Cells

Structure and Function of Cell Organelles:

First off, an organelle is classified as any number of specialized or organized structures within a living cell. Here we will learn how these organelles help the cell function as well as some common structures. There are three major divisions to every eukaryotic cell (we will discuss prokaryotic cells shortly); the nucleus, cytoplasm and the surface.

Nucleus

There is at least 1 nucleus in every eukaryotic cell however some can contain more than one. They are usually spherical in shape and denser than the surrounding cytoplasm. Inside the nucleus you will find chromosomes which contain genetic information, are composed of DNA, and thicken for cellular division. Every species has a specific number of chromosomes, for example, humans have 23 sets.

The nucleus is surrounded by a nuclear membrane which is double layered and has many openings for nuclear traffic. It also contains the nucleolus which is the chief structure in the nucleus. The nucleolus makes ribosome which transfers out of the nucleus and contain RNA for protein manufacturing.

Cytoplasm

Next, we will discuss the cytoplasm which is the collective word for cytosol and the organelles that are encompassed within it. The cytosol is mostly made up of water and free-floating molecules. Its viscosity continually changes and the proteins within it control cell metabolism.

One kind of organelle inside the cytoplasm is known as the centrioles which are paired and lie near the nucleus. They are made up of 9 tubules and are involved in cellular division. They also lie at right angles to each other and are only found in animal cells.

Next, we will consider the cytoskeleton which is in both animal and plant cells. As the name may indicate, cytoskeletons give cells support and provide shape. Its main function however, is to aid in the movement of materials in and out of the cell in their microtubules.

The endoplasmic reticulum is an organelle with two different varieties. The smooth type lacks ribosome and the rough type has them ingrained in the surface. Both are a network made from tubes that are fused to the nuclear membrane. They also separate and serve as the cell’s transport system.

The Golgi apparatus is comprised of various layers that form a sac. Essentially, the Golgi apparatus is a packaging factory for proteins.

Lysosomes are where proteins, lipids and carbohydrates are digested. They are rare in plant cells but common in animal cells. They also take away undigested material and differ in shape depending on what is being carried out. Lysosomes explode if the cell breaks down.

Mitochondria control the level of water and other materials in the cell. It recycles and breaks down proteins, carbohydrates and fats. Like the nucleus, it has a double layered membrane, the inner of which has a lot of folds that are called cristae. The cristae is where sugar is combined with oxygen to produce the cell’s energy.

Every cell accommodates thousands of ribosomes which can be embedded in the rough endoplasmic reticulum or can be mobile. Ribosomes are very heavy and equal 25% of the entire cell’s mass. Their job is to generate proteins.

And the final organelles that are contained in the cytosol are the vacuoles. Also bound in a membrane, these sacs are used for waste removal, storage and digestion. They also accommodate a solution of water. In plant cells, they are often very large and have more jobs such as making the cell bigger, regulating pressure, and producing plant rigidity.

The Surface

The third and final division of the cell is its outer most layers or the surface. The cell wall is only found in plants and is an extra cellular structure that surrounds the plasma membrane and acts as the primary cell wall. It is extremely elastic.

The plasma membrane is found in both animal and plant cells and controls the cellular traffic. It contains proteins that also allow materials to pass through it.

Properties of Cell Membranes:

The cell membrane or plasma membrane is a collective or common feature of all cells. The membranes are made up of proteins and lipids and control how materials move through the cell. Some substances can pass more easily than others because the plasma membrane regulates the movement of different substances.

The membranes are made up of a phospholipid-bilayer (a lipid with one or more phosphate group attached). This bilayer can be partially penetrated by water and some other small molecules. Phospholipids have 2 parts; a “head” which is hydrophilic (can mix with water) and 2 “fatty acid tails” which repel water (hydrophobic).

Osmosis is one function of the cell membrane. This is the term used for the penetration of water through the semipermiable cell membrane. Osmosis can develop pressure known as osmotic pressure.

The membrane is very complex due to the diverse number of proteins that are essential for the cell’s functions. These proteins are capable of receiving tastes, odors, and hormones. They also must regulate the motion of substances entering and exiting the cell like calcium, sodium and chloride.

Diagrams of cell membranes

Comparison of Prokaryotic and Eukaryotic Cells :

All cells can be put into two main categories, prokaryotic and eukaryotic. Both of these divisions of cell types have commonalities and differences but they have more similarities than irregularities so let’s start by talking about their shared qualities.

They both have DNA as their genetic material and are covered by a cell membrane. They also both have RNA and are composed of the same basic chemicals; carbohydrates, proteins, minerals, nucleic acid, vitamins and fats. Another likeness is that their proteins are made in ribosomes and both cells regulate the flow of waste and nutrients that exit and enter the cell. Each prokaryotic and eukaryotic cells have similar life processes and require an energy supply.

Now we will discuss some of the differences between prokaryotic and eukaryotic cells. The two most prominent being structure and age. Prokaryotic cells arrived on a “newly” formed earth around 3.5 billion years ago. Almost 2 billion years before eukaryotic cells. There is also some compelling data that suggests that eukaryotic cells evolved from groupings of prokaryotic cells that progressed to be interdependent on each other.

Now let’s learn about the structural differences. Eukaryotic cells have two important aspects that are absent in prokaryotic cells, a nucleus and organelles with membranes. And although both types of cells contain DNA, in eukaryotic cells the DNA is stationary in the nucleus where as in prokaryotic cells they float freely in an unorganized soup. And because of the organelles in the eukaryotic cells, the cell is capable of carrying out more complex functions. Size is another key difference. Eukaryotic cells are generally bigger, up to ten times the prokaryotic size. Also, their cell walls and flagella arrangements are different in composition and the manner in which they function.

The following site has a great Cell Interactive Diagram

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