6.18: Ribosomes, Mitochondria, and Peroxisomes - Biology

6.18: Ribosomes, Mitochondria, and Peroxisomes - Biology

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Ribosomes are the cellular structures responsible for protein synthesis. Amino acids are the building blocks of proteins.

Because proteins synthesis is an essential function of all cells, ribosomes are found in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function.


Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. ATP represents the short-term stored energy of the cell. Cellular respiration is the process of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need a lot of energy to keep your body moving. When your cells don’t get enough oxygen, they do not make a lot of ATP. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid.

Mitochondria are oval-shaped, double membrane organelles (Figure 2) that have their own DNA and ribosomes (we’ll talk about these later!). Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.


Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. Many of these oxidation reactions release hydrogen peroxide, H2O2, which would be damaging to cells; however, when these reactions are confined to peroxisomes, enzymes safely break down the H2O2 into oxygen and water. For example, alcohol is detoxified by peroxisomes in liver cells. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars.

Peroxisomes - Another Enzyme Package

There are many ways that peroxisomes are similar to lysosomes. They are small vesicles found around the cell. They have a single membrane that contains digestive enzymes for breaking down toxic materials in the cell. They differ from lysosomes in the type of enzyme they hold. Peroxisomes hold on to enzymes that require oxygen (oxidative enzymes). Lysosomes have enzymes that work in oxygen-poor areas and lower pH.

Peroxisomes absorb nutrients that the cell has acquired. They are very well known for digesting fatty acids. They also play a part in the way organisms digest alcohol (ethanol). Because they do that job, you would expect liver cells to have more peroxisomes than most other cells in a human body. They also play a role in cholesterol synthesis and the digestion of amino acids.

Cytoplasmic organelles

The cytoplasm is the substance of the cell other than the nucleus and cell membrane, and it is basically fluid in nature, “Organelles” are the functional elements within the cytoplasm. Some of the most important organelles that cytoplasm contains are the ribosomes, mitochondria, proteins, the endoplasmic reticulum, lysosomes, and the Golgi apparatus.


The mitochondria are powerhouses of the cell as they are the sites of adenosine triphosphate (ATP) production.

LM picture

Mitochondria, when present in large numbers contribute to cytoplasmic eosinophilia because of the large amount of membrane they contain. Mitochondria can be visualized in tissues by using special stains e.g. silver stain.

EM picture

Mitochondria are membrane-bounded organelles, present in all types of cells except in RBCs and terminal keratinocytes of skin epidermis. They have variable shapes and their number ranges from few to several hundred depending on the energy needs of the various cell types. Each mitochondrion is surrounded by two membranes: outer and inner, which define two mitochondrial compartments

  • The intermembranous space, the compartment located between the two membranes.
  • The matrix space, the compartment enclosed by the inner membrane.

Mitochondrial membranes contain distinct protein composition related to its function, The outer membrane is smooth and porous. It allows easy passage of small molecules due to the presence of specific transmembrane proteins called mitochondrial porins.

Cell structure

The inner membrane

  • It is folded into numerous cristae which greatly increase its total surface area, the number of cristae is greater in cells of greater demand for ATP as in cardiac muscle fibers.
  • Most mitochondria have flat, lamellar cristae in their interiors, whereas cells that secrete steroids frequently contain tubular cristae.
  • It is highly impermeable to ions and small molecules due to the presence of specific phospholipid called cardiolipin.
  • It contains the enzymatic components of the electron transport system (respiratory chain enzymes) and the ATP synthase. These enzymes from the elementary particles projecting from the mitochondrial cristal membrane into the matrix.

Matrix space is surrounded by the inner mitochondrial membrane. It contains the following:

  • Numerous soluble enzymes involved in specialized mitochondrial functions as a citric acid cycle.
  • Mitochondrial DNA and few ribosomes.
  • Matrix granules that store calcium ions (Ca + ) thus play a role in mitochondrial regulation of Ca + intracellular concentration.

Intermembranous space contains substrates diffusing from the cytoplasm through the outer membrane and ions pumped out of the matrix space through the inner membrane.

The genetic system of mitochondria: Mitochondria are self-replicating organelles, the mitochondrial genome is a circular DNA molecule with limited coding capacity. It represents 1% of the total DNA of the cell. Mitochondria can synthesize some of their structural proteins by their own RNAs. Most of the mitochondrial proteins are encoded by the nuclear DNA and are synthesized in the cytoplasm and subsequently imported into mitochondria.

