Cellular Biology and Organelles Explained

Studying the cell, the most basic unit of all life, was made possible was made possible by the invention and perfection of the microscope by Antoni van Leeuwenhoek and Robert Hooke in the early 1600s. After first observing the checkerboard-like structure of cork under a microscope, Hooke called these structures “cells”, which literally means “little rooms”. These early scientists could see the anatomy, or structure, of cells. They did not understand the cells’ physiology, or function, and little did they know that they were looking at the basic unit of all life. The three tenants of modernly accepted Cell Theory that provide the basis for our study in cellular anatomy and physiology are (1) All living things are made of cells, (2) Cells are the fundamental unit of structure in living things, and (3) Cells only arise from pre-existing cells.

I. Semi-Permeable Plasma Membrane: Essential Structure Common to All Cells

The Semi-Permeable Plasma Membrane is the only structure that all cells, regardless of type, have in common. The plasma membrane is a phospholipid bilayer composed of two layers of organic phospholipids interspersed with receptors and transport proteins. The plasma membrane separates the neutral pH of the cytosol, or inside of the cell, from the environment outside the cell.

A single phospholipid has a hydrophilic phosphate head and two hydrophobic fatty tails. Because the heads are hydrophilic, meaning they are attracted to H2O, the heads will always face outward, while the fatty tails make up the internal area of the membrane. Keep in mind that there are two layers of these phospholipid molecules, so the heads will face outward to the exterior of the cell on one side, and outward to the interior of the cell on the other sidelike making a sandwich where the heads are the bread and the fatty tails are the center. According to the Fluid Mosaic Model, the molecules in this conformation can move freely about the membrane.

Physiologically speaking, the plasma membrane is responsible for the transport of water, minerals, proteins, wastes, and all other molecules into and out of the cell. The plasma membrane is “selectively permeable”, meaning that it allows some substances, such as water, to pass freely through the membrane while blocking other substances, or allowing substances only by the use of special activated protein channels. Free movement of water through the membrane by osmosis, or the movement of water from areas of higher concentration to lower concentration, is critical to the plasma membrane’s function.

II. Two Types of Cells: Eukaryotic and Prokaryotic

Eukaryotes and prokaryotes are the two basic forms of cells. Unicellular creatures are known as prokaryotes are made of only one single cell. Eukaryotes are generally more complex and can be unicellular (single celled) or multicellular (having many cells). Animals, plants, fungi, protozoans, and algae all possess eukaryotic cell types. Only bacteria have prokaryotic cell types. We refer to the interior of both types of cells as the cytoplasm, however, the structures contained in the cytoplasm of each cell are very different.

III. Eukaryotic Cells

Eukaryotic cells contain many membrane-bound organelles, a defined nucleus containing X-shaped chromosomes, and a cytoskeleton.
When we say “membrane-bound” we means that these organelles (or the cell’s “organs”) have their own membranes, much like the plasma membrane that surrounds the cell. Many scientists theorize that what are now organelles of eukaryotic cells were once free-swimming prokaryotic cells that formed a symbiotic relationship, evolving into their current forms and losing their ability to live on their own.

Eukaryotic Cells: Nucleus

The nucleus of a eukaryotic cell is contained by a semi-permeable double membrane much like the one of the exterior of the cell.
This nuclear membrane is known as the envelope. The nuclear pores are small holes and transport proteins that allow for transfer of material into and out of the nucleus to the cytoplasm. The nucleus contains the genetic material of the cell, known as the DNA (deoxyribonucleic acid), which is critical to reproduction of the cell. DNA is normally represented in loose strings called chromatin, though during replication it takes on the familiar X shape referred to as chromosomes.

Eukaryotic Cells: Ribosomes

Ribosomes are the cells protein production center. They are found attached to the outer membrane of the endoplasmic reticulum (called Bound Ribosomes) or free in the cytoplasm (called Free Ribsomes).
Ribosomes are composed of a large and small subunit made of rRNA and proteins produces in the nucleolus.

Eukaryotic Cells: Endoplasmic Reticulum

The endoplasmic reticulum (ER) exists in two forms that appear as a large folded membrane in the cytoplasm. Rough ER contains protein-producing ribosomes and is primarily a site of protein production and transport. Smooth ER contains no ribosomes and is primary responsible for lipid synthesis and transport. The ER transports substances inside the cell, particularly substances to be secreted.

Eukaryotic Cells: Golgi Apparatus

The golgi apparatus is a stack of membrane-enclosed sacs that receives vesicles, or membrane-bound packages, from the smooth ER and modifies, repackages, and distributes them. Molecules destined to be secreted from the cell are packaged into secretory vesicles by the golgi apparatus and prepare to be released by a process known as exocytosis.

Eukaryotic Cells: Vesicles and Vacuoles

Vesicles and vacuoles are membrane-bound sacs involved in transport or storage of molecule in cells. Vacuoles are larger and more likely to be found in plant cells.

