Cell Membrane Transport

Selective Permeability

• Cell membranes allow some substances to pass and block others
• Small nonpolar molecules (O2, CO2) and small uncharged polar molecules (H2O) can cross easily
• Large molecules and charged ions (Na+, K+) need assistance to cross, due to their size or repulsion by the hydrophobic core

Membrane Potential

• Membrane potential refers to the voltage difference across a membrane
• It occurs because of uneven ion distribution
• Measured using microelectrodes to compare inside vs. outside charge

Transporters vs Channels

• Transporters bind specific molecules and change shape to move them across membranes
• Channels form pores for specific ions or water to flow through
• Channels are usually faster

Types of Transport

• Simple diffusion is the movement of molecules from high to low concentration without help
• Facilitated transport is passive movement using channel or transporter proteins
• Coupled transport is active transport where one molecule moves with its gradient to help move another against its gradient

Water Entry

• Water enters cells through osmosis
• This happens when there's a solute concentration difference
• Aquaporins (water channels) facilitate this process when solute concentration is higher inside the cell

GLUT1

• GLUT1 facilitates passive glucose transport down its concentration gradient, without energy

Active Transporters

• Four classes of active transporters ("ATP Pumps") exist
• P-type pumps (e.g., Na+/K+ pump)
• F-type pumps (e.g., ATP synthase in mitochondria)
• V-type pumps (vacuolar pumps)
• ABC transporters which use ATP to transport various molecules

Na+/K+ Pump

• The Na+/K+ pump uses ATP
• It exports 3 Na+ ions and imports 2 K+ ions
• It maintains low Na+ and high K+ inside the cell
• This helps set up membrane potential

Na+ Gradient

• The Na+ gradient provides energy for coupled symporters
• This brings substances (like glucose) against their concentration gradient
• Done by moving Na+ down its gradient

Patch Clamping

• Patch clamping measures ion channel activity
• It attaches a microelectrode to a small membrane patch to detect current flow

Membrane-Enclosed Organelles

• Membrane-enclosed organelles include the nucleus, ER, Golgi apparatus, lysosomes, endosomes, and peroxisomes
• Each has unique functions like protein processing, transport, or degradation

Protein Transport Mechanisms

• Three mechanisms transport protein into organelles:
• Gated transport (e.g., nucleus)
• Transmembrane transport (e.g., mitochondria, ER)
• Vesicular transport (e.g., between ER and Golgi)

Protein Localization

• Proteins have signal sequences that act like zip codes
• Nuclear localization signals guide proteins into the nucleus
• ER signal sequences direct entry into the ER
• Start/stop-transfer sequences help embed proteins into membranes

Vesicular Transport

• Vesicles bud off one organelle and fuse with another
• Transports materials especially between ER, Golgi, and plasma membrane

Endomembrane System

• The endomembrane system includes the ER, Golgi, lysosomes, endosomes, and plasma membrane
• It is a network of membrane-bound organelles for protein and lipid transport

Anterograde vs Retrograde Transport

• Anterograde transport moves from ER to Golgi to membrane
• Retrograde transport moves from membrane or Golgi back to ER

Golgi Cisternal Maturation Model

• Golgi cisternae mature as they move from the cis to the trans face
• They carry and modify proteins

Protein Transport in Vesicles

• Proteins are packaged into vesicles that bud from one compartment and fuse with another
• Coat proteins help vesicles form
• SNAREs help them dock and fuse

SNARE Proteins

• SNARE proteins ensure vesicles fuse with the correct target membrane
• They do this by forming a tight complex that brings the vesicle and target membranes together

Signal Transduction Pathways

• Signal transduction is how cells respond to external signals
• It involves receptors, second messengers, and intracellular pathways

Types of Signaling

• Endocrine signaling: long-distance signaling via hormones in the bloodstream
• Paracrine signaling: local signaling to nearby cells
• Autocrine signaling: cells signal to themselves

Hydrophilic and Hydrophobic Molecules

• Hydrophobic molecules (like steroids) can cross membranes and bind intracellular receptors
• Hydrophilic molecules cannot cross and must bind to extracellular receptors on the cell surface

Cell Surface Receptors

• Three primary classes of cell surface receptors exist:
• Ion-channel-coupled receptors: open or close ion channels in response to a signal
• G-protein-coupled receptors (GPCRs): activate G-proteins that trigger internal signaling cascades
• Enzyme-coupled receptors: often receptor tyrosine kinases that phosphorylate proteins to relay signals

