Thylakoid membranes contain chlorophyll and other pigments that capture light so plants can conduct photosynthesis. The thylakoids are arranged in stacks inside the chloroplasts of a plant, with each stack connected to the others by a system of tubules.
The chlorophyll in the thylakoid membrane absorbs the red and blue spectrum of light, enabling the eye to see the unabsorbed green light. The absorbed light is turned into chemical energy in a process known as a light reaction. Once energy is harvested from the sun by the thylakoid membrane, the plant proceeds to use this energy for a process known as a dark reaction. During the dark reaction, carbon dioxide is converted into sugar in another part of the chloroplast known as the stroma.
The sugar produced by plants during photosynthesis is necessary for the process of cellular respiration. Cellular respiration converts the sugar made during photosynthesis into energy needed for cells to carry out their daily activities. Both plants and animals need to perform cellular respiration. Though plants can get the energy they need from the sugar they make, animals do not have chloroplasts and cannot make their own sugar. They must get the sugar they need for cellular respiration from eating plants or other animals.