Topic 2 notes – Organisms and energy
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AEROBIC RESPIRATION
All cells in the body need energy - this energy is released in a process known as
‘respiration’
Cells that are more active need more energy - e.g during exercise, muscles
contract to cause movement – this requires a lot of energy from respiration
Aerobic respiration – respiration in the presence of oxygen:
o Equation: glucose + oxygen  carbon dioxide + water (+ energy released)
o The energy released during aerobic respiration can then be used by cells
o Aerobic respiration takes place inside organelles called mitochondria
Delivering glucose and oxygen to respiring cells:
In humans, glucose and oxygen needed for aerobic respiration are carried around
the body by the blood
The blood also carries waste carbon dioxide (product of aerobic respiration)
Glucose, oxygen and carbon dioxide move between respiring cells and tiny (one
cell thick) blood vessels called capillaries by a process called ‘diffusion’…
o Diffusion is the movement of particles from an area of high concentration
to an area of lower concentration
 i.e particles diffuse down a concentration gradient
o Respiring cells produce carbon dioxidecarbon dioxide concentration is
higher in respiring cells than in the bloodcarbon dioxide diffuses down
its concentration gradient into the blood
o Respiring cells use up oxygen and glucoseconcentrations of glucose and
oxygen are higher in the blood than in the respiring cellsglucose and
oxygen diffuse down their concentration gradients into the respiring cells
Gas exchange:
Tiny air sacs called ‘alveoli’ in the lungs are surrounded by capillaries
Alveoli are the sites of ‘gas exchange’:
o As we discussed above, in the body…
 oxygen diffuses from the blood into cells for use in respiration
 waste carbon dioxide produced during respiration diffuses out of
respiring cells and into the blood
o blood arriving at the alveoli has a high concentration of carbon dioxide
and a low concentration of oxygen…:
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The concentration of carbon dioxide is higher in the blood than the
alveolicarbon dioxide moves into the alveoli by diffusion
 Carbon dioxide is then removed from the body when we
breathe out (‘exhale’)
 As we breathe in through the lungs, oxygen enters alveoli…
concentration of oxygen in the alveoli is higher than in the blood
oxygen moves into the blood by diffusion
o blood leaving the alveoli has a high concentration of oxygen and a low
concentration of carbon dioxide…
 The oxygen is then transported in the bloodstream (in ‘red blood
cells’) to cells for use in respiration...and process repeats…
EXERCISE AND ANAEROBIC RESPIRATION
The amount of blood pumped out by the heart in one minute is called the ‘cardiac
output’
The cardiac output depends on the…:
o Heart rate – beats per minute
o Stroke volume – volume of blood pumped out of the heart each beat
o The higher the stroke volume and the heart rate, the higher the cardiac
output
 equation: cardiac output = stroke volume x heart rate
During exercise…
o Muscles need more energythey need more oxygen from respiration
o 1. the heart rate and stroke volume increase
 cardiac output is increased – i.e more blood reaches respiring
tissues faster
o However, this wouldn’t be much use if the blood reaching the respiring
tissues didn’t contain enough oxygen
o 2. breathing rate also increases…
 more oxygen enters alveolimore oxygen diffuses into the
blood
o So more oxygen reaches muscle cells fasterfaster rate of aerobic
respiration in muscle cellsmore energy released
During intense exercise:
o even increasing the heart rate and the breathing rate isn’t enough to supply
oxygen to muscle cells quickly enough
o anaerobic respiration starts to happen (alongside aerobic respiration)
inside muscle cells
Anaerobic respiration breaks down glucose without using oxygen…the waste
product lactic acid is produced:
o Equation: glucose  lactic acid (+ energy released)
o The energy released by anaerobic respiration is less than the energy
released by aerobic respiration
Breaking down the lactic acid:
The waste lactic acid produced during anaerobic respiration is toxicmust be
broken down as soon as possible
Lactic acid can be broken down by oxygen into carbon dioxide and water…
o after exercise has finished, increased oxygen is needed so that the lactic
acid can be broken down
(the lactic acid can’t be broken down during exercise because there isn’t
enough oxygen available - it is being used in aerobic respiration)
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o This requirement for additional oxygen after exercise is called ‘excess
post-exercise oxygen consumption’ (EPOC)
o Note: the increased oxygen available after exercise can also be used for
other processes in cells (e.g cell repair)
The extra oxygen needed after exercise is obtained by keeping the breathing rate
and the heart rate high for a few minutes after exercise (i.e until the oxygen has
broken down all the lactic acid produced into carbon dioxide and water)
The time taken for the heart rate to return to normal (resting) after exercise is the
‘recovery time’ - the faster the recovery time, the fitter the person
PHOTOSYNTHESIS
