Lesson 3.3 Eukaryotic Cells

3.3.3 Plant and Animal Cell Organelles

Organelles shared by both animal and plant cells

Nucleus

Figure 3.3.3 shows a three dimensional representation of the nucleus of a typical eukaryotic cell.

The DNA of a eukaryotic cell is located in the nucleus, which codes for all the proteins of the cell.

This organelle possesses a double membrane, referred to as the nuclear envelope, composed of lipids and proteins. Pores are scattered throughout this envelope, allowing small molecules and ions to enter and to leave.

These pores are crucial to the cell functioning, as they allow ribonucleic acid (RNA) molecules to leave the nucleus. These RNA molecules contain the codes for the creation of proteins. Without creating proteins, a cell will not survive. In addition, carrier proteins are also embedded in the nuclear envelope to allow active transport of larger molecules.

The nucleus possesses a nuclear lamina to support its structure. This lamina is composed of specific types of proteins, and together, they function like a scaffold for the nucleus—very similar to the function of the cytoskeleton for the entire cell.

The materials found within the nucleus are not homogeneous. Areas called sub-nuclear bodies exist. These bodies can contain unique proteins, RNA molecules, and specific parts of chromosomes. An example of such an area is the nucleolus. Ribosomes are created here and are required for the production of proteins.

Mitochondrion

All cells require energy in order to carry out their life functions. This energy needs to be obtained directly by the cell or needs to be created by the cell. Eukaryotic cells use an organelle called the mitochondrion (plural: mitochondria) to fill this need.

Figure 3.3.4 is a diagram presenting the basic structure of a mitochondrion.

Mitochondria are very small, rod-shaped, double membrane structures. The total number of these organelles varies per cell, dependent primarily on the cell’s energy needs, or, in the case of multi cellular organisms, the organism’s energy needs. Human liver cells, for example possess over one thousand mitochondria per cell. In contrast, yeast cells contain up to fifty mitochondria per cell.

The primary function of a mitochondrion is to create energy in the form of adenosine triphosphate (ATP). This can be accomplished chemically through the process of oxidative phosphorylation, which occurs in several different types of chemical pathways within the cell.

The concentration of calcium ions is also under the control of the mitochondria. These ions are important in the process of cell communication and cell signaling. Calcium ions play an important role in human muscles and bones. In addition, mitochondria are also associated with the preprogrammed death of individual cells (known as apoptosis) and the aging process.

Structurally, the mitochondrion has an outer and inner membrane, separated by an inter-membrane space. The outer membrane is composed of phospholipids and proteins. Most notably, a group of cell membrane proteins, called porins, required for transportation of materials through the cell membrane are found in large numbers here.

The outer membrane of the mitochondrion is permeable to ATP, ADP, ions and certain types of nutrient molecules.

The inner membrane is highly folded, and in such a form, is referred to as the cristae. The highly folded nature of this membrane increases its surface area significantly. It is here that the processes required to produce ATP can take place. Only oxygen and ATP are able to cross the inner membrane of the mitochondrion. Oxygen is required for the creation large numbers of ATP molecules.

The inter-membrane space contains a cytoplasm-like materials that contains a high concentration of water—a required molecule for the creation of ATP.

An unusual characteristic of mitochondria is that they contain their own DNA, different from the cell’s DNA. In addition, they also are able to synthesis their own proteins using their own RNA, not the cell’s.

Golgi apparatus (also known as Golgi body or Golgi complex)

The Golgi apparatus is very active metabolically and has been identified to have several functions within the cell. It acts as the manufacturing and shipping organelle for the cell, modifying, sorting and packaging both proteins and lipids. These proteins and lipids come from another cell organelle called the endoplasmic reticulum.

Figure 3.3.5 is a diagram illustrating the basic structure of a Golgi apparatus.

