Cellular Transport, Membrane Potential, Cellular Respiration, and Cell Cycle: ANP Study Guide

Cellular Transport and Gradients

Types of Gradients in the Human Body

Gradients are differences in physical or chemical properties across a space, driving movement of substances in the body. Four main types exist:

• Concentration Gradient: Difference in solute concentration between two areas; solutes move from high to low concentration.

• Osmotic Gradient: Difference in water concentration across a semipermeable membrane; water moves toward higher solute concentration.

• Pressure Gradient: Difference in hydrostatic pressure; fluids move from high to low pressure (e.g., blood flow).

• Electrical Gradient: Difference in electrical charge across a membrane; ions move toward opposite charge.

Factors Affecting Rate of Diffusion (Fick's Law)

The rate at which substances diffuse across membranes depends on several factors, as described by Fick's Law:

• Concentration Gradient: Larger gradient increases diffusion rate.

• Surface Area: Greater membrane surface area increases diffusion rate.

• Solubility: Lipid-soluble substances diffuse faster through membranes.

• Molecular Size: Smaller molecules diffuse faster; larger molecules diffuse slower.

• Membrane Thickness: Thicker membranes slow diffusion.

Fick's Law Equation:

$\text{Rate of diffusion} = \frac{(\text{Surface Area}) \times (\text{Concentration Gradient}) \times (\text{Solubility})}{\text{Membrane Thickness} \times (\text{Molecular Size})}$

Active vs. Passive Transport Across Cell Membranes

Transport mechanisms move substances across cell membranes, classified as passive or active:

• Passive Transport: No ATP required; moves substances down their gradient (high to low). Includes simple diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires ATP; moves substances against their gradient (low to high). Uses carrier proteins or pumps.

Types of Membrane Transport

Membrane transport is classified by energy use and solute movement:

• Passive Transport:

  • Simple Diffusion: Direct movement through lipid bilayer (e.g., O2, CO2).

  • Facilitated Diffusion: Movement through channel or carrier proteins; still down gradient.

  • Osmosis: Diffusion of water across a membrane, often via aquaporins.

• Active Transport:

  • Primary (Direct) Active Transport: Uses ATP directly (e.g., Na+/K+ ATPase pump).

  • Secondary (Indirect) Active Transport: Uses energy stored in ion gradients; includes symport and antiport mechanisms.

Types of Ion Channels

Ion channels regulate movement of ions across membranes:

• Leak Channels: Always open; allow passive ion movement; major role in resting membrane potential.

• Ligand-Gated Channels: Open when a specific chemical (ligand) binds.

• Voltage-Gated Channels: Open in response to changes in membrane potential.

• Mechanically Gated Channels: Open due to physical deformation (stretch, pressure, vibration).

Tonicity: Effects of Solutions on Cells

Tonicity describes how a solution affects cell volume:

• Hypotonic: Lower solute concentration outside the cell; water enters, cell swells (may lyse).

• Isotonic: Equal solute concentration; no net water movement, cell unchanged.

• Hypertonic: Higher solute concentration outside; water leaves, cell shrinks (crenates).

Key Rule: Water moves toward higher solute concentration.

Transmembrane Vesicular Transport

Cells use vesicles to move large substances across membranes:

• Endocytosis: Vesicle brings substances into the cell.

  • Phagocytosis: Engulfment of large particles (“cell eating”).

• Exocytosis: Vesicle fuses with plasma membrane to release substances outside the cell.

Direction: Endocytosis = into cell; Exocytosis = out of cell.

Membrane Potential and Ion Gradients

Resting Membrane Potential (RMP)

The resting membrane potential is the electrical charge difference across the plasma membrane of a resting cell, typically about −70 mV, with the inside of the cell negative relative to the outside.

Establishing Membrane Potential

Membrane potential is established by:

• Unequal Ion Distribution:

  • Na+ high outside

  • K+ high inside

• Selective Permeability: Membrane is more permeable to K+ due to leak channels; K+ diffuses out more than Na+ diffuses in, creating a net negative charge inside the cell.

