Topic 6 - Plant structures and their functions

what are the main produces of food

plants and algae are the main producers of food, which they synthesise from sunlight in a process called photosynthesis. they are also therefore the primary producers of biomass in all food webs and food chains

photosynthesis

• photosynthesis occurs in plants and algae

• it is an endothermic reaction, meaning that it takes in more energy than it releases

• light energy from the enviroment is transferred to chloroplasts in leaves

equation for photosynthesis

carbon dioxide + water —> (light) glucose + oxygen

CO₂ + H₂O —> C₆H₁₂O₆ + O₂

what factors affect the rate of process

temperature, light intensity and carbon dioxide concentration

effect of temperature on the rate of the process

• with an increase in temperature, the rate of photosynthesis increases

• as the reaction is controlled by enzymes, this trend continues up to a certain temperature until the enzymes begin to denature and the rate of reaction decreases

effect of light intensity on the rate of the process

• for most plants, the higher the light intensity, the faster the rate of the reaction

effect of carbon dioxide concentration on the rate of the process

• carbon dioxide is also needed to make glucose

• as the concentration of carbon dioxide increases, the rate of reaction increases

what may happen to any of the factors

any of the factors above may become a limiting factor. this is an enviromental condition (such as light intensity) which, in low levels, restricts any increase in the rate of photosynthesis. despite increases in other factors (such as temperature or carbon dioxide concentration), the rate of photosynthesis will not increase any more. this can be seen on a graph as the curve levelling off.

interaction of limiting factors in photosynthesis

by carrying out an experiment measure the oxygen production of a plant, you can calculate the rate of photosynthesis

• pondweed is placed in a test tube full with water. the top is sealed with a bung. a capillary tube also containing water leads into the test tube, and it is attached to a syringe

• a lamp is placed at a measured distance from the test tube

• as it is photosynthesis, oxygen is produced, forming a gas tube in the capillary tube

• the distance the bubble has moved is measured using a ruler to calculate the volume of oxygen produced

• many variables can be changed to observe their effect on photosynthesis: the temperature (using a water bath), time the pondweed is left, the light intesity (varied by the distance the lamp is from the plant)

• it is important to control all factors that may affect photosynthesis except your independent variable (the one you want to observe), so it is a valid experiment

any of the factors above may become a limiting factor. this is an enviromental condition (such as light intensity) which, in low levels, restricts any increase in the rate of photosynthesis. despite increases in other factors (such as temperature or carbon dioxide concentration), the rate of photosynthesis will not increase any more.

this can be seen on a graph as the curve levelling off

• a graph involves one limiting factor if it has two limiting factor if it has one line which levels off, with the factor on the horizontal axis and rate of photosynthesis on the vertical axis

• a graph with two lines represents two limiting factors in two experiments

light intensity is measure in lux

farmers can use the knowledge of limiting factors to enhance the conditions in the greenhouse for a greater rate of photosynthesis. this will increase growth leading to increased profits

Core Practical: light intensity and rate of photosynthesis

we can set up a simple experiment to measure the rate of photosynthesis in a plant:

we need a sealed 100ml flask filled with water at room temperature, a gas syringe, a small amount of pondweed, a small lamp and a 1m ruler

1. use the ruler to place the flask and pondweed 15cm from the lamp

2. leave the apparatus for around 10 minutes to allow the pondweed to adjust

3. connect the gas syringe to the flask and record the change in volume on the syringe after 5 minutes

4. move the lamp 10cm further away and measure the volume change again. repeat.

plot your results on a graph of distance from the lamp on the x-axis, and change in gas volume on the y-axis

relationship between light intensity and rate of photosynthesis

light intensity is directly proportional to the rate of photosynthesis.

this is because the greater the intensity of light, the more photons (light energy) hit the chloroplasts in the leaf, and the more photosynthesis can occur at once

relationship between rate of photosynthesis and the distance from the light source

inverse proportion describes a relationship between two factors which involves one increasing whilst one decreasing. as the distance between the light source and the plant increases, the light intensity decreases. the light intensity is inversely proportional to the square of the distance - called the inverse square law

root hair cells

specialised to take up water by osmosis and mineral ions by active transport from the soil as they are found in the tips of roots

