When we think of life, we often imagine animals in motion, yet the green world of plants holds its own drama—one that unfolds quietly in every breath, cell, and tissue. Unit 5 of biology takes us into this fascinating world, where respiration fuels energy, growth regulators shape development, cells operate as microscopic factories, and tissues form the architecture of life.
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Plant Respiration: Energy Behind the Silence
Though plants appear still, they are constantly engaged in respiration, a process as vital to them as it is to humans. Unlike photosynthesis, which makes food, respiration breaks down that food to release energy.
Glycolysis: The First Step
Respiration begins with glycolysis, a universal pathway that occurs in the cytoplasm. Here, one molecule of glucose is split into two molecules of pyruvate, releasing a modest amount of ATP—the energy currency of the cell.
Fermentation: Life Without Oxygen
In the absence of oxygen, plants can switch to fermentation, also called anaerobic respiration. This process produces less energy but ensures survival under oxygen-deficient conditions such as waterlogged soils. For example, yeast uses fermentation to produce ethanol, a process harnessed by humans in baking and brewing.
Through respiration, plants keep their cells alive and their systems running, proving that even quiet greenery is powered by invisible sparks of energy.
Plant Growth and Development
From tiny seedlings pushing through the soil to towering trees, growth in plants is a story of phases, conditions, and regulators.
Phases and Rate of Growth
Plant growth occurs in three distinct phases:
Lag Phase – a slow start, as cells prepare for active division.
Log Phase – a rapid increase in size and number of cells.
Stationary Phase – a plateau, where growth slows as maturity is reached.
The rate of growth can be arithmetic or geometric. In arithmetic growth, one cell divides and only one daughter cell continues to divide. In geometric growth, both daughter cells divide, leading to exponential expansion.
Conditions for Growth
Growth depends on key conditions: availability of water, nutrients, oxygen, suitable temperature, and light. Any imbalance—like drought or nutrient deficiency—can halt development.
Plant Growth Regulators
Enter the plant growth regulators (PGRs), chemical messengers that act like hormones in animals. Auxins promote elongation, gibberellins stimulate stem growth, cytokinins trigger cell division, abscisic acid regulates dormancy and stress, while ethylene helps in fruit ripening. These regulators ensure plants adapt and thrive in changing environments.
The Cell: The Unit of Life
Every towering tree or delicate flower begins with the smallest structural and functional unit—the cell.
Structure and Functions
A plant cell is enclosed by a rigid cell wall, giving it strength and shape. Inside lies the plasma membrane regulating entry and exit of materials, and the cytoplasm where countless reactions occur. The nucleus acts as the control center, holding genetic information. Organelles like mitochondria generate energy, chloroplasts capture sunlight, endoplasmic reticulum and Golgi apparatus manage protein synthesis and transport, while vacuoles maintain water balance.
Cell Division
For plants to grow, cells must divide. Two types of cell division occur:
Mitosis, producing identical daughter cells for growth and repair.
Meiosis, producing gametes (sex cells) with half the chromosome number, essential for reproduction.
Together, these cellular processes ensure both continuity and diversity in plant life.
Plant Tissues: Building Blocks of Structure
While cells are the basic units, tissues represent teamwork—groups of cells working together for specific functions.
Types of Tissues
Plant tissues are broadly classified into two categories:
Meristematic Tissues – Found in growing regions like shoot tips and root tips, these cells actively divide and contribute to primary growth.
Permanent Tissues – Once cells specialize, they form permanent tissues that carry out defined roles. These include:
Parenchyma (storage and photosynthesis)
Collenchyma (flexible support)
Sclerenchyma (rigid support due to lignin)
Xylem (water transport)
Phloem (food transport).
Location and Functions
Root tissues absorb water and anchor plants.
Stem tissues provide support and transport.
Leaf tissues capture sunlight and exchange gases.
Vascular tissues (xylem and phloem) ensure movement of vital substances throughout the plant body.
In short, tissues transform individual cells into organized, functional systems that keep plants alive and efficient.