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this is a site on remodeling organelles

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Organelle remodeling at membrane contact sites

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Cellular organelles must execute sophisticated biological processes to persist, and often communicate with one another to exchange metabolites and inf...

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describes the cell structure

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All living organisms on Earth are divided into cells. The main concept of cell theory is that cells are the basic structural unit for all organisms. Cells are small compartments that hold the biological equipment necessary to keep an organism alive and successful. Living things may be single-celled or they may be very complex such as a human being.

There are smaller pieces that make up cells such as macromolecules and organelles. A protein is an example of a macromolecule while a mitochondrion is an example of an organelle. Cells can also connect to form larger structures. They might group together to form the tissues of the stomach and eventually the entire digestive system. However, in the same way that atoms are the basic unit when you study matter, cells are the basic unit for biology and organisms.

In larger organisms, the main purpose of a cell is to organize. Cells hold a variety of pieces and each cell type has a different purpose. By dividing responsibilities among different groups of cells, it is easier for an organism to survive and grow.

If you were only made of one cell, you would be very limited. You don't find single cells that are as large as a cow. Cells have problems functioning when they get too big. Also, if you were only one cell you couldn't have a nervous system, no muscles for movement, and using the internet would be out of the question. The trillions of cells in your body make your way of life possible.

There are many types of cells. In biology class, you will usually work with plant-like cells and animal-like cells. We say "animal-like" because an animal type of cell could be anything from a tiny microorganism to a nerve cell in your brain. Biology classes often take out a microscope and look at single-celled microbes from pond water. You might see hydra, amoebas, or euglena.

Plant cells are easier to identify because they have a protective structure called a cell wall made of cellulose. Plants have the wall; animals do not. Plants also have organelles such as the green chloroplast or large, water-filled vacuoles. Chloroplasts are the key structure in the process of photosynthesis.

Cells are unique to each type of organism. If you look at very simple organisms, you will discover cells that have no defined nucleus ( prokaryotes) and other cells that have hundreds of nuclei ( multinucleated).

Humans have hundreds of different cell types. You have red blood cells that are used to carry oxygen (O ₂) through the body and other cells specific to your heart muscle. Even though cells can be very different, they are basically compartments surrounded by some type of membrane.

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Biology4Kids.com! This tutorial introduces cell structure. Other sections include plants, animal systems, invertebrates, vertebrates, and microorganisms.

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toms wife the animal cell

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An insightful padlet

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this is a picture tom the mitochondria

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a video of cells organelles and their functions

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Cell Organelles And Their Function Animation (BOTH 3D AND MICROSCOPIC VIEWS )

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Shows every organelle and tell everything needed to know bout them

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Supports and protects cell organelles -Animal and plant cell -Smooth ER: produces the lipids and hormones-Rough ER: transports proteins ou...

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this infographic shows you the different parts that make up a prokaryotic cell.

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a photo of cells organelles and information about it

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This infographic shows the cell organelles.

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It has photos of the types of organelles

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An infographic describing an animal cell

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A website that teaches all bout the organelles and all their functions

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A chart describing eukaryotic plant cells.

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What is a cell

Right now your body is doing a million things at once. It's sending electrical impulses, pumping blood, filtering urine, digesting food, making protein, storing fat, and that's just the stuff you're not thinking about! You can do all this because you are made of cells - tiny units of life that are like specialized factories, full of machinery designed to accomplish the business of life. Cells make up every living thing, from blue whales to the archaebacteria that live inside volcanos. Just like the organisms they make up, cells can come in all shapes and sizes. Nerve cells in giant squids can reach up to 12m [39 ft] in length, while human eggs (the largest human cells) are about 0.1mm across. Plant cells have protective walls made of cellulose (which also makes up the strings in celery that make it so hard to eat) while fungal cell walls are made from the same stuff as lobster shells. However, despite this vast range in size, shape, and function, all these little factories have the same basic machinery.

There are two main types of cells, prokaryotic and eukaryotic. Prokaryotes are cells that do not have membrane bound nuclei, whereas eukaryotes do. The rest of our discussion will strictly be on eukaryotes. Think about what a factory needs in order to function effectively. At its most basic, a factory needs a building, a product, and a way to make that product. All cells have membranes (the building), DNA (the various blueprints), and ribosomes (the production line), and so are able to make proteins (the product - let's say we're making toys). This article will focus on eukaryotes, since they are the cell type that contains organelles.

What's found inside a cell

An organelle (think of it as a cell's internal organ) is a membrane bound structure found within a cell. Just like cells have membranes to hold everything in, these mini-organs are also bound in a double layer of phospholipids to insulate their little compartments within the larger cells. You can think of organelles as smaller rooms within the factory, with specialized conditions to help these rooms carry out their specific task (like a break room stocked with goodies or a research room with cool gadgets and a special air filter). These organelles are found in the cytoplasm, a viscous liquid found within the cell membrane that houses the organelles and is the location of most of the action happening in a cell. Below is a table of the organelles found in the basic human cell, which we'll be using as our template for this discussion.

Our DNA has the blueprints for every protein in our body, all packaged into a neat double helix. The processes to transform DNA into proteins are known as transcription and translation, and happen in different compartments within the cell. The first step, transcription, happens in the nucleus, which holds our DNA. A membrane called the nuclear envelope surrounds the nucleus, and its job is to create a room within the cell to both protect the genetic information and to house all the molecules that are involved in processing and protecting that info. This membrane is actually a set of two lipid bilayers, so there are four sheets of lipids separating the inside of the nucleus from the cytoplasm. The space between the two bilayers is known as the perinuclear space.

