Control and Coordination: Brain, Hormones, and Plant Tropisms Class 10

When you accidentally touch a scorching hot iron pan, your hand pulls back instantaneously before your conscious brain even realizes what happened. When a green seedling germinates inside a dark drawer with a single pinhole of light, its delicate stem bends towards the beam while its roots plunge downwards into the soil. And when a charging dog barks at you, your heart pounds at double speed, pumping oxygen to prepare your legs for flight.

In living organisms, survival depends on rapid communication and coordinated physiological responses. In CBSE Class 10 Science, Chapter 6 (Control and Coordination) explores the biological hardware of survival: the Human Central Nervous System and Brain, the Endocrine System and Hormones, and directional Plant Tropisms.

What You Will Learn

• Structural anatomy and functional divisions of the Human Brain
• Cerebrum vs. Cerebellum vs. Medulla Oblongata: Who controls what?
• Plant movements: Directional Tropisms vs. Non-directional Nastic Movements
• Role of Auxin in stem phototropism
• Major Phytohormones: Auxins, Gibberellins, Cytokinins, and Abscisic Acid
• Major Animal Endocrine Glands: Thyroid (Thyroxin), Pancreas (Insulin), Adrenal (Adrenaline), and Pituitary (Growth Hormone)
• Negative feedback mechanism regulating insulin secretion

1. The Human Brain: Master Control Center

The human brain is divided into three primary anatomical regions, protected by the bony cranium (skull) and cushioned by cerebrospinal fluid (CSF):

                                  The Human Brain
                                         |
       +---------------------------------+---------------------------------+
       |                                 |                                 |
FOREBRAIN                         MIDBRAIN                          HINDBRAIN
Cerebrum                          Controls auditory &               +-------+-------+
- Thinking, logic, intelligence   visual involuntary reflexes       |       |       |
- Voluntary muscle actions                                        Cerebellum  Pons  Medulla
- Memory, sensory perception                                      (Balance)  (Breath) Oblongata
                                                                                    (Heart/BP)

1. Forebrain (Cerebrum):

• The largest and most complex part of the brain.
• Functions: <u>The center of conscious thought, intelligence, memory, sensory interpretation (vision, hearing, smell), and control of all voluntary muscle movements (walking, writing, talking).</u>

2. Midbrain:

• Connects the forebrain to the hindbrain.
• Functions: Coordinates involuntary reflex movements of the head, neck, and eyes in response to visual and auditory stimuli.

3. Hindbrain (Cerebellum, Pons, Medulla):

• Cerebellum: <u>Responsible for maintaining precision of voluntary actions, physical body posture, and equilibrium (balance) while walking, riding a bicycle, or picking up a pencil!</u> (Alcohol directly impairs the cerebellum, causing staggering gait).
• Medulla Oblongata: Controls all vital, life-sustaining involuntary functions: blood pressure, salivation, heartbeat, swallowing, coughing, and vomiting.
• Pons: Regulates respiratory rhythm and breathing rate.

2. Coordination in Plants: Tropisms and Phytohormones

Plants have no nervous system and no muscular tissue, yet they respond to environmental stimuli through chemical coordination:

A. Directional Growth Movements (Tropisms):

A tropism is a growth movement of a plant part in response to an external directional stimulus:

1. Phototropism: Response to light (stems show positive phototropism, roots show negative phototropism).
2. Geotropism: Response to gravity (roots show positive geotropism, stems show negative geotropism).
3. Hydrotropism: Growth movement of roots towards water.
4. Chemotropism: Growth movement in response to chemical stimuli (e.g., <u>growth of a pollen tube towards the ovule during plant fertilization</u>).
5. Thigmotropism: Growth response to physical contact/touch (e.g., tendrils coiling around supports).

B. How Does Auxin Cause Phototropism? (CBSE High-Frequency Question)

Why does a plant stem bend towards light?

    Uniform Sunlight:                         Directional Sunlight (from Right):
        \   |   /                                                 / / /
         \  |  /                                                 / / /
       +---------+                                            +---------+
       |  Auxin  | (Evenly distributed)                       |  Auxin  | (Auxin diffuses to SHADED side!)
       +---------+                                            +---------+
            |                                                     |
       Grows Straight Up!                                     Cells on shaded left side ELONGATE FASTER!
                                                              Stem BENDS TOWARDS THE LIGHT!

1. Auxin is synthesized at the growing tip of the shoot.
2. When light falls from one side of the plant, auxin diffuses towards the shaded side of the shoot.
3. The higher concentration of auxin stimulates cells on the shaded side to elongate faster than cells on the illuminated side.
4. <u>Because the shaded side grows longer than the sunny side, the shoot bends smoothly towards the source of light!</u>

C. The Four Major Plant Hormones (Phytohormones):

3. Hormones in Animals: The Endocrine System

Hormones are chemical messengers secreted directly into the bloodstream by ductless endocrine glands:

The Feedback Mechanism of Insulin:

Hormone levels are self-regulated by negative feedback: $\text{Blood sugar rises} \longrightarrow \text{Pancreatic cells detect rise} \longrightarrow \text{Insulin secreted} \longrightarrow \text{Sugar converted to glycogen}$ $\text{Blood sugar falls to normal} \longrightarrow \text{Pancreatic cells stop secretion!}$

4. Summary and Examination Tips

Exam Tip: In questions asking "Why is iodized salt recommended in our diet?": Write that iodine is strictly required by the thyroid gland to synthesize thyroxin hormone; lack of iodine causes thyroxin deficiency, leading to goitre (enlarged thyroid gland)!

Common Mistake: Confusing cerebellum with cerebrum. Cerebrum is for thinking and conscious memory; cerebellum is strictly for posture, motor coordination, and equilibrium!