9.1 & 9.2 The Need for Energy and Photosynthesis

Cell Energy Energy is essential to life. All living organisms obtain energy from their environment. Plants trap light energy from the sun while animals eat food to obtain energy. Several cell activities require energy. A quick source of energy for any organelle to access when needed is ATP (adenosine triphosphate). Energy is stored in the chemical bonds of the ATP molecule and can be used quickly and easily.

Forming and Breaking Down ATP ATP is made up of an adenosine molecule and 3 phosphate groups of charged particles. A small amount of energy is required to bond one phosphate group to the adenosine and this molecule is called AMP (adenosine monophosphate). More energy is required to bond the second phosphate group and this molecule is called ADP (adenosine diphosphate). A greater amount of energy is required to bond the third phosphate group (ATP).

Forming and Breaking Down ATP continued… The energy in ATP is available when the bond between the second and third phosphate groups is broken and energy is released. This results in ADP, which can bond to another phosphate group and make ATP again. The formation and breakdown of ATP is cyclic and this recycling process relieves the cell from always needing ATP right away. As long as phosphate groups are available ATP can be formed.

The Formation and Breakdown of ATP

How Cells Use ATP When ATP is broken down and the energy is released cells use this energy for regular cell activities. Cells have proteins that have specific sites where the ATP molecule can bind to it and release its energy for use. This can be compared to batteries being placed into an electronic device to power it. Just like ATP when the energy from the batteries is all used up they can be removed and replaced with new ones.

Uses of Cell Energy Energy is used in cells for activities such as: making new molecules (proteins, enzymes, molecules that build membranes and organelles), maintaining homeostasis, transporting molecules through the plasma membrane, movement, to produce light, etc.

Trapping Energy from Sunlight Green organisms use energy in sunlight and must store it for later use. The process that uses the sun’s energy to make simple sugars is call photosynthesis. The simple sugars are then converted into complex carbohydrates (starches), which store energy. Photosynthesis happens in two phases: light-dependent reactions and light-independent reactions. Light-dependent reactions convert light energy into chemical energy (ATP) and the ATP is used to fuel light-independent reactions to produce simple sugars.

Where Photosynthesis Occurs Photosynthesis occurs in chloroplast The light-dependent reactions take place within the membranes of the thylakoid discs where pigments are stored. Pigments absorb specific wavelengths of sunlight, the most common one being chlorophyll. Chlorophyll is what gives leaves a green color. In the fall leaves stop producing chlorophyll and change color.

Light-Dependent Reactions The first phase of photosynthesis requires sunlight. As sunlight strikes the chlorophyll molecules the energy in the light is transferred to highly energized electrons. The excited electrons are passed to an electron transport chain in the thylakoid membrane. Each protein in the chain passes the electrons to the next protein and energy is lost.

Light-Dependent Reactions Continued… The electrons are re-energized and then passed to a second electron transport chain. The electrons are then transferred to the stroma of the chloroplast by an electron carrier molecule called NADP+ NADP+ can combine with two excited electrons and a hydrogen ion to become NADPH, which stores the energy until it is transferred to a stroma.

Photolysis During the beginning of photosynthesis electrons are lost from chlorophyll molecules when light is absorbed. If these electrons were not replaced the chlorophyll would not be able to absorb light and the light-dependent reaction and production of ATP would stop. Electrons are replaced by a process called photolysis where molecules of water are split to form oxygen, hydrogen ions and 2 electrons. The oxygen produced is released into the air that we breathe, the electrons are returned to the chlorophyll and the hydrogen ions provide energy to make more ATP.

Light-Independent Reactions The second phase, called the Calvin cycle, of photosynthesis does not require light. The Calvin cycle is a series of reactions that use carbon dioxide to form sugars and takes place in the stroma of chloroplast.

The Calvin Cycle

The Calvin Cycle Carbon fixation – one molecule of carbon dioxide is added to one molecule of RuBP to form a six-carbon sugar. In a series of reactions using ATP and NADPH from the light-dependent reactions the six-carbon sugar breaks down into two three-carbon sugars called PGAL. Five PGAL molecules are used to form three RuBP molecules and the sixth PGAL is used to make sugars, complex carbohydrates, and other organic compounds.