OCR GCSE Biology A Gateway revision notes
Scaling up Revision Notes
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Scaling up
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Transport of Substances Across Cell Membranes Revision Notes
Diffusion: Passive Movement Down a Gradient
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. This process is passive, meaning it does not require energy from the cell. It occurs in gases and liquids due to the random motion of particles. Important examples include the movement of oxygen into cells and carbon dioxide out of cells during respiration. The rate of diffusion is affected by temperature (higher temperature, faster rate), surface area (larger area, faster rate), concentration gradient (steeper gradient, faster rate), and diffusion distance (shorter distance, faster rate).
Osmosis: Water Movement Across Selectively Permeable Membranes
Osmosis is a specific type of diffusion involving the net movement of water molecules across a selectively permeable membrane. Water moves from a region of higher water potential (a dilute solution, meaning a higher concentration of water molecules) to a region of lower water potential (a concentrated solution, meaning a lower concentration of water molecules). This is also a passive process, requiring no energy. Osmosis is critical for maintaining water balance in plant and animal cells. For instance, plant roots absorb water by osmosis. Animal cells can swell and burst in pure water or shrivel in very concentrated solutions.
Active Transport: Energy-Dependent Movement Against a Gradient
Active transport is the movement of particles from a region of lower concentration to a region of higher concentration, against a concentration gradient. This process requires energy, which is typically supplied by respiration. Cells use active transport to absorb essential substances even when their external concentration is lower than internally. Examples include the absorption of mineral ions by plant root hair cells from dilute soil solution and the uptake of glucose by cells lining the small intestine, even if glucose concentration in the intestine is lower than inside the cells.
Practical Investigations of Transport Mechanisms (PAGs B6 and B8)
Practical activities for investigating transport include observing osmosis in plant tissue (PAG B6) and diffusion in agar blocks (PAG B8). For osmosis, potato cylinders can be weighed before and after immersion in sugar solutions of varying concentrations to observe mass change. The independent variable is the sugar solution concentration, and the dependent variable is the change in mass of the potato. Control variables include temperature, potato cylinder size, and immersion time. For diffusion, agar blocks containing an indicator can be placed in acid, with colour change indicating the rate of diffusion. The independent variable is often temperature or surface area to volume ratio, and the dependent variable is the time for the colour change to reach the centre or the distance the colour change has moved.
Adaptations of Red Blood Cells and Plasma for Transport Revision Notes
Red Blood Cell Adaptations for Oxygen Transport
Red blood cells are highly specialised for carrying oxygen. Their biconcave disc shape provides a large surface area-to-volume ratio, maximising the rate at which oxygen can diffuse in and out. They are also flexible, allowing them to bend and squeeze through narrow capillaries, ensuring oxygen delivery to all tissues. Crucially, red blood cells lack a nucleus and most other organelles. This adaptation creates more internal space, enabling them to pack in a large amount of haemoglobin, the iron-containing protein that reversibly binds with oxygen, effectively increasing their oxygen-carrying capacity. Without a nucleus, red blood cells have a limited lifespan (around 120 days) and cannot repair themselves.
Plasma Adaptations for General Transport
Plasma is the pale yellow liquid component of blood, making up about 55% of its total volume. Its primary adaptation is being mostly water (around 90%), making it an excellent solvent. This solvent property allows plasma to dissolve and transport a vast array of substances. It carries dissolved nutrients, such as glucose, amino acids, vitamins, and mineral ions, from the digestive system to all body cells for energy and growth. Plasma also transports waste products, including carbon dioxide (mostly as hydrogen carbonate ions) from respiring cells to the lungs for excretion, and urea from the liver to the kidneys for removal. Additionally, hormones, antibodies, and heat are transported dissolved within the plasma.
Transport of Nutrients and Waste by Plasma
The watery nature of plasma is key to its role in transporting essential substances. Glucose, absorbed from digested food, is dissolved in plasma and delivered to cells for respiration. Amino acids, the building blocks of proteins, are also transported to cells for protein synthesis. Vitamins and mineral ions, crucial for various bodily functions, are similarly carried. For waste removal, carbon dioxide produced during cellular respiration diffuses into the plasma and is converted into hydrogen carbonate ions, which are transported to the lungs. Urea, a toxic waste product from protein metabolism, is carried by plasma from the liver to the kidneys for filtration and excretion as urine.
