General Biology Study Guide: Cell Structure, Membranes, and Communication

Topic 1: Cell Structure, Subcellular Components

Overview

Cells contain a variety of subcellular structures, each with specialized functions that contribute to the life of the cell. Understanding these components is fundamental to cell biology.

• Subcellular Components: Structures within cells, often membrane-bound, that perform specific functions.

• Organelles: Specialized structures within eukaryotic cells, such as the nucleus, mitochondria, and Golgi apparatus.

Key Points

• Ribosomes: Complexes of RNA and protein that synthesize proteins. Found in both prokaryotes and eukaryotes.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein (rough ER) and lipid (smooth ER) synthesis.

• Golgi Complex: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: Organelles responsible for ATP production through cellular respiration. Contain their own DNA.

• Lysosomes: Contain hydrolytic enzymes for digestion of macromolecules.

• Vesicles: Small membrane-bound sacs that transport substances within the cell.

• Chloroplasts: Organelles in plants and algae that conduct photosynthesis.

• Compartmentalization: Eukaryotic cells use internal membranes to create specialized environments for different functions.

Examples and Applications

• Example: The rough ER is abundant in cells that produce large amounts of protein for export, such as pancreatic cells.

• Comparison: Eukaryotic cells have membrane-bound organelles; prokaryotic cells do not.

Additional info: Compartmentalization increases efficiency by isolating incompatible reactions and concentrating substrates and enzymes.

Topic 2: Cell Size

Overview

Cell size is limited by the surface area-to-volume ratio, which affects the rate of material exchange and metabolic efficiency.

• Surface Area-to-Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting the rate of exchange with the environment.

• Adaptations: Cells may have elongated or flattened shapes to increase surface area relative to volume.

Key Points

• Smaller cells have a higher surface area-to-volume ratio, allowing for more efficient exchange of materials.

• Large organisms are composed of many small cells rather than a few large ones.

Example

• Example: Intestinal epithelial cells have microvilli to increase surface area for absorption.

Equation:

$\text{Surface Area of a Sphere} = 4\pi r^2$

$\text{Volume of a Sphere} = \frac{4}{3}\pi r^3$

Additional info: The efficiency of diffusion and transport is critical for cell survival and function.

Topic 3: Cell Membranes

Overview

Cell membranes are composed of a phospholipid bilayer with embedded proteins, providing a selective barrier and mediating communication with the environment.

• Phospholipid Bilayer: Double layer of phospholipids with hydrophilic heads and hydrophobic tails.

• Fluid Mosaic Model: Describes the dynamic and flexible nature of the membrane, with proteins and lipids moving laterally.

• Membrane Proteins: Integral and peripheral proteins serve functions such as transport, signaling, and structural support.

Key Points

• Membrane fluidity is influenced by temperature, fatty acid composition, and cholesterol content.

• Selective permeability allows certain molecules to cross more easily than others.

Example

• Example: Aquaporins are channel proteins that facilitate water transport across the membrane.

Additional info: The "fluid mosaic model" was first proposed by Singer and Nicolson in 1972.

Topic 4: Membrane Transport

Overview

Membrane transport mechanisms regulate the movement of substances into and out of cells, maintaining homeostasis and enabling communication.

• Passive Transport: Movement of substances down their concentration gradient without energy input (e.g., diffusion, osmosis, facilitated diffusion).

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

• Bulk Transport: Endocytosis and exocytosis move large molecules or particles via vesicles.

Key Points

• Simple diffusion allows small, nonpolar molecules to cross the membrane.

• Facilitated diffusion uses transport proteins for polar or charged molecules.

• Osmosis is the diffusion of water across a selectively permeable membrane.

• Active transport (e.g., Na+/K+ pump) maintains concentration gradients essential for cell function.

Example

• Example: The Na+/K+ ATPase pump moves sodium out of and potassium into animal cells, consuming ATP.

Additional info: Endocytosis includes phagocytosis (cell eating) and pinocytosis (cell drinking).

Topic 5: Osmoregulation

Overview

Osmoregulation is the process by which organisms maintain water and solute balance, crucial for survival in varying environments.

• Osmoregulation: Regulation of water and solute concentrations to prevent excessive water loss or gain.

• Osmosis: Movement of water from areas of low solute concentration to high solute concentration across a membrane.

Key Points

• Organisms in freshwater environments must expel excess water; those in saltwater must conserve water.

• Specialized structures (e.g., contractile vacuoles in protists, kidneys in vertebrates) aid in osmoregulation.

Example

• Example: Paramecium uses a contractile vacuole to pump out excess water.

Additional info: Osmoregulation is vital for maintaining cell turgor in plants and blood osmolarity in animals.

Topic 6: Neurons as an Example of Membrane Function

Overview

Neurons use specialized membrane properties to transmit electrical signals, illustrating the importance of membrane structure and function.

• Neurons: Nerve cells that transmit information via electrical and chemical signals.

• Membrane Potential: The voltage difference across a cell membrane due to ion distribution.

Key Points

• Ion channels and pumps maintain resting membrane potential and enable action potentials.

• Neurotransmitters are released at synapses to communicate with other cells.

Example

• Example: The rapid influx of Na+ during an action potential depolarizes the neuron membrane.

Additional info: The Na+/K+ pump is essential for restoring ion gradients after nerve impulses.

Table: Comparison of Eukaryotic and Prokaryotic Cells

Feature	Eukaryotic Cells	Prokaryotic Cells
Membrane-bound organelles	Present	Absent
Nucleus	Present	Absent (nucleoid region)
Cell size	Larger (10–100 μm)	Smaller (0.1–5 μm)
Examples	Animals, plants, fungi, protists	Bacteria, archaea