete chemicals from the cell eg the secretion of insulin from the β cells in the
Islets of Langerhans of the pancreas.
(i) explain what is meant by osmosis, in terms of water potential. (No calculations of water
potential will be required);
AS Biology p.26-27
Osmosis:
• A special case of diffusion.
• The movement of water molecules from a region of high water potential
to a region of low water potential across a partially permeable membrane.
Water potential (symbol ψ):
•
•
•
The concentration of ‘free’ water molecules ie water molecules that are
able to diffuse.
Pure water has a water potential of 0kPa.
Any solute lowers the water potential of a solution ie makes it more
negative.
Isotonic – solutions that have the same ψ.
Hypertonic – a solution that has a lower ψ than a second solution.
Hypotonic – a solution that has a higher ψ than a second solution.
(j) recognise and explain the effects that solutions of different water potentials can have upon
plant and animal cells (HSW3).
AS Biology p.27
Animal cells:
•
In a hypotonic solution, animal cells will gain water by osmosis because
the ψ of their cytoplasm is lower than that of the surrounding medium
(ψi<ψe); eventually, the cell will burst (lysis).
•
In a hypertonic solution, animal cells will lose water by osmosis because
the ψ of their cytoplasm is higher than that of the surrounding medium
(ψi>ψe); the cell will shrink (crenation).
Plant cells:
•
In a hypotonic solution, plant cells will gain water by osmosis because
the ψ of their cytoplasm is lower than that of the surrounding medium
(ψi<ψe). This continues until the pressure of the cell wall prevents the
further uptake of water, and the cell is described as turgid.
•
In a hypertonic solution, plant cells will lose water by osmosis because the
ψ of their cytoplasm is higher than that of the surrounding medium
(ψi>ψe); the cell’s cytoplasm and vacuole shrink until the cell surface
membrane pulls away from the cell wall (plasmolysis).
1.1.3 Cell division, cell diversity and cellular organisation
During the cell cycle, genetic information is copied and passed to daughter cells.
Microscopes can be used to view the different stages of the cycle.
In multicellular organisms, stem cells are modified to produce many different types of
specialised cell. Understanding how stems cells can be modified has huge potential in
medicine.
To understand how a whole organism functions, it is essential to understand the importance
of cooperation between cells, tissues, organs and organ systems.
Students should be able to:
(a) state that mitosis occupies only a small percentage of the cell cycle and that the
remaining percentage includes the copying and checking of genetic information;
AS Biology p.28-29
Cell cycle – the sequence of events by which a parent cell divides to produce
two daughter cells. The cell cycle can be as short as 30 minutes, or take
several days.
The cell cycle is divided into four stages:
•
•
•
•
Interphase, during which the DNA replicates, and is checked for copying
errors (gene mutations)
Mitosis – the nuclear division
Cytokinesis – the division (cleavage) of the cytoplasm
A growth phase
The nuclear division only occupies 5-10% of the entire cell cycle.
(b) describe, with the aid of diagrams and photographs, the main stages of mitosis
(behaviour of the chromosomes, nuclear envelope, cell membrane and centrioles);
AS Biology p.38-31
(Hint: the names of the stages can be remembered using the acronym IPMAT)
Interphase:
•
•
•
•
•
•
90-95% of cycle
organelle synthesis/replication
DNA replication and histone synthesis
Duplication of centrioles
Increased ATP synthesis
Cell growth
Prophase:
•
•
•
•
•
Condensation of replicated chromosomes, each of which is comprised of
two sister chromatids (the products of DNA replication) held together by
the centromere
Disappearance of nucleolus
Centrioles move to opposite poles of cell
Nuclear membrane breaks into vesicles and disperses through the
cytoplasm
Spindle forms from microtubules in the cytoplasm
Metaphase:
•
•
Chromosomes line up on equator of spindle
Held in place by kinetochore microtubules radiating from the centromere
Anaphase:
•
•
•
Centromeres divide
spindle fibres pull chromatids to opposite poles of spindle
each chromatid now becomes a chromosome
Telophase:
•
•
•
•
Chromsomes decondense
Spindle disintegrates
Nuclear envelopes reforms around each pole, forming two new nuclei
Nucleolus reappears
Cytokinesis:
•
•
Division of cytoplasm
Organelles become evenly distributed
•
Differs in plants and animals:
Animals:
•
•
Cell membrane invaginates, forming a cleavage furrow
Cytoplasm pinches in two
Plants:
•
•
•
•
Spindle fibres remain across centre of cell
Vesicles from Golgi coalesce to form the cell plate
New cell wall is laid down inside the cell plate
The membrane of the cell plate forms the new cell membrane
Differences between mitosis in animal and plant cells:
ANIMALS
PLANTS
centrioles
no centrioles
no cell plate
cell plate
cleavage furrow
no cleavage furrow
most somatic cells (exceptions:
muscle, nerve, red blood cells)
usually only MERISTEMATIC CELLS
(c) explain the meaning of the term homologous pair of chromosomes;
AS Biology p.33, 261
Homologous pair of chromosomes:
•
•
the paternal and maternal copies of each chromosome in a diploid
organism
homologous chromosomes carry the same genes in the same position, or
locus (although not necessarily the same alleles)
Diploid – an organism or cell in which there are 2 copies of each chromosome,
one from the mother, and one from the father (symbol 2n).
