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
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.