Photosynthesis - Chp 11

Photosynthesis

The process in which plants make food (carobohydrates) and oxygen using energy from the sun, water from the soil, and carbon dioxide from the atmosphere
Light energy → Chemical energy

Role of photosynthesis

• plants use it to make food

• animals get their food from plants

• produces oxygen for respiration to release energy

• responsible for forming fossil fuels

• removes carbon dioxide from the air

Balanced equation for photosynthesis

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Where does photosynthesis occur

the leaf's cells in the chloroplast

Where are most chloroplasts found

along the edge of the palisade (top) layer of the leaf to absorb sunlight

What pigment is required for photosynthesis

chlorophyll

What adaptations do leaves have for photosynthesis

Stomata - allow for gaseous exchange, open during the day to allow CO₂ in for photosynthesis, closed at night which reduces transpiration
Air spaces - allow for diffusion of CO₂ and H₂O withing the leaf
Thin - for rapid diffusion of CO₂ and O₂ out, allows all cells to capture light
Cuticles prevents excessive water loss, transparent, allows light through for photosynthesis
Leaf flattened - gives large surface area for maximum absorption of light and CO₂

What 4 things are required for photosythesis

• carbon dioxide

• water

• light energy

• chlorophyll

What are the products of photosythesis

• glucose

• oxygen

Photophosphorylation

process of adding a phosphate group to ADP to form ATP using light energy

Key events of photosynthesis

• light is absorbed

• water is split

• products are produced

• light energised electrons

• glucose is formed

Event 1 - light is absorbed

• light reaches the plant and is trapped by the chlorophyll in the chloroplast

• trapped light provides the energy the plants need to make glucose

Event 2 - water is split

• the trapped light energy is used to split 2 water molecules into oxygen molecules (O₂), protons (H⁺), and electrons (e⁻)
  2H₂O → 4H⁺ + 4e⁻ + O₂

Event 3 - products of split water are stored

• electrons passed to the chlorophyll

• protons are stored in the proton pool in the chloroplast for later use

• oxygen may pass out of the leaf into the atmosphere or may be used for plant respiration

Event 4 - light energises electrons

• electrons that were passed to the chlorophyll become energised by some of the trapped light energy, and become high-energy electrons

Event 5 - glucose is formed

• high energy electrons along with protons from the proton pool are combined with carbon dioxide to form glucose

Sources of light for plants

• sunlight is a natural source of light for plants

• artificial light can also be used, particularly in greenhouses to stimulate growth

• plants absorb all colours of white light other than green, which they will reflect, this is why most plants are green

Sources of carbon dioxide for plants

• get most of their carbons dioxide from the atmosphere, which is external

• a small amount is gained internally from their own cellular respiration

What can be used to increase photosynthesis in an agricultural environment?

• artificial light will increase growth up to a certain saturation point where no more light will be absorbed and the rate of photosynthesis will level off

• artificial sources of carbon dioxide are used, such as burning gas

Sources of water for plants

water is absorbed from the soil by the roots of the plant, then transported through the xylem

Limiting factors to photosynthesis

• slowed down if necessary factors such as light, CO₂, or water are short in supply

• light will increase the rate until it reaches its saturation point, at which it levels off

• maintain an optimum temperature

Role of NADP

electron acceptor during the light stage

Role of NADP-

proton acceptor during the light stage

Overview: The light stage

• light dependant stage

• 2 pathways within this stage: Cyclic photophosphorylation (pathway 1) and Non-cyclic photophosphorylation (pathway 2)

• Makes ATP and NADPH

• not controlled by enzymes

Overview: The dark stage

• light independent stage

• Uses ATP and NADPH

• is controlled by enzymes and can be affected by temperature

Where does the light stage occur?

the granum of the chloroplast

Where does the dark stage occur?

the stroma of the chloroplast

Cyclic phosphorylation (pathway 1) process

• electrons pass from the first electron acceptor to a series of other electron acceptors and back again to the chlorophyll

• as the electrons are passed around as they lose energy

• this energy is used to join a phosphate to ADP to form the high energy carrier ATP

• water is also formed in this process

• the electron travels in a cycle and returns to its original chlorophyll

Non-cyclic phosphorylation (pathway 2) process

• 2 high energy electrons at a time are passed from the chlorophyll

• they do not return to the chlorophyll

• they lose energy as they pass from electron acceptor to electron acceptor and this energy is used to make more ATP

• the 2 electrons become low energy electrons and combine with NADP⁺ to form NADP-

• the chlorophyll molecule is now short of electrons and gains more from the splitting of water

• protons that were stored in the proton pool are attracted to NADP- and combine with it to form NADPH
  H⁺ + 2e⁻ + NADP⁺ → NADPH

Photolysis

the splitting of water into O₂, H⁺ and electrons using light energy

What are the end products of the light stage?

• ATP which provides energy for the dark stage

• NADPH this will provide protons + energised electrons for the dark stage

• Oxygen is made when the water is split

Dark stage process

• carbon dioxide from the air enters the chloroplast where they combine with protons + electrons to form glucose

• this requires energy and protons + electrons

• it uses energy got from breaking down ATP

• it uses hydrogen ions and electrons from breaking NADPH into NADP⁺

• carbon dioxide is reduced to glucose

What plant is used in the experiment for photosynthesis?

Elodea (pond weed)

Why is the plant used in the experiment?

it is an underwater plant, so it is possible to see the bubbles of oxygen released by the plant as it respires, passing through the water
the number of bubbles indicates the rate of photosynthesis

How is the rate of reaction observed?

count the bubbles per minute

What is the independent variable?

distance between the lamp and the plant

What are constants in the experiment?

• temperature is kept constant using a water bath and thermometer
• CO₂ concentration is kept constant using sodium bicarbonate in excess to make sure water is saturated with CO₂ keeping levels constant