Cell Structure and Function

Eukaryotic Cells

  • Eukaryotic cells are characterized by organelles that are surrounded by membranes.
    • This feature makes eukaryotic cells more complex and larger than prokaryotic cells.

Compartmentalization

  • Eukaryotic cells display compartmentalization, meaning that various cellular processes occur in distinct membrane-enclosed organelles.

    • Examples of processes include:
      • Cellular respiration
      • Photosynthesis
      • DNA synthesis

  • Compartmentalization allows for specialization within different areas of the cell, which enhances efficiency.

    • Analogy:
    • Prokaryotic cells can be likened to a one-room schoolhouse, where all processes happen in one shared space, leading to inefficiency.
    • Eukaryotic cells, contrastingly, allow for division of tasks akin to modern educational settings, where subjects and grades are separated, thereby enhancing efficiency.

Cytoskeleton

  • Prokaryotic cells do not require a cytoskeleton due to their smaller size and structural cell wall.
  • Eukaryotic cells, being larger, require an internal framework (cytoskeleton) for structural support.

Quiz Preparation

  • Important: Students will have a quiz requiring labeling of the following structures:
    • Parts of a bacterial cell
    • Parts of an animal cell
    • Parts of a plant cell
  • Refer to slide numbers 19 and 20 for visuals of animal and plant cells.

Cellular Structures

Nucleus
  • The nucleus is the largest and most prominent organelle in cells.
    • It is typically located at the cell's center, though not exclusively.
    • Contains a darker region known as the nucleolus, where ribosomes are assembled.
  • Surrounded by a double membrane (two phospholipid bilayers).
  • Nuclear pores allow substances like RNA to pass between the nucleus and cytoplasm.
  • DNA exists in the form of chromatin within the nucleus, which is not visible at low magnification (light microscope).
Ribosomes
  • Ribosomes do not have a membrane and consist of ribosomal RNA and proteins.
  • Function: Synthesize proteins for cellular use
    • Can be found free-floating in the cytoplasm or attached to the surface of certain organelles.

Endomembrane System

  • The following organelles are part of the endomembrane system, which organizes internal membranes to help process materials:
Endoplasmic Reticulum (ER)

  • Composed of two types:

    • Rough ER:
      • Characterized by ribosomes attached to its surface, giving it a bumpy appearance.
      • Captures, processes, and transports proteins synthesized by ribosomes.
    • Smooth ER:
      • Lacks ribosomes, hence not bumpy.
      • Functions vary by cell type:
      • In muscle cells, it stores calcium for contractions.
      • In liver cells, it detoxifies substances (alcohol, drugs) and synthesizes lipids.
      • The structure and amount of rough vs. smooth ER depend on cell specialization.

  • Rough ER's location: Just outside the nuclear envelope.

  • Smooth ER appears more tubular and is found farther out in the cytoplasm from the nucleus.

Golgi Apparatus
  • Composed of a series of flattened membrane-bound spaces.
  • Analogy: Functions like an Amazon shipping and packaging center, ensuring proteins reach their destinations.
  • Structure:
    • Cis face: Faces toward the nucleus; receives incoming vesicles containing proteins.
    • Trans face: Faces the cell membrane; where vesicles are released containing processed proteins.
  • Vesicle formation:
    • Process is likened to blowing a bubble, where the membrane extends and seals off into a vesicle (process known as blebbing).
Lysosomes
  • Membrane-bound organelles filled with digestive enzymes that:
    • Break down macromolecules (e.g. digests food, recycles old organelles).
  • Example of function: White blood cells use lysosomes to destroy bacteria they engulf.
Peroxisomes
  • Small organelles containing enzymes, including catalase, that:
    • Break down hydrogen peroxide (H₂O₂) into water and oxygen gas, preventing cellular damage.
  • Mention: Hydrogen peroxide causes bubbling when applied to cuts due to this reaction.
Vacuoles
  • Larger than lysosomes and are involved in various functions:
    • Storage Vacuoles: Stores substances like fat.
    • Central Vacuole: Present in plant cells, takes up significant space, filled with water, and surrounded by a membrane called the tonoplast.
    • Contractile Vacuole: Present in protozoan species (like paramecium), pumps excess water out to avoid cell rupture.

Energy-Transforming Organelles

Mitochondria
  • Present in all cells due to their role in energy production (ATP synthesis).
  • Function: Involved in oxidative metabolism (cellular respiration).
  • Structure:
    • Outer membrane (bean-shaped) and inner membrane that is highly folded, creating compartments: the intermembrane space and the matrix.
    • Contains its own DNA, similar to prokaryotes.
Chloroplasts
  • Unique to plant cells and responsible for photosynthesis.
  • Structure includes:
    • Two outer membranes and an inner membrane system that forms thylakoids (stacked into structures called granum).
    • Chlorophyll within thylakoids gives chloroplasts their green color.
    • Also contains its own DNA.

Endosymbiotic Theory

  • This theory posits that both mitochondria and chloroplasts originated from free-living prokaryotic cells that were engulfed by larger cells, losing the ability to live independently while retaining some genetic material.

    • Types of engulfed prokaryotic cells identified:

  • Mitochondrium came from proteobacterium (capable of cellular respiration).

  • Chloroplast originated from cyanobacteria (photosynthetic bacteria).

Conclusion

  • Review of the endomembrane system emphasizes its components: ER, vesicles, vacuoles, lysosomes, peroxisomes, mitochondria, and chloroplasts.
  • Lab work to include examination of prokaryotic cells (Cyanobacteria) illustrating the foundational concepts of the material discussed.