Receptor Tyrosine Kinases (RTKs)

• RTKs dimerize upon ligand binding
• They activate their kinase domains and autophosphorylate
• Resulting phosphates act as docking sites for proteins that continue the signal pathway

GPCRs

• GPCRs activate heterotrimeric G-proteins
• Upon ligand binding, the GPCR exchanges GDP for GTP on the G-protein, activating it
• Activated subunits then influence downstream enzymes or channels

Cellular Respiration

• Cellular respiration is how cells harvest energy from glucose
• It converts glucose into ATP

Activated Carrier

• An activated carrier is a molecule (ATP, NADH, or FADH2)
• It stores energy or electrons used in metabolic reactions

Glucose Energy Harvesting

• Harvesting energy in steps allows cells to capture and store energy more efficiently
• This prevents it from being lost as heat

Catabolism Stages

• Catabolism occurs in three stages:
• Breakdown of large macromolecules into simple subunits
• Breakdown of subunits into Acetyl CoA
• Complete oxidation of Acetyl CoA in the citric acid cycle and electron transport chain

Glycolysis

• Glycolysis is a 10-step pathway
• It breaks glucose into 2 pyruvate molecules
• Glycolysis occurs in the cytoplasm

Glycolysis Products

• Starting products: Glucose, 2 NAD+, 2 ATP
• Net ending products: 2 pyruvate, 2 NADH, 2 ATP (net gain)

Glycolytic Enzymes

• Kinase: adds phosphate
• Isomerase: rearranges molecules
• Dehydrogenase: removes hydrogen (oxidation)
• Mutase: shifts chemical groups within a molecule

Glycolysis Summary

• Figure 13-5 visually summarizes the 10 steps of glycolysis
• It shows inputs, outputs, and energy generation

Enzymatic Coupling

• Enzymatic coupling links an energetically unfavorable reaction with a favorable one (like ATP hydrolysis)
• It drives the reaction forward

Pyruvate without Oxygen

• Without oxygen, pyruvate undergoes fermentation
• This produces lactate in animals, or ethanol and CO2 in yeast

Pyruvate Oxidation

• Pyruvate is transported into mitochondria
• It is converted to Acetyl CoA by the pyruvate dehydrogenase complex
• Produces CO2 and NADH

Citric Acid Cycle

• The Citric Acid Cycle is a cyclic series of reactions
• It completes glucose oxidation and generates NADH, FADH2, and GTP
• It occurs in the mitochondrial matrix

Citric Acid Cycle Products

• Starting products: Acetyl CoA, NAD+, FAD, GDP
• Ending products: CO2, NADH, FADH2, GTP
• Activated carriers: NADH and FADH2

Citric Acid Cycle Summary

• Figure 13-12 summarizes the 8 steps of the cycle, highlighting where energy carriers are produced

General Enzymes

• Dehydrogenase: oxidation-reduction
• Synthase: builds larger molecules
• Mutase: rearranges chemical groups

Electron Carriers

• NADH and FADH2 donate electrons to the electron transport chain
• This generates ATP in oxidative phosphorylation

Carbon Fixation Cycle

• The carbon fixation cycle, also called the Calvin cycle
• Occurs in the stroma
• Uses ATP and NADPH to convert CO2 into G3P (a sugar precursor)

Plasmolysis

• Plasmolysis happens when plant cells lose water in a hypertonic solution
• This causes the plasma membrane to pull away from the cell wall
• In an experiment, Elodea leaves in salt water showed this effect, supporting selective membrane permeability
• Water left the cells due to osmosis, but salt ions did not freely enter

Aseptic Technique

• Aseptic technique prevents contamination from pathogens
• It is essential in cell culture to maintain sterile conditions and ensure experimental results are not compromised

Cell Culture

• Cells used in cell culture are typically mammalian cells like fibroblasts or epithelial cells
• The best conditions are: Warm (37°C), humidified incubator with 5% CO2
• Confluent means cells cover the entire surface of the culture dish
• Trypsin detaches adherent cells from the dish surface for subculturing

Healthy Cell Culture

• To maintain a healthy cell culture: Maintain sterility, change media regularly, avoid over-confluency, and use proper aseptic technique