Nerve Cells and Synapses: A* understanding for iGCSE Biology

Neurones are the cells in the nervous system that are adapted to send nerve impulses. 
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!

Homeostasis: A* understanding for iGCSE Biology

Homeostasis is one of the life characteristics shared by all organisms.  Living things all inhabit a world in which the external environment changes from hour to hour, from day to day, from month to month.  Even organisms living in the most stable aquatic environments may be subject to changing oxygen concentrations, changing water pH, changing light intensities and so on.  

This changing external environment poses a challenge for life since how can life processes operate at optimal levels in all these differing conditions.   Life has solved this by allowing organisms to keep their internal environments much more constant than the ever-fluctuating external environment.

A definition to learn:
“Homeostasis is the set of processes occurring in an organism to maintain a constant internal environment”

Examples of Homeostasis in Humans

A whole variety of factors are maintained at constant values in the body by homeostasis.  For example (there are many more….):

• Blood pH
• Blood temperature
• Blood dilution
• Blood oxygen concentration
• Blood carbon dioxide concentration
• Blood glucose concentration
• Blood pressure

This introduces the first area of common confusion in students’ exam answers.  For some reason many students think that homeostasis is a word for the maintenance of body temperature in humans.  I hope you can see it is a much more general term than that.
But…. the systems that maintain a constant body temperature in endothermic animals are one example of homeostasis.  In fact this example (thermoregulation) is one of the two from the list above that you need to understand for your exam.  The other one you might be asked about is osmoregulation (the maintenance of a constant dilution of the blood).

All homeostatic control systems have some common features.  The variable that is going to be regulated needs to be measured somewhere in the body.  A change in this variable is called a stimulus and is measured by a cell called a receptor.  The measured value needs to be compared with a “set value” and this is done by an integrating centre that then controls an effector.  The effector is an organ that can bring about a response.  

But what kind of response do you want in the process of negative feedback?

A common process involved in homeostasis is negative feedback.  This is quite tricky to define but in fact it is a really simple idea.  If you want things to stay the same, any change must be corrected. That’s negative feedback in a nutshell.
For example a school might want students walking round the campus at a sensible speed:  not to fast to knock people over, not to slow or people are late for lessons…  Imagine a particular group of children who start to run around the place, causing mayhem and injuries to fellow students.  Well this will first be detected by the system.  There may be a particular teacher who comes out and sees the students running, the school nurse might report an increase in cuts and bruises.  However it happens, a change in the system (a stimulus) is detected.  There will be an integrating centre in this control system too, probably in the form of a stern deputy head.  She will compare the measured speed to her own “set value” of how fast students should move.  And she will initiate a response:  probably a loud telling off to the entire school in assembly, lots of dire warnings about future conduct and an after school detention for all the rule breakers.  The net response of this will be that students will start moving slower around the school….  Eventually of course people will start moving too slowly and will be late for lessons.  How do you think the system will react to this new stimulus?  This process where the response tends to reduce the stimulus is called negative feedback.

Levels of Organisation: A* understanding for iGCSE Biology

Living things (or organisms to be precise) are complex entities.  Even the simplest organism will be made up of millions of different molecules arranged in an organised and complex way.  Human beings are organisms made up of about 10 trillion cells of roughly 210 different cell types all put together in a organised and systematic way.  It makes it much easier to study such complexity if we have a system to break the complexity down into constituent parts.  This is what scientists mean by levels of organisation.
So, starting with the smallest things that might be of interest to a biologist……
 
All matter on earth including the matter of living things is made of atoms (e.g. a carbon atom, an oxygen atom etc.).  Atoms can combine together in a variety of ways to form molecules (a water molecule H₂O, a carbon dioxide molecule CO₂,)  How atoms combine to form molecules is chemistry, and the levels of organisation smaller than an atom forms part of physics, so we won’t worry too much about them….

But molecules in an organism are interesting and worth studying – you learn about carbohydrates, lipids, proteins, DNA in your iGCSE course.  These molecules can be grouped together to form structures inside cells called organelles.  If you are being really precise with your terminology, an organelle is a membrane-bound compartment inside a eukaryotic cell (remember bacterial cells have no organelles at all).  Examples of organelles are structures like the nucleus, chloroplasts, mitochondria and so on.

Cells are structures enclosed by a cell membrane that contain many different organelles.  You have probably looked at a human cheek cell using a light microscope at some point in the past.  In multicellular organisms, cells of the same type are often attached together to form a Tissue.  A tissue is a group of similar cells often attached to each other that carry out the same function in an organism. Tissues are grouped together to form larger structures called Organs.  For example, the lungs are an organ made up of a particular arrangement of epithelial tissues together with some blood and connective tissues.   

Organs can be grouped into Organ Systems based on their function such as the Digestive System (oesophagus, tongue, stomach, pancreas, liver, intestines etc.)  An Organism such as you or I is made up of many organ systems (nervous system, cardiovascular system, digestive system, excretory system, etc)

Mineral ions in Plants: A* understanding for iGCSE Biology

The roots anchor the plant in the ground and so prevent it toppling over due to wind.  But their main function is to do with the absorption of materials from the soil into the cells of the plant.  The question is what exactly is taken up in the roots?
Well most people remember that water is absorbed in the roots by osmosis.  The best candidates will remember the microscopic root hair cells in the root that massively increase the surface area for the uptake of water.  This absorbed water is transported into the xylem tissue in the centre of the root and then moved up the plant to the leaves by transpiration pull.

Roots also absorb mineral ions from the soil by active transport.  Active transport is the process where energy from respiration in the cell is used to pump material across the cell membrane against the concentration gradient.  Mineral ions absorbed included nitrate ions (needed to make amino acids and proteins), magnesium ions (needed to make chlorophyll) and phosphate ions (needed to make DNA)

So where is the common misconception?  

This all seems sensible and fairly straightforward.  Roots absorb water by osmosis and mineral ions by active transport. Whenever root function is tested in exams, many candidates get in a pickle as they confuse mineral ions (nitrate, phosphate, magnesium, potassium) with food molecules.  Plants do NOT absorb food molecules through their roots.  There are very few food molecules such as glucose, amino acids, and lipids in soil.  If there were, more animals would eat soil as a source of nutrition……  Plants do not need to absorb food molecules of course:  the big idea you learn is that plants can make their own food molecules in the leaves in the process of photosynthesis.