Download Chapter 1 • Lesson 1 Cell Structure Objectives: 1,1.1,1.1.2 Key

Chapter 1 • Lesson 1
Objectives: 1,1.1,1.1.2
Cell Structure
Key Words
• cell • eukaryote • organelle • prokaryote • cytoplasm • nucleus • nuclear membrane
• chromosome • plasmid • plasma membrane • ribosome • mitochondria • cell wall
• chloroplast • vacuole
Getting the Idea
While living organisms share a few common structures and functions, they exhibit an
amazing diversity of sizes, shapes, and features. The same is true of cells, the basic
units of structure and function in all organisms. Each cell is a complete system that
carries out all the processes necessary for life. All cells, both single-celled organisms
and cells that are part of larger organisms, have some features in common. However,
cells also have a wide range of specialized structures that aid in their survival.
Prokaryotic Cells and Eukaryotic Cells
The two basic types of cells are prokaryotic cells and eukaryotic cells. Each kind of cell
performs similar functions in the same ways. Both kinds of cells are enclosed by an outer
structure called a plasma membrane. Both are filled with cytoplasm and contain structures
called ribosomes, in which proteins are synthesized. How then do the two kinds of cells
differ? You can see some of the differences in the diagram below.
One obvious difference is that the eukaryotic cell is more complex than the prokaryotic
cell. The two kinds of cells have very different internal structures. For example, the
eukaryotic cell has a distinct nucleus and other cell structures, called organelles, that are
enclosed within membranes. Organisms whose cells have a distinct nucleus and
membrane-bound organelles are called eukaryotes. The cells of eukaryotes are larger
than prokaryotic cells. A typical eukaryotic cell ranges in size from about 2 to 100
micrometers. By contrast, most prokaryote cells are only 0.5 to 2 micrometers in size.
Organelles are cell structures that are specialized for different functions. Each type of
organelle has a structure that is suited to its function. You will learn more about how
organelle structure is related to function as you read about the different types of organelles.
Many eukaryotic organisms consist of large numbers of cells that work together. Plants,
animals, protists, and fungi are all eukaryotes. All plants and animals, and some protists
and fungi, are multicellular, or made up of many cells. In eukaryotes, the organelles interact
to carry out most of the cell's life processes.
Notice that the prokaryotic cell does not have a nucleus or membrane-bound organelles.
Organisms whose cells lack a nucleus and membrane-bound organelles are prokaryotes.
In prokaryotes, most of the processes of life occur in the cytoplasm. The cytoplasm is the
thick, semifluid material that is enclosed by the plasma membrane. In eukaryotes, the
cytoplasm contains the nucleus and the organelles that carry out most of the cell's life
processes. For example, eukaryotes have specialized organelles that release energy. In
prokaryotes, the processes that release energy occur in the cytoplasm or the plasma
membrane. All prokaryotes are one-celled organisms—either bacteria or archaea.
The Nucleus
The largest structure in most eukaryotic cells is the nucleus. The nucleus directs and
controls most cellular activities. It is enclosed by a structure called the nuclear membrane,
or nuclear envelope. This membrane controls the passage of materials between the nucleus
and the cytoplasm.
The nucleus contains deoxyribonucleic acid, or DNA. The DNA molecules control protein
production and cell functions. DNA also stores the genetic information that is passed from
parent to offspring during reproduction. In eukaryotes, DNA is bundled into structures called
chromosomes, which are located in the nucleus. You will read more about DNA and
chromosomes in Lessons 5 and 20. Prokaryotic cells lack nuclei, but they do contain DNA.
Their DNA is found floating in the cytoplasm. In addition to a large, tangled piece of DNA,
many prokaryotes have ring-shaped pieces of DNA called plasmids, which are also
suspended in the cytoplasm.
Structures Common to Most Eukaryotic Cells
The organelles common to most eukaryotic cells are shown in the diagram of the animal cell
below.
Plasma Membrane
The plasma membrane is a thin, flexible layer that surrounds the cytoplasm. This
membrane supports and protects the cell and gives it shape. The plasma membrane is
made up of two layers of lipids, in which protein molecules are embedded. Tiny openings in
the plasma membrane enable it to control which materials enter and leave the cell, either
from the external environment or from other cells of the same organism. The plasma
membrane is also called the cell membrane. You will learn more about the role of the
plasma membrane in the movement of materials into and out of cells in the next lesson.
Ribosomes
Structures called ribosomes are scattered throughout the cytoplasm of a cell. Ribosomes
make a variety of proteins that are used throughout the cell. Unlike most organelles,
ribosomes are not enclosed by membranes. Many ribosomes float freely within the
cytoplasm of both prokaryotes and eukaryotes. In eukaryotes, ribosomes may also be
attached to the endoplasmic reticulum (ER). The endoplasmic reticulum is a network of
membranes and sacs that transports molecules from one part of the cell to another.
Mitochondria
Mitochondria (singular mitochondrion) are the organelles that carry out cellular respiration.
