Electron Transport Chain

By chia | February 26, 2009

(or Oxidative Phosphorylation)

The final part of cellular respiration is called the electron transport chain, where oxidative phosphorylation occurs. Oxidative phosphorylation refers to phosphorylation of ADP to ATP using energy released by oxidation of NADH + H+.

The electron transport chain takes place on the inner mitrochondrial membrane, using NADH + H+ made by glycolysis and the Kreb’s cycle.

The transport of electrons from one electron carrier to another establishes a concentration gradient of H+

NADH + H+ goes from the mitochondrial matrix to the inner membrane and passes 2 electrons to the first electron carrier. This releases some energy that allows 2 protons (H+) to be transferred across the inner mitochondrial membrane into the intermembrane space. NAD+ is recycled back to the matrix for use in glycolysis and the Kreb’s cycle.

The electrons are passed from one carrier to another, and each time this happens, more energy is released. This energy is used to pump more protons (H+) into the intermembrane space. The concentration of H+ is therefore much higher in the intermembrane space compared to the matrix, resulting in a concentration gradient of H+.

Oxygen is required for aerobic respiration

The electrons are finally passed to a terminal electron acceptor: oxygen.
O + 2e- –> O2-

The oxygen ion very quickly reacts with H+ in the matrix to form water.
O2- + 2H+ –> H2O

(This is the reason why we need to breathe oxygen, and also why we breathe out carbon dioxide and water vapor!)

Without oxygen, the electron transport chain cannot occur and ATP can only be produced anaerobically.

H+ concentration gradients allow synthesis of ATP

Back to the H+ concentration gradient….. This is the important step in ATP production. Embedded in the inner mitrochondrial membrane are ATP synthase enzymes. H+ ions go from the intermembrane space (high H+ concentration) into the ATP synthase structure and is released into the mitochondrial matrix (low H+ concentration). ATP synthase uses this energy to combine ADP and inorganic phosphate (Pi) to form ATP ( 2 H+s are needed to make 1 ATP). This flow of H+ is also known as chemiosmosis.

Additional notes:

  • 1 NADH  can pump 6 H+s into the intermembrane space. Since 2 H+s are needed to make 1 ATP, 1 NADH is able to produce 3 ATPs.
  • FADH2 can pump 4 H+s. Therefore each FADH2 can produce 2ATPs.

Animations

2 excellent animations have been produced by the NDSU Virtual Cell Lab:

The electron transport chain

ATP Synthase: How H+ concentration gradients lead to ATP synthesis

These videos are quite large (~30MB) so if you don’t have a really fast internet connection, you could check out these simpler animations:

Anaerobic respiration

By chia | February 26, 2009

The first phase of cellular respiration, glycolysis, is said to be anaerobic. Anaerobic means it doesn’t need oxygen (the opposite is aerobic - requiring oxygen).

During intense exercise, oxygen in the blood gets used up quickly and sometimes there isn’t enough. The lack of oxygen blocks oxidative phosphorylation (see electron transport chain). NADH + H+ that was made during glycolysis and the Kreb’s cycle cannot be oxidized and remains in the matrix of the mitochondria.

For muscle cells to continue working, ATP is needed, and the only way to make it without oxygen is by glycolysis.

However, remember that glycolysis also needs free NAD+. NAD+ is in limited supply in the cell so there needs to be a way to convert NADH into NAD+. The regeneration of NAD+ can be done by converting pyruvate into lactic acid, or lactate — a process known as lactic acid fermentation. The sharp pains you sometimes feel during exercise is due to lactic acid build up in your muscles.

Some bacteria can also use lactose (sugar found in milk) for anaerobic respiration, producing lactic acid as a waste product. Lactic acid reduces the pH of the milk and causes denaturation of proteins in the milk. Yoghurt and cheese production use such bacteria!

Some eukaryotic cells like yeast also carry out anaerobic respiration. They convert glucose into pyruvate for ATP, and pyruvate is then converted into ethanol and carbon dioxide. This process is called ethanol fermentation and is the key step in brewing wine and beer. Yeast is also used in bread-making, where the carbon dioxide gas produced causes the dough to rise.

