Organization of the Cell Study Notes

Chapter 4: Organization of the Cell

Cell Theory

  • Cells are the basic living units of organization and function in all organisms.

  • All cells arise from pre-existing cells.

  • All living cells share a common origin, exhibited by structural and molecular similarities.

Characteristics of Cells (1 of 2)

  • The cell is the smallest unit that can carry out all activities associated with life.

    • Most prokaryotes and many protists and fungi consist of a single cell, while most plants and animals are composed of millions of cells.

  • Cells are:

    • Building blocks of complex multicellular organisms.

    • Extraordinarily diverse and versatile.

Characteristics of Cells (2 of 2)

  • Cells maintain homeostasis:

    • The plasma membrane serves as a selective barrier between cellular contents and the environment, supporting homeostasis.

  • Cells possess specialized organelles that perform specific tasks.

  • Each cell contains genetic instructions encoded in DNA.

  • All substances that enter or leave a cell must pass through its plasma membrane.

    • The ratio of surface area to volume is critical for cell function.

  • The size and shape of a cell are adapted to its specific function.

Discovery of Cells

  • Robert Hooke first described cells in 1665 using a microscope he made.

  • Antonie van Leeuwenhoek discovered bacteria, protists, blood cells, and sperm cells with small lenses he crafted.

  • By the late 19th century, microscopes were sufficiently advanced for biologists to study cells in detail.

Prokaryotes

  • Bacteria and Archaea are classified as prokaryotic cells.

    • DNA is located in a region known as the nucleoid.

    • Prokaryotic cells lack membrane-enclosed organelles.

    • Most prokaryotes have cell walls situated outside of the plasma membrane.

    • Many feature flagella for movement.

    • The interior contains ribosomes and storage granules.

Structure of a Prokaryotic Cell

  • Components of a prokaryotic cell include:

    • Fimbriae

    • Storage granule

    • Flagellum

    • Ribosome

    • Cell wall

    • Plasma membrane

    • DNA

    • Nuclear area

    • Capsule

Eukaryotes

  • Eukaryotic cells are characterized by highly organized, specialized membrane-enclosed organelles.

    • The nucleus is a key organelle.

    • Membrane-enclosed compartments enable simultaneous occurrence of different cellular activities.

    • Chemical reactions within cells are facilitated by enzymes bound to membranes.

    • Membranes support energy storage.

    • Eukaryotic cells are typically larger than prokaryotic cells.

Structure of Eukaryotic Cells

  • Components of a eukaryotic cell include:

    • Ribosomes

    • Nuclear envelope

    • Nucleus

    • Nucleolus

    • Chromatin

    • Endoplasmic Reticulum (ER)

    • Rough ER

    • Smooth ER

    • Mitochondria

    • Peroxisomes

    • Vacuoles

    • Chloroplasts (in plant cells)

    • Golgi complex

    • Centrioles

    • Plasma membrane

    • Cell wall (in plant cells)

Nucleus (1 of 4)

  • The nucleus serves as the control center of the cell:

    • Nuclear envelope: a double membrane that separates the nuclear contents from the cytoplasm.

    • Nuclear pores regulate the passage of materials between the nucleoplasm and the cytoplasm.

    • Nuclear lamina: aids in organizing nuclear contents, DNA duplication, and regulating the cell cycle.

Nucleus (2 of 4)

  • Nuclear components:

    • Nuclear pores

    • Chromatin

    • Nucleolus

    • Nucleoplasm

Nucleus (3 of 4)

  • Nuclear DNA:

    • Forms chromatin when DNA associates with RNA and proteins.

    • DNA molecules compact into chromosomes within the nucleus.

    • In humans, there are 46 chromosomes (23 pairs), containing about 2 meters of DNA.

Nucleus (4 of 4)

  • Most nuclei contain one or more nucleoli, which synthesize ribosomal RNA (rRNA):

    • Ribosomal proteins are synthesized in the cytoplasm and imported into the nucleolus.

    • rRNA and proteins combine to form ribosomal subunits that exit the nucleus via nuclear pores.

Ribosomes and Protein Manufacturing

  • Ribosomes: organelles located freely in the cytoplasm or bound to specific membranes:

    • Contain the enzyme necessary for forming peptide bonds, linking amino acids into polypeptides.

