Lesson Notes By Weeks and Term v5 - Grade 8
Systems for transporting substances in plants and animals – Week 4 focus
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Systems for transporting substances in plants and animals – Week 4 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Natural Sciences
Class: Grade 8
Term: 3rd Term
Week: 4
Theme: General lesson support
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The efficient transportation of substances is crucial for the survival of all living organisms, from the smallest plants to the largest animals. In South Africa, understanding these systems is particularly important as it helps us appreciate the delicate balance of our ecosystems and how human activities can impact them. Imagine trying to build a house without a reliable transportation system to bring in bricks, cement, and other materials – it would be impossible! Similarly, plants need water and nutrients transported from the soil to their leaves for photosynthesis, and animals need oxygen and nutrients delivered to their cells, while waste products must be removed.
2.1 Transport in Plants: Xylem and Phloem Plants have two main transport tissues: xylem and phloem. These tissues work together to distribute essential substances throughout the plant.
Xylem: Xylem tissue is responsible for transporting water and dissolved mineral salts from the roots to all other parts of the plant, including the leaves. Think of it as the plant's "water pipeline." Xylem vessels are essentially dead, hollow tubes that are strengthened by a substance called lignin. The movement of water up the xylem is mainly driven by two processes: Transpiration: This is the evaporation of water from the leaves, primarily through tiny pores called stomata. As water evaporates, it creates a "pull" or suction that draws water up the xylem from the roots. Imagine sucking on a straw – the plant uses a similar process. Transpiration is influenced by environmental factors like temperature, humidity, and wind. On a hot, dry day in the Karoo, transpiration rates will be much higher than on a cool, humid day in Durban.
Root Pressure: This is a pressure generated in the roots due to the active absorption of mineral ions. These ions draw water into the root cells by osmosis, creating a pressure that pushes water up the xylem.
Phloem: Phloem tissue transports sugars (produced during photosynthesis in the leaves) to all other parts of the plant for energy and growth. Think of it as the plant's "food delivery system." Phloem consists of living cells called sieve tubes and companion cells. Unlike xylem, transport in phloem (called translocation) can occur in both directions – up and down the plant, depending on the needs of different parts. Sugars are loaded into the phloem at the source (e.g., leaves) and unloaded at the sink (e.g., roots, fruits, growing tips).
Example 1: Imagine a maize plant growing in a field in Mpumalanga. The roots absorb water and minerals from the soil. The xylem vessels transport these substances up the stem to the leaves. In the leaves, photosynthesis occurs, producing sugars. The phloem then transports these sugars down the stem to the roots, where they are used for growth and storage. 2.2 Transport in Animals: The Human Circulatory System The circulatory system in humans is responsible for transporting oxygen, nutrients, hormones, and waste products throughout the body.
It consists of three main components: The Heart: The heart is a muscular organ that acts as a pump, propelling blood around the body.
It has four chambers: two atria (receiving chambers) and two ventricles (pumping chambers). The heart beats continuously, pushing blood through a network of blood vessels. The heartbeat is controlled by electrical signals that originate in the heart itself.
Blood Vessels: There are three main types of blood vessels: Arteries: These carry oxygenated blood away from the heart to the body's tissues and organs. They have thick, elastic walls to withstand the high pressure of the blood pumped by the heart. The pulmonary artery is an exception; it carries deoxygenated blood from the heart to the lungs.
Veins: These carry deoxygenated blood back to the heart from the body's tissues and organs. They have thinner walls than arteries and contain valves to prevent blood from flowing backward. The pulmonary vein is an exception; it carries oxygenated blood from the lungs to the heart.
Capillaries: These are tiny, thin-walled vessels that connect arteries and veins. They are the site of exchange of oxygen, nutrients, and waste products between the blood and the body's cells.
Blood: Blood is a fluid connective tissue that transports substances throughout the body.
It consists of: Red blood cells (erythrocytes): These contain haemoglobin, a protein that binds to oxygen and transports it to the body's tissues.
White blood cells (leukocytes): These are part of the immune system and help fight off infections.
Platelets (thrombocytes): These help with blood clotting to prevent excessive bleeding.
Plasma: This is the fluid portion of blood, which carries dissolved nutrients, hormones, waste products, and other substances. 2.3 The Flow of Blood Through the Human Circulatory System The circulatory system works in a continuous loop: Deoxygenated blood enters the right atrium of the heart from the body. The right atrium pumps the blood into the right ventricle. The right ventricle pumps the blood through the pulmonary artery to the lungs, where it picks up oxygen and releases carbon dioxide. Oxygenated blood returns to the left atrium of the heart through the pulmonary vein. The left atrium pumps the blood into the left ventricle. The left ventricle pumps the oxygenated blood through the aorta (the largest artery) to the rest of the body. The blood travels through arteries, arterioles, and capillaries, delivering oxygen and nutrients to the body's cells and picking up waste products. Deoxygenated blood then travels through venules and veins back to the right atrium of the heart, completing the cycle.