Nov 29, 2009

Dear all my students...

When you were in school, you couldn't wait to be out of it. Now that you're leaving school soon, you will miss all your friends and even your teachers.

When you're young, you don't appreciate a lot of things because you're busy having fun. Only when you're older do you get wiser and wish that you had spent more time doing things right the first time round. But everyone has to go through this order to understand that life is not perfect, but it is still your life, so make full use of it......






Quick Revision

Nucleus - contains DNA

Nucleolus - in nucleus, manufactures ribosomes

Endoplasmic reticulum - move materials from one part of the cell to another

Golgi apparatus - where proteins are converted to their final form

Lysosomes - contain digestive enzymes

Chloroplasts - contain pigments important to photosynthesis

Mitochondria - site of ATP production

Ribosomes - manufacture of proteins

Centrioles and microtubules - support, cell locomotion, forming spindle in nuclear division

Cell wall - support, prevents cell from bursting by taking in too much water

Osmosis: the net movement of water molecules through a partially permeable membrane from an area of higher water potential to an an area of lower water potential.

Facilitated diffusion: This is where polar molecules are transported across membranes. Molecules bind with transport proteins which change shape and move the molecules across the membrane. No metabolic energy is required.

Examples of active transport: the calcium pump (skeletal muscles), the sodium-potassium pump (nerve cells).

Endocytosis: An active process whereby substances are taken into the cell by infoldings of the surface membrane. (Exocytosis is similar.)

Mitosis: Mitosis is a type of cell division where the daughter cells have the same number of chromosomes as the parent cell and are genetically identical to the parent cell. Mitosis takes place in four stages: prophase, metaphase, anaphase and telophase.

During prophase, each chromosome forms two chromatids joined by a centromere. Two centrioles begin to move forming a spindle and the nuclear envelope breaks down.

During metaphase, the chromosomes are attached to the spindle fibres and line up at the equator of the cell.

During anaphase, the centromeres split and the chromatids are pulled to opposite poles of the cell.

During telophase, the nuclear envelope reforms and the cell membrane narrows at the middle, forming two daughter cells
 
Sucrose = glucose + fructose; main form in which carbohydrate is transported in plants
Maltose = 2 glucose; found in some germinating seeds eg barley
Lactose = glucose + galactose; found in milk

Lipids - insoluble in water, soluble in organic solvents

Fats & oils - compounds of glycerol and fatty acids

Structure of proteins
Primary - Order of the amino acids
Secondary - The way the chain folds/turns on itself due to hydrogen bonding
Tertiary - Cross-links including hydrogen bonds, inonic bonds and sulphur bridges
Quaternary - The arrangement of two or more polypeptides eg haemoglobin
Collagen - fibrous protein; great tensile strength; found in bones, tendons, skin etc; structure = triple helix
Insulin - globular protein, folded chain held together by 2 disulphide bridges with the loop removed

Nov 26, 2009

Gene Idea







Alleles, alternative versions of a gene. A somatic cell has two copies of each chromosome (forming a homologous pair) and thus two alleles of each gene, which may be identical or different. This figure depicts an F1 pea hybrid with an allele for purple flowers, inherited from one parent, and an allele for white flowers, inherited from the other parent.


Mendel’s law of segregation. This diagram shows the genetic makeup of the generations in Figure 14.3. It illustrates Mendel’s model for inheritance of the alleles of a single gene. Each plant has two alleles for the gene controlling flower colour, one allele inherited from each parent. To construct a Punnett square, list all the possible female gametes along one side of the square and all the possible male gametes along an adjacent side. The boxes represent the offspring resulting from all the possible unions of male and female gametes.



Phenotype versus genotype. Grouping F2 offspring from a cross for flower color according to phenotype results in the typical 3:1 phenotypic ratio. In terms of genotype, however, there are actually two categories of purple–flowered plants, PP (homozygous) and Pp (heterozygous), giving a 1:2:1 genotypic ratio.




Pedigree Analysis
Unable to manipulate the mating patterns of people, geneticists must analyze the results of matings that have already occurred. They do so by collecting information about a family’s history for a particular trait and assembling this information into a family tree describing the interrelationships of parents and children across the generations—the family pedigree shows a three–generation pedigree that traces the occurrence of a pointed contour of the hairline on the forehead. This trait, called a widow’s peak, is due to a dominant allele, W. Because the widow’s–peak allele is dominant, all individuals who lack a widow’s peak must be homozygous recessive (ww ). The two grandparents with widow’s peaks must have the Ww genotype, since some of their offspring are homozygous recessive. The offspring in the second generation who do have widow’s peaks must also be heterozygous, because they are the products of Ww × ww matings. The third generation in this pedigree consists of two sisters. The one who has a widow’s peak could be either homozygous (WW ) or heterozygous (Ww ), given what we know about the genotypes of her parents (both Ww).

Cystic Fibrosis
The most common lethal genetic disease in the United States is cystic fibrosis, which strikes one out of every 2,500 people of European descent but is much rarer in other groups. Among people of European descent, one out of 25 (4%) is a carrier of the cystic fibrosis allele. The normal allele for this gene codes for a membrane protein that functions in chloride ion transport between certain cells and the extracellular fluid. These chloride transport channels are defective or absent in the plasma membranes of children who inherit two recessive alleles for cystic fibrosis. The result is an abnormally high concentration of extracellular chloride, which causes the mucus that coats certain cells to become thicker and stickier than normal. The mucus builds up in the pancreas, lungs, digestive tract, and other organs, leading to multiple (pleiotropic) effects, including poor absorption of nutrients from the intestines, chronic bronchitis, foul stools, and recurrent bacterial infections. Recent research indicates that the extracellular chloride also contributes to infection by disabling a natural antibiotic made by some body cells. When immune cells come to the rescue, their remains add to the mucus, creating a vicious cycle.

If untreated, most children with cystic fibrosis die before their fifth birthday. Gentle pounding on the chest to clear mucus from clogged airways, daily doses of antibiotics to prevent infection, and other preventive treatments can prolong life. In the United States, more than half of the people with cystic fibrosis now survive into their late 20s or even 30s and beyond.

Sickle–Cell Disease
The most common inherited disorder among people of African descent is sickle–cell disease, which affects one out of 400 African–Americans. Sickle–cell disease is caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells. When the oxygen content of an affected individual’s blood is low (at high altitudes or under physical stress, for instance), the sickle–cell hemoglobin molecules aggregate into long rods that deform the red cells into a sickle shape (see Figure 5.21). Sickled cells may clump and clog small blood vessels, often leading to other symptoms throughout the body, including physical weakness, pain, organ damage, and even paralysis. The multiple effects of a double dose of the sickle–cell allele are another example of pleiotropy. Regular blood transfusions can ward off brain damage in children with sickle–cell disease, and new drugs can help prevent or treat other problems, but there is no cure.

Although two sickle–cell alleles are necessary for an individual to manifest full–blown sickle–cell disease, the presence of one sickle–cell allele can affect the phenotype. Thus, at the organismal level, the normal allele is incompletely dominant to the sickle–cell allele. Heterozygotes, said to have sickle–cell trait, are usually healthy, but they may suffer some sickle–cell symptoms during prolonged periods of reduced blood oxygen. At the molecular level, the two alleles are codominant; both normal and abnormal (sickle–cell) hemoglobins are made in heterozygotes.

About one out of ten African–Americans has sickle–cell trait, an unusually high frequency of heterozygotes for an allele with severe detrimental effects in homozygotes. One explanation for this is that a single copy of the sickle–cell allele reduces the frequency and severity of malaria attacks, especially among young children. The malaria parasite spends part of its life cycle in red blood cells (see Figure 28.11), and the presence of even heterozygous amounts of sickle–cell hemoglobin results in lower parasite densities and hence reduced malaria symptoms. Thus, in tropical Africa where infection with the malaria parasite is common, the sickle–cell allele is both boon and bane. The relatively high frequency of African–Americans with sickle–cell trait is a vestige of their African roots.

Mating of Close Relatives
When a disease–causing recessive allele is rare, it is relatively unlikely that two carriers of the same harmful allele will meet and mate. However, if the man and woman are close relatives (for example, siblings or first cousins), the probability of passing on recessive traits increases greatly. These are called consanguineous (“same blood”) matings, and they are indicated in pedigrees by double lines. Because people with recent common ancestors are more likely to carry the same recessive alleles than are unrelated people, it is more likely that a mating of close relatives will produce offspring homozygous for recessive traits—including harmful ones. Such effects can be observed in many types of domesticated and zoo animals that have become inbred.

There is debate among geneticists about the extent to which human consanguinity increases the risk of inherited diseases. Many deleterious alleles have such severe effects that a homozygous embryo spontaneously aborts long before birth. Still, most societies and cultures have laws or taboos forbidding marriages between close relatives. These rules may have evolved out of empirical observation that in most populations, stillbirths and birth defects are more common when parents are closely related. Social and economic factors have also influenced the development of customs and laws against consanguineous marriages.

Dominantly Inherited Disorders
Although many harmful alleles are recessive, a number of human disorders are due to dominant alleles. One example is achondroplasia, a form of dwarfism with a prevalence of one among every 25,000 people. Heterozygous individuals have the dwarf phenotype



Therefore, all people who are not achondroplastic dwarfs—99.99% of the population—are homozygous for the recessive allele. Like the presence of extra fingers or toes mentioned earlier, achondroplasia is a trait for which the recessive allele is much more prevalent than the corresponding dominant allele.

Dominant alleles that cause a lethal disease are much less common than recessive alleles that do so. All such lethal alleles arise by mutations (changes to the DNA) in a sperm or egg; presumably, such mutations occur equally often whether the mutant allele is dominant or recessive. However, if a lethal dominant allele causes the death of offspring before they mature and can reproduce, the allele will not be passed on to future generations. In contrast, a lethal recessive allele can be perpetuated from generation to generation by heterozygous carriers who have normal phenotypes. These carriers can reproduce and pass on the recessive allele. Only homozygous recessive offspring will have the lethal disease.

A lethal dominant allele can escape elimination if it causes death only at a relatively advanced age. By the time the symptoms become evident, the individual may have already transmitted the lethal allele to his or her children. For example, Huntington’s disease , a degenerative disease of the nervous system, is caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35 to 45 years old. Once the deterioration of the nervous system begins, it is irreversible and inevitably fatal. Any child born to a parent who has the allele for Huntington’s disease has a 50% chance of inheriting the allele and the disorder. (The mating can be symbolized as Aa × aa, with A being the dominant allele that causes Huntington’s disease.) In the United States, this devastating disease afflicts about one in 10,000 people.

Until relatively recently, the onset of symptoms was the only way to know if a person had inherited the Huntington’s allele. This is no longer the case. By analyzing DNA samples from a large family with a high incidence of the disorder, geneticists tracked the Huntington’s allele to a locus near the tip of chromosome 4.



Meiotic nondisjunction. Gametes with an abnormal chromosome number can arise by nondisjunction in either meiosis I or meiosis II.




Alterations of chromosome structure. Vertical arrows indicate breakage points. Dark purple highlights the chromosomal parts affected by the rearrangements.



Down syndrome.The child exhibits the facial features characteristic of Down syndrome. The karyotype shows trisomy 21, the most common cause of this disorder.

Conception and Embryonic Development

The sperm fuses the egg (fertilisation) in the fallopian tube.


About 24 hours later, the resulting zygote begins dividing, a process called cleavage. Cleavage continues, with the embryo becoming a ball of cells by the time it reaches the uterus 3 to 4 days after fertilisation. By about 1 week after fertilisation, cleavage has produced an embryonic stage called the blastocyst, a sphere of cells containing a cavity. In a process that takes several more days for completion, the blastocyst implants into the endometrium.

The embryo secretes hormones that signal its presence and control the mother′s reproductive system. One embryonic hormone, human chorionic gonadotropin (HCG), acts like pituitary LH to maintain secretion of progesterone and oestrogens by the corpus luteum through the first few months of pregnancy. In the absence of this hormonal override, the decline in maternal LH due to inhibition of the pituitary would result in menstruation and loss of the embryo. Levels of HCG in the maternal blood are so high that some is excreted in the urine, where it can be detected in pregnancy tests.

First Trimester
Human gestation can be divided for convenience into three trimesters of about three months each. The first trimester is the time of most radical change for both the mother and the embryo. Let′s take up our story where we left off, at implantation. The endometrium responds to implantation by growing over the blastocyst. Differentiation of the embryo′s body structures now begins.

During its first 2 to 4 weeks of development, the embryo obtains nutrients directly from the endometrium. Meanwhile, the outer layer of the blastocyst, called the trophoblast, grows out and mingles with the endometrium, eventually helping to form the placenta. This disk–shaped organ, containing both embryonic and maternal blood vessels, grows to about the size of a dinner plate and can weigh close to 1 kg. Diffusion of material between maternal and embryonic circulations provides nutrients, exchanges respiratory gases, and disposes of metabolic wastes for the embryo. Blood from the embryo travels to the placenta through arteries of the umbilical cord and returns via the umbilical vein.




From the fourth week of development until birth, the placenta, a combination of maternal and embryonic tissues, transports nutrients, respiratory gases, and wastes between the embryo or foetus and the mother. Maternal blood enters the placenta in arteries, flows through blood pools in the endometrium, and leaves via veins. Embryonic or foetal blood, which remains in vessels, enters the placenta through arteries and passes through capillaries in fingerlike chorionic villi, where oxygen and nutrients are acquired. As indicated in the drawing, the foetal (or embryonic) capillaries and villi project into the maternal portion of the placenta. Foetal blood leaves the placenta through veins leading back to the foetus. Materials are exchanged by diffusion, active transport, and selective absorption between the fetal capillary bed and the maternal blood pools.

The heart begins beating by the fourth week and can be detected with a stethoscope by the end of the first trimester. By the end of the eighth week, all the major structures of the adult are present in rudimentary form. (It is during organogenesis that the embryo is most sensitive to such threats as radiation and drugs that can cause birth defects.) At 8 weeks, the embryo is called a foetus. Although well differentiated, the foetus is only 5 cm long by the end of the first trimester.

Meanwhile, the mother is also undergoing rapid changes. High levels of progesterone initiate changes in her reproductive system. These include increased mucus in the cervix that forms a protective plug, growth of the maternal part of the placenta, enlargement of the uterus, and (by negative feedback on the hypothalamus and pituitary) cessation of ovulation and menstrual cycling. The breasts also enlarge rapidly and are often quite tender. (Hmmm)

Second Trimester
During the second trimester, the foetus grows to about 30 cm and is very active. The mother may feel movements during the early part of the second trimester, and foetal activity may be visible through the abdominal wall by the middle of this time period. Hormone levels stabilise as HCG declines, the corpus luteum deteriorates, and the placenta completely takes over the production of progesterone, which maintains the pregnancy. During the second trimester, the uterus grows enough for the pregnancy to become obvious.

Third Trimester
The final trimester is one of growth of the foetus to about 3–4 kg in weight and 50 cm in length. Foetal activity may decrease as the foetus fills the available space within the embryonic membranes. As the foetus grows and the uterus expands around it, the mother′s abdominal organs become compressed and displaced, leading to frequent urination, digestive blockages, and strain in the back muscles. A complex interplay of local regulators (prostaglandins) and hormones (chiefly oestrogen and oxytocin) induces and regulates labour, the process by which childbirth occurs. The mechanism that triggers labor is not fully understood, but shows one model. Oestrogen, which reaches its highest level in the mother′s blood during the last weeks of pregnancy, induces the formation of oxytocin receptors on the uterus. Oxytocin, produced by the foetus and the mother′s posterior pituitary, stimulates powerful contractions by the smooth muscles of the uterus. Oxytocin also stimulates the placenta to secrete prostaglandins, which enhance the contractions. In turn, the physical and emotional stresses associated with the contractions stimulate the release of more oxytocin and prostaglandins, a positive feedback system that underlies the process of labour.

Transpiration

Transpiration is the loss of water vapour through evaporation in plants. The loss of water is replaced by the absorption of water from the soil by the plant roots.
Only 1% - used by plants for photosynthesis; 99% - evaporates
90% of transp. occurs through the stomata - also through lenticels in woody stems

Trans.
-helps in absorption and transport of water and mineral ions from roots to different parts of plants
-produce cooling effect in plants
-helps to supply water to all plant cells for metabolic processs
-helps to prevent plants from wilting and maintaining cell turgidity



1. The surfaces of the mesophyll cells are covered by a thin layer of water.
2. Heat from the sun causes the water on the external surfaces of the mesophyll cells to evaporate, thus saturating the air spaces in the mesophyll with water vapour.
3. Outside the stomata, the air in the atmosphere is less saturated.
4. This means the that the concentration of water vapour in the atmosphere is lower than the concentration of water vapour in the air spaces of the leaves.
5. Hence, the water vapour in the air spaces evaporates and the water vapour diffuses from the plant cells through the stomata.
6. The movement of air carries water vapour away from the stomata.
7. The loss of water from mesophyll cell makes the cell hypertonic to an adjacent cell.
8. Water from the adjacent cell diffuses into the mesophyll cell by osmosis.
9. In the same way, water continues to diffuse from the neighbouring cells into the adjacent cells.
10. Eventually, water is drawn from the xylem vessels in the veins.
11. A pulling force is thus created to pull water up the xylem vessels as a result of the evaporation of water vapour from the mesophyll cells.
12. This pull is called the transpirational pull.

The external conditions that affect the rate of transpiration are
a) light intensity
b) temperature
c) relative humidity
d) air movement

Light intensity
-an increase in light intensity increases the rate of transp.
-light stimulates the opening of the stomata
-stomata open wider; more water vapour evaporates

Temperature
-temperature increases; rate of transp. increases
-temp. increases; rate of evaporation of water from the surfaces of the mesophyll cells increases; rate of diffusion of water through the stomata increases

Air movement
-faster air movement helps removing the water vapour
-air movement increases the concentration gradient between the water vapour in the leaf and that outside the leaf; this increases the transp. rate.
-when the air is still, the transp. rate decreases/stops

Relative humidity
-high humidity surrounding the leaves reduces the evaporation of water from the stomata; transp. slows down
-a rise in temp. lowers the relative humidity of surrounding air; rate of transp. increases

Nov 25, 2009

Human activities - endanger the ecosystems

Human activities often affect whole ecosystems.
Conflicts normally arise between the need to meet the immediate human demands in short term and the need to protect and conserve ecosystems from long-term damage.

Deforestation
-soil erosion
-landslides
-flash floods
-climatic changes
-the loss of biodiversity
-the greenhouse effect and global warming

Burning of fossil fuels
-greenhouse effect
-global warming
-air pollution

Overuse of fertilisers in intensive farming
-eutrophication
-water pollution

Dumping of domestic and industrial waste
-water pollutions





Nov 23, 2009

How To Study for Biology Exams

Biology exams can seem intimidating and overwhelming to biology students. The key to overcoming these obstacles is preparation. By learning how to study for biology exams you can conquer your fears.
Remember, the purpose of an exam is for you to demonstrate that you understand the concepts and information that have been taught. Below are some excellent tips to help you learn how to study for biology exams.

Here's How:

Get Organised
An important key for success in biology is organization. Good time management skills will help you to become more organized and waste less time preparing to study.
Items such as daily planners and semester calendars will help you to know what you need to do and when you need to have it done.

Start Studying Early
It is very important that you start preparing for biology exams well in advance.
I know, I know, it is almost tradition for some to wait until the last minute, but students who implore this tactic don't perform their best, don't retain the information, and get worn out.

Review Notes
Be sure that you review your notes before the exam. You should start reviewing your notes on a daily basis. This will ensure that you gradually learn the information over time and don't have to cram.

Review the Biology Text
Your biology textbook/reference book is a wonderful source for finding illustrations and diagrams that will help you visualise the concepts you are learning. Be sure to reread and review the appropriate chapters and information in your textbook. You will want to make sure that you understand all key concepts and topics.

Get Answers To Your Questions
If you are having difficulty understanding a topic or have unanswered questions, discuss them with your teacher.
You don't want to go into an exam with gaps in your knowledge.

Quiz Yourself
To help prepare yourself for the exam and find out how much you know, give yourself a quiz. You can do this by using prepared flash cards or taking a sample test.
You can also use online biology games and quiz resources.

Find a Study Buddy
Get together with a friend or classmate and have a study session. Take turns asking and answering questions. Write your answers down in complete sentences to help you organise and express your thoughts.

Attend a Review Session
If your teacher holds a review session, be sure to attend. This will help to identify specific topics that will be covered, as well as fill in any gaps in knowledge. Help sessions are also an ideal place to get answers to your questions.

Relax
Now that you have followed the previous steps, it's time to rest and relax. You should be well prepared for your biology exam.
It's a good idea to make sure you get plenty of sleep the night before your exam. You have nothing to worry about because you are well prepared.

Transport Systems in Plants

Plants don’t have a circulatory system like animals, but they do have a sophisticated transport system for carrying water and dissolved solutes to different parts of the plant, often over large distances.


Epidermis : One cell thick. In young plants the epidermis cells may secrete a waterproof cuticle, and in older plants the epidermis may be absent, replaced by bark.
Cortex : Composed of various “packing” cells, to give young plants strength and flexibility, and are the source of plant fibres such as sisal and hemp.
Vascular Tissue : This contains the phloem and xylem tissue, which grow out from the cambium. In dicot plants (the broad-leafed plants), the vascular tissue is arranged in vascular bundles, with phloem on the outside and xylem on the inside. In older plants the xylem bundles fuse together to form the bulk of the stem.
Pith : The central region of a stem, used for food storage in young plants. It may be absent in older plants (i.e. they’re hollow).



Epidermis : A single layer of cells often with long extensions called root hairs, which increase the surface area enormously. A single plant may have 1010 root hairs.
Cortex : A thick layer of packing cells often containing stored starch.
Endodermis : A single layer of tightly-packed cells containing a waterproof layer called the casparian strip. This prevents the movement of water between the cells.



Pericycle : A layer of undifferentiated meristematic (growing) cells.
Vascular Tissue : This contains xylem and phloem cells, which are continuous with the stem vascular bundles. The arrangement is different, and the xylem usually forms a star shape with 2-6 arms.



 Xylem Tissue

Xylem tissue is composed of dead cells joined together to form long empty tubes. Different kinds of cells form wide and narrow tubes, and the end cells walls are either full of holes, or are absent completely. Before death the cells form thick cell walls containing lignin, which is often laid down in rings or helices, giving these cells a very characteristic appearance under the microscope. Lignin makes the xylem vessels very strong, so that they don’t collapse under pressure, and they also make woody stems strong.



Phloem Tissue

Phloem tissue is composed of sieve tube cells, which form long columns with holes in their end walls called sieve plates. These cells are alive, but they lose their nuclei and other organelles, and their cytoplasm is reduced to strands around the edge of the cells. These cytoplasmic strands pass through the holes in the sieve plates, so forming continuous filaments. The centre of these tubes is empty. Each sieve tube cell is associated with one or more companion cells, normal cells with nuclei and organelles. These companion cells are connected to the sieve tube cells by plasmodesmata, and provide them with proteins, ATP and other nutrients.

Water Transport in Plants
Vast amounts of water pass through plants. A large tree can use water at a rate of 1 dm³ min-1. Only 1% of this water is used by the plant cells for photosynthesis and turgor, and the remaining 99% evaporates from the leaves and is lost to the atmosphere. This evaporation from leaves is called transpiration.
The movement of water through a plant can be split into three sections: through the roots, stem and leaves:




Water moves through the root by two paths:
The Symplast pathway consist of the living cytoplasms of the cells in the root (10%). Water is absorbed into the root hair cells by osmosis, since the cells have a lower water potential that the water in the soil. Water then diffuses from the epidermis through the root to the xylem down a water potential gradient. The cytoplasms of all the cells in the root are connected by plasmodesmata through holes in the cell walls, so there are no further membranes to cross until the water reaches the xylem, and so no further osmosis.

The Apoplast pathway consists of the cell walls between cells (90%). The cell walls are quite thick and very open, so water can easily diffuse through cell walls without having to cross any cell membranes by osmosis. However the apoplast pathway stops at the endodermis because of the waterproof casparian strip, which seals the cell walls. At this point water has to cross the cell membrane by osmosis and enter the symplast. This allows the plant to have some control over the uptake of water into the xylem.

The uptake of water by osmosis actually produces a force that pushes water up the xylem. This force is called root pressure, which can be measured by placing a manometer over a cut stem, and is of the order of 100 kPa (about 1 atmosphere). This helps to push the water a few centimetres up short and young stems, but is nowhere near enough pressure to force water up a long stem or a tree. Root pressure is the cause of guttation, sometimes seen on wet mornings, when drops of water are forced out of the ends of leaves.

Movement through the Stem
The xylem vessels form continuous pipes from the roots to the leaves. Water can move up through these pipes at a rate of 8m h-1, and can reach a height of over 100m. Since the xylem vessels are dead, open tubes, no osmosis can occur within them. The driving force for the movement is transpiration in the leaves. This causes low pressure in the leaves, so water is sucked up the stem to replace the lost water. The column of water in the xylem vessels is therefore under tension (a stretching force). Fortunately water has a high tensile strength due to the tendency of water molecules to stick together by hydrogen bonding (cohesion), so the water column does not break under the tension force. This mechanism of pulling water up a stem is sometimes called the cohesion-tension mechanism.

The very strong lignin walls of the xylem vessels stops them collapsing under the suction pressure, but in fact the xylem vessels (and even whole stems and trunks) do shrink slightly during the day when transpiration is maximum.

Movement through the Leaves



The xylem vessels ramify in the leaves to form a branching system of fine vessels called leaf veins. Water diffuses from the xylem vessels in the veins through the adjacent cells down its water potential gradient. As in the roots, it uses the symplast pathway through the living cytoplasm and the apoplast pathway through the non-living cell walls. Water evaporates from the spongy cells into the sub-stomatal air space, and diffuses out through the stomata.



Factors affecting Transpiration
The rate of transpiration can be measured in the lab using a potometer (“drinking meter”):



A potometer actually measures the rate of water uptake by the cut stem, not the rate of transpiration; and these two are not always the same. During the day plants often transpire more water than they take up (i.e. they lose water and may wilt), and during the night plants may take up more water than they transpire (i.e. they store water and become turgid). The difference can be important for a large tree, but for a small shoot in a potometer the difference is usually trivial and can be ignored.


The potometer can be used to investigate how various environmental factors affect the rate of transpiration.
Light : Light stimulates the stomata to open allowing gas exchange for photosynthesis, and as a side effect this also increases transpiration. This is a problem for some plants as they may lose water during the day and wilt.
Temperature : High temperature increases the rate of evaporation of water from the spongy cells, and reduces air humidity, so transpiration increases.
Humidity : High humidity means a higher water potential in the air, so a lower water potential gradient between the leaf and the air, so less evaporation.
Air movements : Wind blows away saturated air from around stomata, replacing it with drier air, so increasing the water potential gradient and increasing transpiration.

Many plants are able to control their stomata, and if they are losing too much water and their cells are wilting, they can close their stomata, reducing transpiration and water loss. So long periods of light, heat, or dry air could result in a decrease in transpiration when the stomata close.

Nov 21, 2009

Quick Revision

http://www.scribd.com/doc/22852141

Ignore the eye structure.

Lonjakan Saujana Bio SPM 2009

http://www.scribd.com/doc/22817421

Credit to PPDMT

Nov 19, 2009

Trial SPM Kelantan 2009 (Answer P1, P2, P3)

http://www.scribd.com/doc/22747087/Answer-P1-P2-P3-2009

The digestive tracts of a herbivore (koala)

The koala′s intestines are much longer, an adaptation that enhances processing of fibrous, protein–poor eucalyptus leaves from which it obtains virtually all its food and water. Extensive chewing chops the leaves into very small pieces, increasing exposure of the food to digestive juices. The koala′s caecum—at 2 m, the longest of any animal of equivalent size—functions as a fermentation chamber where symbiotic bacteria convert the shredded leaves into a more nutritious diet.





Ruminant digestion

The stomach of a ruminant has four chambers. Because of the microbial action in the chambers, the diet from which a ruminant actually absorbs its nutrients is much richer than the grass the animal originally ate. In fact, a ruminant eating grass or hay obtains many of its nutrients by digesting the symbiotic microorganisms, which reproduce rapidly enough in the rumen to maintain a stable population.



Major Human Endocrine Glands and Some of Their Hormones


Nov 18, 2009

Homeostasis

Homeostasis Provides a Constant Internal Environment and Independence from

Fluctuating External Conditions
- Features that influence internal environment have a set level → norm
- Any changes from the norm is called deviation
- Negative feedback/caused by deviation from norm/change results in return to norm
- External environment is changing → experienced by body

Homeostatic system even out variations experienced by body
- Liver can store or release glucose
- Blood is kept at a constant, ideal state
- Glucose conc. of 80mg cm-3
- Tissue fluid surrounds working cell with constant ideal conditions
- Optimum glucose for respiration

Negative Feedback Tends to Restore Systems to their Original Level
- Homeostasis is achieved by a negative feedback and involves
      Change in level of an internal factor (change from norm level)
      Detected by receptors / impulse send to hypothalamus
      Activates effectors / stimulates corrective mechanism
      Level of factor returns to norm
 - Factors in blood and tissue fluid must be kept constant :
      Temp and pH
          Change affects rate of enzyme-controlled/biochemical reactions
          Extreme changes denatures proteins
          Humans maintain constant core body temp between 36-37.8°C
          Body temp refers to core body temp → limbs may be cooler than 37°C
      Water potential / avoids osmotic problems → cellular disruption
      Concent. of ions (Na, K, Ca)

Hypothermia
Mechanisms Involved in Heat Production, Conversation, and Heat Loss.
The Role of the Hypothalamus and the Autonomic Nervous System in Temperature Control

Blood flows through receptors in the hypothalamus
Deviation causes the autonomic nervous system to initiate an appropriate response

DEFICIENCY/DROP IN CORE BODY TEMP BY DECREASING HEAT LOSS/INCREASING HEAT PRODUCTION
- Receptors in hypothalamus detect increase in core temp/temp of blood
- Heat conversation centre stimulated
- VASOCONSTRICTION of arterioles
- Arterioles leading to capillaries in the skin narrow
- SHUNT VESSELS DILATE
- Less blood flows to skin surface / less heat is lost by RADIATION
- Hair raising / greater insulation / humans have less dense hair \ no effect
- Shivering / rapid contraction and relaxation of muscles / heat produced by RESPIRATION
- Adrenaline INCREASES METABOLIC RATE of cells //Mammals in cold climates can increase     secretion  of thyroxine / hormone increases metabolic rate on a more permanent basis
- VOLUNTARY CENTRE: put on clothes / seek warmer areas / warm drink

EXCESS/RISE IN CORE BODY TEMP BY INCREASING HEAT LOSS/REDUCING HEAT PRODUCTION
- Receptors in hypothalamus detect increase in core temp/temp of blood
- Heat loss centre stimulated
- VASODILATION of arterioles
- Arterioles leading to capillaries in the skin dilate (expand)
- SHUNT VESSELS CONSTRICT
- More blood flows to skin surface (capillaries) / heat loss by RADIATION
- Heat loss by EVAPORATION of sweat / by using energy
- High(er) rate of sweating leads to a low(er) skin temp
- VOLUNTARY CENTRE: remove clothing / seek cooler area / cold drink

The Role of Temperature Receptors in the Skin
- Hypothalamus detects temp fluctuation inside the body/internal environment
- Skin receptors detect temp changes in external environment
- Information is sent by nerves to voluntary centres of the brain
- Voluntary activities (jogging, moving into a shade) are initiated
- Changes behaviour of human

The Structure and Role of the Skin in Temp Regulation
- Surface area is very large and in direct contact to external environment
- Skin is divided into two layers: outer epidermis and inner dermis
- MALPIGHIAN layer is the boundary between these two layers
- Cells of this layer divide repeatedly by mitosis
- Older cells are pushed towards the surface/EPIDERMIS
- Cytoplasm of old cells becomes full of granules / cells die
- Cells become converted into scales of keratin (waterproof)
- DERMIS is thicker than epidermis and contains
- Nerve endings (temp receptors)
- Blood vessels held together by connective tissue
- Beneath dermis is a region which contains some subcutaneous fat
- Adipose tissue (fat storage tissue) provides vital insulations in humans

Hypothermia
- Body temp falls dangerously below normal
- Heat energy is lost from body more rapidly than it can be produced
- Brain is affected first → person becomes clumsy and mentally sluggish
- As body temp falls, metabolic rate falls as well
- Makes body temp fall even further, causing a POSITIVE FEEDBACK
- Temp is taken further away from the norm
- Death when core body temp is below ≈25°C / by ventricular fibrillation / normal beating of the
   heart is replaced by uncoordinated tremors
- Most at risk are (1) babies and (2) elderly
- (1) High surface area:volume ratio, undeveloped temp regulation mechanisms
- (2) Detoriated thermoregulatory mechanisms
- Deliberate hypothermia is sometimes used in surgical operations on heart
Patient is cooled by
- Circulating blood through a cooling machine
- Placing ice packs in contact with the body
- Reduces metabolic rate / O2 demand by brain + other vital tissues is lowered
Heart can be stopped without any risks of the patient suffering brain damage through lack of O2
Tissues may be permanently damaged if patient is cooled to long

Diabetes
The Factors which Influence Blood Glucose Concentration
Digestion of carbohydrates in diet
Digestion → polysaccharide → glucose
Fluctuation of glucose blood level depend on amount + type of carbohydrate eaten
Breakdown of glycogen
Excess glucose → glycogen → glucose
Storage polysaccharide made from excess glucose by glycogenesis
Glycogen is abundant in liver + muscles
Conversion of non-carbohydrates to glucose by gluconeogenesis
Oxidation of glucose by respiration
Glucose → ATP → energy
Rate of respiration varies for different activities
This affects glucose uptake from blood into cells
Brain is unable to store carbohydrates
Lack of glucose in blood → no respiratory substrate → insufficient energy for brain
Short period of time already causes brain to malfunction
Normal glucose level in blood ≈90mg per 100cm2
After a meal it rarely exceeds 150mg per 100cm2

Role of Hormones in Activating Enzymes Involved in Interconversion of Glucose and Glycogen

The Role of Insulin and Glucagon in Controlling Blood Glucose
The Pancreas
Endocrine role is to produce hormones
Contains islets of Langerhans → sensitive to blood glucose conc
Islet cells contain
α-cells → secrete glucagon and β-cells → secrete insulin
capillaries into which hormones are secreted
delta cells → produce hormone somatostatin → inhibits secretion of glucagon
Insulin mainly affects muscles, liver, adipose tissue
Exocrine role is to produce digestive enzymes
Active trypsin damages pancreas / digests proteins that make up pancreas / amylase leaks into blood from damaged tissues / amylase conc in blood higher

High Blood Glucose Concentration
Detected by β-cells in islet of Langerhans (receptor) → secrete insulin
Increase in insulin secretion (corrective mechanism → effectors bring about a return to norm)
Speeds up rate of glucose uptake by cells from blood
Glucose enters cells by facilitated diffusion via glucose carrier proteins
Cells have vesicles with extra carrier molecules present in their cytoplasm
Insulin binds to receptor in plasma membrane
Chemical signal → vesicles move towards plasma membrane
Vesicle fuses with membrane → increases glucose carrier proteins
Activates enzymes / Converts glucose to glycogen / Promotes fat synthesis

Low Blood Glucose Concentration
Detected by α-cells in islets of Langerhans → secrete glucagon
Increase in glucagon secretion
Hormone activates enzymes in the liver → convert glycogen to glucose
Stimulates formation of glucose form other substances such as amino acids
Glucose passes out of cells into blood, raising blood glucose conc until norm is reached
Diabetes and its Control with Insulin and Manipulation of Carbohydrate Intake
Diabetes mellitus → inability of control of blood glucose level
High levels of blood glucose because
Pancreas becomes diseased → fails to secrete insulin
Target cells lose responsiveness to insulin
Kidney is unable to reabsorb back into blood all the glucose filtered into its tubules
Glucose secreted into urine
Craving for sweet food and persistent thirst
DIAGNOSTIC: glucose tolerance test
Patient swallows glucose solution
Blood glucose level measured at regular intervals

Two Types of Diabetes Mellitus
Type I → insulin dependant/juvenile-onset
Occurs in childhood
Autoimmune reaction → immune system attacks and destroys own cells
Destroys β-cells in islet of Langerhans → unable to produce insulin
TREATMENT: insulin given must match glucose intake and expenditure
Overdose causes hypoglycaemia (to much glucose withdrawn from blood)
Diabetics need to manage their diet and levels of exercise
Need to monitor blood glucose conc
Type II → insulin independent/late-onset
Occurs late in life, more common than type I
Causes by gradual loss in responsiveness of cells to insulin
TREATMENT: regulated diet
Sugar intake must balance with amount of exercises taken
Glycogen levels are lower
Little insulin / no glucose to glycogen
Insulin receptors no longer functional / less glucose taken up by cells
Glycogen is an effective storage molecule
Insoluble → no osmotic effect
Large → cannot diffuse out of cell
Branched → easy to break down / hydrolyse to glucose
Compact → large amount of glucose stored in small space

Insulin Patches
Insulin → peptide chains → digested if swallowed by peptidase → had to be injected
Treat skin area with ultrasound → disrupts underlying fat tissues
Insulin is not soluble in fat
Disrupting tissues allows movement through skin
Apply patch containing insulin to that area


Enzymes

What are enzymes?
All enzymes are globular proteins → spherical in shape (Fig 1)
Control biochemical reactions in cells
They have the suffix "-ase"
Intracellular enzymes are found inside the cell
Extracellular enzymes act outside the cell (e.g. digestive enzymes)
Enzymes are catalysts → speed up chemical reactions (Fig 2, Fig 3)
Reduce activation energy required to start a reaction between molecules
Substrates (reactants) are converted into products
Reaction may not take place in absence of enzymes (each enzyme has a specific catalytic action)
Enzymes catalyse a reaction at max. rate at an optimum state


Lock and key theory
Only one substrate (key) can fit into the enzyme's active site (lock)
Both structures have a unique shape
Induced fit theory (Fig 4)
Substrate binds to the enzyme's active site
The shape of the active site changes and moves the substrate closer to the enzyme
Amino acids are moulded into a precise form
Enzyme wraps around substrate to distort it
This lowers the activation energy
An enzyme-substrate complex forms → fast reaction

E + S → ES → P + E

Enzyme is not used up in the reaction (unlike substrates).





Enzyme Activity

Changes in pH
Affect attraction between substrate and enzyme
Ionic bonds can break and change shape → enzyme is denatured
Charges on amino acids can change → ES complex cannot form
Optimum pH (enzymes work best)
pH 7 for intracellular enzymes
Acidic range (pH 1-6) in the stomach for digestive enzymes (pepsin)
Alkaline range (pH 8-14) in oral cavities (amylase)
pH measures the conc. of hydrogen ions → higher conc. will give a lower pH

Increased Temperature
Increases speed of molecular movement → chances of molecular collisions → more ES complexes
At 0-42°C rate of reaction is proportional to temp
Enzymes have optimum temp. for their action (usually 37°C in humans)
Above ≈42°C, enzyme is denatured due to heavy vibration that breaks -H bonds
Shape is changed → active site can't be used anymore

Decreased Temperature
Enzymes become less and less active, due to reductions in speed of molecular movement
Below freezing point
Inactivated, not denatured
Regain their function when returning to normal temperature

Nov 15, 2009

To all SPM & STPM Candidates 2009.....


Carbon dioxide transport in the blood


Tracheal system

The tracheal system of insects, made up of air tubes that branch throughout the body, is one variation on the theme of a folded internal respiratory surface. The largest tubes, called tracheae, open to the outside. The finest branches extend to the surface of nearly every cell, where gas is exchanged by diffusion across the moist epithelium that lines the terminal ends of the tracheal system .


The structure and function of fish gills



A fish continuously pumps water through its mouth and over gill arches, using coordinated movements of the jaws and operculum (gill cover) for this ventilation. (A swimming fish can simply open its mouth and let water flow past its gills.) Each gill arch has two rows of gill filaments, composed of flattened plates called lamellae. Blood flowing through capillaries within the lamellae picks up oxygen from the water. Notice that the countercurrent flow of water and blood maintains a concentration gradient down which O2 diffuses from the water into the blood over the entire length of a capillary.

The composition of mammalian blood


The mammalian cardiovascular system: An overview



There are two types of circulatory systems in animals : Open system and Closed system.

Open system - The organs/cells are bathed in the blood. (eg. in insects such as locusts)
Closed systems - The organs/cells are provided with blood via vessels. (eg. in mammals such as kangaroos )

Closed System
Single Circulation is a circulatory system in which the blood passes through the heart once, in its passage around the body. eg. Fish
Blood leaves the heart and is oxygenated in the gills.
The oxygenated blood goes to the rest of the body, and then returns back to the heart.
The heart has one atrium and one ventricle. Remember the ventricle is at the bottom.
The Blood which arrives at the organs of the body are at a low pressure.

Double Circulation is when blood flows through the heart twice during its journey around the body. eg. Mammals
Blood leaves the right hand side of the heart, via the pulmonary artery ( right hand side meaning the person's right, ie, the left hand side as you look at it in a book) . The oxygenated blood returns to the heart to the left hand side, via the pulmonary vein. The pulmonary artery is the only artery in the body that carries deoxgenated blood, and the pulmonary vein is the only vein that carries oxygenated blood. The overall system is known as pulmonary circulation.

The Heart Beat
The mammalian heart is myogenic which means the heart beat starts at the heart itself.
The sinoatial node (SAN), which is located in the right atrium, sends an electrical impulse across the muscle in the right atria to another node called the atrioventricular node (AVN).
This electric impulse causes the muslce to contract, so as it goes across the atria, they contract forcing blood through into the ventricles, via the bicuspid and the tricuspid valves.
The AVN is located in the middle of the two ventricles in the septum, when the impulse reaches this node, it spreads along the specialised fibres (The Bundle of His). This causes the ventricles to contract forcing the blood to leave via the semi lunar valves to the arteries.

Things to remember :-
The aorta takes oxygenated blood TO the body.
The vena cavae bring deoxygenated blood BACK to the heart.
The pulmonary artery takes deoxygenated blood TO the lungs.
The pulmonary vein brings oxygenated blood back to the heart.

Regulation of the cardiac output -There are two nerves running to the heart, 1. The vagus (a.k.a. parasympathetic nerve) and 2. the sympathetic nerve. These two nerves bring impulses from the Medulla Oblongata (more precisely the cardiovascular centre from the medulla oblongtata).

The vagus nerve is used to slow down the heart. It sends its impulses to the SAN and to the AVN.

The sympathetic is used to speed up the heart. It sends its impulses to the walls of the heart. These two nerves work in opposite to each other.

Adrenaline - is a hormone and it is secreted from the adrenal glands in time of fear, stress or nervous anticipation (eg. SPM result day). Adrenaline speeds up the cardiac cycle so that more oxygen can be provided to muscles and cells which need it. Similar effects are seen when the sympathetic nerve is stimulated.

These two nerves can be effected by many different things :-

Blood pressure  - Baroreceptors/stretch receptors in the aorta and also in the walls of the carotoid artery are sensitive to any changes in pressure. When the blood pressure changes these receptors send messages to the medulla oblongta, which then reacts accordingly.

The concentration of carbon dioxode - If there is a low pH then there's an increase in CO2 levels, chemoreceptors in the brain, aorta and carotoid arteries detect these changes and then send messages to the medulla oblangta. N.B. The concentration of oxygen does NOT affect the heart rate, the carbon dioxide concentration does.

Feedback Mechanisms in Thermoregulation


Their Main Components and Functions in Mammals


Phototropism


Water Transport


There are two types of vessels in plants :-

Xylem -- These vessels take water and mineral ions from the roots to the stem and the leaves.
Phloem -- takes iorganic substances and sugars from the leaves to the parts of the leaves that require them eg. the flowers, fruits and roots.
Xylem travels only upwards, whereas phloem travels in both directions.

Movement in xylem vessels
The cells which make up a xylem vessel are dead. They are joined together by a sticky substance called lignin, these cells are therefore said to be "lignified". This causes the xylem vessels to be impermeable.
There are three mechanisms which contribute to the movement of water through the xylem vessel.

Capillarity --- Xylem vessels are often very small in plants and therefore water is able to travel up them via capillary action. The water molecules stick to the side of the vessel and slowly "climbs up". However this mechanism does not account for the greater distance that water can travel in trees.
Root pressure ---- Some plants can produce a water potential gradient by actively transporting mineral ions to the top of the plant. The water potential on the top of the plant is much greater than the bottom of the plant, therefore the water moves up to the top.
Cohesion-tension --- As leaves transpire, water evaporates to the dry surroundings outside the plant. Water molecules stick to each other by hyrdogen bonds. This is known as cohesion. So as they leaves transpire the water molecules are all pulled up the plant. Water molecules also stick to the sides of the vessel which helps to speed this mechanism up. This is known as adhesion.















An overview of transport in a vascular plant.


Life cycle of Angiosperms


Nov 13, 2009

Gametes Development and Pollination


Gametophyte Development and Pollination

Anthers and ovules bear sporangia, structures where spores are produced by meiosis and gametophytes develop. Pollen grains, each consisting of a mature male gametophyte surrounded by a spore wall, are formed within pollen sacs (microsporangia) of anthers. An egg–producing female gametophyte, or embryo sac, forms within each ovule.

In angiosperms, pollination is the transfer of pollen from an anther to a stigma. If pollination is successful, a pollen grain produces a structure called a pollen tube, which grows and digests its way down into the ovary via the style and discharges sperm in the vicinity of the embryo sac, resulting in fertilization of the egg. The zygote gives rise to an embryo, and as the embryo grows, the ovule that contains it develops into a seed. The entire ovary, meanwhile, develops into a fruit containing one or more seeds, depending on the species. Fruits, which disperse by dropping to the ground or being carried by wind or animals, help spread seeds some distance from their source plants. When light, soil, and temperature conditions are suitable, seeds germinate and the embryo carried in the seed grows and develops into a seedling.

Keep in mind, however, that there are many variations in the details of these processes, depending on the species.

Within the microsporangia (pollen sacs) of an anther are many diploid cells called microsporocytes, also known as microspore mother cells
Each microsporocyte undergoes meiosis, forming four haploid microspores, each of which can eventually give rise to a haploid male gametophyte.

A microspore undergoes mitosis and cytokinesis, producing two separate cells called the generative cell and tube cell. Together, these two cells and the spore wall constitute a pollen grain. During maturation of the male gametophyte, the generative cell passes into the tube cell. The tube cell now has a completely free–standing cell inside it (the generative cell). The tube cell produces the pollen tube, a structure essential for sperm delivery to the egg. During elongation of the pollen tube, the generative cell usually divides and produces two sperm cells, which remain inside the tube cell. The pollen tube grows through the long style of the carpel and into the ovary, where it then releases the sperm cells in the vicinity of an embryo sac.

One or more ovules, each containing a megasporangium, form within the chambers of the ovary. One cell in the megasporangium of each ovule, the megasporocyte (or megaspore mother cell), grows and then goes through meiosis, producing four haploid megaspores.

The details of the next steps vary extensively, depending on the species. In most angiosperm species, only one megaspore survives. This megaspore continues to grow, and its nucleus divides by mitosis three times without cytokinesis, resulting in one large cell with eight haploid nuclei. Membranes then partition this mass into a multicellular female gametophyte—the embryo sac. At one end of the embryo sac are three cells: the egg cell and two cells called synergids. The synergids flank the egg cell and function in the attraction and guidance of the pollen tube to the embryo sac. At the opposite end of the embryo sac are three antipodal cells of unknown function. The remaining two nuclei, called polar nuclei, are not partitioned into separate cells but instead share the cytoplasm of the large central cell of the embryo sac. The ovule, which will eventually become a seed, now consists of the embryo sac and two surrounding integuments (layers of protective sporophytic tissue that eventually develop into the seed coat).

Pollination, the transfer of pollen from anther to stigma, is the first step in a chain of events that can lead to fertilization. This step is accomplished in various ways. In some angiosperms, including grasses and many trees, wind is a pollinating agent. In such plants, the release of enormous quantities of pollen compensates for the randomness of this dispersal mechanism. At certain times of the year, the air is loaded with pollen grains, as anyone plagued with pollen allergies can attest. Some aquatic plants rely on water to disperse pollen. Most angiosperms, however, depend on insects, birds, or other animals to transfer pollen directly to other flowers.


Leaf Anatomy


Human Lymphatic System


The lymphatic system consists of lymphatic vessels, through which lymph travels, and various structures that trap “foreign” molecules and particles. These structures include the adenoids, tonsils, lymph nodes, spleen, Peyer′s patches, and appendix. The flow of lymph is traced in steps 1–4.

A vertebrate neurone

Arising from the cell body are two types of extensions: numerous dendrites and a single axon. Dendrites (from the Greek dendron, tree) are highly branched extensions that receive signals from other neurones. The axon is typically a much longer extension that transmits signals to other cells, which may be neurones or effector cells. Some axons, such as the ones that reach from your spinal cord to muscle cells in your feet, may be over a meter long. The conical region of an axon where it joins the cell body is called the axon hillock; as we will see, this is typically the region where the signals that travel down the axon are generated. Many axons are enclosed by a layer called the myelin sheath. Near its end, an axon usually divides into several branches, each of which ends in a synaptic terminal. The site of communication between a synaptic terminal and another cell is called a synapse. At most synapses, information is passed from the transmitting neurone (the presynaptic cell) to the receiving cell (the postsynaptic cell) by means of chemical messengers called neurotransmitters.