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AP Biology Chapter 6: A Tour of the Cell

Overview of Cells

  • All organisms are made of cells. This reminds me of how fundamental life is, from the smallest bacteria to the largest whale, we all share this basic building block. It also makes me think about how continuous life is, as all cells come from other cells.

  • A cell is the simplest collection of matter that can be alive.

  • Cell structure is correlated to cellular function. This is like how the design of a tool matches its purpose; a hammer is for nails, just as a nerve cell is shaped to transmit signals.

  • All cells are related by their descent from earlier cells.

Microscopy and Cell Observation

Measurements of Cell and Organelles

  • Measurement ranges from:

    • Atoms (0.1 nm)

    • Small molecules (1 nm)

    • Ribosomes (10 nm)

    • Smallest bacteria (1 μm)

    • Mitochondrion (10 μm)

    • Most bacteria (1 μm)

    • Nucleus (10 μm)

    • Most plant and animal cells (100 μm)

    • Human egg (100 μm)

    • Frog egg (1 cm)

Types of Microscopy

  • Light Microscopy (LM): Magnifies effectively to about 1,000x the size of the actual specimen; however, cannot visualize most organelles.

  • Electron Microscopy (EM):

    • Scanning Electron Microscopes (SEMs): Focus a beam of electrons onto the surface of the specimen, providing 3-D images. This reminds me of Google Earth, but for microscopic surfaces! You can zoom in and see the tiny textures as if you were truly there.

    • Transmission Electron Microscopes (TEMs): Focus a beam of electrons through a specimen to study the internal structure of cells.

  • Super-resolution microscopy: Achieves resolution beyond the limits of traditional light microscopy.

Cell Fractionation

  • Cell fractionation is a technique used to take cells apart and separate major organelles. This is like disassembling a complex machine, like a computer, to understand each individual component and its function, or like sorting laundry into different piles based on color and fabric type.

  • Utilizes centrifuges to fractionate cells into their component parts:

    1. Homogenization to create homogenate (cell mixture).

    2. Differential centrifugation with various forces:

    • 1,000 g for 10 min: Pellet is rich in nuclei and cellular debris.

    • 20,000 g for 20 min: Supernatant poured into the next tube.

    • 80,000 g for 60 min: Pellet rich in mitochondria (and chloroplasts in plant cells).

    • 150,000 g for 3 hr: Pellet rich in ribosomes and cellular membranes.

Comparison of Cell Types: Prokaryotes vs Eukaryotes

Feature

Prokaryotes

Eukaryotes

Plasma Membrane

Yes

Yes (phospholipid bilayer)

Cytosol

Yes

Yes

Ribosomes

Yes

Yes

Chromosomes

Yes (unbound DNA)

Yes (bounded by nuclear envelope)

Organelles

No

Yes

Examples

Bacteria and Archaebacteria

Protists, Fungi, Plants, Animals

Cell Size Limitations

  • As cell surface area increases with a factor of n², volume increases with a factor of n³. This concept is crucial for understanding why cells generally remain small; it's a constant balancing act between taking in nutrients and expelling waste. Think of it like trying to feed a giant group of people through a small door—it becomes inefficient quickly as the group (volume) grows faster than the entryway (surface area).

  • Metabolic requirements set upper limits on the size of cells; larger cells require more energy exchange.

  • Surface-to-volume (S-to-V) ratio:

    • Total Surface Area:

      \text{Total Surface Area} = (\text{height} \times \text{width}) \times \text{number of boxes}

    • Total Volume:

      \text{Total Volume} = \text{height} \times \text{width} \times \text{length} \times \text{number of boxes}

    • Surface-to-volume Ratio:

      \text{S-to-V Ratio} = \frac{\text{Surface Area}}{\text{Volume}}

Animal and Plant Cell Structures

Animal Cell Structures

  • Organelles Include:

    • Nucleus

    • Nucleolus

    • Rough ER

    • Smooth ER

    • Golgi apparatus

    • Mitochondria

    • Lysosomes

    • Peroxisomes

    • Microvilli

    • Cytoskeleton (microtubules, intermediate filaments, microfilaments)

    • Centrosome

Plant Cell Structures

  • Organelles Include:

    • Cell wall

    • Central vacuole

    • Chloroplast

    • Nucleus

    • Nucleolus

    • Smooth ER

    • Rough ER

    • Golgi apparatus

    • Mitochondria

    • Peroxisomes

    • Microtubules

    • Intermediate filaments

    • Microfilaments

Nucleus: Structure and Function

  • Nucleus: Contains most of the cell's genes, typically the most conspicuous organelle. This is the 'brain' or central library of the cell, holding all the instructions for its existence. It reminds me of the hard drive in a computer, storing all the essential programs.

  • Nuclear Envelope: Double lipid bilayer separating the nucleus from the cytoplasm. It's like a highly regulated security fence or a protective casing around the computer's CPU, controlling what goes in and out.

  • Nuclear Pores: Regulate molecular entry and exit from nucleus.

  • Chromatin: DNA and associated proteins forming chromosomes. This relates to our genetic inheritance, how traits get passed down through generations, shaping who we are.

  • Nucleolus: Site for ribosomal RNA synthesis.

  • Ribosomes: Made of rRNA and protein, facilitating protein synthesis in the cytosol (free ribosomes) or on the ER (bound ribosomes).

Endoplasmic Reticulum (ER) and Golgi Apparatus

Endomembrane System Components

  • Nuclear envelope

  • Endoplasmic reticulum

  • Golgi apparatus

  • Lysosomes

  • Vacuoles

  • Plasma membrane

Smooth and Rough ER Functions

  • Smooth ER:

    • Synthesizes lipids.

