Large particles cannot diffuse accross the cell membrane. They enter the cell through a process called endocytosis.
The particle pushes against the membrane and forms an indentation. The cell membrane wraps around the particle and engulfs it into a vacuole.
Exocytosis is a process by which the cell expels the contents of a vacuole.
The vacuole fuses with the cell membrane and the contents (usually waste products) are pushed into the external environment.
Phagocytosis refers to the endocytosis of a solid particle.
Pinocytosis refers to teh endocytosis of a liquid and its dissolved molecules.
Energy, exergonic, and endergonic reactions
Energy is the ability to cause a change.
Energy comes in many forms and can easily change from one form to another.
To perform processes, cells must use energy. They store this energy in molecular bonds; it is called chemical energy.
When molecular bonds are broken, energy is released which can be used to power various reactions.
Exergonic reactions result in the release of energy.
They usually occur spontaneously since only a minimal input of energy (the activation energy) is required to start the reaction.
Endergonic reactions absorb energy, so they do not occur spontaneously.
They can only occur when enough energy is available to fulfill the reaction's activation energy.
How cells store energy: ATP
Cells store chemical energy in a molecule called adenosine triphosphate (ATP).
An ATP molecule has three phosphate groups. Energy which can power an endergonic reaction is released as the bonds which hold the phosphate groups together are broken.
When food molecules are broken down, energy is released which cells use to form ATP molecules. In this way, energy is stored for later use.
Aerobic organisms create ATP much more efficiently than do anaerobes.
Glycolysis occurs in the cytoplasm of all cells.
The reactants of glycolysis are a molecule of glucose, two molecules of NAD-, and two molecules of ATP.
The products of glycolysis are two molecules of pyruvic acid, two molecules of NADH, and four molecules of ATP.
In anaerobes, the pyruvic acid enters fermentation, whereas it enters the Krebs cycle in aerobes.
Anerobic processes occur in anaerobes all the time and in aerobes when there is not sufficient oxygen for an aerobic process.
Carbon dioxide is removed from the pyruvic acid molecules, forming acetaldehyde.
NADH (formed during glycolysis) releases its hydrogen atom to the acetaldehyde. The NADH molecule becomes NAD+, and the acetaldehyde becomes ethyl alcohol.
The NAD+ molecule formed is then used in glycolysis so that more ATP can be synthesized.
In a second anaerobic process, the NADH releases its hydrogen directly to the pyruvic acid molecule, forming lactic acid.
The Krebs cycle and electron transport chain
The Krebs cycle
The Krebs cycle occurs in the mitochondria of aerobes.
A molecule of carbon dioxide is removed from each pyruvic acid molecule (forming acetyl coenzyme A) before they enter the Krebs cycle.
The cycle begins when the acetyl coenzyme A bonds with oxaloacetic acid.
A series of reactions ensue, producing ATP, NADH, FADH2, and oxaloacetic acid.
The oxaloacetic acid formed is used in the next turn of the Krebs cycle.
The electron transport chain
The molecules of NADH FADH2 were given high energy electrons during glycolysis and the Krebs cycle.
The electrons are passed to carrier molecules, releasing energy during the process.
At some steps in the chain, more energy is released than in others. This energy is used to power chemiosmosis.
Overview of chemiosmosis
The theory of chemiosmosis was proposed in 1961 by Peter Mitchell.
It explains how ATP is synthesized from the energy released in the electron transport chain.
Chemiosmosis occurs at the cell membrane in prokaryotes, at the crystae in heterotrophic eukaryotes, and at the thylakoid membrane in autotrophic eukaryotes.
Steps in chemiosmosis
Carrier molecules use the energy from the electron transport chain to separate H+ ions and electrons and to move the H+ ions accross the membrane.
A pH gradient and an electrical gradient forms accross the membrane as the H+ ions accumulate on one side.
Eventually, the H+ ions gush through the F0 channel into the F1 unit where ATP synthetase it located.
Using the energy from the movement of the H+ ions, ATP sythetase drives together ADP and a phosphate group to form ATP.
Energy yield for aerobic respiration
For each molecule of glucose, glycolysis produces a net gain of 2 ATP molecules directly.
2 NADH molecules are formed during glycolysis, providing the energy for the production of 3 ATP molecules each through the electron transport chain.
