CELL ORGANELLES

INTRODUCTION

Plants are eukaryotes, multicellular organisms that have membrane-bound organelles. Unlike prokaryotic cells, eukaryotic cells have a membrane-bound nucleus. A plant cell is different from other eukaryotic cells in that it has a rigid cell wall, a central vacuole, plasmodesmata, and plastids. Plant cells take part in photosynthesis to convert sunlight, water, and carbon dioxide into glucose, oxygen, and water. Plants are producers that provide food for themselves (making them autotrophs) and other organisms.

 

Parts

These are some of the parts common to plant cells:

•Cell Wall- smooth layer that provides DNA and protection from osmotic swelling.

•Cell (Plasma) Membrane- it is composed of a phospholipid lipid bilayer (including polar hydrophilic heads facing outside and hydrophobic tails facing each other inside) that makes it semipermeable and thus capable of selectively allowing certain ions and molecules in/out of the cell.

•Cytoplasm- it consists of the jelly-like fluid in and around the organelles.

•Cytoskeleton- is made up of microtubules, intermediate filaments, and microfilaments. It provides shape the shape of the cell and helps in transporting materials in and out of the cell.

•Golgi Apparatus (body/complex)- it is the site where membrane-bound vesicles are packed with proteins and carbohydrates. These vesicles will usually leave the cell through secretion.

•Vacuole- stores metabolites and degrades and recycles macromolecules.

•Mitochondria- is responsible for cellular respiration by converting the energy stored in glucose into ATP.

•Ribosome- contain RNA and proteins for protein synthesis. One type is embedded in Rough ER and another type puts proteins directly into the cytosol.

•Rough Endoplasmic Reticulum (roughER)- covered with ribosomes, it stores, separates, and transports materials through the cell. It also produces proteins in cisternae, which then go to the Golgi apparatus or insert it into the cell membrane.

•Smooth Endoplasmic Reticulum (smooth ER)- it has no ribosomes embedded in its surface. Lipids and proteins are produced and digested here. Smooth ER buds off from rough ER to move newly-synthesized proteins and lipids. The proteins and lipids are transported to the Golgi apparatus (where they are made ready for export) and membranes.

•Peroxisome- is involved in metabolizing certain fatty acids and producing and degrading hydrogen peroxide.

•Nuclear Membrane (envelope)- the an extension of the endoplasmic reticulum that wraps around the nucleus. Its many gaps allow traffic in/out of the nucleus.

•Nucleus - it contains DNA in the form of chromosomes or chromatin and controls protein synthesis.

•Nucleolus - it is the site of ribosomal RNA synthesis.

•Centrosome- consisting of a dense center and radiating tubules, it organizes the microtubules into a mitotic spindle during cell division.

•Chloroplast- conducts photosynthesis and produces carbohydrates, oxygen, and internally ATP and NADPH from captured light energy.

•Starch Granule- temporarily stores produced carbohydrates from photosynthesis. Depending on the organism, it can be inside or outside of the chloroplast (if present).

Exclusive to Plant Cells

Cell Wall

The cell wall is a tough, usually flexible but fairly rigid layer that surrounds the plant cells. It is located just outside the cell membrane and it provides the cells with structural support and protection. A major function of the cell wall is to act as a pressure vessel, preventing over-expansion when water enters the plant cells. The strongest component of the cell wall is a carbohydrate called cellulose, a polymer of glucose.

The cell wall gives rigidity and strength to the plant cells which offers protection against mechanical stress. It also permits the plants to build and hold its shape. It limits the entry of large molecules that may be toxic to the cell. It also creates a stable osmotic environment by helping to retain water, which helps prevent osmotic lysis.

While the cell wall is rigid, it is still flexible and so it bends rather than holding a fixed shape due to its tensile strength. The rigidity of primary plant tissues is due to turgor pressure and not fromrigid cell walls. This is evident in plants that wilt since the stems and leaves begin to droop and in seaweed that bends in water currents. This proves that the cell wall is indeed flexible. The rigidity of healthy plants is due to a combination of cell wall construction and turgor pressure. The rigidity of the cell wall is also affected by the inflation of the cell contained. This inflation is a result of the passive uptake of water.

