Cell Structure And Transport Biology Essay

Cells can be split into two types, prokaryotic (see figure 1) and eukaryotic (See figure 2). Karyose originates from a Greek, meaning kernel, as in a kernel of grain. In physiology, it is used to refer to the nucleus of a cell. Pro means before, and eu means true. Therefore you could say that prokaryotic means “before a nucleus” and eukaryotic means “possessing a true nucleus.” (Wikipedia)

Prokaryotic and eukaryotic cells have many differences and few similarities.

Eukaryotic cells and lager than prokaryotic cells, a eukaryotic cell is usually 10nm in diameter where as a prokaryotic cell in 1nm in length.(Class Notes)

Eukaryotic cells contain a nucleus, protected by a double membrane. Prokaryotic cells do not contain a nucleus. The purpose of the nucleus is to sequester the DNA-related functions of the big eukaryotic cell into a smaller chamber, for improved efficiency. This function is unnecessary for the prokaryotic cell as the cell is smaller and all materials within the cell are close together. However prokaryotic cells do have DNA and DNA functions.

Eukaryotic DNA contains proteins called histones, and these are organised into chromosomes; prokaryotic DNA has no histones, there are no chromosomes in a prokaryotic cell. A eukaryotic cell contains a number of linear chromosomes; a prokaryotic cell contains only one circular DNA molecule and a varied assortment of much smaller circlets of DNA called plasmids. A prokaryotic cell is smaller and simpler; it requires far fewer genes to operate than the eukaryotic cell. (Rowlands, 2000)

Eukaryotic cells contains large numbers of mitochondria, the number of these varies according to the size and type of cell. Mitochondria support the protein of the electron transport chain of the aerobic respiration within the cell supplying the cell with ATP, but this organelle is not located in a prokaryotic cell. (Williams)

Eukaryotic cells also contain endoplasmic reticulum, however there is none present in a prokaryotic cell. Endoplasmic reticulum comes in two forms, rough and smooth. Rough endoplasmic reticulum helps to compartmentalize the cell, and it serves as routes for the transport of materials from one part of the cell to another. It is also associated with lipid synthesis and protein synthesis. Smooth endoplasmic reticulum is responsible for generating new layers for golgi bodies. Golgi bodies are stacks of sacs called cisternae. The function of golgi bodies is to process materials manufactured by the cell. The other kind of golgi vesicle contains materials to be secreted from the cell. These materials are not waste products but chemicals manufactured by the cell for export (hormones and pheromones for example).

Prokaryotic and eukaryotic cells have many ribosomes, however the ribosomes of the eukaryotic cells are larger and more complex than those of the prokaryotic cell. Ribosomes are made out of a special class of RNA molecules and a specific collection of different proteins. A eukaryotic ribosome is made up of five types of rRNA and approximately eighty types of proteins. However a prokaryotic cell¿½s ribosomes are made up of only three types of rRNA and approximately fifty types of protein.

The cytoplasm of eukaryotic cells contains a complex collection of organelles and many of the organelles are enclosed in their own membranes. However the prokaryotic cell contains no membrane-bound organelles that are independent of the plasma membrane.

Some prokaryotic cells contain flagella. These organisms which are usually single celled and are used for locomotion. Unlike the flagellum of a eukaryotic cell, this mechanism lacks an internal structure. The prokaryotic flagellum is shaped like a corkscrew and propels the cell where as in a eukaryotic cell where the flagellum is more complex it moves in a lashing action. (Wikipedia)

Prokaryotic and eukaryotic cells also differ in regards to what they store. A prokaryotic cell store specialised compounds in the form of granules or droplets. Where as a eukaryotic cell stores glycogen, starch, lipid and in some cases specialised materials of particular organisms.

Prokaryotic cells are found in bacteria and blue green algae, eukaryotic cells are found in fungi plants and animals. (Class Notes)

To summarise, prokaryotic and eukaryotic cells are similar in the fact that they are both are contained by plasma membranes, filled with cytoplasm and also contain ribosomes. Both contain the same kind of DNA as the genetic code for a prokaryotic cell is the same for a eukaryotic cell. However they have many materials present in a eukaryotic cell which are not present in a prokaryotic cell. This is because a eukaryotic cell is much more complex and has more complex process to complete.

Microscopic appearance of squamous cells

2. The attached diagram represents an electron micrograph of a liver cell. Identify the structures:

Label

A

Rough Endoplasmic reticulum

B

Mitochondria

C

Nucleus

D

Nucleus Membrane

E

Cell Membrane

F

Smooth endoplasmic reticulum

G

Cytoplasm

3. Using appropriate detail with diagrams and sizes where needed, explain how the structure of the following organelles is related to their function:

A: Rough Endoplasmic reticulum

Most ribosomes are attached to the surface of the rough endoplasmic reticulum. The endoplasmic reticulum is a series of interconnecting flattened tubular tunnels, which are continuous with the outer membrane of the nucleus. It runs through the cytoplasm of all eukaryotic cells. The ER of a cell often takes up more than a tenth of the total cell volume. Rough ER appears rough due to the presence of ribosomes on the membrane. The rough ER is concerned with the transportation of proteins, which are made by the ribosomes on the surface of the membranes. The membranes form a series of sheets known as lamellae of reticulum, and these enclose flattened sacs called cisternae. The purpose of this is to form a maze-like structure so that there is a very large surface area for chemical reactions to take place. Information coded in DNA sequences in the nucleus is transcribed as messenger RNA. Messenger RNA exits the nucleus through pores in the nuclear envelope to enter the cytoplasm. At the ribosomes on the rough ER, the messenger RNA is translated into proteins. The newly made proteins are taken into the tubes of the rough ER so that they cannot escape into the cytoplasm, and are threaded through pores in the membrane to accumulate in the cisternal space where they can fold into their normal three-dimensional shape. Proteins made by the rough ER are then either secreted or used where they are needed within the cell. (Roberts, 2009)

