IGCSE edexcel Biology UNIT 3 - Plant Physiology chapters 10-12

Photosynthesis

Conversion of light energy from the sun into chemical energy.

what is fertilisation?

the fusion of gamete nuclei

In plants, fertilisation occurs when the pollen grain nucleus fuses with the ovum nucleus

what is pollination?

transfer of pollen from an anther to a stigma > so the male gamete and female gamete can fuse

> can happen by wind or insects

flowers are the _____ of plants.

- what do they produce

- how does pollen transfer

Flowers are the reproductive organs of plants - usually contain both male and female reproductive parts

- produce pollen, which contains a nucleus inside that is the male gamete > unlike sperm pollen is not capable of locomotion so plants have to have mechanisms in place to transfer pollen from the anther (male part of the flower) to the stigma (female part of the flower) = known as pollination > can be transferred by wind or insects

describe the functions of

sepal, petal, anther, stigma, ovary & ovule

sepal: protects unopened flower

petal: brightly coloured in insect pollinated plants to attract insects

anther: produces and releases the male gamete (pollen grain)

stigma: top of the female part of the flower, collects pollen grains

ovary: produces female gamete ovum

ovule: contains the female gametes inside ovary

How are insect pollinated flowers adapted for pollination?

stamen & stigma: enclosed within the flower so that the insect must make contact

type of stigma: sticky so pollen grains attach from the insect

petal size & colour: large and brightly coloured to attract insects

scent/nectaries: produce nectar = sweet liquid containing sugars to entice insect to visit the flower and push past stamen to get nectar

pollen grains: larger sticky grains or grains with hooks to stick to insects bodies

what are insect pollinated flowers? Describe the process of insect pollination

flowers where the pollinating agents are insects (e.g. bees)

- Insects often visit flowers to collect nectar (sugary substance produced by flowers at base of petals > provides insects with energy)

- As insect enters the flowers for nectar, often brushes against the anthers > deposit sticky pollen onto the insect's body

- When insect visits another flower, may brush against the stigma of 2nd flower > in the process, may deposit some of the pollen from the first flower = pollination

* structures of the flower ensure this process goes well

how do wind-pollinated flowers fertilise?

*process of pollination is more random than for insect-pollinated flowers

- When ripe, anthers open & shed their pollen into the open air

- pollen then blown by wind or carried by air currents until it (by chance) lands on the stigma of a plant of the same species = pollination

* structures of the flower ensure this process goes well

describe the structures of an wind-pollinated flower and how its adapted for pollination?

scent/nectar: absent - no need to waste energy producing these as no need to attract insects

large number of pollen grains: most grains arent transferred to another flower so the more produced = better chance of pollination occurring

pollen grains: smooth, small, light so easily carried by wind

anthers: outside flower, swing loose on log filaments to release pollen grains easily

stigma: outside the flowers and feathery to catch drifting pollen grains

the growth of the pollen tube followed by fertilisation leads to what?

to seed and fruit formation

in plants what does the ovary contain? Whats an ovule

In plants, the ovary contains one or more ovules

The ovules = structures that eventually develop into seeds

Each ovule contains an ovum (egg cell) that contains the female nucleus that a male pollen nucleus can fuse with

How does the plant male gamete reach the ovum nucleus? (has no tail to swim towards the ovary)

- the pollen grain grows a pollen tube (only happens if pollen grain has landed on the right kind of stigma like one of the same species as the flower the pollen came from)

- nucleus inside the pollen grain moves down the tube as it grows down the style towards the ovary (which contains the ovule > contains the ovum)

- Once the nucleus of the pollen grain and the nucleus of the ovum have fused (joined together), that particular ovule has been fertilised

- zygote has been formed which will then start to divide and eventually develops into an embryo plant

after fertilisation what happens to the ovule that contains the zygote?

- the ovule develops into the seed

- wall of the ovule develops into the seed coat, known as the testa

- parts of the flower surrounding the ovule (mainly ovary walls) develop into the fruit, which contains the seeds

- The fruit provides a mechanism for seed dispersal (getting the seeds away from parent plant)

>> Some fruits are eaten by animals, which disperse the seeds in their droppings (tough outer coat of seeds stops them being digested)

>> some fruits have sticky hocks that get caught in animal fur

- diff plants have diff number of ovules = diff plants have diff seeds

What's germination? What are the 3 factors required for it? (WOW)

the start of growth in the seed

Water, Oxygen, Warmth

Name the 3 factors are required for successful germination?

Water: allows the seed to swell up = causes seed coat (testa) to burst = allowing the growing embryo plant to exit the seed. + allows the enzymes in the embryo to start working so that growth can occur (increases metabolic activity)

Oxygen: required for respiration, so energy can be released for germination

Warmth: germination improves as temperature rises (up to a certain point) > bc reactions which take place are controlled by enzymes, which function better when temps are high

list the method for said practical: investigating factors of germination in seeds

- 4 test tubes each containing 10 cress seeds on cotton wool - Label the test tubes A, B, C and D

tube A: leave the cotton wool dry

tube B: add enough water to the cotton wool = becomes moist

tube C: enough water to cover cotton wool and seeds, then carefully add a layer of oil on top of the water

tube D: enough water to cotton wool so that it becomes moist

- Leave tubes A, B and C at room temperature or incubated at a specific temperature (e.g. 20°C)

- tube D in a fridge (approximately 4°C)

- Leave all tubes for a set period of time (e.g. 3 - 5 days) (Ensure cotton wool in tubes B and D remains moist throughout the time by adding more drops of water if required)

- Compare the results and see which tube has the greatest number of germinated seeds

list the results for practical: investigating factors of germination in seeds

- test tubes are set up so that each of the factors required for germination (water, oxygen and warmth) can be tested, by selectively removing each in turn

tube A: water is removed

tube B: control tube, where all factors are present

test tube C: oxygen is removed (oxygen cant pass through the oil & water layers on top of the seeds)

tube D: warmth is removed

- would be expected only seeds in the control tube will germinate > bc germination cannot occur if the conditions are not right

CORMS IN conditions of seed germination practical

C- abiotic conditions in which the seeds are germinating

O - cress seeds will all be taken from the same parent plant (or at least from the same species of cress plant)

R - investigation repeated several times to ensure results are reliable

M1 - We will record how many seeds in each test tube germinate

M2 - ...after a set time period (e.g. 3 days)

S - control the temperature for tubes A, B and C.

- control the type of water used (i.e. sterile water, made by boiling then cooling water)

- number of cress seeds used

how germinating seeds utilise food reserves until the seedling can carry out photosynthesis?

- When the seed germinates embryo begins to grow into young seedling

- Structures known as cotyledons surround the embryo (Some plants have 1 whereas others have 2)

- The cotyledons contain food reserves that supply the young seedling with food (and, therefore, energy for growth) when the seed starts to germinate

- The cotyledons fulfil this role until the young plant grows its own leaves and becomes capable of making its own food via photosynthesis

how can plants reproduce?

sexually and asexually - natural and artificial by humans for their own uses

Asexual reproduction = involves 1 parent + all offspring produced are exact genetic copies of each other and the parent plant - they are clones (genetically identical)

how can plants reproduce asexually naturally? (runners)

- Some plants grow side branches (runners), that have small plantlets at their ends

↳Runners are horizontal stems that grow sideways out of the parent plant

- when runners touch soil the plantlets will grow roots and the new plantlets will grow and become independent from the parent plant

how can plants reproduce asexually artificially? (cuttings)

- Gardeners take cuttings from good parent plants (those that are healthiest and best-looking) > A section of the parent plant with a new bud is cut off

- cutting can be placed into water until new roots grow or can sometimes be placed directly into soil

