MEANS OF TRANSPORT
- Water, nutrients, organic compounds, and plant growth regulators are essential substances that need to be transported within plants.
- Plants lack a circulatory system like animals, so they rely on various mechanisms to move substances over short and long distances.
- Short-Distance Transport:
- Within cells and across membranes, substances move through diffusion and cytoplasmic streaming.
- Active transport supplements short-distance movement.
- Long-Distance Transport - Xylem and Phloem:
- Xylem primarily transports water and minerals unidirectionally from roots to stems.
- Phloem facilitates multidirectional transport of organic and mineral nutrients.
- Translocation is the term for long-distance transport through the vascular system.
- Xylem transport (water and minerals) is unidirectional, from roots to stems.
- Organic and mineral nutrients show multidirectional transport.
- Hormones and growth regulators may exhibit polarized or unidirectional transport.
- Organic compounds synthesized in photosynthetic leaves move to various plant parts, including storage organs.
- Mineral nutrients are absorbed by roots and transported upwards to stems, leaves, and growing regions.
- Nutrient withdrawal from senescent regions redirects resources to growing parts.
- Cellular transport involves various mechanisms facilitating the movement of substances across cell membranes. Three primary methods include diffusion, facilitated diffusion, and active transport.
Diffusion
- Diffusion, a passive process, occurs without any expenditure of energy.
- Spontaneous movement of molecules from regions of higher concentration to lower concentration.
- Scope of Movement:
- It operates within cells.
- Extends between cells.
- Facilitates movement over short distances, such as from intercellular spaces to the external environment.
- Molecules exhibit random motion, leading to a net movement from regions of higher concentration to regions of lower concentration.
- Diffusion is characterized by its slow pace and is not reliant on a 'living system' for its occurrence.
- While obvious in gases and liquids, it is more likely to occur in solids than the diffusion of solids itself.
- In plants, diffusion plays a pivotal role as the primary means for gaseous movement within the plant body.
- It is indispensable for various physiological processes, contributing significantly to plant functionality.
- Factors Influencing Diffusion Rates:
- The rate of diffusion is influenced by the gradient of concentration.
- Additionally, the permeability of the membrane, temperature, and pressure impact the efficiency of diffusion.
- Diffusion, fundamental to cellular processes, affects the distribution of substances within a biological system, ensuring the equilibrium of essential molecules.
Facilitated diffusion
- Facilitated diffusion relies on an existing gradient.
- The rate is influenced by substance size, with smaller substances diffusing more rapidly.
- The diffusion of a substance across a membrane is affected by its solubility in lipids.
- Lipid-soluble substances efficiently diffuse through the membrane.
- Substances with a hydrophilic part face difficulty passing through the membrane. Membrane proteins assist in their movement without establishing a concentration gradient.
- Facilitated diffusion involves special proteins aiding substance movement across membranes.
- This process does not require ATP energy expenditure.
- Facilitated diffusion does not drive net transport from low to high concentration; energy input is required for such movement.
- Transport rate peaks when all protein transporters are in use, reaching saturation.
- Facilitated diffusion is highly specific, enabling cells to selectively uptake substances.
- Sensitive to inhibitors interacting with protein side chains.
- Membrane proteins form channels for molecule passage.
- Porins, large proteins forming pores in outer membranes, allow the passage of molecules up to the size of small proteins.
- Figure illustrates an extracellular molecule binding to a transport protein.
- The transport protein rotates and releases the molecule inside the cell, exemplified by water channels composed of various aquaporins.
- Certain carrier or transport proteins facilitate diffusion only when two types of molecules move together.
- In a symport, both molecules traverse the membrane in the same direction.
- Conversely, in an antiport, the involved molecules move in opposite directions.
- When a molecule independently moves across a membrane without reliance on other molecules, the process is termed uniport.
- Symports and antiports provide specialized channels for the coordinated movement of specific molecules, expanding the repertoire of facilitated diffusion.
- Carrier proteins play a crucial role in orchestrating symport and antiport dynamics, ensuring precise and coordinated transport across the membrane.
- These transport variations contribute to the functional diversity of facilitated diffusion, accommodating scenarios where molecules need to move together or in opposite directions.
Active Transport
- Active transport expends energy to move molecules against their natural concentration gradient. It is an energy-dependent process moving molecules against their concentration gradient.
- Membrane proteins are instrumental in facilitating active transport.
- Different proteins within the membrane play crucial roles in both active and passive transport processes.
- Pumps, a type of protein, utilize energy to transport substances across the cell membrane.
- These pumps can effectively move substances from regions of low concentration to high concentration, essentially performing 'uphill' transport.
- The transport rate reaches its maximum when all available protein transporters are actively engaged or saturated.
- Similar to enzymes, carrier proteins involved in active transport exhibit specificity in the substances they transport.
- Carrier proteins in active transport are highly specific, selectively transporting particular substances across the membrane.
- These proteins are sensitive to inhibitors that interact with specific protein side chains.
- Active transport stands in contrast to passive transport mechanisms, showcasing the cell's ability to actively regulate and control the movement of molecules.
- Active transport plays a critical role in various cellular processes by allowing the cell to move essential substances against their natural flow.
