Download Structure of cells

Xylem is a term applied to woody
(lignin-impregnated) walls of
certain cells of plants. Xylem cells
tend to conduct water and
minerals from roots to leaves. It
consists of tracheary elements,
tracheids and wood vessels and of
additional xylem fibres. Tracheids
are the more primitive of the two
cell types, occurring in the earliest
vascular plants. Tracheids are long
and tapered, with angled endplates that connect cell to cell.
Vessel elements are shorter, much
wider, and lack end plates. They
occur only in angiosperms, the
most recently evolved large group
of plants.
Xylem:
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Phloem:
•
Phloem cells conduct
food from leaves to rest
of the plant. They are
alive at maturity and tend
to stain green. The two
most common cells in the
phloem are the
companion cells and
sieve cells. Companion
cells retain their nucleus
and control the adjacent
sieve cells. Dissolved
food, as sucrose, flows
through the sieve cells.
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Structure of plant cell:
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Mitochondria:
Functions:
•
•
In plant cells, mitochondria
convert oxygen and nutrients into
adenosine triphosphate to provide
energy, when light isn't available
for the chloroplasts to produce
energy.
In inner membrane, which is highly
convoluted so that a large number
of infoldings called cristae are
formed, also allows only certain
molecules to pass through it and is
much more selective than the outer
membrane. To make certain that
only those materials essential to
the matrix are allowed into it, the
inner membrane utilizes a group of
transport proteins that will only
transport the correct molecules.
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Cell Nucleus:
Functions:
• It stores the cell's
hereditary material, or
DNA, and it
coordinates the cell's
activities, which
include growth,
intermediary
metabolism, protein
synthesis, and cell
division.
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The nucleus is a specialized
organelle that serves as the
information processing and
administrative center of the
cell.
Chloroplasts:
Photosynthesis occurs in almost all plant species and is carried out in
specialized organelles known as chloroplasts. All of the green structures in
plants, including stems and unripened fruit, contain chloroplasts, but the majority
of photosynthesis activity in most plants occurs in the leaves.
Chloroplast is enclosed in a double membrane and the area between the two
layers that make up the membrane is called the intermembrane space. The
outer layer of the double membrane is much more permeable than the inner
layer, which features a number of embedded membrane transport proteins.
Enclosed by the chloroplast membrane is the
stroma, a semi-fluid material that contains
dissolved enzymes and comprises most of the
chloroplast's volume. Since, like mitochondria,
chloroplasts possess their own genomes, the
stroma contains chloroplast DNA and special
ribosomes and RNAs as well. In higher plants,
lamellae, internal membranes with stacks of
closed hollow disks called thylakoids, are also
usually dispersed throughout the stroma. The
numerous thylakoids in each stack are thought to
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be connected via their lumens.
Endoplasmic Reticulum:
The endoplasmic reticulum membrane allows molecules to be selectively
transferred between the lumen and the cytoplasm, and since it is connected
to the double-layered nuclear envelope, it further provides a pipeline between
the nucleus and the cytoplasm.
There are two basic kinds of endoplasmic reticulum morphologies:
rough and smooth
The surface of rough endoplasmic reticulum
is covered with ribosomes. This type of
endoplasmic reticulum is involved mainly
with the production and processing of
proteins that will be exported, or secreted,
from the cell. The ribosomes assemble
amino acids into protein units, which are
transported into the rough endoplasmic
reticulum for further processing.
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Plant Cell Vacuoles:
In mature plant cells, vacuoles tend to be very large and are
extremely important in providing structural support, as well as
serving functions such as storage, waste disposal, protection, and
growth.
When a plant receives adequate
amounts of water, the central vacuoles
of its cells swell as the liquid collects
within them, creating a high level of
turgor pressure, which helps maintain
the structural integrity of the plant,
along with the support from the cell wall.
In the absence of enough water,
however, central vacuoles shrink and
turgor pressure is reduced,
compromising the plant's rigidity so that
wilting takes place.
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Golgi Apparatus:
The Golgi apparatus (GA), also called Golgi body or Golgi
complex and found universally in both plant and animal cells.
The Golgi apparatus is often considered the distribution and
shipping department for the cell's chemical products. It
modifies proteins and lipids that have been built in the
endoplasmic reticulum and prepares them for export outside
of the cell or for transport to other locations in the cell.
Proteins and lipids built in the smooth and rough endoplasmic
reticulum bud off in tiny bubble-like vesicles that move
through the cytoplasm until they reach the Golgi complex. The
vesicles fuse with the Golgi membranes and release their
internally stored molecules into the organelle. Once inside,
the compounds are further processed by the Golgi apparatus,
which adds molecules or chops tiny pieces off the ends.
When completed, the product is extruded from the GA in a
vesicle and directed to its final destination inside or outside
the cell. The exported products are secretions of proteins or
glycoproteins that are part of the cell's function in the
organism. Other products are returned to the endoplasmic
reticulum or may undergo maturation to become lysosomes.
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Microfilaments:
Common to all eukaryotic cells, these filaments are primarily
structural in function and are an important component of the
cytoskeleton, along with microtubules and often the intermediate
filaments. Microfilaments range from 5 to 9 nanometers in
diameter and are designed to bear large amounts of tension. In
association with myosin, microfilaments help to generate the
forces used in cellular contraction and basic cell movements. The
filaments also enable a dividing cell to pinch off into two cells and
are involved in amoeboid movements of certain types of cells.
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Microtubules:
These straight, hollow cylinders are found throughout the cytoplasm
of all eukaryotic cells (prokaryotes don't have them) and carry out a
variety of functions, ranging from transport to structural support.
Microtubules, which are about 25 nanometers in diameter, form part
of the cytoskeleton that gives structure and shape to a cell, and
also serve as conveyor belts moving other organelles throughout
the cytoplasm. In addition, microtubules are the major components
of cilia and flagella, and participate in the formation of spindle fibers
during cell division (mitosis). The length of microtubules in the cell
varies between 200 nanometers and 25 micrometers, depending
upon the task of a particular microtubule and the state of the cell's
life cycle.
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Ribosomes:
•
•
•
Ribosomes are small dot-like structures in cells.
They are often associated closely with endoplasmic
reticulum(ER), forming rough ER.
They are the site of protein synthesis in cells.
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Strcuture:
Ribosomes are made up of proteins and ribonucleic acid(RNA). These
molecules are arranged into two subunits called the large and small subunits.
These subunits are attached to each other and together form the entire
ribosome.
Function:
The only function of ribosomes is to build proteins. The plan for the protein
in the form of mRNA passes through a groove in the ribosome. The
ribosome "reads" the plan and assembles the proper sequence of amino
acids to build the protein. There are two kinds of ribosomes, those attached
to the endoplasmic reticulum and those floating in the cell cytoplasm.
Attached ribosomes make proteins that are used in the ER or transported
within the ER. Free ribosomes make proteins that are used in the cytoplasm.