Mitochondria
A mitochondrion (mitochondria, plural), is a membrane-enclosed organelle that is found in most eukaryotic cells. Mitochondria are called the "power plants" of the cell because they use energy from organic compounds to make ATP. ATP is the cell's energy source that is used for such things such as movement and cell division. Some ATP is made in the cytosol of the cell, but most of it is made inside mitochondria. The number of mitochondria in a cell depends on the cell’s energy needs. For example, active human muscle cells may have thousands of mitochondria, while less active red blood cells do not have any.
Figure 3.19
Electron micrograph of a single mitochondrion within which you can see many cristae. Mitochondria range from 1 to 10 m in size. This model of a mitochondrian shows the organized arrangement of the inner and outer membranes, the protein matrix, and the folded inner mitochondrial membranes.
As Figure above (a) and (b) shows, a mitochondrion has two phospholipids membranes. The smooth outer membrane separates the mitochondrion from the cytosol. The inner membrane has many folds, called cristae. The fluid-filled inside of the mitochondrian, called matrix, is where most of the cell’s ATP is made.
Although most of a cell's DNA is contained in the cell nucleus, mitochondria have their own DNA. Mitochandria are able to reproduce asexually and scientists think that they are descended from prokaryotes. According to the endosymbiotic theory, mitochondria were once free-living prokaryotes that infected ancient eukaryotic cells. The invading prokaryotes were protected inside the eukaryotic host cell, and in turn the prokaryote supplied extra ATP to its host.
Endoplasmic Reticulum
The endoplasmic reticulum (ER) (plural, reticuli) is a network of phospholipid membranes that form hollow tubes, flattened sheets, and round sacs. These flattened, hollow folds and sacs are called cisternae. The ER has two major functions:
Transport: Molecules, such as proteins, can move from place to place inside the ER, much like on an intracellular highway.
Synthesis: Ribosomes that are attached to ER, similar to unattached ribosomes, make proteins. Lipids are also produced in the ER.
There are two types of endoplasmic reticulum, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).
Rough endoplasmic reticulum is studded with ribosomes which gives it a "rough" appearance. These ribosomes make proteins that are then transported from the ER in small sacs called transport vesicles. The transport vesicles pinch off the ends of the ER. The rough endoplasmic reticulum works with the Golgi apparatus to move new proteins to their proper destinations in the cell. The membrane of the RER is continuous with the outer layer of the nuclear envelope.
Smooth endoplasmic reticulum does not have any ribosomes attached to it, and so it has a smooth appearance. SER has many different functions some of which are: lipid synthesis, calcium ion storage, and drug detoxification. Smooth endoplasmic reticulum is found in both animal and plant cells and it serves different functions in each. The SER is made up of tubules and vesicles that branch out to form a network. In some cells there are dilated areas like the sacs of RER. Smooth endoplasmic reticulum and RER form an interconnected network.
Figure 3.20
Image of nucleus, endoplasmic reticulum and Golgi apparatus, and how they work together. The process of secretion from endoplasmic reticuli (orange) to Golgi apparatus (pink) is shown.
Ribosomes
Ribosomes are small organelles and are the site of protein synthesis (or assembly). They are made of ribosomal protein and ribosomal RNA. Each ribosome has two parts, a large and a small subunit, as shown in Figure below. The subunits are attached to each other. Ribosomes can be found alone or in groups within the cytoplasm. Some ribosomes are attached to the endoplasmic reticulum (as shown in Figure above), and others are attached to the nuclear envelope.
Ribozymes are RNA molecules that catalyzes chemical reactions, such as translation. Translation is the process of ordering the amino acids in the assembly of a protein, and more will be discussed on translation in a later chapter. Briefly, the ribosomes interact with other RNA molecules to make chains of amino acids called polypeptide chains, due to the peptide bond that forms between individual amino acids. Polypeptide chains are built from the genetic instructions held within a messenger RNA molecule. Polypeptide chains that are made on the rough ER are inserted directly into the ER and then are transported to their various cellular destinations. Ribosomes on the rough ER usually produce proteins that are destined for the cell membrane.
Figure 3.21
The two subunits that make up a ribosome, small organelles that are intercellular protein factories.
Golgi Apparatus
The Golgi apparatus is a large organelle that is usually made up of five to eight cup-shaped, membrane-covered discs called cisternae, as shown in Figure above. The cisternae look a bit like a stack of deflated balloons. The Golgi apparatus modifies, sorts, and packages different substances for secretion out of the cell, or for use within the cell. The Golgi apparatus is found close to the nucleus of the cell where it modifies proteins that have been delivered in transport vesicles from the RER. It is also involved in the transport of lipids around the cell. Pieces of the Golgi membrane pinch off to form vesicles that transport molecules around the cell. The Golgi apparatus can be thought of as similar to a post office; it packages and labels "items" and then sends them to different parts of the cell. Both plant and animal cells have a Golgi apparatus. Plant cells can have up to several hundred Golgi stacks scattered throughout the cytoplasm. In plants, the Golgi apparatus contains enzymes that synthesize some of the cell wall polysaccharides.