Though part of the function of the nucleus is to separate the DNA from the rest of the cell, molecules must still be able to move in and out (e.g., RNA). Proteins channels known as nuclear pores form holes in the nuclear envelope. The nucleus itself is filled with liquid (called nucleoplasm) and is similar in structure and function to cytoplasm. It is here within the nucleoplasm where chromosomes (tightly packed strands of DNA containing all our blueprints) are found.

A nucleus has interesting implications for how a cell responds to its environment. Thanks to the added protection of the nuclear envelope, the DNA is a little bit more secure from enzymes, pathogens, and potentially harmful products of fat and protein metabolism. Since this is the only permanent copy of the instructions the cell has, it is very important to keep the DNA in good condition. If the DNA was not sequestered away, it would be vulnerable to damage by the aforementioned dangers, which would then lead to defective protein production. Imagine a giant hole or coffee stain in the blueprint for your toy - all of a sudden you don't have either enough or the right information to make a critical piece of the toy.

The nuclear envelope also keeps molecules responsible for DNA transcription and repair close to the DNA itself - otherwise those molecules would diffuse across the entire cell and it would take a lot more work and luck to get anything done! While transcription (making a complementary strand of RNA from DNA) is completed within the nucleus, translation (making protein from RNA instructions) takes place in the cytoplasm. If there was no barrier between the transcription and translation machineries, it's possible that poorly-made or unfinished RNA would get turned into poorly made and potentially dangerous proteins. Before an RNA can exit the nucleus to be translated, it must get special modifications, in the form of a cap and tail at either end of the molecule, that act as a stamp of approval to let the cell know this piece of RNA is complete and properly made.

Within the nucleus is a small subspace known as the nucleolus. It is not bound by a membrane, so it is not an organelle. This space forms near the part of DNA with instructions for making ribosomes, the molecules responsible for making proteins. Ribosomes are assembled in the nucleolus, and exit the nucleus with nuclear pores. In our analogy, the robots making our product are made in a special corner of the blueprint room, before being released to the factory.

Endoplasmic Reticulum

Endoplasmic means inside (endo) the cytoplasm (plasm). Reticulum comes from the Latin word for net. Basically, an endoplasmic reticulum is a plasma membrane found inside the cell that folds in on itself to create an internal space known as the lumen. This lumen is actually continuous with the perinuclear space, so we know the endoplasmic reticulum is attached to the nuclear envelope. There are actually two different endoplasmic reticuli in a cell: the smooth endoplasmic reticulum and the rough endoplasmic reticulum. The rough endoplasmic reticulum is the site of protein production (where we make our major product - the toy) while the smooth endoplasmic reticulum is where lipids (fats) are made (accessories for the toy, but not the central product of the factory).

Rough Endoplasmic Reticulum

The rough endoplasmic reticulum is so-called because its surface is studded with ribosomes, the molecules in charge of protein production. When a ribosome finds a specific RNA segment, that segment may tell the ribosome to travel to the rough endoplasmic reticulum and embed itself. The protein created from this segment will find itself inside the lumen of the rough endoplasmic reticulum, where it folds and is tagged with a (usually carbohydrate) molecule in a process known as glycosylation that marks the protein for transport to the Golgi apparatus. The rough endoplasmic reticulum is continuous with the nuclear envelope, and looks like a series of canals near the nucleus. Proteins made in the rough endoplasmic reticulum as destined to either be a part of a membrane, or to be secreted from the cell membrane out of the cell. Without an rough endoplasmic reticulum, it would be a lot harder to distinguish between proteins that should leave the cell, and proteins that should remain. Thus, the rough endoplasmic reticulum helps cells specialize and allows for greater complexity in the organism.

Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum makes lipids and steroids, instead of being involved in protein synthesis. These are fat-based molecules that are important in energy storage, membrane structure, and communication (steroids can act as hormones). The smooth endoplasmic reticulum is also responsible for detoxifying the cell. It is more tubular than the rough endoplasmic reticulum, and is not necessarily continuous with the nuclear envelope. Every cell has a smooth endoplasmic reticulum, but the amount will vary with cell function. For example, the liver, which is responsible for most of the body's detoxification, has a larger amount of smooth endoplasmic reticulum.

Golgi apparatus (aka Golgi body aka Golgi)

We mentioned the Golgi apparatus earlier when we discussed the production of proteins in the rough endoplasmic reticulum. If the smooth and rough endoplasmic reticula are how we make our product, the Golgi is the mailroom that sends our product to customers . It is responsible for packing proteins from the rough endoplasmic reticulum into membrane-bound vesicles (tiny compartments of lipid bilayer that store molecules) which then translocate to the cell membrane. At the cell membrane, the vesicles can fuse with the larger lipid bilayer, causing the vesicle contents to either become part of the cell membrane or be released to the outside.

Different molecules actually have different fates upon entering the Golgi. This determination is done by tagging the proteins with special sugar molecules that act as a shipping label for the protein. The shipping department identifies the molecule and sets it on one of 4 paths:

1. Cytosol: the proteins that enter the Golgi by mistake are sent back into the cytosol (imagine the barcode scanning wrong and the item being returned).

2. Cell membrane: proteins destined for the cell membrane are processed continuously. Once the vesicle is made, it moves to the cell membrane and fuses with it. Molecules in this pathway are often protein channels which allow molecules into or out of the cell, or cell identifiers which project into the extracellular space and act like a name tag for the cell.

3. Secretion: some proteins are meant to be secreted from the cell to act on other parts of the body. Before these vesicles can fuse with the cell membrane, they must accumulate in number, and require a special chemical signal to be released. This way shipments only go out if they're worth the cost of sending them (you generally wouldn't ship just one toy and expect to profit).

4. Lysosome: The final destination for proteins coming through the Golgi is the lysosome. Vesicles sent to this acidic organelle contain enzymes that will hydrolyze the lysosome's content.

