Protists, fungi, plants, and animals are eukaryotic organisms, meaning their cells contain a nucleus and other specialized structures called organelles that carry out important functions. The nucleus acts as the cell’s command center, directing the activities of all organelles. It contains the cell’s DNA, which holds the instructions for life. Surrounding the nucleus is the nuclear envelope, a membrane with thousands of tiny pores that allow materials to move in and out.

Inside the nucleus, chromatin—long strands of DNA—can condense to form chromosomes, which store and pass genetic information to offspring. The nucleolus, also located within the nucleus, produces ribosomes, which are responsible for building proteins based on instructions from the DNA.

Outside the nucleus is the cytoplasm, a thick, gel-like fluid that fills the cell and supports its organelles. While many cells, including those of plants, fungi, and some protists, have a cell wall for added support and structure, animal cells do not. The cell wall is a rigid layer outside the plasma membrane that helps maintain the cell’s shape and protect it.  Several organelles work together to build, transport, and store materials within the cell. The endoplasmic reticulum (ER) is a network of membranes where important molecules are assembled. The rough ER, covered in ribosomes, helps produce proteins, while the smooth ER is responsible for making lipids. Once proteins are made, they are sent to the Golgi apparatus, which modifies, sorts, and packages them into small sacs called vesicles for transport to their proper destinations.

Cells also contain structures for storage and cleanup. The vacuole stores water, food, and other materials; in plant cells, a large central vacuole helps maintain structure and support. Lysosomes contain digestive enzymes that break down waste and old cell parts. The mitochondria convert energy from food into a usable form for the cell, which is why they are often called the “powerhouse” of the cell. In plant cells and some protists, chloroplasts capture energy from sunlight and convert it into chemical energy through the process of photosynthesis.

A cell’s structure, movement, and communication are supported by several specialized components. The cytoskeleton is a network of protein filaments that helps the cell maintain its shape and internal organization. It is made up of structures such as microtubules and microfilaments, which also play a role in moving materials within the cell.

Centrioles, which are composed of microtubules, are important for cell division because they help organize the movement of chromosomes. Centrioles are found in animal cells but are not present in plant cells.

Some cells also have structures that allow for movement. Cilia are short, hair-like projections that move in a coordinated, wave-like motion to help the cell move or to move substances across its surface. In contrast, flagella are longer, whip-like structures that propel the cell forward.

Review:

Identify the parts of the cell below.