Skin and homeostasis

When the body is cold
1. The small capillaries near the surface of the skin constrict (vasoconstriction) so less blood flows nearer to the surface, and less heat is lost through he blood.
2. The hairs on the skin stand erect by the hair erector muscles tightening. The hairs trap a layer of insulating air therefore less heat is lost to the surroundings.
3. Sweat Glands produce less sweat so less heat is lost through evaporation.
The muscles also vibrate very quickly when it is cold, so heat is produced to keep the body temperature at the correct level. This is also known as shivering!
When the body is hot
1. The small capillaries near the surface of the skin dilate (vasodilation) so more blood flows nearer to the surface, and more heat is lost through he blood. The body therefore cools down.
2. The hairs on the skin lie flat because the hair erector muscles slacken. No layer of insulating air is trapped therefore heat can be lost more easily.
3. Sweat Glands produce more sweat so more heat can be lost through evaporation

Responding to changes in the environment

Organisms have receptors (such as eyes, ears, tongues, auxins in plants etc) to detect external stimuli (such as light, sound-waves and chemicals). This enables them to detect what is going on around them and so helps improve their chances of survival.

The human nervous system is made up of the central nervous system (the brain and the spinal cord that are able to made decisions) and many nerves (made up of many nerve cells called neurons) that carry messages from receptors and to effectors. The effector is the organ that carries out the response. Sometimes this can be a gland, but it is usually a muscle. Motor neurons are nerve cells that carry an impulse from the central nervous system to a muscle (you need to know the structure of a motor neuron). A neuron can be very long, with the longest ones reaching from the tip of your toes up to your spine.

It takes a short time for a message to reach your brain, for your brain to think of a response and then for the message to travel to the effector and for it to react. Sometimes this would result in a response that is a little too slow to allow the body to protect itself. This is why, sometimes, the spine can make a simple response of its own. This is known as a spinal reflex arc. The impulse travels up the sensory nerve to the spine. It is then joined by a connector neuron to the motor neuron that immediately sends an impulse to respond. This cuts down the response time.

The rods and cones are cells in the retina that detect light. Rods detect how bright the light is. They are good for seeing at night, but only give a black-and-white image. Cones give a clearer image in colour. There are 3 types of cone: one blue, one green and one red. From these all the other colours can be formed. The fovea has only cones. This is where light is focused when you stare directly at something. It gives a clear coloured image (except when its dark). Around the fovea there is a mixture of rods and cones, giving good vision both in the light and in darkness. The outer parts of the retina have only rods. This is why you cannot see colours in your peripheral vision (although you are usually not aware of this). The rods and cones convert the light stimulus into an impulse, which passes down the optic nerve to the brain, which forms it into an image. There are no rods or cones at that part of the retina where the optic nerve leaves the eye. This area is called the blind spot.

The amount of light entering the eye depends on the size of the pupil, which is controlled by muscles in the iris. Light passes through the cornea then the liquid aqueous humour, followed by the pupil (the hole surrounded by the iris), then the lens, then through the liquid vitreous humour to land on the retina at the back of the eye. In order to see an image this light needs to be focused. The cornea, humours and lens all help to focus the light. However, the lens can vary how much it bends the light. When it is thin (stretched by the ciliary muscles) it does not bend the light very much, so it is good for focusing light from distant objects. When it is fatter it bends the light more, so it is better at focusing on close objects. This ability of the eye to change its focal length is called accommodation.

Plants also have receptors to detect stimuli in their environment and have effectors to respond. One example is phototropism, where a plant grows towards a light source. A hormone called auxin causes this to happen. Auxin is produced in the tips of shoots and promotes growth in the area just below the tip. When light falls on the shoot the auxin moves to the opposite side, away from the light. It then promotes the growth of the cells on that side only, resulting in the shoot bending towards the light.

nutrients in food

There are many different kinds of nutrient in our diet.

Protein
This is an essential body-building food. Much of the structure of cells is protein. Muscles are mostly proteins as are enzymes and hormones. Meat, fish, egg white and milk contain lots of protein. A good source of protein for vegetarians is Soya beans. In many undeveloped countries there is not enough protein in the diet which can prevent normal growth and development.

Fat
Cell membranes are made of fats. Fat also provides a long term store of energy. It also acts as insulation in many animals. Meat, milk and egg yolk contain a lot of fat. Oils are liquid fats. Many plants produce oils such as sunflower oil and olive oil which is used in cooking and to make margarine. It is much healthier to eat plant oils than to eat animal fats, because animal fats are usually much higher in cholesterol, which can cause heart attacks.

