IB DP Biology C3.1 Integration of body systems Study Notes - New Syllabus
IB DP Biology C3.1 Integration of body systems Study Notes
IB DP Biology C3.1 Integration of body systems Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of
- What are the roles of nerves and hormones in integration of body systems?
- What are the roles of feedback mechanisms in regulation of body systems?
Standard level and higher level: 5 hours
Additional higher level: 2 hours
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
C3.1.1—System integration
System Integration: Orchestrating Complexity
- Life as a Network of Systems: All organisms, from single cells to complex multicellular organisms, are composed of multiple systems that work together to perform vital functions. These systems interact and coordinate with each other to ensure the overall functioning of the organism.
- Interdependence and Communication: Effective system integration relies on clear and efficient communication between different components within and between systems. This communication can involve simple feedback loops or more complex networks of interactions.
- Example: Wind Turbine Control: The image illustrates the concept of system integration using the example of a wind turbine.
- Sensors: Sensors monitor wind speed and direction.
- Actuator: The pitch actuator adjusts the angle of the turbine blades.
- Controllers: Controllers process the sensor data and send signals to the actuator to adjust the blade pitch.
- Torque Controller: This component manages the power output of the turbine.
- Engineering Analogy: This example highlights how the principles of system integration are also relevant in engineering and technology, such as in the design and control of complex systems like wind turbines.
System integration is a fundamental principle in biology and engineering. It ensures that multiple components work together in a coordinated and efficient manner to achieve a desired outcome.
C3.1.2—Cells, tissues, organs and body systems as a hierarchy of subsystems that are integrated in a multicellular living organism
Key Points:
- Hierarchy of Organization: Multicellular organisms exhibit a hierarchical organization, with cells being the basic building blocks. Cells group together to form tissues, tissues form organs, and organs work together to form organ systems. These organ systems then coordinate to carry out the functions of the whole organism.
- Tissues: Groups of Cells with Specialized Functions: Tissues are groups of cells with similar structures and functions. They are organized to perform specific tasks within an organ. For example, epithelial tissue lines the surfaces of the body, while muscle tissue is specialized for contraction.
- Organs: Integrated Groups of Tissues: As discussed before, organs are composed of multiple tissues that work together to perform a specific function. The image provides the example of the trachea, which is an organ of the respiratory system. It is composed of various tissues, including:
- Epithelium: Lines the inner surface of the trachea.
- Cartilage: Provides structural support to the trachea.
- Smooth Muscle: Allows for changes in the diameter of the trachea to regulate airflow.
- Connective Tissue: Connects and supports the various tissues of the trachea.
- Organ Systems: Interacting Groups of Organs: Organ systems are groups of organs that work together to perform a specific bodily function. The image provides examples of several organ systems, including the nervous system and the endocrine system.
- Nervous System: Consists of the brain, spinal cord, and nerves, responsible for coordinating and controlling bodily functions.
- Endocrine System: Consists of glands that secrete hormones, which regulate various physiological processes.
- Organs: Integrated Groups of Tissues: As discussed before, organs are composed of multiple tissues that work together to perform a specific function. The image provides the example of the trachea, which is an organ of the respiratory system. It is composed of various tissues, including:
C3.1.3—Integration of organs in animal bodies by hormonal and nervous signalling and by transport of materials and energy
Key Points:
- Interdependence of Organ Systems: Organ systems within an animal body are not isolated entities. They work together in a coordinated manner to maintain the overall functioning of the organism. This integration involves communication and the transport of materials and energy.
- Two Major Communication Systems:
- Hormonal Signaling: Hormones are chemical messengers released by endocrine glands into the bloodstream. They travel throughout the body and can affect target cells in various tissues. Hormonal signaling is typically slower and has a longer duration of action.
- Nervous Signaling: Neurons transmit electrical signals (nerve impulses) to specific target cells. This type of signaling is very rapid and has a short duration.
