This lesson describes the cell structure and two different types of cells. The lesson also explores the functions of various cell parts.
All living things are made of cells. Cells are the smallest structural units and basic building blocks of living things. Cells contain everything necessary to keep living things alive. Varying in size and shape, cells carry out specialized functions. Robert Hooke discovered the first cells in the mid-eighteenth century. Many years later, after advancements in microscopy, the cell theory was formed. This theory, or in-depth explanation, about cells consists of three parts:
Did You Know?
More than a trillion cells and at least 200 different types of cells exist in the human body!
Many different types of cells exist. Because of this, cells are classified into two general types: prokaryotic cells and eukaryotic cells. The following comparison table lists key differences between prokaryotes and eukaryotes:
|Cell size||Around 0.2–2.0 mm in diameter||Around 10–100 mm in diameter|
|Organelles||Absent||Several present, ranging from ribosomes to the endoplasmic reticulum|
|Flagella||Simple in structure||Complex in structure|
As shown in the image, prokaryotic cells lack nuclei. Their DNA floats in the cytoplasm, which is surrounded by a plasma membrane. Very simplistic in structure, these cells lack organelles but do have cell walls. Organelles are specialized structures with a specific cellular function. They also may have ribosomes that aid in protein synthesis. Also, these cells have a flagellum that looks like a tail attached to the cell. Flagella aid in locomotion. The pili, or hair-like structures surrounding the cells, aid in cellular adhesion. Bacteria and Archaea are the most common prokaryotes. Most prokaryotes are unicellular, or made of a single cell, but there are a few multicellular organisms.
Eukaryotic cells contain a membrane-bound nucleus where DNA is stored. Membrane-bound organelles also exist in eukaryotic cells. Similar to prokaryotic cells, eukaryotic cells have cytoplasm, ribosomes, and a plasma membrane. Eukaryotic organisms can be either unicellular or multicellular. Much larger than prokaryotes, examples of eukaryotic organisms include fungi and even people.
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What is an organelle?