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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Need for Transport Systems
Just like humans need a circulatory system to move nutrients and oxygen around our bodies, plants need transport systems too! Plants may look like they're just sitting there, but inside they're buzzing with activity, moving water, minerals and food to every cell.
Key Definitions:
Plants need transport systems for three main reasons:
Small plants can rely on diffusion to move substances, but larger plants need specialised transport systems. As plants grow taller, the distance substances need to travel increases. Diffusion is too slow for these longer distances, so transport tissues evolved to move materials quickly and efficiently.
Plants collect water and minerals from the soil through their roots, but photosynthesis happens in the leaves. They need to move these resources between different parts. Water and minerals must go up, while the sugars made in leaves need to travel down to growing roots and other parts.
Plants face changing conditions - sometimes there's plenty of water, other times there's drought. An efficient transport system helps plants survive these changes by making the most of available resources and storing extras for when they're needed.
Plants have two main transport tissues: xylem and phloem. Think of them as the plant's plumbing system!
Xylem transports water and dissolved minerals from the roots up to the stems and leaves. It's a one-way system that works against gravity. The xylem vessels are made of dead cells that form hollow tubes. Their cell walls contain a substance called lignin which makes them strong and waterproof.
Key features:
Phloem transports dissolved sugars and other organic compounds throughout the plant. Unlike xylem, phloem is made of living cells and can transport substances both up and down the plant. The main cells in phloem are sieve tube elements, which are connected by sieve plates.
Key features:
Water transport in plants is a fascinating process that relies on several physical principles rather than the plant using energy.
Water enters the root hair cells by osmosis. Root hair cells have a large surface area to absorb more water. Minerals enter by active transport, which requires energy.
As water evaporates from the leaves (transpiration), it creates a pulling force that draws water up through the xylem vessels - like drinking through a straw.
Water molecules stick to each other (cohesion) and to the sides of the xylem vessels (adhesion), creating a continuous water column that doesn't break.
Coast redwoods can grow over 100 meters tall! Their transport systems must move water from the roots all the way to the highest leaves against gravity. That's like lifting water to the top of a 30-story building without using a pump!
The rate at which water moves through a plant depends on how quickly it evaporates from the leaves. Several factors affect this:
More light means the stomata open wider for photosynthesis, allowing more water to evaporate. On bright, sunny days, transpiration rates increase.
Wind blows away the water vapour around the leaf, maintaining a steep concentration gradient that increases evaporation and transpiration.
Higher temperatures give water molecules more energy, increasing evaporation and transpiration rates. This is why plants need more water on hot days.
Low humidity means the air is dry and can hold more water vapour, increasing the rate of transpiration. High humidity reduces transpiration as the air is already saturated with water.
While water transport in xylem is passive, the movement of sugars in phloem requires energy. This process is called translocation.
Sugars move from 'sources' (where they're made, usually leaves) to 'sinks' (where they're used or stored, like roots, fruits, or growing tips).
The annual rings you see in tree trunks are actually layers of xylem tissue! Each year, trees grow new xylem vessels. In spring and summer when there's plenty of water, the cells are larger, creating lighter-coloured wood. In autumn and winter, smaller cells create darker rings. By counting these rings, scientists can determine a tree's age and even learn about past climate conditions!
Different plants have adapted their transport systems to suit their environments:
Cacti and other desert plants have adaptations to reduce water loss. They often have fewer stomata, thick waxy cuticles and modified leaves (spines). Their xylem vessels may be narrower to prevent air bubbles forming during drought conditions.
Water lilies and other aquatic plants have reduced xylem tissue since they're surrounded by water. Instead, they have more air spaces (aerenchyma) to help with buoyancy and gas exchange. Their stomata are usually on the upper surface of leaves that float on water.
Plant transport systems are essential for survival, allowing plants to:
Without these sophisticated transport systems, plants would be limited to small, simple forms. The evolution of xylem and phloem has allowed plants to colonize nearly every habitat on Earth and grow into the diverse range of forms we see today, from tiny mosses to massive redwood trees!
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