AQA GCSE Biology 9-1 -- Photosynthesis

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Chloroplasts
- Cells in algae and plant leaves are full of them
- Contain chlorophyll that ABSORBS the light
Energy during photosynthesis
- Energy is transferred FROM the environment to the chloroplasts by light
- Is then transf. to convert CO₂ from the air and water from the soil into GLUCOSE
- The chemical reaction also produces O₂ as a by product
Photosynthesis
- Is an endothermic reaction

Carbon dioxide + Water → (light) → Glucose + Oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Glucose
- Some is used immediately by the cells for RESPIRATION
- A lot of it is converted to an insouble starch and stored
Leaf adaptations
- Are broad, giving them a BIG surface area for light to fall on
- Are thin so diffusion distances for gases are SHORT
- Veins to bring water in the XYLEM to the leaves' cells and REMOVE products of photosynthesis in the phloem
- Air spaces to allow CO₂ to GET to cells, and O₂ to LEAVE by diffusion
- Guard cells that open and close the stomata to regulate gas exchange
Algae photosynthesis
- Are aquatic so are adapted to photosynthesising in water
- Large surface area
- Absorb CO₂ dissolvedd* in the water around them
- The oxygen they produce dissolves in the water around them
Limiting factors
- When light, warmth or CO₂ is in short supply it can LIMIT the amount of photosynthesis a plant can manage
Light
- If there is plenty, lots of photosynthesis will occur
- If there is very little or none, photosynthesis will STOP despite the other conditions
Brighter light = Faster rate
Temperature
- Affects ALL chemical reactions
Temperature rise = Rate increase as reaction SPEEDS up
- Photosynthesis is controlled by ENZYMES, most of which denature around 40-50 degrees
Temperature too high = Denatured enzymes = Fallen rate
CO₂ concentration
- Plants need CO₂ to makee* GLUCOSE
- The atmosphere is only 0.04% CO₂ which means it OFTEN limits the rate
Increased concentration = Increased rate
CO₂ concentrations in specific conditions
- Sunny day - is the most COMMON limiting factor
- Nighttime - tend to rise as a plant RESPIRES but doesn't photosynthesis
- Science lab or greenhouse means the levels can be increased ARTIFICIALLY
Amount of chlorophyll in a leaf
- If limited, LESS photosynthesis will take place
- The leaves of some plants have WHITE, chlorophyll-free areas
- These plants grow less vigorously
- If they are in permanently dim light, the leaves will turn COMPLETELY green
Plants lacking in minerals
- Cannot make chlorophyll (esp. if lacking in magnesium)
- The rate of photosynthesis will DROP and eventually the plant will die
Light Intensity
= 1
−−−−−−−−−−−−−−−
distance²
- As the distance of the light from the plant INCREASES, the light intensity DECREASES
- The relationship is not linear
Respiration
- Done by all plant cells and algal cells
- Uses the glucose produced in photosynthesis
- Is chemically the reverse of photosynthesis
What is respiration used for?
- Building SMALLER molecules into BIGGER molecules e.g cellulose
Plants and glucose conversion
- G. produced in photosynthesis in converted into STARCH to be stored
- Glucose is SOLUBLE in water so it would affect the way water moves in and out of cells by osmosis
Starch
- Insoluble in water
- Has no effect on the water BALANCE of the plant
- Can be stored in large amounts in the cells
- Found in cells of the leaves
- Plants can produce tubers and bulbs full of stored starch to help them survive through the winter
Amino acids
- Made using some of the glucose
- Combination of sugars with nitrate and mineral IONS
- They are built up into proteins to be used in the plant cells in many ways
Importance of minerals
- Very few plants can survive if the soil is LOW in minerals
- Some carnivorous plants are ADAPTED to live in nitrate-poor soil
- They survive as they obtain most of their nutrients from the insects they catch
Lipids
- Uses the glucose from photosynthesis to build up fats ad oils
- Sometimes used in cells as an energy STORE or in the cell wall to make them STRONGER
- Used as an energy store in their seeds which provides food for the new plant to respire as it GERMINATES
More photosynthesis
= More BIOMASS it makes = Faster it grows
Greenhouse
- Glass/plastic building means the environment is much more controllable
- The atmosphere is warmer INSIDE than outside
- This SPEEDS up photosynthesis, so the plants grow faster
Commercial greenhouses
- Take advantage of limiting factors
- Controls the temp, light levels and CO₂
- Conditions are controlled using a PC SOFTWARE
Hydroponics
- Plants are grown in water with a perfect balance of NUTRIENTS instead of soil
Greenhouse economics
- Electricity and gas are used to maintain lighting and temps.
- Expensive monitoring equipment and PCs are needed to maintain conditions within NARROW boundaries
- Alarms are vital if things go wrong
- Less staff are needed
- Final crop is larger and cleaner