Plant cell

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The cell is the basic unit of life. All organisms are made up of cells (or in some cases, a single cell). Most cells are very small; most are invisible without using a microscope.

The following is a glossary of plant cell terms:

Amyloplast: An organelle in some plant cells that stores starch. Amyloplasts are found in starchy plants like tubers and fruits.

ATP: ATP is short for adenosine triphosphate; it is a high-energy molecule used for energy storage by organisms. In plant cells, ATP is produced in the cristae of mitochondria and chloroplasts.

Cell wall: The rigid cell wall of plants is made of fibrils of cellulose embedded in a matrix of several other kinds of polymers such as pectin and lignin which gives the cell most of its support and structure. The cell wall also bonds with other cell walls to form the structure of the plant. Like their prokaryotic ancestors, plant cells have a rigid wall surrounding the plasma membrane. It is a far more complex structure, however, and serves a variety of functions, from protecting the cell to regulating the life cycle of the plant organism.

The linear nature of cellulose molecules and the many opportunities for side-to-side intermolecular hydrogen bonding provide just what one would want to build long, stiff fibrils.

Primary cell walls:
The cell walls of parenchyma and meristems are uniform in thickness.
Although each cell appears encased within a box, in fact primary cell walls are perforated permitting plasmodesmata to connect adjacent cells.

Secondary cell walls:
The cells of sclerenchyma, collenchyma and xylem have secondary deposits of lignified cellulose which provide mechanical strength to the tissue.

Vacuoles: Each plant cell has a large, single vacuole that stores compounds, helps in plant growth, and plays an important structural role for the plant.Vacuoles are bounded by a single membrane. Young plant cells often contain many small vacuoles, but as the cells mature, these unite to form a large central vacuole.It helps maintain the shape of the cell.
Vacuoles serve several functions, such as

• storing foods (e.g., proteins in seeds)
• storing wastes
• storing malic acid in CAM plants
• storing various ions (e.g., calcium, sodium, iron) which, among other functions, helps to
• maintain turgor in the cell.

Plant cells avoid bursting in hypotonic surroundings by their strong cell walls. These allow the build-up of turgor within the cell. Loss of turgor causes wilting.

Cell membrane: The thin layer of protein and fat that surrounds the cell, but is inside the cell wall. The cell membrane is semipermeable, allowing some substances to pass into the cell and blocking others.

The Plasma Membrane: All living cells have a plasma membrane that encloses their contents. One universal feature of all cells is an outer limiting membrane called the plasma membrane.

In addition, all eukaryotic cells contain elaborate systems of internal membranes which set up various membrane-enclosed compartments within the cell.

Cell membranes are built from lipids and proteins.

The plasma membrane serves as the interface between the machinery in the interior of the cell and the extracellular fluid (ECF) that bathes all cells.

The lipids in the plasma membrane are chiefly phospholipids like phosphatidyl ethanolamine and cholesterol. Phospholipids are amphiphilic with the hydrocarbon tail of the molecule being hydrophobic; its polar head hydrophilic. As the plasma membrane faces watery solutions on both sides, its phospholipids accommodate this by forming a phospholipid bilayer with the hydrophobic tails facing each other.

Centrosome - (also called the "microtubule organizing center") a small body located near the nucleus - it has a dense center and radiating tubules. The centrosomes is where microtubules are made. During cell division (mitosis), the centrosome divides and the two parts move to opposite sides of the dividing cell.

Chloroplasts: The most important characteristic of plants is their ability to photosynthesize, in effect, to make their own food by converting light energy into chemical energy. This process is carried out in specialized organelles called chloroplasts. It is an elongated or disc-shaped organelle containing chlorophyll.

Chloroplasts are the most familiar plastids. They are usually disk-shaped and about 5-8 µm in diameter and 2-4 µm thick. A typical plant cell has 20-40 of them.
Chloroplasts are green because they contain chlorophylls - the pigments that harvest the light used in photosynthesis.

Chloroplasts are probably the descendants of cyanobacteria that took up residence in the ancestor of the plants.

Plant cells that are not engaged in photosynthesis also have plastids that serve other functions, such as

• storing starch (when they are called leucoplasts)
• storing the carotenoids that give flowers and fruits their color (when they are called chromoplasts).

Endoplasmic Reticulum: The endoplasmic reticulum is a network of sacs; a vast system of interconnected, membranous, infolded and convoluted tubes that are located in the cell's cytoplasm that manufactures, processes, and transports chemical compounds for use inside and outside of the cell. It is connected to the double-layered nuclear envelope, providing a pipeline between the nucleus and the cytoplasm. In plants, the endoplasmic reticulum also connects between cells via the plasmodesmata. The space within the ER is called the ER lumen. Smooth ER transport materials through the cell. It contains enzymes and produces and digests lipids (fats) and membrane proteins; smooth ER buds off from rough ER, moving the newly-made proteins and lipids to the Golgi body and membranes.

Golgi Apparatus: The Golgi apparatus is flattened, layered, sac-like organelle that looks like a stack of pancakes and is located near the nucleus. It is the distribution and shipping department for the cell's chemical products. It modifies proteins and fats built in the endoplasmic reticulum and prepares them for export as outside of the cell.

