There are two theories in biology. One is the theory of evolution by natural selection. The other is known as the cell theory.
The cell theory has three main components. First, it states that the cell is the basic unit of life. The second principle of cell theory is that all cells came from the other cells.
But which really came first, the chicken or the egg? Well, who cares really? It has nothing to do with cell theory. And of all the cells that are out there, the cell theory states that they all came from other cells.
One consequence of this is that it defines viruses, which lack a membrane, to not be considered alive, even though they exhibit all the other properties of life. They have order and respond to the environment, they use energy to grow, develop, and reproduce, and they evolve. So are viruses alive?
Well, it is debatable, but most biologists would argue that they aren't, and they would invoke the cell theory to back up their claims. When the first cells were discovered, the microscopes were so rudimentary that only the most basic understanding of the cell could be seen. It was obvious that all cells were contained within a sac-like structure that was termed the cell membrane. They could also see discrete structures within the cell, which became known as organelles. They were thought to have specific functions for the cell, analogous to the specific functions of the organs in our body.
It was also noticed that these organelles were swimming within the cell in a medium that has been termed Cytoplasm. Cells are so small that you need a microscope to see them. Why?
To answer this question we have to understand that in order to survive, cells must constantly interact with their surrounding environment. Gases and food molecules dissolved in water must be absorbed and the waste products must be eliminated. For most cells, this passage of all materials in and out of the cell must occur through the plasma membrane. Each internal region of the cell has to be served by part of the cell's surface. As the cell grows bigger, its internal volume enlarges and the cell membrane expands.
Unfortunately, the volume increases more rapidly than does the surface area, and so the relative amount of surface area available to pass material into and out of a cell per unit volume of the cell steadily decreases. Finally, at some point, there is just enough surface area available to service all of the interior. If it is to survive, the cell must stop growing. The important point is that as the surface area to the volume ratio gets smaller, the cell gets larger. Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume.
When this happens, the cell must divide into smaller cells with favorable surface area to volume ratios or cease to functions. And that is why cells are so small. Cells that lack a membrane-bound nucleus are called prokaryotes, from the Greek meaning before nuclei. These cells have few internal structures that are distinguishable under a microscope.
Cells in the Monera kingdom, such as bacteria and cyanobacteria, are prokaryotes. Prokaryotic cells differ significantly from eukaryotic cells. Well, we already said that they don't have a membrane-bound nucleus, and instead of having a chromosomal DNA, their genetic information is not known. is in a circular loop called a plasmid.
DNA sends messages as RNA to the ribosomes within the cell, which synthesize proteins. Bacteria cells are very small, roughly the size of an animal mitochondrion. Prokaryotic cells feature three major shapes, rod-shaped, spherical, and spiral.
Instead of going through an elaborate replication process like eukaryotes, bacteria cells divide by binary fission. In other words, they just split in half. Eukaryotic cells, from the Greek meaning truly nuclear, compose of all the life kingdoms except monera. They can be easily distinguished through a membrane-bound nucleus. Whereas prokaryotes have circular DNA, DNA in the eukaryotic cell is linear and enclosed within the nucleus.
Eukaryotic cells also contain many internal membrane-bound structures called organelles. These organelles, such as the mitochondrion or the chloroplast, serve to perform metabolic functions and energy conversion. Other organelles, like intracellular filaments, provide structural support and cellular mobility.
Eukaryotes can be either single-cellular or multicellular, whereas prokaryotes are only unicellular. And now we'll watch a video about the different components of cells. In the next several slides we will look at the structure and functions of different organelles of the cell. And we have to start with the boss, the nucleus. The nucleus is the warehouse for DNA and DNA is truly the code of life.
It gives the instructions in the form of RNA to the rest of the cell, which allows it to live and reproduce. This is a picture of what the nucleus actually looks like, and it's made up of several structures. It is bound by a nuclear envelope.
Punctuated within the nuclear envelope are nuclear pores. This is where stuff are shuttled in and out, kind of like the postal service on a good day. Within the nucleus are chromatin, and chromatin is the DNA. When a cell is reproducing, the chromatin can dense and look like distinct X's, which we know as chromosomes.