- Sometimes, stem of the cutting may first be dipped into 'rooting powder', which contains plant growth regulators (rooting hormones) that encourage new root growth

- cuttings are then planted and eventually grow into adult plants that are genetically identical to the original plant > Plants cloned by taking cuttings can be produced cheaply and quickly

what is photosynthesis?

process where energy from sunlight is absorbed by chlorophyll (a green pigment inside chloroplasts in green plants) to make glucose from the raw materials carbon dioxide and water + makes oxygen as a waste product

what do plants use glucose for?

used in respiration to provide energy for the cell

Produce starch for storage

Synthesise lipids for energy source in seeds

make cellulose > make cell walls

Produce amino acids > make proteins when combined with nitrogen & other mineral ions absorbed by roots

made into sucrose to transport around plant

Chemical equation for photosynthesis

6CO2 + 6H2O ------> C6H12O6 + 6O2

word equation for photosynthesis

Carbon dioxide + water -> glucose + oxygen

CO2 = diffuses into the leaf through the stomata

6H2O = Taken up by the roots and transported through the xylem to the leaves

C6H12O6 = Used to make substances needed by the plant; used in respiration for energy

6O2 = Diffuses out of the leaf through stomata; used in respiration

whats a limiting factor? Name the 3 main limiting factors of photosynthesis

limiting factor: something present in the environment in such short supply that it restricts life processes

- Temperature

- Light intensity

- Carbon dioxide concentration

why arent water or the number of chloroplasts considered limiting factors of photosynthesis despite them affecting the rate of photosynthesis?

- water: necessary for photosynthesis but not considered limiting factor as amount needed is relatively small compared to the amount of water transpired from a plant so rarely is there not enough water for photosynthesis

- number of chloroplasts or amount of chlorophyll in chloroplasts: can also affect the rate of photosynthesis

how does temperature affect photosynthesis?

temp of environment affects how much kinetic energy particles have > temp increases kinetic energy of particles = increasing likelihood of collisions between reactants and enzymes = results in formation of products > so temp affects the speed that CO2 and H2O move through a plant

- lower temperature = less kinetic energy particles have = fewer successful collisions occurring over a period of time

- higher temperatures = enzymes control processes of photosynthesis can be denatured > reduces the overall rate of photosynthesis

temperature and rate of photosynthesis graph / relationship

1. increasing rate as increase in rate of collisions between substrates and enzymes increase

2. optimum temperature

3. rate starts to decrease as enzymes begin to denature

how does light intensity affect photosynthesis?

- light intensity available affects the amount of energy a plant has to carry out photosynthesis

- more light a plant receives = faster the rate of photosynthesis as it has more energy available

- trend continues until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

light intensity and rate of photosynthesis graph / relationship

1. increase in rate is linear as light intensifies

2. at some point another factor becomes limiting

3. graph levels off, rate becomes constant

how does carbon dioxide concentration affect photosynthesis?

CO2 = one of the raw materials required for photosynthesis = means the more carbon dioxide present = faster the reaction can occur because more raw materials are there

- trend continues until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

carbon dioxide concentration and rate of photosynthesis graph / relationship

1. at lower concentrations the increase in rate is linear

2. at some point another factor becomes limiting

3. graph levels off, rate becomes constant

how does Chlorophyll affect the rate of photosynthesis?

The number of chloroplasts will affect the rate of photosynthesis as they contain pigment chlorophyll which absorbs light energy for photosynthesis

- more chloroplasts a plant has = faster rate of photosynthesis

- amount of chlorophyll can be affected by:

> Diseases (such as tobacco mosaic virus)

> Lack of nutrients (such as magnesium)

> Loss of leaves (fewer means fewer chloroplasts)

describe the structure of a leaf cell

Top down: waxy cuticle, upper epidermis, palisade mesophyll layer, spongy mesophyll layer, lower epidermis, guard cell, stomata + vascular bundle (xylem and phloem)

describe the following features of a leaf:

- wax cuticle

- upper epidermis

- palisade mesophyll

- spongy mesophyll

- wax cuticle: protective layer on top of leaf, prevents water evaporating

- upper epidermis: thin and transparent to allow light to enter the palisade mesophyll layer underneath

- palisade mesophyll: column shaped cells tightly packed with chloroplasts to absorb more light, maximizing photosynthesis

- spongy mesophyll: contains internal air spaces that increase the surface area to volume ratio for the diffusion of gases (mainly CO2)

describe the following features of a leaf:

- lower epidermis

- guard cell

- stomata

- xylem

- phloem

vascular bundle = contains the xylem and phloem to transport substances to + from the leaf

- lower epidermis: contains guard cell and stomata

- guard cell: absorbs and loses water to open and close the stomata to allow CO2 to diffuse in and oxygen out

- stomata: hole where gas exchange and water evaporation takes place, opens during the day and closes at night. Often found on the underside of the leaf to reduce water loss

- xylem: transports water into the leafs for mesophyll cells to use in photosynthesis + transpiration from stomata

- phloem: transports sucrose and amino acids around the plant

list Adaptations of Plant Leaves for Photosynthesis

- large surface area of leaf: increases surface area for the diffusion of CO2 and absorption of light for photosynthesis

- thin and transparent epidermis: allows more light to reach + CO2 to diffuse to palisade mesophyll cells quickly

-spongy layer: air spaces allow CO2 to diffuse through the leaf increasing the surface area

- palisade mesophyll layer at top of leaf: maximizes absorption of light as it will hit chloroplasts in the cell directly

- thin cuticle made of wax: protects leaf from too much water evaporating without blocking out sunlight

why do plants require mineral ions?

for growth

- Photosynthesis provides source of carbs but plants contain/require other types of biological molecule; such as proteins, lipids and nucleic acid (DNA)

- plants do not eat so need to make these substances themselves

- Two mineral ions required by plants are nitrogen and magnesium without them plants cannot photosynthesise or grow properly

- they obtain these elements in the form of mineral ions absorbed from the soil by root hair cells

need for magnesium in plants + what does lack of it cause?

- needed to make chlorophyll which is needed to absorb light for photosynthesis

- lack causes yellowing between the veins of leaves

need for nitrate in plants + what does lack of it cause?

- source of nitrogen used to make amino acids to build proteins for growth

- causes stunted growth and yellowing of leaves

2.23 practical: investigate photosynthesis, showing the evolution of oxygen from a water plant, the pro

PAPER 2 whats the importance of diffusion in gas exchange?

- Diffusion is the process by which gas exchange occurs

- unicellular organisms can exchange gases sufficiently by simple diffusion through the cell membrane

- Multicellular organisms have exchange surfaces and organ systems that maximise the exchange of materials

> These organs increase the efficiency of exchange in a number of ways:

> Having a large surface area to increase rate of transport

> A short diffusion distance for substances to move across created because the barrier that separates two regions is as thin as possible

What does a coordinated response require?

A stimulus, a receptor and an effector

what do plants need to grow in response

- GRAVITY: to ensure that shoots grow upwards and roots grow downwards

- LIGHT: to ensure their leaves can absorb light for photosynthesis

whats tropism? Whats geotropism and phototropism?

directional growth responses made by plants in response to light and gravity

phototropism: response to light

geotropis/gravitropism: response to gravitypositiv

positive and negative tropism

shoots and roots display what type of tropism?

positive tropism: If the growth is towards the stimulus

negative tropism: growth is away from the stimulus

shoots: grow upwards away from gravity, towards light = positive phototropic response + negative geotropic response

roots: grow downwards into the soil, away from light and towards gravity (to anchor the plant and absorb water and minerals from the soil) = negative phototropic response + a positive geotropic response

What are auxins?

plant growth regulators (similar to hormones in animals) that coordinate/control tropism

- produce in the tips of shoots: then diffuses down to the region where cell division occurs = only the region behind the tip of a shoot is able to contribute to growth by cell division and cell elongation

shoots:

Auxins are produced in the tips of the shoots + roots; diffuse to cells below the tips and:

In the shoots: auxins promote cell elongation (growth); more auxin = more cell elongation = more growth

In the roots: auxins inhibit cell elongation (growth); more auxin = less cell elongation = less growth

what affects the distribution of auxins in the shoots vs in the roots

shoots: affected by light and gravity

roots: is primarily affected by gravity alone

Unequal distributions of auxin cause unequal growth rates in plant roots and shoots

explain how auxins cause roots to grow downwards and shoots to grow upwards

SHOOTS: lower side grows faster than upper (more auxin = more cell elongation), so the shoot grows upwards

ROOTS: lower side grows slower than the upper (bc auxin inhibits cell elongation + growth in roots), so root grows downwards

- If a shoot/root is placed on its side auxins will accumulate along lower side as a result of gravity; so the uppermost side has a lower auxin concentration

how does auxin work?

stimulates the cells in this region (Where cell division occurs just below the tip) to elongate (get larger); the more auxin there is = faster they will elongate/grow

- light shines all around the tip = auxin distributed evenly throughout + cells in the shoot grow at the same rate - (what normally happens)

- light shines on the shoot predominantly from one side = auxin produced in the tip concentrates on the shaded side, making the cells on that side elongate and grow faster than the cells on the sunny side

↳ unequal growth on either side of the shoot = shoot bends and grows in the direction of the light

what are the waste products in a plant + what affects the waste products in plants + what happens to the waste products?

Oxygen

Carbon dioxide

Water/water vapour

- amount/intensity of light affects the waste products within plants

- some can be used up in other processes in the plant while some must exit the plant via the leaf organ

what happens during the day in plants when theres sufficient light?

- rate of photosynthesis > rate of respiration

- More oxygen is released than used in respiration

- Less carbon dioxide is released than used in photosynthesis

NET EFFECT: oxygen is in excess and a waste product

what happens during the night in plants when theres insufficient light?

- no photosynthesis, only respiration

- Oxygen used in respiration and carbon dioxide is produced

- No photosynthesis = no carbon dioxide is used

NET EFFECT: CO2 is in excess and a waste product

whats oxygen and carbon dioxide in a plant?

both reactants and waste products within a plant

- excretion of gases in plants is by diffusion = passive process

- whatever gas thats in excess diffuses out of the plant via the leaf organ through the stomata

what is water vapour in plants? When is it excerted?

- majority of water vapour lost from a plant is not a waste product of metabolism but water that has been drawn up from the roots in the transpiration stream

- water vapour is excerted night and day through transpiration

how are chemical waste products in plants removed?

- Plant cells can break down molecules into chemical substances no longer required by the plant

- Some of them cant be converted into another useful compound and so must be removed from the plant

- they can be stored in the dying tissues of a plant

- When the dying tissue falls off the plant the substances are removed

E.g. in autumn the leaves of deciduous trees turn a variety of colours due to the presence of chemical waste products

how does oxygen diffuse from outside to inside the leaf?

oxygen diffuses down the concentration gradient from a high concentration (outside the leaf) to a low concentration (inside the leaf) > leaf cells use oxygen in respiration so the concentration is always low inside the respiring cells

how does CO2 diffuse from inside to outside the leaf?