The lysosome is the cell's recycling center. These organelles are spheres full of enzymes ready to hydrolyze (chop up the chemical bonds of) whatever substance crosses the membrane, so the cell can reuse the raw material. These disposal enzymes only function properly in environments with a pH of 5, two orders of magnitude more acidic than the cell's internal pH of 7. Lysosomal proteins only being active in an acidic environment acts as safety mechanism for the rest of the cell - if the lysosome were to somehow leak or burst, the degradative enzymes would inactivate before they chopped up proteins the cell still needed.

Peroxisome

Like the lysosome, the peroxisome is a spherical organelle responsible for destroying its contents. Unlike the lysosome, which mostly degrades proteins, the peroxisome is the site of fatty acid breakdown. It also protects the cell from reactive oxygen species (ROS) molecules which could seriously damage the cell. ROSs are molecules like oxygen ions or peroxides that are created as a byproduct of normal cellular metabolism, but also by radiation, tobacco, and drugs. They cause what is known as oxidative stress in the cell by reacting with and damaging DNA and lipid-based molecules like cell membranes. These ROSs are the reason we need antioxidants in our diet.

Mitochondria

Just like a factory can't run without electricity, a cell can't run without energy. ATP (adenosine triphosphate) is the energy currency of the cell, and is produced in a process known as cellular respiration. Though the process begins in the cytoplasm, the bulk of the energy produced comes from later steps that take place in the mitochondria.

Like we saw with the nuclear envelope, there are actually two lipid bilayers that separate the mitochondrial contents from the cytoplasm. We refer to them as the inner and outer mitochondrial membranes. If we cross both membranes we end up in the matrix, where pyruvate is sent after it is created from the breakdown of glucose (this is step 1 of cellular respiration, known as glycolysis).The space between the two membranes is called the intermembrane space, and it has a low pH (is acidic) because the electron transport chain embedded in the inner membrane pumps protons (H+) into it. Energy to make ATP comes from protons moving back into the matrix down their gradient from the intermembrane space.

Mitochondria are also somewhat unique in that they are self-replicating and have their own DNA, almost as if they were a completely separate cell. The prevailing theory, known as the endosymbiotic theory, is that eukaryotes were first formed by large prokaryotic cells engulfing smaller cells that looked a lot like mitochondria (and chloroplasts, more on them later). Instead of being digested, the engulfed cells remained intact and the arrangement turned out to be advantageous to both cells, which created a symbiotic relationship.

So far we've discussed organelles, the membrane-bound structures within a cell that have some sort of specialized function. Now let's take a moment to talk about the scaffolding that's holding all of this in place - the walls and beams of our factory.

Cytoskeleton

Within the cytoplasm there is network of protein fibers known as the cytoskeleton. This structure is responsible for both cell movement and stability. The major components of the cytoskeleton are microtubules, intermediate filaments, and microfilaments.

Microtubules

Microtubules are small tubes made from the protein tubulin. These tubules are found in cilia and flagella, structures involved in cell movement. They also help provide pathways for secretory vesicles to move through the cell, and are even involved in cell division as they are a part of the mitotic spindle, which pulls homologous chromosomes apart.

Intermediate Filaments

Smaller than the microtubules, but larger than the microfilaments, the intermediate filaments are made of a variety of proteins such as keratin and/or neurofilament. They are very stable, and help provide structure to the nuclear envelope and anchor organelles.

Microfilaments

Microfilaments are the thinnest part of the cytoskeleton, and are made of actin [a highly-conserved protein that is actually the most abundant protein in most eukaryotic cells]. Actin is both flexible and strong, making it a useful protein in cell movement. In the heart, contraction is mediated through an actin-myosin system.

Plants and Platelets

So far we've covered basic organelles found in a eukaryotic cell. However, not every cell has each of these organelles, and some cells have organelles we haven't discussed. For example, plant cells have chloroplasts, organelles that resemble mitochondria and are responsible for turning sunlight into useful energy for the cell (this is like factories that are powered by energy they collect via solar panels). On the other hand, platelets, blood cells responsible for clotting, have no nucleus and are in fact just fragments of cytoplasm contained within a cell membrane.

Eukaryotes vs Bacteria vs Archaea

It is also important to keep in mind that organelles are found only in eukaryotes, one of the three major cell divisions. The other two major divisions, Bacteria and Archaea are known as prokaryotes, and have no membrane bound organelles within.

Consider the Following:

Some diseases can be traced back to organelle lack / malformation. For example, inclusion-cell (I-cell) disease occurs due to a defect in the Golgi. In order to mark enzymes that should be sent to lysosomes to help degrade unwanted molecules, the Golgi has to bind them with a mannose 6-phosphate tag, like a shipping label. However, in patients with I-cell disease, one of the proteins that make this tag is mutated, and cannot do its job, like a broken label machine. This means that proteins cannot be targeted to lysosomes. These untagged proteins are the enzymes that are responsible for chopping up other proteins. What happens is the inactivated enzymes end up being sent outside the cell, while lysosomes clog up with undigested material. This disease is congenital, and usually fatal before patients reach 7 years of age.

An interesting idea is that mitochondria can be used to trace maternal ancestry. Since mitochondria are self-replicating and have their own DNA, they are not determined by the genes found in the nucleus. Instead, your mitochondria have developed from the mitochondria present in the female ovum (egg) that you developed from. Defects in mitochondrial DNA cause hereditary diseases that pass only from mother to children.

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Cellular organelles and structure (article) | Khan Academy

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Learn for free about math, art, computer programming, economics, physics, chemistry, biology, medicine, finance, history, and more. Khan Academy is a nonprofit with the mission of providing a free, world-class education for anyone, anywhere.

