Plant Nutrition Notes

Plant Nutrition

6.1 Types of Nutrition

All Organisms Feed

All living organisms require various substances for building new parts, repairing old ones, and releasing energy. This process of taking in useful substances is called feeding or nutrition.

Key definition: Nutrition is the intake of nutrients, including organic substances and mineral ions, which provide raw materials or energy for growth and tissue repair, involving absorption and assimilation.

Animals Take Complex Substances from Plants

Animals and fungi cannot produce their own food and rely on organic substances made by plants. Even animals that consume other animals ultimately depend on plants, as the substances are initially produced by plants.

Green Plants Can Make Complex Substances from Simple Chemicals

Green plants are autotrophs, meaning they produce their own food. They use simple inorganic substances such as carbon dioxide, water, and minerals from the air and soil to synthesize complex organic materials like carbohydrates, lipids, proteins, and vitamins.

6.4 Photosynthesis

Photosynthesis Is a Chemical Process

Green plants synthesize glucose (a carbohydrate) from carbon dioxide and water, with oxygen as a byproduct. This process requires energy, typically obtained from sunlight, and is called photosynthesis.

Key definition: Photosynthesis is the fundamental process where plants manufacture carbohydrates from raw materials using light energy.

Chlorophyll Absorbs Sunlight

Sunlight energy must be captured by a substance called chlorophyll, a pigment that gives plants their green color. Chlorophyll is located inside the chloroplasts within plant cells. When sunlight strikes a chlorophyll molecule, the energy is absorbed and used to combine carbon dioxide with water, facilitated by enzymes inside the chloroplast. This converts light energy into chemical energy stored in glucose.

Photosynthesis Can Be Shown as an Equation

The balanced equation for photosynthesis:

6CO2 + 6H2O \xrightarrow[chlorophyll]{sunlight} C6H{12}O6 + 6O2

6.7 Leaves: Food Factories

Photosynthesis occurs within chloroplasts, where enzymes and chlorophyll are present to catalyze the reaction and supply energy. Leaves are the primary sites of photosynthesis in plants and are adapted for this purpose.

Structure of Leaves

A leaf consists of a broad, flat lamina connected to the plant by a petiole (leaf stalk). Vascular bundles within the petiole form veins in the leaf, containing tubes for transporting substances to and from the leaf.

The balanced equation:

6CO2 + 6H2O \xrightarrow[chlorophyll]{sunlight} C6H{12}O6 + 6O2

6.8 Leaf Structure in Detail

A leaf comprises several cell layers, visible in a transverse section (TS) under a microscope.

The epidermis, a layer of closely fitting cells, covers the top and bottom of the leaf and protects the inner layers. Epidermal cells lack chloroplasts. The upper epidermis often secretes a waxy cuticle to prevent water evaporation.

Stomata, small openings, are present in the lower epidermis, each surrounded by a pair of guard cells that regulate the opening and closing of the stomata. Unlike other epidermal cells, guard cells contain chloroplasts.

The mesophyll lies between the upper and lower epidermis and contains chloroplasts. The palisade layer, located nearer the top of the leaf, consists of cells arranged like a fence. The spongy layer, beneath the palisade layer, is composed of loosely arranged, rounder cells with large air spaces between them.

Veins run through the mesophyll, containing xylem vessels (thick-walled) for water transport and phloem tubes (thin-walled) for carrying sucrose and other substances.

6.9 Adaptations of Leaves

Obtaining Carbon Dioxide

Leaves are adapted to efficiently absorb carbon dioxide from the air. The leaf's large surface area maximizes exposure to the air, despite carbon dioxide constituting only about 0.04% of the air. Carbon dioxide enters the leaf through the stomata via diffusion and then diffuses through air spaces between spongy mesophyll cells to reach all cells.

Obtaining Water

Water is absorbed from the soil by root hairs and transported to the leaf via xylem vessels. It then moves from the xylem vessels to the mesophyll cells by osmosis.

Obtaining Sunlight

The leaf's position and broad, flat surface area maximize sunlight absorption. Leaves are arranged to minimize light blockage from one another. Epidermal cells are transparent, allowing sunlight to penetrate to the mesophyll cells. Chloroplasts in mesophyll cells arrange themselves to optimize sunlight capture; they may lie broadside on in low light but arrange themselves end-on in strong sunlight to reduce light absorption. Chlorophyll is arranged on flat membranes inside the chloroplasts to maximize exposure to sunlight.

