Chapter 6 — Cells

Surface Area to Volume Ratio & Cellular Diffusion

The surface area to volume (SA:V) ratio is a key factor influencing how efficiently cells exchange materials with their environment. As cells grow, their volume increases faster than their surface area, affecting diffusion and transport processes.

• Surface Area to Volume Ratio (SA:V): Measures how much surface (membrane area) a cell has compared to its internal volume.

• Cell Growth: As a cell grows, its volume increases faster than its surface area, resulting in a lower SA:V ratio.

• Diffusion Efficiency: Smaller cells have a larger SA:V ratio, allowing more efficient diffusion of materials (e.g., oxygen, nutrients, waste) in and out of the cell.

• Cell Size Limitation: Larger cells have a smaller SA:V ratio, which slows diffusion and makes it harder for the cell to transport materials quickly.

• Adaptations: Cells stay small or form folds and extensions (like microvilli) to increase surface area.

• Example: Intestinal epithelial cells have microvilli to maximize absorption efficiency.

Mitochondria, Chloroplasts & the Endosymbiotic Theory

The Endosymbiotic Theory explains the origin of mitochondria and chloroplasts in eukaryotic cells, suggesting they evolved from prokaryotic cells engulfed by a larger host cell.

• Endosymbiotic Theory: Proposes that mitochondria and chloroplasts originated as independent prokaryotes engulfed by a host cell.

• Mutualistic Relationship:

  • Engulfed cells provided energy (ATP or glucose).

  • Host cell provided protection and nutrients.

• Evidence:

  • Both have double membranes.

  • Both contain their own circular DNA, similar to bacteria.

  • Both have 70S ribosomes, like prokaryotes.

  • Both replicate independently of the host cell (binary fission).

Compartmentalization in Eukaryotic Cells

Compartmentalization refers to the division of eukaryotic cells into membrane-bound organelles, each with specialized functions.

• Benefits:

  • Increases efficiency by separating incompatible reactions.

  • Allows for specialized environments (e.g., different pH levels).

  • Enables simultaneous processes (e.g., protein synthesis in rough ER, energy production in mitochondria).

  • Prevents interference between chemical reactions.

Pathway of Protein Production (Secretory Pathway)

The secretory pathway describes the process by which proteins are synthesized and transported out of the cell.

1. Nucleus: DNA holds instructions for proteins; mRNA copies the gene.

2. Ribosomes on Rough ER: mRNA travels to the rough ER, where ribosomes translate it into a protein.

3. Rough Endoplasmic Reticulum (Rough ER): Newly made proteins fold and are packaged into transport vesicles.

4. Golgi Apparatus: Vesicles fuse with the Golgi; proteins are modified, sorted, and repackaged into vesicles.

5. Vesicles: Carry the modified proteins to the cell membrane.

6. Cell Membrane: Vesicles fuse with the membrane and release the proteins (exocytosis) into the extracellular space.

Major Cell Parts & Their Functions

Cells contain various organelles, each with specific functions essential for cell survival and activity.

Parts Unique to Plant vs. Animal Cells

Plant and animal cells share many organelles but also have unique structures.

Chapter 7 — Cell Membrane & Transport

Fluid-Like Component of the Cell Membrane

The Fluid Mosaic Model describes the cell membrane as a flexible, dynamic structure composed of lipids, proteins, and carbohydrates.

• Fluid: Refers to the movement of phospholipids and proteins within the bilayer.

• Mosaic: Refers to the mix of lipids, proteins, and carbohydrates.

• Function: This fluidity helps with transport, cell movement, and self-repair.

Saturated vs. Unsaturated Phospholipid Tails

The fluidity of the cell membrane is influenced by the types of fatty acids in phospholipids.

• Saturated Fatty Acid Tails: Straight, no double bonds—tightly packed, less fluid.

• Unsaturated Fatty Acid Tails: Kinked (double bonds)—loosely packed, more fluid.

• Temperature Adaptation: More unsaturated tails = greater membrane fluidity (especially in cold temps).

Distinguishing Animal Cells by Their Plasma Membranes

Animal cell membranes have unique glycoproteins and glycolipids that act as cell "ID tags" for recognition, important for immune response and tissue formation.

Six Functions of Membrane Proteins

Membrane proteins perform a variety of essential functions:

1. Transport: Move substances across membrane (channels or pumps).

2. Enzymatic Activity: Catalyze reactions on the inner or outer surface.

3. Signal Transduction: Receive and transmit chemical signals (e.g., hormone receptors).

4. Cell Recognition: Glycoproteins act as ID tags for cell identity.

5. Intercellular Joining: Help form cell junctions (tight, gap, desmosomes).

6. Attachment to Cytoskeleton & ECM: Anchor the cell and maintain shape.

Components of the Cell Membrane

The cell membrane is composed of several key components, each with specific functions.

Isotonic, Hypertonic, and Hypotonic Solutions

Cells respond differently to solutions with varying solute concentrations.

Plasmolysis, Turgid, Flaccid (Plant Cells)

Plant cells respond to water movement in specific ways:

• Plasmolysis: Cell membrane pulls away from cell wall due to water loss in a hypertonic solution.

• Turgid: Cell is swollen with water in a hypotonic solution; ideal condition for plant cells.

• Flaccid: Cell has equal water movement (isotonic solution); limp, not firm.

Passive vs. Active Transport

Cells use different mechanisms to move substances across membranes, depending on energy requirements and direction of movement.

Key Equations

• Surface Area of a Sphere:

• Volume of a Sphere:

• Surface Area to Volume Ratio: