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Biology lesson
Cell structure and function
The cell is the basic unit of life.
All organisms are made up of cells.
Unicellular organisms are
organisms that only have one
cell
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When people talk about multicellular
organisms, they refer to organisms that
have more than one cell. This is the case for
most animals and plants that can be seen
without the use of a microscope.
In such organisms, cells are usually
specialised. All the cells with the same
function are grouped together. Such a group
of cells is then called tissue.
Different types of cell
Sperm cells move by inducing
a wave-like deformation in a
thin filament, called a
flagellum.
The one-celled organism
amoeba proteus
A single-celled bacteria
of the type: E. coli
Other type of organism
propel themselves using cilia
(this picture is of a
Paramecium).
A human red blood cell
A plant cell from the leaf
of a tree
Prokaryotic cell
A cell whose DNA
is not located in
the membranebound organelle
known as a
nucleus.
Prokaryotes are
microscopic
forms of life, and
all are either
bacteria or
archaea.
DNA in the bacterial cell is
generally confined to this
central region (nucleoid).
Though it isn't bounded by
a membrane, it is visibly
distinct (by transmission
microscopy) from the rest
of the cell interior.
Ribosomes give the cytoplasm
of bacteria a granular
appearance in electronic
micrographs. Though smaller
than the ribosomes in eukaryotic
cells, these organelles are the
site of protein synthesis.
Cell wall structure
A cell wall is a tough, flexible and sometimes fairly rigid layer
surrounding a cell, located external to the cell membrane, which
provides the cell with structural support, protection, and acts as a
filtering mechanism.
They are found in plants, bacteria, fungi, algae, and some archaea.
Animals and protozoa do not have cell walls.
Cell wall composition is one of the most
important factors in bacterial species
analysis and differentiation.
There are two major types of walls: Grampositive and Gram-negative.
The cell wall of bacteria consists in a
different organization of many
polymer layers of peptidoglycan
connected by amino acid bridges.
CAPSULE
This layer of polysaccharide (sometimes proteins) protects the
bacterial cell and is often associated with pathogenic bacteria because
it serves as a barrier against phagocytosis by white blood cells.
APPENDAGES
Bacteria may have the following
appendages:
PILI: These are hollow, hairlike structures made of
protein allowing bacteria to attach to other cells.
FLAGELLA: are long appendages which rotate by
means of a "motor" located just under the
cytoplasmic membrane. Bacteria may have one,
a few, or many flagella in different positions on
the cell. They are composed of microtubes that
function in cell movement.
Three domains
Eukaryotic cell
A cell whose primary
complement of DNA is
contained within a
membrane - lined nucleus.
Eukaryotic cells have
several other organelles in
addition to the nucleus. All
organisms except bacteria
and archaea are single
eukaryotic cells or are
composed of more
eukaryotic cells.
Eukaryotic cells
PLANT CELL
ANIMAL CELL
Organelle
A highly organized structure
within a cell that carries out
specific cellular functions.
Almost all organelles
(meaning "tiny organs") are
bound by membranes. The
only exception is the
ribosome, which has no
membrane and is the single
organelle possessed by
prokaryote cells (bacteria and
archaea). The ribosome, like
the plasma membrane,
cytoskeleton and centrioles
are composed of molecular
structure. Organelles in
eukaryotic cells include the
cell nucleus, mitochondria,
lysosomes, chloroplasts, and
ribosomes, ecc…
Animal cell
Plasma Membrane structure
A membrane forming the outer boundary of many cells, composed of a
phospholipid bilayer interspersed with proteins and cholesterol molecules and
coated, on its exterior face, with short carbohydrate chains associated with
proteins and lipids.
Model of plasma membrane
INTEGRAL PROTEIN is a protein of the plasma membrane that crosses the
membrane's hydrophobic interior.
PERIPHERAL PROTEIN is a protein of the plasma membrane that lies on the
inside or outside of the membrane but it doesn’t cross the membrane's
hydrophobic interior
Nucleus of cell
The membrane-lined
compartment that
encloses the primary
complement of DNA in
eukaryotic cells.