 Other functions associated with the Golgi apparatus are:

1. Chemically labeling products created to ensure they either get to other specific parts of the cell or removed from the cell by exocytosis.
2. Creates secretory vesicles to deliver materials throughout the cell
3. Synthesizes specific simple carbohydrates and specific polysaccharides
4. Creates lipoproteins from lipids and proteins provided by the endoplasmic reticulum which are then used by the cell
5. Chemically bonds carbohydrates with proteins provided by the endoplasmic reticulum which are then used by the cell
6. Produces hormones from endocrine glands
7. Modifies proteins and phospholipids
8. Creates lysosomes used by the cell to get rid of unnecessary materials

The number and size of the Golgi varies dependent upon the organism. Animal cells, for instance, will average 2-3 of these organelles, while plant cells will have up to 10.

The structure of the Golgi apparatus resembles a series of curved, pancake- like membranes, with several interconnections. Each membrane separates what is inside the Golgi from the cytoplasm of the cell. One end of the stack, called the “cis” end, receives materials, while the other end of the stack, called the “trans” end, ships materials. As you might expect, the receiving end is closely associated with its supplier of materials, the endoplasmic reticulum.

During cell division, this organelle disassembles itself into vesicles that are then divided into the daughter cells. During the last phase of division, these vesicles will reassemble to create the Golgi apparatus. The steps in the process that needs to take place in order for this to occur have not yet been determined.

Endoplasmic reticulum

As a whole, the endoplasmic reticulum is responsible the creation and export of proteins, as well as membrane lipids. This organelle extends from the cell membrane through the cytoplasm and forms a continuous connection with the nuclear membrane. Two forms of this organelle exist in eukaryotic cells: smooth endoplasmic reticulum ( also known as SER) and rough endoplasmic reticulum (also known as RER). The number of each form found within a particular cell varies and is influences by changes in the metabolic activity of the cell.

Figure 3.3.6 illustrates the location and structural components of the smooth and rough endoplasmic reticula. The key to the numbered structures follows.

1 Nucleus 2 Nuclear pore 3 Rough endoplasmic reticulum (RER) 4 Smooth endoplasmic reticulum (SER) 5 Ribosome on the rough ER 6 Proteins that are transported 7 Transport vesicle 8 Golgi apparatus 9 Cis face of the Golgi apparatus 10 Trans face of the Golgi apparatus 11 Cisternae of the Golgi apparatus

Smooth endoplasmic reticulum

The smooth endoplasmic reticulum is specifically responsible for the creation of lipids, phospholipids, and sterols, such as cholesterol. These, in turn, are used by the cell for cell membrane repair, communication between cells, metabolism, and detoxification.

Structurally, the smooth endoplasmic reticulum is composed of folded membranes and contains a tubular network acting as connections between these membranes. It is a separate structure from the rough endoplasmic reticulum ( Figure 3.3.7, letter a).

Rough endoplasmic reticulum

The rough endoplasmic reticulum is a highly folded, flattened, sealed sac that occupies a large portion of the cell and connects directly to the nuclear envelope. Ribosomes are embedded throughout this organelle, thus giving it the appearance of being rough. (Figure 3.3.7. letter b). This organelle is found in higher density near the nucleus and Golgi apparatus.

Proteins and amino acids from the cytoplasm of the cell are taken in by the rough endoplasmic reticulum to continue protein finishing. Once completed, chemical identification labels are attached to ensure the finished protein gets to its proper destination. The RER creates proteins for several parts of the cell including the plasma membrane, Golgi apparatus, lysosomes, and the endoplasmic reticulum itself, among others.

An animation illustrating the creation of a protein and its movement into the RER.

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Figure 3.3.7

Figure 3.3.7 presents a diagram of the rough and smooth endoplasmic reticula (a), as well as a photomicrograph of the actual rough endoplasmic reticulum (b) and the smooth endoplasmic reticulum (c) in a cell.

Peroxisomes

Peroxisomes are small membrane bound structures that contain enzymes required for a variety of chemical reactions of the cell. In animal cells, they are responsible for the breakdown of fatty acids (lipids). In plant cells, they are needed to provide the raw materials and energy for growth of germinated seeds. In addition, peroxisomes are used by plants in the process of photorespiration.