Electron carriers

By chia | February 26, 2009

Electron carriers in cells accept and give away electrons to facilitate reduction (gain of electrons) and oxidation (loss of electrons). They are crucial in cell respiration and are found on the inner membrane of mitochondria and chloroplasts (the energy producing organelles of animals and plant cells respectively).

NAD (Nicotinamide Adenine Dinucleotide) is the main electron carrier in animal cell respiration, and phosphorylated NAD (NADP) in photosynthesis of plant cells.

NAD exists as positively-charged in cells : NAD+
NAD+ can be reduced by removing 2 hydrogen atoms from an oxidized substance. (Remember, a hydrogen atom is one proton and one electron.) NAD+ accepts 1 proton and 2 electrons from the 2 hydrogen atoms, and releases one of the protons (H+).

The overall reduction of NAD+ can be shown as:

NAD+ + 2H+ + 2e- —> NADH + H+

The electrons remove the positive charge of NAD+.

NAD+ reduction takes place in glycolysis and the Kreb’s cycle (electrons are added), and NADH oxidation (removal of electrons) occur in the electron transport chain to produce ATP for the cell.

Cellular Respiration

By chia | February 2, 2009

In order for a cell to carry out its functions, it requires energy in the form of ATP. Cellular respiration is the way cells make this ATP, using food as its fuel. Glucose is the most common starting point for cellular respiration (although other sugars, fats and proteins can also be used).

The overall equation for cellular respiration is

Glucose + Oxygen –> Carbon Dioxide + Water + Energy

C6H12O6 + O2 –> CO2 + H2O + ATP

The energy released from glucose is step-wise, releasing small amounts of energy each time that can be used. If we released all the energy contained in glucose it would be too much for the cells to handle!

Cellular respiration involves 3 different stages:
1 Glycolysis
2 The Kreb’s cycle (or citric acid cycle)
3 The electron transport chain

Glycolysis

By chia | February 2, 2009

Glycolysis is the breaking down (lysis) of glucose (glyco) into pyruvate (also known as pyruvic acid).

It is the first part of cellular respiration, where cells use glucose as fuel for energy (making ATP).
Glycolysis takes place in the cytoplasm of the cell, and some energy is made in the process. This part of respiration does not need oxygen, so it is anaerobic.

Glycolysis converts glucose (a 6-carbon molecule) into pyruvate (a 3-carbon molecule) in a few steps.

Step 1

Glucose is first phosphorylated (phosphate group, P, added) by a kinase enzyme, to Glucose-6-phosphate, using 1 ATP molecule. (Read about ATP and ADP)

Step 2

Glucose-6-phosphate is then rearranged (by an isomerase enzyme) into fructose-6-phosphate.

Step 3

Another ATP molecule is used to add a second P to fructose-6-phosphate, making fructose-1,6-bisphosphate.

Step 4

Fructose-1,6-bisphosphate (6 carbons) is not stable, so it divides into 2 molecules : glyceraldehyde 3-phosphate (3 carbons)

Now comes the “pay-off phase” where ATP is actually made, instead of consumed!

Step 5

Each 3-carbon molecule undergoes phosphorylation (this time with inorganic phosphate) and a hydrogen is removed from each molecule. An oxygen carrier, NAD+ takes this hydrogen and forms NADH + H+. (NADH produced here goes to the electron transfer chain of cellular respiration). 2 NADHs are produced per glucose (since 1 glucose –> 2 glyceraldehyde 3-phosphates).

Step 6

The P that was just added is enzymatically transferred to ADP, forming ATP. This step is called substrate-level phosphorylation. ADP is required for this step to happen! 2 ATPs are produced per glucose molecule.

Step 7

Water is also removed from each 3-carbon molecule.

Step 8

A final step converts the 3-carbon molecule into pyruvate, by transferring the last phosphate group to ADP, forming another ATP.

So in summary:

2 ATPs are first used to “activate” glucose.

In the pay-off phase:

4 ATPs
2 H2Os
and 2 NADH + H+s
are made.

Therefore, on the whole:
glycolysis forms 2 ATPs, 2 water molecules and 2 NADH+ H+s (which makes another 4 ATPs in the electron transfer chain).

