Eukaryotic Cell Structures

EUKARYOTIC CELL STRUCTURES

Eukaryotic cells have each of the things found in a prokaryotic cell plus a membranebound nucleus containing DNA, multiple membrane-bound organelles, and several chromosomes. The organelles include Golgi-apparatus, chloroplasts, endoplasmic reticulum, vacuoles, and mitochondria. Each organelle has a specialized role with the membranes allowing for compartmentalizing of the different cellular functions. Figure 12 shows the interior of a eukaryotic cell:

Figure 12.

The nucleus is the largest and most prominent organelle in the cell. The DNA is completely surrounded by the nuclear envelope. The DNA within the cell’s nucleus

directs the synthesis of proteins and ribosomes, which are responsible for protein synthesis. The envelope is a double membrane—each consisting of phospholipid bilayers. There are pores within the envelope that control the passage of molecules, RNA, and ions through the membranes.

The nucleoplasm is the fluid within the nucleus. It is semisolid in structure and contains the chromosomes made from DNA. Figure 13 shows the human cell nucleus:

Figure 13.

Surrounding the nuclear envelope is the endoplasmic reticulum, which is continuous with the nuclear envelope. This configuration allows for RNA to exit the nucleus and take part in the synthesis of proteins in the rough endoplasmic reticulum, which is itself studded with protein-producing ribosomes. The smooth endoplasmic reticulum, on the other hand, does not contain ribosomes and is responsible for lipid synthesis.

Mitochondria in the eukaryotic cells are oval in shape and double-membrane bound. They have their own DNA and protein-producing ribosomes. They make the ATP that is necessary for cellular energy by engaging in cellular respiration. The Golgi apparatus is the “post office” of the cell. It sorts, labels, packages, and distributes the substances made by the cell. Peroxisomes are single-membrane bound organelles that participate in redox reactions that break down amino acids and fatty acids, detoxifying cellular toxins.

There are many vesicles and vacuoles inside the cell. These are single-layer membranous sacs that transport and store substance within the cell. Some will take up extracellular substances, while others will relieve the cell of waste products by combining with the cell membrane, allowing the waste products to leave the cell.

All living things have a plasma membrane, which is part of what defines a living thing. It separates the inside of the cell from the extracellular space. While containing a phospholipid bilayer, as mentioned, it also consists of cholesterol and multiple plasma membrane proteins that have many functions. Everything that enters and leaves the cells comes through the plasma membrane. Figure 14 shows what the cell membrane looks like:

Figure 14.

The cell membrane is not static but is fluidic. The phospholipid molecule is extremely important in making the cell membrane because it has a polar end and a nonpolar end. The integral proteins that exist within the membrane have hydrophobic and hydrophilic components that allow them to imbed within the cell membrane. Some of these proteins form pores that allow certain ions, nutrients, and amino acids to enter or leave the cell, depending on the circumstances. The water-soluble substances, such as electrolytes, amino acids, and glucose, cannot pass through by themselves so they need an ion channel or other hydrophilic condition in order to pass through.

There are substances that pass through actively (requiring energy) or passively (not requiring energy). Passive transport involves simple diffusion or facilitated diffusion. Water passes through via osmosis, which only works for water. Water passes down through its concentration gradient. Those things that require energy are transported through active transport, which involves substances like ATP to provide energy.

Endocytosis and exocytosis involve the movement of substances via vesicles that fuse with the membrane rather than being passed through membrane itself.

The purpose of the nucleus of the cell is to store DNA plus the proteins that together form chromatin. In eukaryotes, the chromatin is not in a circular format but is linear and found in several separate chromosomes. In humans, there are 46 chromosomes. Other organisms have more or fewer chromosomes than this. They are generally invisible unless they are in the process of dividing. DNA is wrapped around histone proteins, which together make chromatin.

