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Transport in plants: 2. Flowering plants have two separate transport systems which must reach all parts of the plant

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(2) Flowering plants have two separate transport systems which must reach all parts of the plant

Reminder of the two types of transportation

Other than for active transport, diffusion, naturally occurs in gases and liquids, because all the particles (molecules or ions) have sufficient kinetic energy to move around freely at random from a region of higher concentration to a region of lower concentration.

Osmosis is a particular spontaneous diffusion through a membrane from a higher water potential region (solution less concentrated in solutes) to a lower water potential region (solution more concentrated in solutes).

Active transport is the movement of particles (molecules or ions) through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration.

Within cell membranes there are carrier proteins which use energy from respiration to transport molecules or ions across the membrane, against the natural concentration gradient, therefore cells that use active transport usually have more mitochondria for respiration compared to other cells.
 

The structure and function of plant xylem and phloem cells - the transport tissues

Plants have two networks of 'fine tubes' to transport molecules and ions (xylem tubes and phloem tubes).

The xylem vessel tissue transports water and minerals from the root hairs to all the rest of plant i.e. through the roots and stem to the very tips of all the leaves.

This xylem vessel process is driven by transpiration.

The phloem vessel tissue transports dissolved sugars from the leaves (where they are made from photosynthesis) to all parts of the plant e.g. for growth of new cells or to storage tissue where they are converted to starch.

This function of the phloem to move sugar molecules (mainly dissolve sucrose), amino acid molecules and mineral ions around the plant is called translocation.

These systems are essential for a plant to be healthy.

In some trees the transport systems run through the bark.

Unfortunately, some animals like to chew this bark.

The transport systems are disrupted and the tree sadly dies!

diagram of xylem and phloem vessels explaining how they work the transportation systems in green flowering plants

Consider the xylem first

The xylem tissue transports water and mineral ions from the roots to the stem and leaves.

Xylem tubes are made of lignified dead cells joined together 'end to end' in such a way they form a complete fine tube through which water and mineral ions are freely transported from the roots, up through the stem to the leaves.

The xylem cells have no end wall but a hole down the middle (lumen) allows the free movement of fluid - but in only one direction.

The strong xylem cell walls are made from cellulose and are strengthened-stiffened by a material called lignin - these give the plant support.

The movement of water from the roots through the xylem and out of the leaves is called the transpiration stream and is caused by the diffusion of water and its subsequent evaporation. The transpiration stream only flows in one direction - up through the plant.

Water evaporating through the stomata in the leaves, causes it to be replaced by water absorbed by the roots and this water moves up via the xylem tube system through the stem and to all the leaves - from the roots to ALL of the plant and carrying essential mineral ions too.

If the stomata pores are open, evaporation of water will always take place because the concentration of water in the air is less than the concentration off water in outer layers of a leaf.

The diffusion and evaporation of water from the leaves produces a water deficiency in the plant, so water is automatically (if available) drawn up through the xylem tube system, so the transpiration stream is driven by this evaporation of moisture from the leaves.

Water is essential to the plant for both transportation and photosynthesis.

Evolution adaptation notes:

Transpiration is a necessary adaptation to work in conjunction with photosynthesising leaves - the stomata allow the gas exchange - carbon dioxide in, and water vapour and oxygen out.

Even the narrow roots of plants are further covered in tiny root hairs that greatly increase the surface area even more, and hence increase the efficiency of water absorption.

This adaptation means the water has only got to move a short distance to the xylem to be transported up through the whole of the plant.

Now consider the phloem cells and compare their structure and function with xylem cells

Phloem cells are elongated living cells and the phloem tube tissues carry dissolved sugars (food - glucose, sucrose) from the leaves to the rest of the plant, including the tissue growing regions and the storage organs.

The phloem cells also transport other important materials like amino acids for protein synthesis and mineral ions for the function of chlorophyll and enzymes.

This process is called translocation and can operate fluid flow in both directions - another useful adaptation.

Two of the principal substances transported by translocation are sucrose and amino acids from regions of production to regions of storage, respiration (sugars) or growth (e.g. protein synthesis).

Note that some parts of a plant may act as a source or a sink at different times during the life of a plan

The sugars from photosynthesis enter the phloem system by active transport and transported around by water which enter the phloem cells by osmosis.

Reminder: Osmosis as the net movement of water molecules from a region of higher water potential (from a more dilute solution) to a region of lower water potential (a more concentrated solution), through a partially permeable membrane (concentrated refers to dissolved molecules or ions).

Phloem cells are elongated with end walls that have pores in them to allow fluids to flow through and the phloem system allows transportation in both directions.

Phloem tubes are sometimes called sieve tube elements and the perforated end-plates allow fluids to pass through.

The phloem cells (sieve tube elements) have no nucleus and can't survive on their own, so each one has a companion cell (not shown, but has nucleus) that controls the living functions for both cells.

The companion cell, with a nucleus, has lots of mitochondria to provide energy for the phloem's transportation function.

They allow the transport of water and dissolved substances to all part of the plants where nutrients are needed for immediate use in growth or converted to starch for storage

The phloem tubes mainly carry the sugars made in the leaves from photosynthesis up and down the stem to all parts of the plant for immediate use in respiration, new growth or to the food storage organs to form starch.

Phloem cells contain a little cytoplasm, but no nucleus and have very few sub-cellular structures, this adaptations increase the efficiency of transportation - also aided by the sieve plates of adjoining cells.

 

Vascular bundles

Throughout the plant the chains of xylem and phloem cells are held together in vascular bundles.

You find them in the roots, stem and in the leaf - where you see them as the veins in leaves.


Keywords, phrases and learning objectives for this part on transport systems in plants

Be able to describe the structure and function of xylem vessel tubes and phloem vessels in transporting materials throughout a flowering plant.

Know that they work as two separate transport systems which must reach all parts of the plant.

Know the role of diffusion, osmosis and active transport in movement of substances around a plant.

Know what vascular bundles are structures that hold the xylem and phloem cell tubular networks together,.


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