- Comparison of Hormonal and Nervous Signaling:
| Feature | Hormonal Signaling | Nervous Signaling |
|---|---|---|
| Type of Signal | Chemical (hormones) | Electrical (passage of ions across membranes) |
| Transmission of Signal | Widespread; to all parts of the body that are supplied with blood | Highly focused – to one specific neuron or group of neurons |
| Destination of Signal | Target cells in any type of tissue | Specific neurons or muscle cells |
| Effectors | Cells throughout the body | Muscles or glands |
| Type of Response | Growth, development, including puberty and reproduction; metabolic rate and heat generation; mood, including thirst, sleep, wakefulness, and sex drive | Responses due to contraction of muscle: skeletal muscle (e.g., locomotion), smooth muscle (e.g., heart rate and sphincter muscles), and cardiac muscle; secretion from glands, e.g., sweat or saliva secretion by exocrine glands, epinephrine secretion by endocrine glands, e.g., adrenal gland |
| Speed of Response | Slow – until the hormone is broken down | Very rapid – unless nerve impulses are sent repeatedly |
| Duration of Response | Longer | Short – unless nerve impulses are sent repeatedly |
Hormonal and nervous signaling play critical roles in coordinating the activities of different organ systems within an animal body. These two systems work together to maintain homeostasis, respond to environmental stimuli, and ensure the proper functioning of the organism as a whole.
C3.1.4—The brain as a central information integration organ
Key Points:
- Central Processing Unit: The brain serves as the central control center for the entire body. It receives information from various sources, processes it, and sends out instructions to coordinate bodily functions.
- Information Sources: The brain receives information from sensory receptors located throughout the body. This includes specialized sense organs like the eyes and ears, as well as receptors in internal organs (e.g., pressure receptors in blood vessels).
- Information Processing and Storage: The brain processes the received information, making sense of it and storing relevant data for later use. This storage capacity is known as memory, which is crucial for learning and adapting to new situations.
- Decision Making: Based on the processed information, the brain makes decisions and sends out signals to other parts of the body, such as muscles and glands, to initiate appropriate responses. These responses can include voluntary movements, involuntary reflexes, hormone release, and other bodily functions.
The brain is the command center of the body, responsible for receiving, processing, storing, and utilizing information to coordinate and regulate all bodily functions.
C3.1.5—The spinal cord as an integrating centre for unconscious processes
Key Points:
- Central Nervous System: The nervous system comprises the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and1 the spinal cord.
- Spinal Cord Structure: The spinal cord is located within the vertebral column (backbone) and is protected by the surrounding vertebrae. Spinal nerves branch out from the spinal cord, connecting it to various parts of the body.
- Integration and Processing: The spinal cord plays a crucial role in integrating information and coordinating unconscious processes. It contains:
- Gray Matter: Contains the cell bodies of motor neurons and interneurons, as well as synapses where information is processed and decisions are made.
- White Matter: Contains myelinated axons that transmit signals between the spinal cord, brain, and other parts of the body.
- Unconscious vs. Conscious Processes: The spinal cord primarily handles unconscious processes, such as reflexes and involuntary movements. These actions occur without conscious thought or control. Conscious processes, on the other hand, involve voluntary control and require conscious decision-making, which is primarily coordinated by the brain.
The spinal cord serves as a vital integrating center, processing information and coordinating unconscious responses. This allows for rapid and efficient responses to stimuli without the need for conscious involvement.
C3.1.6—Input to the spinal cord and cerebral hemispheres through sensory neurons
Key Points:
- Sensory Receptors: Changes in the external or internal environment are detected by sensory receptors. These receptors are located throughout the body, including the skin, eyes, ears, and internal organs.
- Types of Sensory Receptors:
- Exteroceptors: Located on the surface of the body, these receptors respond to external stimuli like touch, temperature, and light.
- Interoceptors: Located within the body, these receptors monitor internal conditions such as blood pressure, body temperature, and blood oxygen levels.
- Sensory Neurons: Sensory neurons transmit signals from the receptors to the central nervous system (CNS). The axons of these neurons can be quite long, depending on the distance between the receptor and the CNS.
- Sensory Pathways: Signals from the body enter the CNS through the spinal nerves or cranial nerves. For example, sensory signals from the eyes enter the brain via the optic nerve, while signals from the skin enter through spinal nerves.
