In this lesson, we will explore the fascinating world of cells. The wings of a butterfly, the shiny layer of our eyes, the meat we eat, and even our eyelashes and fingernails are all made up of cells. Everything around us, including orange juice and the wood in our pencils, is produced by cells. We will take a closer look at the internal structure of cells and learn how different types of cells work together to form tissues.

Cell Wall

Not all living organisms have cell walls; for example, animals and many protists do not. A cell wall is a strong, non-living structure located outside the plasma membrane, providing shape, strength, protection, and support to the living material inside the cell. In plant cells, the outer layer of the cell wall is called the primary wall, which is mainly made of cellulose. Some plant cells, like xylem cells, have an additional thicker layer called the secondary wall, which contains lignin and other substances. There are tiny openings, known as plasmodesmata, in the cell walls that connect the cytoplasm of adjacent cells.

Fungi and many protists also have cell walls, but these do not contain cellulose. Instead, fungi have cell walls made of chitin. In prokaryotes, the cell wall is composed of peptidoglycan, which is a complex of amino acids and sugars.

Cell Membrane

All prokaryotic and eukaryotic cells are surrounded by a thin and flexible cell membrane that covers the cytoplasm. The cell membrane acts as a semi-permeable barrier, allowing only a select few molecules to pass through while keeping most substances contained within the cell. This function helps maintain the internal environment of the cell, ensuring it remains stable and balanced.

Cell Organelles

Organelles are small structures within cells that perform specific functions. Eukaryotic cells typically contain about a dozen different types of organelles, which we’ll explore here.

Nucleus

The nucleus is a prominent organelle found in eukaryotic cells. In animal cells, it is usually located at the center, while in mature plant cells, it is pushed to one side by a large central vacuole. The nucleus is surrounded by a double membrane called the nuclear envelope, which has small pores that allow it to act as a semi-permeable barrier. Inside the nuclear envelope is a fluid called nucleoplasm, which contains one or two nucleoli and chromosomes.

The nucleolus is a dark spot within the nucleus where ribosomal RNA is produced and assembled into ribosomes. Chromosomes are visible only during cell division; during the non-dividing phase (interphase), they exist as fine thread-like structures called chromatin. Chromosomes are made up of DNA and proteins. In contrast, prokaryotic cells do not have a distinct nucleus; their DNA is located in the cytoplasm and is not surrounded by a membrane.

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