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click to see the many different eukaryotic species

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How Many Species Are On Earth? | Visual.ly

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This infographic communicates a prediction of the number of eukaryotic species on Earth in relation to the number of known species.

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a photo of a plant cell and the organelles inside

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This padlet shows the different organelles and their functions.

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A website on prokaryotes and their viruses.

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this news article is about organelles being discovered

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Super-resolution microscopy reveals surprising details of endoplasmic reticulum's architecture

Textbook drawings of the cell's largest organelle might need to be updated based on new images. Super-resolution shots of the endoplasmic reticulum reveal tightly packed tubes where previous pictures showed plain flat sheets, scientists report in the Oct. 28 Science.

The finding helps explain how the endoplasmic reticulum, or ER, reshapes itself in response to changing conditions, says study coauthor Jennifer Lippincott-Schwartz, a cell biologist at the Howard Hughes Medical Institute's Janelia Research Campus in Ashburn, Va.

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Scientists need to redraw picture of cell's biggest organelle

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Super-resolution microscopy reveals surprising details of endoplasmic reticulum's architecture Textbook drawings of the cell's largest organelle might need to be updated based on new images. Super-resolution shots of the endoplasmic reticulum reveal tightly packed tubes where previous pictures showed plain flat sheets, scientists report in the Oct. 28 Science.

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cell organelles fictions of the organelle 

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cell organelles fictions of the organelle 

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this news article talks about cell organelle

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This video gives the definition and functions of organelles.

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Definition of an Organelle

Living things are made of cells, or biological units also referred to as the building blocks of life. Some organisms consist of just one cell, while others are made of trillions of cells. Each one of these cells functions as a tiny factory, with individual parts that work together to keep the cell alive and, in turn, keep the organism going. These parts are called organelles.

Organelles and Their Functions

The nucleus is arguably the most important organelle in the cell. It is the control center, telling all of the other organelles what to do and when to do it.

The nucleus also contains all of the cell's genetic material, or its DNA. This material has all the instructions the cell needs for making proteins and many other important molecules.

The nucleus is surrounded by two membranes. These membranes have many openings in them, which allow for the transport of materials into and out of the nucleus. RNA, proteins, and other molecules move out of the nucleus into the rest of the cell.

The cell's chromosomes are also found in the nucleus. These condensed strands of DNA are what carry all of the cell's genetic information.

When the cell is in its growing phase, the DNA is elongated into very thin, hair-like structures. When the cell is getting ready to divide, the DNA condenses into chromosomes and the material gets duplicated.

Within the nucleus of eukaryotic cells is a structure called the nucleolus. This is the site of ribosome formation.

Prokaryotic cells lack a nucleus. In these organisms (which include the bacteria), the genetic material is free-floating within the cell membrane. The genetic material of prokaryotes is a different shape than that of eukaryotes, but it serves the same function.

Outside of the nucleus but within the cell membrane is a gel-like substance called cytoplasm (also called cytosol). It is made mostly of water and dissolved salts. It bathes the organelles and keeps them healthy. It is also the medium through which materials move about the cell.

Within the cytoplasm is a network of tiny tubes called the cytoskeleton. These tubes are used to give the cell structure and also to support the organelles by holding them into place. Some parts of the cytoskeleton also work to transport certain things between different parts of the cell.

The cytoskeleton is made from two different components: microtubules and microfilaments. Microtubules are hollow and made of protein. They are very important in helping to maintain the cell's shape. They are also important during cell division. When this occurs, the microtubules form structures called spindle fibers, which allow the chromosomes to separate. Microtubules are also part of the different projections coming off of the cell surface. These structures, called cilia and flagella, assist with cell locomotion.

Microfilaments are very thin threads made of protein. Like the microtubules, they help the cell keep its shape.

Making proteins is a very important job for a cell. Ribosomes are small pieces of RNA found throughout the cytoplasm and on some other organelles. Their only job is to assemble proteins.

DNA coding tells them which proteins to make. Prokaryotic cells can have tens of thousands of ribosomes. Eukaryotic cells can have hundreds of thousands, if not millions of them, all making proteins.

Once proteins are made, they need to be moved to different parts of the cell. The endoplasmic reticulum (E.R.) is a collection of lipid membranes that work to move the proteins from one area of the cell to another.

The E.R. comes in two different forms - rough and smooth. Rough E.R. has ribosomes attached to it. These ribosomes make proteins that are scheduled to leave the cell. The E.R. transports these proteins to another organelle that will package them up and ship them out. The smooth E.R. does not have any ribosomes on its surface. This is where the lipid part of the cell membrane is assembled.

Once the ribosomes on the E.R. have made the proteins for exporting from the cell, they need to be packaged in such a way that they can leave the cell and be taken in by the parts of the body that need them. The organelle responsible for this is called the Golgi apparatus (or Golgi bodies), which looks like a stack of pancakes. The Golgi apparatus changes, sorts and packages the proteins as they leave the E.R. so they are ready to leave the cell. Because it is so important to shipping things out of the cell, the Golgi apparatus is always found near the cell membrane.

Cell parts wear out and die. These pieces of 'garbage' need to be disposed of somehow. That is the job of the lysosomes. These structures are filled with digestive enzymes that break down those items that would become toxic if they were left in the cell. Lysosomes also break down proteins, lipids and carbohydrates into more usable forms for the cell.

Living things need energy in order to perform daily functions. The mighty mitochondria are the organelles that make energy by converting food into more usable forms for the cell.

The mitochondria have a double membrane, just like the nucleus. The inner membrane is folded into many twists and turns that increase the surface area where energy can be created.

In a cell, storage of materials is carried out by vacuoles and vesicles. These structures are large, sac-like and enclosed by membranes.