- Receptive Fields: Each sensory neuron has a specific area of the body that it receives input from. This area is called the receptive field of the neuron.
Sensory neurons play a critical role in transmitting information about the internal and external environment to the central nervous system, enabling the organism to interact with and respond to its surroundings.
C3.1.7—Output from the cerebral hemispheres to muscles through motor neurons
Key Points:
- Motor Cortex and Voluntary Movement: The cerebral hemispheres, particularly the primary motor cortex, play a crucial role in controlling voluntary movements. Signals from the motor cortex are transmitted via motor neurons to striated muscles throughout the body.
- Striated Muscle: Striated muscle is attached to bones and is responsible for locomotion, posture, and other voluntary movements. It can be consciously controlled.
- Motor Neurons: Motor neurons have their cell bodies and dendrites located in the gray matter of the cerebral hemispheres. They receive signals from other neurons and transmit these signals to the cell body.
- Long Axons: The axons of motor neurons can be very long, extending from the cell body in the brain down to the spinal cord and then to the target muscle.
- Synaptic Transmission: At the neuromuscular junction, the motor neuron releases neurotransmitters that stimulate the muscle fibers to contract.
The motor cortex initiates voluntary movements by sending signals through motor neurons to striated muscles throughout the body. This complex process involves multiple neurons and synaptic transmissions, allowing for precise and coordinated control of movement.
C3.1.8—Nerves as bundles of nerve fibres of both sensory and motor neurons
Key Points:
- Nerve Structure: A nerve is a bundle of nerve fibers (axons and dendrites) enclosed in a protective sheath. The size of a nerve varies depending on the number and type of nerve fibers it contains.
- Examples:
- Sciatic Nerve: The sciatic nerve is one of the largest nerves in the human body, with a diameter of approximately 20 mm.
- Optic Nerve: The optic nerve, which transmits visual information from the eye to the brain, is estimated to contain between 770,000 and 1.7 million nerve fibers.
- Sensory and Motor Neurons: Most nerves contain a mixture of both sensory and motor neurons. However, some nerves contain only sensory neurons (e.g., the optic nerve) and some contain only motor neurons (e.g., the oculomotor nerve).
- Innervation: All organs and tissues in the body are served by one or more nerves, providing them with neural connections for sensory input and motor output.
Nerves are complex structures that facilitate communication between the central nervous system and the rest of the body. They transmit sensory information from the body to the brain and motor commands from the brain to muscles and glands.
C3.1.9—Pain reflex arcs as an example of involuntary responses with skeletal muscle as the effector
Key Points:
- Reflex Arcs: Reflexes are rapid, involuntary responses to specific stimuli. They involve a simple neural pathway, allowing for quick responses in situations where immediate action is necessary, such as avoiding harm.
- Components of a Reflex Arc:
- Receptors: Sensory receptors detect the stimulus (e.g., pain).
- Sensory Neurons: Transmit the sensory signal from the receptor to the central nervous system (spinal cord or brain).
- Interneurons: Located within the central nervous system, interneurons process the sensory information and make decisions about the appropriate response. In simple reflexes, there may only be one interneuron.
- Motor Neurons: Receive signals from interneurons and transmit them to the effector (e.g., skeletal muscle).
- Effector: The effector is the muscle or gland that carries out the response. In the case of the pain reflex, the effector is the skeletal muscle that contracts to withdraw the limb from the source of pain.
Example: Pain Reflex
- When you step on a sharp object, pain receptors in your foot are activated.
- Sensory neurons transmit the pain signal to the spinal cord.
- Interneurons in the spinal cord process the signal and quickly generate a motor response.
- Motor neurons transmit the signal to the muscles in your leg, causing them to contract and lift your foot away from the painful stimulus.
The pain reflex arc demonstrates how the nervous system can generate rapid and involuntary responses to protect the body from harm. It highlights the coordinated action of sensory receptors, sensory neurons, interneurons, motor neurons, and effector muscles in producing a quick and effective response.
