Nitrogen Cycle

Very few organisms can use gaseous nitrogen directly.

Instead depend on soil minerals especially nitrates, as their source of nitrogen.
Supply of nitrates is variable not least because they are soluble.
Deficiency of nitrates often limiting factor to plant growth.
Surrounded by an atmoshpere abundant in, the growth of many plants is stunted by a lack of it.
Deforestation (esp. tropical rainforests) also led to nitrate deficiency.
Trees gone = large scale erosion due to high rainfall in these areas = washes off the suface.
Roots help to bind the soil particles together.
Combination of soil erosion and leaching of nitrates has impoverished the land.
Nitrogen cycle depends on activities of various bacteria.
Nitrate in the soil is built up by plants into protein which is then passed on to animals.
Nitrogn from protein returned to enviroment as ammonia, either from excretory matter or action of decomposing bacteria or dead bodies.
Combined actions of various nitrifying bacteria converts ammonia back to nitrate.

Nitrifying Bacteria

Found in Soil.
Through their metabolic activities they enrich the soil in available nitrogen.
Some of these bacteria, specifically Nitrosononas obtain E by oxidizing ammonia to nitrate.
(NH₄)₂CO₃ + 3O₂ --> 2HNO₂ + CO₂ + 3H₂O + E
Another nitrifying bacterium, Nitrobacter, oxidizes nitrites to nitrates.
Ca(NO₂ + O₂ --> Ca(NO₃)₂ + E
In all cases the E released is used for the synthesis of orgainic compounds.
Nitryfying bacteria from part of natural system in which nitrogen compounds are converted from one to another step by step.

Gas exchange

In Plants

To enable gas exchange to take place efficiently:

The leaf is thin so diffusion paths for gases are short.
Spongy mesophyll allows for the circulation of gases.
Leaves have a good S.A / V ratio.

In Fish

Organisms get Oxygen directly from the atmosphere or dissolved in water.
The Oxygen content of the air is 1%, whereas that of water is 0.8%.
Aquatic organisms must pass a much greater volume of water over the reparatory surface to acquire the Oxygen they need.
Water is 700 times denser than air, and 10 times more viscous. (meaning)
Therefore more energy is needed to pass the water over the respiratory surface.
Oxygen diffuses 1000 time slower through water, so it is harder to maintain a steep concentration gradient.
Just like the lungs, the gills are constructed to provide a large S.A., well supplied with blood, in contact with the water.
The gill can be thought of as a sheet of thin epithelium tissue which is folded once to form lamellae/filaments and again to form gill plates.

Ecology and Ecosystems

Ecosystems - A natural unit of living (biotic) components in a given area, as well as the non-living (abiotic) factors with which they interact. Two main areas of study:
- The flow of energy through the system.
- The cycling of matter with in the system.
Population - A group of organisms of a single spicies occuping a particular area.
Community - Populations of different spicies in a given area.

Population Growth

Factors which limit Growth

Availibility of Light
Food!
Preditors (predaion)
Toxic Waste

Density Dependant Growth

How size of population will affect the population.
population reaches certain size then stablises.
e.g.
- Food!
- toxic wate
- disease
- parasitism
In small population, little food used and little toxic wate build up.
In high population, small available food and more toxic wate build up.

Density Inderpendant Growth

Population continues to grow until a factor arises causing sudden decrease. Effect regardless of population size.
e.g.
- temperature change (sudden fall or rise)
- Fire!
- Storms
(Sudden abiotic factor)

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Phloem

Made up from 4 different parts:

Phloem fibres.
Phloem parendyma.
Sieve tubes.
Companion cells.

The sieve tubes and companion cells are both involved with the mass flow hypothesis.

Sieve Tubes

Cell membrane broken down.
Fluid cytoplasm.
No vacuole.
No nucleus.

With the end of the sieve wall broken down it allows inter cellular movement.

Have plasmodesmata.
Very metabolicly active.
High number of mitochondria.
May provide energy for traslocation (mass movement).