Functions of Peroxisomes

Peroxisomes contain at least 50 different enzymes, which are involved in a variety of biochemical pathways in different types of cells. Peroxisomes originally were defined as organelles that carry out oxidation reactions leading to the production of hydrogen peroxide. Because hydrogen peroxide is harmful to the cell, peroxisomes also contain the enzyme catalase, which decomposes hydrogen peroxide either by converting it to water or by using it to oxidize another organic compound. A variety of substrates are broken down by such oxidative reactions in peroxisomes, including uric acid, amino acids, and fatty acids. The oxidation of fatty acids (Figure 10.25) is a particularly important example, since it provides a major source of metabolic energy. In animal cells, fatty acids are oxidized in both peroxisomes and mitochondria, but in yeasts and plants fatty acid oxidation is restricted to peroxisomes.

Figure 10.25

Fatty acid oxidation in peroxisomes. The oxidation of a fatty acid is accompanied by the production of hydrogen peroxide (H2O2) from oxygen. The hydrogen peroxide is decomposed by catalase, either by conversion to water or by oxidation of another organic (more. )

In addition to providing a compartment for oxidation reactions, peroxisomes are involved in lipid biosynthesis. In animal cells, cholesterol and dolichol are synthesized in peroxisomes as well as in the ER. In the liver, peroxisomes are also involved in the synthesis of bile acids, which are derived from cholesterol. In addition, peroxisomes contain enzymes required for the synthesis of plasmalogens𠅊 family of phospholipids in which one of the hydrocarbon chains is joined to glycerol by an ether bond rather than an ester bond (Figure 10.26). Plasmalogens are important membrane components in some tissues, particularly heart and brain, although they are absent in others.

Figure 10.26

Structure of a plasmalogen. The plasmalogen shown is analogous to phosphatidylcholine. However, one of the fatty acid chains is joined to glycerol by an ether, rather than an ester, bond.

Peroxisomes play two particularly important roles in plants. First, peroxisomes in seeds are responsible for the conversion of stored fatty acids to carbohydrates, which is critical to providing energy and raw materials for growth of the germinating plant. This occurs via a series of reactions termed the glyoxylate cycle, which is a variant of the citric acid cycle (Figure 10.27). The peroxisomes in which this takes place are sometimes called glyoxysomes.

Figure 10.27

The glyoxylate cycle. Plants are capable of synthesizing carbohydrates from fatty acids via the glyoxylate cycle, which is a variant of the citric acid cycle (see Figure 2.34). As in the citric acid cycle, acetyl CoA combines with oxaloacetate to form (more. )

Second, peroxisomes in leaves are involved in photorespiration, which serves to metabolize a side product formed during photosynthesis (Figure 10.28). CO2 is converted to carbohydrates during photosynthesis via a series of reactions called the Calvin cycle (see Figure 2.39). The first step is the addition of CO2 to the five-carbon sugar ribulose-1,5-bisphosphate, yielding two molecules of 3-phosphoglycerate (three carbons each). However, the enzyme involved (ribulose bisphosphate carboxylase or rubisco) sometimes catalyzes the addition of O2 instead of CO2, producing one molecule of 3-phosphoglycerate and one molecule of phosphoglycolate (two carbons). This is a side reaction, and phosphoglycolate is not a useful metabolite. It is first converted to glycolate and then transferred to peroxisomes, where it is oxidized and converted to glycine. Glycine is then transferred to mitochondria, where two molecules of glycine are converted to one molecule of serine, with the loss of CO2 and NH3. The serine is then returned to peroxisomes, where it is converted to glycerate. Finally, the glycerate is transferred back to chloroplasts, where it reenters the Calvin cycle. Photorespiration does not appear to be beneficial for the plant, since it is essentially the reverse of photosynthesis—O2 is consumed and CO2 is released without any gain of ATP. However, the occasional utilization of O2 in place of CO2 appears to be an inherent property of rubisco, so photorespiration is a general accompaniment of photosynthesis. Peroxisomes thus play an important role by allowing most of the carbon in glycolate to be recovered and utilized.

Figure 10.28

Role of peroxisomes in photorespiration. During photosynthesis, CO2 is converted to carbohydrates by the Calvin cycle, which initiates with the addition of CO2 to the five-carbon sugar ribulose-1,5-bisphosphate. However, the enzyme involved sometimes (more. )

6.18: Ribosomes, Mitochondria, and Peroxisomes - Biology

Scanning electron
microscopy (SEM)

Transmission electron
microscopy (TEM)

Longitudinal section
of cilium

Figure 6.3 Exploring: Microscopy

Comparing Prokaryotic and Eukaryotic Cells

  • Basic features of all cells
    • Plasma membrane
    • Semifluid substance called cytosol
    • Chromosomes (carry genes)
    • Ribosomes (make proteins)

    © 2011 Pearson Education, Inc.

