How Plants Make Food (or Photosynthesis)

Introduction

This lecture we will be talking about how plants make the food we eat. Scientifically, this topic goes by the name photosynthesis. We could spend a great deal of time going into the very detailed reactions and chemical names associated with the reactants. However, I would like to help you understand the concepts involved in the process rather than have you memorize a detailed chain of reactions. If you do find you need to know this subject in more detail, guess what, you can do a web search for photosynthesis and find the information you need.

Carbon Cycle

Let's start today's lecture on how plants make food, by looking at this figure which depicts the Carbon Cycle. As depicted in the figure, in the atmosphere, carbon is found as carbon dioxide. The process of photosynthesis utilizes carbon dioxide to create sugar. In a previous lecture, we discussed how a process called aerobic respiration returns carbon dioxide to the atmosphere.

Photosynthetic autotrophs or producers (plant, protistan, and bacterial members) harness light energy. They are "self-nourishing" and obtain carbon from carbon dioxide.

Heterotrophs or consumers feed on autotrophs, each other, and on organic wastes. Heterotrophs acquire carbon and energy from autotrophs. Heterotrophs include animals, protistans, bacteria, and fungi.

This animation (Audio - Important) describes the carbon cycle.

This animation describes the linkeage of photosynthesis and respiration

Photosynthesis

In photosynthesis, there are two series of reactions which occur in the chloroplasts of plant cells

  1. The light-dependent reactions occur in the grana (stack of thylakoids) of the chloroplasts and convert light energy to chemical energy. This chemical energy is stored in two nucleotides called ATP and NADPH. These energy carrying molecules are used in the second series of reactions. The thylakoids, are flattened sacs, interconnected by channels that weave through the semifluid interior (stroma).

  2. The light-independent or dark reactions occur in the stroma of the chloroplasts and assemble sugars and other organic molecules using ATP and NADPH as energy sources.

This figure summarizes the light and dark reactions.

The equation for photosynthesis which occurs in the leaves of plants is:

6H2O + 6CO2 ----------> C6H12O6+ 6O2

In plain English, this translates into: six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen.

In the chloroplast, the newly formed glucose has a phosphate group attached. The glucose phosphate molecules react further to form sucrose, starch, and cellulose.

This animation (Audio - Important) gives an overview of photosynthesis.

REVIEW: The internal membrane system of the chloroplast is called a

REVIEW: Thylakoid disks are

REVIEW: Stroma and grana are portions of

REVIEW: What do plants need to carry on photosynthesis?

REVIEW: Organisms that derive their chemical energy either from the process of chemosynthesis or photosynthesis are classified as

REVIEW: The carbon source for organisms that derive their energy from photosynthesis is

REVIEW: Most carbon enters the web of life through

REVIEW: The oxygen released in photosynthesis comes from

REVIEW: Plants need which of the following to carry on photosynthesis?

REVIEW: Four of the five answers listed below are heterotrophs. Select the exception.
a. fungus
b. carrot
c. earthworm
d. lobster
e. parasite

REVIEW: The ultimate source of energy for living things is the

REVIEW: How many membranes surround a chloroplast from a plant cell?

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