8.2 Cells and Cellular Processes

8.2 Cells and Cellular Processes on NMAT Biology

Within CEM NMAT Biology (30 items, ~30 minutes), Cells and Cellular Processes is a dense scoring region: structure–function of organelles, how materials cross membranes, how enzymes and pathways manage energy, how cells divide, and how signals change cell behavior. College introductory premed depth means you should trace a process, not only match a word to a one-line definition.

Quick frame: Ask four questions on every stem: Where does it happen? What crosses or transforms? Energy in or energy out? Same chromosome number or half?

Organelles and functions (eukaryotic map)

Protein secretory path (high-yield sequence): ribosome on RER → lumen folding → transport vesicle → Golgi → secretory vesicle → exocytosis at plasma membrane. Membrane proteins follow related routes.

Worked conceptual scenario A
A mutation blocks Golgi sorting. Which products fail first? Secreted enzymes and plasma-membrane proteins that require Golgi processing — not necessarily all cytosolic glycolytic enzymes made on free ribosomes.

Worked conceptual scenario B
Which organelle pair supports endosymbiotic origin ideas? Mitochondria and chloroplasts — double membranes, circular DNA, 70S-type ribosomes, binary fission-like division.

Membrane transport

Tonicity (animal cells): hypotonic solution → water in → swell/lyse risk; hypertonic → water out → crenate; isotonic → no net water movement. Plant cells in hypotonic medium become turgid (wall prevents lysis); in hypertonic medium they plasmolyze.

Worked conceptual scenario C
Red blood cells placed in distilled water swell. Mechanism: osmosis into the higher intracellular solute environment; no ATP pump is required for the water movement itself.

Worked conceptual scenario D
Intestinal glucose absorption can continue when intracellular glucose exceeds lumen glucose because secondary active transport couples Na⁺ influx (down its gradient maintained by Na⁺/K⁺ ATPase) to glucose uptake.

Enzymes

Enzymes are biological catalysts, mostly proteins (some RNA ribozymes). They:

• Lower activation energy without changing overall ΔG of the reaction
• Bind substrates at an active site (induced fit)
• Show specificity and saturation kinetics (Vmax when enzyme is limited)
• Are sensitive to temperature and pH (denaturation outside optimum)
• Are regulated by competitive inhibitors (active-site competition; overcome by more substrate) and noncompetitive / allosteric inhibitors (bind elsewhere; change shape/function)

Worked conceptual scenario E
Adding more substrate restores rate despite an inhibitor present. That pattern fits competitive inhibition better than irreversible active-site destruction.

Cellular respiration overview vs photosynthesis

Both are energy-transforming pathways with linked redox chemistry, but they are not simple reverse twins in every detail. At NMAT depth:

Cellular respiration (typical aerobic summary in eukaryotes)
Glucose + O₂ → CO₂ + H₂O + ATP (and heat). Stages:

1. Glycolysis (cytosol): glucose → 2 pyruvate; net 2 ATP + NADH
2. Pyruvate oxidation + citric acid cycle (mitochondrial matrix): complete oxidation to CO₂; NADH/FADH₂
3. Electron transport chain + oxidative phosphorylation (inner membrane): O₂ final electron acceptor; major ATP via chemiosmosis

Anaerobic options: fermentation regenerates NAD⁺ so glycolysis continues; net ATP remains low (2 per glucose in classic lactic acid/alcoholic schemes).

Photosynthesis (plants/algae/cyanobacteria)
Light energy + CO₂ + H₂O → sugars + O₂ (oxygen-evolving organisms).

1. Light reactions (thylakoid membrane): light → ATP + NADPH; H₂O split → O₂
2. Calvin cycle (stroma): CO₂ fixation into carbohydrates using ATP + NADPH

Worked conceptual scenario F
A sealed illuminated plant system raises O₂ and later, in dark with stored sugar, consumes O₂. Illuminated phase dominated by photosynthesis net O₂ release; dark phase by respiration.

Worked conceptual scenario G
Cyanide blocks electron transfer to O₂. Immediate effect: ETC/oxidative phosphorylation fails, ATP from mitochondria collapses, NADH is not reoxidized efficiently — glycolysis may continue briefly only if fermentation pathways compensate.

Cell cycle, mitosis vs meiosis

Interphase: G1 (growth), S (DNA replication — sister chromatids form), G2 (prepare to divide). M phase: mitosis + cytokinesis. Checkpoints (G1/S, G2/M, spindle) protect genome integrity.

Chromosome counting caution: After S phase, a human cell still has 46 chromosomes counted by centromeres, but 92 chromatids. In anaphase of mitosis, sister separation makes daughter nuclei 46 each.

Worked conceptual scenario H
A diploid cell (2n = 4) completes meiosis correctly. Products: four cells with n = 2. If homologs fail to separate in meiosis I (nondisjunction), gametes with extra or missing chromosomes result — bridge to genetics section abnormalities.

Worked conceptual scenario I
Skin fibroblast division for wound repair uses mitosis. Spermatogenesis uses meiosis. Mixing these purposes is a common stem trap.

Signal transduction (intro level)

Cells respond to chemical signals without every ligand entering the nucleus directly:

1. Reception — ligand binds membrane or intracellular receptor (specificity)
2. Transduction — relay molecules, often a phosphorylation cascade or second messengers (cAMP, Ca²⁺, IP₃)
3. Response — change in gene expression, enzyme activity, cytoskeleton, secretion, etc.

High-yield properties:

• Amplification: one ligand → many second-messenger molecules → large response
• Specificity: receptor presence determines which cells respond
• Reversibility: phosphatases and signal degradation turn pathways off
• Hydrophilic ligands (peptides, epinephrine) typically use membrane receptors; steroid hormones often use intracellular receptors acting as transcription factors

Worked conceptual scenario J
Epinephrine binds a G protein–coupled receptor on a hepatocyte → cAMP rises → kinase cascade → glycogen breakdown. The hormone need not enter the nucleus; second messengers carry the message. Blocking the receptor blocks response even if blood epinephrine is high.

Integration scenarios (exam style)

Scenario K — organelle + transport: Insulin triggers glucose uptake in muscle via GLUT transporter insertion. This is facilitated diffusion of glucose after a signal transduction event — not active pumping of glucose in the classic Na⁺-coupled gut sense.

Scenario L — energy + enzymes: Fever elevates temperature; enzyme rates may rise modestly, but extreme fever risks denaturation. Homeostasis keeps enzymes near optima.

Scenario M — cycle control: A drug freezes microtubules. Mitotic spindle fails → cells arrest in mitosis (spindle checkpoint). Meiosis would also fail chromosome segregation.

Error traps checklist

Section checkpoint

You are ready when you can: (1) route a secretory protein from ribosome to exterior, (2) classify a transport problem as passive vs active with gradient logic, (3) state where O₂ is produced vs consumed, (4) contrast mitosis and meiosis on purpose, number of divisions, and ploidy, and (5) outline ligand → receptor → cascade → response in one breath.