iGCSE Biology Section C

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Photosynthesis

carbon dioxide + water --> glucose + oxygen
              6CO2 + 6H2O --> C6H12O6 + 6O2

Rate of photosynthesis is affected by CO2 concentration, light intensity and temp.
Limiting factor: the component of a reaction that is in short supply so it limits the rate at which the reaction can take place.

The strucutre of a leaf is adapted for photosynthesis and gas exchange with chloroplasts, a large surface area for diffusion of gasses and stomata to allow the exchange of gases between the lead and the air.

The glucose that is produced is converted into sucrose to be transported, starch to be stored, cellulose for cell walls, proteins and DNA, lipids as an energy store in seeds and chlorophyll. 

DESTARCHED PLANT EXPERIMENT

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Leaf structure

(http://images.tutorvista.com/content/feed/u2044/leafcross%20section.GIF)

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Transport in plants

Osmosis: the movement of water down a concentration gradient of water through a partially permeable membrane.

Water moves into and out of living cells by osmosis
Plant cells need to be turgid to support the plant.
Water is taken up from the soil by root hair cells and moves across the root by osmosis.
Water and mineral ions are transported around the plant in the xylem
Sucrose and amino acids are transported from the leaves around the plant by the phloem.

Transpiration: the evaporation of water from the surface of a plant
Rate of transpiration is affected by; humidity, wind speed, temp and light intensity.

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Osmosis in plant cells

The cell membrane is partially permeable. The cytoplasm and cell sap in the vacuole contain many solutes. If the plant cell is bathed in pure water or a dilute solution, water moves into the plant cell by osmosis. The vacuole and cell swell up and the cytoplasm pushes against the cell wall. The plant cell is turgid. When the plant isn't turgid, the plant will wilt and become flaccid.
(http://wizznotes.com/wp-content/uploads/2011/01/image0012.jpg)

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Transport tissue (vascular tissue)

Xylem - transports water and mineral ions up from the roots to the rest of the plant.
Phloem - transports sugars for energy (particulary sucrose) and amino acids for cell building around the plant.
These two are called the vascular bundle

Uptake of water and mineral ions by the roots
The outside off the roots are covered in root hair cells which increase the surface area of the root for uptake of water. The conc. of mineral ions in the soil water is much lower than in the root cell cytoplasm. Water enters the root hair cells by osmosis; it moves across the root cellls to the xylem.
Mineral ions are at a low conc in the soil water and are moved into the root hair cells against a concentration gradient, using active transport using energy from cell respiration. The mineral ions move across the root mainly by diffusion and enter the xylem.

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Transpiration - the transpiration stream

  • Water leaves the leaf as water vapour through the stomata.
  • It evaporates from the mesophyll, cooling the leaf.
  • Water moves from the xylem to the mesophyll cells by osmosis.
  • As water is lost by evapouration, the cytoplasm of that mesophyll cell becomes concentrated so water moves in by osmosis and water now moves into them from cells closer to the xylem. 

The rate of transpiration is controlled by the stomata. They open and close to allow gas exchange and to control the rate of water loss.
It is affected by: light intensity, temp, humidity and wind speed.

USE A POTOMETER TO MEASURE THE RATE OF WATER UPTAKE

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Phototropism

  • Plants respond to stimuli (tropisms)

Phototropism: response to light
Stems have positive phototrophic responses - they grow towards the light
Roots usually have a negative phototrophic response - grow away from the light
AUXINS - a hormone which builds up on the 'dark' side of the shoot. The more auxins, the more growth. This makes the plant bend and face the light.
(http://plantphys.info/plant_physiology/images/darwintip.gif)

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Geotropisms

Geotropism: response to the force of gravity
Stems have a negative geotrophic response - they grow away from the direction of gravity
Roots have a positive geotrophic response - they grow towards gravity.

(http://4.bp.blogspot.com/-oCfe-1IoCB4/TZBtkSjYP4I/AAAAAAAAAEM/8zlN9GVfuVk/s400/geotropism.png)

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Reproduction in plants

Plants can reproduce both asexually and sexually.

Asexual reproduction involves only one parent. The offspring are identical to the parent. Result of mitosis.

Sexual reproduction involves the joining of two special sex cells or gametes. The offspring are not the same as the parents. Pollen is carried frrom one flower to another. Flowers can be pollinated by the wind or by insects. After pollenation a pollen tube grows down into the ovary to carry the male nucleus to the ovules. A zygote is formed. Gametes are formed by meiosis.
Wind pollenated and insect pollenated flowers have different features.
                       Insect pollinated                 Wind pollinated.
stamens:        Inside petals                         outside petals
stigma:           Inside petals + sticky            outside petals + feathery
petals:            large and bright                    small and green
nectaries        present                                 absent
pollen grains: large and sticky                    small, smooth and light.

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Flower structure

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Seeds, fruit and germination

Each seed contains the embryo plant.
radicle - root
plumule - shoot
seed leaves - cotyledons  (act as a food store)
seed coat - testa

Germination - Where the embryo plant can photosynthesise independently

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