Vacuoles store water, proteins, carbohydrates and salts. In those creatures that live in the water, the vacuole is the way the amount of water coming and going from its body is controlled.

Vesicles are smaller structures found in eukaryotic cells. They store and move things between the different organelles, as well as between the cell and its outside environment.

How Plant Cells Are Different

Plants are eukaryotic, just like animals. In this regard, they have all the aforementioned organelles. However, because plants have some different characteristics and processes than other eukaryotes, their cells contain a few additional organelles.

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organelles in cells: definition & functions - video & lesson transcript | study.com

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Organelles in Cells: Definition & Functions - Video & Lesson Transcript | Study.com

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Living things are made of cells, or biological units also referred to as the building blocks of life. Some organisms consist of just one cell, while others are made of trillions of cells. Each one of these cells functions as a tiny factory, with individual parts that work together to keep the cell alive and, in turn, keep the organism going.

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The differences between a prokaryote and eukaryotes

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A padlet page describing prokaryotes.

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This image shows a cell organelle diagram.

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A website telling the definition of eukaryotic.

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Eukaryotic definition, any organism having as its fundamental structural unit a cell type that contains specialized organelles in the cytoplasm, a membrane-bound nucleus enclosing genetic material organized into chromosomes, and an elaborate system of division by mitosis or meiosis, characteristic of all life forms except bacteria, blue-green algae, and other primitive microorganisms. See more.

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A news article on the redrawing of the cells largest organelle.

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Super-resolution microscopy reveals surprising details of endoplasmic reticulum's architecture

Textbook drawings of the cell's largest organelle might need to be updated based on new images. Super-resolution shots of the endoplasmic reticulum reveal tightly packed tubes where previous pictures showed plain flat sheets, scientists report in the Oct. 28 Science.

The finding helps explain how the endoplasmic reticulum, or ER, reshapes itself in response to changing conditions, says study coauthor Jennifer Lippincott-Schwartz, a cell biologist at the Howard Hughes Medical Institute's Janelia Research Campus in Ashburn, Va.

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Scientists need to redraw picture of cell's biggest organelle

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Super-resolution microscopy reveals surprising details of endoplasmic reticulum's architecture Textbook drawings of the cell's largest organelle might need to be updated based on new images. Super-resolution shots of the endoplasmic reticulum reveal tightly packed tubes where previous pictures showed plain flat sheets, scientists report in the Oct. 28 Science.

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This is a 3D picture showing the organelles.

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This news article is about the nucleus.

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this website explains the science of prokaryotic cells

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Basic Structures of Prokaryotic Cells

Prokaryotes, found in both Domain Archaea and Bacteria, are unicellular organisms that lack membrane-bound organelles and a defined nucleus.

Learning Objectives

Describe the basic structure of a typical prokaryote

Key Takeaways

Key Points

Prokaryotic cells lack a defined nucleus, but have a region in the cell, termed the nucleoid, in which a single chromosomal, circular, double-stranded DNA molecule is located.

Archaeal membranes have replaced the fatty acids of bacterial membranes with isoprene; some archaeal membranes are monolayer rather than bilayer.

Prokaryotes can be further classified based on the composition of the cell wall in terms of the amount of peptidoglycan present.

Gram-positive organisms typically lack the outer membrane found in gram-negative organisms and contain a large amount of peptidoglycan in the cell wall, roughly 90%.

Gram-negative bacteria have a relatively thin cell wall composed of a few layers of peptidoglycan.

Gram-negative bacteria have a relatively thin cell wall composed of a few layers of peptidoglycan.

Key Terms

nucleoid: the irregularly-shaped region within a prokaryote cell where the genetic material is localized

plasmid: a circle of double-stranded DNA that is separate from the chromosomes, which is found in bacteria and protozoa

osmotic pressure: the hydrostatic pressure exerted by a solution across a semipermeable membrane from a pure solvent

The Prokaryotic Cell

Prokaryotes are unicellular organisms that lack organelles or other internal membrane-bound structures. Therefore, they do not have a nucleus, but, instead, generally have a single chromosome: a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid. Most prokaryotes have a cell wall outside the plasma membrane.

The composition of the cell wall differs significantly between the domains Bacteria and Archaea, the two domains of life into which prokaryotes are divided. The composition of their cell walls also differs from the eukaryotic cell walls found in plants (cellulose) or fungi and insects (chitin). The cell wall functions as a protective layer and is responsible for the organism's shape. Some bacteria have a capsule outside the cell wall. Other structures are present in some prokaryotic species, but not in others. For example, the capsule found in some species enables the organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and increases its resistance to our immune responses. Some species also have flagella used for locomotion and pili used for attachment to surfaces. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea.

The Plasma Membrane

The plasma membrane is a thin lipid bilayer (6 to 8 nanometers) that completely surrounds the cell and separates the inside from the outside. Its selectively-permeable nature keeps ions, proteins, and other molecules within the cell, preventing them from diffusing into the extracellular environment, while other molecules may move through the membrane. The general structure of a cell membrane is a phospholipid bilayer composed of two layers of lipid molecules. In archaeal cell membranes, isoprene (phytanyl) chains linked to glycerol replace the fatty acids linked to glycerol in bacterial membranes. Some archaeal membranes are lipid monolayers instead of bilayers.

The Cell Wall

The cytoplasm of prokaryotic cells has a high concentration of dissolved solutes. Therefore, the osmotic pressure within the cell is relatively high. The cell wall is a protective layer that surrounds some cells and gives them shape and rigidity. It is located outside the cell membrane and prevents osmotic lysis (bursting due to increasing volume). The chemical composition of the cell walls varies between archaea and bacteria. It also varies between bacterial species.