The nucleolus resides within the nucleus. It is an area of condensed chromatin that is responsible for the synthesis of ribosomes. Ribosomes are complexes of RNA and protein that make proteins for the cell. They are made in the nucleolus and travel to the rough endoplasmic reticulum. Their job is to read the messenger RNA message, which has been transcribed from the DNA of the cell, and use transfer RNA to add one amino acid at a time to a growing peptide chain. Figure 15 shows what a ribosome looks like:

Figure 15.

The mitochondria are double-membrane structures, with an inner membrane that has many infoldings called cristae. This inner membrane contains many proteins and enzymes that are responsible for aerobic respiration. This is where ATP is synthesized. Figure 16 is what the structure of mitochondria looks like:

Figure 16.

As you can see by the figure, mitochondria have their own DNA, which is usually circular. They also have their own protein-making ribosomes. Because of the shape and structure of mitochondria, it is believed that mitochondria were evolutionarily once prokaryotic cells that have been incorporated into the structure and function of eukaryotic cells. They are essential to the life of the cell.

Mitochondria, as you will learn more about later, make ATP energy as part of cellular respiration. There is a series of chemical reactions that start with simple sugars, fatty

acids, and amino acids, making carbon dioxide and water, plus many molecules of ATP, which are later used as cellular fuel. Oxygen is necessary for this process to occur. Carbon dioxide and water are the waste products after these reactions have completed.

Cells that require a great deal of ATP energy, such as muscle cells, have a great many mitochondria in order to do this job. Cells that do not require a lot of ATP energy do not have many mitochondria. Red blood cells, for example, do not have mitochondria and do not participate in aerobic respiration. Instead, they participate in anaerobic metabolism, sometimes giving off lactic acid as a byproduct.

The centrosome is something found in animal cells but not in plant cells. Each centrosome contains one pair of centrioles, which are perpendicular to one another. The centriole consists of a cylinder of microtubules. The centrosome is where all the microtubules of the cell come from; it divides when the cell divides in order to pull chromosomes to opposite ends of the dividing cell during cell division.

Animal cells also have lysosomes, which are not found in plant cells. Lysosomes are where waste products accumulate and are taken care of by the body. This process in plant cells happens in the vacuoles of the cell instead. There are enzymes in the lysosomes that break down worn-out organelles, proteins, carbohydrates, lipids, and nucleic acids no longer needed by the cells. The environment inside lysosomes is highly acidic, allowing for their breakdown.

There are some differences between plant cells and animal cells. Plant cells have microtubule organizing centers, just like animal cells, but they do not have centrosomes. Plant cells also do not have lysosomes but have vacuoles instead. Plant cells have cell walls and chloroplasts, which are not seen in animal cells. Figure 17 describes what a plant cell looks like:

Figure 17.

Plant cells contain chloroplasts, which are the structures of the cells that participate in photosynthesis. Chloroplasts also have their own DNA and ribosomes, similar to mitochondria. The act of photosynthesis involves using carbon dioxide, water, and the energy of the sunlight in order to make sugar or glucose plus oxygen as a byproduct. Plant cells are autotrophs because they make their own food, while animal cells are heterotrophs that must ingest food in order to survive.

There is an inner and outer membrane in a chloroplast, similar to mitochondria. The biggest difference is that within the space inside the chloroplast are stacked membranous sacs called thylakoids. Each stack is called a granum. The stroma is the fluid around the thylakoids. The chloroplasts contain chlorophyll, which captures the light necessary for photosynthetic reactions. Figure 18 describes what a chloroplast looks like:

Figure 18.

The central vacuole is also present in plant cells but not animal cells. It allows for the regulation of the concentration of water inside the cell with differences in environmental conditions. The central vacuole will shrink under dry conditions in order to support the cell. It also supports the cell in conditions of increased water in the environment by holding more water under these circumstances.

The cell wall covers and protects the cell. It is present in protists and fungal organisms. In plants, the cell wall consists of cellulose, which is nothing more than repeating units of glucose. It allows for the crunchy sensation you notice when biting into a fresh fruit or vegetable.