CHR4 notes the

Cell Structure & Function

What is a Cell?

  • Definition: A cell is the smallest unit of life that can function independently and perform all the necessary functions of life, including reproducing itself.

Cell Theory

  • All organisms are composed of cells.

  • Cells are the basic units of structure and function in organisms.

  • Cells come only from preexisting cells because cells are self-reproducing.

  • Key Contributors:

    • Matthias Schleiden

    • Theodor Schwann

    • Rudolph Virchow

Cell Size

  • Significance of Size: Smaller cells have a higher surface area relative to volume.

  • High surface area facilitates the rapid exchange of materials with their surroundings.

  • Surface Area to Volume Ratio: Denoted as SA/V=RatioSA/V = Ratio

Egg Sizes and Costs

  • Examples of Large Cells:

    • Ostrich eggs: over 3 pounds each.

    • Beluga sturgeon eggs: 700700 per ounce.

    • Human eggs: thousands of dollars per egg.

    • Sperm cells valued at 1¢1¢ per 20,000 cells.

  • Conclusion: Eggs are the largest and most expensive cells in the world.

Types of Cells

1. Eukaryotic Cells

  • Features:

    • Contain a central control structure called a nucleus, which houses the cell’s DNA.

2. Prokaryotic Cells

  • Features:

    • Lack a nucleus, therefore, the DNA resides in the nucleoid region within the cell.

  • Classification: Organisms are categorized as either eukaryotes or prokaryotes.

Basic Structural Features of Prokaryotes

  • Four Basic Structural Features:

    1. Plasma Membrane:

    • Encloses cell contents including DNA, ribosomes, and cytoplasm.

    1. Cytoplasm:

    • A jelly-like fluid inside the cell.

    1. DNA:

    • Comprised of circular loops containing genetic information.

    1. Ribosomes:

    • Granular bodies in the cytoplasm that convert genetic information into protein structure.

  • Additional Structures:

    • Pili: Hair-like projections aiding in surface attachment and possibly in DNA transfer.

    • Cell Wall: Provides protection and shape.

    • Capsule: A protective outer coating.

    • Flagellum: Whip-like projections aiding in cellular movement.

Origin of Eukaryotic Cells

  • Endosymbiosis Theory:

    • Provides the best explanation for the presence of certain organelles (chloroplasts and mitochondria) in eukaryotes.

    • Asserts that two different types of prokaryotes worked in close partnerships.

Supporting Evidence for Endosymbiosis

  • Observations:

    • Chloroplasts and mitochondria are similar in size to prokaryotic cells and replicate via binary fission.

    • Both organelles contain ribosomes.

    • Both have small amounts of circular DNA.

    • The DNA of chloroplasts and mitochondria is more closely related to bacterial DNA than to eukaryotic DNA.

Endosymbiosis vs. Invagination

  • Endosymbiosis Process:

    1. An ancestral eukaryote engulfs a prokaryote.

    2. Eukaryote and prokaryote merge.

    3. Over time, the prokaryote evolves into an organelle.

  • Invagination Process:

    1. Plasma membrane folds in on itself.

    2. This creates inner compartments (organelles).

  • Conclusion: Organelles may develop through endosymbiosis, invagination, or a combination of both processes.

Eukaryotic Cells

  • Features of Typical Eukaryotic Cells:

    • DNA contained in a nucleus.

    • Cytoplasm contains specialized structures known as organelles.

    • Typically larger than prokaryotes—at least 10 times bigger.

  • Features of Typical Prokaryotic Cells:

    • No nucleus; DNA remains in the cytoplasm.

    • Internal structures are not organized into compartments.

    • Much smaller than eukaryotes.

Structures in Animal Cells

  • Structures Found in Both Cells:

    • Lysosome

    • Nucleus

    • Plasma membrane

    • Ribosome

    • Mitochondria

    • Rough endoplasmic reticulum

    • Smooth endoplasmic reticulum

    • Cytoplasm

    • Cytoskeleton

    • Golgi apparatus

  • Structure Not Found in Plant Cells:

    • Centriole

Structures in Plant Cells

  • Structures Found in Both Cells:

    • Lysosome

    • Nucleus

    • Plasma membrane

    • Ribosome

    • Mitochondria

    • Rough endoplasmic reticulum

    • Smooth endoplasmic reticulum

    • Cytoplasm

    • Cytoskeleton

    • Golgi apparatus

  • Structures Not Found in Animal Cells:

    • Chloroplast

    • Cell wall

    • Vacuole (occasionally found in animal cells)

The Cell as a Factory

  • Functions of Organelles:

    • Plasma Membrane: Regulates what enters and leaves; contacts with external environment.

    • Nucleus: Controls cell activities; determines protein production.

    • Cytoplasm: Contains organelles; site of most cell activity.

    • Ribosomes: Build proteins.

    • Golgi Apparatus: Prepares proteins for use or export.

    • Lysosomes: Break down and absorb materials.

  • Support Structures:

    • Cytoskeleton: Provides structural support and shape.

    • Mitochondria: Transforms energy forms.

Plasma Membrane

  • Structure: Composed of two layers filled with pores, molecules, and channels.

  • Functions:

    • Holds cell contents in place.