Cell Transport & Plant Nutrition: Essential Biological Processes

Diffusion

Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration. This is a passive cell movement, meaning cells do not need to use energy for it.

Concentration Gradient

The concentration gradient is the difference in the concentration of molecules between two places.

Factors Affecting Diffusion

• Distance molecules travel (shorter distances enhance diffusion)

• Concentration gradient (a steeper gradient increases the rate)

• Surface area (larger areas facilitate more diffusion)

• Temperature (higher temperatures increase kinetic energy, speeding up diffusion)

• Size of molecule (smaller molecules diffuse more rapidly)

Osmosis

Osmosis is a special type of diffusion, specifically the passage of water molecules from a region of high water concentration to a region of lower water concentration through a partially permeable membrane.

Solution Composition

A solution is composed of a solute dissolved in a solvent.

Osmosis and Animal Cells

When placed in pure water, animal cells can burst. This occurs because water molecules diffuse into the cell through its partially permeable membrane.

Conversely, animal cells will shrink in concentrated solutions as water diffuses out of the cell through the partially permeable membrane.

Osmosis is potentially damaging to animal cells due to their inability to withstand significant changes in water potential, leading to bursting (lysis) or shrinking (crenation).

Osmosis and Plant Cells

Unlike animal cells, if water enters a plant cell, the cytoplasm will swell, but the rigid cell wall prevents it from bursting. If water leaves the cell, the cytoplasm will shrink, and the cell may become flaccid or even plasmolyzed.

Plant Nutrition

Photosynthesis: The Basis of Plant Nutrition

Photosynthesis is a fundamental process by which plants manufacture carbohydrates from raw materials, utilizing sunlight energy.

Photosynthesis Chemical Equation

The chemical equation for photosynthesis is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

(Requires sunlight and chlorophyll)

The Process of Photosynthesis

Green plants absorb carbon dioxide (CO₂) through their leaves via diffusion. Water (H₂O) is absorbed through the roots via osmosis and transported to the leaves through xylem vessels. Within the chloroplasts, chlorophyll traps light energy, which is used to break up water molecules and form glucose. Glucose is typically converted to sucrose for transport throughout the plant or to starch for storage. Oxygen (O₂) is released as a byproduct or utilized by the plant for respiration.

Factors Limiting Photosynthesis

1. Light intensity: Photosynthesis increases with brighter light up to a certain point. Beyond this saturation point, the rate of photosynthesis remains constant as chloroplasts cannot trap any more light.

2. Carbon dioxide (CO₂) concentration: A higher CO₂ concentration generally leads to a faster rate of photosynthesis. Atmospheric air contains approximately 0.04% CO₂. The rate increases up to a point, after which it remains constant because another factor becomes limiting.

3. Temperature: The rate of photosynthesis decreases at excessively high temperatures because the enzymes within chloroplasts become denatured, losing their functional structure.