Carbohydrate
Starch and sugar are the main types of carbohydrate in the diet. They provide a more immediate source of energy than fats (eg glucose that is used in respiration). The staple diet (the main source of energy) for any population is always starch. In Asia the staple diet is mostly rice; in Britain it is mostly bread (or other products made from wheat flour) or potatoes; in Italy it is pasta and in Central America it is traditionally mainly Maize. Anything sweet contains sugar, such as cakes, sweets etc. Although starch and sugars contain basically the same thing, it is better to eat mostly starch as too much sugar can upset the osmotic balance in your body. Any carbohydrate that is not used is converted into fat for storage.

Vitamins
These are essential chemicals that you need to eat in order for your body to function properly. Vitamin C is needed to help bind the cells together. Without you can get scurvy. Vitamin D is needed so that your body can deposit calcium correctly. A lack of vitamin D leads to Rickets, a disease where the bones lack strength, resulting in the legs bending. Good sources of vitamins are fruits, vegetables and milk.

Minerals
These are inorganic compounds. This means that they do not need to be made by a living thing. If you went to the moon you would find minerals, but you would not find any of the substances mentioned above. Calcium is an example of a mineral that is an important part of our bones and teeth. Iron is another mineral; in this case it is an important part of the haemoglobin in our blood. A good source of calcium is milk. Iron is particularly common in liver and spinach. Many vegetables provide smaller amounts of these vitamins.

Roughage (fibre)
Much of the food we eat cannot be digested and passes right through the gut and is egested in the faeces. Much of this is the cellulose in the cell walls of plants. However, this is still useful because it gives some solid substance to the half digested food in the intestine which makes digestion easier. Without roughage a person can get ulcers, colon cancer and can suffer from constipation. Most roughage is found in fruits and vegetables. Constipation is often the result of eating too much meat and not enough vegetables. Modern packaged foods are often lacking roughage.

A balanced diet includes all of these nutrients in the required amounts. It also includes water. They are not all required in large amounts. Only small amounts of vitamins and minerals are required, but they are still very important. Eating the wrong amounts of these nutrients can be a problem, leading to malnutrition. Eating too much is often as bad as eating too little. In many developed countries people suffer from obesity (being overweight) because they eat too much sugar and fatty foods in their diet. Other parts of the world are lacking one aspect of a balanced diet or they have too little food altogether. The world produces enough food for everyone, but it is often in the wrong places and there are difficulties in distributing it to everyone.

Foods with lots of carbohydrates or fats are high energy foods. If a person eats a lot of this they can put on weight, unless they do a lot of exercise. If you take in more energy than you use you put on weight. If you use more than you take in you lose weight. When someone is dieting they should lower the amounts of fatty foods that they eat and do a bit more exercise. This ensures that they stay healthy. Harsh diets are very bad for the body and can even lead to you putting on more weight in the long run.

Respiration

Glucose is a store of chemical energy. When glucose is broken down into simpler chemicals the energy is released and the cell can use it for such things as movement, growth and warmth. To get the maximum energy out of glucose it is best to react it with oxygen to produce carbon dioxide and water. This is called Aerobic respiration.

Aerobic respiration

Glucose + Oxygen ----Carbon dioxide + Water + released energy

For short spells it is possible for human cells to respire without any oxygen. For instance, when an athlete runs 100m there is no time for the body to get enough oxygen to all of the muscles. In this case the cells do anaerobic respiration, which is respiration without oxygen. The cells convert the glucose into lactic acid and a small amount of energy is released. Lactic acid is harmful in the body, so humans cannot respire this way for very long. Afterwards the lactic acid needs to be removed and this requires oxygen (it is known as the oxygen dept). This is why an athlete pants after a race.

Respiration is one of the characteristics of living things. If a cell does not respire then it is not alive. Therefore, plant cells respire and animal cells respire and so do bacteria cells and fungi cells.

Gaseous Exchange in Animals

Larynx ? this is the Adam?s apple, which is where the tube in your throat splits up into 2: the trachea that takes air to and from the lungs and the esophagus that takes food to the stomach.
Trachea ? this tube has rings of cartilage that stop it from closing over (you can feel them in your throat. This ensures that air can always pass through.
Epiglottis ? this is a flap that is found in the larynx. Usually it closes over the oesophagus, but when you swallow it closes over the trachea to stop food going into your lungs (sometimes food or drink does go down the wrong tube and makes you cough).
Bronchi ? there are 2 lungs, so the trachea splits up into 2 bronchi: one for each lung.
Bronchioles ? the bronchi divide up into a network of tiny tubes called bronchioles.
Alveoli ? the bronchioles end on groups of small air sacs called alveoli (one is called an alveolus). This is where gas exchange takes place.
Pleural membranes ? it is important that the lungs do not rub on the inside of the chest. The pleural membranes prevent this.
Diaphragm ? this thin layer of muscle separates your thorax (chest cavity) from your abdomen (the area with most of your guts). When it contracts it increases the volume of the thorax and causes air to enter the lungs.
Intercostal muscles ? these are the muscles between the ribs. When they contract, they pull the ribs upwards and outwards to increase the volume of the thorax.

