SAT Study Guide 2026

Insulin is a vital hormone produced by the pancreas that plays a central role in regulating blood glucose (sugar) levels. After a meal, when blood sugar rises, insulin signals cells to absorb glucose from the bloodstream for energy or storage. This action effectively lowers blood sugar, maintaining the body's internal balance.

Meiosis halves the chromosome number, so a diploid cell with 46 chromosomes produces haploid cells with 23 chromosomes.

In skeletal muscle contraction, an action potential triggers the release of Ca2+ from the sarcoplasmic reticulum. These calcium ions then bind to troponin, a protein complex on the actin filament. This binding causes a conformational change that moves the tropomyosin protein, uncovering the sites on actin where myosin heads can bind, thus initiating the cross-bridge cycle and muscle contraction.

Meiosis II resembles mitosis in that sister chromatids separate (rather than homologs), though the cells entering meiosis II are already haploid.

DNA synthesis (replication) occurs exclusively during the S (synthesis) phase of interphase.

When a neuron reaches its threshold potential, voltage-gated sodium channels open rapidly. This allows a massive influx of positively charged sodium ions (Na+) to rush into the cell, down their electrochemical gradient. This rapid influx of positive charge causes the membrane potential to shoot up from negative to positive, an event known as depolarization, which is the rising phase of the action potential.