Haploid – an organism or cell in which there is only one copy of each
chromosome (symbol n).
(d) explain the significance of mitosis for growth, repair and asexual reproduction in plants
and animals;
AS Biology p.30
•
•
•
•
Mitosis produces cells that are genetically identical to the parent cell.
Mitosis is used to produce more body (somatic) cells to allow growth,
repair of damaged tissues, and replacement of dead cells.
Unicellular eukaryotes (not prokaryotes) use mitosis to reproduce
asexually by binary fission eg Amoeba or by budding eg yeast.
Some multicellular organisms use mitosis to produce structures used for
asexual reproduction, eg budding in Hydra, yeast, Bryophyllum, runners
in strawberry and spider plants.
(e) outline, with the aid of diagrams and photographs, the process of cell division by budding
in yeast;
AS Biology p.33
Cytokinesis produces smaller cells, which bud off the parent cells.
(f) state that cells produced as a result of meiosis are not genetically identical (details of
meiosis are not required);
AS Biology p.33
Meiosis:
•
•
•
•
•
A type of cell division that produces four haploid cells from a diploid
parent cell (germ cell).
Used by organisms to produce gametes or spores (plants), therefore
linked to sexual reproduction.
During meiosis, the alleles on the homologous pairs of chromosomes are
recombined, producing chromosomes that are different in allele sequence
to those of the parent cell.
The products of meiosis are genetically different to the parent cell, and to
one another.
An important source of genetic variation for populations.
(g) define the term stem cell;
AS Biology p.32
Stem cells:
•
•
Undifferentiated cells potentially capable of becoming any of the different
types of cells found in an organism.
Found only in small numbers in adults, but found in large numbers in
embryos and umbilical cord blood.
Totipotent/omnipotent – stem cells capable of differentiating into any kind of
cell eg embryonic stem cells.
Pluripotent – stem cells capable of differentiating into a narrower range of cell
types eg cord blood stem cells.
(h) define the term differentiation, with reference to the production of erythrocytes (red blood
cells) and neutrophils derived from stem cells in bone marrow, and the production of xylem
vessels and phloem sieve tubes from cambium;
AS Biology p.34-35; 70-71
Differentiation - the changes occurring in the cells of a multicellular organism
as they become specialised.
Specialisation – the acquisition of certain features that adapts the cell to
perform a certain function within the organism.
Specialisation involves:
•
•
•
Changes in the number of certain organelles
Changes in the shape of the cell
Changes in the contents of the cell
often resulting from the switching on/off of specific genes within the
differentiating cell.
Red blood cells (erythrocytes):
•
•
•
•
•
Produced from stem cells in the bone marrow
Loss of the nucleus, mitochondria, Golgi apparatus, RER
Become packed full of haemoglobin
Assume a biconcave disc shape (large SA)
Transport of respiratory gases
Neutrophils:
•
•
•
•
Produced from stem cells in the bone marrow
Nucleus retained
Cytoplasm appears granular because of presence of many lysosomes
Ingest invading microorganisms by phagocytosis
Xylem:
•
•
•
•
•
Produced from cambium cells in vascular bundles
Cell walls become impregnated with waterproof lignin
Cell contents die, leaving a hollow tube with a wide lumen
End walls break down
Transport of water and minerals from roots to the rest of the plant
Phloem:
•
•
•
Produced from cambium cells in vascular bundles
Sieve tube elements lose most of their cytoplasm and organelles
Transport of glucose and amino acids from photosynthesising cells to the
rest of the plant
(i) describe and explain, with the aid of diagrams and photographs, how cells of multicellular
organisms are specialised for particular functions, with reference to erythrocytes (red blood
cells), neutrophils, epithelial cells, sperm cells, palisade cells, root hair cells and guard cells;
AS Biology p.34-37
Erythrocytes and neutrophils – see (h) above
Epithelial cells - form layers and linings; underlain by mesh of collagen and
glycoprotein fibres called the basement membrane.