However, most of the time it looks like a bowl of spaghetti. In that form it is known as chromatin. The nucleolus is the dark core of the nucleus, and it has the distinction of producing the ribosomes for the cell. In 1873, Levi Strauss invented genes.
Oh, wait, that's another lecture. Let's get back to biology now. A gene is a molecular unit of heredity in a living organism, and it's the name given to some stretches of DNA and even RNA that have a specific function in that organism. And all living beings depend on genes, as they specify the proteins that we need to live and survive.
Genes hold the information to build and maintain an organism's cell, and pass genetic traits to offspring. And all organisms have many, many, many genes corresponding to the various biological traits, such as eye color or the number of limbs. They can also do things that we can't see, like the thousands of biochemical processes that we need in order to survive. Ooh, the endoplasmic reticulum.
It's a great word to use on Scrabble. If you could actually use it on Scrabble. Anyway, it's also known as the ER, and it's an organelle of cells and eukaryotic organisms that it forms an interconnected network of tubules, vesicles, and folded structures known as cisternae.
The general structure of the endoplasmic reticulum is an extensive membrane network of cisternae, or sac-like structures, held together by the skeleton. These kind of look like a sheet that's been folded over on itself again and again and again. The function of the endoplasmic reticulum vary greatly depending on the exact type of endoplasmic reticulum and the type of cell in which it resides.
There are two main types of endoplasmic reticulum. Rough endoplasmic reticula synthesize proteins, while smooth endoplasmic reticula synthesize lipids and steroids, as well as metabolize carbohydrates. You can think of the ER as the internal delivery service of the cell. If the DNA is the boss of the cell and the RNA are the messages of the cell, the ER is like the mail and the copy guy of an office. The messages get sent by the DNA in the form of messenger RNA traveling outside the nuclear pore and into the rough ER.
The mRNA then hooks to a ribosome on the rough ER and synthesizes proteins through a process known as translation. The surface of the rough endoplasmic reticulum is studded with protein manufacturing ribosomes, giving it a rough appearance, hence its name. However, the ribosomes bound to the rough ER at any one time are not a stable part of its organelle structures, as ribosomes are constantly being bound and released from the membrane.
The membrane of the rough ER is continuous with the outer layer of the nuclear envelope. Although there is no continuous membrane between the ER and the Golgi apparatus, which we'll discuss in a minute, membrane-bound vesicles shuttle proteins between these two compartments. The rough ER works in concert with the Golgi apparatus to target new proteins to their proper destinations. The rough endoplasmic reticulum is also involved in membrane synthesis, and this is where it combines phospholipids with proteins, creating phospholipid bilayer embedded with proteins that is used in the cellular membrane and the intracellular organelles within the cell. And this is what endoplasmic reticulum really looks like.
On the bottom you see it's connected to the nucleus, and then it looks like folded sheets one on top of another. These are known as cisternae. And if you look really close at the blue sections, you can see little, little brown dots. These are the ribosomes. The smooth endoplasmic reticulum has no ribosomes.
but it has a lot of different functions. Smooth ER is involved in several metabolic processes, including the synthesis of lipids, which produces steroids such as hormones, as well as the phospholipids for the cell membranes. It is also involved in the metabolism of carbohydrates and has a dominating function in liver cells.
It can also detoxify drugs and regulates calcium concentrations within the cells, and it is also connected to the nuclear envelope. Smooth endoplasmic reticulum is found in a variety of cell types, both plant and animal. and it serves different functions in each.
If the endoplasmic reticulum were Batman, the Golgi apparatus would be its Robin. Now if you think of the endoplasmic reticulum as the internal mailing system within an organization, which packages up products and sends them out, the Golgi apparatus is like UPS. It takes those packages and it modifies them, much like a UPS truck would run over your package and say, well you should have gotten insurance.
and then ships it to its new owner, which gets the cappuccino maker and is broken into a thousand pieces and sad and crying and pissed off. Well, in other words, the Golgi apparatus is integral in modifying and sorting and packaging the macromolecules for cell secretion, also known as exocytosis, or also for use within the cell. It primarily modifies proteins delivered from the rough endoplasmic reticulum, but is also involved in the transport of lipids around the cell, and the creation of lysosomes.
