Nutrition in Plants

Photosynthesis equation

Word: water + carbon dioxide → glucose + oxygen

Chemical: 6H₂O + 6CO₂ → C₆H₁₂O₆ + 6O₂

Aerobic respiration equation

Word: glucose + oxygen → water + carbon dioxide + energy

Chemical: C₆H₁₂O₆ + 6O₂ → 6H₂O + 6CO₂ + 36ATP

Leaf structure: Upper epidermis

Structure:

• One cell thick, cells have no chloroplasts

• Covered by a waxy cuticle

• Fewer stomata than lower epidermis

Function:

• Waxy cuticle and fewer stomata reduces water loss via evaporation

• Protects against mechanical damage

• Transparent to allow light to pass through to palisade layer

Leaf structure: Palisade mesophyll

Structure:

• Densely packed cylindrical-shaped cells, arranged at right angles to upper epidermis

• Palisade cells have thin cell walls and the most chloroplasts

Function:

• Main site of photosynthesis due to highest concentration of chloroplasts

• Thin cell wall and cytoplasm allows for rapid diffusion of water and carbon dioxide into cells

Leaf structure: Spongy mesophyll

Structure:

• Irregularly shaped cells containing fewer chloroplasts than palisade mesophyll

• Loosely packed cells form a loose network of large intercellular air spaces

Function:

• Allows for rapid gaseous exchange

• Some photosynthesis

Leaf structure: Vascular tissue

Structure:

• Veins in leaf contain xylem and phloem tissues

• Xylem comprises of lignified vessels, above phloem

• Phloem consists of sieve tubes and companion cells, under xylem

Function:

• Veins provide structural support

• Xylem transports water and dissolved mineral salts

• Phloem transports synthesized food (eg. sucrose and amino acids)

Leaf structure: Lower epidermis

Structure:

• One cell thick

• Covered by a waxy cuticle

• More stomata than upper epidermis

Function:

• Has stomata for gaseous exchange between intercellular air spaces and surroundings

• Protects underlying cells

Leaf structure: Stomata (singular: stoma)

Structure:

• Pore surrounded by specialised guard cells with some chloroplasts

• Guard cells have thicker cell walls around pore

Function:

• Guard cells regulate the size of stomata to control gaseous exchange

• Also controls water loss due to evaporation

How does light intensity affect the rate of photosynthesis?

As light intensity increases, the rate of photosynthesis increases from point 0 to X. As light intensity continues to increase beyond point X, light intensity is no longer the limiting factor, hence the rate of photosynthesis remains the same.

How does light quality affect the rate of photosynthesis?

Different wavelengths of light are absorbed by chlorophyll during photosynthesis. Blue and red lights are absorbed most effectively by chlorophyll a and b, hence they cause the highest rate of photosynthesis. Green light is absorbed least effectively by chlorophyll, hence it causes the lowest rate of photosynthesis.

How does concentration of carbon dioxide affect the rate of photosynthesis?

As carbon dioxide concentration increases from 0.00% to 1.00%, the rate of photosynthesis increases. As carbon dioxide concentration increases beyond 1.00%, carbon dioxide is no longer the limiting factor, hence the rate of photosynthesis remains the same.

How does temperature affect the rate of photosynthesis?

Photosynthesis is an enzyme-controlled reaction, thus as the temperature increases to the optimum temperature, the rate of photosynthesis increases. As the temperature increases beyond optimum temperature, the enzymes are denatured, hence the rate of photosynthesis decreases.

Explain how guard cells regulate gaseous exchange between the leaf and its surroundings during the day.

In the presence of sunlight, the concentration of potassium ions increases in the guard cells. Chloroplasts in the guard cells photosynthesize. The light energy is converted into chemical energy to pump potassium ions into the guard cells from neighbouring epidermal cells, lowering the water potential in the guard cells. Water from neighbouring epidermal cells enter the guard cells via osmosis, causing the guard cells to swell and become turgid. The guard cells have a thicker cell wall on the side around the stomatal pore, hence the swollen guard cells become more curved and pull the stoma open.

Explain how guard cells regulate gaseous exchange between the leaf and its surroundings during the night.

In the absence of sunlight, the potassium ions accumulated in the guard cells during the day diffuse to the neighbouring cells, increasing the water potential in the guard cells. Water leaves the guard cells into the neighbouring cells via osmosis, causing the guard cells to become flaccid and the stoma pore to close.

Effects of warmer temperatures on plants

• Warmer springs cause some plants to start producing pollen earlier

• Warmer falls cause some plants, like ragweed, to extend their growing seasons

• Warmer temperatures causes some plants, like ragweed, to produce larger quantities of pollen, potentially causing more allergic reactions in people

Effect of increased carbon dioxide on plants

• Increase in rate of photosynthesis

• Trees are favoured over tropical grasses, leading to tree invasions in savannahs

• Increase in leaf sugars, affecting growth of plant

• Stomata may close more frequently, reducing transpiration and increasing efficient use of water