    • Prokaryotic cells – Archae and Bacteria
      • No nucleus
      • DNA in an unbound region called the nucleoid
      • No membrane-bound organelles
      • Cytoplasm bound by the plasma membrane
      • DNA in a nucleus that is bounded by a membranous nuclear envelope
      • Membrane-bound organelles
      • Cytoplasm in the region between the plasma membrane and nucleus
      • Larger than prokaryotes

      © 2011 Pearson Education, Inc.

      Figure 6.5 A prokaryotic cell.

      A thin section
      through the
      bacterium Bacillus
      coagulans (TEM)

      Figure 6.5 A prokaryotic cell.

      • The plasma membrane - a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell
        • The general structure of a biological membrane is a double layer of phospholipids
        • 30-300 μm circumference

        © 2011 Pearson Education, Inc.

        (b) Structure of the plasma membrane

        Figure 6.6 The plasma membrane.

        Surface area increases while
        total volume remains constant

        Total surface area
        [sum of the surface areas
        (height × width) of all box
        sides × number of boxes]

        Total volume
        [height × width × length
        × number of boxes]

        (S-to-V) ratio
        [surface area ÷ volume]

        Figure 6.7 Geometric relationships between surface area and volume.

        A Panoramic View of the Eukaryotic Cell

        • A eukaryotic cell has internal membranes that partition the cell into organelles
        • Plant and animal cells have most of the same organelles

        © 2011 Pearson Education, Inc.

        Figure 6.8 Exploring: Eukaryotic Cells

        Human cells from lining
        of uterus (colorized TEM)

        Yeast cells budding
        (colorized SEM)

        A single yeast cell
        (colorized TEM)

        Figure 6.8 Exploring: Eukaryotic Cells

        Figure 6.8 Exploring: Eukaryotic Cells

        Cells from duckweed
        (colorized TEM)

        (colorized SEM)

        (colorized TEM)

        Figure 6.8 Exploring: Eukaryotic Cells

        Concept 6.3: Nucleus, DNA, and Ribosomes

        • The nucleus contains most of the DNA in a eukaryotic cell
        • Ribosomes use the information from the DNA to make proteins
        • 5 μm

        © 2011 Pearson Education, Inc.

        The Nucleus: Information Central

        • Nucleus - contains most of the cell’s genes and is the most conspicuous organelle
        • nuclear envelope - encloses the nucleus, separating it from the cytoplasm

        A double membrane each membrane consists of a lipid bilayer

        © 2011 Pearson Education, Inc.

        Figure 6.9 The nucleus and its envelope.

        Surface of nuclear

        Figure 6.9 The nucleus and its envelope.

        Pores- regulate the entry and exit of molecules from the nucleus

        The shape of the nucleus is maintained by the nuclear lamina , which is composed of protein

        Figure 6.9 The nucleus and its envelope.

        • In the nucleus, DNA is organized into discrete units called chromosomes
        • Each chromosome is composed of a single DNA molecule associated with proteins
        • The DNA and proteins of chromosomes are together called chromatin
        • Chromatin condenses to form discrete chromosomes as a cell prepares to divide
        • The nucleolus is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis

        © 2011 Pearson Education, Inc.

        • Ribosomes are particles made of ribosomal RNA and protein
        • Ribosomes carry out protein synthesis in two locations
          • In the cytosol (free ribosomes)
          • On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes)

          © 2011 Pearson Education, Inc.

          For the Cell Biology Video Staining of Endoplasmic Reticulum, go to Animation and Video Files.

          Ribosomes: Protein Factories

          • Ribosomes are particles made of ribosomal RNA and protein
          • Ribosomes carry out protein synthesis in two locations
            • In the cytosol (free ribosomes)
            • On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes)

            Concept 6.4: The endomembrane system - regulates protein traffic and performs metabolic functions in the cell

            • Components of the endomembrane system
              • Nuclear envelope
              • Endoplasmic reticulum
              • Golgi apparatus
              • Lysosomes
              • Vacuoles
              • Plasma membrane

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              The Endoplasmic Reticulum: Biosynthetic Factory

              • The endoplasmic reticulum (ER) accounts for more than half of the total membrane in many eukaryotic cells
              • The ER membrane is continuous with the nuclear envelope
              • There are two distinct regions of ER
                • Smooth ER , which lacks ribosomes
                • Rough ER , surface is studded with ribosomes

                © 2011 Pearson Education, Inc.

