Ch. 6: Cell Structures

Organelles: Structure & Function

Cells contain specialized structures called organelles that perform distinct functions necessary for cellular life. Understanding the structure and function of each organelle is fundamental to cell biology.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Mitochondria: Site of cellular respiration and energy (ATP) production.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

• Lysosomes: Contain digestive enzymes to break down waste.

• Chloroplasts: (in plant cells) Site of photosynthesis.

• Cell Membrane: Regulates entry and exit of substances.

Example: The mitochondria are often called the "powerhouse" of the cell because they generate most of the cell's supply of ATP, used as a source of chemical energy.

Cell Types: Prokaryotes vs. Eukaryotes

Cells are classified as either prokaryotic or eukaryotic based on their structural differences.

• Prokaryotes: Lack a nucleus and membrane-bound organelles; DNA is found in the nucleoid region. Example: Bacteria.

• Eukaryotes: Have a nucleus and membrane-bound organelles. Examples: Plants, animals, fungi, protists.

Comparison Table:

Endosymbiosis

The endosymbiotic theory proposes that some organelles, such as mitochondria and chloroplasts, originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells. This theory is supported by similarities in DNA, double membranes, and reproduction methods.

• Evidence: Mitochondria and chloroplasts have their own DNA and reproduce independently within the cell.

• Significance: Explains the origin of key eukaryotic cell features.

Surface Area-to-Volume Ratio

The surface area-to-volume ratio is crucial for cell function, as it affects the rate of diffusion of materials into and out of the cell.

• As a cell grows, its volume increases faster than its surface area, limiting the efficiency of transport.

• Cells remain small to maximize this ratio and maintain efficient exchange with their environment.

Formula:

Example: A small cell with a high surface area-to-volume ratio can absorb nutrients and expel wastes more efficiently than a large cell.

Ch. 7: Cell Membrane & Transport

Osmosis and Water Potential

Osmosis is the diffusion of water across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration.

• Direction of flow: Water moves toward the region with higher solute concentration (lower water potential).

• Key terms: Hypertonic (higher solute), Hypotonic (lower solute), Isotonic (equal solute).

Water Potential (): A measure of the potential energy of water in a system, influencing the direction of water movement.

Formula:

• = Solute potential

• = Pressure potential

Example: If a plant cell is placed in a hypotonic solution, water enters the cell by osmosis, causing it to swell.

Determining Unknown Solutions: Osmosis Lab

In laboratory settings, osmosis can be used to determine the concentration of unknown solutions by observing the movement of water into or out of cells or artificial membranes.

• When a cell is placed in a solution, the direction and amount of water movement can indicate the relative concentrations of solutes inside and outside the cell.

• By measuring changes in mass or volume, students can estimate the solute concentration of the unknown solution.

Example: Placing potato slices in different sucrose solutions and measuring their mass change to determine the molarity at which there is no net water movement (isotonic point).

Cell Membrane Structure & Function

The cell membrane is a selectively permeable barrier that controls the movement of substances into and out of the cell. It is composed primarily of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipid bilayer: Provides fluidity and flexibility; hydrophilic heads face outward, hydrophobic tails face inward.

• Proteins: Serve as channels, carriers, receptors, and enzymes.

• Cholesterol: Stabilizes membrane fluidity.

• Carbohydrates: Involved in cell recognition and signaling.

Functions:

• Regulates transport of materials

• Facilitates communication with other cells

• Maintains homeostasis

Example: The sodium-potassium pump is a membrane protein that actively transports Na+ and K+ ions across the membrane, crucial for nerve impulse transmission.

Comparing Diffusion, Facilitated Diffusion, and Active Transport

Cells use several mechanisms to move substances across their membranes:

Additional info: Facilitated diffusion requires membrane proteins but does not use cellular energy, while active transport requires both proteins and energy input.