Eukaryotic Cells: Lysosomes

Lysosomes are specialized vesicles containing a hydrolytic enzymes at a pH of approximately 5, which is much more acidic than the normal cytosol inside a cell. Think of the lysosome as the “stomach” of the cell. The lysosome breaks down nutrients and other molecules, digesting them to a point that the can be used or reused by a cell.

Eukaryotic Cells: Microbodies

Microbodies are specialized vesicles used for storage.
A common type of microbody in plant cells is the glyoxysome, which stores the sugars produces in photosynthesis.

Eukaryotic Cells: Mitochondria

Mitochondria are the energy production centers for a cell.
Unlike all other organelles, mitochondria are semiautonomous, possessing their own DNA and produce some of their own proteins. Also, during cellular reproduction mitochondria reproduce themselves by binary fission, much like bacteria or prokaryotic cells.

Eukaryotic Cells: Cytoskeleton

The cytoskeleton is made up of hollow microtubules and provides support to the cell and its internal structures. Specializes microtubule structures, such as hair-like cilia and tail-like flagella may also allow the cell to move; however, these structures are not present in all cells.

Eukaryotic Cells: DNA

The DNA of eukaryotic cells is much longer and more complex than prokaryotic DNA, and in order to keep it organized into the condensed structure used in the cell, it is paired with complex proteins known as histones. When DNA is replicated it goes through a complex process before it gets into its final condensed structure, as found in most mature cells. It is first wrapped around a nucleosome (achieving a packing ratio of about 6), then it is placed into a its final helical structure (achieving a packing ratio of about 40), and finally it is coiled into loops (achieving a packing ratio of about 1000).

Eukaryotic Cells: Reproduction by Mitosis

Eukaryotic cells reproduce through a process known as mitosis, where the cell duplicates its genetic material and generates two identical daughter cells. This process is generally divided into (1)Interphase (2)Prophase (3)Metaphase (4)Anaphase, and (5) Telophase. Interphase is the longest phase, where the cell prepares for division. During prophase loose chromatin DNA coils into the recognizable X shaped chromosomes. In metaphase bone-like spindle fibers form through the nucleus and connect to kinetochores on the DNA. The X chromosomes are split into two individual sister chromatids. In Telophase two new nuclear envelopes form. Simultaneously to telophase, the cell divides in a process known as cytokinesis, forming two daughter cells.

Eukaryotic Cells: Condensing DNA

The centromeres are condensed regions of DNA essential to proper separation of DNA for reproduction. If DNA is thought of as a capital X, then the centromeres is located at the junction point of the sister chromatids, or the center of the X.
The telomeres are regions at the end of each DNA strand that protect the strand from binding with other strands during replication. Think of the telomeres as a disposable buffer zone protecting your DNA. As you age, some telomeres are reduced are destroyed, and it is possible that telomeres play a critical role in aging. When telomeres are destroyed, genetic rearrangements can cause critical conditions, such as cancer.

Eukaryotic Cells: Animals Cells vs. Plant Cells

Though animal and plant cells contain fundamentally the same structures, animal cells are often more amorphously shaped, while plant cells are generally rigid. The Cell Wall is a structure unique to plant cells. It is a hard outer shell that provides structural support and helps to maintain the internal pressure of the plant cell when water levels outside are too high or too low. The cell wall is primarily composed of a polymer of glucose called Cellulose. Animal cells also have a unique structure not found in plant cells, called the Centrioles.
The centrioles, found in the centrisome, are a specialized type of microtubules used in cellular division only in animal cells.

IV. Prokaryotic Cells

Unique properties of prokaryotic cells include the nucleoid region, circular DNA rings, and plasmids. These structures are only found in single-celled prokaryotes, but prokaryotes also contain a cell wall, small flagella, ribosomes, and a cell membrane, which many be found in both eukaryotic and prokaryotic cells.
Because prokaryotes contain no nucleus, they reproduce differently from eukaryotic cells.

Prokaryotic Cells: Nucleoid Region

The nucleoid region, like the nucleus of eukaryotic cells, contains the DNA for the cells. Unlike the nucleus, which contains X-shaped chromosomes, DNA found in the nucleoid region is one continuous circle called a genophore.
Genophores are generally much smaller than chromosomes, and can contain as few as 580,000 individual base pairs.
In addition to the primary genophore, addition self-replicating DNA circles called plasmids may by found in this region.

Prokaryotic Cells: Reproduction by Binary Fission

Because prokaryotic cells do not contain membrane-bound organelles, a structured nucleus, or complex super-coiled chromosomes, they reproduce by a simpler method called Binary Fission. During Binary Fission, DNA replicates itself and then cytokinesis, or pinching of the plasma membrane that results in an eventual split into two individual cells, occurs.

Prokaryotic Cells: Locomotion

About half of all bacteria are capable of locomotion, or moving on their own. The most common means of locomotion for a bacteria is one or more flagella, or whip-like tail structures composed of a helical flagellin structure and microtubules.
Gram-negative and gram-positive bacteria have distinctly different flagella, but both are normally smaller than Eukaryotic flagella.