    • Metabolizes carbohydrates.

    • Detoxifies drugs and poisons. This reminds me of the liver in our bodies; it's the cell's detox center, breaking down harmful substances.

    • Stores calcium ions.

  • Rough ER:

    • Distributes transport vesicles.

    • Serves as a membrane factory for the cell.

    • Bound ribosomes secrete glycoproteins.

Golgi Apparatus Functions

  • Modifies ER products.

  • Manufactures certain macromolecules.

  • Sorts and packages materials into transport vesicles:

    • Cis Face: Receiving side. This is like the 'inbox' of the cell's post office, where new packages arrive.

    • Trans Face: Shipping side. This is the 'outbox', where processed packages are dispatched to their destinations.

Ribosomes in Eukaryotic Cells

Characteristics of Ribosomes

  • Free Ribosomes: Float in the cytoplasm.

  • Bound Ribosomes: Attached to the ER.

  • Function: Translate mRNA, link amino acids, export polypeptides, temporary assembly for specific protein production. Ribosomes are like tiny factories building proteins essential for everything we do, from moving our muscles to digesting food. They are the universal builders of life's essential molecules.

Lysosomes

  • Defined as a sac of hydrolytic enzymes that digest macromolecules. Lysosomes are the 'recycling centers' or 'waste disposal units' of the cell, breaking down old parts and waste, much like how we recycle plastics and paper at home to create new products or dispose of unwanted items responsibly.

  • Fuses with food vacuoles from phagocytosis for molecule digestion.

  • Engages in autophagy, recycling the cell's own organelles and macromolecules.

  • Plays a role in waste removal within the cell.

Vacuoles

  • Types of Vacuoles:

    • Food Vacuoles: Formed by phagocytosis.

    • Contractile Vacuoles: In freshwater protists; pump excess water out of cells.

    • Central Vacuoles: Hold organic compounds and water in plant cells. This reminds me of a storage unit or a water tower for a plant, crucial for maintaining turgor and storing nutrients for growth and survival.

Mitochondria

  • Function: Convert chemical energy from food into ATP. These are the 'powerhouses' of the cell, generating the energy currency (ATP) that allows us to walk, talk, and think. Every bite of food we eat contributes to fueling these tiny energy factories, directly impacting our daily energy levels and activities.

  • Structure:

    • Smooth outer membrane.

    • Inner membrane folded into cristae creating large surface area for ATP synthesis.

    • Compartments include matrix and intermembrane space.

Chloroplasts

  • Function: Photosynthesis using chlorophyll and enzymes. Only in plants, these are like miniature solar panels converting sunlight into energy, sustaining life on Earth through food production. They remind me of green leaves soaking up the sun's energy to grow, making the oxygen we breathe and the food we eat possible.

  • Key Components:

    • Thylakoids: Membranous sacs stacked into a granum.

    • Stroma: Internal fluid surrounding thylakoids.

Peroxisomes

  • Specialized metabolic compartments producing hydrogen peroxide and converting it to water. These are like mini hazardous waste treatment plants, handling toxic byproducts inside the cell, ensuring its internal environment remains safe and functional.

Cytoskeleton

Function

  • A network of fibers organizing cell structures and activities, anchoring organelles, maintaining cell shape, and enabling motility with motor proteins. This reminds me of the skeleton and muscles in our own bodies, providing structure, movement, and support. It's the cell's internal scaffolding and transport system.

Properties of Cytoskeletal Components

  • Microtubules: Hollow tubes with a diameter of 25 nm; formed from tubulin.

    • Functions: Maintenance of cell shape, motility, chromosome movement, and organelle movement.

  • Microfilaments (Actin Filaments): 7 nm; structures involved in maintaining cell shape, motility, muscle contraction, and cytokinesis. Our muscle contractions, like lifting a weight or walking, rely on these tiny filaments sliding past each other, a direct link to our physical movements.

  • Intermediate Filaments: 8-12 nm; maintain cell shape, anchor organelles, and form the nuclear lamina.

Intercellular Junctions

  • Facilitate contact between neighboring cells in tissues:

    • Plasmodesmata: Channels in plant cell walls for molecule passage.

    • Tight Junctions: Prevent leakage between cells. These are like the caulking between tiles, creating a waterproof seal, essential for things like our skin barrier or the lining of our intestines to prevent unwanted substances from entering.

    • Desmosomes: Anchor cells into strong sheets. They're like rivets holding sheets of metal together, giving tissues strength and stability, especially in areas that experience mechanical stress like muscle tissue.

    • Gap Junctions: Provide communication channels between cells. These are like tiny telephone lines or interconnected WhatsApp groups, letting cells directly communicate with each other, crucial for coordinated functions like heartbeats or tissue development.

Endosymbiont Theory

  • A theory proposing ancestral eukaryotic cells engulfed non-photosynthetic prokaryotes, eventually leading to a symbiotic relationship forming mitochondria through evolution. This incredible theory explains how complex life likely evolved, reminding me of a historical merger where two separate entities became one, benefiting both to create something entirely new and more capable, much like a successful business partnership.

  • Evidence for Endosymbiosis:

    • Mitochondria and chloroplasts have double membranes.

    • Possess ribosomes similar to bacteria.

    • Contain circular DNA and reproduce via binary fission-like processes.

Summary

  • The cell represents a living unit greater than the sum of its parts; reliance on the integration of structures and organelles is essential for cellular function. This truly emphasizes that a cell is a complex, perfectly orchestrated system, much like an entire city where