Photosynthesis in Plants

• All living organisms need nutrients for growth, repair, and to release energy.
• Autotrophs make their own food from inorganic substances, while heterotrophs (like animals) obtain organic substances based on carbon.
• Photosynthesis uses light to synthesize carbohydrates from carbon dioxide and water, producing glucose and oxygen.
• Oxygen is released as a waste product during photosynthesis.
• The reaction requires energy obtained from sunlight, trapped by chlorophyll.
• Water is taken up by roots to the leaves through the xylem, while carbon dioxide diffuses into the leaf through the stomata.
• Light + 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ + Chlorophyll.
• Chlorophyll pigments absorb sunlight, converting it into chemical energy and helping combine CO₂ and H₂O to create glucose.
• Different pigments absorb different wavelengths of light; light energy is transferred into chemical energy.
• Photons of light "excite" electrons in plant pigments, and these electrons carry the absorbed energy.

Uses of Glucose

• Glucose is released by cell respiration.
• Glucose is turned into starch for energy storage.
• Starch is water-insoluble and not reactive, found in granules, and stored in chloroplasts.
• The starch is changed back into glucose when dissolved from the cell.

Leaf Structure

• A single cell layer of transparent cells allows light to pass through, maximizing light for photosynthesis.
• Contain many stomata and guard cells to regulate gas exchange.
• Tightly packed cells near the upper surface contain an increased number of chloroplasts to increase light absorption.
• Open: CO₂ enters, O₂ exits. Closed: vice versa.
• Contain xylem and phloem to transport substances.
• Xylem transports water and minerals from roots to leaves.
• Phloem transports sugars and other organic substances made by photosynthesis from leaves to other parts of the plant for storage or use.

Leaf Adaptations

• Leaves are adapted in several ways for photosynthesis.
• Large Surface Area increases light energy absorption by maximizing the absorption of light energy and CO₂ capture.
• Thin leaves allow sunlight to easily diffuse, and all parts of leaf are close to stomata, facilitating gas exchange.
• Contain light absorbing pigment chlorophyll to trap more light - they will position themselves flat against the cell wall to increase light adsorption or cluster to reduce damage in strong light.
• Stomata: Small openings/pores that facilitate gas exchange and regulate water loss through transpiration.
• Guard cells open and close stomata, controlling water loss.

Limiting Factors of Photosynthesis

• Photosynthesis is affected by sunlight, CO₂, water, and temperature.
• A limiting factor restricts the rate of photosynthesis; one factor will limit the process if others are in sufficient supply.
• Main Limiting Factors: Temperature, carbon dioxide concentration, light intensity.
• Water is not usually a limiting factor because plants need only a small amount compared to overall water needs.
• Optimum temperature is the temperature at which enzymes begin to denature.

Investigating Gas Exchange

• Plants photosynthesize at a faster rate during the day than they respire.
• In the dark, plants only respire.
• Changes in net gas exchange can be observed in aquatic plants using a pH indicator.
• Carbon dioxide is an acidic gas.
• Hydrogen carbonate indicator changes colors depending on the carbon dioxide concentration in the solution.

Mineral Needs

• Carbohydrates contain carbon, hydrogen, and oxygen, but plants need different elements for growth, obtained from the soil.
• Mineral is any naturally occurring inorganic substance.
• Provides building material.
• Minerals are integrated into products like proteins, which use nitrogen.

Effects of Fertilizers

• Varying the amounts and types of fertilizer affect plant growth.
• Nitrogen is the most important mineral ion because plants use it to make amino acids for proteins, so the plant can't grow without it.

Iodine Test For Starch

• Although plants make glucose in photosynthesis, leaves cannot be tested for it because it is quickly used or converted into other substances and temporarily stored as starch
• Starch is stored in the chloroplasts, impeding the proper view of iodine and green colour
• Boiling water disrupts cell membranes, allowing for better access of iodine
• Removal of colour is achieved by introducing the leaf into boiling ethanol
• Starch will turn blue/black in the presence of iodine

Experimental Controls

• Controlled variables are kept the same with experimental groups.
• If two groups (experimental & control) are treated exactly the same, then any differences can be affected by the 'independent variable'.

Need for Light Investigation

• A plant needs to be destarched before an investigation, by placing it in a dark cupboard for 24 hours and allow the leaves to use up all starch
• After this process, a leaf of the plant can be partially covered with aluminium foil and the plant placed sunlight for a day
• It can then be removed and tested for starch using iodine