support and movement

When plant cells take up lots of water by osmosis they become rigid, like a blown up balloon. We say they are turgid. This provides a lot of support to plants. When a plant dries out the cells lose their turgidity, which is why the leaves start to wilt.


Most of the largest plants we see, such as trees, have wood in them. Wood provides a lot of support and prevents branches from breaking or from wilting if they dry out. Lignin is the substance produced in the cells of woody plants that gives them this structure.

The skeleton of a mammal is made of bone and cartilage. Bone is a lot stronger than cartilage, because it needs to provide support for the body and a firm structure for muscles to attach to so that they can move parts of the body. Cartilage is the softer substance found in your ears, in your nose and between the joints. Cartilage does not contain as much calcium as bone, so it is more flexible which is why you can bend your ears. It is also smoother, which is why it is found in joints to stop the bones from rubbing together.

You need to know the names and positions of the following parts of the arm: the ulna, radius, humerus, scapula, tendons, biceps an triceps. The two bones in the forearm are the radius and the ulna. The ulna is the one that forms the elbow. The humerus is the bone in the upper arm. The scapulas are also known as the shoulder blades and are the plates located on your upper back. The biceps and the triceps are found either side of the humerus. When the biceps contracts it pulls on the bones in the forearm and bends the arm. When the triceps contracts it pulls on the part of the ulna that sticks out to make the elbow and this straightens the arm.

The contraction of these muscles produces a turning effect, with the elbow acting as the pivot. You may remember from your physics that the ?moment? (or turning force) is the force multiplied by its perpendicular distance (distance at right angles) from the pivot. This means that there is a bigger moment if the force is not acting near the pivot. This is why door handles are not near the hinge. The design of the human arm has the muscles attached very close to the pivot. Therefore they need to produce very large forces in order to move the arm. The arm could lift much heavier weights if the muscle was attached near the hand, but of course it would get in the way. You might be asked to make some moments calculations to do with the arm.

A joint occurs wherever two bones meet. Not all joints can move (for instance the various bones in your skull are fixed in place). A synovial joint allows the movement of two bones without the bones rubbing together and causing damage. The bones have a layer of cartilage at the ends and synovial fluid in the gap between the bones helps reduce friction, a bit like oil in a motor.

Organisms in their Environment

A population is all the individuals of a particular species that live in one area. For instance, you can have the population of fleas on a dog, or the population of fleas in El Salvador. If the conditions are perfect a population will grow. Eventually the population will get so big that there is not enough food, or not enough space, or too much disease, or too many predators, or some other factor that limits the population size, so the population stops growing.

We can draw graphs of population size against time which often have characteristic shapes. When nothing in the environment limits the growth of the population we see an exponential curve. This is where the graph gets steeper and steeper and steeper. This is what has happened to human populations throughout history. We have not yet reached our limit (though we will soon), but usually the graph stops getting steeper and flattens out, giving a characteristic sigmoid shape (sigmoid is like a flattened S shape).

In any habitat there will be many different populations that all have an effect on each other. This is called the community. The habitat will also affect the community because of such things as temperature, amount of water etc. The community and habitat together are known as the ecosystem.

An ecosystem gets all of its energy from sunlight. Plants absorb sunlight during photosynthesis and use it to produce complex chemicals such as glucose and proteins. An animal cannot make these chemicals, so to get them it needs to consume plants, or other animals that have consumed plants. When these animals and plants die the energy they contain is not wasted. Other organisms such as bacteria and worms feed on the decomposing remains of dead organisms. In this way, energy passes along a food chain from sunlight to plants (which we call producers), to animals (which we call consumers) to decomposers.

At the end of the food chain very little of the original energy from the sunlight remains for the decomposers. This is because much of the energy has been used up by the plants and animals further down the chain (of course remembering that we cannot create or destroy energy, only transfer it from one form into another ? in this case the organisms have transferred it mostly into heat energy). This is why the animals at the top of the food chain (the tertiary consumers), such as eagles, jaguars, wolves etc are generally quite rare, whereas plants and primary consumers (the animals that eat plants) are much more common.