• have a large surface area due to root hairs, meaning more water can move in

• the large permanent vacuole affects the speed of movement of water from the soil to the cell

• mitochondria to provide energy from respiration for the active transport of mineral ions into the root hair cell

xylem cells

specialised to transport water and mineral ions up the plant from the roots to the shoots

• upon formation, a chemical called lignin is deposited which causes the cells to die. we say that these dead cells have become lignified. they become hollow and are joined end to end to from a continuous tube so water and mineral ions can move through

• lignin is deposited in spirals which helps the cells withstand the pressure from the movement of water

phloem cells

specialised to carry the products of photosynthesis (food) to all parts of the plants

• cell walls of each cell from structures called sieve plates when they break down, allowing the movement of substances from cell to cell

• unlike xylem, these cells within phloem are alive

• despite losing many sub-cellular structures, the energy these cells need to be alive is supplied by the mitochondria of the companion cells

• these cells use this energy to transport sucrose (the sugar that plants use around the plant)

transpiration

transpiration is the loss of water or water vapour from the leaves and stems of the plant. it is a consequence of gaseous exchange, as the stomata are open so that this can occur

• water also evaporates at the open stomata (pores) on the leaf surfaces

• as water molecules are attracted to each other, when some molecules leave the plant the rest are pulled through the xylem

• this results in more water being taken up from the soil resulting in a continuous transpiration stream through the plant

what are guard cells

guard cells close and open the stomata

• they are kidney shaped

• they have thin outer walls and thick inner walls

• when lots of water is available to the plant, the cells fill and change shape, opening stomata (they are also light sensitive)

• this allows gases to be exchanged and more water to leave the plant via evaporation

• more stomata are found on the bottom of the leaf, allowing gases to be exchanged whilst minimising water loss by evaporation as the lower surface is shaded and cooler

what is translocation

translocation is the movement of food substances (such as sucrose) made in the leaves up or down the phloem, for use immediately or storage

• translocation only occurs in the phloem, not the xylem or any other tissues in the plant

• translocation of sucrose occurs from the sources (the places where it is made) to the sinks (the places where it is used or stored)

• the location of the sources and sinks can depend on the season. for example, in spring the source could be located in the root, and the sink in the leaf - and in summer this could be the other way round

leaf adaptations: stomata

they are able to close to minimise water loss and open to increase evaporation and transpiration. stomata also allow gas exchange to occur when they are open

leaf adaptations: chlorophyll

chlorophyll is green, which is the most efficient colour for absorbing light. this means that the most light possible is absorbed

leaf adaptations: thinness

leaves are very thin, meaning that carbon dioxide only has a short distance to travel to enter the leaf (and work in photosynthesis) and oxygen only has a short distance to diffuse out

leaf adaptations: large surface area

having a large surface area means that the leaf can absorb more light at once, maximising the rate of photosynthesis

effect on rate of transpiration from increase in temperature

the molecules move faster, resulting in evaporation happening at a faster rate and therefore the rate of transpiration increases.

the rate of photosynthesis increases, meaning more stomata are open for gaseous exchange, so more water evaporates and the rate of transpiration increases

effect on rate of transpiration from increase in relative humidity (the measure of the concentration of water vapour in the air in comparison to the total concentration of water that air can hold)

if the relative humidity is high, then there will be a reduced concentration gradient between the concentrations of water vapour inside and outside the leaf, resulting in a slower rate of diffusion. this will decrease the rate of transpiration

effect on rate of transpiration from increased air movement

if more air is moving away from the leaf due to it being blown away, then the concentration of water vapour surrounding the leaf will be lower. this will mean there will be a steeper concentration gradient resulting in diffusion happening faster. this will increase the rate of transpiration

effect on rate of transpiration from increase in light intensity

this leads to an increased rate of photosynthesis, so more stomata open to allow gaseous exchange to occur. this means more water can evaporate, leading to an increased rate of transpiration

rate calculations for transpiration

• measuring the uptake of water by the plant gives an indication to the rate of transpiration, because water is only taken up if water leaves the plant