                For the Cell Biology Video ER and Mitochondria in Leaf Cells, go to Animation and Video Files.

                Figure 6.11 Endoplasmic reticulum (ER).

                Figure 6.11 Endoplasmic reticulum (ER).

                • The rough ER
                  • Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to carbohydrates)
                  • Distributes transport vesicles, proteins surrounded by membranes
                  • Is a membrane factory for the cell
                  • Synthesizes lipids – sex cells and adrenal glands
                  • Metabolizes carbohydrates
                  • Detoxifies drugs and poisons- liver cells
                  • Stores calcium ions - muscles

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                  The Golgi Apparatus: Shipping and
                  Receiving Center

                  • The Golgi apparatus consists of flattened membranous sacs called cisternae
                  • Functions of the Golgi apparatus
                    • Modifies products of the ER – glycoproteins and phospholipids
                    • Manufactures certain macromolecules – pectin
                    • Sorts and packages materials into transport vesicles –molecular tagging for docking sites

                    © 2011 Pearson Education, Inc.

                    For the Cell Biology Video ER to Golgi Traffic, go to Animation and Video Files.

                    For the Cell Biology Video Golgi Complex in 3D, go to Animation and Video Files.

                    For the Cell Biology Video Secretion From the Golgi, go to Animation and Video Files.

                    cis face
                    (“receiving” side of
                    Golgi apparatus)

                    trans face
                    (“shipping” side of
                    Golgi apparatus)

                    Figure 6.12 The Golgi apparatus.

                    Lysosomes: Digestive Compartments

                    • A lysosome is a membranous sac of hydrolytic enzymes that can digest macromolecules
                    • Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids
                    • Lysosomal enzymes work best in the acidic environment inside the lysosome
                    • Some types of cell (amoeba and white blood cells) can engulf another cell by phagocytosis this forms a food vacuole
                    • fuses with the food vacuole and digests the molecules
                    • use enzymes to recycle the cell’s own organelles and macromolecules, a process called autophagy

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                    Vesicle containing
                    two damaged

                    Vesicle containing
                    two damaged

                    Vacuoles: Diverse Maintenance Compartments

                    • A plant cell or fungal cell may have one or several vacuoles , derived from endoplasmic reticulum and Golgi apparatus
                    • Food vacuoles are formed by phagocytosis
                    • Contractile vacuoles , found in many freshwater protists, pump excess water out of cells
                    • Central vacuoles , found in many mature plant cells, hold organic compounds and water

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                    Figure 6.14 The plant cell vacuole.

                    Figure 6.15 Review: relationships among organelles of the endomembrane system.

                    Figure 6.15 Review: relationships among organelles of the endomembrane system.

                    Figure 6.15 Review: relationships among organelles of the endomembrane system.

                    Concept 6.5: Mitochondria and chloroplasts change energy from one form to another

                    • Mitochondria are the sites of cellular respiration, a metabolic process that uses oxygen to generate ATP
                    • Chloroplasts , found in plants and algae, are the sites of photosynthesis
                    • Peroxisomes are oxidative organelles

                    © 2011 Pearson Education, Inc.

                    For the Cell Biology Video ER and Mitochondria in Leaf Cells, go to Animation and Video Files.

                    For the Cell Biology Video Mitochondria in 3D, go to Animation and Video Files.

                    For the Cell Biology Video Chloroplast Movement, go to Animation and Video Files.

                    The Evolutionary Origins of Mitochondria and Chloroplasts

                    • Mitochondria and chloroplasts have similarities with bacteria
                      • Enveloped by a double membrane
                      • Contain free ribosomes and circular DNA molecules
                      • Grow and reproduce somewhat independently in cells

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                      • The Endosymbiont theory
                        • An early ancestor of eukaryotic cells engulfed a nonphotosynthetic prokaryotic cell, which formed an endosymbiont relationship with its host
                        • The host cell and endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion
                        • At least one of these cells may have taken up a photosynthetic prokaryote, becoming the ancestor of cells that contain chloroplasts

                        © 2011 Pearson Education, Inc.

                        Ancestor of
                        eukaryotic cells
                        (host cell)

                        Engulfing of oxygen-
                        using nonphotosynthetic
                        prokaryote, which
                        becomes a mitochondrion

                        Figure 6.16 The endosymbiont theory of the origin of mitochondria and chloroplasts in eukaryotic cells.