Vesicles
A vesicle is a small, spherical compartment that is separated from the cytosol by at least one lipid bilayer. Many vesicles are made in the Golgi apparatus and the endoplasmic reticulum, or are made from parts of the cell membrane. Vesicles from the Golgi apparatus can be seen in Figure above. Because it is separated from the cytosol, the space inside the vesicle can be made to be chemically different from the cytosol. Vesicles are basic tools of the cell for organizing metabolism, transport, and storage of molecules. Vesicles are also used as chemical reaction chambers. They can be classified by their contents and function.
Transport vesicles are able to move molecules between locations inside the cell. For example, transport vesicles move proteins from the rough endoplasmic reticulum to the Golgi apparatus.
Lysosomes are vesicles that are formed by the Golgi apparatus. They contain powerful enzymes that could break down (digest) the cell. Lysosomes break down harmful cell products, waste materials, and cellular debris and then force them out of the cell. They also digest invading organisms such as bacteria. Lysosomes also break down cells that are ready to die, a process called autolysis.
Peroxisomes are vesicles that use oxygen to break down toxic substances in the cell. Unlike lysosomes, which are formed by the Golgi apparatus, peroxisomes self replicate by growing bigger and then dividing. They are common in liver and kidney cells that break down harmful substances. Peroxisomes are named for the hydrogen peroxide (H2O2) that is produced when they break down organic compounds. Hydrogen peroxide is toxic, and in turn is broken down into water (H2O) and oxygen (O2) molecules.
Vacuoles
Vacuoles are membrane-bound organelles that can have secretory, excretory, and storage functions. Many organisms will use vacuoles as storage areas and some plant cells have very large vacuoles. Vesicles are much smaller than vacuoles and function in transporting materials both within and to the outside of the cell.
Special Structures in Plant Cells
Most of the organelles that have been discussed are common to both animal and plant cells. However, plant cells also have features that animal cells do not have; they have a cell wall, a large central vacuole, and plastids such as chloroplasts.
Plants have very different lifestyles from animals, and these differences are apparent when you examine the structure o
f the plant cell. Plants make their own food in a process called photosynthesis. They take in carbon dioxide (CO2) and water (H2O) and convert them into sugars. The features unique to plant cells can be seen in Figure below.
Figure 3.22
In addition to containing most of the organelles found in animal cells, plant cells also have a cell wall, a large central vacuole, and plastids. These three features are not found in animal cells.
Cell Wall
A cell wall is a rigid layer that is found outside the cell membrane and surrounds the cell. The cell wall contains not only cellulose and protein, but other polysaccharides as well. In fact, two other classes of polysaccharides, hemicelluloses and pectic polysaccharides, can comprise 30% of the dry mass of the cell wall. The cell wall provides structural support and protection. Pores in the cell wall allow water and nutrients to move into and out of the cell. The cell wall also prevents the plant cell from bursting when water enters the cell.
Microtubules guide the formation of the plant cell wall. Cellulose is laid down by enzymes to form the primary cell wall. Some plants also have a secondary cell wall. The secondary wall contains a lignin, a secondary cell component in plant cells that have completed cell growth/expansion.
Central Vacuole
Most mature plant cells have a central vacuole that occupies more than 30% of the cell's volume, but can also occupy as much as 90% of the volume of certain cells. The central vacuole is surrounded by a membrane called the tonoplast. The central vacuole has many functions. Aside from storage, the main role of the vacuole is to maintain turgor pressure against the cell wall. Proteins found in the tonoplast control the flow of water into and out of the vacuole. The central vacuole also stores the pigments that color flowers.
The central vacuole contains large amounts of a liquid called cell sap, which differs in composition to the cell cytosol. Cell sap is a mixture of water, enzymes, ions, salts, and other substances. Cell sap may also contain toxic byproducts that have been removed from the cytosol. Toxins in the vacuole may help to protect some plants from being eaten.
Plastids
Plant plastids are a group of closely related membrane-bound organelles that carry out many functions. They are responsible for photosynthesis, for storage of products such as starch, and for the synthesis of many types of molecules that are needed as cellular building blocks. Plastids have the ability to change their function between these and other forms. Plastids contain their own DNA and some ribosomes, and scientists think that plastids are descended from photosynthetic bacteria that allowed the first eukaryotes to make oxygen. The main types of plastids and their functions are:
Chloroplasts are the organelle of photosynthesis. They capture light energy from the sun and use it with water and carbon dioxide to make food (sugar) for the plant. The arrangement of chloroplasts in a plant’s cells can be seen in Figure below.
Chromoplasts make and store pigments that give petals and fruit their orange and yellow colors.
Leucoplasts do not contain pigments and are located in roots and non-photosynthetic tissues of plants. They may become specialized for bulk storage of starch, lipid, or protein. However, in many cells, leucoplasts do not have a major storage function; instead they make molecules such as fatty acids and many amino acids.
