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.
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.
By chia | January 8, 2009
Insulin is a hormone that is secreted by the pancreas in response to 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).
By chia | January 6, 2009
A myelin sheath is an insulating layer that is wrapped around an axon.
In the peripheral nervous system, each myelin sheath is formed by an individual Schwann cell which wraps itself around the axon length. In the central nervous system, oligodendrocytes send out numerous processes (arms!) to wrap multiple axons. So myelin is actually just layers and layers of phospholipid membrane!
The insulating properties of myelin allow the action potential to be conducted along the axon at high velocity. This is because action potentials are formed only at the points along the axon which are not myelinated (called the nodes of Ranvier). Voltage-gated Na+ channels are located in these gaps. An action potential at one node causes an action potential in the neighboring node, thus “skipping” the myelinated section. This propagation of action potential is called saltatory conduction.
The loss of myelin can cause severe physical problems such as blindness, motor problems, imbalance and speech impairments. Multiple sclerosis is one such disease.
By chia | January 6, 2009
Neurons communicate with each other by releasing chemical messengers called neurotransmitters. Neurotransmitters are found at the end of a neuron axon, called the presynaptic terminal.
These chemicals are released in response to an electrical stimulation (the traveling action potential). Another neuron then detects the neurotransmitters by specific receptors at its postsynaptic membrane.
Activation of these receptors by an excitatory neurotransmitter causes Na+ channels to open, which starts of another action potential in the second neuron.
Activation by an inhibitory neurotransmitter stops the flow of action potential.
- Examples of excitatory neurotransmitters are such as acetylcholine, glutamate, dopamine, epinephrine and norepinephrine.
- An example of an inhibitory neurotransmitter is GABA.
To recap, an electrical flow that reaches the end of a neuron causes the release of neurotransmitters. A second neuron detects the neurotransmitter, which could cause an action potential to start and flow in the second neuron.
* Neurotransmitters are NOT taken up by the second neuron. They are either destroyed or recycled back to the presynaptic terminal of the first neuron, after release from the receptor on the second neuron.
* At the presynaptic terminal: When an action potential reaches the terminal, it opens up voltage-gated Ca2+ channels and causes Ca2+ ions to enter into the terminal of the neuron. Ca2+ then causes the synaptic vesicles to fuse with the presynaptic membrane, thus releasing the neurotransmitter into the synapse.
By chia | January 6, 2009
Amino acids are the building blocks of proteins. Our bodies need amino acids in order to make new cells and enzymes. Out of the 20 types of amino acids that exist, we can make 10. The other 10 must come from our diet.
An amino acid has an amino group (NH2) and a carboxyl group (COOH), and a side chain (R group) that makes one amino acid type different from another.
A chain of amino acids is called a polypeptide. To link up amino acids, an amino group from one amino acid reacts with a carboxyl group of another amino acid, and they are joined by removal of a water molecule (called a dehydration reaction) to form a peptide bond. Amino acids can be further added to the sides of the chain to form a long polypeptide.
By chia | December 16, 2008
Prokaryotes are the oldest living organisms on Earth, dating back to 3.5 billion years ago. They function as single cells; in other words, they are unicellular. Each cell is only about 1 micrometer (μm) in size!
Prokaryotes are part of the kingdoms Archaea & Eubacteria.
Prokaryotes have this basic structure:
Prokaryotes have:
- a plasma membrane (or cell membrane), which controls the movements of molecules in and out of the cell. (read about Membranes)
- a cell wall, which provides the cell with additional support and shape. Some bacteria have a further mucous-like outer layer made of polysaccharides, called an outer membrane or capsule, that protects the cell from phagocytosis.
- projections such as flagella to help them move, and pili for adhering to surfaces and genetic exchange (“sex”).
Inside a prokaryotic cell, there are:
- free DNA. The DNA of prokaryotes is circular, like a rubber band. The region that contains the DNA is called the nucleoid (there is no nucleus!).
- plasmids, which are small DNA circles that contain genes that give the bacteria an advantage (e.g. antibiotic resistance). Plasmids are “read” separately from the chromosomal DNA and are used in genetic engineering.
- free ribosomes that are involved in making proteins (protein synthesis).
Prokaryotes do NOT have any membrane-bound organelles, such as mitochondria or endoplasmic reticulum.
Prokaryotes may have other components in their cytoplasm that allow them to carry out special metabolic processes such as photosynthesis, nitrogen fixation and fermentation.
E. Coli bacteria
This electron microscope image shows rod-shaped Escherichia Coli (E. Coli) bacteria that can be found in our intestines. Some strains are harmless and help us make Vitamin K, but some strains are pathogenic and can cause food poisoning. Each cell is about 2 micrometers (?m) in length. In this image, you can see some cells that are replicating by binary fission, where the cell first replicates its DNA, and then divides into 2 separate cells.