Week 3: Organization of the Cell

Introduction to Organization of a Cell

This concept will describe what cell theory is, explain what cells are, explore the differences between prokaryotic and eukaryotic cells, and identify the three major components of a eukaryotic cell.

Prokaryotic cells are exclusively unicellular, consisting of a single cell, whereas eukaryotic cells can exist as unicellular or multicellular, contingent upon the specific organism in question. Prokaryotic cells do not have a nucleus or membrane-bound organelles.

The components present in all cells include the cell membrane (also called the plasma membrane), ribosomes, cytoplasm, and DNA.

Exclusively found in eukaryotic cells, the nucleus is a membrane-enclosed organelle that contains DNA. Prokaryotic cells possess a nucleoid, a distinct region within the cell that harbors the genetic material in DNA.

What Is a Cell?

A cell is an organized unit that can live and reproduce on its own by utilizing energy, raw materials, and DNA instructions.​

• Cells are the basic building block of life.​

• Our bodies are made of approximately three trillion cells! 

Principles of Cell Theory

• The smallest structural and functional unit capable of carrying out life processes is the cell. No smaller unit of life can reproduce and grow, respond to stimuli, maintain homeostasis, take in and use environmental materials, and adapt to the environment. ​

• Every organism is made up of one or more cells. ​

• Cells can only form from pre-existing cells.

Prokaryotic Cells

• "Pro" means "before."​

• "Karyon" means "nucleus."​

• Cells are more “primitive.”​

• Cells are always unicellular.​

• The internal environment of a cell is not divided into membrane-bound compartments.​

• Example of prokaryotic organisms: bacteria.

Eukaryotic Cells

• "Eu" means "normal."​

• "Karyon" means "nucleus."​

• Cells can exist as unicellular or multicellular, contingent upon the specific organism in question. ​

• The internal environment is divided into membrane-bound compartments called organelles.​

• Example of a eukaryotic organism: the human cell. 

Four Structures Common to Most Cells

​Most cells, whether prokaryotic or eukaryotic, share four structures in common:

• The plasma membrane (cell membrane) surrounds all living cells, separating internal and external aspects. ​

• DNA is the genetic material that controls an organism's composition and can replicate with the cell, resulting in new cells that contain the same genetic material as the parent cell. ​In eukaryotic cells, DNA is found in the nucleus.

• Cytoplasm is a rich, semifluid material found in cells. The cytoplasm is found between the nucleus and the cell membrane.​

  • Cytosol is the liquid part of the cytoplasm.​

• Ribosomes are the site of protein synthesis.

Bacteria are prokaryotic organisms. Prokaryotes are always unicellular.

Most cells share four structures in common:​

• A plasma membrane (cell membrane) surrounds all living cells, separating internal and external aspects. ​

• DNA is the genetic material that controls an organism's composition and can replicate with the cell, resulting in new cells that contain the same genetic material as the parent cell. ​

• Cytoplasm is a rich, semifluid material found in cells. The cytoplasm is found between the nucleus and the cell membrane.​

  • Cytosol is the liquid part of the cytoplasm.

• Ribosome is the site of protein synthesis.

Eukaryotic Human Cell

What exactly do our cells look like, and what do they do? ​

All human cells are made of the same materials:​

• The plasma (cell) membrane is the outer layer of cells made of a phospholipid bilayer embedded with protein channels.​

• Cytoplasm is the dense liquid that fills a cell and stores carbohydrates, organelles, and enzymes. This is found between the nucleus and the cell membrane.​

  • Cytosol is the liquid part of the cytoplasm.​

• The nucleus is the double-membraned enclosed organelle that stores your DNA.​

• The cytoskeleton is what gives our cells shape and protection.​

All our cells help to form our bodies and help us to move, think, eat, sleep, and excrete. Without cells, you wouldn’t be you!​
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Cytosol Versus Cytoplasm

Some texts refer to these terms synonymously:​

• Cytosol is the clear gel within the cell sometimes referred to as intracellular fluid (ICF),​

• Whereas cytoplasm is all the fluid and components between the nucleus and the cell membrane. The cytoplasm includes the cytosol as well as the cytoskeleton.

Gamete and Somatic Cells

There are two main types of eukaryotic cells in the human body:​

• Gamete (sex) cells consist of

  • the sperm in the male and

  • the ovum (egg) in the female.​

• Somatic (body) cells include all other cells in the body.​

  • Examples: red blood cells (RBC), epithelial cells, nerve cells, bone cells, muscle cells

Cytosol is the liquid part of the cytoplasm.​

• The plasma (cell) membrane is the outer layer of a cell made of a phospholipid bilayer embedded with protein channels.​

• Cytoplasm is the dense, semifluid material that fills a cell and stores carbohydrates, organelles, and enzymes. This is found between the nucleus and the cell membrane.​

• The nucleus is the double-membraned enclosed organelle that stores your DNA.​

• The cytoskeleton gives our cells shape and protection.

• The ribosome is the site of protein synthesis.

• The cytoplasm spans from the nucleus to the cell membrane.

DNA is the genetic material that controls an organism's composition.​

• The nucleoid is a region that houses DNA and is found only in prokaryotic cells.​

• Cytosol is the fluid component of cytoplasm.

• The nucleolus is found in the nucleus of eukaryotic cells and is responsible for synthesizing ribosomes.

"Karyon" means "nucleus."​

"Pro" means "before."​

"Eu" means "after."

Remember that the nucleoid is a region housing DNA found only in prokaryotic cells. Bacteria are an example of an organism composed of prokaryotic cells. 

All cells (prokaryotic and eukaryotic) share four structures in common:​

• Ribosome

• Plasma membrane (cell membrane)

• DNA ​

• Cytoplasm ​

The pronucleus is the nucleus found specifically in a sperm or ovum (egg) during fertilization.

Eukaryotic cells can be unicellular or multicellular.​

• Prokaryotic cells are always unicellular.​

• The nucleoid is a region unique to prokaryotic cells only.​

• Ribosome and DNA are shared by both prokaryotic and eukaryotic cells.