organelle sutructure and function

           Both prokaryotic and eukaryotic cells have a plasma membrane , a double layer of lipids that separates the cell interior form the outside environment . this double layer consists largely of specialized lipids called phospholipids .The plasma membrane is the border between the interior and     exterior of a cell. As such, it controls passage of various     molecules—including sugars, amino acids, ions, and water—into and out of the cell. The cell surface area of the plasma membrane limits the exchange of materials between a cell and its environment . it is where the cell is identified by surrounding cells . 

 In eukaryotic cells, which have a nucleus, the cytoplasm is everything between the plasma membrane and the nuclear envelope. In prokaryotes, which lack a nucleus, cytoplasm simply means everything found inside the plasma membrane .The  major component of the cytoplasm in both prokaryotes and eukaryotes is the gel-like cytosol, a water-based solution that contains ions, small molecules, and macromolecules .The cytosol contains a rich broth of macromolecules and smaller organic molecules, including glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, and fatty acids. Ions of sodium, potassium, calcium, and other elements are also found in the cytosol. Many metabolic reactions, including protein synthesis, take place in this part of the cell.

The nucleus houses the cell’s genetic material, or DNA, and is also the site of synthesis for ribosomes, the cellular machines that assemble proteins. Inside the nucleus, chromatin is stored in a gel-like substance called nucleoplasm.   Enclosing the nucleoplasm is the nuclear envelope, which is made up of two layers of membrane: an outer membrane and an inner membrane. Each of these membranes contains two layers of phopholipids, arranged with their tails pointing inward. There’s a thin space between the two layers of the nuclear envelope, and this space is directly connected to the endoplasmic reticulum. 

Nuclear pores are the  small channels that span the nuclear envelope, let substances enter and exit the nucleus. Each pore is lined by a set of proteins, called the nuclear pore complex, that control what molecules can go in or out .The Nucleolus  the site in which new ribosomes are assembled.

Chromosomes are only visible as distinct structures when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, the chromosomes instead resemble an unwound, jumbled bunch of threads. In this form, the DNA is accessible to the enzymes that transcribe it into RNA, allowing the genetic information to be put to use. In both their loose and compact forms, the DNA strands of chromosomes are bound to structural proteins, including a family of proteins called histones . These DNA-associated proteins organize the DNA and help it fit into the nucleus, and they also play a role in determining which genes are active or inactive. The complex formed by DNA and its supporting structural proteins is known as chromatin.

The endoplasmic reticulum (ER) plays a key role in the modification of proteins and the synthesis of lipids. It consists of a network of membranous tubules and flattened sacs. The discs and tubules of the ER are hollow, and the space inside is called the lumen.

The rough endoplasmic reticulum  gets its name from the bumpy ribosomes attached to its cytoplasmic surface. As these ribosomes make proteins, they feed the newly forming protein chains into the lumen. Some are transferred fully into the ER and float inside, while others are anchored in the membrane.   Inside the ER, the proteins fold and undergo modifications, such as the addition of carbohydrate side chains. These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles—or secreted from the cell. If the modified proteins are not destined to stay in the ER, they will be packaged into vesicles, or small spheres of membrane that are used for transport, and shipped to the Golgi apparatus. The rough ER also makes phospholipids for other cellular membranes, which are transported when the vesicle forms.

The smooth endoplasmic reticulum (smooth ER) is continuous with the rough ER but has few or no ribosomes on its cytoplasmic surface. the functions of the smooth ER include:

• Synthesis of carbohydrates, lipids, and steroid hormones
• Detoxification of medications and poisons
• Storage of calcium ions

In muscle cells, a special type of smooth ER called the sarcoplasmic reticulum is responsible for storage of calcium ions that are needed to trigger the coordinated contractions of the muscle cells.

When vesicles bud off from the ER, where do they go? the lipids and proteins in the transport vesicles need to be sorted, packaged, and tagged so that they wind up in the right place.This sorting, tagging, packaging, and distribution takes place in the Golgi apparatus, an organelle made up of flattened discs of membraneAs proteins and lipids travel through the Golgi, they undergo further modifications. Short chains of sugar molecules might be added or removed, or phosphate groups attached as tags .  Finally, the modified proteins are sorted and packaged into vesicles . Some of these vesicles deliver their contents to other parts of the cell where they will be used, such as the lysosome or vacuole. Others fuse with the plasma membrane, delivering membrane-anchored proteins that function there and releasing secreted proteins outside the cell.   Cells that secrete many proteins,such as salivary gland cells that secrete digestive enzymes, or cells of the immune system that secrete antibodies,have many Golgi stacks. In plant cells, the Golgi apparatus also makes polysaccharides,some of which are incorporated into the cell wall.

The lysosome is an organelle that contains digestive enzymes and acts as the organelle-recycling facility of an animal cell. It breaks down old and unnecessary structures so their molecules can be reused. Lysosomes are part of the endomembrane system, and some vesicles that leave the Golgi are bound for the lysosome.

Mitochondria (singular, mitochondrion) are often called the powerhouses or energy factories of the cell. Their job is to make a steady supply of adenosine triphosphate (ATP), the cell’s main energy-carrying molecule . The mitochondria are suspended in the jelly-like cytosol of the cell. They are oval-shaped and have two membranes: an outer one, surrounding the whole organelle, and an inner one, with many inward protrusions called cristae that increase surface area.  The space between the membranes is called  the intermembrane space, and the compartment enclosed by the inner membrane is called the mitochondrial matrix. The matrix contains mitochondrial DNA and ribosomes. Although mitochondria are found in most human cell types (as well as most cell types in other animals and plants), their numbers vary depending on the role of the cell and its energy needs. For instance, muscle cells typically have high energy needs and large numbers of mitochondria, while red blood cells, which are highly specialized for oxygen transport, have no mitochondria at all.

