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Summary International Baccalaureate Biology all chapters HL
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International Baccalaureate Biology all chapters study notes and flashcards HL
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Topic 1: Cell Biology1.1 Introduction to cells
The cell theory-
● Cells are the basic unit of life
● New cells are always formed from pre-existing cells
● Cells and cell products make up the structures of all living things
Characteristics of cells-
● Nutrition- obtaining food, to provide energy and the materials needed for growth
● Metabolism- chemical reactions inside the cell, including respiration to release energy
● Growth- an irreversible increase in size
● Response- the ability to react to changes in the environment
● Excretion- getting rid of waste products of the metabolism
● Homeostasis- keeping conditions inside the organism within tolerable limits
● Reproduction- producing offspring either sexually or asexually
Limitations of cell size
So: if the surface area to volume ratio is too small then it make take too long for substances to enter
the cell and waste products will accumulate because they will build up more quickly than they can be
excreted. If the surface area to volume ratio is too small then the cell may overheat as metabolism
will produce heat faster than it is lost out of the cell’s surface
The bigger the cell the smaller the surface area to volume ratio
Unicellular and Multicellular organisms-
Unicellular organisms consist only of one cell and carry out all the functions of life, because of this
the structure of unicellular organisms is more complex than most cells in multicellular organisms.
As a cell grows larger its surface area to volume ratio becomes smaller. Large organisms therefore
have to be multicellular.
Being multicellular also allows the division of labour. Different groups of cells can become specialised
for different functions by the process of differentiation.
Specialised tissues can develop by cell differentiation in multicellular organisms
Multicellular organisms can have:
● Developed circulatory systems to deliver nutrients and oxygen and remove waste products
● Evolved exchange surfaces which help to achieve this
Multicellular organisms show emergent properties:
● They are organised and have an input of energy
● The abilities of the multicellular organisms will be greater than the sum of its parts- the
components of a system together have more properties than they have as individual
elements.
● These properties emerge from the interaction of the cellular components.
Cell differentiation-
1
,Groups of cells will differentiate from non-specialised cells that can divide to produce new cells.
Undifferentiated cells are called stem cells.
An organisms entire set of genes is its genome. In a multicellular organism each cell has a full
genome, so it has the instructions to develop into any kind of cell.
When a gene is being used in a cell, we say it’s being expressed, this means that it is actively being
used to make a protein or other gene product. Cell differentiation happens because a different
sequence of genes is expressed in different cell types.
During differentiation a cell only uses the genes it needs to follow it pathway of development. Once a
pathway of development has begun in a cell, it is usually fixed and the cell cannot switch to a
different pathway.
Stem cells-
● Stems cells have the capacity to divide and differentiate along different pathways
● Stem cells can divide again and again to produce copious quantities of new cells. They are
therefore useful for the growth of tissues or the replacement of cells that have been lost or
damaged.
● Stem cells aren’t fully differentiated. They can differentiate in different ways, to produce
different cell types.
Embryonic stem cells are therefore potentially very useful. They could be used to produce
regenerated tissue or heal diseases.
Stargardt’s macular dystrophy is a genetic disease that develops in children between the ages of 6
and 12. It causes the membrane protein used for active transport in retina cells to malfunction.
Researchers have found methods of developing embryonic stem cells into retina cells, to be injected
into the eye and improve the patient’s vision.
Leukaemia is a type of cancer where a mutation in the bone marrow cells have caused it to produce
large numbers of white blood cells.
Arguments for and against the use of stem cells in medicine:
Against:
● Some argue that an embryo is a human life even at the earliest stage of development and
that the benefits from therapies don’t justify the taking of a human life.
For:
● Early stage embryos are little more than balls of cells that have yet to develop the essential
features of human life.
● Early stage embryos lack a nervous system so do not feel pain or suffering during procedures.
● If embryos are produced deliberately, no individual that would otherwise have the chance of
living is denied the chance of life
● Large numbers of embryos implanted by IVF are never implanted so do not get the chance of
life, so we should use these to cure diseases rather than kill them.
2
,Microscopes
Light/optical microscopes are useful for viewing things at a low magnification. At high magnification
is lost.
● Magnified size = real size x magnification
● Magnification = magnified size (measured with a ruler) / real size (the number on the scale
bar)
● Real size = size on image (measured with a ruler) / magnification
● The power of the eye piece lens x the power of the objective lens = the magnification
Electron microscopes allow us to see the full structure of a cell
Resolution is the ability of the microscope to show two close objects separately in the image.
● The resolution of an image depends on the wavelength of the rays used to form the image
● The shorter the wavelength the higher the resolution
● Electrons have a much shorter wavelength than light, so electron microscope have a higher
resolution than light microscopes.
Transmission electron microscopes (TEM) are used to view ultra-thin sections
Scanning electron microscopes (SEM) produce an image of the surfaces of cells
The advantage of electron microscopes is that we can get a much greater resolution and
magnification.
The disadvantage is that there is no colour, it is very technical and very expensive. Only dead material
can be used as it has to be treated. These microscopes also have a very small field of view.
1.2 Ultrastructure of cells
Prokaryotic cell structure
3
, Cell division in prokaryotes
Prokaryotes divide by binary fission. They use asexual reproduction to produce new cells:
● The single circular chromosome is replicated so that there are two identical copies
● The two copies of the chromosome move to opposite sides of the cell.
● Division of the cytoplasm of the cell quickly follows
● Each of the daughter cells contains one copy of the chromosome so they are genetically
identical.
Eukaryotic cells
Eukaryotes have a compartmentalised cell structure. The partitions are single or double membranes.
Each organelle in a eukaryotic cell has a distinctive structure and function.
There are several advantages of being compartmentalised:
● Enzymes and substrates for a particular process can be much more concentrated than if they
were spread through the cytoplasm.
● Substances that could cause damage to the cell can be kept inside the membrane of an
organelle. The digestive enzymes of lysosome would digest a cell from its inside if it wasn’t
contained by the lysosome membrane
● Conditions such as PH can be maintained at an ideal level for a particular process, which may
be different to the levels need for other processes in a cell
● Organelles with their contents can be moved around within the cell.
Diagram of a Eukaryotic cell
4
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