Bacterial cell walls contain peptidoglycan composed of polysaccharide chains that are cross-linked by unusual peptides containing both L- and D-amino acids, including D-glutamic acid and D-alanine. Proteins normally have only L-amino acids; as a consequence, many of our antibiotics work by mimicking D-amino acids and, therefore, have specific effects on bacterial cell wall development. There are more than 100 different forms of peptidoglycan. S-layer (surface layer) proteins are also present on the outside of cell walls of both archaea and bacteria.

Bacteria are divided into two major groups: gram-positive and gram-negative, based on their reaction to gram staining. Note that all gram-positive bacteria belong to one phylum; bacteria in the other phyla (Proteobacteria, Chlamydias, Spirochetes, Cyanobacteria, and others) are gram-negative. The gram-staining method is named after its inventor, Danish scientist Hans Christian Gram (1853-1938). The different bacterial responses to the staining procedure are ultimately due to cell wall structure. Gram-positive organisms typically lack the outer membrane found in gram-negative organisms. Up to 90 percent of the cell wall in gram-positive bacteria is composed of peptidoglycan, with most of the rest composed of acidic substances called teichoic acids. Teichoic acids may be covalently linked to lipids in the plasma membrane to form lipoteichoic acids. Lipoteichoic acids anchor the cell wall to the cell membrane. Gram-negative bacteria have a relatively thin cell wall composed of a few layers of peptidoglycan (only 10 percent of the total cell wall), surrounded by an outer envelope containing lipopolysaccharides (LPS) and lipoproteins. This outer envelope is sometimes referred to as a second lipid bilayer. The chemistry of this outer envelope is very different, however, from that of the typical lipid bilayer that forms plasma membranes.

Prokaryotic Reproduction

Prokaryotes reproduce asexually by binary fission; they can also exchange genetic material by transformation, transduction, and conjugation.

Learning Objectives

Distinguish among the types of reproduction in prokaryotes

Key Takeaways

Key Points

Binary fission is a type of reproduction in which the chromosome is replicated and the resultant prokaryote is an exact copy of the parental prokaryate, thus leaving no opportunity for genetic diversity.

Transformation is a type of prokaryotic reproduction in which a prokaryote can take up DNA found within the environment that has originated from other prokaryotes.

Transduction is a type of prokaryotic reproduction in which a prokaryote is infected by a virus which injects short pieces of chromosomal DNA from one bacterium to another.

Conjugation is a type of prokaryotic reproduction in which DNA is transferred between prokaryotes by means of a pilus.

Key Terms

transformation: the alteration of a bacterial cell caused by the transfer of DNA from another, especially if pathogenic

transduction: horizontal gene transfer mechanism in prokaryotes where genes are transferred using a virus

binary fission: the process whereby a cell divides asexually to produce two daughter cells

conjugation: the temporary fusion of organisms, especially as part of sexual reproduction

pilus: a hairlike appendage found on the cell surface of many bacteria

Reproduction

Reproduction in prokaryotes is asexual and usually takes place by binary fission. The DNA of a prokaryote exists as as a single, circular chromosome. Prokaryotes do not undergo mitosis; rather the chromosome is replicated and the two resulting copies separate from one another, due to the growth of the cell. The prokaryote, now enlarged, is pinched inward at its equator and the two resulting cells, which are clones, separate. Binary fission does not provide an opportunity for genetic recombination or genetic diversity, but prokaryotes can share genes by three other mechanisms.

In transformation, the prokaryote takes in DNA found in its environment that is shed by other prokaryotes. If a nonpathogenic bacterium takes up DNA for a toxin gene from a pathogen and incorporates the new DNA into its own chromosome, it, too, may become pathogenic. In transduction, bacteriophages, the viruses that infect bacteria, sometimes also move short pieces of chromosomal DNA from one bacterium to another. Transduction results in a recombinant organism. Archaea are not affected by bacteriophages, but instead have their own viruses that translocate genetic material from one individual to another. In conjugation, DNA is transferred from one prokaryote to another by means of a pilus, which brings the organisms into contact with one another. The DNA transferred can be in the form of a plasmid or as a hybrid, containing both plasmid and chromosomal DNA.

Reproduction can be very rapid: a few minutes for some species. This short generation time, coupled with mechanisms of genetic recombination and high rates of mutation, result in the rapid evolution of prokaryotes, allowing them to respond to environmental changes (such as the introduction of an antibiotic) very rapidly.

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Prokaryotes, found in both Domain Archaea and Bacteria, are unicellular organisms that lack membrane-bound organelles and a defined nucleus. Describe the basic structure of a typical prokaryote Key Points Prokaryotic cells lack a defined nucleus, but have a region in the cell, termed the nucleoid, in which a single chromosomal, circular, double-stranded DNA molecule is located.

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The image shows the organelles in a eukaryotic cell.

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1 – News Article
2 – Images
1 – Video (YouTube)
1 – Padlet
1 – Website
1 – Infographic

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A great video that walks you through the main organelles inside an eukaryotic and prokaryotic cell.

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Introduction to Cells: The Grand Cell Tour

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A view of the parts that make up the mitochondria - the energy creator in the cell.

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A student created poster on the Mighty Mitochondria and how it creates energy for the cell.

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Mitochondria Campaign Visual by Ellery and Simon

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Mitochondria are specialized structures unique to the cells of animals, plants and fungi. They serve as batteries, powering various function...

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This article explains how lysosomes are part of cell autophagy (a process in which a cell breaks down and processes waste).

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Nobel awarded for unveiling how cells recycle their trash

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Cell biologist Yoshinori Ohsumi has won the 2016 Nobel Prize for physiology or medicine for discovering how cells take care of housekeeping.