C3.1.10—Role of the cerebellum in coordinating skeletal muscle contraction and balance
Key Points:
- Fine-Tuning Movements: The cerebellum plays a vital role in coordinating and fine-tuning skeletal muscle contractions. While it doesn’t initiate the decision to move, it ensures that the timing and sequence of muscle contractions are precise and coordinated.
- Balance and Posture: The cerebellum is crucial for maintaining balance and posture. It helps to adjust muscle activity to maintain equilibrium, especially when standing or moving.
- Motor Learning: The cerebellum is also involved in motor learning. It helps us to acquire and refine motor skills, such as riding a bike or typing on a keyboard. These skills require precise coordination and timing of muscle contractions, which the cerebellum helps to refine.
The cerebellum is a critical structure for coordinating movement and maintaining balance. It works in conjunction with other brain regions to ensure smooth and precise motor control.
C3.1.11—Modulation of sleep patterns by melatonin secretion as a part of circadian rhythms
Key Points:
- Circadian Rhythms: Humans, like most organisms, exhibit circadian rhythms, which are daily cycles of biological activity that occur approximately every 24 hours. These rhythms persist even in the absence of external cues like light and dark, indicating an internal biological clock.
- Suprachiasmatic Nucleus (SCN): The SCN, a cluster of neurons in the hypothalamus, is the primary circadian pacemaker in mammals. It generates a roughly 24-hour rhythm even when isolated in culture.
- Melatonin Secretion: The SCN controls the secretion of melatonin, a hormone produced by the pineal gland. Melatonin levels rise in the evening, promoting sleepiness, and fall in the morning, contributing to wakefulness.
- Light as a Cue: Light is a crucial cue for synchronizing the SCN and the circadian rhythm. Specialized cells in the retina detect light and transmit signals to the SCN, allowing it to adjust melatonin secretion to match the day-night cycle.
- Sleep-Wake Cycle: Melatonin plays a key role in regulating the sleep-wake cycle. High melatonin levels promote sleep, while low levels contribute to wakefulness.
- Core Body Temperature: Melatonin also influences core body temperature. Melatonin levels are associated with a drop in core body temperature at night.
Melatonin, under the control of the SCN, plays a vital role in regulating the sleep-wake cycle and other circadian rhythms. This hormonal signal helps to synchronize our internal biological clock with the external day-night cycle.
C3.1.12—Epinephrine (adrenaline) secretion by the adrenal glands to prepare the body for vigorous activity
Epinephrine: The “Fight-or-Flight” Hormone
- Function: Epinephrine is a hormone secreted by the adrenal glands. Its primary role is to prepare the body for situations that require a burst of energy, such as responding to threats (“fight”) or fleeing from danger (“flight”).
- Effects on the Body: Epinephrine triggers a cascade of responses throughout the body, including:
- Increased Glucose Availability:
- Muscle cells break down glycogen into glucose for energy production.
- Liver cells release glucose into the bloodstream.
- Enhanced Oxygen Delivery:
- Bronchioles in the lungs dilate, increasing airflow.
- Heart rate increases, boosting blood circulation.
- Blood flow is redirected to muscles and the liver, while blood flow to less essential organs like the gut and skin is reduced.
- Increased Glucose Availability:
Epinephrine orchestrates a series of physiological changes that prepare the body for intense physical activity. It increases the availability of energy sources, enhances oxygen delivery, and redirects blood flow to the muscles, allowing for a rapid and sustained response to stress or danger.
C3.1.13—Control of the endocrine system by the hypothalamus and pituitary gland
Hypothalamus and Pituitary Gland: A Master Control System
- Hypothalamus: This small but crucial region of the brain acts as a central control center for many bodily functions. It links the nervous and endocrine systems.
- Pituitary Gland: Located beneath the hypothalamus, the pituitary gland is often referred to as the “master gland” due to its influence on various hormonal functions. It has two main parts:
- Anterior Pituitary: Produces and releases hormones like growth hormone (GH), thyroid-stimulating hormone (TSH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin.1
- Posterior Pituitary: Stores and releases hormones produced by the hypothalamus, including antidiuretic hormone (ADH) and oxytocin.2
Key Functions of the Hypothalamus-Pituitary Axis:
- Hormone Regulation: The hypothalamus controls the release of hormones from both lobes of the pituitary gland.