Osmotic Pressure

The movement of solutes from a high hydrostatic pressure to a low hydrostatic pressure.

Note: [ Translocation from Source (e.g. sugar in leaves) to Sink (roots) ]

Simplified diagram of water and energy movement in a Plant

(Click to image to enlarge)

Evidence to support Mass Flow hypothesis

Solution under pressure.
Evidence for concentration gradient.
Observation of sieve tubes.
Movement of virus through plant.
No movement of virus when no photosynthesis.

Evidence to against Mass Flow hypothesis

Get the impression of steady flow, this is wrong.
Certain things move different ways.

Diagram showing a Phloem in a Root

Diagram of the Heart showing basic areas

Blood & The Circulatory System

The circulatory system of a mammal is a closed, double circulation and a heart with two atria and two ventricles. From the heart, blood gets pumped into arteries, these then split up into smaller vessels called arterioles, from which blood passes into capillaries. From there the blood travels through veins back to the heart.

Plasma

Clear, yellowy in colour.

Carries:
- Leucocytes - White Blood Cells
- Thrombocytes - Platelets
- Erythrocytes - Red Blood Cells (or cells being corpuscles)
- CO₂ - Some in the form of hydrogen carbonate
- hormones
- digested food
  - glucose
  - amino acids
  - fatty acids
  - glycerol
  - vitamins
- urea
- mineral ions
- fibrinogen
Distribute heat

White Blood Cells - Leucocytes

There are two types of leucocytes, granulocytes and agranularcytes. Granulocytes engulf bacteria whereas agranularcytes produce anti-bodies and anti-toxins.

Red Blood Cells - Erythrocytes

These are in a biconcave shape for a better surface area to volume ratio and are flexible to get through capillaries.

Note: [ 1mm³ of blood = 5,000,000 Red Blood Cells ]

Haemoglobin + O₂ = Oxyhemoglobin
No Nucleus - not needed
Carbonic anhydrase - carries CO₂ in cell.

CO₂ Levels

5% in Plasma
10% attacted to hemoglobin
85% carried as hydrogen carbonate

The Heart

Note: [ Remember, remember the 5th of ... No, Remember Right is Left! ]

The heart, part of the circulatory system, pumps blood around the body through, arteries, veins and capillaries. The heart is made of thick muscle which does not tire.

Speed Control

The heart muscle is myogenic (the heart beat is initiated from within the muscle itself, and not from nervous stimulation).

Diagram of the Heart showing Speed Change and autonomic system

Signals from the brain reaching the heart through the Vagus nerve enter the SAN, Sinoatrial node, this is the impulse generating (pacemaker) tissue located in the right atrium of the heart. From there signals get set across the heart to the AVN, Atrioventricular node, and then across the ventricles via the purkinje fibers.

Note: [ Cardiac Output = Volume Pumped x Number of beats ]

The speed of the heart is controlled by the sympathetic nerve, from the spinal cord and vagus nerve, from the brain. Stimulation of the sympathetic nerve speeds up the heart and stimulation of the vagus nerve slows it down, they are both controlled brain which reacts to the CO₂ content.

Agricultural Exploitation

Agriculture is the prime means by which food is produced for human consumption.
The human population is rapidly rising and therefore so is the demand for food. Because of this the demands on that agriculture are ever-increasing. To meet these demands the industry is becoming more intensified. This does not come without some environmental impacts.
Large areas of forest are cleared for intensive farming, destroying habitats and destabilising the soil. In a typical rainforest, the roots of the trees bind the soil together and over thousands of years a fertile layer of topsoil has developed. When deforestation occurs, the soil is exposed to wind and rain, and the top soil is quickly eroded by leaching. This leaves the area much more infertile and it is very difficult for the area to recover from this.
Monoculture is the repeated growth of the same crop year after year. This depletes the amount of certain nutrients consumed by that crop and its yield reduces annually. Also, this creates an ideal environment for crop’s pests and parasites. A means to overcome this problem is crop rotation. This makes conditions unfavourable for pest and parasites.

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