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explanation of Eukaryotic  cells

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this video is an introduction to pokaryote by the ameba sisters

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Cell Organelles And Their Function Animation (BOTH 3D AND MICROSCOPIC VIEWS )

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explanation of prokaryotic and eukaryotic cells

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Prokaryotic and eukaryotic cells. Bacteria and archaea.

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Organelle video of their functiond

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Cell Organelles And Their Function Animation (BOTH 3D AND MICROSCOPIC VIEWS )

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Short video on prokaryotes and their functions.

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Biology: Cell Structure I Nucleus Medical Media

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Short explanation of Eukaryotes and prokaryotes

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pic of eukaryotic cell

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video on intro to Eukaryotic cells and prokaryotic cells

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Picture of an animal  cell and its organelles

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A informative video on organelles

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An article of fun facts on Eukaryotes.

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News article- Eukaryotic Cells

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news article on eukaryotes

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A news article on prokaryotic microbes with Eukaryote-like genes.

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In cell biology, an organelle is one of several structures with specialized functions, suspended in the cytoplasm of a eukaryotic cell.

Eukaryotes are the most structurally complex known cell type, and by definition are in part organized by smaller interior compartments, that are themselves enclosed by lipid membranes that resemble the outermost cell membrane.

The larger organelles, such as the nucleus and vacuoles, are easily visible with moderate magnification (although sometimes a clear view requires the application of chemicals that selectively stain parts of the cells); they were among the first biological discoveries made after the invention of the microscope.

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cell biology; molecular biology; genetics; developmental biology; biology; biochemistry research

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In cell biology, an organelle is one of several structures with specialized functions, suspended in the cytoplasm of a eukaryotic cell. Eukaryotes are the most structurally complex known cell type, and by definition are in part organized by smaller interior compartments, that are themselves enclosed by lipid membranes that resemble the outermost cell membrane.

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website on eukaryotes

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this video is
really great it describes the main organelles inside the eukaryotic and prokaryotic cells

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The ribosome-associated organelle consists of tightly packed tubes, not flat sheets as previously believed, according to new super-resolution microscopy images.

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Photo of a plant and its organelles

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A detailed diagram of a Eukaryotic cell

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this is the newest article on prokaryotes

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Did endosymbiosis-and the innovations in membrane bioenergetics it engendered-make it possible for eukaryotic life to evolve?

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Cell organelles and their functions animation.

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Article containing where eukaryotes  can be be found and compare and contrasts from prokaryotes

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literally a picture of a prokaryote

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Depiction of what a Eukaryotic Cell looks like.

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This is a picture of a Organelle and it's inside system

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Biology: Cell Structure I Nucleus Medical Media

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eukaryote pic

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This is a picture of a Organelle and this shows the cell structure and the inside of the organelle

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News Article on organelles

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A picture comparing Eukaryotes and Prokaryotes.

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literally a picture of a prokaryote

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A picture locating the different parts of a prokaryote.

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another persons topic

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Structural Details of endoplasmic reticulum

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Structural Details of endoplasmic reticulum

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Cell Organelles | Piktochart Visual Editor

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other persons padlet

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this explains what parts are in the organelle

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Organelles are small, specialized structures in cells which operate like organs by carrying out specific tasks.

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A picture showing what a prokaryote looks like.

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detailed Eukaryote with labeled organelles

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This padlet tells thecharacteristic of Eukaryotes

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Realistic Eukaryote

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Plant and Animal Cell Organelles

Eukaryotes (protozoa, plants and animals) have highly-structured cells. These cells tend to be larger than the cells of bacteria, and have developed specialized packaging and transport mechanisms that may be necessary to support their larger size. Use the following interactive animation of plant and animal cells to learn about their respective organelles.

Nucleus: The nucleus is the most obvious organelle in any eukaryotic cell. It is enclosed in a double membrane and communicates with the surrounding cytosol via numerous nuclear pores. Within each nucleus is nuclear chromatin that contains the organism's genome. The chromatin is efficiently packaged within the small nuclear space. Genes within the chromatin are made of deoxyribonucleic acid (DNA). The DNA stores the organism's entire encoded genetic information. The DNA is similar in every cell of the body, but depending on the specific cell type, some genes may be turned on or off - that's why a liver cell is different from a muscle cell, and a muscle cell is different from a fat cell. When a cell is dividing, the nuclear chromatin (DNA and surrounding protein) condenses into chromosomes that are easily seen by microscopy. For a deeper understanding of genetics, visit our companion site, GeneTiCs Alive!

Nucleolus: The prominent structure in the nucleus is the nucleolus. The nucleolus produces ribosomes, which move out of the nucleus and take positions on the rough endoplasmic reticulum where they are critical in protein synthesis.

Cytosol: The cytosol is the "soup" within which all the other cell organelles reside and where most of the cellular metabolism occurs. Though mostly water, the cytosol is full of proteins that control cell metabolism including signal transduction pathways, glycolysis, intracellular receptors, and transcription factors.

Cytoplasm: This is a collective term for the cytosol plus the organelles suspended within the cytosol.

Centrosome: The centrosome, or MICROTUBULE ORGANIZING CENTER (MTOC), is an area in the cell where microtubules are produced. Plant and animal cell centrosomes play similar roles in cell division, and both include collections of microtubules, but the plant cell centrosome is simpler and does not have centrioles.

During animal cell division, the centrioles replicate (make new copies) and the centrosome divides. The result is two centrosomes, each with its own pair of centrioles. The two centrosomes move to opposite ends of the nucleus, and from each centrosome, microtubules grow into a "spindle" which is responsible for separating replicated chromosomes into the two daughter cells.

Centriole (animal cells only): Each centriole is a ring of nine groups of fused microtubules. There are three microtubules in each group. Microtubules (and centrioles) are part of the cytoskeleton. In the complete animal cell centrosome, the two centrioles are arranged such that one is perpendicular to the other.

