Grade 8 Biology: Responding to the Environment and Body Systems

Responding to the Environment

  • Stimulus (The "Cause"): Defined as any change in an organism's environment that causes the organism to react. The plural form is stimuli.     * Types of Stimuli:         * External: Originating from the outside environment.         * Internal: Originating from within the organism.

  • Response (The "Effect"): Defined as how an organism reacts to a stimulus.     * Types of Responses:         * Voluntary: Controlled consciously.         * Involuntary: Occurring automatically without conscious control.

Tropism in Plants

  • Definition: Tropism is plant growth in response to a stimulus. It is derived from the word "tropo," meaning "to turn."

  • Directionality:     * Positive Tropism: Growth towards the stimulus.     * Negative Tropism: Growth away from the stimulus.

  • Specific Types of Tropisms:     * Phototropism: Growth in response to light.     * Geotropism: Growth in response to gravity.     * Thigmotropism: Growth in response to touch.     * Hydrotropism: Growth in response to water.     * Chemotropism: Growth in response to chemical stimuli.     * Thermotropism: Growth in response to heat.

  • Plant Organ Responses to Specific Stimuli:     * Gravity (Geotropism):         * Shoot: Negative response (grows away from gravity).         * Root: Positive response (grows toward gravity).     * Unilateral Light (Phototropism):         * Shoot: Positive response (grows toward light).         * Root: Negative response (grows away from light).     * Water (Hydrotropism):         * Shoot: No response.         * Root: Positive response (grows toward water).

Anatomy and Physiology of the Human Eye

  • Protective and Structural Components:     * Sclerotic Layer: A tough, white-colored protective layer (noted as pink in diagrams) that surrounds the eye and helps maintain its shape.     * Conjunctiva: A clear, thin layer covering the front of the eye. Infection here is known as conjunctivitis.     * Cornea: A clear "window" in the sclerotic layer located under the conjunctiva that allows light to enter the eye.     * Aqueous Humour: A watery liquid filling the space between the cornea and the lens; it helps maintain the shape of the eye.     * Vitreous Humour: A jelly-like substance that helps maintain the shape of the eyeball.     * Choroid: A black layer forming part of the inside lining behind the lens. It prevents internal light reflection and provides nourishment via blood and oxygen.

  • Functional Components for Vision:     * Iris: Located just in front of the lens. It changes size to control the pupil's diameter and provides the eye's color.     * Pupil: A hole in the center of the iris.         * Dim light/Darkness: The pupil opens up (dilates) to allow more light in.         * Bright light: The pupil shuts down to a small hole.     * Lens: A clear, jelly-like "window" that focuses images onto the back surface of the eye.     * Suspensory Ligaments: Structures that hold the lens in place.     * Ciliary Muscles: Muscles that change the shape of the lens to bring images into focus.     * Retina: A layer containing over 100100 million light-sensitive cells that transmit messages to the brain.     * Yellow Spot (Fovea): A section of the retina directly behind the pupil containing a high concentration of color-sensitive cells; looking directly at an object utilizes this spot for maximum clarity.     * Blind Spot: The point where blood vessels and the optic nerve join the eyeball; it lacks light-sensitive cells to detect image information.     * Optic Nerve: Joins the eye to the brain, passing image information for processing so images are perceived the right way up.

Vision Conditions and Disorders

  • Myopia (Near Sighted):     * Physical Cause: The eyeball is "too long."     * Symptom: Distance vision is blurry; near objects are clear.     * Clinical Note: Patients may feel they do not need reading glasses as they age, provided they remove their distance corrective lenses.

  • Hyperopia (Far Sighted):     * Physical Cause: The eyeball is "too short."     * Symptom: Distant objects may be clearer than near objects.     * Clinical Note: Children may not notice issues until adulthood unless severe, which may require glasses at a young age to reduce eye strain.

  • Astigmatism (Two Focal Points):     * Physical Cause: The front surface of the eye (cornea) is irregularly shaped, often described as "football shaped."     * Symptom: A smudged appearance for both near and far objects; may cause glare and halos around lights.

  • Presbyopia (Age Related):     * Physical Cause: The lens loses elasticity and cannot accommodate (focus).     * Symptom: Inability to focus on objects up close; typically requires reading glasses.

  • Glaucoma:     * Definition: An eye disorder damaging the optic nerve that carries info to the brain.     * Cause: High pressure built up inside the eye, known as "Ocular Hypertension," often caused by a blocked drainage canal.     * Outcome: No communication between the eye and brain once the nerve is damaged.

  • Cataracts:     * Definition: A cloudy area in the lens.     * Cause: Proteins making up the lens begin to clump together.     * Symptom: Distortion of light passing through, damaging vision; develops slowly with age.