    • Each ribosome is composed of a large subunit and a small subunit that join to assemble polypeptides.

    • The cell can adjust the number of ribosomes based on its metabolic requirements.

Endomembrane System

  • A network of membrane-enclosed organelles exchanging materials via small membrane-enclosed transporting vesicles:

    • Each vesicle is equipped with proteins embedded in its membrane acting as routing signals for destination organelles.

Endoplasmic Reticulum (1 of 2)

  • The Endoplasmic Reticulum (ER) constitutes a significant portion of the total cytoplasmic volume:

    • Smooth ER (SER) is responsible for synthesizing lipids, breaking down toxins, and storing calcium ions.

    • Rough ER (RER) synthesizes proteins that are secreted, transmembrane, or lysosomal in nature.

Endoplasmic Reticulum (2 of 2)

  • Structure:

    • Rough ER has ribosomes attached.

    • Smooth ER lacks ribosomes.

Golgi Complex (1 of 3)

  • The Golgi complex consists of stacks of flattened membranous sacs called cisternae:

    • Responsible for processing, sorting, and modifying proteins.

    • Each Golgi stack has three regions:

    • Cis face: entrance for substances.

    • Trans face: exit surface.

    • Medial region: located in between.

Golgi Complex (2 of 3)

  • Glycoprotein Processing Sequence:

    1. Polypeptides are synthesized on ribosomes.

    2. Carbohydrate components are added in the lumen of rough ER.

    3. Transport vesicles convey glycoproteins to the Golgi (cis face).

    4. Glycoproteins are modified in the Golgi.

    5. At the trans face, glycoproteins are packaged in transport vesicles.

    6. Glycoproteins are transported to the plasma membrane.

    7. Contents are released from the cell.

Lysosomes (1 of 2)

  • Lysosomes are small sacs containing digestive enzymes dispersed throughout the cytoplasm of animal cells:

    • Comprise around 40 different hydrolytic enzymes.

    • Maintain an interior pH of approximately 5.

    • Primary lysosomes bud off from the Golgi complex and fuse with vesicles to form secondary lysosomes.

    • Lysosomal enzymes receive unique modifications in the Golgi complex, such as a mannose-6-phosphate tag.

Lysosomes (2 of 2)

  • Lysosome Types:

    • Primary lysosome

    • Secondary lysosome

Vacuoles (1 of 2)

  • Vacuoles are membrane-enclosed sacs with diverse functions:

    • Food vacuoles fuse with lysosomes to aid in food digestion.

    • Contractile vacuoles expel excess water from the cell.

Vacuoles (2 of 2)

  • Central Vacuole in Plants:

    • Single large central vacuole surrounded by a tonoplast membrane.

    • Critical in storage, controlling turgor pressure, and waste breakdown.

Peroxisomes

  • Contain enzymes that transfer hydrogen from various compounds to oxygen:

    • Assist in breaking down fatty acid molecules.

    • Synthesize phospholipids.

    • Degrade alcohol in yeast; detoxify toxic compounds in human liver and kidney cells.

    • Convert stored fats to sugars in plant seeds.

Mitochondria and Chloroplasts Are Energy Converting Organelles (1 of 2)

  • These organelles specialize in converting energy from one form to another:

    • Involved in cellular respiration and photosynthesis.

    • Contain their own DNA and ribosomes, supporting the endosymbiont theory.

Mitochondria and Chloroplasts Are Energy Converting Organelles (2 of 2)

  • Chemical Reactions:

    • Mitochondria convert glucose and oxygen into ATP, carbon dioxide, and water.

    • Chloroplasts convert carbon dioxide, water, and ATP into glucose and oxygen using light energy.

Mitochondria (1 of 3)

  • Mitochondria engage in aerobic respiration, converting chemical energy in food to ATP:

    • Have a double membrane resulting in two compartments: intermembrane space and matrix.

    • The outer membrane is smooth, allowing small molecules to pass freely.

    • The inner membrane strictly manages the transport of molecules.

Mitochondria (2 of 3)

  • Mitochondria play a crucial role in apoptosis, which is a normal part of development and tissue maintenance:

    • Example: In human embryos, apoptosis is responsible for removing webbed tissue between the fingers.

    • Mechanism: Cytochrome c activates caspases, leading to the degradation of essential cellular components.