Figure 3.23
Plant cells with visible chloroplasts (left). Starch-storing potato leucoplasts (right).
Figure 3.24
The internal structure of a chloroplast, with a granal stack of thylakoids circled.
Chloroplasts capture light energy from the sun and use it with water and carbon dioxide to produce sugars for food. Chloroplasts look like flat discs that are usually 2 to 10 micrometers in diameter and 1 micrometer thick. A model of a chloroplast is shown in Figure above. The chloroplast is enclosed by an inner and an outer phospholipid membrane. Between these two layers is the intermembrane space. The fluid within the chloroplast is called the stroma, and it contains one or more molecules of small circular DNA. The stroma also has ribosomes. Within the stroma are stacks of thylakoids, the sub-organelles which are the site of photosynthesis. The thylakoids are arranged in stacks called grana (singular: granum). A thylakoid has a flattened disk shape. Inside it is an empty area called the thylakoid space or lumen. Photosynthesis takes place on the thylakoid membrane.
Within the thylakoid membrane is the complex of proteins and light-absorbing pigments, such as chlorophyll and carotenoids. This complex allows capture of light energy from many wavelengths because chlorophyll and carotenoids both absorb different wavelengths of light. You will learn more about how chloroplasts convert light energy into chemical energy in the Photosynthesis chapter.
Organization of Cells
Biological organization exists at all levels in organisms. It can be seen at the smallest level, in the molecules that made up such things as DNA and proteins, to the largest level, in an organism such as a blue whale, the largest mammal on Earth. Similarly, single celled prokaryotes and eukaryotes show order in the way their cells are arranged. Single-celled organisms such as an amoeba are free-floating and independent-living. Their single-celled "bodies" are able to carry out all the processes of life such as metabolism and respiration without help from other cells. Some single-celled organisms such as bacteria can group together and form a biofilm. A biofilm is a large grouping of many bacteria that sticks to a surface and makes a protective coating over itself. Biofilms can show similarities to multicellular organisms. Division of labor is the process in which one group of cells does one job (such as making the "glue" that sticks the biofilm to the surface) while another group of cells does another job (such as taking in nutrients). Multicellular organisms carry out their life processes through division of labor and they have specialized cells that do specific jobs. However, biofilms are not considered a multicellular organism and are instead called colonial organisms. The difference between a multicellular organism and a colonial organism is that individual organisms from a colony or biofilm can, if separated, survive on their own, while cells from a multicellular organism (e.g., liver cells) cannot.
Figure 3.25
Colonial algae of the genus .
Colonial Organisms
Colonial organisms were probably one of the first evolutionary steps towards multicellular organisms. Algae of the genus Volvox are an example of the border between colonial organisms and multicellular organisms.
Each Volvox, shown in Figure above, is a colonial organism. It is made up of between 1000 to 3000 photosynthetic algae that are grouped together into a hollow sphere. The sphere has a distinct front and back end. The cells have eyespots, which are more developed in the cells near the front. This enables the colony to swim towards light.
Origin of Multicellularity
The oldest known multicellular organism is a red algae Bangiomorpha pubescens, fossils of which were found in 1.2 billion year old rock. However, the first organisms were single celled. How multicellular organisms developed is the subject of much debate.
Scientists think that multicellularity arose from cooperation between many organisms of the same species. The Colonial Theory proposes that this cooperation led to the development of a multicellular organism. Many examples of cooperation between organisms in nature have been observed. For example, a certain species of amoeba (a single-celled animal) groups together during times of food shortage and forms a colony that moves as one to a new location. Some of these amoebas then become slightly differentiated from each other. Volvox, shown in Figure above, is another example of a colonial organism. Most scientists accept that the Colonial theory explains how multicellular organisms evolved.
Multicellular organisms are organisms that are made up of more than one type of cell and have specialized cells that are grouped together to carry out specialized functions. Most life that you can see without a microscope is multicellular. As discussed earlier, the cells of a multicellular organism would not survive as independent cells. The body of a multicellular orga
nism, such as a tree or a cat, exhibits organization at several levels: tissues, organs, and organ systems. Similar cells are grouped into tissues, groups of tissues make up organs, and organs with a similar function are grouped into an organ system.
Levels of Organization in Multicellular Organisms
The simplest living multicellular organisms, sponges, are made of many specialized types of cells that work together for a common goal. Such cell types include digestive cells, tubular pore cells; and epidermal cells. Though the different cell types create a large organized, multicellular structure—the visible sponge—they are not organized into true interconnected tissues. If a sponge is broken up by passing it through a sieve, the sponge will reform on the other side. However, if the sponge’s cells are separated from each other, the individual cell types cannot survive alone. Simpler colonial organisms, such as members of the genus Volvox, as shown in Figure above, differ in that their individual cells are free-living and can survive on their own if separated from the colony.
Figure 3.26
This roundworm, a multicellular organism, was stained to highlight the nuclei of all the cells in its body (red dots).
CK-12 Biology I - Honors Page 17