Peroxisomes: Microbodies are a diverse group of organelles that are found in the cytoplasm, roughly spherical and bound by a single membrane. There are several types of microbodies but peroxisomes are the most common.

Peroxisomes are about the size of lysosomes (0.5–1.5 µm) and like them are bound by a single membrane. They also resemble lysosomes in being filled with enzymes.

However, peroxisomes bud off from the endoplasmic reticulum, not the Golgi apparatus (that is the source of lysosomes).

Peroxisomes present in plant cells participate in such functions as symbiotic nitrogen fixation and photorespiration.

Plasmodesmata: Plasmodesmata are small tubes that connect plant cells to each other, providing living bridges between cells.

Ribosomes: All living cells contain ribosomes, tiny organelles composed of approximately 60 percent RNA and 40 percent protein. In eukaryotes, ribosomes are made of four strands of RNA. In prokaryotes, they consist of three strands of RNA.

Microfilaments: Microfilaments are solid rods made of globular proteins called actin. These filaments are primarily structural in function and are an important component of the cytoskeleton.

Microtubules: These are straight, hollow cylinders whose wall is made up of a ring of 13 "protofilaments" and 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. They have a diameter of about 25 nm. They are variable in length but can grow 1000 times as long as they are wide, built by the assembly of dimers of alpha tubulin and beta tubulin.

Microtubules grow at each end by the polymerization of tubulin dimers (powered by the hydrolysis of GTP), and shrink at each end by the release of tubulin dimers (depolymerization).

However, both processes always occur more rapidly at one end, called the plus end. The other, less active, end is the minus end.
Microtubules participate in a wide variety of cell activities. Most involve motion. The motion is provided by protein "motors" that use the energy of ATP to move along the microtubule.

Microtubule motors:
There are two major groups of microtubule motors:

• kinesins (most of these move toward the plus end of the microtubules) and
• dyneins (which move toward the minus end).

In plant cells, microtubules are created at many sites scattered through the cell.



Mitochondria: Mitochondria are spherical to rod-shaped organelles with a double membrane found in the cytoplasm of all eukaryotic cells. The inner membrane is infolded many times, forming a series of projections (called cristae). Their number within the cell ranges from a few hundred to, in very active cells, thousands. The mitochondrion converts the energy stored in glucose into ATP (adenosine triphosphate) for the cell by breaking down carbohydrate and sugar molecules to provide energy, particularly when light isn't available for the chloroplasts to produce energy.

Cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water. The energy released is trapped in the form of ATP for use by all the energy-consuming activities of the cell.

The process occurs in two phases:

• glycolysis, the breakdown of glucose to pyruvic acid
• the complete oxidation of pyruvic acid to carbon dioxide and water

In eukaryotes, glycolysis occurs in the cytosol. (Link to a discussion of glycolysis). The remaining processes take place in mitochondria.
Mitochondria have:

• an outer membrane that encloses the entire structure
• an inner membrane that encloses a fluid-filled matrix
• between the two is the intermembrane space
• the inner membrane is elaborately folded with shelflike cristaematrix. projecting into the
• a small number (some 5–10) circular molecules of DNA

The Outer Membrane:
The outer membrane contains many complexes of integral membrane proteins that form channels through which a variety of molecules and ions move in and out of the mitochondrion.

The Inner Membrane:
The inner membrane contains 5 complexes of integral membrane proteins:

• NADH dehydrogenase (Complex I)
• succinate dehydrogenase (Complex II)
• cytochrome c reductase (Complex III; also known as the cytochrome b-c1 complex)
• cytochrome c oxidase (Complex IV)
• ATP synthase (Complex V)

Christae: Christae (singular crista) is the multiply-folded inner membrane of a cell's mitochondrion that are finger-like projections. The walls of the cristae are the site of the cell's energy production (it is where ATP is generated).

Stroma: Stroma are part of the chloroplasts in plant cells, located within the inner membrane of chloroplasts, between the grana.

Thylakoid disk: Thylakoid disks are disk-shaped membrane structures in chloroplasts that contain chlorophyll. Chloroplasts are made up of stacks of thylakoid disks; a stack of thylakoid disks is called a granum. Photosynthesis (the production of ATP molecules from sunlight) takes place on thylakoid disks.



Rough endoplasmic reticulum - (rough ER) a vast system of interconnected, membranous, infolded and convoluted sacks that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane). Rough ER is covered with ribosomes that give it a rough appearance. Rough ER transport materials through the cell and produces proteins in sacks called cisternae (which are sent to the Golgi body, or inserted into the cell membrane).

Cytoplasm: It is the jellylike material outside the cell nucleus in which the organelles are located.

Granum: (plural grana) A stack of thylakoid disks within the chloroplast is called a granum.

Nuclear membrane: The membrane that surrounds the nucleus.

Nucleus: The nucleus is a spherical body containing many organelles, including the nucleolus. It serves as the information processing and administrative center of the cell. This organelle has two major 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 reproduction (cell division). The nucleus is surrounded by the nuclear membrane. The nucleus is the hallmark of eukaryotic cells; the very term eukaryotic means having a "true nucleus".