AQA GCSE Biology Unit 3

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  • Created by: Georgia
  • Created on: 09-05-13 13:59


DIFFUSION is the movement of molecules from an area of high concentration (dilute solution) to an area of low concentration (concentrated solution).

OSMOSIS is the diffusion of water over a partially permeable membrane.

Both DIFFUSION and OSMOSIS go along the concentration gradient.

The movement of water molecules requires NO ENERGY.

In animals, the cytoplasm becomes more concentrated if a cell uses up its water in chemical reactions so more water moves in by osmosis. PROBLEM: The cell could swell and may burst. If the cytoplasm becomes too dilute as more water is made in chemical reactions, water will leave the cell by osmosis. PROBLEM: If the cytoplasm becomes too concentrated, it may shrivel up.

Plants rely on water entering the plant by osmosis to support the stem and leaves. The pressure caused by the vacuole swelling and pressing the cytoplasm against the plant cell walls makes the leaves and stems of plants rigid and firm.

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Active Transport

ACTIVE TRANSPORT is the movement of a substance against a concentration gradient from an area of low concentration to an area of high concentration. This uses up energy from 'cellular respiration' (found in the intestines and the kidneys) for absorption and re-absorption.

Examples are root hair cells and cell lining in your gut.

Lots of mitochondria > lots of energy > lots of respiration > lots of active transport.

Active Transport requires ENERGY.

Cells can absorb ions from very dilute solution and can actively absorb substances e.g. sugar (glucose) and salt against a concentration gradient.

Also, by using active transport, plants can absorb mineral ions found in soil as these are normally dilute solutions.

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The Lungs

OXYGEN diffuses INTO the bloodstream while CARBON DIOXIDE diffuses OUT.

The lungs have a very large surface area provided by millions of air sacs called alveoli. The lungs are moist and thin so that diffusion takes place quickly.

Alveoli provide a large, moist surface area (spherical shape) with a rich blood supply and have thin walls (shorter distance) to make diffusion as efficient as possible. 

Oxygen diffuses into the many capillaries surrounding the alveoli and carbon dioxide diffuses back out into the lungs.

Breathe out: Intercostal muscles contract... Ribcage down and out, Diaphragm up (domed shape) forcing carbon dioxide out of the lungs, Decreased volume = incresed pressure in chest.

Breathe in: Intercostal muscles relax...  Ribcage up and out, Diaphragm down (flattened), Increased volume = lower pressure in chest.

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Artificial Breathing Aids

Several types of breathing aid have been developed:

Iron lung

Used for people with polio who were paralysed.
Person lay with chest sealed in a large metal cylinder.

When air was drawn out of cylinder the person’s chest moved out and they breathed in.
vacuum which was formed inside cylinder created negative pressure.
air was pumped back into cylinder, pressure created and forced air out of persons lungs.

Positive Pressure Breathing

Breathing aids which force measured amount of air into the lungs use positive pressure. Bags of air linked to masks can force air down the trachea.

Positive pressure aids are often smaller, easier to manage in the home and can be linked to computers for control

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The Gut

The food we eat is digested in the gut into small, soluble molecules to be absorbed by the blood in the small intestine.

The small intestine has a rich blood supply to carry away the digested food to the cells and maintain a steep concentration gradient. For this to take place efficiently a large surface area is needed.

In the small intestine, small ‘finger-like projections’ called villi increase the surface area for absorption to take place.

Food is absorbed by diffusion when there is a lower concentration of that molecule in the blood. However it is absorbed by active transport when there is a higher concentration of that molecule in the blood, therefore moving against the concentration gradient. This is to make sure that all food is absorbed and none is wasted.

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Plants and Transpiration

Plants need...
DIFFUSION: to get carbon dioxide needed for photosynthesis

OSMOSIS:  to take up water from the soil
ACTIVE TRANSPORT: to obtain minerals from the soil.

The surface area of the roots is increased by root hairs. The surface area of the leaves is increased by the flattened shape and internal air spaces BUT leaves are also well adapted to control the INPUT and OUTPUT of carbon dioxide

Plants have STOMATA that allow them to obtain carbon dioxide from the air however this causes water to be lost. To prevent uncontrolled water loss, leaves have waxy cuticle layers that waterproof the plant.

Guard cells can control the size of the stomata and can close them to prevent water loss.