We can represent the number or organisms in a food chain by using a pyramid of numbers. We call it a pyramid because usually there are less individuals the further up the food chain that you go. However this is not always the case. For example you can have thousands of insects living on one tree because the tree is so big. In this case it is better to use a pyramid of biomass (the dry mass of the organisms at each step in the food chain). This is because even though there are more insects than trees, there is a far greater mass of tree than there is of insects.

The pyramid of numbers can also tell us something about how humans can get more food from the land. If energy is lost on its way up the food chain then humans will get more energy out of their land if they grow crops than if they raise animals. An area of land may grow enough lettuces to feed one hundred people, but if you feed the lettuces to rabbits and then eat the rabbits you may only feed about 10 people, because typically only about 10% of the energy is passed from one level in the food chain to the next. Occasionally, it will still be important to eat some animals because they provide an important source of protein which only certain plants can provide.

Inheritance

The individuals of any species are never exactly the same. This is partly because they have different genes and partly because they have had different experiences in their lives or lived in different environments. For example, a single gene determines the colour of a persons eyes and the environment has no effect. In contrast, any scars a person has are only the result of the environment. Some things are a mixture of both; for instance a tall person may have genes that help make them tall, but they also need to eat the correct food to help them grow.

The difference between continuous and discontinuous variation often confuses people. Continuous variation means that a characteristic can be almost anything between two extremes. For example, some people are very tall and some people are very short. These are the extremes. Other people can be ANY size in between. On the other hand, discontinuous variation means that there are only specific categories that you can belong to. An example is blood group. A person is either A, B, AB, or O and can be either positive or negative. You cannot be half way between the two, you are either completely one or completely another.

The nucleus in cells contains a chemical called DNA. This is found in long strands called chromosomes. Humans have 46 chromosomes, whilst other organisms have more, less or even the same number as us. The DNA contains sets of coded instructions for making proteins. These proteins will help determine the characteristics of an organism. A set of coded instructions to make a protein is called a gene. Humans have about 100,000 different kinds of gene (we actually have up to two versions of each gene). Almost all of the cells in a person?s body will have exactly the same 46 chromosomes containing the same 100,000 genes. Eye colour is controlled by a single gene, because the colour is caused by a single protein. Some more complicated features, such as the brain, require many proteins so are controlled by many genes.

The only cells that are different are the red blood cells, because they don?t have a nucleus so don?t have any DNA, and the sex cells (sperm and eggs). Each sperm cell only contains half of a man?s chromosomes (23). Equally, each egg contains only half of a woman?s chromosomes. This is important because it means that when an egg and a sperm join together during fertilization, the new zygote that forms will have the correct number of 46 chromosomes; half of which were obtained from each parent.
Earlier I mentioned that we have up to two copies of each gene. One copy of each of the 100,000 genes we received from our father and the other copy of each of the 100,000 genes we received from our mother. This means that we might receive a different version of a gene from each parent. The different versions of a gene are called the alleles. There is a singe gene that controls our eye colour. However, there are several different alleles that we can have, ie blue, brown, green etc. We might get, say, a brown allele from one parent and a blue allele from the other parent. Eye colour has discontinuous variation, so we can only be one colour or the other ? we cannot be a mixture of the two. What happens is that one allele is DOMINANT to the other allele, which we call RECESSIVE. In humans, the brown eye colour allele is always dominant to the blue eye colour allele. In this case the child would have brown eyes, even though they also carry the allele for blue eyes. In this case we say they are HETEROZYGOUS, because the two alleles for that gene are different. A person who receives the allele for blue eyes from both parents will have blue eyes. Both of the alleles are the same, so we say they are HOMOZYGOUS. A good way to remember which is which is to remember that a homosexual fancies people from the same sex and a heterosexual fancies people from a different sex. We can see from this that by looking at the type of features a person has (their PHENOTYPE) we can not necessarily tell what type of genes a person has (their GENOTYPE).