- Osmoregulation: The hypothalamus monitors blood solute concentration and regulates water balance through the release of ADH.
- Puberty: The hypothalamus initiates puberty by releasing GnRH, which stimulates the release of hormones from the pituitary gland that trigger the development of sexual characteristics.
The hypothalamus and pituitary gland form a complex interconnected system that plays a crucial role in regulating various bodily functions, including growth, development, metabolism, and stress response.
C3.1.14—Feedback control of heart rate following sensory input from baroreceptors and chemoreceptors
Key Points:
- Sinoatrial Node (SAN): The SAN, located in the right atrium, is the pacemaker of the heart, initiating the heartbeat.
- Neural Control of Heart Rate:
- Sympathetic Nerve: Signals from the sympathetic nerve increase the heart rate.
- Vagus Nerve: Signals from the vagus nerve decrease the heart rate.
- Baroreceptors and Chemoreceptors: These sensory receptors provide feedback to the cardiovascular center in the medulla oblongata, allowing for the regulation of heart rate.
- Baroreceptors: Located in the walls of the aorta and carotid arteries, they monitor blood pressure. They send signals to the cardiovascular center to adjust heart rate in response to changes in blood pressure.
- Chemoreceptors: Also located in the aorta and carotid arteries, they monitor blood oxygen and carbon dioxide levels. They send signals to the cardiovascular center to adjust heart rate to maintain appropriate levels of blood oxygen and carbon dioxide.
- Epinephrine’s Influence: Epinephrine, released by the adrenal glands in response to stress, stimulates the sinoatrial node, increasing heart rate.
The heart rate is regulated through a complex feedback mechanism involving the cardiovascular center, baroreceptors, chemoreceptors, and the sympathetic and parasympathetic nervous systems. This ensures that the heart rate is adjusted to meet the body’s changing needs.
C3.1.15—Feedback control of ventilation rate following sensory input from chemoreceptors
Key Points:
- Ventilation Rate and Cell Respiration: The overall rate of cell respiration in the body determines the body’s demand for oxygen and the need to remove carbon dioxide.
- Maintaining Blood pH: Blood pH is tightly regulated. An increase in carbon dioxide concentration in the blood leads to a decrease in pH (respiratory acidosis), which can have harmful consequences.
- Role of Chemoreceptors: Chemoreceptors located in the aorta and carotid arteries monitor blood pH and carbon dioxide levels. They send signals to the respiratory centers in the brainstem to adjust the ventilation rate.
- Feedback Mechanism:
- Increased CO2: When chemoreceptors detect an increase in blood CO2 (and thus a decrease in pH), they send signals to the respiratory centers to increase the ventilation rate. This increases the removal of CO2 from the lungs, bringing blood pH back to normal.
- Decreased CO2: When chemoreceptors detect a decrease in blood CO2 and a rise in pH, they reduce the signals to the respiratory centers, leading to a decrease in ventilation rate.
- Oxygen Monitoring: Chemoreceptors also monitor blood oxygen levels. If oxygen levels are low (hypoxia), they send signals to the respiratory centers to override pH-related signals and increase ventilation rate to ensure adequate oxygen supply to the brain.
The feedback control mechanism involving chemoreceptors and the respiratory centers ensures that the body maintains appropriate levels of oxygen and carbon dioxide in the blood, thereby regulating blood pH and supporting cellular respiration.
C3.1.16—Control of peristalsis in the digestive system by the central nervous system and enteric nervous system
Key Points:
- Peristalsis: Peristalsis is the wave-like muscular contractions that propel food through the digestive tract. It involves coordinated contractions of the longitudinal and circular muscle layers in the gut wall.
- Two Layers of Muscle: The gut wall contains two layers of smooth muscle:
- Longitudinal Muscle: Fibers run lengthwise along the gut.
- Circular Muscle: Fibers encircle the gut.
- Coordinated Contractions: The coordinated contractions of these two muscle layers create a wave-like motion that propels food forward through the digestive system.
