Ibrahim, you will learn more about this as you proceed. Your body for instance, has many different kinds of cells. Though they might look different under a microscope, most cells have chemical and structural features in common. In humans, there are about <strong>200 different types</strong> of cells, and within these cells there are about 20 different types of structures or organelles.

<p>By the end of this lesson and the next few, you should be able to:</p>
<ul>
<li>Describe the role of cells in organisms</li>
<li>Compare and contrast light <a data-lesson="microscopy" data-subject="biology" data-tutorial="introducing-the-cell" href="https://app.ulearngo.com/biology/introducing-the-cell/microscopy">microscopy</a> and electron microscopy</li>
<li>Summarize cell theory</li>
</ul>
<h2>Introducing Cells</h2>
<p>Close your eyes and picture a brick wall. What is the basic building block of that wall? A single brick, of course. Like a brick wall, your body consists of basic building blocks, and the building blocks of your body are cells.</p>
<div class="wp-caption alignnone" id="attachment_95626" style="width: 1000px"><a href="https://data.ulearngo.com/assets/5a0d2659-a910-4269-a17d-5ce2f76260dd"><img alt="different-types-of-cells" class="size-full wp-image-95626" height="335" src="https://data.ulearngo.com/assets/5a0d2659-a910-4269-a17d-5ce2f76260dd" width="1000"/></a><p class="wp-caption-text">(a) Nasal sinus cells (viewed with a light microscope), (b) onion cells (viewed with a light microscope), and (c) Vibrio tasmaniensis bacterial cells (seen through a scanning electron microscope) are from very different organisms, yet all share certain characteristics of basic cell structure. Image Attribution: (a) modification of work by Ed Uthman, MD; (b) modification of work by Umberto Salvagnin; (c) modification of work by Anthony D'Onofrio, William H. Fowle, Eric J. Stewart, and Kim Lewis of the Lewis Lab at Northeastern University; scale-bar data from Matt Russell</p></div>
<p>Your body has many kinds of cells, each specialized for a specific purpose. Just as a home is made from a variety of building materials, the <a data-lesson="human-body" data-subject="biology" data-tutorial="human-biology" href="https://app.ulearngo.com/biology/human-biology/human-body">human body</a> is constructed from many cell types. For example, epithelial cells protect the surface of the body and cover the organs and body cavities within. Bone cells help to support and protect the body. Cells of the immune system fight invading bacteria. Additionally, <a data-lesson="blood-2" data-subject="biology" data-tutorial="human-biology" href="https://app.ulearngo.com/biology/human-biology/blood-2">blood</a> and <a data-lesson="blood-2" data-subject="biology" data-tutorial="human-biology" href="https://app.ulearngo.com/biology/human-biology/blood-2">blood cells</a> carry nutrients and <a data-lesson="transport-of-oxygen-in-the-blood" data-subject="biology" data-tutorial="respiratory-system" href="https://app.ulearngo.com/biology/respiratory-system/transport-of-oxygen-in-the-blood">oxygen</a> throughout the body while removing <a data-lesson="carbon-cycle" data-subject="biology" data-tutorial="ecology-overview" href="https://app.ulearngo.com/biology/ecology-overview/carbon-cycle">carbon</a> dioxide.</p>
<p>Each of these cell types plays a vital role during the growth, development, and day-to-day maintenance of the body. In spite of their enormous variety, however, cells from all organisms—even ones as diverse as bacteria, onion, and human—share certain fundamental characteristics.</p>
<h2>What is a cell?</h2>
<p id="fs-id1488921">A cell is the smallest unit of a living thing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism. Thus, cells are the basic building blocks of all organisms.</p>
<p id="fs-id1754058">Several cells of one kind that interconnect with each other and perform a shared function form tissues, several <a data-lesson="complex-tissue-structure" data-subject="biology" data-tutorial="introducing-animal-diversity" href="https://app.ulearngo.com/biology/introducing-animal-diversity/complex-tissue-structure">tissues</a> combine to form an organ (your stomach, heart, or brain), and several organs make up an organ system (such as the digestive system, circulatory system, or nervous system). Several systems that function together form an organism (like a human being). Here, we will examine the structure and function of cells.</p>
<p id="fs-id1325976">There are many types of cells, all grouped into one of two broad categories: prokaryotic and eukaryotic. For example, both animal and <a data-lesson="summarizing-asexual-reproduction" data-subject="biology" data-tutorial="plant-reproduction" href="https://app.ulearngo.com/biology/plant-reproduction/summarizing-asexual-reproduction">plant</a> cells are classified as eukaryotic cells, whereas bacterial cells are classified as prokaryotic. Before discussing the criteria for determining whether a cell is prokaryotic or eukaryotic, let’s first examine how biologists study cells in the next lesson.</p>

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Introducing the Cell

Biology studies life and living organisms. Biology can be divided into different sections that cover the morphology, physiology, anatomy, behavior, and so on of living organisms.

Biology

Discover the structures and functions of cells including prokaryotic and eukaryotic cells, organelles like the nucleus, mitochondria, and ribosomes, and learn about the endomembrane system, cytoskeleton, extracellular matrix and intercellular connections.


Introducing the Cell

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