Download Chapter 6 – A Tour of the Cell

BIOL 1406
HCC-SW/Stafford Campus
J.L. Marshall, Ph.D.
Chapter 6 – A Tour of the Cell*
*Lecture notes are to be used as a study guide only and do not represent the comprehensive information you will need to know for
the exams.
The Fundamental Units of Life
Cells are fundamental to the living systems of biology (Figure 6.1). All organisms are made of cells. There are
simple, single cells like bacteria, and multicellular organisms like plants and humans. Even though there are
many different types of cells, they all share basic features.
Concept 6.1: Biologists use microscopes and the tools of biochemistry to study cells
Cells are too small to be seen with the naked eye (Figure 6.2).
Microscopy
The early study of cells was started by the invention of the microscope. Cells were first seen in the microscope
by Robert Hooke in 1665. Antoni van Leeuwenhoek crafted lenses to help refine seeing the cells in 1674.
The most commonly used microscope in the biology student classes is the light microscope (LM). The light
microscope uses visible light to view the specimen (Appendix D-1 and Figure 6.3). This microscopy can be used
to study living cells. Today, new techniques like fluorescent markers make it possible to study more details.
Three (3) important parameters in microscopy are: 1) magnification – the ratio of an objects image size to its
real size; 2) resolution – measures the clarity of the image, able to view close points clearly; 3) contrast –
accentuates differences in parts of the sample.
The electron microscope (EM) uses electron beams to visualize internal eukaryotic cells parts such as
organelles (Appendix D-1 and Figure 6.3). Organelles are membrane enclosed structures in eukaryotic cells.
Electron microscopes have revealed many organelles and other subcellular structures that were impossible to
resolve with the light microscope.
Scanning electron microscope (SEM) is useful in studying the topography (surface) of a specimen.
Transmission electron microscope (TEM) is used to study internal structures.
Microscopy is an important tool for cytology, the study of cell structure. It is also important to study the
chemical processes of the cell, which is called biochemistry.
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Cell Fractionation
Cell fractionation is the process of taking cells apart, and separating their organelles from their subcellular
structures (Figure 6.4). This process enables scientists to study specific components of the cell.
Concept 6.2: Eukaryotic cells have internal membranes that compartmentalize their
functions
Cells are of two (2) distinct types: prokaryotic and eukaryotic.
Comparing Prokaryotic and Eukaryotic Cells
All cells have certain basic features:
1.
2.
3.
4.
They are all bounded by a selectively permeable barrier called the plasma membrane.
Inside all cells is a semi-fluid jelly like substance called the cytosol.
All cells contain chromosomes.
All cells have ribosomes that carry out protein synthesis.
A major difference between prokaryotic and eukaryotic cells is the location of their DNA. In a eukaryotic cell
most of the DNA is in the nucleus. In a prokaryotic cell the DNA is concentrated in a region of the cell called a
nucleoid (Figure 6.5).
The interior of both types of cells is called the cytoplasm. Various organelles are found in the eukaryotic cell’s
cytoplasm. At the boundary of every cell is the plasma membrane, it is a selectively permeable barrier that
allows the passage of gasses, nutrients, and wastes into and out of the cell (Figure 6.6).
Most of the discussion of cell structure that follows in this chapter applies to eukaryotic cells.
A Panoramic View of the Eukaryotic Cell
Figure 6.8 – Two eukaryotic cells: 1) animal cell, and 2) plant cell. Note the similarities and differences
between each.
Concept 6.3: The eukaryotic cell’s genetic instructions are housed in the nucleus and carried
out by the ribosomes
The nucleus is one of the cellular components that houses the genetic control of the cell.
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The Nucleus: Information Central
The nucleus contains most of the genes in a eukaryotic cell. Other genes are located in the mitochondria and
chloroplast. The nucleus is enclosed in the nuclear envelope (Figure 6.9).
The nuclear pores of the nuclear membrane allows for passage of materials into and out of the nucleus. The
nuclear side of the envelope is lined by the nuclear lamina, protein filaments that maintain the shape of the
nucleus. The chromosomes are in the nucleus, and carry the genetic information. Chromatin is the complex of
DNA and proteins. When the cell is not dividing, then the chromatin is diffuse. When the cell initiates the
division process, the chromatin condenses to chromosomes.
A prominent structure within the non-dividing nucleus is the nucleolus. The ribosomal RNA (rRNA) is
synthesized here. Protein subunits that help to comprise the ribosomes are imported in to build the ribosome.
