Sunlight as the Source of Energy.
For photosynthetic cells, the main energy source is Sun.
The cell is the basic unit of life. All organisms are made of cells. In the hierarchy of biological organization, the cell is the simplest collection of matter that can live. There are diverse forms of life existing as single–celled organisms. More complex organisms, including plants and animals, are multicellular; their bodies are cooperatives of many kinds of specialized cells that could not survive for long on their own. However, even when they are arranged into higher levels of organization, such as tissues and organs, cells can be singled out as the organism\'s basic units of structure and function.
The contraction of muscle cells moves your eyes as you read this sentence; when you decide to turn to next page, nerve cells will transmit that decision from your brain to the muscle cells of your hand. Everything an organism does occurs fundamentally at the cellular level. Life at the cellular level arises from structural order, reinforcing the themes of emergent properties and the correlation between structure and function. For example, the movement of an animal cell depends on an intricate interplay of the structures that make up a cellular skeleton. The study of energy through living systems begins with the cell and how energy is obtained and used. In this unit we study major ideas: metabolism, respiration and photosynthesis.
Cells manage a wide range of functions in their tiny package – growing, moving, housekeeping, and so on – and most of those functions require energy. Cells get this energy and use it in the most efficient manner. Cells, like humans, cannot generate energy without locating a source in their environment. However, whereas humans search for substances like fossil fuels to power their homes and businesses, cells seek their energy in the form of food molecules or sunlight. In fact, the Sun is the ultimate source of energy for almost all cells, because photosynthetic prokaryotes, algae, and plant cells harness solar energy and use it to make complex organic food molecules. Other cells rely on these food molecules for the energy required to sustain growth, metabolism, and reproduction.