• this is observed by using a potometer, which involves placing a plant in a capillary tube in water, and measuring the distance travelled by a bubble

• we can place a leaf shoot in one end of the potometer, and use a ruler to measure how far the bubble travels up the capillary tube in a set time (eg. 1 minute)

• the further the bubble moves in this time, the greater the rate of transpiration and thus the greater the rate of water uptake

extreme adaptations

leaf shape and size - many desert plants do not have leaves, or have very small leaves. this reduces the amount of water lost as a result of transpiration

presence of a waxy cuticle - many leaves have a waxy cuticle on top, preventing evaporation of water in environments where water is scarce

stomata - stomata are small pores on the surface of a leaf. they can be closed to prevent evaporation of water in extreme environments, and opened when carbon dioxide is needed for photosynthesis. this is useful, as it means that the plant can adapt when water is scarce

what do plants need for growth and what are they needed for

plants need hormones to coordinate and control growth. they are needed for tropisms

examples of tropisms

examples of these include phototropism, the response to light, and gravitropism or geotropism, the response to gravity. hormones move from the place they are made to where they are needed in order to produce the appropriate response

auxins for growth

most plants show positive phototropism because they grow towards the light source

• the plant is exposed to light on one side

• auxin, a growth hormone, moves to the shaded side of the shoot

• auxin stimulates cells to grow more here

• this means the shoot bends towards the light

• the plant recieves more light, meaning photosynthesis can occur at a faster rate

most shoots show negative gravitropism as they grow away from gravity. if a shoot is horizontal:

• auxin moves to the lower side

• the cells of the shoot grow more on the side with most auxin, so it stimulates cells to grow more here

• this makes the shoot bend and grow away from the ground

• this is beneficial as light levels are likely to be higher further away from the ground

most roots show positive gravitropism as they grow towards gravity. if a root is horizontal:

• auxin moves to the lower side

• the cells of the root grow more on the side with less auxin, so it stimulates cells to grow on the upper side

• this makes the root bend and grow downwards

• this is beneficial as there are more likely to be increased levels of water and nutrients lower down, and it provides stability for the plant

when the auxin distribution becomes equal on both sides it grows straight in that direction

how to investigate the light or gravity on newly germinated seedlings

you can investigate the effect of light or gravity on newly germinated seedlings by varying conditions

• placing in cardboard box and shining light from one side

• attaching a petri dish containing the seedlings to a wall (effects of gravity)

commercial uses of plant hormones

humans can use plant hormones to alter plant growth. they are used in areas such as agriculture and horticulture for many reasons: to increase yield, obtain desirable features and to lower costs

Auxin

1. as weed killers

   • many weeds are broad-leaved

   • weed killers, containing auxin, have been synthesised so they only affect broad-leaved plants

   • the increased amount of auxin causes the cells to grow too rapidly

   • this results in the weed dying

2. as rooting powders

   • plants with desirable features are cloned to make more plants with the same feature

   • one way to clone a plant is to take a cutting from the original plant

   • rooting powder containing auxin is applied to it and it is placed in the ground

   • roots grow and the new plant begins to grow very quickly

3. to promote growth in tissue culture

   • another way to clone a plant is to use tissue culture

   • cells from the plant are take and placed in a growth medium containing lots of nutrients

   • hormones such as auxins are added

   • the cells begin to from roots and shoots

Gibberellins are used in germination, for fruit and flower:

• gibberellins allow seed germination to occur by breaking seed dormancy (the period before germination)

• they allow fruits to grow heavier and larger, increasing yields

• they encourage flowering plants to flower at a faster rate

as ethene controls ripening, it is used in the food industry

• fruit is picked when it is not ripe

• it is firm which means that during transport it gets less bruised and damaged

• when it is needed to be sold, it is exposed to ethene and warmer temperatures

• ethene is involved in controlling cell division and stimulates enzymes that result in fruit ripening

• this reduces wastage as more fruit is suitable to be sold and it does not ripen too early