NeuroExplorer

Glycolysis can be divided into two phases: the energy investment phase and the energy payoff phase. The energy investment phase requires an input of two molecules of ATP. In the energy payoff phase (a continuation of the energy investment phase), four molecules of ATP are produced for a net total of two ATP during glycolysis.  The steps of the energy payoff phase are explained below. Notice that all of the steps are catalyzed by enzymes. The enzyme triose phosphate dehydrogenase  catalyzes the oxidation of G3P. Electrons are transferred from G3P to NAD+, forming NADH. Using the energy from this exergonic redox reaction, a phosphate group is attached to the oxidized product, forming 1, 3-biphosphoglycerate . The enzyme phosphoglycerokinase  catalyzes the transfer of a phosphate group from  1, 3-biphosphoglycerate  to ADP, forming ATP. This process is known as substrate-level phosphorylation. Phosphoglyceromutase  (an enzyme) relocates the remaining phosphat...

Organic chemistry is the study of molecules containing the element carbon. Carbon is one of the most important elements in living things as it makes up the backbones of many molecules. There are some prefixes that you should know that indicate the number of carbons in a molecule. Meth- indicates one carbon, eth-  indicates two, prop-  indicates three, but-  is four, pent-  is five and hex-  is six. For example, methane has one carbon, ethane has two, propane has three, butane has four, a pentose sugar has five carbons and a hexose has six. A helpful mnemonic to remember these prefixes: meat eaters prefer buttered, peppered ham.                                                                                                ...

Erwin Chargaff was a biochemist who analyzed the base compositions of DNA. His findings became known as Chargaff's rules, which will be explained later. To understand his rules, it is important to first understand DNA structure. DNA consists of units called nucleotides, which are made of three components: a nitrogen-containing (nitrogenous) base, a phosphate group, and the deoxyribose sugar. There are four types of bases: adenine (A), thymine (T), cytosine (C) and guanine (G). Erwin Chargaff made an interesting discovery about the ratio of nitrogenous bases; the number of adenines approximately equaled the number of thymines and the number of cytosines equaled the number of guanines. This discovery constitutes his first rule: 1) For each species, the percentages of A and T bases are equal, as are those of C and G bases. Chargaff also conducted research regarding the base concentrations among different species. His findings led to his second rule: 2) DNA base compositions vary betwe...

Glycolysis is the first step of cellular respiration in which glucose, a six-carbon sugar, is split into two different three-carbon sugars. The three-carbon sugars are then oxidized and rearranged to form two molecules of pyruvate (an ionized form of pyruvic acid). There are two phases in glycolysis: the energy investment phase and the energy payoff phase. The energy investment phase requires an input of two molecules of ATP. In the energy payoff phase, four molecules of ATP are produced for a net total of two ATP during glycolysis. The steps of the energy investment phase are explained below. Notice that all of the steps are catalyzed by enzymes. The enzyme hexokinase  catalyzes the transfer of a phosphate group from ATP to glucose, releasing ADP in the process. Glucose becomes glucose 6-phosphate, which is more chemically reactive than glucose. It is trapped in the cell because its phosphate group carries a negative charge. The enzyme phosphoglucoisomerase  converts glucose ...

Linear electron flow produces ATP and NADPH through a series of reactions that occur in the photosystems embedded in the thylakoid membranes of chloroplasts. Cyclic electron flow is the alternative mechanism to linear electron flow that only produces ATP. Cyclic electron flow is a short circuit that only uses PSI, not PSII. Electrons cycle back from ferredoxin (in the second transport chain) to the cytochrome complex (in the first transport chain). They then travel via the plastocyanin molecule to the P700 chlorophyll in the PSI reaction center complex. By traveling along the 1st transport chain, an electrochemical gradient is produced, which then powers the production of ATP by chemiosmosis. However, there is no NADPH produced because electrons are not transferred to NADP+ via the enzyme NADP+ reductase.

Linear electron flow is the process that produces ATP and NADPH during the light reactions of photosynthesis. These products are then used for the next process, the Calvin cycle . Linear electron flow occurs in the photosystems that are embedded in the thylakoid membrane within a chloroplast. There are two photosystems (PSI and PSII), each made of a reaction-center complex surrounded by a light-harvesting complex , both of which will be discussed later. Photosystem II actually comes before photosystem I (they were named in order of discovery). The following steps describe linear electron flow in detail. A photon of light strikes the light-harvesting complex of photosystem II (PSII). The light-harvesting complex consists of various pigment molecules (e.g. chlorophyll a , chlorophyll b, and carotenoids) bound to proteins. The photon of light boosts an electron to a higher energy state. As it falls back to the ground state, it releases energy. This energy is then passed along to the ne...

The Calvin cycle is a stage of photosynthesis that occurs during the light-independent reactions. This process allows autotrophs to convert carbon dioxide into sugar (an anabolic process). This sugar is not glucose; it is a three-carbon sugar known as glyceraldehyde 3-phosphate (G3P). Three turns of the cycle produce a net total of one molecule of G3P; therefore, three molecules of CO2 are required. The net synthesis of one G3P also requires nine ATP molecules and six NADPH molecules. The ATP and NADPH are produced during the light-dependent reactions. The Calvin cycle can be divided into three phases: carbon fixation, reduction and regeneration. Carbon Fixation A molecule of carbon dioxide is incorporated into a five-carbon sugar known as RuBP (ribulose bisphosphate). This step is catalyzed by the enzyme rubisco, short for RuBP carboxylase-oxygenase. It forms a six-carbon intermediate which is energetically unstable, so it splits in half to form two molecules of 3-phosphoglycerate. Re...

The electron transport chain of cellular respiration is a series of molecules embedded within the inner mitochondrial membrane of a eukaryotic cell's mitochondria. Most of the molecules (electron carriers) are proteins with a tightly bound prosthetic group (a nonprotein component essential for enzymatic function). The molecules exist in multiprotein complexes numbered I to IV. The electron transport chain carries out a series of redox reactions that creates an electrochemical gradient, which then powers the production of ATP by chemiosmosis. A redox reaction occurs as the electron carriers transition from oxidized to reduced states by accepting electrons and then donating them to their neighbors. It is important to note that electronegativity (the measure of a molecule's attraction to electrons) increases as electrons progress through the chain. In other words, the first electron carrier is the least electronegative, and the last electron carrier is the most electronegative. ...

Huntington's Disease (HD) is a neurological disorder that impairs voluntary movement and cognition. Adult onset of the disease is usually in the 30s and 40s, but HD can occur in juveniles. Juvenile HD patients usually die 10 to 15 years after their symptoms appear. Huntington's Disease causes changes in mood, impaired cognition, and motor symptoms (such as chorea , small involuntary movements). Huntington's disease affects the basal ganglia and the cerebral cortex. Huntington's disease can be traced to genetics. It is caused by a dominant inheritance for a mutation of the  huntingtin (HTT) gene  located on chromosome 4, which codes for the huntingtin protein . The mutation involves an abnormal amount of repeats for a three-part snippet of DNA. Normally, people have 10 to 35 repeats of the sequence CAG (cytosine, adenine, guanine). HD patients, however, can have 36 to 120 of these repeats, known as a trinucleotide repeat . Potential HD biomarke...

Epilepsy is a condition characterized by seizures that result from irregular activity in brain cells. These seizures can last for five minutes or more. Most people associate seizures with collapsing, shaking, and losing consciousness. However, some seizures are milder and include staring spells or rapid blinking. Epileptic seizures are classified by their location in the brain.     Generalized seizures  affect both sides of the brain. They include: Absence or petit-mal   seizures cause rapid blinking or a few seconds of staring into space Tonic-clonic or grand-mal   seizures cause muscle spasms, loss of consciousness or suddenly crying out  Local or partial seizures  are localized to one area of the brain. They include Simple focal seizures  cause twitching or changes in sensation Complex focal seizures  can leave a person confused and unable to follow directions. There are also secondary generalized seizures , which b...

Deep brain stimulation (DBS) is a surgical procedure used for patients who do not respond to medication. This technique can be used for patients with neurological disorders, such as Parkinson's disease , to reduce symptoms such as tremors or muscular rigidity. DBS involves implanting a small device (an electrode) similar to a pacemaker that sends electrical signals to interfere with and block abnormal brain signals. The device is connected to a pulse stimulator attached to the back or the chest. Before beginning the procedure, the neurosurgeon determines where to implant the device by imaging the brain using MRI or CT scans. Since Parkinson's patients have damaged neurons in parts of the basal ganglia, (read more) the device is usually placed in the globus pallidus or the subthalamic nucleus (pictured below).                                                ...

The cerebrum is the largest part of the human brain that is divided into two hemispheres. These hemispheres are connected by a bundle of nerve fibers known as the corpus callosum , which allows the hemisphere to communicate with each other. The cerebrum is divided into four lobes that each have a unique function (pictured below). The frontal lobe  is directly above the eyes at the front of the brain. It controls voluntary movement, speech, memory, emotion, and higher cognitive skills such as planning and problem-solving. The parietal lobe , located behind the frontal lobe, receives sensory signals and processes taste. The occipital lobe located at the back of the cerebrum processes visual information and the temporal lobe , located at the side of the brain, interprets auditory information.                                                     ...

Down syndrome is a neurological childhood disorder that is prevalent in about 250,000 people in the United States. Children with Down syndrome have distinctive facial features including a flattened face and bridge of the nose, eyes that slant upward, and small ears. They usually have low to moderate intellectual ability. Down syndrome is caused by an extra copy of the 21st chromosome in a person's cells, known as trisomy 21 . This means that, instead of having two copies of chromosome 21, they have three copies (pictured below). In rare cases, the extra copy may not be present in every cell — a condition known as mosaic down syndrome.  People with mosaic Down syndrome have milder symptoms and a longer life expectancy. There is no clear cause for trisomy 21, although maternal age can be a risk factor. People with Down syndrome are at a higher risk of developing early-onset Alzheimer's disease. Chromosome 21 contains the gene that codes for the amyloid precursor prot...