- CO2 diffuses down the concentration gradient from a high concentration (inside the leaf) to a low concentration (outside the leaf) > leaf cells use carbon dioxide in photosynthesis so the concentration is always low inside the photosynthesizing cells

the structure of a leaf is adapted to maximize..?

gas exchange of the 3 main gases:

Carbon dioxide: released in respiration used in photosynthesis

Oxygen: released in photosynthesis but used in respiration

Water vapour: released in respiration and transpiration

Adaptations of the whole leaf for gas exchange

- are thin = short diffusion distance

- are flat = provides a large surface area to volume ratio

- many stomata = allow movement of gases in and out of the air spaces inside the leaf to maintain a steep concentration gradient

Adaptations of the internal leaf structure/tissues for gas exchange

- Air spaces = allow gas movement around the loosely packed mesophyll cells

- Many stomata in the lower epidermis open in sunlight to allow gas movement in and out of the leaf

- Thin cell walls = gases move into the cells easily

- Moist air = gases can dissolve into for easier movement into and out of cells

- The close contact between cells and the air spaces allows efficient gas exchange for photosynthesis and respiration

whats the route of diffusion of carbon dioxide into a leaf

1. high CO2 in atmosphere than in leaf

2. CO2 diffuses into leaf through open stoma into air space in spongy mesophyll

3. CO2 diffuses through cell membrane and wall of mesophyll cells > dissolves in cytoplasm and diffuses into chloroplasts

- Gases always diffuse down a concentration gradient (from high concentration to low concentration)

when do stomata open and close

Stomata open = when water moves (by osmosis) into the guard cells causing them to become turgid

> allows gases to diffuse in and out of the leaf through the stomata pore

> Stomata tend to open when there is plenty of water and sunlight

Stomata close = when guard cells lose water (by osmosis) to the neighbouring epidermal cells and they become flaccid

> prevents any diffusion into or out of the leaf

> Stomata tend to close due to low water availability or low sunlight

where are stomata and what are they?

Stomata: spaces found between two guard cells usually on the lower epidermis of the leaf

- guard cells are responsible for opening and closing of the stomatal pore which controls gas exchange and water loss

when can plants photosynthesize or do gas exchange? Differences in gas exchange during daytime, night time and low light intensities

- only photosynthesize when they have access to light, however, cells respire all the time = gas exchange in plants varies throughout a 24 hour period

DAYTIME: plants both respire and photosynthesize. rate of photosynthesis usually > rate of respiration (unless there is a low light intensity) = net diffusion of carbon dioxide into the plant and net diffusion of oxygen out of the plant during the day

NIGHTIME: plants only respire = there is a net movement of oxygen into the plant and net diffusion of carbon dioxide out of the plant during the night time

LOW LIGHT INTENSITIES: rate of photosynthesis = the rate of respiration > means there is no net movement of oxygen or carbon dioxide in either direction

CORMS: practical The Effect of Light on Gas Exchange in Plants

Change: the availability of light for each boiling tube (not wrapped, wrapped in foil, wrapped in gauze)

Organisms: leaves be from the same species/age of the plant, they should be approximately the same size

Repeat: repeat investigation 3 times to ensure reliable results

Measurement 1: observe change in the hydrogen carbonate indicator

Measurement 2: after 30 minutes

Same: control volume of hydrogen carbonate indicator, the number of leaves, the temperature of the environment

method of practical: The Effect of Light on Gas Exchange in Plants

- Measure out 20 cm3 hydrogencarbonate indicator into 4 boiling tubes

- Put cotton wool into each boiling tube

- Label boiling tubes A-D and set them up as follows:

> Tube A - No leaf (control tube)

> Tube B - leaf in the tube and leave in the light

> Tube C - leaf in the tube and wrap it in aluminium foil to block out the light

> Tube D - leaf in the tube and wrap it in gauze to allow partial light

- Put bung into the top of each tube

- Leave all 4 tubes in the light for 30 minutes

- observe change in hydrogen carbonate indicator after

results of practical: The Effect of Light on Gas Exchange in Plants

Tube A (control): remain ORANGE/RED colour to show that the carbon dioxide is at atmospheric levels = no net movement

Tube B: indicator will turn PURPLE as less CO2 than atmospheric levels (tube was placed in the light with a photosynthesizing and respiring leaf and the rate of photosynthesis > the rate of respiration = less CO2)

Tube C: indicator turns YELLOW = (tube had a leaf but was wrapped in aluminium foil = no sunlight could reach the leaf > No light means > leaf wont photosynthesize but will still be respiring and therefore producing CO2 = more CO2).