Cell rigidity can be increased by the presence of a second cell wall, which is a thicker additional layer of cellulose. This additional layer can be formed containing lignin in xylem cell walls or containing suberin in cork cell walls. These compounds are rigid and waterproof, making the secondary cell wall very stiff. Secondary cell walls are present in both wood and bark cells of trees.

The primary cell wall of most plant cells is semi-permeable so that small molecules and proteins are allowed passage into and out of the cell. Key nutrients, such as water and carbon dioxide, are distributed throughout the plant from cell wall to cell wall via apoplastic flow.

The major carbohydrates that make up the primary cell wall are cellulose, hemicellulose, and pectin. The secondary cell wall contains a wide range of additional compounds that modify their mechanical properties and permeability. Plant cell walls also contain numerous enzymes, such as hydrolases, esterases, peroxidases, and transglycosylases, that cut, trim, and cross-link wall polymers. The relative composition of carbohydrates, secondary compounds, and protein varies between plants and between the cell type and age.

There are up to three strata, or layers, that can be found in plant cell walls:

The middle lamella, which is a layer rich in pectins. This is the outermost layer that forms the interface between adjacent plant cells and keeps them together.

The primary cell wall is generally a thin, flexible layer that is formed when the cell is growing.

The secondary cell wall which is a thick layer that is formed inside the primary cell wall after the cell is fully grown. It is only found in some cell types.

Vacuole

The vacuole is essentially an enclosed compartment that is filled with water containing inorganic and organic molecules including various enzymes in solution. Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these vesicles. This organelle does not have a basic shape or size since its structure is determined by the needs of the cell. The functions of the vacuole in the plant cell include isolating materials that may be harmful to the cell, containing waste products, maintaining internal hydrostatic pressure within the cell, maintaining an acidic internal pH, containing small molecules, exporting unwanted substances from the cell, and allowing plants to support structures such as leaves and flowers. Vacuoles also play an important role in maintaining a balance between biogenesis and degradation of many substances and cell structures in the organism. Vacuoles aid in the destruction of invading bacteria or of misfolded proteins that are building up within the cell. They have the function of storing food and assist in the digestive and waste management process for the cell.

Most mature plant cells have a single large central vacuole that takes up approximately 30% of the cell's volume. It is surrounded by a membrane called the tonoplast, which is the cytoplasmic membrane separating the vacuolar contents from the cell's cytoplasm. It is involved in regulating the movements of ions around the cell, and isolating substances that may be harmful to the cell.

Other than storage, the main function of the central vacuole is to maintain turgor pressure against the cell wall. The proteins that are found in the tonoplast control the flow of water into and out of the vacuole through active trasnport, pumping potassium ions into and out of the vacuolar interior. Because of osmosis, water will flow into the vacuole placing pressure on the cell wall. If there is a significant amount of water loss, there is a decline in turgor pressure and the cell will plasmolyse. Turgor pressure exerted by the vacuole is required for cellular elongation as well as for supporting plants in the upright position. Another function of the vacuole is to push all contents of the cell's cytoplasm against the cellular membrane which helps keep the chloroplasts closer to light.

Plasmodesmata

Plasmodesmata are microscopic channels that traverse the cell walls of plant cells enabling the transport and communication between the cells. Plasmodesmata enable direct, regulated intercellular transport of substances between the cells. There are two forms of plasmodesmata, primary ones that form during cell division and secondary ones that form between mature cells. They are formed when a portion of the endoplasmic reticulum is trapped across the middle lamella as a new cell wall is laid down between two newly divided plant cells and this eventually becomes the cytoplasmic connection between the two cells. It is here that the cell wall is thickened no further and depressions or thin areas known as pits are formed in the walls. Pits usually pair up between adjacent cells.

Plasmodesmata are constructed of three main layers, the plasma membrane, the cytoplasmic sleeve, and the desmotubule. The plasma membrane part of the plasmodesmata is an extension of the cell membrane and it is similar in structure to the cellular phospholipid bilayers. The cytoplasmic sleeve is a fluid-filled space that is enclosed by the plasma membrane and is an extension of the cytosol. The trafficking of molecules and ions through the plasmodesmata occurs through this passage. Smaller molecules, such as sugars and amino acids, and ions can pass through the plasmodesmata via diffusion without the need for additional chemical energy. Proteins can also pass through the cytoplasmic sleeve but it is not yet known just how they are able to pass through. Finally, the desmotubule is a tube of compressed endoplasmic reticulum that runs between adjacent cells. There are some molecules that are known to pass through this tube but it is not the main route for plasmodesmatal transport.