                        Mitochondria: Chemical Energy Conversion

                        • Found in eukaryotes
                        • smooth outer membrane and an inner membrane folded into cristae
                        • The inner membrane creates two compartments: intermembrane space and mitochondrial matrix
                        • Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix
                        • Cristae present a large surface area for enzymes that synthesize ATP

                        © 2011 Pearson Education, Inc.

                        (a) Diagram and TEM of mitochondrion

                        Network of mitochondria in a protist
                        cell (LM)

                        Figure 6.17 The mitochondrion, site of cellular respiration.

                        Chloroplasts: Capture of Light Energy

                        • Chloroplast structure includes
                          • Thylakoids , membranous sacs, stacked to form a granum
                          • Stroma , the internal fluid

                          © 2011 Pearson Education, Inc.

                          (a) Diagram and TEM of chloroplast

                          (b) Chloroplasts in an algal cell

                          Figure 6.18 The chloroplast, site of photosynthesis.

                          • specialized metabolic compartments bounded by a single membrane
                          • Remove H+ to O+ which produces hydrogen peroxide and convert it to water
                          • Glyoxysomes – found in plant seed and fatty acids to sugar for the cotyledon until photosynthesis begins

                          © 2011 Pearson Education, Inc.

                          Concept 6.6: The cytoskeleton is a network of fibers that organizes structures and activities in the cell

                          • Extends throughout the cytoplasm
                          • Organizes the cell’s structures and activities, anchoring many organelles
                          • Composed of three types of molecular structures
                            • Microtubules
                            • Microfilaments
                            • Intermediate filaments

                            © 2011 Pearson Education, Inc.

                            For the Cell Biology Video The Cytoskeleton in a Neuron Growth Cone, go to Animation and Video Files

                            For the Cell Biology Video Cytoskeletal Protein Dynamics, go to Animation and Video Files.

                            Figure 6.20 The cytoskeleton.

                            Receptor for
                            motor protein

                            Figure 6.21 Motor proteins and the cytoskeleton.

                            Components of the Cytoskeleton

                              • Microtubules – thickest organelle movement secretory vesicles to plasma membrane mitosis
                              • Microfilaments – thinnest (also called actin ) –supports cell shape microvilli forms bridge with myosin for muscle contractioncell contraction during cell cleavage pseudopodia movement (amoeba and WBC)
                              • Intermediate filaments -diameters in a middle range – contains protein keratin reinforces position of nucleusnuclear lamina axons

                              © 2011 Pearson Education, Inc.

                              For the Cell Biology Video Actin Network in Crawling Cells, go to Animation and Video Files.

                              For the Cell Biology Video Actin Visualization in Dendrites, go to Animation and Video Files.

                              Fibrous subunit (keratins
                              coiled together)

                              Table 6.1 The Structure and Function of the Cytoskeleton

                              Table 6.1 The Structure and Function of the Cytoskeleton

                              Centrosomes and Centrioles

                              • centrosome near the nucleus microtubules grow from it
                              • In animal cells , the centrosome has a pair of centrioles , each with nine triplets of microtubules arranged in a ring

                              © 2011 Pearson Education, Inc.

                              section of
                              one centriole

                              Cross section
                              of the other centriole

                              Figure 6.22 Centrosome containing a pair of centrioles.

                              • Microtubules control the beating of cilia and flagella , locomotor appendages of some cells
                              • Cilia and flagella differ in their beating patterns
                              • Cilia and flagella share a common structure
                                • A core of microtubules sheathed by the plasma membrane
                                • A basal body that anchors the cilium or flagellum
                                • A motor protein called dynein , which drives the bending movements of a cilium or flagellum

                                © 2011 Pearson Education, Inc.

                                Direction of organism’s movement

                                Power stroke Recovery stroke

                                Figure 6.23 A comparison of the beating of flagella and motile cilia.

                                Longitudinal section
                                of motile cilium

                                Cross section of
                                motile cilium

                                proteins between
                                outer doublets

                                Cross section of
                                basal body

                                Figure 6.24 Structure of a flagellum or motile cilium.

                                • How dynein “walking” moves flagella and cilia
                                  • Dynein arms alternately grab, move, and release the outer microtubules
                                  • Protein cross-links limit sliding
                                  • Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum

                                  © 2011 Pearson Education, Inc.

                                  For the Cell Biology Video Motion of Isolated Flagellum, go to Animation and Video Files.

                                  For the Cell Biology Video Flagellum Movement in Swimming Sperm, go to Animation and Video Files.

                                  (a) Effect of unrestrained dynein movement

                                  Figure 6.25 How dynein “walking” moves flagella and cilia.

