OCR GCSE Combined Science A Gateway revision notes
Scaling up Revision Notes
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Scaling up
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Adaptations of Xylem and Phloem for Transport in Plants Revision Notes
Xylem Structure and Function
Xylem tissue is responsible for the transport of water and dissolved mineral ions from the roots up to the leaves and other parts of the plant. It is primarily composed of dead cells called xylem vessels. These vessels are long, continuous, hollow tubes formed by cells joining end-to-end and losing their end walls. The cell walls of xylem vessels are strengthened and waterproofed by lignin deposition, which provides structural support to the plant and prevents the vessels from collapsing under the suction pressure generated by transpiration. The narrow lumen and continuous, unobstructed pathway allow for efficient bulk flow of water.
Phloem Structure and Function
Phloem tissue transports sugars, primarily sucrose, produced during photosynthesis from the leaves (source) to other parts of the plant where they are needed for growth or storage (sink). Phloem is composed of living cells, mainly sieve tube elements and companion cells. Sieve tube elements are elongated cells that form continuous tubes with perforated end walls called sieve plates, allowing sap to flow through. They have reduced cytoplasm and no nucleus to maximise space for sap. Companion cells are located next to sieve tube elements, possess a nucleus and dense cytoplasm, and are metabolically active, providing the energy and controlling the functions of the sieve tube elements.
Adaptations for Efficient Transport
Both xylem and phloem have specific adaptations for their roles. Xylem's lignin-reinforced walls provide strength and prevent collapse, ensuring a continuous water column. The hollow, dead cells remove obstruction for water movement. Phloem's sieve plates allow for the flow of sugary sap, while the absence of a nucleus in sieve tube elements maximises the cross-sectional area for transport. Companion cells, with their rich metabolic activity, actively load and unload sugars into the sieve tube elements, ensuring efficient translocation.
The Importance of Cell Differentiation Revision Notes
What is Cell Differentiation?
Cell differentiation is the process by which a less specialised cell becomes a more specialised cell type. This means that cells change their structure and function to perform specific roles within the body. All cells initially develop from a single fertilised egg, which divides to form an embryo. The cells in early embryos are stem cells, meaning they are unspecialised and have the potential to differentiate into many different cell types. As development progresses, these cells undergo differentiation, permanently changing their characteristics based on the genes they express and suppress.
Why is Differentiation Important for Multicellular Organisms?
Differentiation is essential for the formation of complex multicellular organisms. If all cells remained identical, an organism would be a simple mass of cells unable to carry out complex functions. Specialisation allows for a division of labour, where different cell types are responsible for different tasks. For example, muscle cells are specialised for contraction, nerve cells for transmitting electrical signals, and red blood cells for oxygen transport. This specialisation leads to the formation of tissues, organs, and organ systems, which are vital for processes like movement, digestion, respiration, and thought, enabling the organism to function efficiently and survive.
Differentiation in Development and Repair
In embryonic development, differentiation is crucial for creating all the various cell types needed to form a complete organism. During this stage, embryonic stem cells differentiate into all the cell types of the body. In mature organisms, differentiation continues to be important for growth, repair, and replacement of damaged or worn-out cells. For example, stem cells in the bone marrow constantly differentiate into new blood cells, and stem cells in the skin differentiate to replace old skin cells. This ongoing process maintains tissue health and function throughout life.
Surface Area to Volume Ratio and Multicellular Organisms Revision Notes
The Challenge for Multicellular Organisms
As organisms increase in size, their volume grows much faster than their surface area. This results in a decreasing surface area:volume ratio. Single-celled organisms have a large surface area relative to their volume, allowing efficient diffusion of substances directly across their outer membrane. However, for larger multicellular organisms, the reduced surface area:volume ratio means that diffusion alone across the outer surface is not enough to meet the needs of all the cells within the body. The distance for diffusion to the innermost cells would be too great, and the rate of exchange too slow.
Specialised Exchange Surfaces
To compensate for the small surface area:volume ratio, multicellular organisms have evolved specialised exchange surfaces. These surfaces are highly adapted to maximise the rate of substance exchange. Common adaptations include a very large total surface area (e.g., millions of alveoli in lungs, extensive root hair systems), thin walls (often just one cell thick) to reduce the diffusion distance, and a rich blood supply (e.g., dense capillary networks) to maintain steep concentration gradients. Examples include the alveoli in mammalian lungs for gas exchange, villi in the small intestine for nutrient absorption, and gills in fish.
The Role of a Transport System
Even with specialised exchange surfaces, substances still need to be moved efficiently to and from all the cells throughout the large body of a multicellular organism. This is where a transport system comes in. In animals, the circulatory system (heart, blood vessels, and blood) acts as this transport system, ensuring that oxygen and nutrients absorbed at exchange surfaces are rapidly delivered to every cell, and waste products like carbon dioxide are transported from cells to the appropriate exchange organs for removal. Without such a system, the internal cells would not receive adequate supplies or be able to eliminate waste effectively.
Substance Transport in and out of Cells Revision Notes
Diffusion: Passive Movement Down a Concentration Gradient
Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration. This is a passive process, meaning it does not require metabolic energy from the cell. Particles move randomly, but the *net* movement is towards the lower concentration until an equilibrium is reached. The concentration gradient, surface area, temperature, and diffusion distance all affect the rate of diffusion. For instance, oxygen diffuses into cells where its concentration is low, and carbon dioxide diffuses out where its concentration is high.
Osmosis: The Diffusion of Water
Osmosis is a specific type of diffusion that involves the movement of water molecules across a partially permeable membrane. Water moves from an area of high water potential (a dilute solution with many free water molecules) to an area of low water potential (a concentrated solution with fewer free water molecules). This is also a passive process. Water potential refers to the tendency of water to move from one area to another due to osmosis. Cells can gain or lose water by osmosis, affecting their volume and turgidity.
Active Transport: Moving Against the Gradient, Requiring Energy
Active transport is the movement of substances across a cell membrane from a region of lower concentration to a region of higher concentration, against their concentration gradient. This process is 'active' because it requires metabolic energy, typically supplied by ATP from respiration. Specific carrier proteins embedded in the cell membrane are responsible for active transport. Examples include the absorption of mineral ions by plant root hair cells and glucose absorption in the small intestine, where the cell needs to accumulate substances even when external concentrations are low.
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