3.1: Cells and Molecules

Molecules of Life

All organisms are composed of four basic types of molecules that are essential for cell structure and function: proteins, lipids, carbohydrates, and nucleic acids (Figure 3.1). Proteins are crucial for cell shape and nearly all cellular tasks, including receiving signals from outside the cell and mobilizing intra-cellular responses. Lipids are a class of organic compounds that include fats, oils, and hormones. As discussed later in the chapter, lipids are also responsible for the characteristic phospholipid bilayer structure of the cell membrane. Carbohydrates are sugar molecules and serve as energy to cells in the form of glucose. Lastly, nucleic acids, including deoxyribonucleic acid (DNA), carry genetic information about a living organism.

Figure 3.1: Information about the four biomolecules. Credit: Biomolecules Table original to Explorations: An Open Invitation to Biological Anthropology (2nd ed.) by Hayley Mann is under a CC BY-NC 4.0 License.

Molecule	Definition	Example
Proteins	Composed of one or more long chains of amino acids (i.e., basic units of protein) Often folded into complex 3D shapes that relate to function Proteins interact with other types of proteins and molecules	Proteins come in different categories including structural (e.g., collagen, keratin, lactase, hemoglobin, cell membrane proteins), defense proteins (e.g, antibodies), enzymes (e.g., lactase), hormones (e.g., insulin), and motor proteins (e.g., actin)
Lipids	Insoluble in water due to hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail	Fats, such as triglycerides, store energy for your body Steroid hormones (e.g., estrogen and testosterone) act as chemical messengers to communicate between cells and tissues, as well as biochemical pathways inside of the cell
Carbohydrates	Large group of organic molecules that are composed of carbon and hydrogen atoms	Starches and sugars, including blood glucose, provide cells with energy
Nucleic Acids	Carries the genetic information of an organism	DNA RNA

Cells

In 1665, Robert Hooke observed slices of plant cork using a microscope. Hooke noted that the microscopic plant structures he saw resembled cella, meaning “a small room” in Latin. Approximately two centuries later, biologists recognized the cell as being the most fundamental unit of life and that all life is composed of cells. Cellular organisms can be characterized as two main cell types: prokaryotes and eukaryotes (Figure 3.2).

Figure 3.2: Prokaryotic cell and eukaryotic cell. Credit: Prokaryote vs. eukaryote original to Explorations: An Open Invitation to Biological Anthropology (2nd ed.) by Mary Nelson is under a CC BY-NC 4.0 License. [Image Description]

Prokaryotes include bacteria and archaea, and they are composed of a single cell. Additionally, their DNA and organelles are not surrounded by individual membranes. Thus, no compartments separate their DNA from the rest of the cell (see Figure 3.2). It is well known that some bacteria can cause illness in humans. For instance, Escherichia coli (E. coli) and Salmonella contamination can result in food poisoning symptoms. Pneumonia and strep throat are caused by Streptococcal bacteria. Neisseria gonorrhoeae is a sexually transmitted bacterial disease. Although bacteria are commonly associated with illness, not all bacteria are harmful. For example, researchers are studying the relationship between the microbiome and human health. The bacteria that are part of the healthy human microbiome perform beneficial roles, such as digesting food, boosting the immune system, and even making vitamins (e.g., B12 and K).

Eukaryotes can be single-celled or multi-celled in their body composition. In contrast to prokaryotes, eukaryotes possess membranes that surround their DNA and organelles. An example of a single-celled eukaryote is the microscopic algae found in ponds (phytoplankton), which can produce oxygen from the sun. Yeasts are also single-celled, and fungi can be single- or multicellular. Plants and animals are all multicellular.

Although plant and animal cells have a surprising number of similarities, there are some key differences (Figure 3.3). For example, plant cells possess a thick outer cell membrane made of a fibrous carbohydrate called cellulose. Animal and plant cells also have different tissues. A tissue is an aggregation of cells that are morphologically similar and perform the same task. For most plants, the outermost layer of cells forms a waxy cuticle that helps to protect the cells and to prevent water loss. Humans have skin, which is the outermost cell layer that is predominantly composed of a tough protein called keratin. Overall, humans have a diversity of tissue types (e.g., cartilage, brain, and heart).

Figure 3.3: Plant cell compared to an animal cell. Credit: Simple plant and animal cell by Tomáš Kebert & umimeto.org has been modified (labels added) and is under a CC BY-NC-SA 4.0 License. [Image Description].

Animal Cell Organelles

An animal cell is surrounded by a double membrane called the phospholipid bilayer (Figure 3.4). A closer look reveals that this protective barrier is made of lipids and proteins that provide structure and function for cellular activities, such as regulating the passage of molecules and ions (e.g., H₂O and sodium) into and out of the cell. Cytoplasm is the jelly-like matrix inside of the cell membrane. Part of the cytoplasm comprises organelles, which perform different specialized tasks for the cell (Figure 3.5). An example of an organelle is the nucleus, where the cell’s DNA is located.

Figure 3.4: A phospholipid bilayer with membrane-bound carbohydrates and proteins. Credit: Cell Membrane (Anatomy & Physiology, Figure 3.4) by OpenStax is under a CC BY 4.0 License. [Image Description].

Figure 3.5: An animal cell with membrane-enclosed organelles. Credit: Organelle by NIH National Human Genome Research Institute is in the public domain. [Image Description].

Another organelle is the mitochondrion. Mitochondria are often referred to as “powerhouse centers” because they produce energy for the cell in the form of adenosine triphosphate (ATP). Depending on the species and tissue type, multicellular eukaryotes can have hundreds to thousands of mitochondria in each of their cells. Scientists have determined that mitochondria were once symbiotic prokaryotic organisms (i.e., helpful bacteria) that transformed into cellular organelles over time. This evolutionary explanation helps explain why mitochondria also have their own DNA, called mitochondrial DNA (mtDNA). All organelles have important physiological functions and disease can occur when organelles do not perform their role optimally. Figure 3.6 lists other organelles found in the cell and their specialized cellular roles.

Figure 3.6: This table depicts the names of organelles and their cellular functions. Credit: Cell Structure table (Figure 3.11) original to Explorations: An Open Invitation to Biological Anthropology by Hayley Mann, Xazmin Lowman, and Malaina Gaddis is under a CC BY-NC 4.0 License.

Cell structure	Description
Centrioles	Assist with the organization of mitotic spindles, which extend and contract for the purpose of cellular movement during mitosis and meiosis.
Cytoplasm	Gelatinous fluid located inside of cell membrane that contains organelles.
Endoplasmic reticulum (ER)	Continuous membrane with the nucleus that helps transport, synthesize, modify, and fold proteins. Rough ER has embedded ribosomes, whereas smooth ER lacks ribosomes.
Golgi body	Layers of flattened sacs that receive and transmit messages from the ER to secrete and transport proteins within the cell.
Lysosome	Located in the cytoplasm; contains enzymes to degrade cellular components.
Microtubule	Involved with cellular movement including intracellular transport and cell division.
Mitochondrion	Responsible for cellular respiration, where energy is produced by converting nutrients into ATP.
Nucleolus	Resides inside of the nucleus and is the site of ribosomal RNA (rRNA) transcription, processing, and assembly.
Nucleopore	Pores in the nuclear envelope that are selectively permeable.
Nucleus	Contains the cell’s DNA and is surrounded by the nuclear envelope.
Ribosome	Located in the cytoplasm and also the membrane of the rough endoplasmic reticulum. Messenger RNA (mRNA) binds to ribosomes and proteins are synthesized.