                                  Cross-linking proteins
                                  between outer doublets

                                  (b) Effect of cross-linking proteins

                                  Figure 6.25 How dynein “walking” moves flagella and cilia.

                                  (a) Myosin motors in muscle cell contraction

                                  Cortex (outer cytoplasm):
                                  gel with actin network

                                  Inner cytoplasm: sol
                                  with actin subunits

                                  (c) Cytoplasmic streaming in plant cells

                                  Figure 6.27 Microfilaments and motility.

                                  • Cytoplasmic streaming is a circular flow of cytoplasm within cells
                                  • This streaming speeds distribution of materials within the cell
                                  • In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming

                                  © 2011 Pearson Education, Inc.

                                  Concept 6.7: Extracellular components and connections between cells help coordinate cellular activities

                                  • Most cells synthesize and secrete materials that are external to the plasma membrane
                                  • These extracellular structures include
                                    • Cell walls of plants
                                    • The extracellular matrix (ECM) of animal cells
                                    • Intercellular junctions

                                    © 2011 Pearson Education, Inc.

                                    For the Cell Biology Video Ciliary Motion, go to Animation and Video Files.

                                    • An extracellular structure that distinguishes plant cells from animal cells
                                    • Prokaryotes, fungi, and some protists also have cell walls
                                    • The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water
                                    • Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein
                                    • Plant cell walls may have multiple layers
                                      • Primary cell wall : relatively thin and flexible
                                      • Middle lamella : thin layer between primary walls of adjacent cells
                                      • Secondary cell wall (in some cells): added between the plasma membrane and the primary cell wall

                                      © 2011 Pearson Education, Inc.

                                      Figure 6.28 Plant cell walls.

                                      Distribution of cellulose
                                      synthase over time

                                      Distribution of
                                      over time

                                      Figure 6.29 Inquiry: What role do microtubules play in orienting deposition of cellulose in cell walls?

                                      The Extracellular Matrix (ECM) of Animal Cells

                                      • Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM)
                                      • The ECM is made up of glycoproteins such as collagen , proteoglycans , and fibronectin
                                      • ECM proteins bind to receptor proteins in the plasma membrane called integrins
                                      • Functions of the ECM
                                        • Support
                                        • Adhesion
                                        • Movement
                                        • Regulation

                                        © 2011 Pearson Education, Inc.

                                        For the Cell Biology Video Cartoon Model of a Collagen Triple Helix, go to Animation and Video Files.

                                        For the Cell Biology Video Staining of the Extracellular Matrix, go to Animation and Video Files.

                                        For the Cell Biology Video Fibronectin Fibrils, go to Animation and Video Files.

                                        Localization of glucose-6-phosphate dehydrogenase activity on ribosomes of granular endoplasmic reticulum, in peroxisomes and peripheral cytoplasm of rat liver parenchymal cells

                                        Glucose-6-phosphate dehydrogenase activity has been localized ultrastructurally in fixed tissues. Activity was found in particular in association with ribosomes of granular endoplasmatic reticulum. Biochemical studies indicated that glucose-6-phosphate dehydrogenase activity is also present in the cytoplasm and in peroxisomes. Fixation may be held responsible for selective inactivation of part of glucose-6-phosphate dehydrogenase activity. In the present study, we applied the ferricyanide method for the demonstration of glucose-6-phosphate dehydrogenase activity in unfixed cryostat sections of rat liver in combination with the semipermeable membrane technique and in isolated rat liver parenchymal cells. Isolated liver parenchymal cells were permeabilized with 0.025% glutaraldehyde after NADP+ protection of the active site of glucose-6-phosphate dehydrogenase. This treatment resulted in only slight inactivation of glucose-6-phosphate dehydrogenase activity. The composition of the incubation medium was optimized on the basis of rapid light microscopical analysis of the formation of reddish-brown final reaction product in sections. With the optimized method, electron dense reaction product was observed in cryostat sections on granular endoplasmic reticulum, in mitochondria and at the cell border. However, the ultrastructural morphology was rather poor. In contrast, the morphology of incubated isolated cells was preserved much better. Electron dense precipitate was found on ribosomes of the granular endoplasmic reticulum, in peroxisomes and the cytoplasm, particularly at the periphery of cells. In conclusion, our ultrastructural study clearly demonstrates that it is essential to use mildly-fixed cells to allow detection of glucose-6-phosphate dehydrogenase activity in all cellular compartments where activity is present.

                                        Animal Cell Organelles

                                        Animal cells contain numerous organelles (literally meaning ‘little organs’) to help them carry out the functions essential to their survival.