Gas exchange
The exchange of gases takes place in the alveoli. Oxygen crosses from the air in the alveoli to the haemoglobin in the blood. Carbon dioxide crosses in the other direction, from the blood to the alveoli.
The alveoli are especially designed to allow this to happen (again, it would help here to check a diagram). There are large numbers of blood vessels surrounding the alveoli; the alveoli and blood vessels both have thin walls (only one cell thick), so it is easy for gasses to diffuse through; the surfaces are moist, which allows the diffusion to take place more easily; the alveoli provide a large surface area for diffusion to occur. If the lungs were just bags, there would only be a very small amount of surface to exchange gasses, but the alveoli provide a surface area equal to about the size of a tennis court.
The gasses move across the surface by diffusion. Remember that this is the movement of particles from an area where it is highly concentrated to an area where it is less concentrated. Remember, also, that the membrane does not carry the particles across, they just move across randomly.

Inhaling and exhaling (again, check your handouts)
When you inhale (breath in), one set of intercostals muscles contracts pulling the ribs upwards and outwards, which increases the volume of the thorax (chest). At the same time the diaphragm contracts, which further increases the volume. This means that the air in the lungs is now taking up a bigger space, so it will be at a much lower pressure. The air outside the body is now at a higher pressure, so when you open your mouth the outside air pressure forces air to enter your lungs.
When you exhale, the opposite happens. The intercostals muscles pull the ribs downwards and inwards and the diaphragm relaxes, causing the volume of the thorax to decrease, which increases the pressure, forcing air out of the mouth.

Keeping the lungs clean
When you breathe in there are often small particles in the air and bacteria that enter your lungs. These can be harmful and need to be cleaned out. The surface of the lung has cells called goblet cells that secrete a sticky substance called mucous. The particles in the air stick to the mucous which can then be removed. This is done by the cilia, which are tiny moving hairs lining the surface of the lungs. The cilia waft the mucous and particles up the bronchi and trachea where it is usually swallowed. Coughing is a way of removing mucous from the lungs.

Smoke and air pollution
Although the lungs can clean some substances from the lungs, if there is too much it can cause serious damage. Many air pollutants can damage the delicate cells inside the lungs. The effect depends a lot on the type of pollutant that is present, but commonly the person will cough a lot and the efficiency of gas exchange will be less.

Smoking
Cigarettes contain a number of chemicals. Nicotine is the main drug that is found in cigarettes and it acts as a depressant, which means that it calms the body. Nicotine is also highly addictive, so many people smoke because they have a need caused by the addiction rather than getting any useful affect. A number of serious problems can be caused by smoking. The biggest danger is from lung cancer, but many other diseases such as emphysema, bronchitis, heart disease and a whole load of other problems, are associated with smoking. One problem is that the cilia lining the lungs stop being formed, so the lungs are not able to clean themselves. This means that the dangerous chemicals build up and have a greater effect. When someone gives up smoking the cilia grow back and start cleaning the lungs again. Smoking also affects the efficiency of the lungs, so smokers often get short of breath very quickly when they do exercise.

cells and function- plant cell

Plant cell: the cell is a highly complex system that is the site of intense energy exchange and which presents vast interphase surfaces. Like all living organisms, it feeds itself, grows, multiplies and dies.

Plasmodesma: intercellular bridge.

Dictyosome: cellular organelle that elaborates sugars and proteins.

Chromatin: a colouring substance in the nucleus of the cell.Nucleus: small spherical body with the cell nucleus.

Nuclear envelope: membrane surrounding the nucleolus.Endoplasmic reticulum: a formation within the cytoplasm that plays a role in the production of various substances.Peroxisome: cytoplasmic organelle which contains enzymes.Chloroplast: granule of chlorophyll, which is needed for photosynthesis.

Mitochondria: granule that plays an important role in the respiration and energy-releasing reactions in living cells.

Cytosol: liquid part of the cytoplasm.

Free ribosome: cytoplasmic organelle which is responsible for protein synthesis.

Tonoplast: vacuolar membrane.Vacuole: space with the cytoplasm of a cell containing various substances.

Cell wall: cell wall.

Plasma membrane: envelope of plasma.

