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How do the Parts of a Cell Work?

Each kind of cell structure has a different function within a cell.

Cell Wall: The cell wall is a rigid layer that surrounds the cells of plants and some other organisms. The cells of animals, in contrast, do not have cell walls. A plant’s cell walls help protect and support the cell. The cell wall is made mostly of a strong material called cellulose. Still, many materials, including water and oxygen, can pass through the cell wall easily.

Cell Membrane: The cell membrane controls which substance pass into and out of a cell. Everything a cell needs, such as food particles, water, and oxygen, enters through the cell membrane. Waste products leave the same way. In addition, the cell membrane prevents harmful materials from entering the cell. All cells have cell membranes. In plant cells, the cell membrane is just inside the cell wall. In cells without cell walls, the cell membrane forms the border between the cell and its environment. (Use a screen on a window analogy.)

Nucleus: A cell doesn’t have a brain, but it has something that functions in a similar way. A large oval structure called the nucleus acts as a cell’s control center, directing all of the cell’s activities. The nucleus is the largest of many tiny cell structures, called organelles that carry out specific functions within a cell. A membrane called the nuclear envelope surrounds the nucleus. Materials pass in and out of the nucleus through pores in the nuclear envelope.

• Chromatin: These thin strands of material fill the nucleus and contain information for directing a cell’s functions. For example, the instructions in the chromatin ensure that leaf cells grow and divide to form more leaf cells.
• Nucleolus: The small, round structure within the nucleus and that makes ribosomes. Ribosomes are small grain-shaped organelles that produce proteins. Proteins are important substances in cells.

Organelles in the Cytoplasm: Most of a cell consists of a thick, clear, gel-like fluid. The cytoplasm fills the region between the cell membrane and the nucleus. The fluid of the cytoplasm moves constantly within a cell, carrying along the nucleus and other organelles that have specific jobs.

• Mitochondria: Floating in the cytoplasm are rod-shaped structures that are nicknamed the “powerhouses” of a cell. Mitochondria convert energy stored in food to energy the cell can use to live and function.
• Endoplasmic Reticulum and Ribosomes: The endoplasmic reticulum (ER) is an organelle with a network of membranes that produces many substances. Ribosomes dot some parts of the ER, while other ribosomes float in the cytoplasm. The ER helps the attached ribosomes make proteins. These newly made proteins and other substances leave the ER and move to another organelle.

Golgi Apparatus: As proteins leave the Endoplasmic Reticulum, they move to a structure that looks like flattened sacs and tubes. This structure can be thought of as a cell’s warehouse. The Golgi Apparatus receives proteins and other newly formed materials from the ER, packages them, and distributes them to other parts of the cell or to the outside of the cell.

Vacuoles: Plant cells often have one or more large, water-filled sacs floating in the cytoplasm. This type of sac, called a vacuole, stores water, food, or other materials needed by the cell. Vacuoles can also store waste products until the wastes are removed. Some animal cells do not have vacuoles, while others do.

Chloroplasts: A typical plant cell contains green structures, called chloroplasts, in the cytoplasm. A chloroplast, captures energy from sunlight and changes it to a form of energy, cells can use in making food. Animal cells don’t have chloroplasts, but the cells of plants and some other organisms do. Chloroplasts make leaves green because leaf cells contain many chloroplasts.

Lysosomes: These saclike organelles contain substances that break down large food particles into smaller ones. Lysosomes also break down old cell parts and release the substances so they can be used again. You can think of lysosomes as a cell’s recycling centers.