Organism level systems Revision Notes

The Role of Auxins in Plant Coordination

Plant growth and development are coordinated by chemical substances called hormones. Auxins are a key type of plant hormone, primarily produced in the growing tips of shoots and roots. They act as chemical messengers, travelling to other parts of the plant where they stimulate or inhibit cell elongation, depending on the tissue and concentration. This controlled cell elongation leads to various growth responses, allowing plants to adapt to environmental stimuli effectively.

Phototropism: Responding to Light

Phototropism is the directional growth of a plant in response to light. Shoots exhibit positive phototropism, meaning they grow towards light. This occurs because auxins migrate to the shaded side of the shoot. On the shaded side, the increased auxin concentration promotes cell elongation, causing the cells there to grow longer than those on the lit side. The differential growth makes the shoot bend towards the light source. Roots, conversely, generally show negative phototropism or are insensitive to light, with high auxin concentrations potentially inhibiting root cell elongation.

Gravitropism: Responding to Gravity

Gravitropism (or geotropism) is the directional growth of a plant in response to gravity. Shoots exhibit negative gravitropism, growing upwards against gravity. Auxins accumulate on the lower side of horizontal shoots, where they promote cell elongation, causing the lower side to grow faster and the shoot to bend upwards. Roots, however, show positive gravitropism, growing downwards with gravity. In roots, auxins also accumulate on the lower side, but here, the higher concentration inhibits cell elongation. This causes the upper side of the root to grow faster, leading the root to bend downwards into the soil.

Investigating Tropisms (PAG B6)

PAG B6 involves investigating tropisms. A common experiment for phototropism uses cress seeds or shoots placed in different light conditions (e.g., unilateral light, all-around light, dark). For gravitropism, seedlings or shoots can be rotated or placed horizontally. Key variables include: independent variable (e.g., light direction, plant orientation), dependent variable (e.g., direction of growth, angle of bend), and control variables (e.g., temperature, water, species, initial size). Observations typically involve measuring the angle of growth or sketching changes over time. Safety precautions include careful handling of plant material and avoiding contamination.

What is Hormonal Coordination?

Hormonal coordination involves the endocrine system, a network of glands that produce and secrete chemical messengers called hormones. These hormones are released directly into the bloodstream and travel to specific target cells or organs throughout the body. Unlike the rapid, short-lived electrical signals of the nervous system, hormonal responses are generally slower in onset but have longer-lasting effects. The endocrine system plays a crucial role in regulating a wide range of bodily functions over the long term, including growth, development, metabolism, reproduction, and maintaining internal balance (homeostasis).

Key Components of the Endocrine System

The endocrine system consists of various glands, each producing specific hormones. Major endocrine glands include the pituitary gland (often called the 'master gland' as it controls other glands), the thyroid gland, the adrenal glands, the pancreas, and the ovaries in females or testes in males. Hormones are chemical substances, typically proteins or steroids, which act as signals. Target cells possess specific receptor molecules that bind only to certain hormones, ensuring that each hormone produces its effect only where needed. Once a hormone binds to its receptor, it triggers a specific response within the target cell.

How Hormones Travel and Act

After being produced by an endocrine gland, hormones are secreted directly into the capillaries that surround the gland. From there, they are transported via the circulatory system (bloodstream) to all parts of the body. While circulating throughout the body, hormones only exert their effects on 'target' cells or organs. These target cells have unique protein receptors on their surface or inside them that are complementary in shape to a specific hormone. This lock-and-key mechanism ensures that hormones only bind to and activate the cells equipped to respond, leading to a coordinated and specific physiological change. For example, insulin targets liver and muscle cells to remove glucose from the blood.

Examples of Hormonal Control in Humans

Hormones control numerous vital processes. For instance, the pancreas produces insulin and glucagon to regulate blood glucose levels. The adrenal glands release adrenaline in response to stress, preparing the body for 'fight or flight' actions like increasing heart rate and diverting blood to muscles. Growth hormone from the pituitary gland regulates growth. Sex hormones like oestrogen and testosterone, produced by the ovaries and testes respectively, are vital for sexual development and reproduction. These examples highlight how hormones orchestrate complex physiological changes to maintain health and respond to the environment.

The Role of Insulin in Glucose Regulation

Insulin is a hormone produced by the pancreas. Its primary role is to lower high blood glucose levels. When you eat, carbohydrates are broken down into glucose, which enters your bloodstream, causing glucose levels to rise. The pancreas detects this increase and responds by releasing insulin. Insulin then acts on body cells, especially liver and muscle cells, to help them absorb glucose from the blood. This process is essential for maintaining a stable internal environment in the body.

How Insulin Lowers Blood Glucose

Once released, insulin travels through the bloodstream to target cells. It increases the permeability of muscle and liver cells to glucose, allowing more glucose to enter them. Insulin stimulates the liver and muscle cells to convert excess glucose into glycogen, a storage polysaccharide. This stored glycogen can be later converted back to glucose if blood sugar levels drop too low. Insulin also promotes the use of glucose by all body cells for respiration, further reducing its concentration in the blood. These actions collectively bring elevated blood glucose levels back to a normal range.

The Importance of Blood Glucose Control

Maintaining stable blood glucose levels is critical for human health. Consistently high blood sugar can damage organs, while persistently low blood sugar can lead to symptoms like confusion, seizures, and even coma, as the brain relies almost exclusively on glucose for energy. The body's ability to regulate these levels through hormones like insulin is an example of homeostatic control, ensuring cells have a constant supply of energy without harmful fluctuations.

Specification Link

This nervous system diagram is used for: Account for some of the limitations in treating damage and disease in the brain and other parts of the nervous system.

What To Explain

Information is passed through the nervous system so the body can coordinate a response.

Exam Answer Move

The mapped objective is: Account for some of the limitations in treating damage and disease in the brain and other parts of the nervous system. The answer should connect structures such as receptors, neurones, synapses or effectors to coordination and response.