Adaptations of Leaves for Photosynthesis

6.10 Uses of Glucose

Glucose is Used in Different Ways

Several things can happen to glucose produced during photosynthesis:

• Used for energy: Glucose may be used in respiration to release energy within the leaf.

• Stored as starch: Glucose can be converted to starch for storage in the leaf.

• Used to make proteins and other organic substances: Glucose serves as a starting point for synthesizing other organic compounds, including sucrose, cellulose, fats, and oils. Plants also use sugars from photosynthesis to create proteins, requiring nitrogen in the form of nitrate ions absorbed from the soil. These nitrate ions combine with glucose to form amino acids, which then combine to form proteins. Magnesium is also needed to produce chlorophyll.

• Changed to sucrose for transport: Glucose is converted to sucrose for transport to other parts of the plant. Sucrose is less reactive than glucose and dissolves in the sap of phloem vessels.

Mineral Ions Required by Plants

6.11 Photosynthesis investigations

Investigations Need Controls

Controlled experiments are essential to determine the substances needed for photosynthesis. A control plant is given all necessary substances, while the experimental plant lacks one. Differences between the plants indicate the effect of the missing substance. At the end of the investigation, leaves from both plants are tested for the presence of starch.

Plants for Photosynthesis Investigations Must Be Destarched

Plants must be destarched before investigations to ensure accurate results. This is achieved by placing the plants in a dark cupboard for at least 24 hours to deplete their starch reserves.

Iodine Solution Can Stain Starch in Leaves

Iodine solution is used to test for starch, indicated by a blue-black color. However, cell membranes must be broken down, and chlorophyll removed by boiling and dissolving in alcohol, to allow the iodine solution to react with the starch.

6.14 Limiting Factors

Many Factors Affect Photosynthesis

The rate of photosynthesis is affected by several factors, including sunlight, carbon dioxide, and temperature. If any of these factors are in short supply, they limit the rate of photosynthesis.

Key definition: A limiting factor is a resource present in the environment in such short supply that it restricts life processes.

Sunlight

In the dark, plants cannot photosynthesize. As light intensity increases, the rate of photosynthesis increases until it reaches a maximum point. Beyond this point, additional light will not increase the rate of photosynthesis.

Carbon Dioxide

The rate of photosynthesis increases with carbon dioxide concentration up to a certain point, beyond which increasing the concentration will not further increase the rate.

Temperature

Photosynthesis occurs more slowly at low temperatures, so plants photosynthesize faster on warm days.

Stomata

If stomata close to prevent water loss, photosynthesis slows down due to reduced carbon dioxide intake.

Conditions for Growing Crops Can Be Controlled in Glasshouses

In glasshouses, conditions such as temperature, light, and carbon dioxide concentration can be controlled to optimize photosynthesis and crop yield.

6.16 Importance to All Living Things

Photosynthesis is vital to all living organisms as it brings the energy of the Sun into ecosystems. It also maintains a constant global level of oxygen and carbon dioxide.

Key Ideas

• Photosynthesis occurs in chloroplasts in plant leaves.

• The word equation for photosynthesis is: carbon dioxide + water -> simple sugars + oxygen

• The balanced equation for photosynthesis is: 6CO2 + 6H2O -> C6H12O6 + 6O2

• Chlorophyll traps energy from light and converts it to chemical energy in carbohydrates.

• Photosynthesis primarily occurs in the palisade mesophyll cells.

• Leaves are thin and have a large surface area to maximize sunlight absorption and carbon dioxide supply.

• Stomata and air spaces allow carbon dioxide to diffuse quickly to chloroplasts.

• Xylem vessels bring water, and phloem tubes carry away the products of photosynthesis.

• Glucose is used in respiration, stored as starch, used to make cellulose for cell walls, transported as sucrose, and combined with nitrate or ammonium ions to make proteins.

• Testing leaves for starch involves boiling to break down cell membranes and using hot alcohol to remove chlorophyll.

• Light and carbon dioxide are limiting factors in photosynthesis.