Cell Biology — Reading Comprehension Lesson

Reading Passage

Cell Biology

Cells are the fundamental units of life. Every living organism — from the smallest bacterium to the largest whale — is made of cells. Some organisms, like bacteria and amoebas, consist of just a single cell. Others, like humans, are made of trillions of cells of hundreds of different types, each specialized for a particular function. The cell theory, one of the foundational principles of biology, states three things: all living organisms are composed of cells; the cell is the basic unit of life; and all cells arise from pre-existing cells.

There are two fundamentally different types of cells. Prokaryotic cells, found in bacteria and archaea, are simple cells that lack a membrane-bound nucleus — their genetic material floats freely in the cytoplasm. Eukaryotic cells, found in plants, animals, fungi, and protists, are more complex and contain a true nucleus enclosed in a membrane, as well as other membrane-bound structures called organelles. The nucleus is often called the control center of the cell because it contains the cell’s DNA, which carries the instructions for building proteins and regulating all cell activities.

Eukaryotic cells contain many specialized organelles. Mitochondria are often called the “powerhouses of the cell” because they produce ATP — the energy currency of the cell — through cellular respiration. The endoplasmic reticulum is a network of membranes involved in protein and lipid synthesis. The Golgi apparatus packages and ships proteins to their destinations inside or outside the cell. Ribosomes, found throughout the cytoplasm and on the endoplasmic reticulum, are the molecular machines that build proteins. Plant cells have additional structures not found in animal cells: a rigid cell wall outside the cell membrane, chloroplasts for photosynthesis, and a large central vacuole for storage.

All cells are surrounded by a cell membrane — a thin, flexible barrier made of a double layer of lipid molecules with embedded proteins. The cell membrane controls what enters and exits the cell, maintaining the internal environment that cells need to function. Small molecules like oxygen and carbon dioxide can pass directly through the lipid bilayer by diffusion. Larger molecules and ions require protein channels or active transport. Cells reproduce by dividing: prokaryotes divide by binary fission, while eukaryotic cells undergo mitosis (for growth and repair) or meiosis (for producing reproductive cells). Understanding cells is the foundation for understanding all of biology, from genetics to medicine.

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