Parkinson's disease (PD) is a neurodegenerative disease in which patients experience tremors, rigidity, and akinesia (the inability to move). After Alzheimer's, it is the second most common neurodegenerative disease (which involves progressive destruction of nerve cells). Parkinson's disease is characterized by the loss of dopaminergic neurons in the substantia nigra, a region of the basal ganglia in the brain (pictured below). As the name suggests, dopaminergic neurons produce dopamine, a critical neurotransmitter associated with motor control. This loss of dopamine leads to the hallmark symptoms of PD — tremors, muscular rigidity, and slow movement. Over time, the symptoms may worsen as patients begin to experience cognitive decline and develop emotional changes such as depression. The exact cause of Parkinson's is not entirely clear, but a combination of genetics and the environment appears to be the culprit. The disease is linked to a mutation in the PARK...

The central dogma is a fundamental concept of biology. It describes the two step process of gene expression: transcription followed by translation. This allows a protein to be created from a gene. DNA  →   RNA   →  Protein During the first step, the genes in a DNA template are transcribed  to create a single strand of mRNA. Once the mRNA leaves the cell's nucleus, it is translated  into a protein at the ribosome in the cell's cytoplasm.

Paul Broca is one of the most influential neuroscientists best known for his patient, Tan. Well, his name wasn't really Tan. His name was Louis Victor Leborgne, and his incredibly unusual neurological disorder settled a debate about the location of language capabilities in the brain. In 1861, Leborgne approached Broca at the Bicetre Hospital to receive surgery for a leg infection. Leborgne had suffered from several medical conditions prior to his surgery; he had epilepsy at a young age and subsequently lost his ability to produce fluent speech. It was Leborgne's language disorder that really caught Broca's attention. Leborgne could think properly, but whenever he tried to communicate with Broca and verbalize his thoughts, all that came out of his mouth was the meaningless word "tan." For this reason, many scholars of neuroscience simply refer to Leborgne as "Tan". Broca realized that he could learn about our language capabilities by studying Leborg...

Take a look at the image above. This is the chemical structure of caffeine, a widely consumed chemical that you are probably familiar with (it's in your coffee). Scientists use this as a shorthand depiction of chemical structures. While this image may be confusing at first, it's actually very simple to understand once you know the rules! And there are only two! The first rule is carbon at the corners . What does this mean? Well, at every corner where you do not see a letter (an atom), there is an implied carbon. This shorthand notation allows us to show the structure without writing in each single carbon. There is also an implied carbon at the end of every line. Using this rule, let's place carbons at the corners and the ends of the lines. Now that we have the carbons in place, let's move to the next rule: hydrogens bonded to carbons are implied.   To understand this rule, we must first understand the bonding properties of carbon. Carbon likes to make fou...

Let's zoom into the synapse that makes neurotransmission possible. Action potentials travel in the direction from the dendrite to the axon. Therefore, in order to transmit signals from one neuron to another, the signal must leave the axon of one neuron and cross the synaptic cleft to reach the receiving dendrite of another neuron. The neuron that delivers the signal is known as the presynaptic neuron , as shown in the image below. The neuron receiving the signal is known as the postsynaptic neuron . The gap between both neurons is the synaptic cleft. Now, don't confuse the terms "synapse" and "synaptic cleft". The synapse includes the presynaptic neuron, the postsynaptic neuron, and the gap in between. The gap is known as the synaptic cleft . Electrical signals travel as action potentials through the axon. When they reach the axon terminal , which is the very end of the axon, neurotransmitters are released into the synaptic cleft. The neurotran...

As we learned about in the blog post about the  Brain Box , the brain is protected by the skull, meninges, and cerebrospinal fluid (CSF). The meninges, which sit below the skull and vertebral column, is a series of three membranes that surround the brain and spinal cord. Its function is to protect and support the brain and spinal cord and contain cerebrospinal fluid (CSF). The three layers of the meninges (from outermost to innermost) are the dura mater, arachnoid mater and pia mater. The dura mater  is a thick, tough layer that adheres to the skull on one side and the arachnoid mater on the other. It is an extra protective layer that attaches the brain to the skull and the spinal cord to the vertebral column. Beneath the dura mater is the arachnoid mater. The arachnoid mater  is named after its appearance that resembles a cobweb. It is made of strands of connective tissue, known as arachnoid trabeculae , that suspend the brain in place. Between the arachno...

The ventricles are cavities throughout the brain that produce and distribute cerebrospinal fluid. Cerebrospinal fluid (CSF) is a clear, colorless fluid that suspends the brain and protects it from strain. Check out this blog post to learn more about cerebrospinal fluid. The ventricles are lined with the choroid plexus , a membrane made of ependymal cells (a glial cell) that secrete CSF. There are four ventricles in the human brain. There are two C-shaped lateral ventricles ; one in each of the hemispheres. The lateral ventricles connect to the third ventricle via an opening known as the interventricular foramen . The third ventricle , which resembles a misshapen donut, is located along the midline of the diencephalon. It connects to the fourth ventricle via the cerebral aqueduct. The fourth ventricle is located between the cerebellum and brainstem. It has three openings that allow the CSF to enter the subarachnoid space (remember the meninges). Therefore, the CSF leaves the ...