Chloroplasts are found only in plants and photosynthetic algae.  The chloroplast's job is to carry out a process called photosynthesis. In photosynthesis, light energy is collected and used to build sugars from carbon dioxide. The sugars produced in photosynthesis may be used by the plant cell, or may be consumed by animals that eat the plant, such as humans. The energy contained in these sugars is harvested through a process called cellular respiration. Chloroplasts are disc-shaped organelles found in the cytosol of a cell. They have outer and inner membranes with an intermembrane space between them. It contains membrane discs known as thylakoids, arranged in interconnected stacks called grana (singular, granum).

 A microtubule is made up of tubulin proteins arranged to form a hollow, straw-like tube, and each tubulin protein consists of two subunits, α-tubulin and β-tubulin with a diameter of about 25 nm . Microtubules, like actin filaments, are dynamic structures: they can grow and shrink quickly by the addition or removal of tubulin proteins. Also similar to actin filaments, microtubules have directionality, meaning that they have two ends that are structurally different from one another. In a cell, microtubules play an important structural role, helping the cell resist compression forces. For the structural support, microtubules play a variety of more specialized roles in a cell. For instance, they provide tracks for motor proteins called kinesins and dyneins, which transport vesicles and other cargoes around the interior of the cell

start superscript, 4, end supeDuring cell division, microtubules assemble into a structure called the spindle, which pulls the chromosomes apart.

A centriole is a cylinder of nine triplets of microtubules, held together by supporting proteins. Centrioles are best known for their role in centrosomes, structures that act as microtubule organizing centers in animal cells. A centrosome consists of two centrioles oriented at right angles to each osurrounded by a mass of "pericentriolar material," which provides anchoring sites for the microtubules .The centrosome is duplicated before a cell divides, and the paired centrosomes seem to play a role in organizing the microtubules that separate chromosomes during cell division. However, the exact function of the centrioles in this process still isn’t clear. Cells with their centrosome removed can still divide, and plant cells, which lack centrosomes, divide just fine.start superscript, 8, end superscrThe centrosome is duplicated before a cell divides, and the paired centrosomes seem to play a role in organizing the microtubules that separate chromosomes during cell division. However, the exact function of the centrioles in this process still isn’t clear. Cells with their centrosome removed can still divide, and plant cells, which lack centrosomes, divide .The centrosome is duplicated before a cell divides, and the paired centrosomes seem to play a role in organizing the microtubules that separate chromosomes during cell division. However, the exact function of the centrioles in this process still isn’t clear. Cells with their centrosome removed can still divide, and plant cells, which lack centrosomes, divide just fine.

 Ribosomes are the molecular machines responsible for protein synthesis. A ribosome is made out of RNA and proteins, and each ribosome consists of two separate RNA-protein complexes, known as the small and large subunits. The large subunit sits on top of the small subunit, with an RNA template sandwiched between the two.   In eukaryotes, ribosomes get their orders for protein synthesis from the nucleus, where portions of DNA (genes) are transcribed to make messenger RNAs (mRNAs). An mRNA travels to the ribosome, which uses the information it contains to build a protein with a specific amino acid sequence. This process is called translation. Prokaryotes lack a nucleus, so their mRNAs are transcribed in the cytoplasm and can be translated by ribosomes immediately. Because protein synthesis is an essential function of all cells, ribosomes are found in practically every cell type of multicellular organisms, as well as in prokaryotes such as bacteria. However, eukaryotic cells that specialize in producing proteins have particularly large numbers of ribosomes. For example, the pancreas is responsible for producing and secreting large amounts of digestive enzymes, so the pancreatic cells that make these enzymes have an unusually high number of ribosomes.

Plant cells, surrounded as they are by cell walls, don’t contact one another through wide stretches of plasma membrane the way animal cells can. However, they do have specialized junctions called plasmodesmata (singular, plasmodesma), places where a hole is punched in the cell wall to allow direct cytoplasmic exchange between two cells .  Plasmodesmata are lined with plasma membrane that is continuous with the membranes of the two cells. Each plasmodesma has a thread of cytoplasm extending through it, containing an even thinner thread of endoplasmic reticulum.

Plants cells do not have lysosomes. Instead, they have another type of organelle called the vacuole. The large central vacuole stores water and wastes wastes, isolates hazardous materials, and has enzymes that can break down macromolecules and cellular components, like those of a lysosome.Plant vacuoles also function in water balance and may be used to store compounds such as toxins and pigments.

Though plants don't make collagen, they have their own type of supportive extracellular structure: the cell wall. The cell wall is a rigid covering that surrounds the cell, protecting it and giving it support and shape.The major organic molecule of the plant cell wall is cellulose, a polysaccharide composed of glucose units. Cellulose assembles into fibers called microfibrils. Most plant cell walls contain a variety of different polysaccharides and proteins. In addition to cellulose, other polysaccharides commonly found in the plant cell wall include hemicellulose and pectin, shown in the diagram above. The middle lamella is a sticky layer that helps hold the cell walls of adjacent plant cells together.

Though plants don't make collagen, they have their own type of supportive extracellular structure: the cell wall. The cell wall is a rigid covering that surrounds the cell, protecting it and giving it support and shape.The major organic molecule of the plant cell wall is cellulose, a polysaccharide composed of glucose units. Cellulose assembles into fibers called microfibrils. Most plant cell walls contain a variety of different polysaccharides and proteins. In addition to cellulose, other polysaccharides commonly found in the plant cell wall include hemicellulose and pectin, shown in the diagram above. The middle lamella is a sticky layer that helps hold the cell walls of adjacent plant cells together.