Nuclear envelope
The double membrane
that lines the nucleus in
eukaryotic cells.
.
Cytoplasm
The region of a cell inside the plasma membrane and outside the
nucleus. Usually this region is filled with the jelly-like cytosol
containing the cell's extra-nuclear organelles.
Cytosol
The protein-rich, jelly-like fluid in which a cell's organelles that are
outside the nucleus are immersed.
CYTOSKELETON
A network of protein filaments that functions
in cell structure, cell movement, and the
transport of materials within the cell.
MicrofiIaments, intermediate filaments, and
microtubules are all parts of the cytoskeleton.
RIBOSOME
An organelle,
located in the cell's
cytoplasm, that is
the site of protein
synthesis. The
translation phase of
protein synthesis
takes place within
ribosomes.
Rough Endoplasmic Reticulum
The endoplasmic reticulum
(ER) is a special membrane
structure found only in
complex cells. Parts of the
ER are studded with
ribosomes-the cell's
protein-making machinery.
Proteins that require special
conditions or are destined
to become part of the cell
membrane are processed in
the ER and then handed off
to another organelle called
the Golgi apparatus. The
Golgi functions as a cellular
post office. Proteins that
arrive there are sorted,
packaged and transported
to various destinations in
the cell.
Lysosome is
an organelle of
eukaryotic cells
that contains
hydrolytic
enzymes
involved in
intracellular
digestion.
They contain a unique collection of about 40 hydrolytic enzymes,
including proteases, nucleases, lipases, glycosidases,
phospholipases, phosphatases, ecc…
They digest excess or worn-out organelles, food particles, and
engulfed viruses or bacteria.
Mitochondria
Mitochondria are an important
part of every cell; they contain
genetic material.(You must
remember the origin of
mitochondria!) Mitochondria
are responsible for processing
oxygen and converting
substances from the foods we
eat into energy for essential
cell functions. Mitochondria
produce energy in the form of
adenosine triphosphate (ATP),
which is then transported to
the cytoplasm of a cell for use
in numerous cell functions.
Plant cell
Plant cells are
eukaryotic cells that
differ in several key
respects from the
cells of other
eukaryotic
organisms. Their
distinctive features
include: vacuole,
chloroplast and
cell wall.
Vacuole
A large central vacuole, a sapfilled volume enclosed by a
membrane known as the
tonoplast maintains the cell's
turgor, controls movement of
molecules between the cytosol
and sap, stores useful material
Vacuoles tend to be large in plant
cells and play a role in turgor
pressure. When a plant is wellwatered, water collects in cell
vacuoles producing rigidity in the
plant. Without sufficient water,
pressure in the vacuole is reduced
and the plant wilts.
CHLOROPLASTS
Chloroplasts are organelles found only in
plant cells. They use sunlight to create
energy. Chloroplasts are the site of
photosynthesis, a process in which the plant
uses carbon dioxide, water and sunlight to
create energy in the form of glucose. This
glucose is used as food by heterotrophs.
Cell wall
The presence of a cell wall above all other characteristics
distinguishes plant cells from animal cells.
Cell walls are not merely outer, inactive covering of the plant cell
itself but are critical in function such as:
a) Prevent enlargement of the plant cell.
b) Also play important roles in the absorption, transport and
secretion of substances in plants.
The most characteristic
component of plant cell
walls is cellulose, which
largely determines their
architecture. Cellulose is
made of repeating
molecules of glucose
attached end to end in a
linkage.
Plant versus Animal Cells
Now that you know some important cell organelles let us identify those that
distinguish plant cells from animal cells
From the descriptions above, we can identify three organelles unique to plant
cells: 1) cell wall (in contrast to a cell membrane in animal cells), 2) central
vacuole (regular vacuoles are found in animal cells) and 3) chloroplasts
(animals do not perform photosynthesis. This is what makes plants autotrophs
and animals heterotrophs.)