Thylakoids: membranous molecular structures involved in photosynthesis.

Starch grain: starch granule.

Photosynthesis

Photosynthesis is the process used by plants to synthesise (make) many of the chemicals they need. Most of the substance of a plant (except the water) was made by photosynthesis. The process is therefore vital for the survival of all the organisms on this planet, including humans which grow plants to eat and to feed to animals. Many chemicals are made by photosynthesis, but the main one we will look at is glucose, which is the main food source for the plant to provide it with energy.

Glucose is made from Carbon, Hydrogen and Oxygen (C6H12O6). Complicated substances like glucose need to be built by an organism, which is why we say that they are organic. If you went to the moon or Venus you would not find any complicated substances like this. You would only find simple substances like Carbon dioxide (CO2) and water (H2O). Plants use the Carbon and Oxygen from CO2 and the Hydrogen from H2O to synthesize molecules of glucose. The Oxygen from the H2O is not needed, so is given off as a waste gas. A lot of energy is needed to convert simple substances into more complicated ones. Plants get this energy from sunlight. This is why glucose (a kind of sugar) contains so much stored chemical energy.

We can summarise photosynthesis with the following word equation (you don?t need to know the chemical equation):

Carbon dioxide + Water + Sunlight 輯font> Glucose + Oxygen

Sunlight is absorbed by a chemical in plants called chlorophyll. Chlorophyll is green, and it is what gives plants their distinctive colour. The reason it is green is because green light cannot be used for photosynthesis (only the red and blue wavelengths of light can be used), so it is reflected.

Like all chemical reactions, photosynthesis works faster in warm conditions than in cold conditions. It also works faster if there is more Carbon dioxide and sunlight and plenty of water. This is why plants grow so quickly in El Salvador in the wet season. If any of these factors is in short supply the rate of photosynthesis will be less and the plant will not grow as fast. Farmers sometimes use greenhouses to increase the temperature, or grow plants on slopes that face the sun, or even pump extra CO2 into their greenhouses to increase the rate of photosynthesis.

Conversely, a lot of photosynthesis can affect the environment, such as aquatic environments and the atmosphere. Most of the Oxygen in the air has come from photosynthesis. The more photosynthesis that takes place, the more Oxygen is produced and the less CO2 remains.

Most photosynthesis takes place in leaves, which are specially designed for the job they do. The flat shape provides a big surface to allow more sunlight to be trapped, but is thin enough to allow CO2 to diffuse in and O2 to diffuse out. The leaves need to have an outer layer of wax, called cuticle, to prevent them from drying out. This is made by the outer protective layer of cells called the epidermis. These cells do not take part in photosynthesis. The water is transported from the roots to the leaves in the xylem vessels, which are found in the veins of the leaf. The veins also contain phloem to transport the glucose (and other products of photosynthesis) to the rest of the plant.

Near the surface of the leaf lie the palisade cells which have large numbers of chloroplasts containing chlorophyll to trap sunlight. The underside of the leaf has small holes called stomata. This is where CO2 and O2 diffuses in and out. The size of the hole can be altered by the guard cells on each side of the stoma. This means that the stomata can be closed to prevent drying out in dry conditions. The diffusion of these gasses around the inside of the leaf is made easier because the spongy mesophyll cells in the lower half of the leaf have air pockets between them to increase the movement of air.

You may remember back to topic 2 where we looked at osmosis. If a cell has lots of dissolved solutes it will tend to absorb more water and could eventually burst. For this reason plants cannot store very much glucose, so they convert it into starch before they store it. Starch is made from about a thousand glucose molecules joined together into a complex chain. The size of the starch molecules means that they cannot dissolve very easily, so they do not cause very much osmosis, which makes them ideal for storage.

You need to remember how to perform starch tests on leaves.
1. Boil leaf in water for one minute to kill the cells.
2. Place the leaf in a boiling tube of alcohol and boil the leaf for 10 minutes (do not use a flame, but place the tube of alcohol in a beaker of boiling water). This removes the chlorophyll and makes the leaf white so that you can see the results of the Iodine test.
3. Dip the leaf in water to soften it.
4. Lay the leaf on a flat surface and add a few drops of Iodine.
5. The presence of a blue/black colour indicates the presence of starch.

We saw how Carbon, Hydrogen and Oxygen are used in photosynthesis to make glucose and other organic substances. Many of these other substances contain other important elements such as Nitrogen. Nitrogen is particularly important to make proteins. However, plants cannot take Nitrogen directly from the air. It first needs to be in the form of nitrate (NO3-) ions. These (and other important ions) are found in the soil, dissolved in the water. When the plant absorbs water through the roots it also takes in the nitrates.