B: Mitochondria

The mitochondrion produces energy in the form of ATP (adenosine triphosphate), which is produced when organic compounds are completely broken down to carbon dioxide and water. One of the main conditions for this process to occur is the presence oxygen. Each mitochondrion has a double-membrane system. The inner membrane is folded repeatedly and it is also known as cristae. The outer membrane faces the cytoplasm. This complex system creates two divisions. In the outer division, enzymes and other proteins stockpile hydrogen ions. These ions then flow into the inner compartment. The energy inherent creates ATP-formation. Hydrogen binds with oxygen to produce the end product, water. (Roberts, 2009)

C: Nucleus

The nucleus is usually spherical and about 10¿½m across, it has two main functions. Firstly, it segregates all the DNA molecules from the cytoplasm. Secondly, the nuclear membranes administer as a boundary where cells control the movement of substances to and from the cytoplasm. The nucleus is made up from a few constituents. The nuclear envelope is a double-membrane system that has two lipid bilayers in which many protein molecules are firmly established. This prevents water-soluble substances to move without restriction into and out of a nucleus. The second component is nucleolus, which is a highly packed cluster of RNA and proteins, which construct to make subunits of ribosomes. (Roberts, 2009)

4. The structures A and F may be functionally associated in extracellular secretions (as for example salivary glands). Give an illustrated answer to show this association.

The rough endoplasmic reticulum is involved in the formation of the Golgi body. The Golgi body is created by the rough endoplasmic reticulum as a sequence of vesicles containing protein. As these come together the protein modifies with carbohydrate molecules attached, this leading to glycopeptides being produced. The glycopeptides are then ¿½pinched¿½ from the ends of the Golgi body and move toward the cytoplasmic membrane. These then fuse together and a material is secreted. (Class Notes)

5. Lysosomes are important in certain intracellular activities. Using a sequence of annotated diagrams, show the signifigance of these structures in the destruction of unwanted organelles.

Lysosomes are organelles that contain digestive enzymes referred to as acid hydrolases. They dissolve and digest excess or worn out organelles,(see figure 4) food particles, and engulfed viruses or bacteria. The membrane surrounding a lysosome prevents the digestive enzymes inside from destroying the cell. Lysosomes fuse with vacuoles and dispense their enzymes into the vacuoles, digesting their contents. They are built in the Golgi apparatus. (humpath.com)

6. Using diagrams describe the basic chemical structure of the cell membrane (the bimolecular components) and explain why this structure is referred to as the ¿½fluid mosaic model¿½.

The cell membrane separates a watery cytoplasm from a watery external environment also referred to as the extracellular fluid. The phospholipids are arranged in a double layer (lipid layer) with the hydrophilic heads facing outward (cytoplasm or extracellular fluid) and the hydrophilic tails turned in towards each other. The unsaturated fatty acids are not bonded to each other therefore forming a fluid. (Williams)

Embedded within the lipid bilayer are different proteins (hydrophobic parts interact with the hydrophobic tails of the phospholipids). The hydrophilic parts of the proteins protrude either in to the cytoplasm or into the extracellular fluid. Some of the proteins are anchored in place while other proteins move about within the bilayer. (Williams)

Various organelles within the cytoplasm are also bounded by membranes similar in structure to the cell membrane except for specific combinations of lipids and proteins depending on the particular functions of the organelle. (Williams)

Some of the proteins extending into the extracellular fluid have carbohydrates (glycoprotein) attached.

Fluid mosaic model (Figure 6)

The term ¿½fluid¿½ is used because the phospholipid molecules and proteins that make up the membrane are free to drift around in fluid motion. The term ¿½mosaic¿½ is used to describe the position of the protein molecules. The molecules are placed randomly and there is no set pattern. (Wikipedia)

b. Describe concisely how the phospholipids and the protein molecules in the cell membrane make the membrane selectively permeable and are involved in cell recognition as the therapy using tissues typing/ blood transfusion.

The cell surface membrane is the boundary between the cell and its environment; it has very little mechanical strength but plays a vital role in controlling which materials pass into and out of the cell.

Cell membranes contain phospholipids, proteins, cholesterol and polysaccharides. The phospholipids are a major constituent of cell membranes. They naturally form membranes in water because they automatically arrange themselves into a bilayer that is practically impermeable to water and anything that is water soluble.¿½ The membrane proteins act as hydrophilic pores; these are water filled channels that allow water-soluble chemicals to pass through. These pores are usually small and highly selective, proteins in the membrane that form pores usually span the entire membrane, but other proteins with other functions can occur only in the top or bottom layer of lipids.

Some membrane proteins serve as carrier molecules in which ions pass through channels in the protein molecule. The movement of ions may occur through facilitated diffusion. The ions may also move against a diffusion gradient in a process known as active transport. Unlike facilitated diffusion, active transport requires ATP. Membrane proteins may also be associated with cell recognition in which patrolling T-cells and antibodies recognize the shape of membrane proteins as “self” or “foreign.” These membrane proteins often contain unique carbohydrate chains (antennae) which are involved in the cell recognition process. Recognition glycoproteins may include receptor sites for some hormones and neurotransmitters and various blood antigens. (Cellsalive.com)