If you want to visualise these steps better, check out some of these animations!
1. A simple animation by Graham Kent of Smith College
2. Another good animation, with short quiz from Anatomy & Physiology, 7e, by Seeley, Stephans & Tate

Disaccharides

By chia | January 28, 2009

Disaccharides are formed when 2 monosaccharides are linked together by a glycosidic linkage.

Maltose used for beer brewing is glucose + glucose.
Lactose, found in milk, is glucose + galactose.
Sucrose, table sugar, is actually glucose + fructose.

Plants (including fruits and vegetables) usually carry carbohydrates from their leaves to other parts in the form of sucrose.

When we eat sucrose, it is quickly broken down into glucose and fructose, causing a rapid rise in blood glucose levels (not good for diabetics!) Polysaccharides (starch) is broken down much more slowly, so it helps us maintain steady blood glucose levels (and keeps us feeling fuller for longer!).

Polysaccharides

By chia | January 28, 2009

Polysaccharides are polymers of sugars, where hundreds or thousands of monosaccharides are joined together by glycosidic linkages.

A glycosidic linkage is a covalent bond that is formed between two monosaccharides by a dehydration reaction — the removal of a water molecule (H + OH).

Polysaccharides are used for:

1. Storage

In plant cells, excess glucose made by photosynthesis is turned into starch.

In animal cells, excess glucose eaten is stored as glycogen.

We (and animals!) can get our source of sugars by eating starch. We have enzymes that can break down plant starch and turn it into glucose for energy.

2. Structural material

Plant cells have tough cell walls that contain cellulose. Like starch, cellulose is also a polymer of glucose. The difference is that it uses a different form of glucose — beta-glucose. When linked into a chain by beta-1,4-glycosidic linkages, beta-glucose forms a straight chain that can form hydrogen bonds with a neighboring chain. This makes it really strong!

Humans don’t have enzymes that can hydrolyze (break) these beta-linkages, which is why we can’t digest cellulose (dietary fiber). Cows, however, have bacteria in their stomachs that can do this, so they can use the nutrients of the cellulose they eat.

Certain animals (arthropods: insects, spiders, crustaceans…) also use a structural polysaccharide, chitin, to form their exoskeleton.

Glycogen

By chia | January 28, 2009

Glycogen is a polysaccharide consisting of glucose. It is a branched polymer — that means it is not linear/straight but has long chains of glucose coming off the sides of another glucose chain.

Animal cells store glucose in the form of glycogen. After eating, glucose levels in the blood increase, causing insulin to be released. Glucose is then taken up by muscle and liver cells, and converted into glycogen granules (glycogenesis).

When energy is needed (exercise), or when blood sugar levels are low (between meals/fasting), glycogen is broken back down into glucose for use, or released back into the blood.

Glucose

By chia | January 8, 2009

Glucose is a simple sugar, a monosaccharide, that has the molecular formula C6 H12 O6. Other forms of monosaccharides are such as fructose and galactose.

Glucose is used by our bodies as fuel for energy; they are the major starting points for cellular respiration which produces ATP.

When glucose levels in the blood are high, they are converted into glycogen in the liver and muscles (glycogenesis), and also stored as fat in adipose tissue. Insulin controls the levels of glucose in the body to maintain homeostasis.

Diabetes mellitus is a metabolic disease which occurs when glucose levels in the blood are too high due to ineffective insulin action or low insulin production.

Insulin

By chia | January 8, 2009

Insulin is a hormone that is secreted by the pancreas in response to glucose in the blood. More specifically, they are produced by beta-cells which are found in an area of the pancreas called the Islets of Langerhans.

When blood glucose levels go above a set point (90mg/100ml), for example after eating carbohydrates, the beta-cells detect this and release insulin into the blood circulation.

Insulin acts on tissues in the body, especially the liver, muscles and fat (adipose tissue), and causes them to take up glucose. After glucose enters the cell, it is converted into glycogen (a process called glycogenesis) and stored.

A low blood glucose level, for example during exercise or fasting, causes glycogen to be broken down back into glucose (glycogenolysis).

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