Notes on Mitochondria, Chloroplasts, Cytoskeleton & ECM, Plant vs Animal Cells, and Cell Junctions

Mitochondria: Structure and Function

• Endosymbiotic origin: mitochondria likely originated from free-living prokaryotes; evidence includes their own DNA and ribosomes and double-membrane structure, suggesting an ancient symbiotic relationship. The transcript mentions mitochondria as evidence for a 1.3 billion-year-old prokaryote origin.
• Ubiquity and abundance: found in nearly all eukaryotic cells (plants, animals, fungi); cells typically contain several mitochondria, though some may have a single large mitochondrion.
• Internal structure and its importance:
  • Outer membrane and inner membrane with folds (cristae) in between; cristae increase surface area for the inner membrane, which is critical for energy production in cellular respiration.
  • The inner membrane folds are called cristae; the increased surface area provides more space for ATP production.
  • The matrix (the space inside the inner membrane) contains protons (H+), electrolytes, water, and enzymes needed for energy conversion.
• Functional goal: primary site of ATP production; all three components (outer membrane, cristae, inner folds, and matrix) collaborate to make energy molecules.
• Quick take on terminology:
  • Two membranes: outer and inner.
  • Cristae: folds of the inner membrane increasing surface area.
  • Matrix: interior fluid containing enzymes and cofactors for energy metabolism.

Chloroplasts: Structure and Function

• Presence and distribution: chloroplasts are found in plant cells (and some algae); Elodea cells show numerous chloroplasts per cell.
• Internal structure and key features:
  • Double membrane envelope (outer and inner membranes) with a space between them.
  • Chloroplasts have their own DNA and ribosomes, indicating an endosymbiotic origin similar to mitochondria.
  • Thylakoids: stacked, coin-like membranes containing chlorophyll; these stacks are called grana. Thylakoids are the sites where chlorophyll resides and where the bulk of photosynthesis occurs.
  • The interior surrounding the thylakoids is the stroma (not explicitly named in the transcript, but it is the surrounding fluid that contains enzymes for photosynthesis) and the thylakoid lumen is the space inside thylakoids.
• Pigments and colors:
  • Chlorophyll (green) is the primary pigment.
  • Other pigments present include anthocyanins (purple), carotenes (orange), and xanthophylls (yellow).
  • The green chlorophyll fades in cooler temperatures with less light in the fall, revealing other pigments (fall colors).
• Functional note:
  • Thylakoids are the sites of the light reactions of photosynthesis where chlorophyll is housed and energy capture occurs.
  • The photosynthetic products (sugars) feed plants and, ultimately, are the sugars we eat from plants.

Cytoskeleton and Extracellular Matrix (ECM)

• Roles and scope:
  • Provide structural support and define cell shape; help organize the cell's internal architecture (anchor organelles and enzymes).
  • Facilitate intracellular transport and communication; act as a network that can be disassembled and rebuilt to change cell shape quickly.
  • Involved in motility: moving vesicles along tracks, forming structures for movement (e.g., flagella or cilia in some cells).
• Microtubules, actin filaments, and intermediate filaments:
  • Microtubules: the thickest, most involved in movement and cell division; hundreds to thousands of microtubule units per cell.
  • Actin filaments: the thinnest; involved in movement and muscle-like contractions.
  • Intermediate filaments: intermediate in thickness between microtubules and actin.
• Movement and division specifics:
  • Microtubules are key in cell division; they form spindle fibers that help move and separate DNA.
  • Centrioles act as organizing centers for microtubules; they produce spindle fibers during division.
  • Centrioles are composed of 9 triplets of microtubules (9×3 arrangement). They resemble pasta (rigatoni) in diagrams; this grouping makes the centriole an especially strong organizing structure.
  • Centrioles appear only when the cell is about to divide; they are not always visible in a typical cell's life.
• Centrosome and spindle apparatus:
  • Centrosomes organize microtubules during division; defective DNA movement can lead to faulty division and potential tumorigenesis.
• Movement tools obtained from cytoskeleton:
  • Flagella (and cilia) are organelles built from microtubules that enable movement.
  • Prokaryotes commonly have flagella that spin like a rotor; eukaryotic sperm cells have a single flagellum for propulsion.
  • Prokaryotes can have up to five flagella; in bacteria, those flagella spin together to move.
  • Cilia (short, numerous) are common on many eukaryotic cells (e.g., respiratory tract, fallopian tubes) and beat in coordinated waves to move substances such as mucus or eggs.
• The cytoskeleton and ECM as a connected network:
  • The ECM in animals provides a sticky, glue-like matrix outside the cell that holds cells together and supports communication.
  • The ECM includes glycoproteins, glycolipids, sugars, and extensions of the cytoskeleton; integrins act as glue-like receptors that help anchor cells to the ECM.
  • The ECM visually appears messy but is essential for tissue structure and organ shape (e.g., consistency of heart, liver, and intestine shapes).
• Membrane and junction context:
  • The cell membrane (phospholipid bilayer) is the inner boundary of the cell; the ECM sits outside this boundary, forming connections between cells.
  • The next chapter will cover membranes in more detail, including the proteins embedded in the membrane that mediate transport and communication.

Plant Cell Walls vs Animal ECM

• Plant cell walls:
  • Provide rigidity and structural support; made primarily of cellulose.
  • In tougher cases, lignin is added to strengthen the exterior, turning plant cells into wood.
  • All plant cells also possess a cell membrane; the cell wall lies outside the membrane.
  • Plant cells remain attached to each other after division, forming continuous tissue; there is little to no extracellular space between plant cells.
• Animal extracellular matrix (ECM):
  • Animals lack a rigid cell wall; instead, they rely on the ECM to hold tissues together and support communication.
  • The ECM comprises a complex, sticky network that anchors cells, provides space for communication, and shapes tissues.
  • Components include glycoproteins, glycolipids, sugars, and extensions of the cytoskeleton; integrins serve as glue-like connectors linking cells to the ECM.
  • A visually “messy” ECM is normal and functional because it supports tissue integrity and signaling.
• Cell–cell connectivity and communication across tissues:
  • In animals, cell junctions are crucial for attachment and communication (see below).
  • In plants, plasmodesmata act as channels that traverse cell walls, connecting adjacent plant cells to exchange proteins, sugars, water, and other materials; functionally analogous to gap junctions but structurally through cell walls.
  • The plasmodesmata and gap junctions allow direct cytoplasmic exchange without crossing two cell membranes.

Cell Junctions: Animal vs Plant

• Animal cell junctions (three main types):

  • Tight junctions: resemble tightly screwed boards; create a seal to prevent movement of materials between cells; important for cells that must remain close or immobile.
  • Desmosomes: looser attachments; provide structural cohesion and flexibility; common in tissues that stretch or move (e.g., heart, intestines).
  • Gap junctions: form tubes ( connexon channels ) that directly connect neighboring cells, allowing the easy flow of ions, proteins, electrolytes, sugars, and other small molecules; bypasses two membrane crossings.

• Plant cell junction analogs:

  • Plant cells lack desmosomes and tight junctions; they rely on plasmodesmata to communicate and exchange materials through cell walls.
  • Plasmodesmata are channels that pass through cell walls, connecting cytoplasm of adjacent plant cells and enabling transport similar in function to gap junctions.

• Practical exam note:

  • These organelles and junctions are excellent flashcard fodder for exams; creating your own study cards is highly recommended to reinforce understanding.

• Next topics preview:

  • Membranes and membrane proteins, transported processes, and further details will be covered in the next chapter.