Question

How does a tree branch provide energy for the deer?

Answer 1

Short answer: the branch’s chemical energy (made by the tree from sunlight) is eaten and digested by the deer and converted into the deer’s usable biochemical energy (ATP) and building blocks for tissues.

More detail, step by step:
- The tree uses photosynthesis to convert sunlight, CO2 and water into carbohydrates (sugars, starch) and other organic compounds that make up leaves, buds, twigs and some bark.
- When a deer browses on a branch (especially soft shoots, buds and leaves), it consumes those carbohydrates, plus protein, fat, minerals and water contained in the plant tissue.
- Deer are ruminants: food first goes into the rumen where microbes (bacteria, protozoa, fungi) ferment plant fiber (cellulose, hemicellulose). The microbes break down fibers into short‑chain fatty acids (volatile fatty acids — mainly acetate, propionate and butyrate), gases and microbial protein.
- The deer absorbs the volatile fatty acids across the rumen wall; those provide most of the deer’s metabolic energy. Microbial protein and any plant protein are digested later in the intestines into amino acids that build and repair tissue.
- Inside deer cells, absorbed sugars and fatty acids are oxidized via cellular respiration (glycolysis, TCA cycle, oxidative phosphorylation) to produce ATP, which powers movement, thermoregulation, growth and reproduction.
- Not all the branch’s energy is converted into deer biomass — a lot is lost as heat, methane and in undigested material. Also woody, lignified parts of branches are poorly digestible, so deer typically prefer buds, young shoots and leaves, which are higher quality.

So the branch supplies stored plant chemical energy and nutrients; the deer’s digestive system (with microbial help) converts those chemicals into the energy and materials the deer needs.