UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 22/05/2023 [SEARCH]

Transport in plants: 3. The structure and function of roots, absorption and necessity of nutrients

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(3) The structure and function of roots, absorption and necessity of nutrients

Structure and function of roots - absorption of minerals - diffusion and active transport

In plants, most of the water and mineral ions (e.g. magnesium ions or nitrate ions) are absorbed by roots, these in turn will be translocated up through the stem to the rest of the plant using the xylem tube system.

I'm concentrating on minerals here, but remember plants need lots of water for their internal transport systems (xylem and phloem cells), photosynthesis and to keep cells turgid and support the physical structure of the plant as a whole.

Whereas most of the plant consists of green photosynthetic tissue, the roots are non-photosynthetic and pale in colour.

Therefore sugars would be translocated from photosynthetic tissues like leaves to non-photosynthetic tissues like roots using the phloem tubes.

Root hairs are single-celled extensions to epidermis cells in the root.

The root hair cells are connected to root cortex cells, which in turn, are closely connected to xylem vessels which transfer minerals and nutrients around the plant efficiently.

Root hair cells are well adapted for their function:

large surface area for absorbing water - which will also contain minerals,

they have no cuticle, but a thin membrane for efficient absorption - short diffusion distance,

they have a thin cell wall, again reduces the distance for water transfer in osmosis,

a large permanent vacuole to absorb and store (temporarily) as much water as possible,

The surface area of the roots is increased by root hairs and the surface area of leaves is increased by the flattened shape and internal air spaces.

Each branch of a root will have millions of root hair cells creating a massive surface area to absorb water and mineral ions.

The cells on the surface of a plant's roots grow into long cells that shape into 'hairs' that stick out into the soil.

The fine root hairs considerably increase the surface area of the roots for absorbing water and minerals and each branch of the root is covered millions of these microscopic hairs.

Root hair cells are very elongated combining into fine hair-like structures, which greatly increases the surface area of contact with the soil through which most of the plant's water and mineral intake are absorbed.

 

Osmosis and active transport

Reminder about water potential: The greater the concentration of water, the more dilute the solution, the greater the water potential.  The lower the concentration of water, the more concentrated the solution, the lesser the water potential.  Therefore in osmosis through a semi-permeable membrane, water moves from a region of higher water potential, to a region of lower water potential.

As long as the water concentration is higher in the soil, the root hairs will naturally absorb water by osmosis.

The water potential of the soil is usually greater than that of the fluid in the root, so water will be naturally be absorbed by osmosis and the large surface area of the root cell hairs aid this.

However, the concentration of minerals in the root hair cells is higher than in the moisture surrounding the roots, and therefore an absorption problem.

This is because the plants cells would naturally lose essential mineral ions by diffusion back into the soil moisture.

Not good, it means the root hair cells can't use diffusion on its own to absorb minerals from the soil, in fact, without active transport, mineral ions would move out of the root hairs.

Therefore, active transport systems must be used by the plant to counteract the natural direction of diffusion from a high mineral concentration in the plant cells to a low mineral concentration in the soil moisture.

The process requires energy from respiration which powers the process by which the plant's root cells can absorb minerals from the soil, even if they are only present in a very dilute solution of the soil's moisture.

So, energy from respiration is usually required to absorb minerals into the roots from the soil moisture by working against the concentration gradient - active transport mechanism.

It has been shown experimentally that when there is an increase in the uptake of minerals by plants, there is an increase in active transport, accompanied by an increase in respiration.

Examples of moving substances by active transport from a low concentration to a higher concentration:

The concentration of nitrate ions (NO3-) is usually greater in the root hairs cells than the surrounding soil water. Therefore the natural diffusion gradient for the movement of nitrate ions is in the direction of root to soil, but plants need the nitrogen in nitrate ions to synthesise proteins. Therefore active transport is used to move nitrate ions from the soil water into the root fluids against their natural diffusion gradient. The active transport process involves protein carrier molecules in the root cell membranes to convey the nitrate ions into the root cells,.

The same argument applies to magnesium ions which chlorophyll molecules need in photosynthesis.

Active transport is much more complicated than 'simple' osmotic water exchange in plant cells. 

See comparison of diffusion, osmosis and active transport


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