In the light-dependent reactions, electrons are energized by light and passed along the photosynthetic electron transport chain until they are donated to NADP+, forming NADPH. This NADPH is the reduced form of NADP+ and serves as the electron carrier that delivers reducing power to the Calvin cycle in the stroma. Oxygen is produced when water is split, not reduced. ATP is generated to provide energy, while G3P is a product of the Calvin cycle, not a direct output of the light reactions. So the molecule that gets reduced and carries electrons through this stage is NADPH.

Gymnosperms reproduce using cones, also called strobili, as their reproductive structures. These cones can be male, bearing pollen, or female, housing ovules on their scales; after pollen reaches an ovule and fertilization occurs, seeds form on the cone. The zygote is the fertilized cell, not the structure itself, the ovule is the structure inside the cone that becomes a seed, and pollination is the event of pollen transfer. So, the reproductive structures are cones.

The timing of a heartbeat is set by the sinoatrial node, the natural pacemaker of the heart. Its pacemaker cells spontaneously generate impulses at a regular rate, establishing the heart’s rhythm. The impulse then travels through atrial tissue to the atrioventricular (AV) node, which adds a brief delay. That delay coordinates atrial contraction with ventricular contraction so the ventricles fill properly before they pump. After the delay, the impulse moves through the His-Purkinje system to rapidly activate the ventricles for synchronized contraction. Atrial tissue mainly serves to conduct and support atrial contraction, not to set the pace. So the primary source that coordinates the heartbeat timing is the SA node, with the AV node refining the sequence by providing the necessary delay.

Sex is determined by chromosomes, and the chromosome responsible for this trait is called the sex chromosome. In humans and many other organisms, these are the X and Y chromosomes: typically XX lines up with female development and XY with male development, though other species can have different systems. The sex chromosomes stand out from autosomal chromosomes, which are the non-sex chromosomes that carry the majority of genes but don’t determine whether an individual is male or female. A mutation is a change in DNA and isn’t about which chromosome determines sex, while species is simply a level of biological classification.