- Role of the Nervous System:
- Enteric Nervous System: The gut has its own intrinsic nervous system called the enteric nervous system. This system regulates local gut functions, including peristalsis.
- Central Nervous System: The central nervous system, particularly the parasympathetic branch, can influence gut motility. The parasympathetic nervous system stimulates gut motility, while the sympathetic nervous system inhibits it.
Peristalsis is a complex process that is regulated by both the enteric nervous system and the central nervous system. The coordinated contractions of the gut muscles ensure the efficient movement of food through the digestive tract.
C3.1.17—Observations of tropic responses in seedlings
Key Points:
- Tropic Responses: Tropic responses are directional growth responses of plants in response to external stimuli. These responses can be either positive (growth towards the stimulus) or negative (growth away from the stimulus).
- Phototropism: Phototropism is the growth response of a plant towards light. Most shoots exhibit positive phototropism, growing towards the light source.
- Gravitropism (Geotropism): Gravitropism is the growth response of a plant in relation to gravity. Most roots exhibit positive gravitropism, growing downwards towards gravity, while most shoots exhibit negative gravitropism, growing upwards away from gravity.
Tropic responses enable plants to adapt and grow optimally in their environment. By responding to stimuli like light and gravity, plants can maximize their access to sunlight and water, essential for photosynthesis and growth.
C3.1.18—Positive phototropism as a directional growth response to lateral light in plant shoots
Key Points:
- Positive Phototropism: Shoots of plants tend to grow towards the light source. This is known as positive phototropism.
- Light Detection: The shoot tip contains photoreceptors that detect the direction of light.
- Differential Growth: When a shoot is not growing towards the brightest light, it responds by growing at a slower rate on the side facing the brighter light. This differential growth causes the shoot to bend towards the direction of the strongest light.
- Mechanism: In the 1920s, it was discovered that a plant hormone, auxin, plays a crucial role in phototropism. Auxin accumulates on the shaded side of the shoot, promoting cell elongation and causing the shoot to bend towards the light.
- Adaptive Significance: Positive phototropism is an adaptive advantage for plants, as it allows them to maximize light absorption for photosynthesis, particularly important in environments where competition for light is intense.
Positive phototropism is a growth response that enables plants to optimize their light capture and enhance photosynthesis.
C3.1.19—Phytohormones as signalling chemicals controlling growth, development and response to stimuli in plants
Key Points:
Phytohormones: Plant Hormones: Phytohormones are chemical messengers produced by plants that regulate various aspects of plant growth, development, and responses to stimuli.
Functions of Phytohormones:
- Growth: Phytohormones can either promote or inhibit growth by influencing cell division and cell elongation. For example, gibberellins promote stem growth, while abscisic acid inhibits growth.
- Development: Phytohormones play a key role in various developmental processes, such as bud development, leaf formation, and flowering. For example, ethylene promotes fruit ripening.
- Responses to Stimuli: Phytohormones also regulate plant responses to stimuli. For example, jasmonic acid plays a role in plant defense responses, such as the production of enzymes to digest captured insects in carnivorous plants.
Major Classes of Phytohormones: The image shows the structures of several major classes of phytohormones, including:
- Auxins: Involved in cell elongation and phototropism.
- Gibberellins: Promote stem elongation and seed germination.
- Cytokinins: Promote cell division and growth.
- Abscisic Acid: Inhibits growth and promotes seed dormancy.
- Ethylene: Promotes fruit ripening and leaf abscission.
- Brassinosteroids: Promote cell elongation and growth.
- Jasmonic Acid: Involved in plant defense responses.
Phytohormones are essential signaling molecules that regulate a wide range of processes in plant growth, development, and responses to the environment.
C3.1.20—Auxin efflux carriers as an example of maintaining concentration gradients of phytohormones
Key Points:
- Auxin Transport and Phototropism: Auxin is a key phytohormone that plays a crucial role in plant growth, particularly in phototropism (growth towards light).
- Auxin Movement: Auxin can move within plant tissues through a combination of passive diffusion and active transport.
- Auxin Efflux Carriers: Plant cells possess specialized membrane proteins called auxin efflux carriers. These proteins actively pump auxin out of the cell.
