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Samenvatting Cellen BMW jaar 1
Samenvatting van het vak cellen
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CH. 1 blz 1 t/m 27Unity and diversity of cells
Cells vary enormously in appearance and function
There are different ways to distinguish different cells:
- Size
- Shape
- Chemical requirements
Because of these differences the cells function differs too.
Living cells all have a similar basic chemistry
Even though all cells are enormously varied from the outside, the inside is
fundamentally the same. For example, every cell carries genetic information
called genes. Thus, every cell has DNA in it. The appearance and function of a
cell is dictated by its protein molecules it transcribes.
Living cells are self-replicating collections of catalysts
It is the central dogma which makes the self-replication possible. Proteins have
an important role in polynucleotide and protein synthesis, but also catalyze loads
of different chemical reactions that keep self-replication active. A cell can break
down nutrients and use its products to synthesize new polypeptides. Only living
cells can self-replicate. Viruses do have DNA and RNA but don’t have the ability
to self-replicate by their own. They need to inhibit a cell to be able to be active.
All living cells have apparently evolved from the same ancestral cell
Cell replication is not always perfect, sometimes a mutation occurs which is a
change in the nucleotide sequence in the DNA. Mutations aren’t always bad.
- Positive mutations: The organism can survive or reproduce easier;
these genes will be in
favor by survival.
- Negative mutations: The organism is less able to survive or reproduce;
these genes will be
eliminated by survival.
- Neutral mutations: The organism differs in genes but is equally
viable; these genes will be
tolerated by survival.
This is the basis of evolution: the process by which living organisms become
gradually modified and adapted to their environment in more and more
sophisticated ways.
Genes provide instructions for the form, function and behavior of cells
and organisms
A cells genome provides a genetic program that instructs a cell how to behave.
However, within a plant or animal there are loads of differentiated cells. These
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,are generated embryonic development from a single fertilized egg. These
differentiated cells carry the whole genome, but only expresses certain genes,
making the cell different from other sorts.
Cells under the microscope
Light microscope: uses light to illuminate specimens. However, the
wavelength of visible light limits the fineness of detail.
Electron microscope: uses beams of electrons to illustrate cells. Because
electrons have a shorter wavelength these instruments
extend the details.
Fluorescence microscope: uses sophisticated methods of illumination and
electronic image processing to see fluorescently labeled
cell components.
The invention of the light microscope led to the discovery of cells
Cell biology was born thanks to 2 publications.
- Matthias Schleiden (botanist, 1838): systematic investigation,
showed that cells are the
building blocks of all living tissues.
- Theodor Schwann (zoologist, 1839):
The principle that all cells are generated only from preexisting cells and inherit
their characteristics from them underlies all of biology and gives the subject a
unique flavor. Charles Darwin provides the key insight that makes this history
comprehensible.
Light microscopes reveals some of a cell’s components
Distinguishing the internal structure of a cell is difficult because the parts are
small, transparent and often colorless. One way to cope with this problem is to
stain the cells with dyes that color particular components differently. Also, the
difference in refractive index can be used to distinguish different parts.
The fine structure of a cell is revealed by electron microscopy
Preparing a sample is a painstaking process:
- Light microscope: Sample needs to be pickled supported by an embedded
wax, cut or
sectioned in thin slices and stained before it can be viewed.
- Electron microscope: Same process as by light microscope, but
requires to be cut way
thinner, and can’t look at living cells.
With the electron microscope you can see distinct organelles and even large
molecules.
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,The type of microscope used to look at thin sections of a tissue is known as a
transmission electron microscope (TEM). Similar to a light microscope this
shoots beams of electrons instead of light.
Scanning electron microscope (SEM) scatters electrons at the surface of the
sample and so is used to look at the surface.
The prokaryotic cell
Organisms whose cells do not have a nucleus are called prokaryotes. Hallmarks
of prokaryotes are;
- Spherical or corkscrew-shaped
- No nucleus
- Small
- Cell wall
Prokaryotes can by divided into 2 classes; bacteria and archaea.
Prokaryotes are the most diverse and numerous cells on earth
Most prokaryotes live as a single-cell organism. Some are aerobic and others not.
The mitochondria are thought to have evolved from aerobic bacteria that took to
living inside the anaerobic ancestors of today’s eukaryotic cells.
Any organic, carbon-containing material can be used as food for bacterium. Some
prokaryotes can even live from inorganic substances like CO 2. And some
prokaryotes perform photosynthesis.
The world of prokaryotes is devided into two domains: bacteria and
archaea
Bacteria: live in the soil or makes us ill
Archaea: Lives in the soil of hostile places like lava.
The eukaryotic cell
Hallmarks of eukaryotes are;
- Nucleus
- Other cell organelles
The nucleus is the information store of the cell
The nucleus is enclosed within a double membrane called the nuclear
envelope. It contains DNA which become visible as chromosomes.
Mitochondria generate usable energy from food molecules
Mitochondria are generators of chemical energy for the cell. They use the
energy freed from the oxidation of food molecules to produce Adenosine
triphosphate or ATP. Because mitochondria use oxygen and can release CO 2
the entire process is called cell respiration. Mitochondria contain their own DNA
and reproduce by dividing. They are thought to derive from bacteria.
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, Chloroplasts capture energy from sunlight
Chloroplasts are green, large organelles found in plant and algae cells. They
possess internal stacks of membrane containing the green pigment chlorophyll.
They carry out photosynthesis – trapping the energy of sunlight in their
chlorophyll molecules and using the energy to synthesize energy-rich sugar
molecules. The release O2 as by-product. They also allow plants to produce food
molecules that mitochondria use to generate ATP. Like mitochondria, chloroplasts
have their own DNA, reproduce by dividing in 2 and are thought to have evolved
from bacteria.
Internal membranes create intracellular compartments with different
functions
The cytoplasm contains more membrane-surrounded organelles. The
endoplasmic reticulum is a maze of interconnected spaces enclosed by a
membrane. It is where most cell-membrane components, as well as materials
destined for export from the cell, are made.
Stacks of membrane-enclosed sacs constitute the Golgi apparatus which
modifies, and packages molecules made in the ER that are destined to be either
secreted from the cell or transported to another cell.
Lysosomes are small irregular shaped organelles in which intracellular digestion
occurs. Releasing nutrients from the digested food molecules and breaking down
unwanted molecules for recycling or excretion.
Peroxisomes are small vesicles that provide a environment for reactions in
which hydrogen peroxide is used to inactivate toxic molecules.
There are loads of materials to be exchanged. The transport is mediated by
transport vesicles that pinch off from the membrane of 1 organelle and fuse with
the membrane of another. Endo cytosis means that the vesicle comes from
outside inside the cell and exocytosis is the other way around.
The cytosol is a concentrated aqueous gel of large and small molecules
Cytosol is the cytoplasm that is not contained in a membrane. The cytosol is the
site of many chemical reactions that are fundamental to the cell’s existence.
The cytoskeleton is responsible for directed cell movements
The cytoskeleton – a system of protein filaments – is composed of three mayor
filament types;
- Actin filament: thinnest of all. Abundant in all cells but especially in
muscle cells where they
serve as central part of the machinery responsible for
contraction.
- Microtubules: Thickest of all. In dividing cells they are responsible for
pulling the duplicated
chromosomes apart.
- Intermediate filaments: Serve to strengthen most animal cells.
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