Plants (like animals) have mitochondria in their cellsthey respirethey need a
supply of glucose
In animals/humans, glucose needed for respiration is obtained from the
breakdown of starch
Unlike animals/humans, plants can make their own glucose by a process called
photosynthesis
Equation for photosynthesis:
Photosynthesis takes place inside plant cell organelles called chloroplasts:
o Chloroplasts contain a green substance called ‘chlorophyll’
o Chlorophyll absorbs sunlight, transferring the light energy into stored
chemical energy in glucose
o chloroplasts and chlorophyll are essential for photosynthesis – without
them, photosynthesis cannot occur
Leaf adaptations:
The leaf is the main plant organ in which photosynthesis occurs
Leaves are adapted for photosynthesis:
o They have chloroplasts containing a substance called chlorophyll that
absorbs light energy from the sun
o They are broad and flathave a large surface area – this helps the leaves
absorb as much light energy as possible
o On the underside of leaves are microscopic pores called stomata – these
are the sites of gas exchange
 Air spaces inside the leaf give cells a large surface area to volume
ratio – this makes gas exchange through the stomata more efficient
Gas exchange in the stomata:
Carbon dioxide from the atmosphere diffuses into the leaf through the stomata
o The carbon dioxide is then taken up by cells in the leaf for use in
photosynthesis
Oxygen produced in photosynthesis diffuses from the inside of the leaf into the
atmosphere through the stomata
Water produced during respiration can evaporate from cells inside a leaf – the
water vapour can then diffuse out of the leaf through the stomata
Stomata open when it’s light and close when it’s dark:
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o When it’s light, stomata openallow gas exchangephotosynthesis can
take place
(if stomata were closed, carbon dioxide could not enter the leaf
photosynthesis would not take place)
o When it’s dark, stomata close for two reasons:
 1. Photosynthesis can’t take placegas exchange is not necessary
 2. Plants still respire in the dark, producing water vapour…
 Water vapour diffuses out of the leaf through the stomata
 stomata close at night to prevent loss of water vapour
Note:
o Water needed for photosynthesis does NOT enter through stomata in the
leaf - it is taken up in the roots and then transported to the leaf in xylem
vessels (see ‘water transport’ section below)
LIMITING FACTORS
The higher the rate of photosynthesis, the more glucose that is produced - glucose
is needed by plants for growth and for respiration
Factors affecting the rate of photosynthesis:
o Concentration of carbon dioxide
o Water
o Light intensity
o Temperature (reactions in photosynthesis are catalysed by enzymes
temperature can also affect the rate of photosynthesis)
In any process that is affected by several factors, the maximum rate at which the
process can occur is controlled by the factor that is in the shortest supply – the
factor in the shortest supply is the ‘limiting factor’
o I.e the limiting factor is the factor that is responsible for slowing down the
rate of photosynthesis
E.g:
o If a plant has lots of carbon dioxide, lots of water, is grown at the right
temperature but in dim light, it will photosynthesise slowly
o Increasing the concentration of carbon dioxide, or giving the plant more
water or increasing the temperature will not increase the rate of
photosynthesis
o Only increasing the amount of light will increase the rate of
photosynthesis…in this example, the limiting factor is light (intensity)
Interpreting graphs:
o At point x:
 Increasing light intensity increases the rate of photosynthesis…
 initially light intensity is the limiting factor
o At point y:
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Increasing light intensity doesn’t increase the rate of
photosynthesis any further…
another factor (i.e temperature, CO2 concentration or water)
must be limiting the rate of photosynthesis at this point
o At point A – increasing the temperature increases the rate of
photosynthesis (the high temperature trace at the same light intensity has a
much higher rate of photosynthesis)
 the limiting factor at this point is temperature
o At point B – increasing the light intensity increases the rate of
photosynthesis
 the limiting factor at this point is light intensity
o At point C – increasing temperature or light intensity doesn’t increase the
rate of photosynthesis any further
 the limiting factor at this point could either be the amount of
carbon dioxide or water available to the plant
WATER TRANSPORT
Roots don’t just anchor plants to the ground – they also take up water and mineral
salts from the soil
‘Root hair cells’ are present in roots – they have two main adaptations that help
them take up water and minerals from the soil:
o 1. They have long, thin extensions that reach into the surrounding soil
o 2. They have a large surface area
Water is taken up by osmosis:
Water enters the root hair cells by osmosis
Osmosis is the movement of water molecules from an area where there is a higher
concentration of water molecules to an area where there is a lower concentration
of water molecules, through a partially permeable membrane
o A partially permeable membrane is a membrane that only lets certain
