Relationships are complicated, even in nature. Two unrelated species living close together and interacting for survival is called symbiosis. There are three types of symbiotic relationships: mutualism, commensalism, and parasitism.

In a mutualistic relationship, both organisms benefit from the interaction. One example is the relationship between honeybees and flowers. Honeybees drink nectar from flowers, collecting and carrying pollen as they fly from one flower to another. Nectar allows bees to make honey, and spreading pollen helps flowers reproduce. Another example of a mutualistic relationship is between clownfish and sea anemones. The sea anemone provides protection and shelter, while clownfish waste provides the sea anemone with nutrients.

Have you ever noticed plastic utensils or paper plates labeled as “biodegradable” and wondered what that meant? Materials are biodegradable when microorganisms such as bacteria can break them down into their building blocks.

Every day, our cells must produce all the various molecules they need to stay alive. But the chemical reactions to create these molecules can’t occur without help—which is where enzymes come in. Enzymes are biological catalysts, meaning they speed up the rate of specific chemical reactions by reducing the amount of energy needed for the reaction to occur. Most enzymes are proteins, but some RNA molecules can also act as enzymes.

Thousands of different enzymes catalyze the vast range of reactions that take place within cells, but each enzyme typically supports one of the following types of tasks:

Nanoparticles may sound like gadgets from a science fiction movie, but they exist in real life. They’re particles of any material that are less than 100 nanometers (one-billionth of a meter) in all dimensions. Nanoparticles appear in nature, and humans have, mostly unknowingly, used them since ancient times. For example, hair dyeing in ancient Egypt involved lead sulfite nanoparticles, and artisans in the Middle Ages added gold and silver nanoparticles to stained-glass windows. Over the past several decades, researchers have studied nanoparticles for their potential uses in many fields, from computer engineering to biology.

A nanoparticle’s properties can differ significantly from those of larger pieces of the same material. Properties that may change include:

While at the grocery store, you’ve likely noticed foods with labels saying they contain antioxidants, but what does that mean? In short, antioxidants are substances that may prevent or delay some types of cell damage. Many foods, including fruits and vegetables, naturally produce antioxidants like vitamins C and E, beta-carotene, and selenium. Our bodies also naturally produce antioxidant molecules such as alpha-lipoic acid, glutathione, and coenzyme Q10.

Antioxidants are united by their ability to donate electrons, which helps them protect the body against reactive oxygen species (ROS). ROS form naturally during exercise, when your body converts food into energy, or during exposure to certain environmental factors such as cigarette smoke, air pollution, and sunlight. These molecules can “steal” electrons from other molecules, and though they aren’t always harmful, consistently high amounts of ROS in your body can cause a condition known as oxidative stress that can damage cells. That cell damage may also lead to chronic diseases, especially if ROS steal electrons from DNA or other important molecules and alter their functions.

Some might think that protein is only important for weightlifters. In truth, all life relies on the activity of protein molecules. A single human cell contains thousands of different proteins with diverse roles, including:

• Providing structure. Proteins such as actin make up the three-dimensional cytoskeleton that gives cells structure and determines their shapes.
• Aiding chemical reactions. Many proteins are biological catalysts called enzymes that speed up the rate of chemical reactions by reducing the amount of energy needed for the reactions to proceed. For example, lactase is an enzyme that breaks down lactose, a sugar found in dairy products. Those with lactose intolerance don’t produce enough lactase to digest dairy.
• Supporting communication. Some proteins act as chemical messengers between cells. For example, cytokines are the protein messengers of the immune system and can increase or decrease the intensity of an immune response.

Just as electricity powers almost every modern gadget, the tiny molecule adenosine triphosphate (ATP) is the major source of energy for organisms’ biochemical reactions. ATP stores energy in the chemical bonds that hold its three phosphate groups together—the triphosphate part of its name. In the human body, ATP powers processes such as cell signaling, muscle contraction, nerve firing, and DNA and RNA synthesis. Because our cells are constantly using and producing ATP, each of us turns over roughly our body weight in the molecule every day!

Our bodies can produce ATP in several ways, but the most common is cellular respiration—a multistep process in which glucose molecules from our diet and oxygen react to form water and carbon dioxide. The breakdown of a single molecule of glucose in this way releases energy, which the body captures and stores in around 32 ATP molecules. Along with oxygen, mitochondria are crucial for producing ATP through cellular respiration, which is why they’re sometimes called the powerhouses of cells.

Have you ever wondered why your cells don’t spill into each other or what keeps your skin separate from your blood? The answer to both is lipids—a diverse group of organic compounds that don’t dissolve in water. They’re one of the four major building blocks of our bodies, along with proteins, carbohydrates, and nucleic acids. Types of lipids include:

• Fats, necessary for our bodies’ long-term energy storage and insulation. Some essential vitamins are fat soluble, meaning they must be associated with fat molecules to be effectively absorbed.
• Phospholipids, which make up a large part of cell and organelle membranes.
• Waxes, which help protect delicate surfaces. For instance, earwax protects the skin of the ear canal.
• Steroids, including cholesterol, a precursor to many hormones, which helps maintain the fluidity of cell membranes.

RNA, though less well known than its cousin DNA, is equally integral to our bodies. RNA molecules are long, usually single-stranded chains of nucleotides. (DNA molecules are also made up of nucleotides but are typically double-stranded.) There are three major types of RNA, which are all involved in protein synthesis:

• Messenger RNA (mRNA) is complementary to one of the DNA strands of a gene and carries genetic information for protein synthesis to the ribosome—the molecular complex in which proteins are made.
• Transfer RNA (tRNA) works with mRNA to make sure the right amino acids are inserted into the forming protein.
• Ribosomal RNA (rRNA), together with proteins, makes up ribosomes and functions to recognize the mRNA and tRNA that are presented to the ribosomal complex.

Nerve cells, also known as neurons, carry information through our bodies using electrical impulses and chemical messengers called neurotransmitters. A nerve cell’s size and shape depend on its role and location, but nearly all nerve cells have three main parts:

• Dendrites that extend like branches and receive signals
• A cell body containing the nucleus that holds the genetic material of the cell and controls its actions
• An axon, a long structure that transmits messages