Mitochondria (3 of 3)

  • Structural components:

    • Inner mitochondrial membrane

    • Outer mitochondrial membrane

    • Matrix

    • Cristae

Chloroplasts (1 of 2)

  • Chloroplasts have a disc-shaped structure featuring a system of folded membranes:

    • Outer and inner membranes encase a fluid-filled stroma containing enzymes for carbohydrate production from CO2 and H2O.

    • The inner membrane contains chlorophyll and additional pigments that capture light energy for photosynthesis.

    • The thylakoid membrane is organized into stacks known as grana, enclosing the thylakoid lumen.

Chloroplasts (2 of 2)

  • Structural components include:

    • Outer membrane

    • Inner membrane

    • Intermembrane space

    • Stroma

    • Thylakoid membrane

    • Granum (stack of thylakoids)

Other Plastids

  • All plastids originate from proplastids:

  • Chromoplasts contain pigments that impart colors to flowers and fruits, helping attract pollinators and seed dispersers.

  • Leukoplasts, including amyloplasts, store starch in various seeds, roots, and tubers.

Cytoskeleton

  • Composed of microtubules, microfilaments, and intermediate filaments, providing structural support and facilitating cell movement.

Microtubules (1 of 3)

  • Microtubules are hollow rods measuring about 25 nm in diameter:

    • Serve functions in cytoskeleton structure and chromosome movement during cell division.

    • Act as tracks for intracellular movement and are components of cilia and flagella.

    • Comprised of protein dimers of α-tubulin and β-tubulin.

Microtubules (2 of 3)

  • Microtubule Structure:

    • Composed of dimers that facilitate polar ends, enabling dynamic instability.

Microtubules (3 of 3)

  • Motor proteins utilize ATP to generate movement:

    • Kinesin moves organelles toward the plus end of microtubules.

    • Dynein facilitates retrograde transport toward the minus end.

Microtubule-Organizing Center

  • The Microtubule-Organizing Center (MTOC) anchors the minus ends of microtubules within the cell:

    • In animal cells, the MTOC is known as the centrosome and contains two centrioles, duplicated before cell division.

Cilia and Flagella

  • Cilia and flagella enable locomotion for unicellular and small multicellular organisms:

    • Cilia are employed to move liquids and particles across the cell surface.

Microfilaments (1 of 2)

  • Microfilaments consist of two intertwined strings of actin filaments:

    • Finer fibers measuring about 7 nm in diameter, supporting the plasma membrane and stabilizing microvilli.

    • Assist during cell division and associate with myosin motor proteins.

Microfilaments (2 of 2)

  • Visual representation showing bundles of microfilaments in fibroblast tissue.

Intermediate Filaments

  • Intermediate filaments are tough, stable fibers about 10 nm in diameter:

    • Provide mechanical strength and stabilize cell shape.

    • Include keratins present in vertebrate epithelial cells.

Cell Coverings (1 of 4)

  • Many cells feature a sugar coat known as glycocalyx:

    • Facilitates cell recognition and interaction, leading to adhesive or communication associations.

    • Contributes to the mechanical strength of multicellular tissues.

Cell Coverings (2 of 4)

  • Many animal cells secrete an extracellular matrix (ECM):

    • The ECM is composed of a gel of carbohydrates and fibrous proteins, primarily collagens.

    • Fibronectins organize the matrix and assist cell attachment.

    • Integrins at the plasma membrane maintain adhesion between ECM and internal microfilaments and intermediate filaments.

Cell Coverings (3 of 4)

  • Structural components of the extracellular matrix include:

    • Collagen

    • Fibronectins

    • Integrin

Cell Coverings (4 of 4)

  • The cells of bacteria, archaea, fungi, and plants are enveloped by a cell wall:

    • Plant cell walls are primarily composed of cellulose.

    • Fungal cell walls contain chitin.

    • Eubacterial cell walls are made of peptidoglycan.

    • Archaeal cell walls consist of pseudopeptidoglycan.

References

  • Solomon, E. P., Martin, C. E., Martin, D. W., Berg, L. R. (2018). Biology (11th Edition). Cengage Learning, Inc. ISBN: 978-1-337-39293-8

  • Cengage Learning, Inc. https://www.cengage.com/