Transpiration is the process through which water vapour is lost from the surface of leaves.

Wilting of the plant can also reduce water loss because the leaves collape and hang down which reduces the surface area.

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The Circulatory System

Oxygenated blood is pumped to the heart from the lungs, and from the body to the heart.
De- oxygenated blood is pumped from the heart to the lungs and from the body back to the heart.

- Thick walls, small lumen, thick layer of muscle and elastic fibres.
- The arteries stretch as blood is forced through them and go back into shape afterwards.

- Relatively thin walls, large lumen, often have valves.
- Valves prevent backflow.

- Walls a single cell thick, tin vessle with narrow lumen.
- Also substances produced by cells pass into blood through capillary walls.

When conory arteries become blocked, doctors can use stents to allow blood to flow freely.

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The Circulatory System continued

The heart has four chambers.

- Blood enters the heart via the atria (the top chambers of the heart).

- De-oygenated blood from the body > right atrium from the VENA CAVA.

- Oxygenated blood from the lungs > left atrium in the PULMONARY VEIN.

- The atria contract together and force blood down the VENTRICLES.

- VALVES close to stop blood flowing backwards out of the heart.

- The ventricles contract and force blood out of the heart.

- The right ventricle forces de-oxygenated blood to the lungs on the PULMONARY ARTERY.

- The left ventricle pumps oxygenated blood around the body in a big artery called AORTA.

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Artificial or Real?

Artificial blood

Pros: - Always available and doesn't always have to be kept in the fridge.
         - Doesn't contain cells so can go into any tissue without having a blood match.

Cons: - Expensive
          - Doesn't carry as much oxygen as whole blood.
          - Can cause unpleasant side effects.

Artificial hearts

Pros: - No wait for donor and no tissue match needed.

Cons: - Size and expense.
         - Problems with blood clotting.

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Transport In The Blood

Blood Plasma:
- Transports carbon dioxide to the lungs and dissolved food molecules to all living cells.
- Also transports urea to the liver and the kidneys. Urea is a breakdown of protiens formed in the liver. The ura id removed from blood to form urine.

Red Blood Cells:
- Only transports oxygen.
- They are biconcave discs > increase SA:V ratio > more diffusion of oxygen.
- Are packed with a red pigment called HAEMOGLOBIN which releases oxygen into cells.
- They have NO NUCLEUS which means they have more space for molecules of oxygen.
Haemoglobin + Oxygen > Oxyhaemoglobin > Oxygen + Haemoglobin (Reversible reaction)

White Blood Cells:
- Part of the body's defence system against harmful micoorganisms. Some form antibodies against microorganisms and others digest invading bacteria and viruses.
PLATELETS HELP THE BLOOD TO CLOT by producing a network of protein threads that capture the red blood cells and more platelets. The clot dries and hardens to form a scab which which protects new skin as it grows and stops bacteria getting into the body.

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Transport Systems in Plants

Phloem - moving food
Phloem tissue transports dissolved sugars made by photosynthesis, from the leaves to the rest of the plant, including the growing regions e.g. the stems and roots. Here sugars are needed for making new plant cells.
- Food is also transported to the storage organs to provide energy to store for winter.
- Phloem is a living tissue.

Xylem - moving water and mineral ions
- Xylem tissue carries water and mineral ions from the soil around the plant.
- Xylem cells are dead.

- All cells need sugars for respiration and to provide materials for growth.
- The mineral ions are needed for the production of proteins and other molecules in the cells.
- Water is needed for photosynthesis and to hold the plant upright.

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Controlling Internal Conditions

UREA, made in the LIVER; URINE, produced by the KIDNEY. URINE CONTAINS UREA.

The internal environment of your body is kept relatively constant by a whole range of processes that together are know as HOMEOSTASIS. Waste products such as urea and carbon dioxide have to be removed from your blood all the time. The water and ion concentration in your blood are constantly controlled and so is your blood sugar level. Your body temperature is kept within a norrow range so your enzymes work efficiently.

Your body temperature is monitored and controlled by the thermoregulatory centre in your brain.

Cooling the body down:
- Blood vessels that supply your skin capillaries dilate allowing more blood flow through the capillaries. Your skin flushes so you lose more energy by radiation, cooling you down.
- Incease rate of sweating. Sweat cools you body down as it evaporates.
- Hairs lie flat.

