Cellular Structure and Function: AP Biology Unit 2 Study Guide

Cellular Structure and Function

Cell Structure and Subcellular Components

Cells are organized into a hierarchy of structural levels, each with specialized functions. Understanding the structure and function of organelles is essential for grasping how cells operate and interact.

• Ribosomes: Composed of ribosomal RNA (rRNA) and proteins. Their main function is protein synthesis by translating messenger RNA (mRNA) into polypeptide chains.

• Universal Presence: All living things contain ribosomes, highlighting their fundamental role in life and supporting the theory of common ancestry.

• Endomembrane System: A network of membranes within eukaryotic cells, including the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles. It is involved in protein and lipid synthesis, transport, and metabolism.

• Endoplasmic Reticulum (ER):

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

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

• Golgi Complex: Consists of flattened membranous sacs (cisternae); modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: Double-membraned organelles responsible for cellular respiration and ATP production. The inner membrane's folds (cristae) increase surface area for the electron transport chain and ATP synthesis. The Krebs (citric acid) cycle occurs in the matrix; electron transport and ATP synthesis occur on the inner membrane.

• Lysosomes: Membrane-bound vesicles containing hydrolytic enzymes for intracellular digestion and recycling of cellular components.

• Vacuoles: Membrane-bound sacs with diverse roles, including storage, waste disposal, and maintaining turgor pressure in plant cells.

• Chloroplasts: Double-membraned organelles found in plants and algae; site of photosynthesis. The thylakoid membranes house light-dependent reactions, while the stroma is the site of the Calvin-Benson cycle (carbon fixation).

Compartmentalization in Eukaryotic Cells: Internal membranes and organelles allow for specialized environments and processes, increasing efficiency and complexity. Eukaryotic cells have extensive compartmentalization, while prokaryotic cells lack membrane-bound organelles. The endosymbiotic theory suggests mitochondria and chloroplasts evolved from engulfed prokaryotes.

Additional info: Compartmentalization enables simultaneous, incompatible reactions and localized concentration of substrates and enzymes.

Cell Size and Surface Area-to-Volume Ratio

The efficiency of material exchange in cells is influenced by their surface area-to-volume (SA:V) ratio. As cells grow, volume increases faster than surface area, limiting resource exchange and waste removal.

• SA:V Ratio: High ratios favor efficient exchange; as cell size increases, the ratio decreases, restricting cell size.

• Equations:

  • For a sphere: $SA = 4\pi r^2$, $V = \frac{4}{3}\pi r^3$, $SA:V = \frac{4\pi r^2}{\frac{4}{3}\pi r^3} = \frac{3}{r}$

  • For a cube: $SA = 6a^2$, $V = a^3$, $SA:V = \frac{6}{a}$

  • For a rectangular solid: $SA = 2(lw + lh + wh)$, $V = lwh$

  • For a cylinder: $SA = 2\pi r(h + r)$, $V = \pi r^2 h$

• Specialized Exchange Structures: Examples include microvilli in intestines, alveoli in lungs, and root hairs in plants, all increasing surface area for exchange.

• Metabolic Rate: Organisms with higher SA:V ratios can exchange materials and thermal energy more efficiently, affecting metabolic rates.

Additional info: Cells may adopt elongated or flattened shapes to maximize SA:V ratio.

Cell Membranes and Cell Walls

Cell membranes are dynamic structures that regulate the internal environment of the cell. Cell walls provide additional support and protection in certain organisms.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming bilayers that create the basic structure of membranes.

• Membrane Proteins: Embedded based on hydrophilic/hydrophobic interactions; serve as channels, receptors, or enzymes.

• Fluid Mosaic Model: Describes the membrane as a flexible, dynamic arrangement of lipids, proteins, and carbohydrates.

• Selective Permeability: Membranes allow some substances (e.g., small nonpolar molecules) to pass freely, while others require transport proteins.

• Cell Wall: Provides structural support, protection, and prevents excessive water uptake. Found in plants (cellulose), fungi (chitin), and bacteria (peptidoglycan).

Membrane Transport

Cells use various mechanisms to transport substances across membranes, maintaining homeostasis and responding to environmental changes.

• 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). Example: Na+/K+ ATPase pump.

• Facilitated Diffusion: Passive movement of molecules via transport proteins (e.g., aquaporins for water, ion channels for charged particles).

• Endocytosis and Exocytosis: Bulk transport mechanisms for large molecules or particles. Endocytosis brings substances into the cell; exocytosis expels them.

• Concentration Gradients: Created by selective permeability and active transport, essential for processes like nerve impulse transmission.

Example Table: Comparison of Passive and Active Transport

Feature	Passive Transport	Active Transport
Energy Required?	No	Yes (ATP)
Direction	Down gradient	Against gradient
Examples	Osmosis, diffusion	Na+/K+ pump

Additional info: Moving charged ions can create membrane potentials, critical for nerve and muscle function.

Osmoregulation

Osmoregulation is the control of water and solute concentrations to maintain homeostasis. It is vital for cell survival in varying environments.

• Osmoregulation: The process by which organisms regulate water and solute balance.

• Environments:

  • Hypotonic: Lower solute concentration outside; water enters cell.

  • Hypertonic: Higher solute concentration outside; water leaves cell.

  • Isotonic: Equal solute concentrations; no net water movement.

• Water Potential Equation: Predicts the direction of water movement.

  • $\Psi = \Psi_s + \Psi_p$

  • Where $\Psi$ is water potential, $\Psi_s$ is solute potential, $\Psi_p$ is pressure potential.

• Homeostasis: Maintained by balancing water and solute movement across membranes.

Neurons as an Example of Membrane Function

Neurons illustrate the importance of membrane structure and function in generating and transmitting electrical signals.

• Neuron Structure: Consists of dendrites, cell body, axon, and synaptic terminals.

• Electrical Signals: Generated by the movement of ions across the axon membrane, creating action potentials.

• Signal Transmission: Signals are passed to neighboring neurons via synapses, involving neurotransmitter release and receptor activation.

Additional info: The Na+/K+ pump and voltage-gated ion channels are essential for action potential generation and propagation.