Cellular respiration is the process by which living things obtain energy from food.
Mitochondria have folded inner membranes that increase the surface area available for the
release of energy. Cells that need a lot of energy, such as muscle cells, have many more
mitochondria than cells with lower energy requirements.
Other Organelles
Two other organelles common to many eukaryotes are the Golgi apparatus (also called
Golgi bodies) and lysosomes. The Golgi apparatus is a system of membranes that modifies
proteins and lipids according to where they will be used. Lysosomes are small, spherical
organelles that break down old organelles and carry out digestion in the cell. Lysosomes
are common in the cells of animals and fungi, but they are rare in plant cells.
Recall that individual cells carry out all of the functions required for life. These functions
include energy production and use, the synthesis and transport of molecules, and the
removal of wastes. Cell organelles must interact to carry out all of these functions. For
example, the DMA in the nucleus directs the ribosomes to assemble proteins that are used
as enzymes in other parts of the cell. Mitochondria use enzymes made by the ribosomes to
generate the energy that is used by other organelles.
See “Wacky History of Cell Theory”
Structures Found in Some Eukaryotic and Prokaryotic Cells
The organelles discussed so far are common to most eukaryotic cells. They are found in
animals, plants, protists, and fungi. The cells of some eukaryotes, particularly plants, have
structures that are not found in other types of eukaryotic cells. Many of these structures are
shown in the diagram of the plant cell below.
Cell Wall
The plant cell shown is surrounded by a cell wall. A cell wall is a rigid structure that
surrounds the plasma membrane of some cells, giving them additional protection and
support. Cell walls are thicker than plasma membranes but also have openings that enable
the cell to control the passage of materials into and out of the cell. The cells of all
prokaryotes, fungi, and plants have cell walls. Some protists also have cell walls. The cell
walls of different kinds of organisms are composed of different materials. Plant cell walls are
made of cellulose, a substance made up of sugars. The cell walls of fungi contain chitin.
Chitin is made from sugars modified by the addition of nitrogen. Bacteria have cells walls
made up mostly of peptidoglycan. This molecule is made up of sugar and protein. The cell
walls of archaea do not contain peptidoglycan. Instead, they are composed of a variety of
other molecules.
Chloroplasts
Plant cells contain chloroplasts. Chloroplasts are organelles that capture the energy of
sunlight and use it for photosynthesis. Photosynthesis is a process in which energy from
sunlight is used to make sugar from water and carbon dioxide. A green pigment called
chlorophyll captures the energy from sunlight. Some protists, including algae, also have
chloroplasts. Some bacteria carry out photosynthesis, but they do not have chloroplasts. In
these bacteria, chlorophyll is scattered through the cytoplasm. Animals and fungi do not
have chloroplasts or carry out photosynthesis.
Vacuoles
Plant cells have a large central vacuole, an organelle that stores water and other important
materials, including salts, proteins, and carbohydrates. In plants, pressure from the liquidfilled vacuole also helps support heavy structures such as leaves and flowers. Many animal
cells have small vacuoles, which store substances and transport them within the cell. The
cells of some protists also contain vacuoles that may store either useful materials or wastes.
Discussion Question
Which cell structures are common to prokaryotic cells, plant cells, and animal cells? Why do you
think these features are found in all types of cells?
Comparing Structures Found in Different Kinds of Cells
The table below summarizes the functions of the main components of prokaryotic cells and
two types of eukaryotic cells.
Internet Search
Find three photograph of eukaryotic cell under the microscope:
Internet Search
Find three photograph of eukaryotic cell under the microscope:
Cell City Analogy
In a far away city called Grant City, the main export and production product is the steel widget.
Everyone in the town has something to do with steel widget making and the entire town is designed
to build and export widgets. The town hall has the instructions for widget making, widgets come in all
shapes and sizes and any citizen of Grant can get the instructions and begin making their own
widgets. Widgets are generally produced in small shops around the city, these small shops can be
built by the carpenter's union (whose headquarters are in town hall).
After the widget is constructed, they are placed on special carts which can deliver the widget
anywhere in the city. In order for a widget to be exported, the carts take the widget to the postal
office, where the widgets are packaged and labeled for export. Sometimes widgets don't turn out
right, and the "rejects" are sent to the scrap yard where they are broken down for parts or destroyed
altogether. The town powers the widget shops and carts from a hydraulic dam that is in the city. The
entire city is enclosed by a large wooden fence, only the postal trucks (and citizens with proper
passports) are allowed outside the city.
Match the parts of the city (underlined) with the parts of the cell.
1. Mitochondria
________________________________________
2. Ribosomes
________________________________________
3. Nucleus
________________________________________
4. Endoplasmic Reticulum
________________________________________
5. Golgi Apparatus
________________________________________
6. Protein
________________________________________
7. Cell Membrane
________________________________________
8. Lysosomes
________________________________________
9. Nucleolus
________________________________________
Using the “cell city analogy activity” as a model, come up with your own analogy that describe
the function of a cell and its organelles: