February 18, 2015

Nerve Cells and Synapses: Grade 9 Understanding for IGCSE Biology 2.88 2.89

There is very little in the iGCSE specification about nerve cells and synapses. This is a shame since neuroscience is going to be one of the massive growth areas in Biology in the 21st century. There is a syllabus point about reflex acs and I draw your attention to this blog post about that: https://pmgbiology.wordpress.com/2014/04/22/a-simple-reflex-arc/

But in this new post I am going to give you a tiny bit more detail about the types of nerve cells (neurones) that you might encounter, together with an explanation about the most important component of the nervous systems: the chemical synapse.

Neurones are the cells in the nervous system that are adapted to send nerve impulses. You won’t fully understand what the nerve impulse is until year 13 but it is correct so that it is a temporary electrical event that can be transmitted over large distances within a cell with no loss of signal strength. The upshot of this is that neurones can be very long indeed…..

There are three basic types of neurone that are grouped according to their function:

Motor neurones (efferent neurones) take nerve impulses from the CNS to skeletal muscle causing it to contract

Sensory neurones (afferent neurones) take nerve impulses from sensory receptors into the CNS

Relay (or sometimes Inter) neurones are found within the CNS and basically link sensory to motor neurones.

These three types of neurone also have different structures although many features are shared….

This is a diagram of a generalised motor neurone: I know it is a motor neurone since the cell body is at one end of the cell. The cell body contains the nucleus, most of the cytoplasm and many organelles. Structures that carry a nerve impulse towards the cell body are called dendrites (if there are lots of them) and a dendron if there is only one. The axon is the long thin projection of the cell that takes the nerve impulse away from the cell body. The axon will finish with a collection of nerve endings or synapses.

Neurones can only send nerve impulses in one direction. In the diagram above these two cells can only send impulses from left to right as shown. This is due to the nature of the junction between the cells, the synapse (see later on….)

The diagram above shows a sensory neurone. You can tell this because it has receptors at one end collecting sensory information to take to the CNS. The position of the cell body is also different in sensory neurones: in all sensory neurones the cell body is off at right angles to the axon/dendron.

You can see from the diagrams that motor and sensory neurones tend to be surrounded by a myelin sheath. Myelin is a type of lipid that acts as an insulator, speeding up the nerve impulse from around 0.5m/s in unmyelinated neurones to about 100 m/s in the fastest myelinated ones. The myelin sheath is made from a whole load of cells (glial cells) but there are gaps between glial cells called nodes of Ranvier. These will become important in Y12/13 when you study how the impulse manages to travel so fast in a myelinated neurone.

Relay neurones, also known as interneurones, have a much simpler structure. They are only found in the CNS, almost always unmyelinated and have their cell body in the centre of the cell.

The diagram above shows the three types of neurone and indeed how they are linked up in a simple reflex arc. The artist hasn’t really shown the interneurone structure very well, but it was the best I could find just now…..

Nerve cells are linked together (and indeed linked to muscle cells) by structures known as synapses. There are a lot of synapses in your nervous system. The human brain contains around 100 billion neurones and each neurone is linked by synapses to around 1000 other cells: a grand total of 100 trillion synapses. 100 000 000 000 000 is a big number.

The big idea with synapses is that the two neurones do not actually touch. There is a tiny gap called the synaptic cleft between the cells. The nerve impulse does not cross this tiny gap as an electrical event but instead there are chemicals called neurotransmitters that diffuse across the synaptic cleft.

The nerve impulse arrives at the axon terminal of the presynaptic neurone. Inside this swelling are thousands of tiny membrane packets called vesicles, each one packed with a million or so molecules of neurotransmitter. When the impulse arrives at the terminal, a few hundred of these vesicles are stimulated to move towards and then fuse with the cell membrane, releasing the neurotransmitter into the synaptic cleft. The neurotransmitter will diffuse rapidly across the gap and when it reaches the post-synaptic membrane, it binds to specific receptor molecules embedded in the post-synaptic membrane. The binding of the neurotransmitter to the receptor often causes a new nerve impulse to form in the post-synaptic cell.

These chemical synapses are really beautiful things. They ensure the nerve impulse can only cross the synapse in one direction (can you see why?) and also they are infinitely flexible. They can be strengthened and weakened, their effects can be added together and when this is all put together, complex behaviour can emerge. I am going to exhibit some complex behaviour now by choosing to take my dogs for a walk… And it all happened due to synapses in my brain!

February 17, 2015

Adrenaline: Grade 9 Understanding for IGCSE Biology 2.94

Adrenaline is a hormone produced in the adrenal glands which are found on top of the kidneys in the abdomen.

A hormone is “a chemical released by a specialised gland called an endocrine gland into the bloodstream. The hormone travels around the body in the blood plasma and then causes an effect elsewhere in the body by binding to receptors found on certain target cells”.

You should know some other examples of hormones – testosterone, oestrogen, progesterone, ADH – to name a few. Please learn this definition too: it would be wonderful if you got a 3 mark question asking you to define a hormone….

There are many cells in the body that contain receptors for adrenaline. This allows the hormone to exert an effect on a wide variety of tissues. For example there are adrenaline receptors in the pacemaker of the heart and adrenaline will cause the heart to beat faster (more beats per minute) and also with more force.

When is adrenaline released by the adrenal glands into the blood?

Adrenaline is secreted into the blood in times of danger or stress. It prepares the body to either run away from the danger or indeed to battle against it. For this reason, adrenaline is often described as a “fight or flight” hormone.

What are some of the effects of adrenaline?

Target Tissue Effect

Heart Increase in heart rate, increase in cardiac output

Lungs Bronchioles dilate (widen)

Muscles Arteries in muscle dilate to allow more blood to flow to muscles

Skin/Digestive system Arteries in skin/digestive system constrict so less blood flows

Liver Liver breaks down glycogen into glucose to raise blood glucose conc.

Iris Radial muscles in iris contract causing pupil dilation

The overall effect is that the skeletal muscles are supplied with more oxygen and more glucose so they can respire aerobically. This allows the muscle to contract more efficiently.

February 17, 2015

Cardiac cycle and the Human Heart: Grade 9 Understanding for IGCSE Biology 2.65 2.66

The human heart is an organ found in the middle of the thorax. It is made from a specialised type of muscle called cardiac muscle and acts as a pump to push blood around the two circulatory systems. In fact it is better to think of the heart as two separate pumps, one for the pulmonary circulatory system to the lungs and one for the systemic system that supplies blood to all the other body organs.

The diagram above shows in a simplified way this double circulatory system. The lungs are supplied with deoxygenated blood direct from the heart in the pulmonary artery but when the blood has passed through the capillaries in the lungs, it travels back to the heart in the pulmonary vein before being pumped in the systemic system around the body. Although cardiac muscle looks similar to the muscle that attaches to bones and moves the skeleton, it differs functionally in one important way. Cardiac muscle is myogenic: this means that the muscle fibres will contract without the need for a nerve impulse from the brain to initiate the contraction. Incidentally this is why a heart transplant is a possible surgical procedure. A transplanted heart will beat happily in the new body even though all the nerves going to the heart will have been cut in the surgery. You cannot have a biceps transplant at the moment because the transplanted biceps muscle would not do anything in the new patient. For the transplanted biceps to contract, the millions of individual neurones going to it would need linking up individually and this is not possible.

This is a simplified diagram showing the structure of the heart. You can see that the left and right sides of the heart are completely separate from each other. This is essential because the right side of the heart contains deoxygenated blood and the left side oxygenated blood. There are four chambers in the heart: two small atria at the top, and two larger ventricles at the bottom. The atria collect blood from the veins, the ventricles pump blood out of the heart into arteries.

There are four sets of valves in the heart: the easiest way to remember where they are is to think that blood has to pass through a set of valves as it leaves each chamber. Valves allow blood to flow through them in one direction only. The AV valves stop blood going back from the ventricle into the atrium when the ventricle contracts, the aortic and pulmonary valves (not labelled on the diagram above for some reason….) prevent blood falling back into the ventricles in between heart beats.

You also need to know the four main blood vessels that are attached to the heart. The vena cava is the largest vein in the body and carries deoxygenated blood from the organs of the body into the right atrium. The pulmonary artery comes out of the right ventricle and pumps this deoxygenated blood to the lungs. Oxygenated blood returns from the lungs to the left atrium in the pulmonary veins, passes into the left ventricle and is then pumped out of the heart in the aorta, the biggest artery in the body.

NB I strongly suggest you do not use the terms bicuspid, mitral or tricuspid valve to label the valves between the atria and ventricles… In an exam it is easy to get these similar words muddled, so I would always call the valve between the left atrium and left ventricle the left atrio-ventricular valve (never bicuspid or mitral valve) and I would always call the valve between the right atrium and right ventricle the right atrio-ventricular valve (never tricuspid). I know many of you will ignore this advice but it’s good to get it off my chest……

I’ve just spent 45 minutes trying to find a good video on heart structure to put into the post but without success…. If anyone knows a really good YouTube clip (must be under 5 minutes) please add a link as a comment at the foot of this post.

Events of the Cardiac Cycle

This is a topic that requires some simplification as it is easy to get confused. The Cardiac Cycle is simply a word for the sequence of events that happen in a heart beat. At the simplest level, the cardiac cycle consists of three phases:

1. Diastole. During this stage the cardiac muscle is relaxed (the heart is between beats) and blood can enter the atria and then fall into the ventricles through the open AV valves.

2. Atrial Systole. This stage in when the cardiac muscle in the atria contract, increasing the atrial pressure and pushing blood down into the ventricles. There is a small region in the wall of the right atrium called the sino-atrial node (or pacemaker) which initiates each heart beat.

3. Ventricular Systole. After a short delay, the cardiac muscle in the ventricles contracts. This increases the blood pressure in the ventricle which in turn causes the AV valve to close, and the aortic or pulmonary semilunar valves to open. As these valves at the exit of the ventricle open, blood gets pushed into the arteries and out of the heart.

Look at the diagram above and make sure you understand the meaning of these three terms.

The diagram below is much more complex and perhaps you should not worry too much about it….. The important bit is to understand when the valves in the heart open and when they close. It is quite simple really although you would be amazed how confused people can get….

The opening and closing of a valve is not “controlled” in any meaningful way in the heart. The valve has a structure that will only allow it to open in one direction. Let’s consider the AV valves. These can open to allow blood to pass from the atria into the ventricles during atrial systole but will close during ventricular systole to stop the blood flowing back where it came from.

The AV valve will be open whenever the blood pressure in the atria is greater than in the ventricle.

The AV valve will be closed whenever the blood pressure in the ventricle is greater than in the atrium.

Look at the graph of pressure changes below. You can see the mitral valve (I hate that name) is closed as soon as the ventricular pressure exceeds the atrial pressure and it opens again during ventricular diastole as the pressure in the ventricle drops.

February 16, 2015

Blood part 2 White Blood Cells – Grade 9 Understanding for GCSE Biology 2.59 2.62 2.63B

The previous post looked at the structure and function of red blood cells and plasma. Now it is time to turn our attention to the rather more complex topic of white blood cells….. This is a topic in which the complexity can put people off but I am deliberately going to keep things simple (I hope!). If you are thinking about revision for GCSE, don’t worry about anything more complicated than in this post.

There are many types of white blood cell found in blood. But let’s keep things simple…. You need to understand the role of lymphocytes and phagocytes in defending the body against pathogens.

A pathogen is defined as “a microorganism that can cause a disease” and pathogens may be bacteria, viruses, protistans or fungi. Can you give me an example of an infectious disease caused by each class of pathogen?

The structure of these two classes of white blood cell is important. The commonest phagocytes in blood are called neutrophils and they are easily recognised by their irregular shaped nucleus and cytoplasm packed full of granules. Lymphocytes are much smaller white cells and are identifiable by their clear cytoplasm and large spherical nucleus that takes up 90% of the volume of the cell.

So now we should look at how these two types of white blood cells defend the body against pathogens. Remember that the account on this post is an over-simplification of what is in reality an extremely complex process.

Let’s start with a phagocyte. These large cells are able to engulf invading pathogens in the blood and tissue fluid by a process called phagocytosis.

The phagocyte pushes out projections of its cytoplasm around the clump of bacteria. These projections are called pseudopodia and when they meet, the cell membrane of the phagocyte fuses together leaving the bacteria enclosed in a tiny membrane packet called a vesicle inside the cytoplasm. The phagocyte then fuses other vesicles that contain powerful digestive enzymes with the vesicle with the bacteria in, leading to the death and destruction of the bacteria. Simple.

The problem for phagocytes is this: how do they know what to engulf and destroy? This is where lymphocytes come in. One class of lymphocyte is able to secrete small soluble proteins called antibodies into the blood. Antibodies are specific to a particular surface marker on the invading pathogen and bind to it because the shape of the antibody and the shape of the surface marker are complimentary.

Now people always get confused between antibodies (the small soluble Y-shaped proteins secreted by lymphocytes) and antigens (the surface markers on the invading pathogen). Make sure you are completely clear on the difference in meaning of these two words….

This diagram shows antibodies (green) binding to surface markers (antigens) on a bacterial cell.

Antibodies produced by lymphocytes will coat the invading pathogen by binding to antigens on its surface. One effect of this is that phagocytes are stimulated to engulf the antibody-coated organism.

There are many different types of lymphocyte and not all can produce antibodies. Another important function of lymphocytes is to kill your own body cells when they are corrupted, either by the presence of a virus or by becoming cancerous.

Finally, can I draw your attention to two previous posts linked to this one. The first is on the role of platelets in blood clotting, the second on the difficult topic of immunity and how lymphocytes are responsible for giving you lifelong protection against certain infectious diseases.

https://pmgbiology.wordpress.com/2014/06/08/platelets-and-blood-clotting-a-understanding-for-igcse-biology/

https://pmgbiology.wordpress.com/2014/04/07/immunity-a-understanding-for-biology-igcse/

As always, please ask me questions either via the comment section below the post or with a tweet…. I will do my best to respond to any questions from anyone who is bothered to read my posts!

February 16, 2015

Blood part 1 Plasma and RBCs: Grade 9 Understanding for IGCSE Biology 2.59, 2.60, 2.61

Blood is a tissue in the body that plays a variety of roles in transport and in defending the body against disease. It is an unusual tissue since it is a liquid, with many different kinds of cells suspended in a watery solution called plasma.

Plasma makes up 55% of the volume of blood and is a solution of many different chemicals in water. For example, the plasma contains dissolved glucose, amino acids and other products of digestion from the intestines. It also transports the waste molecule urea from the liver where it is made to the kidney where it is excreted. Blood plasma contains dissolved carbon dioxide, mostly in the form of hydrogencarbonate ions. Many hormones (for example testosterone, ADH, adrenalin) are transported in the blood plasma and because the plasma is mostly water, it provides a good way of moving heat around the body from respiring muscles to the skin where it can be lost.

The most common cell in blood are the red blood cells (or erythrocytes). These tiny cells are adapted for the transport of oxygen. Each red blood cell contains around 270 million molecules of a transport protein, haemoglobin. Each molecule of haemoglobin can bind up to four molecules of oxygen in the lungs and then unload the oxygen when the red blood cell passes through a capillary in an actively respiring tissue.

(Don’t worry too much about the structure of the protein – this is A level stuff really…. Just remember haemoglobin is a transport protein for oxygen found in red blood cells)

As well as being packed full of haemoglobin molecules, red blood cells have other adaptations for transporting oxygen. Red blood cells lose their nucleus during their development as this allows more haemoglobin to be packed into each cell. Having no nucleus means the red blood cell cannot divide nor repair damage to its structure. This is why each red blood cell only lives for 100-120 days in the body.

Red blood cells have a characteristic shape. It is called a biconcave disc and they have an especially flexible shape. Remember that a capillary is actually smaller in diameter than a red blood cell, so the cells have to squeeze through capillaries in single file…..

February 15, 2015

Blood vessels – Grade 9 Understanding for IGCSE Biology 2.68

In this post, I will look at the structure and function of the three main types of blood vessel in the human circulatory system. Although this is not the most difficult topic, there are a few things that can catch out even A* GCSE students in the heat of an exam.

Arteries are the blood vessels that take blood away from the heart. Because the blood is coming straight from the ventricles of the heart, it will be at a high blood pressure and will flow in pulses. This means that arteries need a thick wall to withstand this high blood pressure. All arteries apart from one carry oxygenated blood. Can you remember which artery is the exception to this rule?

The artery wall has a narrow lumen (the space where the blood flows) as this helps to maintain the high blood pressure within. There are also many elastic fibres in the middle tissue (tunica media) of the artery wall. This elastic tissue is important because the blood flows in pulses. The artery wall needs to stretch as the pulse of blood passes and the elastic recoil of the wall helps to push the blood along in between heart beats.

The tunica media also contains a lot of smooth muscle. Why do arteries need muscle in their walls? When this muscle contracts it narrows the lumen of the vessel. This will increase the blood pressure and so one reason for muscle in arteries is to regulate the blood pressure. But there is something more… Arteries carry blood into the organs of the body and the pattern of blood flow to different organs can vary depending on the conditions. For example, when you are running, you need more blood to go to your skeletal muscles (to carry oxygen for respiration and to remove heat and carbon dioxide) and less to go to the digestive system. This is brought about by the smooth muscle in the artery taking blood to the intestines and stomach contracting so that less blood can flow through the vessel. The smooth muscle in the arteries in the exercising muscles will relax so that more blood can pass. This shift in the pattern of blood flow is the second key significance of arteries having lots of muscle in the walls.

Veins have the same tissues in their walls as arteries but they are much thinner. The blood is flowing at a much lower pressure in veins as all the pressure from the heart has been lost in the extensive capillary beds in the tissues. Veins return blood to the heart and all bar one (the pulmonary vein) contain deoxygenated blood. As there is low blood pressure in veins, this can cause problems moving blood back to the heart especially when against gravity. Veins contain valves which only allow blood through in one direction thus preventing the blood falling back. The thin walls of veins also mean that they can be compressed by the action of skeletal muscles. When the muscles that move the skeleton contract, they can squeeze on veins and help to return blood to the heart.

Capillaries are the smallest of the three types of blood vessel. They are found in the tissues throughout the body and are beautifully adapted to ensure the exchange of materials between the cells of the body and the blood. The lumen of a capillary is less than the width of a red blood cell and so red blood cells pass through capillaries in single file and only be squeezing along. This ensures the speed of blood flow in capillaries is very very slow.

The lining of a capillary is made up of a single layer of cells called the endothelium. Arteries and Veins have an endothelium too but in the capillary the endothelial cells have gaps between them called pores. This allows the fluid component of blood and various white blood cells to leak out of capillaries to form tissue fluid. This leaky nature of capillaries is very important as it provides the fluid that bathes the tissues of the body.

February 15, 2015

Beware YouTube – “Sexual Reproduction in Plants Video”

https://www.youtube.com/watch?v=CkBNEM2mD30

This video illustrates clearly why you have to be careful using YouTube to find information for revision. It is produced by a company in Australia and is clearly presented, includes the right level of detail for GCSE in the UK and is easy to follow. But….

The video reinforces one of the commonest areas of confusion in this topic by its choice of images to accompany the text. In the section when the voice over is describing the role of animals in seed-dispersal, it has an image of a bee feeding on pollen in a flower. This is NOT seed dispersal!

Later in the video when the voice over talks about wind-pollinated flowers, there is an image of the seeds of a dandelion being blown by the wind. This is NOT pollination!

It might seem like a small point but when you have marked exam questions on this topic for 20 years and seen many students confuse these two separate processes, it starts to take on more significance. So please watch YouTube for science videos – there are some great resources on there…. But be critical and remember, just because it is on a video, it doesn’t mean it is correct!

February 15, 2015

Asexual Reproduction in Plants – Grade 9 Understanding for IGCSE 3.1 3.7

The previous posts have explained the processes involved in sexual reproduction in plants. But many species of plant can also reproduce asexually and this post is going to explain how and why this might occur… Now this is not a topic that is so exciting that it keeps many GCSE students awake at night but there is some good biology in here so pay attention!

Asexual reproduction is the term used for any reproductive strategy that produces genetically identical offspring. The term for a group of genetically identical organisms is a clone and so asexual reproduction is also called cloning. In animals and plants, asexual reproduction only involves one type of cell division, mitosis.

Sexual reproduction on the other hand always introduces genetic variation into the offspring. In the majority of cases, it involves the formation of two haploid gametes (produced in a specialised type of cell division called meiosis) which then fuse together in fertilisation to form a zygote.

This post is not the correct place to discuss the advantages and disadvantages of the two types of reproduction. But sexual reproduction comes at a cost for an organism: in plants this cost is the energy spent making attractive petals and scent to attract insect pollinators, the cost of wasting millions of pollen grains just to ensure some are transferred, the cost of making sweet tasty fruits for animals to eat. I am writing this the day after Valentine’s Day in the UK which illustrates the courtship costs for animals quite well…..

Asexual Reproduction in Plants:

Plants have evolved a variety of asexual strategies shown in the picture above. We only really need to consider one for your iGCSE exam and that is runners. Some plants, the classic example is the strawberry grow long horizontal stems outwards from the parent plant. When this “runner” touches the ground, root development is switched on and a new plant starts to grow upwards. When the runner dies back, you are left with two genetically identical plants, hence Asexual Reproduction.

(This diagram is a little misleading…. The runner is not the name of the offspring plant, it is the long horizontal structure growing outwards just above the soil from the parent plant)

If you are really interested in learning more about asexual strategies in plants, well you should probably get out more… But you could study how tubers (such as in potato) and bulbs (such as the onion) allow plants both to over-winter and also produce clones.

Artificial Methods of Asexual Reproduction in Plants

This is quite weird if you think about it….. A human can cause asexual reproduction in many species of plant by “taking a cutting“. As the name suggests this involves cutting off a small part of the plant (including a leaf and part of the stalk) and then sticking it into soil to grow a new plant. The only type of cell division in this process is mitosis and so the plant produced from the cutting will be a clone of its parent (genetically identical).

This is an example of artificial asexual reproduction in plants. It is a useful strategy for gardeners as it allows you to produce lots of new plants for your garden without shelling out hard-earned cash at the garden centre….

Cuttings work much better if in between taking the cutting and planting it in a small pot of compost, the cut end of the stalk is dipped in a mixture of chemicals calling a “rooting powder”

Rooting powder contains a mixture of plant growth substances (sometimes incorrectly called hormones) that can switch on the genetic programme of root production.

I hope you find this post useful. It probably holds the record for the dullest site anywhere on the World Wide Web…. Typing this has made me feel sleepy, so I am going to lie down…..

Please add comments/questions or tweet me if anything is unclear.

February 14, 2015

Sexual Reproduction in Plants (2) – Grade 9 Understanding for IGCSE Biology 3.4

In the previous post, I looked at the flower structure of both insect and wind pollinated flowers and explained the process of pollination. Now we need to ask “What happens next….?”

When a flower has been pollinated, there will be pollen grains that have landed on the stigma. These might be from a different species of plant in which case, nothing will happen. If they are from a different individual of the same species of plant, then triggered by sugary chemicals on the stigma the pollen grain starts to grow a tube called a pollen tube (imaginative people these plant biologists…..) which grows down through the style.

Pollen tube growth through the style is a complex process and the exact mechanism by which the pollen tube “knows” in which direction to grow is not completely understood. But it does know and grows down through the style and enters the ovule through a tiny opening in the ovule called the micropyle.

Now this is where it gets complicated …… but luckily for your exam, you don’t need to learn about the weird way plants undergo fertilisation. But to give you a taste, the male gamete which is a nucleus in the pollen grain divides on the way down the pollen tube to form two sperm nuclei (see diagram above). Each of these nuclei will fertilise a different nucleus in the ovule, hence the name double fertilisation.

But let’s keep it simple. There is a haploid female gamete called an egg cell inside the ovule and one of the haploid sperm nuclei from the pollen grain will fuse with it in the process of fertilisation. This produces a diploid cell called the zygote that will later develop into the embryo plant.

Quick reminder: Haploid is a term that refers to a cell that only has one member of each pair of chromosomes. Gametes are haploid cells and when two gametes fuse they produce a cell with pairs of chromosomes and this cell is described as Diploid.

The egg cell inside the ovule is now fertilised. It has a full set of chromosomes and is now called a zygote. So what happens to the structures in the flower…? After fertilisation the petals, sepals, stamens, stigma and style all dry out and wither. The ovary develops into a structure called the fruit and inside the fruit, each ovule develops into a seed.

Seeds are tough structures that have evolved to allow the embryo plant to undergo a period of dormancy before the seed germinates. The function of fruit is seed dispersal. It is vital for the parent plant that its offspring do not start to grow right next to themselves as they will be in direct competition with the parent for water and minerals from the soil and for sunlight. For animals it is easy for the parent to get rid of their offspring – they simply kick them out of the nest or send them to boarding school to get them out of the house…. Plants need to rely on more ingenious strategies….

In some plants the fruit has evolved to disperse the seed using the wind. Sycamore seeds have a propellor blade to slow down their fall from the tree. Dandelions give each seed a tiny parachute and can be carried for many miles in the wind.

But animals are more commonly used as couriers to get the seeds away from the parent plant. The fruit may be sweet and attractive to eat; the fruit may have hooks or barbs to get stuck to the animals body. Many seeds are dispersed by animals in a wide variety of ways…

It is really important not to get confused between the role of animals as pollinators of flowers and their separate role in seed dispersal. Keep these two processes (pollination and seed dispersal) clearly separated in your notes and in your mind. In the stress of the exam, candidates often get muddled and so write nonsense….. This is something to avoid if possible!

PMG tip: organise your notes on plant reproduction into the following subheadings to keep things separate.

Flower Structure
Pollination
Fertilisation
Seed Dispersal
Germination

February 14, 2015

Sexual Reproduction in Plants (1) Grade 9 Understanding for IGCSE Biology 3.3

Sexual reproduction in plants is a topic that some students find difficult at iGCSE. Perhaps it is the plethora of jargon terms, perhaps there are one or two complex ideas to master, perhaps it is just because some people just aren’t interested in plants (more fool them…) Anyway this post will attempt to cover all the main ideas needed for an A* understanding…. Flower structure The flower is the reproductive organ of the plant. A key difference in reproductive biology between animals and plants is that the majority of plants are hermaphrodite. Hermaphrodite means “an organism able to produce both male and female gametes”.

The male gamete in plants is a nucleus found inside a pollen grain, the female gamete is an egg cell nucleus found inside a structure called an ovule in the ovary of the flower. The male parts of the flower are called stamens. Stamens are made up of the pollen-producting anther supported on a stalk called the filament. The female part of the flower is called the carpel. The carpel is made of a stigma (adapted for receiving pollen), a thin style and a swelling at the base called the ovary. Inside the ovary are one or more smaller structures called ovules that contain the egg cells, ova that are the female gamete. The flower also has petals, often brightly coloured and scented to attract insects and that are protective structures that cover the flower when it is still a bud.

Pollination is the “transfer of pollen from the anther to the stigma“. Please learn this definition! A few plants (such as the garden pea) self-pollinate. This means pollen grains from one flower stick to the stigma of the same flower. If you want to irritate me in class, tell me that this is a type of asexual reproduction because only one parent is involved. I will respond with a strange facial grimace and a low guttural growl…… You have been warned. A plant that self-pollinates is still undergoing sexual reproduction as it is making gametes (by meiosis so every gamete will be genetically different) and then fertilising them in a random process. The offspring of self-pollination will still be genetically different from each other but the total genetic variation will be less than if DNA from two different individuals is used.

The vast majority of plants cross-pollinate. They transfer pollen from the anther of one flower to the stigma of a flower on a different plant. Cross-pollination can be brought about by a variety of mechanisms but the commonest two in the UK are by insects and by the wind.

The diagrams above all show flowers that are pollinated by insects. But look at this flower.

The stamens are large and hang outside the flower. The petals are green and small. The stigma is also hanging outside the flower and is large and feathery. This is a flower of a grass plant and typical of a wind-pollinated species.

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