Atoms bond together to make molecules, and macromolecules form organelles. These organelles work together to maintain the environment within the cell. Cells are considered the fundamental units of life. The smallest living things are single-cell bacteria.

Microscopes are necessary for visualizing cells. Microscopes enlarge the objects being viewed to allow for study of appearance and behavior. Light microscopes are used in teaching labs. Electron microscopes are for more detailed magnification and study. In a scanning electron microscope, a beam of electrons moves back and forth across a cell’s surface, creating details of cell surface characteristics. In a transmission electron microscope, the electron beam penetrates the cell and provides details of a cell’s internal structures. As you might imagine, electron microscopes are significantly more bulky and expensive than light microscopes.

By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory, which states that all living things are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells.

Prokaryotic Cells 

Prokaryotic cells do not have membrane-bound DNA, which means there is no nucleus. They contain DNA organized into a single circular chromosome. Prokaryotic cells are typically smaller than eukaryotic cells. Bacteria are prokaryotes and do not contain any membrane-bound organelles.

This figure shows the generalized structure of a prokaryotic cell. All prokaryotes have chromosomal DNA localized in a nucleoid, ribosomes, a cell membrane, and a cell wall. The other structures shown are present in some, but not all, bacteria.

Eukaryotic Cells  

Eukaryotic cells have membrane-bound DNA within a nucleus. Eukaryotes have membrane-bound organelles. Plants, protists, fungi and animals are all eukaryotes. All cells (both prokaryotes and eukaryotes) have an external membrane called a plasma membrane surrounding their contents that determines which molecules are permitted to enter and leave the cell. This membrane is considered selectively permeable. The plasma membrane is composed of a phospholipid bilayer embedded with proteins.

The membrane must take in enough nutrients to sustain the cell and must release enough waste to detoxify the cell. For this reason, cells do not get very large. The laws of physics limit the size of cells. When necessary, cells reproduce by dividing into two smaller cells. This gives a better ratio of surface area to volume.

Notice that as a cell increases in size, its surface area-to-volume ratio decreases. When there is insufficient surface area to support a cell’s increasing volume, a cell will either divide or die. 

All cells are filled with a gel-like cytoplasm containing many molecules necessary for cellular reactions. It is a semi-fluid matrix containing the organelles. All cells contain genetic information. Chromosomes are composed of condensed DNA. In eukaryotes, the chromosomes are held within the nuclear membrane.

Organelles

It can be difficult to visualize a cell in three dimensions. The video for this chapter contains an animation of cellular activities that allows you to see the organelles in motion. The following PDF is linked to provide a comprehensive visual of the cell with descriptions.

https://www.yourgenome.org/wp-content/uploads/2000/01/downloads/activities/cell-snap/cellsnap-information.pdf

The nucleus is the largest of the organelles and is at the centre of the cell. It is the storage site of the cell’s DNA. The human genome contains 3 billion bases and provides all the information needed to make a human. It is packaged into chromosomes inside the nucleus.

Within the nucleus, the nucleolus aggregates ribosomal RNA (rRNA) with associated proteins to assemble the ribosomal subunits that are transported through nuclear pores to the cytoplasm. Ribosomes are proteins that provide a location for assembly of amino acids into proteins. Some ribosomes are free ribosomes, and float in the cytoplasm. Other ribosomes are attached to the Rough Endoplasmic Reticulum (RER). Whether free or bound, ribosomes are the location where proteins are assembled. Proteins can stay within the cell or be exported through the plasma membrane to other cells.

A large subunit (top) and a small subunit (bottom) comprise ribosomes. During protein synthesis, ribosomes assemble amino acids into proteins.

Eukaryotic cells contain many membranes, including the nuclear membrane, rough ER, smooth ER, Golgi apparatus, lysosomes, and the plasma membrane.  Vesicles are membrane-bound sacs that function in storage and transport. Vesicle membranes can fuse with either the plasma membrane or other membrane systems within the cell. Vesicles bud in and out of the membrane, due to the properties of phospholipids. The movement of a vesicle by a transport protein is highlighted in the chapter video and is shown on the video preview screen.

Smooth ER does not have any bound ribosomes. It sustains an environment for the generation of lipid molecules, including phospholipids and steroid hormones. This organelle helps break down pharmaceuticals, toxins and stores calcium ions. 

Rough ER has a surface studded with ribosomes, where protein are assembled. Assembled proteins are secreted to the inside of Rough ER to be folded and packaged in vesicles for transport. Most transport vesicles will deliver proteins to Golgi apparatus for further processing.

Golgi apparatus receives, packages, modifies, and sends materials both internally and externally. Secretion cells have extensive Golgi apparatus.

Lysosomes are small membrane sacks of digestive enzymes. The enzymes break down macromolecules into smaller molecules for recycling. Acidic pH of the lysosome contributes to the process.

Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body.

Mitochondria have a double membrane and their own DNA. Human mitochondria follow maternal inheritance and are responsible for the capture and release of energy. Cellular respiration is the process of making ATP using the chemical energy in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide and water you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct. This is a foundational concept in Module 2.

Centrioles are a pair of organelles found in the cell consisting of small protein tube structures known as microtubules. These organelles play an important role in cell division. The centrioles organize fibers called microtubules into spindles which attach to the chromosomes and move them during cell division.

Plant cells contain all of the organelles described above as well as vacuoles and chloroplasts. Vacuoles are membrane-bound sacs that function in storage and transport. Vacuoles are somewhat larger than vesicles and contain enzymes  to break down macromolecules. Although similar to vesicles, vacuoles do not fuse with the membrane system. Chloroplasts are the location for photosynthesis — a series of reactions using carbon dioxide, water, and light energy to produce glucose and oxygen. Like mitochondria, chloroplasts have outer and inner membranes. The ability to complete photosynthesis is the difference between producers and consumers. Plants are producers capable of making their own food (chemical energy). Consumers use the organic compounds generated by producers for their food (chemical energy). Chloroplasts also have their own DNA and contain chlorophyll.

Endosymbiosis is a theory explaining the origins of mitochondria and chlorophyll based on their similarities to bacteria. Bacteria have their own DNA and ribosomes, just like mitochondria and chloroplasts.

Structural Components

The cytoskeleton is a network of protein tracks and microtubules. These proteins work to support cellular structure, aid in cell division, transport, and move cells. There are 3 main types of cytoskeleton fibers:  microtubules, intermediate filaments, and microfilaments. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell (also shown on chapter video preview screen), and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles, flagella, and cilia. Microfilaments provide rigidity and shape to the cell and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place.

Cell walls are found in prokaryotes, fungi, some protists and all plants. They are considered extracellular structures, because they are outside the cell membrane. Cell walls protect, support and contain cellulose. ECM (extracellular matrix) is an exterior network providing supports to maintain a cell’s shape.  Cells attached to other cells to form tissues.