Practical Investigation of Blood Components (PAG B1)
A simple way to observe blood components is through centrifugation. A blood sample is placed in a centrifuge and spun at high speed. This separates the components based on their density. The densest components, red blood cells, settle at the bottom. Above them, a thin layer called the 'buffy coat' contains white blood cells and platelets. The top layer is the plasma, which is the least dense. This practical demonstrates the different components of blood and their relative proportions, highlighting that plasma is the largest volume component and the medium through which other components are suspended or dissolved for transport.
Xylem and Phloem: Structure and Function Revision Notes
Xylem: Water Transport System
Xylem tissue is specially adapted for the efficient transport of water and dissolved minerals from the roots up to the leaves. It is made up of dead cells that form continuous, narrow, hollow tubes. These tubes are reinforced with lignin, a strong, waterproof substance, which gives them structural support and prevents collapse. The continuous, hollow nature of the tubes, without end walls, allows for an uninterrupted flow of water, facilitating the transpiration stream.
Phloem: Sugar Transport System
Phloem tissue is responsible for transporting sugars, primarily sucrose, produced during photosynthesis from the leaves to other parts of the plant, such as roots, fruits, and growing tips. This process is called translocation. Phloem consists of living cells: sieve tube elements and companion cells. Sieve tubes are arranged end-to-end, forming long tubes with perforated end walls called sieve plates, which allow sap to flow.
Adaptations of Xylem Structure
Key adaptations of xylem include: 1. Hollow tubes: Provides a clear pathway for water. 2. Lignified walls: Strengthens the tubes to prevent collapse under pressure and provides structural support for the plant. Lignin also makes the walls waterproof. 3. No end walls: Creates a continuous column for efficient water flow. 4. Dead cells: Reduces resistance to water flow.
Adaptations of Phloem Structure
Key adaptations of phloem include: 1. Sieve tube elements: Form long tubes with perforated sieve plates for sap flow. They have reduced cytoplasm and no nucleus to maximize lumen for transport. 2. Companion cells: Located adjacent to sieve tubes, they are metabolically active, providing energy (via mitochondria) for the active transport of sugars into and out of the sieve tubes, maintaining the pressure gradient for translocation. The plasmodesmata connect companion cells to sieve tubes.
Understanding Stem Cells and Meristems Revision Notes
Embryonic Stem Cells in Animals
Embryonic stem cells are found in early-stage embryos. These cells are pluripotent, meaning they have the remarkable ability to differentiate into almost any type of specialised cell in the body. This characteristic makes them incredibly significant for potential medical treatments, such as regenerating damaged tissues or organs. Ethical considerations are often raised regarding the use of embryonic stem cells due to their origin.
Adult Stem Cells in Animals
Adult stem cells are found in various developed tissues and organs throughout an animal's life, such as bone marrow, skin, and the brain. Unlike embryonic stem cells, adult stem cells are generally multipotent; they can differentiate into a limited range of cell types, typically those found within the tissue they originate from. Their primary role is to replace dead or damaged cells and repair tissues, maintaining the organism's health.
Meristems in Plants
Meristems are regions in plants where cell division occurs continuously, acting as the plant's equivalent of stem cells. They contain undifferentiated cells that can divide and then differentiate into various specialised plant cells, such as those forming roots, stems, leaves, and flowers. Apical meristems are found at the tips of roots and shoots, enabling primary growth (increase in length). Lateral meristems, like the cambium, allow for secondary growth (increase in girth).
Functions of Stem Cells and Meristems
The fundamental function of both animal stem cells and plant meristems is to provide a continuous supply of new cells for growth, repair, and regeneration. In animals, this involves replacing worn-out blood cells, repairing skin damage, or potentially regenerating entire organs. In plants, meristems ensure constant growth throughout the plant's lifespan, allowing for the formation of new structures and repair of damaged parts, enabling adaptation to environmental changes.
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