Found within the cytoplasm of both plant and animal cells, the Golgi apparatus is composed of stacks of membrane-bound structures known as cisternae. The cisternae stack as four functional groups, the cis-Golgi network, the medial Golgi, the endogolgi, and the trans-Golgi. The vesicles that leave the rough endoplasmic reticulum are transported to the cis-face of the Golgi apparatus where they can fuse with the Golgi membrane and then they empty their contents into the lumen. The lumen is basically the inside of the Golgi apparatus.
Once inside the lumen, the molecules are modified and then sorted for transport to their next destination, from the transface of the Golgi. These vesicles are like trucks that ship the packages to the other parts of the cell. And here is a picture of an actual Golgi apparatus. Can you tell which is the cis face and which is the trans face? Which is the receiving end and which is the shipping end?
Lysosomes are membrane-bound cellular organelles that contain enzymes to break down waste materials and cellular debris. They can be described as the stomach of the cell. Lysosomes are the cell's waste disposal system and can digest some compounds.
They are used for the digestion of macromolecules, from the ingestion of other dying cells, or larger extracellular material like foreign invading microbes such as bacteria or viruses. One of the lysosomes main function is to ingest the cellular materials of the cell it resides in, and breaks it down into its fundamental components. In this way, you can think of the lysosome as a recycling center of a cell.
This process is known as autophagy. Lysosomes are frequently nicknamed suicide sacks by particularly humorous cell biologists because lysosomes can cause programmed cell destruction of the cell. Lysosomes are thought to kill cells that are no longer needed, such as those in the tails of tadpoles or in the web from fingers between 3 and 6 month old fetuses in humans.
Thus, lysosomes are an important factor in the development of many multicellular organisms. And here we can see a cell and the little black dots that are in this cell are the lysosomes. So they're really, really tiny.
But they serve a really important function as well, just as your stomach does and your body. A vacuole is a membrane-bound organelle which is present in all plants and fungal cells and some protease, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules.
though in certain cases they may contain solids which have been engulfed by the cell. Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these. The organelle has no basic shape or size and its structure varies according to the need of the cell. The function and importance of vacuoles varies greatly according to the type of cell in which they are present, having much greater prominence in the cells of plants and fungi. and certain protease than those of animals and bacteria.
Bacchules have several different functions in a cell. They are involved in isolating materials that may be harmful or a threat to the cell, They contain and dispose of waste products, they store water in plant cells, they regulate pH, and in seeds, stored proteins needed for germination are kept in protein bodies, which are basically just modified vacuoles. They also hold colored chemicals in plants, creating colors for flowers and fruit, which are beneficial in reproductive fitness.
And in proteins, the vacuoles can expel water, giving them an ability to move. These are known as contractions. vacuoles. And here's a picture of what a vacuole would actually look like in a plant cell.
And you can see that it's very large because plants have to store water for long periods of time in order to go through their metabolic processes. A mitochondrion is a membrane-enclosed organelle found in nearly all eukaryotic organisms. Mitochondria are sometimes described as the cellular power plants.
because they generate most of the cell's supply of adenosine triphosphate, ATP, used as the source of chemical energy for nearly all life on Earth. In addition to supplying cellular energy, mitochondria are involved in a range of other processes such as the control of the cell cycle and cell growth. Mitochondria are bound by a double membrane. Each of these membranes is a phospholipid bilayer with embedded proteins.
The outermost membrane is smooth while the inner membrane has many folds. These folds are called cristae. The folds enhance the productivity of cellular respiration by increasing the available surface area. The double membranes divide the mitochondrion into two distinct parts, the intermembrane space and the mitochondrial matrix.
The intermembrane space is the narrow part between the two membranes while the mitochondrial matrix is the part enclosed by the innermost membrane. Several of the steps in cellular respiration occur in the matrix due to its high concentration of enzymes. Mitochondria are semi-autonomous in that they are only partially dependent on the cell in which they live to replicate and grow.
