chp 4 organisms and their environment

Question 1

what is a population

Answer 1

A population is defined as a group of organisms of one species, living in the same area at the same time

Question 2

What is a community

Answer 2

A community is defined as all of the populations of different species in an ecosystem

Question 3

what is an ecosystem

Answer 3

An ecosystem is defined as a unit containing the community of organisms and their environment, interacting together (eg a decomposing log, a lake)

Question 4

how do populations expand at the expense of others

Answer 4

Those which are the best adapted to their environments generally increase their populations at the expense of those less well adapted

Question 5

factors affecting population

Answer 5

Food supply
Predation
Disease
changing temperature or light
immigration (individuals moving into the area)
emigration (individuals moving out of the area)

Question 6

reasons fro human population grwoth

Answer 6

Improved technology leading to an abundance of food = rapid increase in birth rate
Improved medicine, hygiene and health care = decrease in death rate

Question 7

phases of sigmoid growth curve

Answer 7

lag
log
stationary
death

Question 8

what is lag phase

Answer 8

organisms are adapting to the environment before they are able to reproduce; in addition, at this stage there are very few organisms and so reproduction is not producing larger numbers of offspring

Question 9

what is log phase

Answer 9

food supply is abundant, birth rate is rapid and death rate is low; growth is exponential and only limited by the number of new individuals that can be produced

Question 10

what is stationary phase

Answer 10

population levels out due to a factor in the environment, such as a nutrient, becoming limited as it is not being replenished; birth rate and death rate are equal and will remain so until either the nutrient is replenished or becomes severely limited

Question 11

what is death phase

Answer 11

– population decreases as death rate is now greater than birth rate; this is usually because food supply is short or metabolic wastes produced by the population have built up to toxic levels

Question 12

what is nitrogen fixing bacteria

Answer 12

Bacteria which can absorb nitrogen as a gas from air spaces in the soil and build it into compounds of nitrogen

Question 13

what are decomposers

Answer 13

Organisms which break down dead animal and plant material, releasing the nutrients into the soil

Question 14

what is nitrfying bacteria

Answer 14

Bacteria which convert ammonium compounds into nitrate

Question 15

what is denitrifying bacteria

Answer 15

Bacteria that use nitrate ions in their metabolism and release nitrogen gas

Question 16

describe the carbon cycle

Answer 16

Carbon is taken out of the atmosphere in the form of carbon dioxide by plants to be used for photosynthesis
It is passed on to animals (and microorganisms) by feeding
It is returned to the atmosphere in the form of carbon dioxide by plants, animals and microorganisms as a result of respiration
If animals and plants die in conditions where decomposing microorganisms are not present the carbon in their bodies can be converted, over millions of years and significant pressure, into fossil fuels
When fossil fuels are burned (the process is known as combustion), the carbon combines with oxygen and carbon dioxide is released into the atmosphere

Question 17

what leads to excess co2 in the atmosphere

Answer 17

Increased use of fossil fuels is contributing to an increase in the carbon dioxide content of the atmosphere
In addition, mass deforestation is reducing the amount of producers available to take carbon dioxide out of the atmosphere by photosynthesis
This problem is exacerbated by the fact that in many areas of the world, deforestation is taking place for land rather than for the trees themselves, and as such they are burnt down, releasing yet more carbon dioxide into the atmosphere

Question 18

Describe the water cycle

Answer 18

ater molecules move between various locations – such as rivers, oceans and the atmosphere – by specific processes
This is possible because water changes state at a relatively low temperature
Water enters the atmosphere as water vapour in one of two processes:
Energy from the Sun heats the Earth’s surface and water evaporates from oceans, rivers and lakes
Transpiration from plants releases water vapour into the air
The warmer air of the lower atmosphere rises, taking the water vapour with it
The moist air cools down as it rises
Water vapour condenses back into liquid water, forming clouds
Water returns to Earth in the form of precipitation
As the water droplets in the cloud get bigger and heavier, they begin to fall as rain, snow and sleet
This is called precipitation

Question 19

what is nitrogen required for

Answer 19

Nitrogen as an element is required to make proteins

Question 20

why cant nitrogen be easily absorbed

Answer 20

Neither plants nor animals can absorb it from the air as N2 gas is very stable and the bonds holding the nitrogen atoms together would need massive amounts of energy to break (the two nitrogen atoms in a nitrogen molecule are held together by a triple covalent bond)

Question 21

how is nitrogen absorbed

Answer 21

Nitrogen fixing bacteria found ‘free living’ in soil and also in the root nodules of certain plants (peas, beans, clover – we call them leguminous plants) take N2 gas and change it into nitrates in the soil
Lightning can ‘fix’ N2 gas, splitting the bond between the two atoms and turning them into nitrous oxides like N2O and NO2 that dissolve in rainwater and ‘leach’ into the soil

Question 22

describe the nitrogen cycle

Answer 22

Plants absorb the nitrates they find in the soil and use the nitrogen in them to make proteins
Animals eat the plants (or other animals) and get the nitrogen they need from the proteins in the plant or animal
Waste (urine and faeces) from animals sends nitrogen back into the soil as ammonium compounds (the urea in urine contains nitrogen)
When the animals and plants die, they decay and all the proteins inside them are broken down into ammonium compounds and put back into the soil by decomposers
The plants can’t absorb ammonium compounds though, so a second type of soil bacteria, nitrifying bacteria, convert the ammonium compounds to nitrites and then to nitrates, which can then be absorbed by plants – and so the cycle goes on
Finally, there is a third, unhelpful type of (anaerobic) bacteria called denitrifying bacteria found in poorly aerated soil (ie not much oxygen)
These bacteria take the nitrates out of the soil and convert them back into N2 gas
Farmers can help reduce the amount of these unhelpful bacteria by ploughing and turning over soil

Question 23

what are producers

Answer 23

organisms that produce their own nutrients usually using energy from sunlight