Electrochemical Gradient

An electrochemical gradient is the combined influence of the concentration gradient and the electrical gradient, determining the direction and magnitude of ion movement across a membrane.

Sodium-Potassium ATPase Pump

The Na+/K+ ATPase pump maintains RMP by:

• Using ATP

• Pumping 3 Na+ out of the cell

• Pumping 2 K+ into the cell

• Maintaining ion concentration gradients

• Contributing to the negative resting membrane potential

Na+/K+ ATPase Equation:

$3\ \text{Na}^+_{\text{(in)}} + 2\ \text{K}^+_{\text{(out)}} + \text{ATP} \rightarrow 3\ \text{Na}^+_{\text{(out)}} + 2\ \text{K}^+_{\text{(in)}} + \text{ADP} + \text{P}_i$

Cellular Respiration

Definition of Cellular Respiration

Cellular respiration is the metabolic process by which cells convert the chemical energy in glucose into ATP, producing carbon dioxide and water as by-products.

Glucose Conversion Pathways

Glucose is converted to ATP through three main pathways:

• Glycolysis: Occurs in cytoplasm; converts glucose to 2 pyruvate, 2 ATP (net), and NADH.

• Citric Acid (Krebs) Cycle: Occurs in mitochondrial matrix; acetyl-CoA is processed to produce 2 ATP, NADH, FADH2, and CO2 per glucose.

• Electron Transport Chain (ETC): Occurs in inner mitochondrial membrane; uses NADH and FADH2 to produce ~28–32 ATP and water.

Overall Cellular Respiration Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\ \text{O}_2 \rightarrow 6\ \text{CO}_2 + 6\ \text{H}_2\text{O} + \text{ATP}$

Role of Oxygen in Aerobic Respiration

Oxygen acts as the final electron acceptor in the electron transport chain. By accepting electrons and hydrogen ions, oxygen allows the ETC to continue operating and enables large-scale ATP production. Without oxygen, the ETC stops.

Aerobic vs. Anaerobic Respiration and Fermentation

Feature	Aerobic Respiration	Anaerobic Respiration / Fermentation
Oxygen Requirement	Requires oxygen	Does not require oxygen
Location	Mitochondria	Cytoplasm
ATP Yield	~30–32 ATP per glucose	2 ATP per glucose
End Products	CO2, H2O	Lactate (humans) or alcohol + CO2 (yeast)
NAD+ Regeneration	ETC regenerates NAD+	Fermentation regenerates NAD+

Cell Cycle and Cell Division

Phases of the Cell Cycle and Key Events

The cell cycle consists of interphase, mitosis, and cytokinesis:

• Interphase:

  • G1: Cell growth

  • S: DNA replication

  • G2: Preparation for division

• Mitosis:

  • Prophase: Chromosomes condense; nuclear envelope breaks

  • Metaphase: Chromosomes align at equator

  • Anaphase: Sister chromatids separate

  • Telophase: Nuclei reform

• Cytokinesis: Cytoplasm divides into two cells

Definitions: Parent Cell, Daughter Cell, Cytokinesis

• Parent Cell: Original cell that undergoes division.

• Daughter Cells: Cells produced after division.

• Cytokinesis: Division of the cytoplasm following mitosis.

DNA Replication

DNA replication is semi-conservative and involves:

1. Helicase unwinds the double helix.

2. DNA polymerase synthesizes new complementary strands (5′→3′).

3. Ligase seals Okazaki fragments on the lagging strand.

Product: Two identical DNA molecules.

DNA Molecule, Chromosome, and Sister Chromatids

• DNA Molecule: Double-helix nucleic acid carrying genetic information.

• Chromosome: Condensed DNA wrapped around histone proteins.

• Sister Chromatids: Two identical copies of a replicated chromosome joined at the centromere.

Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	One	Two (I and II)
Number of Cells Produced	2	4
Genetic Identity	Identical diploid cells	Genetically different haploid cells
Function	Growth and repair	Gamete production
Crossing Over	No	Yes, in Prophase I