•
•
Squamous epithelium – thin, flattened cells which often line tubes eg
blood vessels, alveoli
Ciliated epithelium – column-shaped cells with cilia in their outer
membrane – line tubes such as trachea and bronchioles, oviducts and
uterus – often coated with mucus, which moved by beating cilia
Sperm cells:
•
•
•
•
•
Haploid
Super-condensed DNA in very small nucleus
Acrosome containing lysozyme for penetrating halo of cells around the
egg, and the egg membrane
Many mitochondria packed into the middle section provide ATP for
movement of the flagellum
Elongated flagellum provides propulsion
Palisade cells:
•
•
Elongated leaf cells located under the upper epidermis - provide maximum
SA for absorbing light and CO2
Packed with chloroplasts
Root hair cells – hair-like projection from their outer cell walls provides a large
SA for absorbing water and minerals.
Guard cells:
•
•
•
Found in lower epidermis of leaves
Contain chloroplasts
Inner cell walls thickened - when the cells absorb water, they bend,
opening the stomata between them
(j) explain the meaning of the terms tissue, organ and organ system;
AS Biology p.35
Tissue - a collection of similar cells that are specialised to perform a particular
function eg ciliated epithelium, xylem.
Organ – collection of tissues working together to perform a function eg leaves,
liver.
Organ system – made of of a number of organs working together to perform a
function eg reproductive and excretory systems.
(k) explain, with the aid of diagrams and photographs, how cells are organised into tissues,
using squamous and ciliated epithelia, xylem and phloem as examples;
AS Biology p.36-37
See section (i) above.
(l) discuss the importance of cooperation between cells, tissues, organs and organ systems;
Each cell needs to play its part in the body of a multicellular organism, so its
response to internal and external stimuli must be coordinated with that of other
cells to ensure that the growth, development and metabolic processes of the
organism promotes its survival.
FURTHER READING: general
Biological Sciences 1&2 (1997) D.J. Taylor, N.P.O. Green, G.W. Stout
Cambridge University Press
Chapter 5 p.128-166 Cells
Chapter 23 p.776-783 Chromosomes and Mitosis
Chapter 6 p.167-191 Histology (tissues)
Cell Biology and Biochemistry (2006) T. Greenwood, L. Shepherd, R.
Allan, D. Butler Biozone Learning Media
Section 2 p.29-58 Cells and cellular organisation
Section 3 p.60-73 Membranes and transport
Section 5 p.92-96 Mitosis and cell differentiation
OCR Biology AS (2008) T. Greenwood, K. Pryor, L. Bainbridge-Smith,
R. Allan Biozone Learning Media
Section 2 p.40-58 Cell Structure
Section 3 p.59-71 Cell Membranes and Transport
Section 3 p. 72-86 Cell Division and Organisation
An Atlas of Histology (1978) W.H. Freeman, B. Bracegirdle Heineman
Educational Books
1.1.1 CELL STRUCTURE
A Cloistered Existence (1991) S. Fry Biological Sciences Review
September 1991 p.8-11
The Golgi Apparatus (1991) G. Warren Biological Sciences Review
November 1991 p.21-24
1.1.3 Cell Division, Cell Diversity and Cellular Organisation
Understanding Division (1996) D. Gull Biological Sciences Review May
1996
Out of Control (1999) J. Itzhaki Biological Sciences Review January 1999
p.36-39
To Divide or Not to Divide? (1999) P. Nurse Biological Sciences Review
March 1999 p.2-5
EXAM QUESTIONS: cell structure
June 2001 2801 q.1
January 2003 2801 q.1
June 2003 2801 q.3
June 2004 2801 q.3
January 2005 2801 q.1
June 2005 2801 q.1
June 2005 2801 q.1
January 2006 2801 q.1
January 2006 2801 q.1
January 2009 F211 q.1
January 2010 F211 q.3
June 2010 F211 q.1
January 2011 F211 q.5
January 2012 F211 q.4
June 2012 F211 q.2
membranes and transport
January 2002 2801 q.2
June 2003 2801 q.6
January 2004 2801 q.2
January 2004 2801 q.4
June 2004 2801 q.6
June 2009 F211 q.2
June 2010 F211 q.3
January 2011 F211 q.6
June 2011 F211 q.2
June 2001 2801 q.5
January 2012 F211 q.6
Mitosis, tissues and organs
January 2001 2801 q.1
June 2001 2801 q.6
January 2002 2801 q.5
June 2002 2801 q.7
June 2002 2801 q.1
June 2003 2801 q.2
January 2004 2801 q.1
June 2004 2801 q.2
January 2005 2801 q.5
January 2009 2801 q.3
June 2009 2801 q.5
January 2010 F211 q.2
June 2010 F211 q.4
January 2011 F211 q.1
June 2011 F211 q. 4
January 2012 F211 q.2
June 2012 F211 q.1
mitosis
mitosis
mitosis and cancer
tissues
mitosis
tissues
mitosis
cancer
stem cells
mitosis
tissues
tissues
mitosis
mitosis
levels of organisation
mitosis