                                        The Nucleus

                                        The nucleus is a key structure in all eukaryotic cells, as it stores all of the cell’s DNA (and therefore, genetic information). The nucleus also controls and regulates all the vital functions of the cell, including protein production, cell division, metabolism, and growth.

                                        DNA molecules also contain the blueprints for every protein in an organism and must be carefully preserved to maintain successful protein production. The nucleus is, therefore, surrounded by a double membrane called the nuclear envelope, which protects the DNA by keeping it separate from the rest of the cell.


                                        Mitochondria are often referred to as the ‘powerhouses of the cell,’ as they release the energy required to power all other cellular functions. These organelles are the site of respiration, a metabolic process in which glucose is broken down to release energy. The energy released by cellular respiration is used to produce ATP (adenosine triphosphate) molecules. ATP is the energy currency of cells and is used to fuel all other essential cellular processes.


                                        Rough Endoplasmic Reticulum (Rough ER)

                                        The rough ER is so-named because its membrane is studded with ribosomes, giving it a ‘rough’ appearance. After these ribosomes have finished assembling a polypeptide chain, the protein is released into the lumen of the RER. Once inside, it is folded into a complex, 3D structure that is specific to the type of protein. The RER is also where proteins are ‘tagged’ for transport to the Golgi apparatus. ‘Tagging’ usually involves the addition of a carbohydrate molecule to the protein, in a process that is known as glycosylation.

                                        Smooth Endoplasmic Reticulum (Smooth ER)

                                        The main difference between the rough ER and the smooth ER is that the smooth ER does not have ribosomes attached to its surface. The smooth ER is not involved in protein synthesis instead, it is the site of lipid and steroid production in the cell.

                                        Golgi Apparatus

                                        Newly synthesized proteins are sent to the Golgi apparatus after they leave the rough ER. The Golgi apparatus (a series of flattened, membrane-bound sacs) is like the ‘mailroom’ of the cell and packages new proteins into tiny, membrane-bound vesicles for distribution. Once packaged, the proteins are sent off to the outer cell membrane, where they either leave the cell or become part of the lipid bilayer.


                                        Some animal cells contain vacuoles, which are typically small organelles used to transport substances in and out of the cell. They are often used to contain and dispose of waste products.


                                        Lysosomes are spherical organelles filled with digestive enzymes, and they have several functions within cells. They are used to break down old or surplus cell parts, destroy invading pathogens, and also play a key role in programmed cell death (AKA apoptosis).


                                        Peroxisomes are similar to lysosomes in that they are spherical organelles that contain digestive enzymes. However, unlike lysosomes (which primarily break down proteins), peroxisomes degrade fatty acids. This is a major source of metabolic energy for the cell, which can be used to fuel other cellular processes.

                                        The Cell Membrane

                                        All cells are surrounded by a cell membrane (AKA the plasma membrane). In eukaryotic cells, cell membranes also surround each of the cell’s organelles. This compartmentalizes the contents of the cell and keeps the vital (but incompatible) metabolic processes of different organelles separate.

                                        The main function of the cell membrane is to create a physical barrier between the interior of the cell and the external environment. However, it also controls the movement of substances in and out of the cell. The cell membrane consists of a semipermeable lipid bilayer that is studded with channels and receptors to allow certain molecules through. Therefore, the cell membrane helps to keep toxins out of the cell, while ensuring that valuable resources (such as nutrients) can enter. It also allows waste and metabolic products to leave the cell.


                                        The cytoplasm is a jelly-like substance that fills up the spaces inside cells. It cushions and protects the organelles, and also gives cells their shape. The cytoplasm is composed of water, salts, and other molecules required for cellular processes.

                                        Peroxisomes Function

                                        In addition to being involved in the oxidation and decomposition of organic molecules, peroxisomes are also involved in synthesizing important molecules. In animal cells, peroxisomes synthesize cholesterol and bile acids (produced in the liver). Certain enzymes in peroxisomes are necessary for the synthesis of a specific type of phospholipid that is necessary for the building of heart and brain white matter tissue. Peroxisome dysfunction can lead to the development of disorders that affect the central nervous system as peroxisomes are involved in producing the lipid covering (myelin sheath) of nerve fibers. The majority of peroxisome disorders are the result of gene mutations that are inherited as autosomal recessive disorders. This means that individuals with the disorder inherit two copies of the abnormal gene, one from each parent.

                                        In plant cells, peroxisomes convert fatty acids to carbohydrates for metabolism in germinating seeds. They are also involved in photorespiration, which occurs when carbon dioxide levels become too low in plant leaves. Photorespiration conserves carbon dioxide by limiting the amount of CO2 available to be used in photosynthesis.