Tube D: remain ORANGE/RED colour to show there was no change in carbon dioxide levels ( tube had a leaf inside and was wrapped in gauze allowing partial light = means rate of photosynthesis = the rate of respiration so there was no net change)

whats the role of the phloem?

transport food materials (mainly sucrose and amino acids) made by plant from photosynthesising leaves to non-photosynthesising regions in roots and stem so that movement can be in any direction around the plant

structure of vascular bundle

phloem:

- The cells are living cells and are not hollow

- Substances move from cell to cell through pores in the end walls of each cell

xylem:

- composed of dead cells which form hollow tubes

- Xylem cells are strengthened by lignin = adapted for the transport of water in the transpiration stream

structure (outside to center):

- root hair

- root cortex cells

- phloem tissue

- xylem tissues

role of the xylem

transport water and minerals from the roots to the stem and leaves

- no cell contents only a continuous column of water

- walls thickened by lignin

what are root hair cells? What do they do?

root hair cells: single-celled extensions of epidermis cells in the root that grow between soil particles and absorb water/minerals from the soil > adapted for efficient uptake of water (by osmosis) and mineral ions (by active transport)

- they increase the surface area to volume ratio significantly > increases the rate of the absorption of mineral ions by active transport

- high proportion of dissolved minerals + sugars in cytoplasm of root hair cell give it a low water potential (less watery) so Water moves into the root hair cell by osmosis

whats the route of water through a plant?

root hair cell → root cortex cells → xylem → leaf mesophyll cells

- Water moves by osmosis into the root hair cells through the root cortex > into the xylem vessels.

- Once water gets into xylem, its carried up to the leaves where it enters mesophyll cells

how can the pathway of water be investigated in a plant?

by placing a plant (like celery) into a beaker of water that has a stain added to it (food colouring)

- After a few hours the leaves of the celery will turn the same colour as the dyed water = proof water is being taken up by the celery

- If cross-section of celery is cut, only certain areas of the stalk is stained the colour of the water, showing that the water is being carried in specific vessels (xylem) through the stem

what is transpiration?

the loss of water vapour from the parts of the plant that are above ground (leaves, stem, flowers)

- through evaporation of water at the surfaces of the spongy mesophyll cells then by diffusion of water vapour through the stomata

- The interconnecting air spaces between the mesophyll cells and the stomata creates a large surface area so evaporation can happen rapidly when the stomata are open

functions of transpiration

- Transporting mineral ions

- Providing water to keep cells turgid in order to support the structure of the plant

- Providing water to leaf cells for photosynthesis

- Keeping the leaves cool, the conversion of water (liquid) into water vapour (gas) as it leaves the cells and enters the airspace requires heat energy. The use of heat to convert water into water vapour helps to cool the plant down

how does water move in plants

- moves through xylem vessels in a continuous transpiration stream from the roots to the leaves via the stem to replace the water that has been lost due to transpiration

- because of this the water in the xylem creates a continuous unbroken column (each individual molecule 'pulls' on the one below it)

- Transpiration produces tension or 'pulls' on the water in the xylem vessels

- if rate of transpiration from the leaves increases, water molecules are pulled up the xylem vessels quicker

what are the environmental conditions that affect the rate of transpiration

AIR MOVEMENT: high = more transpiration: good air flow removes water vapour from air surrounding the leaf and sets up concentration gradient between leaf and air increasing water loss;t

HUMIDITY: high = less transpiration: humidity is measure of moisture in air; when air is saturated with water vapour concentration gradient is weaker so less water is lost

LIGHT INTENSITY: high = more transpiration: guard cells are responsive to light; when its high theyre turgid and the stomata open allowing water to be lost

TEMPERATURE: high = more transpiration: high temps particles have more kinetic energy so transpiration occur faster rate as molecules evaporate from the mesophyll and diffuse away faster than at low temps

whats a potometer?

