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The cell
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Cells are complex
microsystems
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From biomolecules to cells
There is a hierarchical organization in
cells: atoms constitute biomolecules;
biomolecules and water are used to
build up structures that work together
so that the cell can grow, develop and
reproduce.
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Why are cells small? /1
Same volume but different surface
Cells are small in order to have a large surface compared to
their volume. This ratio allows for efficient exchanges with the
external environment.
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Why are cells small? /2
Volume
64 cm3
64 cm3
64 cm3
Surface
96 cm2
192 cm2
384 cm2
Surface/Volume
96/64 = 1.5
192/64 = 3
384/ 64 = 6
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Observing cells
Cells can be observed with the light microscope or the
electron microscope which has a higher resolving power.
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Prokaryotic cells /1
Prokaryotes are unicellular
organisms that belong to
Archaea or Bacteria domains.
Prokaryotic cells always
present four structures:
• plasma membrane;
• cytoplasm;
• nucleoid with a circular DNA
molecule;
• ribosomes.
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Prokaryotic cells /2
Many prokaryotic cells present
additional structures:
• cell wall;
• capsule;
• flagella;
• fimbriae or pili.
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Eukaryotic cells
Eukaryotes are organisms that can be unicellular or
multicellular.
Eukaryotic cells, besides the base structures of prokaryotic
cells, have membrane-bound organelles and a nucleus.
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Animal and plant cells /1
Both animal and plant cells
have common structures:
•mitochondria;
•Golgi apparatus;
•nucleus;
•endosplamic reticulum;
•cytoskeleton.
Animal cells also have
lysosomes and centrioles.
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Animal and plant cells /2
Plant cells also present a cell wall, vacuoles
and chloroplasts.
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Organelles
in eukaryotic cells
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Nucleus and nucleolus
The nucleus contains the DNA with
the genetic information of the
organism. The DNA controls the
synthesis of proteins and it replicates
during cell division.
The subunits of ribosomes are
assembled in the nucleolus.
The synthesis of proteins takes place
in the cytoplasm, on ribosomes.
nucleolus
nucleus
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Mitochondria and cell energy
The energy necessary for the cell is produced by cell
respiration in the mitochondria.
These organelles contain their own DNA and ribosomes.
outer membrane
Intermembrane
space
matrix
inner membrane
They are divided into
compartments:
• the outer membrane;
• the intermembrane
space;
• the inner membrane;
• the matrix.
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The endoplasmic reticulum:
rough and smooth
The main function of the rough
endoplasmic reticulum is to
store and modify proteins.
The smooth endoplasmic
reticulum is important for the
synthesis of many lipids and for
the degradation of toxic
substances and metabolic waste.
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The Golgi apparatus
The Golgi apparatus is a
membrane system. It receives
the molecules synthesized in the
endoplasmic reticulum, modifies
them and stores them in vesicles.
The vesicles transport the
molecules to different parts
of the cell.
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Lysosomes
Lysosome
(red colored)
Lysosomes are
vesicles formed in the
Golgi apparatus.
They contain enzymes
which degrade
macromolecules and
metabolic waste or
destroy bacteria.
nucleus
(green colored)
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Cytoskeleton, cilia and flagella
The cytoskeleton consists of a dense network of filaments
and microtubules.
It provides shape and support to the cell and an anchoring site
for organelles. It is involved in the transport of organelles and
molecules inside the cell.
Many cells can present external,
mobile structures, called cilia and
flagella.
They are involved in cell
movement and they can be used
to move the external fluids.
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Plant cells: the cell wall
The cell wall in plant cells
provides support and
protection and reduces
water loss.
It is mainly made up of
cellulose and surrounds
the cell membrane.
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Plant cells: vacuoles
Up to 90% of a plant cell’s
volume may be occupied by
the vacuole.
This organelle stores nutrients, degrades waste, accumulates
toxic substances that repel herbivores and insects, contains
pigments that attract pollinators and provides mechanical
support for the plant.
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Plant cells: chloroplast
outer membrane
thylakoids
The chloroplast, which is
present in plants and algae,
is the organelle responsible
for photosynthesis - the
transformation of light
energy into chemical energy.
stroma
inner membrane
granum
It contains chlorophyll and
possesses its own DNA.
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Cell adhesion
and communication
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Cells communicate each other /1
Nearly all the cells of multicellular organisms are connected
to, and communicate with, other cells.
In animal cells, the main types of cell adhesion are:
• tight junctions – they form a thin watertight layer to keep
the cells locked together;
• desmosomes – they connect cells in robust layers and
bundles;
• gap junctions – they form open channels between two cells
and are important for intercellular communication.
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Cells communicate to each other /1
In plants, cells can exchange substances through
plasmodesmata: they are microscopic channels that enable
communication and transportation between cells.
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