Jul 27, 2011

Double Fertilisation

After landing on a receptive stigma, a pollen grain absorbs moisture and germinates; that is, it produces a pollen tube that extends down between the cells of the style toward the ovary.

The nucleus of the generative cell divides by mitosis and forms two sperm. Directed by a chemical attractant, possibly calcium, the tip of the pollen tube enters the ovary, probes through the micropyle (a gap in the integuments of the ovule), and discharges its two sperm near or within the embryo sac.

The events that follow are a distinctive feature of the angiosperm life cycle. One sperm fertilises the egg to form the zygote. The other sperm combines with the two polar nuclei to form a triploid (3n) nucleus in the centre of the large central cell of the embryo sac. This large cell will give rise to the endosperm, a food–storing tissue of the seed. The union of two sperm cells with different nuclei of the embryo sac is called double fertilisation. Double fertilisation ensures that the endosperm will develop only in ovules where the egg has been fertilised, thereby preventing angiosperms from squandering nutrients.

The tissues surrounding the embryo sac have prevented researchers from being able to directly observe fertilisation in plants grown under normal conditions. Recently, however, scientists have isolated sperm from germinated pollen grains and eggs from embryo sacs and have observed the merging of plant gametes in vitro (in an artificial environment). The first cellular event that takes place after gamete fusion is an increase in the cytoplasmic calcium (Ca2+) levels of the egg, as also occurs during animal gamete fusion. Another similarity to animals is the establishment of a block to polyspermy, the fertilisation of an egg by more than one sperm cell. Thus, maize (Zea mays ) sperm cannot fuse with zygotes in vitro. In maize, this barrier to polyspermy is established as early as 45 seconds after the initial sperm fusion with the egg.

From Ovule to Seed
After double fertilisation, each ovule develops into a seed, and the ovary develops into a fruit enclosing the seed(s). As the embryo develops from the zygote, the seed stockpiles proteins, oils, and starch to varying extents, depending on the species. This is why seeds are such major sugar sinks. Initially, these nutrients are stored in the endosperm, but later in seed development in many species, the storage function of the endosperm is more or less taken over by the swelling cotyledons of the embryo.

Endosperm Development
Endosperm development usually precedes embryo development. After double fertilisation, the triploid nucleus of the ovule’s central cell divides, forming a multinucleate “supercell” having a milky consistency. This liquid mass, the endosperm, becomes multicellular when cytokinesis partitions the cytoplasm by forming membranes between the nuclei. Eventually, these “naked” cells produce cell walls, and the endosperm becomes solid. Coconut “milk” is an example of liquid endosperm; coconut “meat” is an example of solid endosperm. The white fluffy part of popcorn is also solid endosperm.

In grains and most other monocots, as well as many eudicots, the endosperm stores nutrients that can be used by the seedling after germination. In other eudicots (including bean seeds), the food reserves of the endosperm are completely exported to the cotyledons before the seed completes its development; consequently, the mature seed lacks endosperm.

Digestive System

Each organ of the mammalian digestive system has specialised food–processing functions.

The general principles of food processing are similar for a diversity of animals, so we can use the digestive system of mammals as a representative example. The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices into the canal through ducts.

After food is chewed and swallowed, it takes only 5–10 seconds for it to pass down the oesophagus and into the stomach, where it spends 2–6 hours being partially digested. Final digestion and nutrient absorption occur in the small intestine over a period of 5–6 hours. In 12–24 hours, any undigested material passes through the large intestine, and faeces are expelled through the anus.

The general principles of food processing are similar for a diversity of animals, so we can use the digestive system of mammals as a representative example. The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices into the canal through ducts.

Peristalsis, rhythmic waves of contraction by smooth muscles in the wall of the canal, pushes the food along the tract. At some of the junctions between specialised segments of the digestive tube, the muscular layer is modified into ringlike valves called sphincters, which close off the tube like drawstrings, regulating the passage of material between chambers of the canal. The accessory glands of the mammalian digestive system are three pairs of salivary glands, the pancreas, the liver and the gallbladder, which stores a digestive juice.

Using the human digestive system as a model, let′s now follow a meal through the alimentary canal, examining in more detail what happens to the food in each of the processing stations along the way.

The Oral Cavity, Pharynx, and Oesophagus
Both physical and chemical digestion of food begin in the mouth. During chewing, teeth of various shapes cut, smash, and grind food, making it easier to swallow and increasing its surface area. The presence of food in the oral cavity triggers a nervous reflex that causes the salivary glands to deliver saliva through ducts to the oral cavity. Even before food is actually in the mouth, salivation may occur in anticipation because of learned associations between eating and the time of day, cooking odours, or other stimuli. Humans secrete more than a liter of saliva each day.

Saliva contains a slippery glycoprotein (carbohydrate–protein complex) called mucin, which protects the lining of the mouth from abrasion and lubricates food for easier swallowing. Saliva also contains buffers that help prevent tooth decay by neutralising acid in the mouth. Antibacterial agents in saliva kill many of the bacteria that enter the mouth with food.

Chemical digestion of carbohydrates, a main source of chemical energy, begins in the oral cavity. Saliva contains salivary amylase, an enzyme that hydrolyses starch (a glucose polymer from plants) and glycogen (a glucose polymer from animals). The main products of this enzyme′s action are smaller polysaccharides and the disaccharide maltose.

The tongue tastes food, manipulates it during chewing, and helps shape the food into a ball called a bolus. During swallowing, the tongue pushes a bolus to the back of the oral cavity and into the pharynx.

The region we call our throat is the pharynx, a junction that opens to both the oesophagus and the windpipe (trachea). When we swallow, the top of the windpipe moves up so that its opening, the glottis, is blocked by a cartilaginous flap, the epiglottis. You can see this motion in the bobbing of the “Adam′s apple” during swallowing. This tightly controlled mechanism normally ensures that a bolus is guided into the entrance of the oesophagus.

Food or liquids may go “down the wrong pipe” because the swallowing reflex didn′t close the opening of the windpipe in time. The resulting blockage of airflow (choking) stimulates vigorous coughing, which usually expels the material. If it is not expelled quickly, the lack of airflow to the lungs can be fatal.

The oesophagus conducts food from the pharynx down to the stomach by peristalsis. The muscles at the very top of the oesophagus are striated (voluntary). Thus, the act of swallowing begins voluntarily, but then the involuntary waves of contraction by smooth muscles in the rest of the esophagus take over.

The Stomach
The stomach stores food and performs preliminary steps of digestion. This large organ is located in the upper abdominal cavity, just below the diaphragm. With accordionlike folds and a very elastic wall, the stomach can stretch to accommodate about 2 L of food and fluid. It is because the stomach can store an entire meal that we do not need to eat constantly. Besides storing food, the stomach performs important digestive functions: It secretes a digestive fluid called gastric juice and mixes this secretion with the food by the churning action of the smooth muscles in the stomach wall.

Gastric juice is secreted by the epithelium lining numerous deep pits in the stomach wall. With a high concentration of hydrochloric acid, gastric juice has a pH of about 2—acidic enough to dissolve iron nails. One function of the acid is to disrupt the extracellular matrix that binds cells together in meat and plant material. The acid also kills most bacteria that are swallowed with food. Also present in gastric juice is pepsin, an enzyme that begins the hydrolysis of proteins. Pepsin breaks peptide bonds adjacent to specific amino acids, cleaving proteins into smaller polypeptides, which are later digested completely to amino acids in the small intestine. Pepsin is one of the few enzymes that works best in a strongly acidic environment. The low pH of gastric juice denatures (unfolds) the proteins in food, increasing exposure of their peptide bonds to pepsin.

What prevents pepsin from destroying the cells of the stomach wall? First, pepsin is secreted in an inactive form called pepsinogen by specialised cells called chief cells located in gastric pits.

Other cells, called parietal cells, also in the pits, secrete hydrochloric acid. The acid converts pepsinogen to active pepsin by removing a small portion of the molecule and exposing its active site. Because different cells secrete the acid and pepsinogen, the two ingredients do not mix—and pepsinogen is not activated—until they enter the lumen (cavity) of the stomach. Activation of pepsinogen is an example of positive feedback: Once some pepsinogen is activated by acid, activation occurs at an increasingly rapid rate because pepsin itself can activate additional molecules of pepsinogen. Many other digestive enzymes are also secreted in inactive forms that become active within the lumen of the digestive tract.

The stomach′s second defense against self–digestion is a coating of mucus, secreted by the epithelial cells of the stomach lining. Still, the epithelium is constantly eroded, and mitosis generates enough cells to completely replace the stomach lining every three days. Gastric ulcers, lesions in this lining, are caused mainly by the acid–tolerant bacterium Helicobacter pylori

Though treatable with antibiotics, gastric ulcers may worsen if pepsin and acid destroy the lining faster than it can regenerate.

About every 20 seconds, the stomach contents are mixed by the churning action of smooth muscles. You may feel hunger pangs when your empty stomach churns. (Sensations of hunger are also associated with brain centres that monitor the blood′s nutritional status and levels of the appetite–controlling hormones discussed earlier in this chapter.) As a result of mixing and enzyme action, what begins in the stomach as a recently swallowed meal becomes a nutrient–rich broth known as acid chyme.

Most of the time, the stomach is closed off at both ends. The opening from the oesophagus to the stomach, the cardiac orifice, normally dilates only when a bolus arrives. The occasional backflow of acid chyme from the stomach into the lower end of the oesophagus causes heartburn. (If backflow is a persistent problem, an ulcer may develop in the oesophagus.) At the opening from the stomach to the small intestine is the pyloric sphincter, which helps regulate the passage of chyme into the intestine, one squirt at a time. It takes about 2 to 6 hours after a meal for the stomach to empty in this way.

The Small Intestine
With a length of more than 6 m in humans, the small intestine is the longest section of the alimentary canal (its name refers to its small diameter, compared with that of the large intestine). Most of the enzymatic hydrolysis of food macromolecules and most of the absorption of nutrients into the blood occur in the small intestine.

Most digestion is completed early in this journey, while the chyme is still in the duodenum. The remaining regions of the small intestine, called the jejunum and ileum, function mainly in the absorption of nutrients and water

Absorption of Nutrients
To enter the body, nutrients in the lumen must cross the lining of the digestive tract. A few nutrients are absorbed in the stomach and large intestine, but most absorption occurs in the small intestine. This organ has a huge surface area—300 m2, roughly the size of a tennis court. Large circular folds in the lining bear fingerlike projections called villi, and each epithelial cell of a villus has many microscopic appendages called microvilli that are exposed to the intestinal lumen

(The microvilli′s shape is the basis of the term brush border for the intestinal epithelium.) This enormous microvillar surface is an adaptation that greatly increases the rate of nutrient absorption.

Penetrating the core of each villus is a net of microscopic blood vessels (capillaries) and a small vessel of the lymphatic system called a lacteal. (In addition to their circulatory system, vertebrates have an associated network of vessels—the lymphatic system—that carries a clear fluid called lymph. Nutrients are absorbed across the intestinal epithelium and then across the unicellular epithelium of the capillaries or lacteals. Only these two single layers of epithelial cells separate nutrients in the lumen of the intestine from the bloodstream.

In some cases, transport of nutrients across the epithelial cells is passive. The simple sugar fructose, for example, apparently moves by diffusion down its concentration gradient from the lumen of the intestine into the epithelial cells and then into capillaries. Other nutrients, including amino acids, small peptides, vitamins, and glucose and several other simple sugars, are pumped against concentration gradients by the epithelial membranes. This active transport allows the intestine to absorb a much higher proportion of the nutrients in the intestine than would be possible with passive diffusion.

Amino acids and sugars pass through the epithelium, enter capillaries, and are carried away from the intestine by the bloodstream. After glycerol and fatty acids are absorbed by epithelial cells, they are recombined into fats within those cells. The fats are then mixed with cholesterol and coated with proteins, forming small globules called chylomicrons, most of which are transported by exocytosis out of the epithelial cells and into lacteals

The lacteals converge into the larger vessels of the lymphatic system. Lymph, containing chylomicrons, eventually drains from the lymphatic system into large veins that return blood to the heart.

In contrast to the lacteals, the capillaries and veins that drain nutrients away from the villi all converge into the hepatic portal vein, a blood vessel that leads directly to the liver. This ensures that the liver—which has the metabolic versatility to interconvert various organic molecules—has first access to amino acids and sugars absorbed after a meal is digested. Therefore, blood that leaves the liver may have a very different balance of these nutrients than the blood that entered via the hepatic portal vein. For example, the liver helps regulate the level of glucose molecules in the blood, and blood exiting the liver usually has a glucose concentration very close to 0.1%, regardless of the carbohydrate content of a meal (see Figure 41.3). From the liver, blood travels to the heart, which pumps the blood and the nutrients it contains to all parts of the body.

The large intestine or colon, is connected to the small intestine at a T–shaped junction, where a sphincter (a muscular valve) controls the movement of material. One arm of the T is a pouch called the caecum. Compared to many other mammals, humans have a relatively small caecum. The human cecum has a fingerlike extension, the appendix, which is dispensable. (Lymphoid tissue in the appendix makes a minor contribution to body defense.) The main branch of the human colon is about 1.5 m long.

A major function of the colon is to recover water that has entered the alimentary canal as the solvent of the various digestive juices. About 7 L of fluid are secreted into the lumen of the digestive tract each day, which is much more liquid than most people drink. Most of this water is reabsorbed when nutrients are absorbed in the small intestine. The colon reclaims much of the remaining water that was not absorbed in the small intestine. Together, the small intestine and colon reabsorb about 90% of the water that enters the alimentary canal.

The wastes of the digestive tract, the faeces, become more solid as they are moved along the colon by peristalsis. The movement is sluggish, and it generally takes about 12 to 24 hours for material to travel the length of the organ. If the lining of the colon is irritated—by a viral or bacterial infection, for instance—less water than normal may be reabsorbed, resulting in diarrhea. The opposite problem, constipation, occurs when peristalsis moves the feces along the colon too slowly. An excess of water is reabsorbed, and the faeces become compacted.

Living in the large intestine is a rich flora of mostly harmless bacteria. One of the common inhabitants of the human colon is Escherichia coli, a favourite research organism of molecular biologists. The presence of Escherica coli in lakes and streams is an indication of contamination by untreated sewage. Intestinal bacteria live on unabsorbed organic material. As by–products of their metabolism, many colon bacteria generate gases, including methane and hydrogen sulfide. Some of the bacteria produce vitamins, including biotin, folic acid, vitamin K, and several B vitamins. These vitamins, absorbed into the blood, supplement our dietary intake of vitamins.

Faeces contain masses of bacteria, as well as cellulose and other undigested materials. Although cellulose fibers have no caloric value to humans, their presence in the diet helps move food along the digestive tract.

The terminal portion of the colon is called the rectum, where faeces are stored until they can be eliminated. Between the rectum and the anus are two sphincters, one involuntary and the other voluntary. One or more times each day, strong contractions of the colon create an urge to defecate.

Jul 21, 2011

A hole in the heart

We always talk about conditions affecting the heart later in life such as hypertension, coronary heart disease and heart failure. We don't talk much of any conditions of the heart which are present at birth, known as congenital heart disease or defects.

Congenital heart disease, also abbreviated to CHD and not to be confused with coronary heart disease, is a type of defect or malformation in one or more structures of the heart or blood vessels that occurs before birth. These defects happen as the foetus is developing.

CHD is more common than one thinks, occurring around 8 to 10 out of every 1,000 births. The cause is unknown in the majority of cases although certain conditions or factors can predispose a child to being born with CHD. They include genetic or chromosomal abnormalities in the child such as Down's Syndrome, viral infections in the mother during pregnancy such Rubella, and consumption of certain drugs or alcohol by the mother during pregnancy.

There are many different types of CHD and they range from a mild form that can go undetected until much later in life to the very severe where the baby is cyanosed - bluish discolouration of the lips and face because of lack of oxygen in the blood - within days of being born.