Ribosomes: Protein Factories
Ribosomes are comprised of rRNA and protein, and carry out protein synthesis (Figure 6.10). Ribosomes exist
in two places in the eukaryotic cell: 1) free ribosomes are in the cytosol; 2) bound ribosomes are attached to
the outside of the endoplasmic reticulum.
Concept 6.4: The endomembrane system regulates protein traffic and performs metabolic
functions in the cell
The different membranes of the eukaryotic cell are part of the endomembrane system, which includes the
nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, various kinds of vesilces and
vacuoles, and the plasma membrane. This system carries out a variety of tasks in the cell. The membranes
either make direct contact with one another, or by the transfer of membrane segments as tiny vesicles. Each
membrane in this system is unique in terms of its composition and function.
The Endoplasmic Reticulum: Biosynthetic Factory
The endoplasmic reticulum (ER) is an extensive network of membranes in the cell. The ER consists of a
network of tubules and sacs called cisternae. The ER membrane is continuous with the nuclear envelope
(Figure 6.11). There are two (2) distinct types of ER: 1) smooth ER: lacks ribosomes; 2) rough ER: studded with
ribosomes on its outer surface.
Functions of Smooth ER Diverse functions that depend on cell type. The processes are: synthesis of
lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions.
For example, the cells in the testes and ovaries produce the sex hormones, respectively.
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Functions of Rough ER Proteins that are secreted are synthesized and then threaded into the ER lumen.
Most secreted proteins have carbohydrate molecules attached to them, glycoproteins. The
carbohydrates are attached in the ER by enzymes in the ER. The glycoproteins are then packaged into
vesicles, and the vesicles move to another part of the cell by transport vesicles. Membrane is also
made in the rough ER.
The Golgi Apparatus: Shipping and Receiving Center
After leaving the ER, many transport vesicles travel to the Golgi apparatus. The Golgi receives, sorts, ships and
manufactures proteins. The proteins of the ER are modified and stored, and then sent to other destinations.
The overall structure of the Golgi is like a stack of flat pancakes (Figure 6.12). The Golgi has two “sides”- a cis
and trans face; they act as the receiving and shipping departments, respectively. Certain polysaccharides are
made in the Golgi. Molecular identification tags are added to the molecules. These tags are like zip codes that
direct the final destination of the molecule.
Lysosomes: Digestive Compartments
A lysosome is a membranous sac of hydrolytic enzymes that an animal cell uses to digest (hydrolyze)
macromolecules. Lysosomal enzymes work in an acidic environment. Lysosomes carry out a variety of
digestion in the cell. Phagocytosis is used by amoeba to consume their food (Figure 6.13a), and by human
white blood cells to destroy foreign material in the body. Some lysosomes use their hydrolytic enzymes t o
recycle cellular structures such as old organelles in a process called autophagy (Figure 6.13b).
Vacuoles: Diverse Maintenance Compartments
Vacuoles are part of the endomembrane system. They perform different functions in different cells. Food
vacuoles are a result from digesting nutrients. Contractile vacuoles pump excess water out of the cells. In
some plants the vacuoles can hold organic compounds and pigments. Most plant cells contain a central
vacuole (Figure 6.14) which is a repository for ions. Central vacuoles can become large and therefore forces
the cytoplasm in to a thin area.
The Endomembrane System: A Review
Figure 6.15 reviews the endomembrane system showing the flow of lipids and proteins through the cell.
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Concept 6.5: Mitochondria and chloroplasts change energy from one form to another
The organelles in a eukaryotic cell that are involved in transforming energy are mitochondria, which is the site
for cellular respiration that generates ATP; and chloroplasts, that are found in plants and algae, are the site of
photosynthesis.
The Evolutionary Origins of Mitochondria and Chloroplasts
Mitochondria and chloroplasts are thought to have come about in evolution through a process called the
endosymbiont theory (Figure 6.16). Part of the evidence for this theory is that each organelle has its own
ribosomes as well as its own DNA, suggesting that initially each organelle existed as a bacterium.
Mitochondria: Chemical Energy Conversion
Mitochondria are found in plant, animal, fungi and most protists cells. The mitochondria have two membranes
(Figure 6.17). The outer membrane is smooth, but the inner membrane has folding called cristae. The
mitochondrial matrix is enclosed by the second membrane. The matrix contains some of the enzymes that
are used in the steps of cellular respiration. Other proteins and enzymes used in cellular respiration are part of
the inner membrane.