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Controlling Internal Conditions continued

Reducing energy loss:
Blood vessels that supply your skin capillaries constrict reducing the blood flow through the
capillaries reducing the energy released by radiation through the surface of the skin.
- Sweat production is reduced. Less sweat evaporates so less energy is released.
- You may shiver as you muscles contract and relax rapidly. Muscle contractions need lots of respiration which releases more energy, raising your body temperature.
- Hairs pulled erect to trap insulating layer of air.

If you core body temperature falls below 35 degrees then you are suffering from hypothermia. Signs of hypothermia are tiredness and not wanting to move; skin feels cold; face goes greyish, blue and puffy with blue lips; feel drowsy; speech becomes slurred.

Heat stroke happens when your body gets too hot. The most common cause is hot, humid weather combined with exercise. Exercise releases energy and when its hot, your sweat cannot evaporate to cool you down. Also if you don't drink enough, you can't make sweat so you don't lose energy.

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The Human Kidney

Kidneys filter water and soluble substances then reabsorb useful substances such as sugar and ions by active transport. Large molecules such as protein cannot be filtered.

Excess protein creates amino acids, which are then broken down into smaller molecules in the liver forming ammonia. Ammonia is very poisonous so it is then converted into the less poisonous urea.

Excess mineral ions and water, along with urea, are removed in the urine.

Selective Reabsorbtion = The varying amount of water and dissolved mineral ions that are taken back into the blood in the kidney, depending on what is needed by the body.

Urobilins = Yellow pigment in the urine, from the breakdown of haemoglobin in the liver.

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We can carry out the function of the kidney artificially using dialysis to clean the blood.

The blood is taken from the patients artery in their arm and flows through tubes made of partially permeable membranes that act as the cell membranes in the kidney. The blood flows into the dialysis machine at a high pressure.

This allows urea and excess mineral ions to diffuse into the dialysis fluid along a concentration gradient and larger molecules like protein and cells to be held back in the blood. The ‘clean’ blood is then pumped back into the patient’s vein, again in their arm as the concentration of substances in the blood has been restored back to normal. THERE IS NO ACTIVE TRANSPORT.

Pros: - If they have a home dialysis machine it is more convenient (however big and expensive).
         - Relatively readily available.

Cons: - Controlled diet.
         - Time consuming as you need regular, long sessions to maintain effectiveness.

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Kidney Transplants

Having a kidney transplant is usually a better option than dialysis but the new kidney must be a very good ‘tissue match’.

The immune system is like to try and reject the new kidney so:

- The patients bone marrow is treated with radiation to stop white blood cell production
- The patient has to take immunosuppressant drugs to suppress the immune system

- The patient must be kept in sterile conditions to prevent infection

The old kidney is usually left in the body. The donor kidney is placed in the groin and attached to the blood vessels and the bladder then the kidney can take over the functions of the failed kidney.

Pros: - If accepted, they can live a normal life.

Cons: - Risk of rejection so you need regular check ups.
         - Don't last forever - normally around 9 years but some last for much longer.
         - You might not get the chance as there aren't many donors.

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Controlling Blood Glucose

GLUCOSE = A sugar found in the blood.
GLYCOGEN = A storage carbohydrate found in the liver and muscles.
GLUCAGON = A hormone.

The PANCREAS constantly monitors and controls your blood glucose concentration using a hormone called INSULIN.

When your blood glucose level rises after you have eaten a meal, insulin is released.
Insulin allows glucose to move fom your blood into the cells where it is used.

Soluble glucose is converted to an insoluble carbohydrate called GLYCOGEN.
Insulin controls the storage of glycogen in your liver.
The glycogen can be converted back into glucose when it is needed.

Insulin REDUCES blood glucose concentration.
Glugagon INCREASES blood glucose concentration.

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Type 1 Diabetes
- The blood glucose may rise to fatally high levels because the pancreas does not secrete enough insulin. Eventually your kidneys excrete glucose in your urine resulting in you producing a lot of urine and feeling thirsty all the time. Without insulin, glucose can't get into the cells of your body so you lack energy and feel tired. You break down fat and protein to use as fuel instead so you lose weight.
- It can be treated by injections of insulin before meals. This allows glucose to be taken in by your body cells and coverted into glycogen by the liver.As the blood glucose levels fall, glycogen is converted back to glucose.
- When you blood concentation levels fall below the ideal range, the pancreas secretes glucagon. Glucagon allows glycogen to be broken down by the liver and converted back into glucose and released into the blood. Doctors can transplant a pancreas successfully however the operations are quite difficult and risky.

