Energy in Cells: How Do the Cells Get Energy?
I. Food for Energy
A. Food is used in the cell as energy to perform its life functions (growth, movement, reproduction,
response to stimuli, and metabolism).
1. metabolism = the total of all the chemical changes that take place in an organism.
II. Obtaining Food: Producers and Consumers
A. The sun is the ultimate source of energy of all living things.
B. producers (also called autotrophs) = organisms that can make food using the process of
photosynthesis.
1. During photosynthesis, the chlorophyll (usually in chloroplasts) absorbs energy from the sun and
uses this energy to break apart water into hydrogen and oxygen.
a. The hydrogen combines with the carbon dioxide to form glucose and oxygen is given off as a
waste product.
b. The energy from the sun is now trapped in the chemical bonds in the glucose molecule.
2. The equation for photosynthesis is
6 CO2
+
6 H2O + energy
C6H12O6
+ 6O2
(carbon dioxide)
(water)
(light)
(glucose sugar)
(oxygen)
3. Producers use some of the food they make to get energy to perform their life functions.
a. They store the rest as carbohydrates and lipids.
C. consumers (also called heterotrophs) = organisms that cannot make their own food.
1. To get energy from the sun, consumers must eat the producers (plants or plant products) or other
consumers that feed on producers.
a. The relationship of “who eats what” is a food chain.
1. Food chains always start with producers.
2. The energy moves up the food chain as one organism is eaten by another.
III. Releasing the Energy in Food
A. To get the energy out of the food, all organisms (including plants) must use cellular respiration.
1. cellular respiration = the process in which glucose and oxygen enter the mitochondria where they
combine, breaking down the chemical bonds in the glucose to release the energy in the form of
ATP molecules.
a. Carbon dioxide and water are given off as wastes.
2. The equation for cellular respiration is
C6H12O6
+
6O2
6 CO2
+ 6 H2O
+
energy
(glucose sugar)
(oxygen)
(carbon dioxide)
(water)
(ATP)
B. If the cells begin to run out of oxygen, they must switch to fermentation.
1. fermentation = the release of energy from glucose without oxygen (takes place in the cytoplasm).
a. Only small amounts of energy are released.
1. Cellular respiration releases 18 times more energy from a food molecule than fermentation
does and is, therefore, a much more efficient process for releasing energy.
b. Carbon dioxide and lactic acid are released as the waste products of fermentation.
1. Lactic acid is what makes your muscles sore when you work them for a long period of time.
2. Yeast and some bacteria use fermentation all of the time.
a. Carbon dioxide and alcohol are released as waste products in these organisms.
1. Bubbles of carbon dioxide are what make bread rise and create the “holes” in bread.
2. The alcohol content in beer comes from the fermentation of the sugars in the grains used to as
ingredients to make the beer.