Geneticists are able to predict what genotypes and phenotypes will occur in the offspring when two organisms are bred together. Any mating of this kind we call a genetic cross. It can be represented using a punnett square. For example, in pea plants, tall plants are dominant to short plants. In this case tall is dominant so we will use the letter T to represent tall and t to represent short (this is the standard way of writing it using the capital letter of the first word to describe the dominant allele and the same letter in lower case to represent the recessive allele ? we do not use T and S). If the male plant had the genotype Tt and the female plant had the genotype tt, then the possible pollen that can be made by the male are T or t. When the female plant produces ovules it will produce only t and t.

We can use a punnett square as follows to show the proportions of each genotype and phenotype that we would expect to find in the offspring.






We can see from this cross that half of the genotypes are heterozygous Tt. The T is dominant to the t, so we can say that half of the offspring will have tall phenotypes. Half of the genotypes are homozygous tt. In this case the recessive characteristic will result in a short phenotype. The ratio of tall plants to short plants in this case would be 1:1.

You need to be able to conduct crosses like this using a punnett square and be able to work out the expected ratios of the different genotypes and phenotypes. Remember that they are only ?expected? ratios. If we actually did the cross shown above with real pea plants and had 1000 offspring, we would expect about 500 to be tall and 500 to be short. However, it would not always be exact because it depends which pollen grain fertilizes which ovule so you could easily find that the numbers are a little bit out, say 485 tall and 515 short.

Sometimes a piece of DNA can be mutated. This means that it becomes slightly changed. Sometimes this occurs naturally, but it is especially common wherever there is radiation, because radioactive particles moving at incredibly high speeds can collide with the complicated DNA structure and alter it. Usually, if a normal body cell is mutated in this way it does not cause too many problems. The exception is when it causes the cell to start dividing uncontrollably into a ball of cells called a tumor. This is called cancer. Another problem is caused when the DNA of a sex cell is damaged. This is worse because the damage is passed on from one generation to the next. We call this a genetic disease.

Sickle-cell anaemia is a genetic disease that affects the red blood cells. When the oxygen levels are low the cells form into a sickle shape and can block the capillaries causing pain and in severe cases severe damage or even death. You might then ask the question ?if its so harmful, why hasn?t sickle cell anaemia disappeared through evolution?? The answer is that it can also be an advantage to have the sickle cell allele. People who are heterozygous do not suffer much from the disease, but they have the advantage that they are immune to Malaria, because the parasite cannot live in their blood. This is why in most parts of the world the sickle cell allele is very rare, but in areas with lots of malaria it is very common.

Genetic engineering is a relatively new method whereby scientists are able to take a gene from one organism and put it into another. One famous example is where the gene to make human insulin was removed from a human cell and inserted into a bacteria cell. These bacteri now make human insulin for diabetics to use. Many crop plants and farm animals can also be altered in this way. There can be problems if mistakes are made which cause harmful results that were not expected.

Modern cloning techniques allow the production of plants with identical genes. This has benefits for agriculture, because farmers know exactly how to look after their crop because there is no variation between the individual plants.

Reproduction in human

You need to know the structure of the female reproductive organs (see the handouts you were given in class) and also the functions of the main parts.

Adolescence (puberty) is controlled by the sex hormones. The male sex hormone is called testosterone and it is produced in the testes. The female sex hormones are called oestrogen and progesterone and they are produced in the ovaries.

Male
Sperm is produced in the testes. A liquid is produced in the seminal vesicles and prostate gland and mixed with the sperm to form semen, which is stored in the seminal vesicles. The man?s penis is made of spongy tissue that fills up with blood when the man is sexually aroused. This makes the penis become erect. During intercourse the penis is inserted in to the woman?s vagina. Stimulation of the penis causes an ejaculation, where the semen is squirted from the seminal vesicles through the urethra (tube in the penis) into the woman?s vagina. The sperm then swim up into the vagina and then into the oviducts to fertilise an egg, if one is present.

Female
An egg is produced every month in one of the ovaries. It passes down the oviduct (or fallopian tube) and then into the uterus (womb). It then goes through the cervix (the part separating the uterus from the vagina) and then passes out through the vagina. However, if the woman has had sexual intercourse there are sperm in the oviducts. Fertilisation takes place when the sperm and the egg fuse to form a zygote.