measure the rate of transpiration

- A mass potometer: measures change in mass of a plant as a measure of amount of water thats evaporated from the leaves and stem

- A bubble potometer measures uptake of water by a stem as a measure of the amount of water that is being lost by evaporation consequently pulling water up through the stem to replace it

method of Practical: Factors Affecting Transpiration

- Cut shoot underwater to prevent air entering the xylem and place in tube

- Set up the apparatus making sure its airtight, using Vaseline to seal any gaps

- Dry the leaves of the shoot since wet leaves will affect the results

- Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water

- Set up a lamp 10cm from the leaf

- Allow the plant to adapt to the new environment for 5 minutes

- Record the starting location of the air bubble and leave for 30 minutes

- Record end location of the air bubble

- Change the light intensity

- Reset the bubble by opening the tap below the reservoir and Repeat the experiment

- Calculate the rate of transpiration by dividing the distance the bubble travelled by the time period

results of Practical: Factors Affecting Transpiration

- As light intensity increases = rate of transpiration increases

> shown by bubble moving a greater distance in the 30 minute time period when the lamp was placed closer to the leaf

- Transpiration rate increases with light intensity because more stomata are open in bright light in order to maximise photosynthesis

> more stomata that are open, more water can be lost by evaporation and diffusion through the stomatal pores

limitations of Practical: Factors Affecting Transpiration

1. potometer equipment has a leak

Solution: Ensure all equipment fits together rightly around the rubber bungs and assemble underwater to help produce a good seal

2. plant cutting has a blockage - air bubbles

Solution: Cut stem underwater and assemble equipment underwater to minimise opportunities for air bubbles to enter the xylem

3. potometer has shown no change during the experiment

Solution: Use plant cuttings as soon as they have been cut, transpiration rates may slow down when the cuttings are no longer fresh

CORMS of Practical: Factors Affecting Transpiration

Change: intensity of the light

Organisms: plants used in each repeat should be the same species, size, age, number of leaves

Repeat: repeat investigation 3 times ensure results reliable

Measurement 1: the distance travelled by the bubble

Measurement 2: in 30 minutes (calculate the rate of transpiration)

Same: control the temperature, wind speed and humidity of the environment

how to investigate environmental factors that affect transpiration c

Light intensity: lamp

Airflow: Set up a fan or hairdryer

Humidity: Spray water in a plastic bag and wrap around the plant

Temperature: Temperature of room (cold room or warm room)

method of Practical: Investigating Light & Photosynthesis

1. destarch

- Destarch the plant by placing it in a dark cupboard for 24 hours to ensure any starch already present in the leaves will be used up and will not affect the results of the experiment

- partially cover a leaf of the plant with aluminium foil and place the plant in sunlight for a day

- Remove the covered leaf and test for starch using iodine using the method below

2 testing starch

- Drop leaf in boiling water to kill the tissue and breaks down the cell walls

- Transfer leaf into hot ethanol in a boiling tube for 5-10 minutes to remove chlorophyll so colour changes from iodine can be seen more clearly

- Dip the leaf in boiling water to soften the leaf tissue after being in ethanol

- Spread the leaf out on a white tile and cover it with iodine solution

results of Practical: Investigating Light & Photosynthesis

- in green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all areas of the leaf

- area of leaf that was covered with aluminium foil will remain orange-brown as it didnt receive any sunlight so couldnt photosynthesise, while the area exposed to sunlight will turn blue-black

> proves that light is necessary for photosynthesis and the production of starch

CORMS Practical: Investigating Light & Photosynthesis

C - We are changing whether there is light or no light

O - The leaves will be taken from the same plant or same species, age and size of the plant

R - We will repeat the investigation several times to ensure our results are reliable

M1 - We will observe the colour change of the leaf when iodine is applied

M2 - ...after 1 day

S - We will control the temperature of the room