Cell Structure, Membrane Function, and Transport: Study Notes for General Biology

Topic 1: Cell Structure, Subcellular Components

Overview of Eukaryotic Cell Structure

Living systems are organized in a hierarchy of structural levels, with each level building upon the previous one. Eukaryotic cells contain various subcellular components, each with specialized functions essential for life.

• Ribosomes: Small structures composed of RNA and proteins; sites of protein synthesis. Found free in cytoplasm or bound to the endoplasmic reticulum.

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

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs.

• Golgi Apparatus: Stack of flattened membranes; modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

• Mitochondria: Double-membraned organelles; site of cellular respiration and ATP production.

• Chloroplasts: Organelles found in plants and algae; site of photosynthesis.

• Lysosomes: Membrane-bound sacs containing digestive enzymes; break down waste materials and cellular debris.

• Vacuoles: Storage organelles; large central vacuole in plants maintains turgor pressure.

Example: The rough ER produces proteins that are then modified in the Golgi apparatus and transported to their final destinations.

Topic 2: Cell Size

Surface Area-to-Volume Ratio and Its Biological Significance

The size and shape of cells are limited by the surface area-to-volume ratio, which impacts the efficiency of material exchange and energy use.

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

• Implications: Cells are typically small and may have specialized structures (e.g., microvilli) to increase surface area.

• Formula:

$\text{Surface Area of a Sphere} = 4\pi r^2$ $\text{Volume of a Sphere} = \frac{4}{3}\pi r^3$ $\text{Surface Area-to-Volume Ratio} = \frac{4\pi r^2}{\frac{4}{3}\pi r^3} = \frac{3}{r}$

• Example: Red blood cells are small and biconcave, maximizing surface area for gas exchange.

Topic 3: Cell Membranes

Structure and Function of the Plasma Membrane

The plasma membrane is a selectively permeable barrier that separates the internal environment of the cell from the external environment.

• Phospholipid Bilayer: Composed of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Membrane Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

• Fluid Mosaic Model: Describes the dynamic and flexible nature of the membrane, with proteins floating in or on the fluid lipid bilayer.

• Cholesterol: Stabilizes membrane fluidity in animal cells.

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

Topic 4: Membrane Transport

Mechanisms of Transport Across Cell Membranes

Cells use various mechanisms to move substances across membranes, maintaining internal conditions distinct from the external environment.

• Passive Transport: Movement of substances down their concentration gradient without energy input.

  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2).

  • Facilitated Diffusion: Movement of larger or polar molecules via channel or carrier proteins.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

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

  • Example: Sodium-potassium pump (Na+/K+ ATPase) maintains electrochemical gradients in animal cells.

• Bulk Transport: Endocytosis (into the cell) and exocytosis (out of the cell) for large molecules or particles.

Example: Glucose enters cells via facilitated diffusion through specific carrier proteins.

Topic 5: Osmoregulation

Maintaining Water and Solute Balance

Osmoregulation is the process by which organisms regulate the balance of water and solutes within their bodies to maintain homeostasis.

• Hypertonic Solution: Higher solute concentration outside the cell; water moves out, cell shrinks.

• Hypotonic Solution: Lower solute concentration outside the cell; water moves in, cell swells.

• Isotonic Solution: Equal solute concentration; no net water movement.

• Water Potential ($\Psi$): Predicts the direction water will flow; water moves from high to low water potential.

$\Psi = \Psi_s + \Psi_p$ Where $\Psi$ is total water potential, $\Psi_s$ is solute potential, and $\Psi_p$ is pressure potential.

Example: Plant cells in hypotonic solutions become turgid due to water influx, which is essential for structural support.

Topic 6: Neurons as an Example of Membrane Function

Structure and Function of Neurons

Neurons are specialized cells that transmit electrical and chemical signals in animals. Their function relies on the properties of cell membranes.

• Structure: Consist of a cell body, dendrites (receive signals), and an axon (transmits signals).

• Signal Transmission: Electrical impulses (action potentials) travel along the axon; neurotransmitters carry signals between neurons at synapses.

• Membrane Potential: The difference in charge across the neuron's membrane is essential for signal transmission.

Example: The sodium-potassium pump helps maintain the resting membrane potential necessary for action potentials.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction (relative to gradient)	Example
Simple Diffusion	No	Down	O2, CO2
Facilitated Diffusion	No	Down	Glucose via carrier protein
Osmosis	No	Down	Water via aquaporins
Active Transport	Yes (ATP)	Up	Na+/K+ pump
Bulk Transport	Yes	Varies	Endocytosis, Exocytosis