How far you should mainline up to depends on your light penetration, and if you side supplement, without getting into too many details, you have at most 2-3 layers of leaf before the red and blue parts of the spectral composition are completely absorbed, after which buds will become larf from too much green and not enough R&B.
Even a leaf with low photosynthetic efficiency can still perform efficient gas exchange. Gas exchange, primarily the intake of carbon dioxide and release of oxygen, is a separate process from photosynthesis, though they are closely linked. Leaves have specialized structures, like stomata, that allow for efficient gas movement regardless of the leaf's photosynthetic capacity. In plants, fan leaves are crucial for both photosynthesis and cellular respiration, the processes that provide the ATP (adenosine triphosphate) needed for growth and other cellular functions. Photosynthesis in chloroplasts produces ATP and NADPH, which are used in the Calvin cycle to produce sugars. These sugars are then broken down through cellular respiration in mitochondria, generating more ATP, along with carbon dioxide and water. This ATP fuels various cellular activities, including protein synthesis, transport, and other metabolic processes.
You can live weeks without food. Days without water. Minutes without oxygen. 16 seconds without ATP. I always enjoy stating that because prior to me reading it I had no idea what it even was, yeeet it is top of the food chain for everything. In its purest form, it is the currency of life. Energy. ATP comes before nutrients, before oxygen, and carbon. While all are essential, ATP is arguably the most fundamental for plant growth as it acts as the primary energy currency for cellular processes. Nutrients, oxygen, and carbon are all necessary for photosynthesis and other metabolic processes, but they require ATP for the energy to be converted into usable forms, like sugars for growth. In essence, ATP is the energy that drives the entire process. Nutrients, oxygen, and carbon are the raw materials and components that are utilized by the plant with the help of ATP. Fan leaves are far more efficient for gas exchange than sugar leaves in cannabis plants. Plants also need to respire, consuming oxygen and releasing carbon dioxide.