                                        The number of mitochondria in cells can vary from a few pieces to thousands of units. Cells, which are making the synthesis of ATP molecules, have a greater number of mitochondria.

                                        Mitochondria have different shapes and sizes, there are rounded, elongated, spiral and cupped representatives among them. How big are mitochondria? Usually, their shape is round and elongated, with a diameter from one micrometer to 10 micrometers long.

                                        Mitochondria can move through the cell (they do this thanks to the cytoplasm) and remain motionless in place. They always move to places where energy production is needed the most.


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                                        Nucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube.

                                        TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube.

                                        A large complex of RNA and protein which acts as the site of RNA translation, building proteins from amino acids using messenger RNA as a template.

                                        A class of biological molecule consisting of linked monomers of amino acids and which are the most versatile macromolecules in living systems and serve crucial functions in essentially all biological processes.

                                        A nucleic acid of which many different kinds are now known, including messenger RNA, transfer RNA and ribosomal RNA.

                                        The smallest unit of life, consisting of at least a membrane, cytoplasm, and genetic material.

                                        Cells which lack membrane-bound structures, specifically a nucleus. Instead they generally have a single circular chromosome located in an area of the cell called the nucleoid.

                                        Cells which have a nucleus enclosed within membranes, unlike prokaryotes, which have no membrane-bound organelles.

                                        A central organelle containing hereditary material.

                                        A tiny cellular structure that performs specific functions within a cell.

                                        The jellylike material that makes up much of a cell inside the cell membrane, and, in eukaryotic cells, surrounds the nucleus. The organelles of eukaryotic cells, such as mitochondria, the endoplasmic reticulum, and (in green plants) chloroplasts, are contained in the cytoplasm.

                                        A double-membrane-bound organelle found in most eukaryotic organisms. Mitochondria convert oxygen and nutrients into adenosine triphosphate (ATP). ATP is the chemical energy "currency" of the cell that powers the cell's metabolic activities.

                                        A network of membranous tubules within the cytoplasm of a eukaryotic cell, continuous with the nuclear membrane. It often has ribosomes attached and is involved in protein and lipid synthesis.

                                        A membrane-bound organelle found in eukaryotic cells made up of a series of flattened stacked pouches with the purpose of collecting and dispatching protein and lipid products received from the endoplasmic reticulum (ER). Also referred to as the Golgi complex or the Golgi body.

                                        A structure within a cell, consisting of lipid bilayer. Vesicles form naturally during the processes of secretion, uptake and transport of materials within the plasma membrane.

                                        A membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. It's function is storage of substances and to maintain the rigidity of plant cells.

                                        Deoxyribonucleic acid - the molecule carrying genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses.

                                        A solution, similar to the cytoplasm of a cell, enveloped by the nuclear envelope and surrounding the chromosomes and nucleolus.

                                        The aqueous component of the cytoplasm of a cell, within which various organelles and particles are suspended.

                                        A structure made up of two lipid bilayer membranes which in eukaryotic cells surrounds the nucleus, which encases the genetic material. Also know as the nuclear membrane.

                                        A protein-lined channel in the nuclear envelope that regulates the transportation of molecules between the nucleus and the cytoplasm.

                                        A structure in the nucleus of eukaryotic cells which is the site of ribosome synthesis/production.

                                        Glucose (also called dextrose) is a simple sugar with the molecular formula C6H12O6. Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using energy from sunlight.

                                        A complex organic chemical that provides energy to drive many processes in living cells, e.g. muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all forms of life, ATP is often referred to as the "molecular unit of currency" of intracellular energy transfer.

                                        Any type of a close and long-term biological interaction between two different biological organisms.

                                        An evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms.

                                        A substance that is insoluble in water. Examples include fats, oils and cholesterol. Lipids are made from monomers such as glycerol and fatty acids.

                                        An organelle found in eukaryotic cells. Its main function is to produce proteins. It is a portion of the endoplasmic reticulum which is studded with attached ribosomes.

                                        An organelle found in eukaryotic cells with the function of making cellular products such as hormones and lipids. The smooth endoplasmic reticulum is a part of the endoplasmic reticulum that does not have attached ribosomes.

                                        The semipermeable membrane surrounding the cytoplasm of a cell.

                                        A cylindrical organelle composed of microtubules located near the nucleus in animal cells, occurring in pairs and involved in the development of spindle fibers in cell division.

                                        The process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle.

                                        A threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.

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