Golgi: The Golgi apparatus is a membrane-bound structure with a single membrane. It is actually a stack of membrane-bound vesicles that are important in packaging macromolecules for transport elsewhere in the cell. The stack of larger vesicles is surrounded by numerous smaller vesicles containing those packaged macromolecules. The enzymatic or hormonal contents of lysosomes, peroxisomes and secretory vesicles are packaged in membrane-bound vesicles at the periphery of the Golgi apparatus.

Lysosome: Lysosomes contain hydrolytic enzymes necessary for intracellular digestion. They are common in animal cells, but rare in plant cells. Hydrolytic enzymes of plant cells are more often found in the vacuole.

Peroxisome: Peroxisomes are membrane-bound packets of oxidative enzymes. In plant cells, peroxisomes play a variety of roles including converting fatty acids to sugar and assisting chloroplasts in photorespiration. In animal cells, peroxisomes protect the cell from its own production of toxic hydrogen peroxide. As an example, white blood cells produce hydrogen peroxide to kill bacteria. The oxidative enzymes in peroxisomes break down the hydrogen peroxide into water and oxygen.

Secretory Vesicle: Cell secretions - e.g. hormones, neurotransmitters - are packaged in secretory vesicles at the Golgi apparatus. The secretory vesicles are then transported to the cell surface for release.

Cell Membrane: Every cell is enclosed in a membrane, a double layer of phospholipids (lipid bilayer). The exposed heads of the bilayer are "hydrophilic" (water loving), meaning that they are compatible with water both within the cytosol and outside of the cell. However, the hidden tails of the phosopholipids are "hydrophobic" (water fearing), so the cell membrane acts as a protective barrier to the uncontrolled flow of water. The membrane is made more complex by the presence of numerous proteins that are crucial to cell activity. These proteins include receptors for odors, tastes and hormones, as well as pores responsible for the controlled entry and exit of ions like sodium (Na+) potassium (K+), calcium (Ca++) and chloride (Cl-).

Mitochondria: Mitochondria provide the energy a cell needs to move, divide, produce secretory products, contract - in short, they are the power centers of the cell. They are about the size of bacteria but may have different shapes depending on the cell type. Mitochondria are membrane-bound organelles, and like the nucleus have a double membrane. The outer membrane is fairly smooth. But the inner membrane is highly convoluted, forming folds (cristae) when viewed in cross-section. The cristae greatly increase the inner membrane's surface area. It is on these cristae that food (sugar) is combined with oxygen to produce ATP - the primary energy source for the cell.

Vacuole: A vacuole is a membrane-bound sac that plays roles in intracellular digestion and the release of cellular waste products. In animal cells, vacuoles are generally small. Vacuoles tend to be large in plant cells and play several roles: storing nutrients and waste products, helping increase cell size during growth, and even acting much like lysosomes of animal cells. The plant cell vacuole also regulates turgor pressure in the cell. Water collects in cell vacuoles, pressing outward against the cell wall and producing rigidity in the plant. Without sufficient water, turgor pressure drops and the plant wilts.

Cell Wall (plant cells only): Plant cells have a rigid, protective cell wall made up of polysaccharides. In higher plant cells, that polysaccharide is usually cellulose. The cell wall provides and maintains the shape of these cells and serves as a protective barrier. Fluid collects in the plant cell vacuole and pushes out against the cell wall. This turgor pressure is responsible for the crispness of fresh vegetables.

Chloroplast (plant cells only): Chloroplasts are specialized organelles found in all higher plant cells. These organelles contain the plant cell's chlorophyll responsible for the plant's green color and the ability to absorb energy from sunlight. This energy is used to convert water plus atmospheric carbon dioxide into metabolizable sugars by the biochemical process of photosynthesis. Chloroplasts have a double outer membrane. Within the stroma are other membrane structures - the thylakoids. Thylakoids appear in stacks called "grana" (singular = granum). Estrella Moumtain Community College provides a good source of information on photosynthesis.

Smooth Endoplasmic Reticulum: Throughout the eukaryotic cell, especially those responsible for the production of hormones and other secretory products, is a vast network of membrane-bound vesicles and tubules called the endoplasmic reticulum, or ER for short. The ER is a continuation of the outer nuclear membrane and its varied functions suggest the complexity of the eukaryotic cell.
The smooth endoplasmic reticulum is so named because it appears smooth by electron microscopy. Smooth ER plays different functions depending on the specific cell type including lipid and steroid hormone synthesis, breakdown of lipid-soluble toxins in liver cells, and control of calcium release in muscle cell contraction.

Rough Endoplasmic Reticulum: Rough endoplasmic reticulum appears "pebbled" by electron microscopy due to the presence of numerous ribosomes on its surface. Proteins synthesized on these ribosomes collect in the endoplasmic reticulum for transport throughout the cell.

Ribosomes: Ribosomes are packets of RNA and protein that play a crucial role in both prokaryotic and eukaryotic cells. They are the site of protein synthesis. Each ribosome comprises two parts, a large subunit and a small subunit. Messenger RNA from the cell nucleus is moved systematically along the ribosome where transfer RNA adds individual amino acid molecules to the lengthening protein chain.

Cytoskeleton: As its name implies, the cytoskeleton helps to maintain cell shape. But the primary importance of the cytoskeleton is in cell motility. The internal movement of cell organelles, as well as cell locomotion and muscle fiber contraction could not take place without the cytoskeleton. The cytoskeleton is an organized network of three primary protein filaments:

microtubules

actin filaments (microfilaments)

intermediate fibers

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This exploration of plant and animal cell organelles and cell structure is presented in a mobile-friendly interactive model with detailed descriptive text.

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All cell organelles and their functions.