The menstrual cycle
In the weeks leading up to ovulation (the release of an egg) an egg develops in one of the ovaries and the uterus develops a thick lining with lots of blood vessels. If an egg is fertilized it will fix itself to this lining and develop into an embryo. The blood supply will provide it with nutrients until the placenta grows. If an egg is not fertilized this thick lining is no longer needed and is passed out through the vagina. This is known as menstruation or the period.

The embryo develops in the uterus and grows into a fetus. It receives nutrients and oxygen from the mother via the placenta. It also gets rid of waste (such as CO2) this way. The mother?s and baby?s blood does not mix in the placenta, but they do pass very close to each other so that these substances can be transferred between them. The fetus develops inside a sac called the amnion that is filled with a liquid called the amniotic fluid. This provides protection for the baby. One of the first signs that a woman is about to give birth is when this sac breaks and the liquid inside passes out of the vagina. Muscular contractions in the uterus build up over the next hour or more until they become so strong that the baby is forced out of the uterus through the vagina. The umbilical cord that attaches the baby to the placenta is cut. The baby no longer receives oxygen from the mother so it needs to start breathing. The placenta is ejected from the uterus shortly after the baby (its often called the afterbirth).

There are more people born in the world each year than there are people who die. This means that the human population is rising rapidly. This is causing many problems, because in some countries there is not enough food or space for everyone. One solution is for people to use some form of birth control (such as contraceptives) to limit the number of children being born.

AIDS is a serious disease that is spreading rapidly throughout the world, especially in some areas such as Africa. AIDS is caused by the HIV virus. The virus is found in the body fluids (such as blood, semen, vaginal secretions etc). Anything that results in the fluids of an infected person entering a healthy person can result in transmission (the virus being passed from person to person). The most common ways of transmission are from sexual intercourse, the sharing of needles by doctors and drug addicts and through blood transfusions (though nowadays the blood is usually tested first). The use of condoms during sexual intercourse and the use of clean needles would help to limit the spread of AIDS.

Reproduction

There are two types of reproduction:
Asexual reproduction
Asexual reproduction produces offspring that are identical to the one parent. One example is the splitting of bacteria into two. Another is when gardeners make cuttings, by removing a small piece from a plant and placing them in the ground to grow into a new plant.

Sexual reproduction
Sexual reproduction involves two parents mixing their genes to produce offspring that are genetically different from them. This is carried out by a mobile gamete, (which contains half of the males genes), such as a sperm or a pollen grain, fusing with a stationary female gamete (containing half of the female?s genes), such as an egg or an ovule.

The fusion of the male and female gametes is called fertilization. Some species (eg fish) use external fertilization. In this case millions of eggs and sperm are released into the water and fertilization takes place in the open water. Usually the adults do not care for the young so the survival rate is low. This is why so many gametes are needed. Some species (eg humans) use internal fertilization, which means that fertilization takes place inside the woman?s body (the fallopian, or egg tubes). This means that less sperm is wasted and also means that the young can grow safely inside the woman?s body. Animals on land cannot use external fertilization, because the sperm needs to be able to swim to the egg.

Insect pollinated flowers (you need to know the structure) produce sweet nectar that the insects feed on. They have brightly coloured petals and smells to attract the insects. The pollen is found on the anther, which is situated at the end of a long filament. When an insect visits the flower the pollen grains stick to the hairs on its body and are carried away when it leaves. When the insect visits another flower, some of the pollen grains are rubbed off onto the stigma, which is positioned at the end of a long stalk called the style at the center of the flower. When the pollen lands on the stigma we say that pollination has taken place. The pollen grows a pollen tube down the length of the style until it reaches the ovary. The nucleus moves down this tube and fuses with the female nucleus (ovule) in the ovary to make a zygote. This fusion of the nuclei is called fertilization.

