OCR GCSE Combined Science A Gateway revision notes
Scaling up Revision Notes
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Scaling up
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Transpiration and Translocation in Plants Revision Notes
What is Transpiration?
Transpiration is the loss of water vapour from the leaves of plants, mainly through stomata. This process creates a 'pulling' force that draws water up from the roots through the xylem vessels. This upward movement of water, known as the transpiration stream, transports water and dissolved minerals throughout the plant. It's also important for cooling the plant. Environmental factors such as temperature, humidity, and light intensity significantly affect its rate.
Importance and Mechanism of Transpiration
Transpiration is essential for maintaining a continuous water supply to the plant, allowing it to photosynthesise and keeping cells turgid. Water enters the plant through root hair cells by osmosis. It then moves into the xylem in the root and stem. From here, it travels to the leaves, moving into the mesophyll cells. Water evaporates from the surface of these cells into air spaces and then diffuses out of the leaf through the stomata. This continuous chain of water molecules acts like a column, drawn upwards by the cohesive forces between water molecules and the adhesive forces to the xylem walls.
What is Translocation?
Translocation is the process of transporting dissolved sugars (produced during photosynthesis) and other plant metabolites from areas of production (sources, like leaves) to areas of use or storage (sinks, like roots, fruits, or growing points). This transport occurs in the phloem tissue. Unlike xylem, phloem transport can be in multiple directions, depending on where the sugars are needed.
Mechanism of Translocation
The movement of substances in the phloem is an active process, requiring energy from ATP to load and unload sugars. Sugars are loaded into the phloem sieve tubes at the source, which increases the solute concentration. This causes water to move into the sieve tubes by osmosis, increasing turgor pressure. This pressure drives the sap along the phloem to a sink where sugars are removed, reducing the concentration and pressure, allowing water to move out of the phloem.
Substances Transported In and Out of Organisms Revision Notes
Essential Substances for Organisms
Organisms constantly exchange materials with their environment to sustain life. Key substances taken into organisms include oxygen, which is essential for aerobic respiration to release energy from food. Nutrients, such as glucose, amino acids, fatty acids, and vitamins, are vital for growth, repair, and energy production. Water is also crucial for all metabolic processes, acting as a solvent and a reactant in many biochemical reactions. Mineral ions, like nitrates for protein synthesis or magnesium for chlorophyll, are also absorbed from the environment.
Waste Products and Their Removal
As organisms carry out metabolic processes, waste products are generated that must be efficiently removed to prevent toxicity. Carbon dioxide is a common waste product of aerobic respiration and is excreted by both plants and animals. Animals also produce nitrogenous wastes, such as urea, from the breakdown of proteins, which is excreted in urine. Excess water and heat are also considered waste products that need to be regulated and expelled from the body to maintain homeostasis.
Transport in Animals
Animals require an efficient transport system, typically a circulatory system, to move substances around their bodies. Oxygen is transported from the lungs to cells, and carbon dioxide from cells to the lungs. Nutrients from digested food are absorbed into the bloodstream and delivered to cells, while waste products like urea are transported to excretory organs (e.g., kidneys) for removal. Water is absorbed from the digestive tract and distributed throughout the body, maintaining hydration and serving as a transport medium.
Transport in Plants
Plants have specialized vascular tissues for transport. Xylem transports water and dissolved mineral ions from the roots up to the leaves. Phloem transports sugars (produced during photosynthesis in the leaves) to all other parts of the plant, including roots for storage and growing regions. Carbon dioxide enters the leaves through stomata for photosynthesis, and oxygen, a byproduct, is released through the same openings. Water is also released as water vapour through stomata during transpiration.
Heart and Blood Vessel Adaptations Revision Notes
Heart Structure and Function
The heart acts as a double pump, ensuring efficient circulation. The right side pumps deoxygenated blood to the lungs, while the left side pumps oxygenated blood to the rest of the body. The atria receive blood, and the ventricles pump it out. The left ventricle has a significantly thicker, more muscular wall than the right ventricle and the atria because it must generate high pressure to distribute blood throughout the systemic circuit. Valves are crucial for preventing the backflow of blood, maintaining a one-way flow through the heart chambers and into the major arteries.
Arteries: Carrying Blood Away from the Heart
Arteries are adapted to transport blood under high pressure away from the heart. Their walls are thick, muscular, and elastic. The thick muscle layer allows them to constrict and dilate to regulate blood flow and pressure. The elastic tissue enables them to stretch as blood is pumped in and recoil, which helps to maintain a continuous flow of blood even when the heart is relaxed. This structural integrity is vital to withstand the forceful pulsations from the ventricles.
Veins: Returning Blood to the Heart
Veins carry blood under low pressure back towards the heart. Their walls are much thinner and less muscular and elastic than arteries. To counteract the low pressure and the effects of gravity, veins contain valves. These valves prevent the backflow of blood, especially in the limbs, ensuring that blood steadily progresses towards the heart. Muscle contractions surrounding the veins also help to squeeze blood along.
Capillaries: Site of Exchange
Capillaries are the smallest and most numerous blood vessels, forming extensive networks within tissues. Their primary function is to facilitate the exchange of substances between blood and body cells. They are uniquely adapted for this with walls that are only one cell thick, which significantly reduces the diffusion distance for oxygen, nutrients, and waste products. Their extremely narrow lumen forces red blood cells to pass in single file, further maximising the surface area and time available for efficient exchange.
Water and Mineral Ion Uptake in Plants Revision Notes
Structure of Root Hair Cells
Root hair cells are specialised epidermal cells located on the surface of plant roots. Their most prominent feature is a long, thin extension, called a root hair, which protrudes into the soil. This extension significantly increases the surface area of the root, allowing for maximum contact with soil water and dissolved mineral ions. They also contain a large permanent vacuole and numerous mitochondria to provide energy for active transport.
Water Uptake by Osmosis
Water is absorbed by root hair cells through a process called osmosis. The cytoplasm and vacuolar sap within the root hair cell have a higher concentration of solutes (lower water potential) compared to the surrounding soil water (higher water potential). This concentration gradient causes water molecules to move across the partially permeable cell membrane from the soil into the root hair cell, down the water potential gradient. This is a passive process, meaning it does not require metabolic energy from the plant.
Mineral Ion Uptake by Active Transport
Mineral ions, such as nitrates, phosphates, and potassium, are often present in the soil at much lower concentrations than inside the root hair cells. Therefore, to absorb these essential nutrients, plants must use active transport. Active transport requires energy, which is supplied by respiration in the root hair cells, to move mineral ions against their concentration gradient from the soil into the cell. Specific protein carriers in the cell membrane are responsible for this selective uptake.
Adaptations for Efficient Absorption
The structural adaptations of root hair cells are crucial for their function. The large surface area provided by the root hair extension maximises the rate of both osmosis (for water) and active transport (for mineral ions). The presence of many mitochondria provides the ATP needed for active transport. The partially permeable cell membrane controls the movement of substances into and out of the cell, allowing selective uptake of mineral ions.
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