Vesicles

Vesicles are found in many eukaryotic cells, including plants and animals. A vesicle is a found in the cytoplasm of a cell which has a specific function. Typically, vesicles can be categorized as: lysosome, secretory, transport, and vacuole, as illustrated in the chart below.

Vesicles tend to be more numerous and smaller in animal cells. Vesicles in plant cells tend to be fewer in number and larger than those in animal cells. Most plant cells will contain a large central vacuole that holds water. The central vacuole is essential to maintaining turgor, which allows plant cells to be rigid. This rigidity is due to the amount of water found in the central vacuole, which is influenced by osmosis. Loss of turgor results in plants wilting. In addition to storage of water, plant cell vacuoles are also involved in detoxification of materials and destruction of old cell organelles.

Structurally, vesicles are simply a membrane composed of a phospholipid bilayer surrounding fluid containing specific materials, dependent upon the vesicle’s function.

Organelles found only in animal cells

Centrioles

In pairs, centrioles play a role in the process of animal cell division (mitosis). They are associated with the organization of protein microtubules needed for the creation of a spindle. The spindle is a structure that enables the movement of chromosomes during the phases of cell division. Centrioles are also needed to ensure the completion of cytokinesis, the physical separation of newly created animal cells.

Structurally, a centriole is composed of protein tubes. Nine microtubule triplets are arranged in a ring, with each triplet arranged at a ninety degree angle to the other, as seen in Figure 3.3.8.

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Figure 3.3.8

Figure 3.3.8 is a diagram of the organization and arrangement of the microtubule proteins that composed the centrioles of an animal cell 

Organelles found only in plant cells

Chloroplast

Only those eukaryotic cells that possess chloroplasts are able to use water, carbon dioxide, and sunlight to create complex sugars, oxygen, and energy. This is accomplished through the process of photosynthesis. Chloroplasts can be found in plant cells, and protists, but not in animal cells.

Figure 3.3.9 is a diagram illustrating the structure of a typical chloroplast.

The primary functions of chloroplasts are to collect light and to create energy in the form of sugars and ATP. Recently, it has also been determined that these organelles also participate in the immune response in plants.

A typical plant chloroplast is composed of three membranes and contains a protein rich material called the stroma. The outer membrane is permeable to small molecules and ions, but not permeable to larger proteins. It provides protection and support to this organelle and is separated from the inner membrane by the inter-membrane space.

The inner membrane forms a border to the stroma, and regulates the passage of materials into and out of the chloroplast. It is also the site for the creation of fatty acids, lipids, and carotenoids (a pigment used by plants for photosynthesis) required by this organelle.

The third set of membranes, the thylakoid system, is a group of flattened sacs arranged in stacks (called grana) throughout the chloroplast. Here, the light reactions of photosynthesis take place to convert radiant energy into chemical energy.

The stroma is a protein rich material that fills the chloroplast, surrounding the grana. Here carbon dioxide is used in the photosynthesis reactions to create carbohydrates for the cell.

As with the mitochondria, chloroplasts possess their own DNA and RNA, different from the cell they are in, and chloroplasts are able to reproduce independently of the cell they are in.

Other cell structures found only in plant cells

Cell wall

Found in some eukaryotic cells, such as fungi, plants, and algae, but not in others, most notably animal cells, the cell wall encloses the entire cell and is outside of the cell membrane. Unlike the plasma membrane, this structure provides physical support for the cell, helps maintain cell shape and prevents the cell from bursting due to osmotic pressure changes.

The composition and structure of the cell all varies dependent upon which eukaryotic cell is observed. Plant cell walls contain primarily cellulose, a large polymer. Typically, the plant cell wall has up to three separate layers, each contributing to the structure of the cell. True fungi have cell walls composed of chitin, glucans (carbohydrate chains), and several types of proteins. Some of the proteins in the fungal cell walls are structural; others are enzymes that help to create and destroy the cell wall.

Additional structures to note

In addition to organelles, eukaryotic cells also possess a cytoskeleton, and some will have external appendages (flagella, pili, or both).