Topic 6

• What is photosynthesis?


  A chemical reaction that takes place inside photosynthetic organisms (e.g. plants, algae) converting light energy into chemical energy
• Write the word equation for photosynthesis

  carbon dioxide+ water -> oxygen + glucose
• Write the symbol equation for photosynthesis.

  6CO2 + 6H2O ------> C6H12O6 + 6O2
• Why is photosynthesis important?

  -used in respiration to release energy

  -converted to starch and stored- broken down to glucose when the plant requires energy

  -used to make complex organic molecules which are used for growth, making up an organisms biomass
• Where does photosynthesis take place?

  chloroplasts
• What type of reaction is photosynthesis?

  endothermic
• What is chlorophyll


  A green pigment found in the chloroplasts of plants, algae, and some bacteria that absorbs light
• Describe the two main stages of photosynthesis

  1. Chlorophyll absorbs light energy which is used to split water into oxygen gas (waste product) and hydrogen ions

  2. Carbon dioxide combines with hydrogen ions to form glucose
• What factors affect the rate of photosynthesis?

  light intensity, carbon dioxide concentration and temperature
• What is a limiting factor?

  A variable that limits the rate of a particular reaction.
• Explain how temperature affects the rate of photosynthesis

  ● Higher temperatures provide more KE for enzymes involved in photosynthesis so the rate increases as temperature rises

  ● The optimum temperature is usually 25°C

  ● If the temperature becomes too high (around 45°C) enzymes become denatured and the rate of photosynthesis decreases
• Explain how light intensity affects the rate of photosynthesis

  Rate of photosynthesis is directly proportional to light intensity ∴ as light intensity increases, the rate of photosynthesis increases.
• Why does the rate of photosynthesis eventually plateau even if light intensity continues to increase?

  Another factor (temperature or CO2 concentration) becomes limiting.
• How can the effect of light intensity on the rate of photosynthesis be measured in the lab?

  Using a light meter

  Using the inverse square law where:
• Why does the rate of photosynthesis decrease as the distance from a light source increases?

  Light intensity is inversely proportional to the square of the distance from the light source. Thus, as the distance increases, light intensity decreases and the rate of photosynthesis decreases.
• Explain how carbon dioxide concentration affects the rate of photosynthesis

  As carbon dioxide concentration increases, the rate of photosynthesis increases
• Why does the rate of photosynthesis eventually plateau even if CO2 concentration continues to increase

  Another factor (temperature or light intensity) becomes limiting.
• Why do plants require water

  - photosynthesis

  - maintenance of structure (turgidity)

  - cooling effect
• Why do plants require mineral ions

  For growth e.g. nitrates are required to produce proteins
• Which structure in plants in adapted for the uptake of water and minerals


  Root hairs
• How is water transported into root hairs?

  - Lower concentration of water in root hair cells than in the soil

  - Water diffuses down its concentration gradient into root hair cells by osmosis
• How are minerals transported into root hairs

  ● Lower concentration of mineral ions in the soil than in the root

  ● Root hair cells take up mineral ions by active transport
• Outline how plant roots are adapted for the absorption of water and minerals

  Plant roots are composed of millions of root hair cells which have:

  - Long hairs that extend from the cell body, increasing the surface area for absorption

  - Many mitochondria which produce ATP for active transport of mineral ions
• Name the two plant transport tissues

  xylem and phloem
• What is the function of the xylem

  transports water and soluble mineral nutrients from the roots throughout the plant
• Describe how the xylem is adapted to its function

  ● Composed of dead cells laid end-to-end to form a long, hollow, continuous column

  ● No end walls which provides little resistance to the passage of water

  ● Thick cell wall strengthened with lignin to provide support
• What is the function of the the phloem


  transports the products of photosynthesis (sugars) throughout the plants by translocation
• What are the two cell types that make up the phloem

  sieve tube elements and companion cells
• Describe how the phloem is adapted to its function

  ● Sieve tube elements are long, thin cells, laid end-to-end with perforated end plates to enable the flow of sugars. They contain no nucleus and little cytoplasm to allow sugars to flow easily.

  ● Companion cells (adjacent to sieve tube elements) contain a dense cytoplasm, nucleus and mitochondria. They provide energy for processes in both cell types.
• What is transpiration?


  The loss of water vapor through the stomata of leaves.
• Where does the majority of transpiration take place

  Leaves
• Describe the process of transpiration

  ● Water evaporates from the mesophyll cell surfaces and diffuses out of the stomata

  ● Water molecules (which have cohesive properties) are drawn up the xylem vessels to replace the water that has been lost

  ● This causes more water molecules to be absorbed from the soil into root hair cells
• How does the transpiration stream work?

  Mineral ions are dissolved in the water
• What are stomata?


  Pores found in the lower epidermis of a leaf which allow gas exchange
• What are guard cells?

  Specialised cells surrounding the stoma that change shape to control the size of the pore
• How do guard cells control the size of stomata?


  To open the stomata:

  Water enters guard cells. They swell and become turgid. They bend and draw away from each other, opening the stomata.

  To close the stomata:

  Water leaves guard cells. They become flaccid, closing the stomata.
• What factors affect the rate of transpiration


  Light intensity, temperature, humidity and wind
• Describe how high light intensity affects the rate of transpiration

  ● High light intensity, greater number of stomata open to allow gas exchange for photosynthesis

  ● Rate of photosynthesis increases so more water is taken up from the soil, pushing water up the xylem

  ● More water vapour diffuses out of the stomata ∴ rate of transpiration increases
• Describe how low light intensity affects the rate of transpiration

  At a low light intensity, fewer stomata are open so the rate of transpiration decreases.
• Describe how temperature affects the rate of transpiration

  ● Temperature increases, water molecules have more KE so rate of diffusion increases

  ● Photosynthesis also increases so more water is taken up from the soil, pushing water up the xylem

  ● More water vapour diffuses out of the stomata ∴ rate of transpiration increases