IGCSE Unit 6: Plant Nutrition

Plant Nutrition - IGCSE Unit 6

• This unit covers functions of leaves, leaf structure, photosynthesis, and mineral ions.

Functions of Leaves

• Leaves serve as the main photosynthetic organ in plants.
• Most leaves have a large surface area to maximize light absorption.
• Leaves have a thin structure that provides a short diffusion distance for gas exchange.
• Stomata are tiny openings in leaves that regulate gas exchange (CO2, O2, and water vapor) with the atmosphere.

Leaf Internal Structure

• The internal structure of a leaf (2D and 3D diagrams) shows several key features.
• The cuticle is a waterproof waxy layer that prevents water evaporation; it is produced by cells in the upper epidermis.
• The upper epidermis comprises thin and transparent cells allowing light to pass through; these cells act as a barrier to bacteria and viruses and do not contain chloroplasts.
• Palisade mesophyll is the primary region for photosynthesis, consisting of columnar cells packed tightly with many chloroplasts, receiving carbon dioxide diffused from air spaces in the spongy mesophyll, and is generally found in the upper layer of the leaf.
• Spongy mesophyll cells are spherical, loosely packed, and contain fewer chloroplasts than palisade cells; air spaces between cells allow gaseous exchange during photosynthesis.
• The vascular bundle is a leaf vein made of xylem and phloem that brings water and minerals to the leaf from the roots and transports sugars and amino acids away (translocation).
• Lower epidermis is a protective layer with stomata, regulating water loss (transpiration) and acting as the site of gas exchange.
• Stomata are surrounded by guard cells controlling the opening/closing; water vapor exits during transpiration, while carbon dioxide enters, and oxygen exits during photosynthesis.

Leaf Adaptation for Photosynthesis

• A thin, waxy cuticle prevents water loss via evaporation.
• A thin, transparent epidermis allows more light to reach the palisade cells.
• There is a network of vascular bundles which supports the leaf and transports water, glucose, amino acids, and salts.
• Intercellular air spaces provide more surface area for gas diffusion.
• Many chloroplasts filled with chlorophyll trap light energy to break up water molecules, resulting in hydrogen and CO2 bonding to form glucose.
• Stomata allow CO2 to diffuse into the leaf and O2 to diffuse out.

Stomata

• Stomata consists of a pair of guard cells, which surrounds the opening (stomatal pore).
• Monocot plants have stomata equally distributed on both leaf sides.
• Most dicot plants only have stomata on the lower epidermis.
• When guard cells gain water by osmosis, turgor pressure increases, causing them to stretch/curve outwards, opening the stomatal pore for gas movement.
• Losing water by osmosis reduces turgor pressure, causing guard cells to move closer together, closing the stomatal pore, stopping gas movement, and preventing water loss.

Photosynthesis

• Photosynthesis synthesizes carbohydrates from raw materials using light energy.
• Chlorophyll interacts with sunlight, converting light energy into chemical energy in molecules.
• The chemical energy synthesizes glucose, used in cellular respiration to provide energy.
• Chloroplasts are the organelles in leaf cells where photosynthesis occurs.
• Biological molecules, like carbohydrates, fats, proteins, and nucleic acids, derive from photosynthesis.
• Glucose is used to make nectar to attract insects for pollination.

Photosynthesis vs. Respiration

• Photosynthesis occurs in chloroplasts in the presence of light, using carbon dioxide and water to produce glucose and oxygen; it stores energy obtained from light.
• Respiration occurs in mitochondria at all times using glucose and oxygen to produce carbon dioxide, water, and energy in the form of chemical bonds; it releases energy.

Experiments Related to Photosynthesis

• Experiments were conducted using a de-starched plant (kept in darkness for 2-3 days).
• Enclose the plant in a clear plastic bag with a chemical (e.g., soda lime) that absorbs CO2.
• Placing a leaf in boiling water kills leaf cells by damaging the membranes and denaturing the enzymes which makes the leaf more permeable to iodine solutions.
• Variegated leaves, those with chlorophyll and those without, are used to make the experiment more observable.
• The parts that were originally green (containing chlorophyll) will turn a dark blue-black color.
• The parts that were originally white (without chlorophyll) will stain a orange-brown color.

Factors Affecting Photosynthesis

• Limiting factors restrict life processes due to their short supply in the environment.
• Three limiting factors affecting the rate of photosynthesis: light intensity, carbon dioxide concentration, and temperature.
• As light intensity increases, the rate of photosynthesis will also increase, the rate of photosynthesis becomes constant, after a certain point.
• As Carbon Dioxide concentration increases, the rate of photosynthesis will also increase, but after a certain concentration, the rate levels off.
• For temperature increases, as temperature increases, the rate of photosynthesis also increases up to a certain point (maximum rate of photosynthesis is reached), if the temperature continues to rise, the rate of photosynthesis rapidly decreases.

Hydrogen Carbonate Indicator

• It is very sensitive to changes in carbon dioxide levels.
• It changes to yellow when a large amount of carbon dioxide is present.
• It becomes purple when a low amount of carbon dioxide is present.