(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 using the active transport mechanism of protein carrier molecules.

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

---

Key points on structure & function of roots and absorption of nutrients

Source of information is based on textbooks and syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10)

Structure and Function of Roots

Root Functions:

1. Anchorage – hold the plant firmly in the soil.

2. Absorption – take up water and mineral ions from the soil.

3. Storage – some roots store food (e.g. carrots, sweet potatoes).

---

Root Structure:

---

Root Hair Cell Structure and Adaptations

Root hair cells are specialised cells found near the tip of roots.

Adaptations:

• Long hair-like extension: increases surface area for more absorption.

• Thin cell wall: allows faster water movement.

• Large vacuole: helps draw in water via osmosis.

• Many mitochondria: provide energy for active transport of minerals.

---

Absorption of Water and Nutrients

Osmosis – Water Uptake:

• Water moves by osmosis from the soil (high water concentration) into the root hair cell (lower water concentration).

• Moves through cells into the xylem for transport up the plant.

Osmosis: diffusion of water through a partially permeable membrane from a high to low water potential.

---

Active Transport – Mineral Ion Uptake:

• Soil has a lower concentration of minerals than inside the root hair cell.

• Active transport uses energy (ATP) to move ions against the concentration gradient.

Active transport: movement of substances from low to high concentration using energy and carrier proteins.

---

Necessity of Nutrients

Plants need minerals for healthy growth:

---

Summary

• Roots absorb water (by osmosis) and minerals (by active transport).

• Root hair cells are adapted with a large surface area and many mitochondria.

• Minerals are essential for growth, photosynthesis, and development.

• Water and minerals move through the xylem to the rest of the plant.

---

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

Be able to describe the function and structure roots and root hair cells

Be able to describe the absorption and necessity of nutrients through the root hair cell membranes with their large surface area.

Be able to explain and describe why active transport may be needed to absorb some nutrients.

Be able to describe the role of diffusion and osmosis in water absorption in the roots.

---

WHAT NEXT?

TOP OF PAGE

INDEX of biology notes on transport in plants

INDEX of all my BIOLOGY NOTES

This is a BIG website, so try using the [SEARCH BOX], it maybe quicker than the many indexes!

email doc brown - comments - query?

Basic Science Quizzes for UK KS3 science students aged ~12-14, ~US grades 6-8

Biology * Chemistry * Physics for UK GCSE level students aged ~14-16, ~US grades 9-10

Advanced Level Chemistry for pre-university age ~16-18 ~US grades 11-12, K12 Honors

Find your GCSE/IGCSE science course for more help links to all science revision notes

---

Revision notes on the absorption and necessity of nutrient uptake by roots  based on the syllabus-specifications for students taking IGCSE/GCSE level biology examinations, summary revision notes and key points on the absorption and necessity of nutrient uptake by roots  for students taking the AQA igcse/gcse biology notes on the absorption and necessity of nutrient uptake by roots , Edexcel gcse biology notes on the absorption and necessity of nutrient uptake by roots ,  OCR 21st century GCSE biology notes on the absorption and necessity of nutrient uptake by roots , OCR gateway GCSE biology notes on the absorption and necessity of nutrient uptake by roots , WJEC gcse biology notes on the absorption and necessity of nutrient uptake by roots , CCEA gcse biology notes on the absorption and necessity of nutrient uptake by roots  for students taking CIE Cambridge igcse biology, exam revision notes on the absorption and necessity of nutrient uptake by roots , useful for US grade 9-10 biology courses, importance of explaining the structure & function of root cells & hairs in GCSE level biology, What you need to know about explaining the structure & function of root cells & hairs for GCSE level biology, Explaining the use of explaining the structure & function of root cells & hairs knowledge in GCSE level biology, Examples of explaining the structure & function of root cells & hairs explained when studying GCSE level biology, What is significant about explaining the structure & function of root cells & hairs, describing the theory of explaining the structure & function of root cells & hairs when studying GCSE level biology, revision notes for explaining the structure & function of root cells & hairs in exams, online exam help for explaining the structure & function of root cells & hairs, revision notes about explaining the structure & function of root cells & hairs, what do I need to learn about explaining the structure & function of root cells & hairs for by GCSE biology exam? help to understand the explaining the structure & function of root cells & hairs topic in preparation for GCSE biology exam question, how to prepare for questions involving explaining the structure & function of root cells & hairs in a GCSE biology examination?

---