plant structures and their functions

• during photosynthesis green plants and algae use energy from the sun to make glucose. some of the glucose is used to make larger, complex molecules that are needed to grow. these make up the organisms biomass - the mass of living material
• the energy stored in the organisms biomass then works its way through the food chain as animals eat them and each other. so photosynthetic organisms are the main producers of food for nearly all life on Earth.
• photosynthesis happens inside chloroplasts - they contain chlorophyll which absorbs light. energy is transferred to the chloroplasts by light
• photosynthesis word equation :

  carbon dioxide + water -- light --> glucose + oxygen
• photosynthesis symbol equation:

  6CO2 + 6H2O --Light---> C6H12O6 + 602
• photosynthesis is an endothermic reaction - energy is taken in during the reaction
• the rate of photosynthesis is affected by the light intensity, concentration of carbon dioxide and the temperate. any of these 3 can be the limiting factor. this means it's stopping photosynthesis from happening any faster
• investigating how light intensity can affect rate of photosynthesis:

  1 ) set up the apparatus with gas syringe empty

  2) a source of light is placed at a specific distance from pondweed

  3) the pondweed is left to photosynthesise for a set amount of time

  4) as it photosynthesises oxygen will be released into the gas syringe so you can accurately measure volume of oxygen

  5) this is repeated with light source at different distances

  control variable - temperature (water bath or heat shield) and carbon dioxide concentration
• light transfers the energy needed for photosynthesis
• at first, as the light intensity is raised, the rate of photosynthesis increases steadily (the rate is directly proportional to light intensity). but this is only true up to a certain point. beyond that it won't make a difference - it'll either be the temperature or carbon dioxide concentration which is the limiting factor
• in the lab you can investigate light intensity by moving a lamp closer to or further away from the plant
• the distance from the lamp and light intensity are inversely proportional to each other - this means as the distance increases, the light intensity decreases. however light intensity decreases in proportion to the square of the distance

  light intensity is proportional to 1/ distance squared
• if you halve the distance, the light intensity will be four times greater and if you double the distance the light intensity will be four times smaller
• CO2 is one of the raw materials needed for photosynthesis. increasing CO2 concentration increases the rate of photosynthesis up to a point. after this the graph flattens out showing that CO2 is no longer the limiting factor. as long as light and CO2 are in plentiful supply then the factor limiting photosynthesis must be temperature
• usually if the temperature is the limiting factor it's because it's too low - the enzymes needed for photosynthesis work more slowly at low temperatures. but if the plant gets too hot, the enzymes it needs for photosynthesis and its other reactions will be denatured. this happens at about 45C
• the cells on the surface of plant roots grow into 'hairs' which stick out into the soil. each branch of a root will be covered in millions of these microscopic hairs. this gives the plant a large surface area for absorbing water and mineral ions from the soil. the concentration of mineral ions is usually higher in the root hair cells than in the soil around them, so mineral ions are absorbed by active transport and water is absorbed by osmosis
• root hair cells take in minerals and water
• phloem tubes transport food
• phloem tubes are made of columns of elongated living cells with small pores in the end walls to allow stuff to flow through. they transport food substances (mainly sucrose) made in the leaves to the rest of the plant for immediate use (in growing regions) or for storage. this process is called translocation and it requires energy from respiration. the transport goes in both directions
• xylem tubes take water up
• xylem tubes are made up of dead cells joined together end to end with no end walls between them and a hole down the middle. they're strengthened with a material called lignin. they carry water and mineral ions from the roots to the stem and leaves. the movement of water from the roots, through the xylem and out of the leaves is called the transpiration stream
• transpiration is the loss of water from the plant
• transpiration is caused by the evaporation and diffusion of water from a plant's surface - at leaves. the loss of water creates a slight shortage of water in the leaf, so more water is drawn up by the rest of the plant through the xylem vessels to replace it. this in turn means more water is drawn up from the roots, and so there's a constant transpiration stream of water through the plant.
• the transpiration stream carries mineral ions that are dissolved in the water along with it
• stomata are tiny pores on the surface of a plant that are mostly found on the lower surfaces of leaves and allow CO2 and oxygen to diffuse directly in and out of a leaf. they also allow water vapour to escape during transpiration
• transpiration is like a side-effect of the way leaves are adapted for photosynthesis. they have to have stomata so that gases can be exchanged easily. because there's more water inside the plant than in the air outside, the water escapes from the leaves through the stomata by diffusion
• stomata are surrounded by guard cells which change shape to control the size of the pore - when the guard cells are turgid (swollen with water) the stomata are open and when the guard cells are flaccid (low on water and limp) the stomata are closed
• guard cells turgid - stoma open
• guard cells flaccid - stoma closed
• the faster the transpiration rate, the faster the water uptake by the plant
• environmental factors that affect transpiration rate:

  humidity

  temperature

  air flow
• light intensity affecting transpiration rate:

  the brighter the light, the greater the transpiration rate. stomata begin to close as it gets darker. photosynthesis can't happen in the dark, so they don't need to be open to let CO2 in. when the stomata are closed, very little water can escape
• temperature affecting transpiration rate:

  the warmer it is, the faster transpiration happens. when it's warm the warm particles have more energy to evaporate and diffuse out of the stomata
• air flow affecting transpiration rate:

  the better the air flow/stronger wind the greater the transpiration rate. if air flow is poor the water vapour just surrounds the leaf and doesn't move away. this means there's a high concentration of water particles outside the leaf as well as inside it so diffusion doesn't happen as quickly. good air flow - water vapour is swept away maintaining a low concentration of water in the air outside the leaf. diffusion then happens quickly from an area of higher concentration to lower concentration
• estimating transpiration rate:

  by using a potometer to estimate transpiration rate. it measures water uptake by a plant but it's assumed that water uptake is directly related to water loss from the leaves
• how to estimate transpiration rate using a potometer:

  1 ) set up the apparatus and then record the starting position of the air bubble.

  2) start a stopwatch and record the distance moved by the bubble per unit time. calculating the speed of air bubble movement gives an estimate of the transpiration rate.

  a potometer can estimate how light intensity, temperature or airflow affect transpiration rate by changing one variable at a time and controlling the rest
• leaves are broad, so there's a large surface area exposed to light, which is needed for photosynthesis
• the palisade layer has lots of chloroplasts. this means that they're near the top of the leaf where they can get the most light
• the upper epidermis is transparent so that light can pass through it to the palisade layer
• the xylem and phloem form a network of vascular bundles, which provide the leaf with water for photosynthesis and take away the glucose produced. they also help support the structure