Wind pollinated flowers (egg the flowers of grass) do not need to attract insects, so they are not brightly coloured, do not produce nectar and have no smell. The anthers hang outside the flower so that the pollen is easily picked up by the wind. The stigma also hangs outside the flower and it has many side branches to act like a net to catch the pollen as it blows past.

The zygote (fertilized ovule) grows to form the seed. The ovary, which usually contains many seeds, will grow into the fruit. The flesh of an apple is a fruit and so is a pod containing beans (which are the seeds). There is little point in a seed growing directly under its parent, because it would be competing for water, sunlight and space. It is best if the seed is dispersed (transported somewhere else). There are many ways of doing this. Some species of plant produce berries that are designed to be eaten by birds. However, the bird cannot digest the seed that passes through the digestive system and out with the faeces (an excellent fertilizer). Some seeds have hooks on the side that stick to the fur of passing animals and only fall off when the animal later cleans itself, which may be many miles from where the seed originated. Other seeds are dispersed by the wind. Some have wings so that they spiral to the ground. Others are very light and have feathery ends so that the wind can carry them great distances. Coconuts have a store of nutrients to survive long journeys as they float across oceans.

You need to know the structure of a bean seed, including: the testa (outer coat), micropyle (small hole where the pollen tube first entered to fertilise the ovule and where water will enter when the seed germinates), cotyledons (the two halves of the bean where starch is stored to feed the growing embryo), plumule (the part of the embryo that will grow upwards to form the shoot) and radicle (the part of the embryo that grows downwards to form the root).

Usually, seeds do not grow as soon as they leave the mother plant. They usually need to wait until the conditions are ideal. For instance, there would be no point in a seed germinating (starting to grow) in freezing conditions or in the middle of the dry season. Such things as temperature, moisture and oxygen are often seed to start germination. Sometimes a seed will lie dormant (still alive, but not growing) for many years before it germinates.

Nervous System

The Vertebrate Nervous System:
1 - receives stimuli from receptors & transmits information to effectors that respond to stimulation
2 - regulates behavior by integrating incoming sensory information with stored information (the results of past experience) & translating that into action by way of effectors
3 - includes billions of nerve cells (or neurons), each of which establishes thousands of contacts with other nerve cells
4 - also includes neuroglia cells that support, nourish, & insulate neurons

Subdivisions of the Vertebrate Nervous System:
1 - Central Nervous System - including the brain & spinal cord
2 - Peripheral Nervous System - including cranial nerves, spinal nerves, & all branches of cranial & spinal nerves

Neurons (or nerve cells):
respond to stimuli & conduct impulses
3 types - all with cell body & processes (axons & dendrites):
multipolar
bipolar
unipolar

Spinal cord:
located in vertebral canal
anatomical beginning is the foramen magnum of the skull
length varies among vertebrates:
in vertebrates with abundant tail musculature, the spinal cord extends to the caudal end of the vertebral column
in vertebrates without tails or without much tail musculature, the spinal cord extends to about the lumbar region of the vertebral column
a cross-section of the spinal cord reveals gray matter & white matter. The gray matter consists of nerve cell bodies, while the white matter consists of nerve cell processes (axons). These processes make up ascending (sensory) and descending (motor) fiber tracts.

Neuronal Structure

Sensory Neurone
The nerve impulses travel from left to right in this diagram of a sensory neurone. A stimulus causes the impulse to be produced by a sense organ. (skin / ears / eyes / tongue / nose)
Dendrites and Synapses are both nerve endings at the ends of neurones. Dendrites are located at the ends that receive the nerve impulses (at the left of diagram above). Synapses are found at the transmitting ends of the neurone where the impulse is transferred to another neurone. Synapses use chemicals to transmit their electrical signal.
Relay Neurone
This neurone does exactly what its name suggests. Relay neurones are situated in the spinal cord. This, along with the brain, acts as the central nervous system.
Reflex actions are caused when a stimulus creates an electrical impulse that is relayed via the relay neurone straight to the effector. The message never actually reaches the brain.
Motor Neurone
A motor neurone is connected to an effector and when an electrical nerve impulse is transmitted, the effector is stimulated into action. (muscles / glands)