Let us just limit the discussion here to atrial and ventricular septal defects, known as ASD and VSD respectively. In layman's term, they are also known as hole-in-the-heart.

You all knoe that the heart is divided into four chambers - two on the left where blood rich in oxygen from the lungs passes through to get to the body - and two on the right where blood lacking in oxygen returning from the body goes to before it gets back to the lungs.

It is vital that the wall separating the two sides called the septum is intact. Any breach in the wall will mean that blood with different oxygen concentrations are mixed together. ASD is when the breach is in the septum between the two atria and likewise, VSD denotes a defect in the ventricular septum.

This may not be a big problem in the beginning as the left chambers which have a higher pressure system than the right will direct blood to the right chambers via the hole or defect in the wall. However, as the pressure rises in the right chambers through the years because of constant flow of high pressure from the left, it can damage the lung causing pulmonary hypertension.

When it comes to a point where both sides have the same blood pressure level, there is then a free mixing of high and low oxygenated blood on both sides resulting in body tissues being compromised of good oxygenation. The person will be cyanosed and breathless and may also have symptoms of heart failure.

Of course, that is the extreme end of the spectrum. For most with ASD or VSD, they are fairly asymptomatic and the condition only gets discovered during a routine medical examination when a murmur - a characteristic sound heard by using a stethoscope placed on the chest over the heart - is detected. The definitive test is an echocardiogram which is an ultrasound scan test of the heart.

When the septal defect is small the doctor will usually advise the person concerned to come back for a checkup every year or so. No regular medication is needed.

He will be advised to take antibiotics as a preventative measure against bacterial infection affecting the heart (endocarditis) if he is going to have a dental procedure or any procedure that may cause bleeding.

If the defect is big from the echocardiogram, then an interventional procedure or surgery to close the hole has to be considered.

If you are a parent and you notice the following in your child - cyanosis of the skin, lips and fingernails, poor weight gain, fast breathing, recurrent lung infections and inability to exercise well, it is suggested that you bring your child to the attention of your doctor or paediatrician. There is a possibility that he or she has a congenital heart disease.

Jul 19, 2011

Photosynthesis (SPM Level)

Photosynthesis – takes place in the chloroplast
There two main stages : light reaction and dark reaction
Light reaction occurs only in the presence of light. Dark reaction occurs during day and night.
Light reaction - occurs in grana.
Chlorophyll captures light energy - excites the electrons to higher energy levels.
Electrons then leave the chlorophyll.
Light energy is also used to split water molecule into hydrogen ions and hydroxyl ions. This is known as photolysis of water.
Hydrogen ions combine with electrons released by chlorophyll to form hydrogen atoms.
The energy from excited electrons is used to form adenosine triphosphate (ATP).
At the same time, hydroxyl ion loses an electron to form hydroxyl group.
This electron is then received by chlorophyll.
The hydroxyl groups then combine to form water and gaseous oxygen.
Oxygen is released into the atmosphere and used for cellular respiration. The ATP molecules provide energy while the hydrogen atoms provide reducing power for the dark reaction.

Dark reaction - also known as Calvin cycle.
Occurs in stroma. Hydrogen atoms are used to fix carbon dioxide into glucose.
The glucose monomers then undergo condensation to form starch which is temporarily stored as starch grains in the chloroplasts.

Light reaction - occurs in the grana (contained chlorophyll) - takes place in the presence of sunlight and chlorophyll - chlorophyll absorbs light; then it becomes activated and this energy is used to :
i) produce energy in the form of – ATP (used for dark reaction)
ii) split up water molecules (photolysis) into hydroxyl ions(OH-) and hydrogen ions (H+) - oxygen is released; but hydrogen enters dark reaction.

Dark reaction (Light independent reaction) - takes place in the stroma -ATP combined with hydrogen atoms (from the light reaction) are used to reduce carbon dioxide to form glucose.

Glucose produced –
i) converted to starch (stored),
ii) transformed - sucrose ; transported to other parts
iii) synthesis of cellulose
iv) converted to a. acids and fatty acids