The Ears and Skin

  • Ear Disorders:     * Ear Infections: General bacterial/viral issues.     * Tinnitus: Characterized by a "roaring" sound in the ears.     * Meniere’s Disease: Result of fluid problems in the inner ear.     * Ear Barotrauma: Injury resulting from changes in barometric (air) or water pressure.

  • Skin Structure and Function:     * Epidermis: Topmost layer; protects from infection.     * Hair Erector Muscle: Contracts to trap air for insulation; relaxes to reduce it.     * Hypodermis: Provides insulation and aids in the production of Vitamin D.     * Sebaceous Gland: Secretes an oily substance to coat hairs.     * Dermis: Layer where blood vessels and nerves are active.     * Melanin: Pigment that protects from the sun's rays.     * Malpighian Layer: Contains melanin to protect against ultraviolet (UV) rays.     * Sweat Gland: Regulates body temperature to cool the skin surface.     * Blood Capillaries: Located in the dermis; constrict (narrow) or dilate (widen) to control heat loss.     * Keratin: Protein that forms a protective, waterproof outer layer.

  • Skin Disorders:     * Liver Spots: Flat brown or black spots appearing on areas exposed to sunlight.     * Scabies: Itchy condition caused by the tiny burrowing mite Sarcoptic scabies.     * Athlete’s Foot (Tinea pedis): Fungal infection usually starting between toes in sweaty feet.     * Acne: Clogging of skin pores forming spots and pimples.

The Nervous System

  • Composition: Made of the brain, spinal cord, and nerves; carries messages between the brain and body.

  • Major Divisions:     * Central Nervous System (CNS): Composed of the brain and spinal cord.     * Peripheral Nervous System (PNS): Composed of nerves branching off the spinal cord to the rest of the body.

  • Nervous System Disorders:     * Cerebral Palsy: A group of conditions affecting movement and posture caused by damage to the developing brain (often due to lack of oxygen before birth).     * Epilepsy: A disease where nerve cells signal improperly, resulting in seizures.     * Paralysis: The inability to move a part of the body.

Respiration and the Respiratory System

  • Cellular Respiration: The process by which plant and animal cells break down sugar to generate energy.

  • Types of Respiration:     * Aerobic Respiration:         * Occurs in the presence of oxygen (O2O_2).         * Glucose is completely broken down into CO2CO_2 and water (H2OH_2O) with a release of energy.         * Occurs in mitochondria of mammals.         * Equation: glucose+oxygen→carbon dioxide+water+energy\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} + \text{energy}     * Anaerobic Respiration:         * Occurs in the absence of oxygen (O2O_2).         * In lower organisms like yeasts (in cytoplasm):             * Equation: glucose→ethanol+carbon dioxide+energy\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide} + \text{energy}         * In higher organisms/human muscles during heavy activities (in cytoplasm):             * Equation: glucose→lactic acid+energy\text{glucose} \rightarrow \text{lactic acid} + \text{energy}

  • Respiratory Anatomy Path:     1. Trachea (Windpipe): Air passes from nose/mouth through here.     2. Bronchi: The trachea splits into two tubes (each a bronchus), leading to the lungs.     3. Bronchioles: Smaller tube branches from the bronchi.     4. Alveoli: Small sacs (singular: alveolus) where gas exchange occurs.

  • Gas Exchange Mechanism:     * CO2CO_2 moves from blood into alveoli.     * O2O_2 moves into red blood cells, combining with haemoglobin to form oxyhaemoglobin.

  • Comparison of Inhaled vs. Exhaled Air:     * Oxygen: Inhaled (21%21\%) vs. Exhaled (17%17\%).     * Carbon Dioxide: Inhaled (Tiny amount) vs. Exhaled (4%4\%).     * Nitrogen/Inert Gases: Both Inhaled and Exhaled at 79%79\%.     * Water Vapour: Inhaled (Not much) vs. Exhaled (Quite a lot).

The Menstrual Cycle

  • Overview: Average cycle is 2828 days long. Involves the uterine lining building up and ovulation.

  • Stages:     1. Menstruation: Loss of uterine lining (occurs at the start if no fertilization happened).     2. Lining Thickening: Rebuilding the uterine wall.     3. Ovulation: Occurs around day 1414; the egg travels down the oviduct.     4. Maintenance: Lining is held ready for a fertilized egg.

  • Hormonal Control:     * Oestrogen: Levels peak just before day 1414. Causes the uterine wall to thicken and the egg to mature.     * Progesterone: Stays low from days 11 to 1414. Rises after ovulation to thicken the lining further. A fall in progesterone levels triggers menstruation.