8 molecules of ATP total are produced through glycolysis.
Oxidation of pyruvic acid
For each molecule of pyruvic acid which is oxidized, one molecule of NADH is formed, resulting in the production of 3 ATP.
Since two pyruvic acid molecules are formed for each molecule of glucose, 6 ATP molecules are formed through the oxidation of pyruvic acid.
The Krebs cycle
For each turn of the Krebs cycle, 1 molecule of ATP is formed directly.
3 molecules of NADH are formed during one turn of the Krebs cycle, resulting in the production of 9 ATP molecules through the electron transport chain.
1 molecule of FADH2 is formed for each turn of the Krebs cycle, providing the energy for the synthesis of 2 ATP molecules.
The Krebs cycle occurs twice for each glucose molecule which enters glycolysis, so the total number of ATP formed through the Krebs cycle is 24.
The total number of ATP molecules formed from the breakdown of one glucose molecule is 38.
Overview of photosynthesis
Photosynthesis is the process which autotrophs use to synthesize their own food.
The reaction requires light energy, carbon dioxide, and water.
The products of photosynthesis are oxygen and glyceraldehyde phosphate.
Noncyclic electron flow and the Calvin cycle
Noncyclic electron flow
Light energy enters a cluster of pigments called Photosystem II.
The energy is absorbed by a chlorophyll a molecule and passed to a carrier molecule.
The electron is passed down through several carrier molecules, releasing energy which is used to form ATP.
The electron transfers its remaining energy to Photosystem I. This and additional light energy is then transferred to another electron.
This electron is passed from carrier to carrier, eventually being accepted by an NADP+ molecule, forming NADPH.
Photosynthesis occurs within the thylakoids of a chloroplast.
The Calvin cycle
The Calvin cycle begins when a molecule of carbon dioxide bonds with the five-carbon sugar ribulose diphosphate.
A series of reactions ensue (which involve the NADPH molecule from noncyclic electron flow and convert it back to an NADP+ molecule), producing another molecule of ribulose diphosphate and a molecule of glyceraldehyde phosphate.
Glyceraldehyde phosphate can be used in the formation of sugars, proteins, and lipids.
Terms to know
activation energy - The input of energy required for certain exergonic and all endergonic reactions to occur.
ATP - Stands for adenosine triphosphate. Cells store energy used to power their processes in the high energy phosphate bonds of ATP molecules.
chemiosmosis - A process which produces ATP for a cell by utilizing energy from a pH and electrical gradient formed from the transport of H+ ions across a membrane.
electron transport chain - The passage of electrons from NADH and FADH2 to carrier molecules. This process results in the release of energy which is harnessed to form ATP.
endergonic reaction - A chemical reaction which requires energy to form chemical bonds.
endocytosis - The engulfing of a large particle by the cell membrane.
energy - The ability to cause a change. It can come in many forms and can easily change from one form to another.
exergonic reaction - A chemical reaction which releases energy, usually by the breaking of molecular bonds.
exocytosis - The fusion of a vacuole with the cell membrane, causing the expulsion of the contents of the vacuole into the external environment.
fermentation - An anaerobic pathway following glycolysis which produces ethanol from glucose and provides NAD+ for glycolysis.
glycolysis - The breaking down of glucose into two molecules of pyruvic acid. Glycolysis is the first step of cellular respiration.
Krebs cycle - A series of reactions which occurs in the mitochondria, resulting in the formation of ATP. The cycle continually regenerates oxaloacetic acid, bonding it with acetyl coenzyme A at the start of each cycle.
noncyclic electron flow - The first stage of photosynthesis, in which light energy is captured by a photosystem which is transmitted to an electron and eventually stored in the bond energy of an NADPH molecule.
phagocytosis - Endocytosis of a solid particle into a vacuole.
photophosphorylation - The chemiosmotic process utilized by autotrophs in the formation of ATP. It occurs at the membranes of the thylakoids.
photosynthesis - The process by which autotrophs produce their own food. Light, carbon dioxide, and water are required for the reaction, resulting in the production of PGAL and oxygen.
photosystem - A cluster of pigment molecules which transfer light energy to an electron.
pinocytosis - Endocytosis of liquid and dissolved molecules into a vacuole.