Anatomy of a Cell Structure and Function
A Framework for Function (Lexile 1080L) 1 Did you know that there are billions of cells moving around in your body at this very moment? Without cells, our bodies would not be able to function. The cell was discovered in 1665, when a scientist named Robert Hooke sat down and curiously peeked through his microscope to study a piece of cork. When he gazed upon the cork, he saw rows and rows of small rectangular holes. Robert Hooke had just discovered cells. This exciting discovery led to the invention of a better microscope by Anton Van Leeuwenhoek in 1673. His microscope was specifically designed to view the interesting “animalcules” described by Hooke. With this improved style of microscope, Van Leeuwenhoek encountered a single-celled protozoa in pond scum and discovered blood cells! It is because of scientists like Van Leeuwenhoek and Hooke that we are able to know so much about cells today. 2 A cell is the smallest unit of life that can perform all life processes. Our basic knowledge of cells is summarized in the cell theory that was proposed by scientists nearly 150 years ago. Cell theory is composed of three major points: all living things are composed of cells, all cells come from pre-existing cells, and cells are the smallest units of life. 3 There are two specific types of cells known—prokaryotes and eukaryotes. Simpler and smaller, prokaryotes only have the membrane and cell wall that enclose the rest of their insides. Eukaryotic cells, like those in your body, are much more complex and have a nucleus. Eukaryotes are up to ten times larger than a typical prokaryote cell. Other important characteristics of the eukaryotic cell are the membrane-enclosed sub-cellular organelles that will be further explained later in this passage. 4 All cells are surrounded by a cell membrane, a boundary that separates the cell from the outside environment. Like the skin on your body, this membrane acts as a protective layer that covers the cell's surface. It allows some things to be excreted out of the cell, such as wastes, and allows some things in, like nutrients and oxygen.
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Anatomy of a Cell Structure and Function
5 Plant cells also have a cell wall, as do fungi and bacteria, but not animal cells. The cell walls give support to the plant cell and provide a basic framework. It is a second layer around the cell membrane and can be described as an extra fence for double protection. The cell wall is different from the cell membrane in that it is rigid and strong. The cell wall is very important because it provides support for plant cells to grow tall. 6 Cytoplasm is a thick, gel-like material that fills all cells and is held in by the cell membrane. This clear, constantly flowing material holds all of the cell's material together on the inside. In eukaryotic cells, it is found between the cell membrane and the nucleus. Organelles move about freely in the cytoplasm, and other cell activity occurs here. In prokaryotic cells, all cellular processes occur in the cytoplasm. 7 Eukaryotic cells contain organelles, but prokaryotes do not. Organelles are structures inside a eukaryotic cell that perform the functions necessary for the cell to survive. There are several different types of organelles within a cell that allow it to survive. Let's get an even closer look at a few of these vital cellular components. 8 If the cell was a ship, then the nucleus would be the captain. The nucleus is an organelle inside the cell that guides all activity. It is the component most important to the eukaryotic cell's existence, because the nucleus contains deoxyribonucleic acid (DNA), which provides specific instructions for the cell's purpose and actions. DNA tells the cell how to produce new cells and perform all the cell's jobs. It also contains hereditary material, which will pass on specific information to future cells. Without DNA, the cell would not know how to operate. The nucleus is in all eukaryotes, while in prokaryotic cells, the DNA floats freely in the cell's cytoplasm. The nucleolus is a special structure within the nucleus, and its function is to make ribosomes, which produce proteins. The nucleus is protected by the nuclear membrane, which serves as an additional guard that controls the movement of materials in and out of this special area. 9 The motors that fuel the entire cell's operation are known as the mitochondria. These are a cell's “power plants.” Located in plant and animal cells, mitochondria produce ATP, the energy source for everything a cell needs to do. ATP is like gas for a car, and the mitochondria are responsible for producing it. They do this by using oxygen to break down sugars in a process known as cellular respiration. 10 Although plants use mitochondria to make ATP as an energy source, these cells also produce their own sugar through unique organelles called chloroplasts. Chloroplasts are organelles that produce chlorophyll, which traps the Sun's energy and then transforms this energy into sugar to be utilized as its own power source. This process is called photosynthesis, and the end waste product is oxygen. Without plant cells and the process of photosynthesis, humans and other organisms would not have oxygen to breathe. 11 These are just some of the remarkable organelles found in the cells of eukaryotes, though you must remember that the cell wall and chloroplasts are found in plant cells, but not animal cells. Who would have guessed there would be so much to see when we take a closer look inside of cells? 2
Anatomy of a Cell Structure and Function
For questions 1 and 2, use this diagram of the cell where one of the mitochondria has been enlarged.
1
2
From the information in the passage and the diagram, what can you conclude about the structure of mitochondria? A
They are membrane-bound organelles with a second, folded membrane inside to provide extra area for cellular respiration.
B
They take instructions from DNA and make proteins.
C
They break down sugars to provide energy for the cell.
D
They are organelles, which are a group of flattened membrane sacs stacked on top of each other.
From the information in the passage and the diagram, what can you conclude about the function of mitochondria? A
They are membrane-bound organelles with a second, folded membrane inside to provide extra area for cellular respiration.
B
They take instructions from DNA and make proteins.
C
They break down sugars to provide energy for the cell.
D
They are organelles, which are a group of flattened membrane sacs stacked on top of each other.
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Anatomy of a Cell Structure and Function
3
4
Which of the following statements best captures the main point of paragraph 3? A
A cell is the smallest unit that is truly alive.
B
All cells are surrounded by a cell membrane.
C
Cytoplasm is gel-like material that fills all cells.
D
Plant cells have cell walls.
In the passage, an analogy was given describing the function of a cell in simple terms by comparing it to a ship. Complete this analogy about the function of cell parts. A cell is to a board game as —
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A
DNA is to the rule book.
B
a nucleus is to the rule book.
C
endoplasmic reticulum is to the rule book.
D
a ribosome is to the rule book.
What does the word sub-cellular in paragraph 3 mean? A
more complex than a cell
B
taken away from a cell
C
hidden underneath a cell
D
located within a cell
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