- Creating Concentration Gradients: The activity of auxin efflux carriers is directional. As a result, auxin can be transported across the plant in a specific direction, creating concentration gradients.
- Phototropism: In phototropism, auxin accumulates on the shaded side of the plant shoot. This higher concentration of auxin on the shaded side stimulates cell elongation, causing the shoot to bend towards the light.
Auxin efflux carriers play a crucial role in establishing and maintaining auxin concentration gradients within plant tissues. These gradients are essential for various plant growth and developmental processes, including phototropism.
C3.1.21—Promotion of cell growth by auxin
Key Points:
- Turgor Pressure: The cell wall provides structural support to plant cells, allowing them to maintain turgor pressure. Turgor pressure is the internal pressure exerted by the cell contents against the cell wall. It is essential for maintaining cell shape and rigidity.
- Cell Wall Structure: The cell wall is composed of cellulose microfibrils, which are bundles of cellulose molecules. These microfibrils are interconnected by other carbohydrates, such as pectin.
- Cell Wall Extension: For a cell to grow, its cell wall must be able to expand. This involves loosening the crosslinks between cellulose microfibrils.
- Role of Auxin: Auxin stimulates the synthesis of proton pumps in the plasma membrane. These pumps actively transport hydrogen ions (H+) from the inside of the cell (cytoplasm) to the outside (apoplast), acidifying the cell wall.
- pH-Dependent Cell Wall Loosening: The decrease in pH weakens the crosslinks between cellulose microfibrils, making the cell wall more flexible and allowing it to expand.
- Growth and Phototropism: Concentration gradients of auxin, such as those established during phototropism, can lead to differential cell wall expansion and growth, causing the plant to bend towards the light.
Auxin promotes cell growth by stimulating the acidification of the cell wall, which weakens the crosslinks between cellulose microfibrils and allows the cell to expand.
C3.1.22—Interactions between auxin and cytokinin as a means of regulating root and shoot growth
Key Points:
- Auxin and Cytokinin: Plant Hormones: Auxin and cytokinin are two major plant hormones that play crucial roles in regulating plant growth and development.
- Distribution: Auxin is primarily produced in shoot tips and transported downwards, while cytokinin is primarily produced in root tips and transported upwards.
- Synergistic and Antagonistic Effects: Auxin and cytokinin can have both synergistic (working together) and antagonistic (opposing) effects on different aspects of plant growth.
- Cell Division and Enlargement:
- Synergistic Effect: Both auxin and cytokinin stimulate cell division and enlargement in shoot and root tips.
- Branch Development:
- Antagonistic Effect: Auxin inhibits the development of lateral buds (branches) in the stem, while cytokinin promotes their development. This is known as apical dominance.
The balance of auxin and cytokinin levels in a plant plays a crucial role in determining the relative growth of shoots and roots, as well as the development of lateral branches.
Example: Apical Dominance
The presence of a growing shoot tip produces auxin, which inhibits the growth of lateral buds. If the main shoot is removed, auxin levels decrease, allowing lateral buds to develop and form new branches.
C3.1.23—Positive feedback in fruit ripening and ethylene production
Key Points:
- Fruit Ripening: Fruit ripening involves a series of changes, including a change in color, softening of the flesh, and the development of sweetness and aroma.
- Role of Ethylene: Ethylene, a plant hormone, plays a key role in initiating and coordinating fruit ripening. It triggers a cascade of biochemical events leading to the ripening process.
- Positive Feedback Loop: Once the ripening process begins, ethylene production is significantly increased. This increased ethylene then stimulates further ripening in the same fruit and can also trigger ripening in neighboring fruits. This creates a positive feedback loop, where the ripening of one fruit accelerates the ripening of others.
- Adaptive Significance: This positive feedback mechanism ensures that a large number of fruits ripen simultaneously. This increases the chances of attracting animals for seed dispersal and ensures the successful propagation of the plant.
The positive feedback loop involving ethylene production is a crucial mechanism in fruit ripening, ensuring that the process occurs efficiently and synchronously, maximizing the chances of seed dispersal.