substances pass through it
E.g:
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o Water moves from a region of higher water concentration to a region of
lower water concentration until ‘equilibrium’ is reached (i.e until the water
concentration is the same on both sides of the partially permeable
membrane)
o Sucrose molecules can’t move because they are too big to fit through the
gaps in the partially permeable membrane
Dissolved minerals can be taken up by active transport:
The concentration of minerals dissolved in soil water is very low compared to the
concentration of minerals in the plant
In order for minerals dissolved in soil water to be taken up into the roots, they
must be absorbed against their concentration gradient – i.e from an area of lower
concentration to an area of higher concentration
Absorbing particles against a concentration gradient is called ‘active transport’
Active transport requires energy – the energy comes from respiration
Transporting substances round the plant – xylem and phloem:
Once water and minerals have entered the root cells, they need to get to all the
plant’s tissues…
o Minerals help plants grow
o Water needs to be transported to the leaves for use in photosynthesis
Xylem vessels:
o transport water and dissolved mineral salts
o also give support to the plant
The glucose made in the leaves during photosynthesis is converted to sucrose and
then transported to other parts of the plant by phloem vessels
Note: glucose can also be stored in the form of starch
How do water and minerals travel up xylem vessels against gravity?
As discussed above, water produced during respiration can evaporate from cells
inside a leaf and then diffuse out of the leaf through the stomata…
o The loss of water from the leaf pulls water and dissolved minerals up
through xylem vessels (against the force of gravity)
 This movement of water and dissolved minerals up through xylem
vessels from the roots is called ‘transpiration’
o Increased rate of evaporationfaster loss of water through stomata
increased rate of transpiration
 I.e the rate of evaporation determines the rate of transpiration
 factors that increase the rate of evaporation will also increase the
rate of transpiration - e.g on a warm and windy day, the rate of
transpiration will be greater
Note: transpiration can be monitored experimentally using a ‘potometer’
ORGANISMS AND THEIR ENVIRONMENT
When ecologists study the biodiversity (i.e the different species of animals and
plants) of living organisms in a habitat (the place where an organism lives), they
want to know:
o Where particular organisms are found (their ‘distribution’) in the habitat
o How many individuals of each species (the ‘population size’) there are in a
habitat
It takes too long to study the whole habitatecologists usually look at just a small
portion of an area – this is called ‘sampling’
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Random sampling - every point within an area has an equal chance of being
selectedthe sample is likely to be representative of the whole area
Sampling techniques:
Pooter - used to catch small invertebrates:
o It has two tubes which are connected to a container
o You suck on one of the tubes and any small invertebrates are sucked up
into the container through the other tube (called the ‘inlet tube’)
Sweep net – this is used in areas of long grass to catch some of the organisms
present
Pond net – can be used to sample aquatic (i.e water) habitats
Pitfall traps:
o Useful for trapping small animals such as spiders, beetles and woodlice
o They can be set up and left overnightpossible to catch nocturnal animals
(i.e animals that are only active at night)
Quadrats - these are square frames, typically used to sample the number
(population) of different plant species in a habitat:
o The quadrat is placed at random locations (e.g by throwing) – at each
location, the number of plants of each type within the quadrat are counted
o For spreading plants (e.g clover), the % of the quadrat area covered by the
plant is estimated
o The mean number of a particular type of plant found in each quadrat can
be used to estimate the population size of that plant in the area
Environmental factors:
Different conditions can sometimes be found within a habitat (e.g at different
points in a field) – this can affect the distribution of organisms:
o E.g understanding how light intensity varies in a habitat might help to
explain the distribution of different species of plants…
 Some species of plant may need lots of lightyou will only find
them in the part of the field that receives the most sunlight
 Other species of plants may be able to survive with less lightyou
might find some in darker, more sheltered parts of the field
Other factors such as temperature or soil/water pH may also have an effect on the
distribution of organisms in a habitat
When investigating changes in a habitat caused by one environmental factor, it’s
sometimes useful to carry out ‘systematic sampling’ along a line:
o Using systematic sampling makes it easier to spot patterns…makes it
easier to see the effect that the environmental factor is having on the
distribution of organisms in a habitat
o Systematic sampling can be done by placing quadrats at regular intervals
along a straight line