Type 2 Diabetes
- Often a result of obesity or lack of exersise. The pancreaus still makes insulin although it makes less and cells stop responding to insulin. It is treated by careful attention to diet, losing weight and more exercise. If this doesn't work, drugs may be needed.

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Population and Pollution

The human population has increased dramatically in the past 200 years. Better farming methods mean we have more food. We can treat and prevent man diseases. The standard of living has also improved. We have no natural predators. All this has allowed to number of humans to increase.

However, more waste is being produced and if it is not handled properly, it can pollute the air, water and land. Toxic chemicals and fertilisers can be washed fom the land into the water.
HERBICIDES are used to kiss weed plants; PESTICIDES kill insect pests.
Eutrophication = When plants die and decompose using up oxygen so the water can no longer sustain animal life.

When fossil fuels are burnt, the pollutant gases sulfur, carbon dioxide and nitrogen oxides are relesed into the atmosphere. The sulfur dioxide and nitrogen oxides dissolve in rainwater and react with oxygen to form nitric and sulfuric acids. This is know as acid rain. Acid rain may damage trees directly. It can make the water to acidic to sustain life.

Air pollution can cause global dimming as tiny solid particles in the air reflect away the sunlight. This could lead to temperatures on the surface of the earth cooling.

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Deforestation and Global Warming

Deforestaion is the destruction or removal of area of forest or woodland. It has led to an increase in the amount of carbon dioxide released into the atmosphere (from slash-and-burn). It has also reduced the rate at which carbon dioxide is removed fom the air by plants. More rice fields and cattle have led to an increase in methane as they both produce it in growth.

Loss of biodiversity = The number and variety of different organisms fond in a specific area being reduced because of loss of habitat.

Peat bogs are made of plant material that cannot decay completely because conditions are very acidic and lack oxygen. Plants here and algae in the sea all use carbon dioxide for photosynthesis. Carbon compounds are then 'locked up' in these plants.

Inceasing levels of carbon dioxide and methane in the atmosphere cause an increased greenhouse effect - energy from the sun is radiated back into space but gases e.g. carbon dioxide and methane absorb some of the energy so it can't escape - leading to global warming - an increase of the surface of the earth.
Global warming may cause; climate change, rise in sea levels as ice caps melt, loss of biodiversity, changes in migration (birds) and distribution of species.

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Biofuels and Biogas

Some land has been deforested so that crops can be grown from which biofuels based on ethanol can be produced. Biofuels can be made from natural products e.g. sugar rich products from cane and maize are fermented anaerobically with yeast.

Pros - ethanol as a fuel: - Efficient, no toxic gases when you burn it > less pollution.
                                      - Carbon neutral
Cons: - A lot of plant material and land  needed to produce that much ethanol.

Biogas - mainly methane - can be produced by anaerobic fermentation of a wide range of plant products and waste materials that contain carbohydrates. Methane can be used for cooking and heating.The generators can be small, to supply a single family, or large, to deal with sewerage for an entire city. They work best in hot countries. The process is exothermic (releases energy) so you need activation energy.

One way of supplying water is to build a dam however they destroy rivers which can cause huge area to dry out. Flood plain with fertile soil may disappear so people can no longer grow the crops they need. There may be a loss of habitat and species aswell as an increase of methane in the atmosphere.

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Food Production

Biomass and energy are reduced at each stage of a food chain. The efficiency of food production is improved by reducing the number of stages in our food chain.
How to manage this:

- Limiting the movement of animals so that they lose less energy in moving so have more biomass available fom their food for growth.
- Controlling surrounding temperature so animals don't use up energy warming up and cooling down.

Cons: - They are close together so disease can spread quickly.
         - They need constant monitoring that costs money.

Birds reared outside is a slower process but the are no electricity costs.

Sustainable food production means producing food in a way which can continue for many years. Fish species in the oceans are declining so it is important to conserve fish stocks at a level where breeing continues or certain stocks may disappear completely. A fungal biomass is harvested and purified where it is dried and processed to make microprotein. The fungus Fusarium is grown on sugar syrup in aerobic conditions to produce mycoprotein foods.

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