Chloroplasts: Capture of Light Energy
Chloroplasts contain a green pigment called chlorophyll, along with other molecules and enzymes that are
used in the process of photosynthesis. These organelles are found in plant leaves and algae (Figure 6.18). The
chloroplast has two membranes. In the internal compartment of the chloroplast there is another membranous
set called thylakoids. Each stack of thylakoids is called a granum. The fluid outside the thylakoid, but inside
the inner membrane is called the stroma. Each compartment function in the overall reaction of
photosynthesis. Chloroplasts are part of a specially related plant organelle called plastids. For example, one
type called amuloplast stores starch.
Peroxisomes: Oxidation
The peroxisome is a metabolic compartment bounded by a single membrane (Figure 6.19). Some peroxisomes
use oxygen to break down fatty acids. How peroxisomes are related to other organelles is still an open
question.
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Concept 6.6 : The cytoskeleton is a network of fibers that organizes structures and activities
in the cell
The cytoskeleton, a network of fibers extending throughout the cytoplasm (Figure 6.20). The cytoskeleton
organizes the structures and activities of the cell. The cytoskeleton is composed of three types of molecular
structures: 1) microtubules, 2) microfilaments, and 3) intermediate filaments.
Roles of the Cytoskeleton: Support and Motility
The function of the cytoskeleton is to give mechanical support to the cell and maintain its shape. It anchors
the many organelles in the cell. The cytoskeleton can be assembled and disassembled in different parts of the
same cell.
Cell motility involves the cytoskeleton, with the interaction of motor proteins (Figure 6.21).
Components of the Cytoskeleton
The three main types of fibers that comprise the cytoskeleton are: 1) microtubules, the thickest; 2)
microfilaments, also called actin, are the thinnest; and 3) intermediate filaments, fibers with diameters in the
middle range (Table 6.1).
1. Microtubules Contained in all eukaryotic cells, hollow rods that are comprised of a dimer called
tubulin. Tubulin can assemble and disassemble. Microtubules shape and support the cell and also serve
as tracks along organelles with motor proteins can move.
Centrosomes and Centrioles Microtubules grow out from a centrosome, a region located near
the nucleus, a microtubule organizing center. Within the centrosome is a pair of centrioles,
each composed of microtubules (Figure 6.22). They function in animal cell division.
Cilia and Flagella Extensions from eukaryotic cells. Flagella and cilia are used for cellular
movement. The sperm cells of animals move using their flagella by an undulating movement.
Cilia are usually located on cell surfaces, and beat back and forth. (Figure 6.23) Motile cilia and
flagella share a common structure (Figure 6.24). The “9+2” pattern is found in nearly all
eukaryotic flagella and motile cilia.
2. Microfilaments (Actin Filaments) Microfilaments are solid rods (Figure 6.25). They are also called
actin filaments because they are built from the protein actin. The role of microfilaments is to bear
tension, it helps to support the cell’s shape (Figure 6.26). They also play a role in cell motility. Actin
functions with myosin in muscle cells that allows the muscle cells to contract (Figure 6.26a).
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Microfilaments also play a role in amoeboid movement by extending pseudopodia (Figure 6.26b). In
plant cells actin-myosin interactions are involved in cytoplasmic streaming (Figure 6.26c).
3. Intermediate filaments Intermediate filaments are larger than microfilaments, but smaller than
microtubules, they also bear tension. They are more permanent than microfilaments and
microtubules. Help to play a role in reinforcing cell shape.
Concept 6.7: Extracellular components and connections between cells help coordinate
cellular activities
The plasma membrane is the boundary of the cell, but there are structures that extend beyond the plasma
membrane.
Cell Walls of Plants
The cell wall is found in plant cells. It protects the plant cell, maintains its shape, and prevents excessive
uptake of water. The plant cell wall is much thicker than the plasma membrane (Figure 6.27).
The Extracellular Matrix (ECM) of Animal Cells
Animal cells have an elaborate extracellular matrix (ECM) that provides support to the cells. The most
abundant glycoprotein of the ECM is collagen. The collagen fibers are embedded in a network woven out of
proteoglycans secreted by the cells (Figure 6.28). Fibronectin and other ECM proteins bind to cell-surface
receptor proteins called integrins that are built into the plasma membrane.
Cell Junctions
Cells in animals and plants are organized into tissues, organs, and organ systems. Neighboring cells are in
direct contact.
Plasmodesmata in Plant Cells Cell walls of plant cells are perforated with plasmodesmata (Figure 6.29).
Cytosol passes through the plasmodesmata of adjacent cells.
Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells Three types of cell junctions: 1) tight
junctions, 2) desmosomes, and 3) gap junctions. All are common in epithelial tissues (Figure 6.30).
The Cell: A Living Unit Greater Than the Sum of Its Parts Figure 6.31 illustrates how the different sub-cellular
components work together to form a fully functioning eukaryotic cell.
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