Xylem: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 end-plates 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.

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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 conver

t 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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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.

The nucleus is a specialized organelle that serves as the information processing and administrative center of the cell.

Cell Nucleus:

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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 photosynth

esis activity in most plants occurs in the leaves.

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 be connected via their lumens.

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.

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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.

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.

There are two basic kinds of endoplasmic reticulum morphologies: rough and smooth

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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. BACK

Golgi Apparatus:

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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.

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:

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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.

Ribosomes:• Ribosomes are small dot-like structures in cells. • They are often associated closely with endoplasm

ic reticulum(ER), forming rough ER. • They are the site of protein synthesis in cells.

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.

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