The plasmodesmata have been shown to transport proteins, short interfering RNA, messenger RNA, and viral genomes from cell to cell. The size of the molecules that can pass through the plasmodesmata is determined by the size exclusion limit. This limit is highly variable and is subject to active modification. There have been several models that have been proposed for the active transport through the plasmodesmata. One suggestion is that such transport is mediated by the interactions with proteins that are localized on the desmotubule, and/or by chaperones partially unfolding proteins which allows them to fit through the narrow passage.

Plastid

Plastids are the site of manufacture and storage of important chemical compounds that are used by the cell. They often contain pigments used in photosynthesis and the types of pigments present can change or determine the color of the cell. Plastids are responsible for photosynthesis, storage of products like starch, and the ability to differentiate between these and other forms. All plastids can be traced back to proplastids, which happen to be present in the meristematic regions of the plant. In plants, plastids may differentiate into several forms depending on what function they need to play in the cell. Undifferentiated plastids, the proplastids, can develop into the following types of plastids:

•Chloroplasts: for photosynthesis

•Chromoplasts: for pigment synthesis and storage

•Leucoplasts: for monoterpene synthesis

Chloroplasts are the organelles that conduct photosynthesis. They capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH. They are observed as flat discs usually 2 to 10 micrometers in diameter and 1 micrometer thick. The chloroplast is contained by an envelope that consists of an inner and outer phospholipid membrane. Between these layers is the intermembrane space. The material within the chloroplast is called the stroma and it contains many molecules of small, circular DNA (though it is often found in branched linear form, such as in corn). Within the stroma are stacks of thylakoids, which are the site of photosynthesis. The thylakoids are arranged in stacks called grana. A thylakoid has a flattened disk shape and has an empty space called the thylakoid space or lumen. The process of photosynthesis takes place on the thylakoid membrane. Embedded in the thylakoid membrane are antenna complexes that consist of the light-absorbing pigments, such as chlorophyll and carotenoids, as well as the proteins that bind the pigments. These complexes increase the surface area for light capture and allows the capture of photons with a wider range of wavelengths. The energy of the incident photons is absorbed by the pigments and funneled to the reaction center of the complex through resonance energy transfer. From there, two chlorophyll molecules are ionized, which produces an excited electron which passes on to the photochemical reaction center.

Chromoplasts are responsible for pigment synthesis and storage. They are found in the colored organs of plants such as fruit and floral petals, to which they give their distinctive colors. This is always associated with a massive increase in the accumulation of carotenoid pigments. Chromoplasts synthesize and store pigments such as orange carotene, yellow xanthophylls and various other red pigments. The most probably main evolutionary role of chromoplasts is to act as an attractant for pollinating animals or for seed dispersal via the eating of colored fruits. They allow for the accumulation of large quantities of water-insoluble compounds in otherwise watery parts of plants. In chloroplasts, some carotenoids are used as accessory pigments in the process of photosynthesis where they act to increase the efficiency of chlorophyll in harvesting light energy. When leaves change color during autumn, it is because of the loss of green chlorophyll unmasking these carotenoids that are already present in the leaves. The term "chromoplast" is used to include any plastid that has pigment, mainly to emphasize the contrast with leucoplasts which are plastids that have no pigments.

Leucoplasts lack pigments and so they are not green. They are located in the roots and non-photosynthetic tissues of plants. They can become specialized for bulk storage of starch, lipid, or protein and are then known as amyloplasts, elaioplasts, or proteinoplasts, respectively. In many cell types, though, leucoplasts do not have a major storage function and are present to provide a wide range of essential biosynthetic functions, including the synthesis of fatty acids, many amino acids, and tetrapyrrole compounds such as haem. Extensive networks of stromules interconnecting leucoplasts have been observed in epidermal cells of roots, hypocotyls, and petals.