In fact, they have their own DNA, they have their own ribosomes, and they can even make their own proteins. And similar to bacteria, Mitochondria have circular DNA and replicate by a process known as fission. This provides tremendous evidence that mitochondria have prokaryotic origins and were held hostage by the eukaryotic cell, which greatly enhanced its energy production ability.
It's kind of like what happened when free agency came to professional sports. And here's a picture of what mitochondrion actually looks like. Can you see its double membrane structure?
Can you see its cisternae? The cytoskeleton is the cellular scaffolding or skeleton contained within a cell's cytoplasm and it's made out of protein. The cytoskeleton is present in all cells.
It was once thought to be unique to eukaryotes but recent research has identified the prokaryotic cytoskeleton. The cytoskeleton has structures such as flagella, which are whip-like structures on the end of certain bacteria, and cilia, which are hair-like structures on other bacteria, and plays important roles in both intracellular transport, the movement of vesicles or organelles within a cell, and also cellular transport of unicellular organisms. And it's also important in cellular division.
And now we'll see a little video on the different parts of a plant cell. Both animals and plants are made up of cells. Their cells have many features in common, but there are a few significant differences. Let's look inside a leaf to take a closer look at a plant cell.
First, we encounter a protective cell wall outside the plasma membrane. The cell wall is made from strong cellulose fibrils. Once inside the plant cell, we see the large central vacuole, which regulates the composition of the cytoplasm, creates the internal pressure that is characteristic of plant cells, and stores various compounds produced by the cell.
Plants make their own food by photosynthesis in chloroplasts. Light passes through the two membranes of the chloroplast and strikes these green disks. where light energy is converted to chemical energy.
The sugar molecules produced by photosynthesis can be made into other molecules or broken down for energy. All plant cells have mitochondria, just like animal cells do. Sugars produced by photosynthesis are broken down and converted to ATP in mitochondria. Most organelles, like mitochondria, are found in both plant cells and animal cells.
So, the next time you pass by a plant, remember that we have more in common than meets the eye. Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy synthesizing ATP through a complex process known as photosynthesis.
Chloroplasts are green because they contain the chlorophyll pigment. Chloroplasts are one of the many different types of organelles in a plant cell, but it's thought that they originated from cyanobacteria and were entrapped, much like the mitochondria. The chloroplast is surrounded by a double-layered membrane with an intermembrane space, which has reticulations or many infoldings filling those inner spaces.
And the chloroplast has its own DNA. However, it's considerably reduced compared to that of free-living cyanobacteria, but the parts that are still present show clear similarities with other cyanobacteria genomes. Chloroplasts are observable as flat disk. The chloroplast is contained by an envelope that consists of an inner and an outer phospholipid membrane.
Between these two layers is the intermembrane space. The material within the chloroplast is called the stroma, corresponding to the cytosol of the original bacterium, and contains one or more molecules of small circular DNA. It also contains ribosomes. However, most of its proteins are encoded by genes contained in the host cell nucleus. with the protein products transported to the chloroplast.
Within the stroma are stacks called thylakaloids, the suborganelles which are the site of photosynthesis. The thylakaloids are arranged in stacks called grana. A thylakaloid has a flattened disc shape.
Inside it is an empty area called the thylakaloid space or lumen. Photosynthesis takes place on the thylakaloid membrane. And here's an example of what chloroplasts actually look like.
Can you identify the stroma? Can you identify the thycolloids? Can you identify the grana? And if you can do that, can you tell me where exactly photosynthesis takes place? And why are chloroplasts green?
What is the molecule that chloroplasts have that make them green? The cell wall is the tough usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to act as a pressure vessel, preventing overexpansion when water enters a cell. They are found in plants, bacteria, fungi, algae, and even some archaea.
Animals Do not have cell walls. Cell walls are particularly important in plants as they form the structural support holding plants upright. Wood is 100% cell wall material. Can you identify the cell walls in this picture?