BIOLOGY NOTES ADVANCED LEVEL (FORM 6)

TRANSPORTATION OF MATERIALS IN LIVING ORGANISM

BIOLOGY NOTES ADVANCED LEVEL (FORM 6): Cover six topics TRANSPORTATION ,GROWTH AND DEVELOPMENT , REPRODUCTION, GENETICS, EVOLUTION & ECOLOGY.

Transportation is the act or process of moving molecules, ions, or substrates across a biological membrane, such as the plasma membrane.

Plants have two transport systems – xylem and phloem . Xylem transports water and minerals. Phloem transports sugars and amino acids dissolved in water.

Why is there a need of special transport system?

Answers:

Over short distances, means of transport are rapid and efficient ie: osmosis, diffusion, endocytosis etc. But in multicellular organisms which have a large surface area to volume ratio; these means are sufficient as cells may be too widely separated from each other. Therefore for these practices to be adequate, specialised long distance transport systems are necessary.

Materials are generally moved by a mass flow system, being the bulk transport of materials from one point to another as a result of pressure difference between the two points.

Example of flow systems in plants and animals are;-

In plants- Vascular system.
In animals – Alimentary canal, respiratory system, etc.

Definition:

A vascular system is one which has tubes which are full of fluid being transported from one place to another.

In animals the blood system is a vascular system.

In plants the xylem and phloem form vascular systems.

Definitions:

1. Osmosis: This is a movement of water molecules from a region of higher water potential or lower solute potential to a region of lower water potential or higher solute potential through a deferentially permeable membrane.

2. Diffusion: This is a movement of materials from a region of higher concentration to a region of lower concentration.

3. Active transport: This is a transportation of materials against concentration gradient. Due to this, the process involves the consumption of energy. Any part of the body where active transport occurs is characterized by;

a) Presence of numerous mitochondria.

b) High rate of metabolism.

c) High concentration of ATP.

Since active transport involves the use of energy, the materials transported actively move faster than those transported passively.

Significance of transportation system

The system of transportation of materials is important for:-

1. Distribution of food materials in the body.

2. Carriage of excretory wastes from their sites of synthesis.

3. Carriage of hormones from their respective glands to their target organs.

4. Distribution of antibodies.

5. Carriage of respiratory gases.

TOPIC 1

TRANSPORTATION

TOPIC 2

GROWTH AND DEVELOPMENT

TOPIC 3

REPRODUCTION

TOPIC 4

GENETICS

TOPIC 5

EVOLUTION

TOPIC 6

ECOLOGY

(I) TRANSPORT IN PLANTS

The movement of substances through the conducting or vascular tissues of plants is called Translocation.

Important application of the study of Translocation:

It is useful to know how herbicides, fungicides, growth regulators and nutrients enter plants and the routes that they take through plants, in order to know how best to apply them and to judge possible effects that they might have.
Plant pathogens are sometimes translocated, and such knowledge could influence treatment or preventive measures.

Terms used:

(i) Water potential, symbol Ψ, Greek latter psi.

The term is used to describe water movement through membranes. It can be described as the tendency of water molecules to move from one place to another. The higher (less negative) the water potential, the greater tendency to leave a system.

Factors affecting water potential of plant cells are:-

Solute concentration and
Pressure generated when water enters and inflates plant cells.

They are expressed in terms of Solute and Pressure Potentials respectively.

NOTE

Pure water has maximum water potential (zero).
Water always moves from a region of higher Ψ to a region of lower Ψ.
All solutions have lower Ψ than pure water, therefore negative values of Ψ

(ii) Solute potential, Ψs

The effect of dissolving solute molecules in pure water is to reduce the concentration of water molecules and hence to lower the water potential.
Solute potential is a measure of the change in water potential of a system due to the presence of solute molecules.

(iii) Pressure potential, Ψp

If pressure is applied to pure water or a solution, its water potential increases. This is because the pressure tends to force water from one place to another.

Ψ = Ψ_s + Ψ_p

(iv) Plasmolysis and Turgidity

If a plant cell is in contact with a solution of lower water potential than its own contents, then water leaves the cell by osmosis through the cell surface membrane. Consequently, the protoplast shrinks and eventually pulls away from the cell wall. The process is called plasmolysis and the cell is said to be plasmolysed.

The point at which plasmolysis is just to happen is called Incipient plasmolysis. At this point, the protoplast has just ceased to exert any pressure against the cell wall, so the cell is Flaccid. Water will continue to leave the protoplast until its contents have the same Ψ as the external solution. No further shrinkage then occurs.

If a plasmolysed cell is placed in pure water or a solution of lower solute potential or higher water potential than the contents of the cell, water enters the cell by osmosis. As the volume of protoplast increases, it begins to exert pressure against the cell wall and stretches it.

The pressure inside the cell rises rapidly, the pressure is called the Ψ_p. As the Ψ_p of the cell increases due to water entering by osmosis, the cell becomes turgid.

Animal cells have no cell wall and the cell surface membrane is too delicate to prevent the cell expanding and bursting in a solution of higher Ψ. They are therefore protected by Osmoregulation.

Question.

What occupies a space between the cell wall and the shrunken protoplasts in plasmolysed cells?
What is the Ψ_p of a flaccid cell?

Answer;

The external solution, since the cell wall is freely permeable to solutions.
Zero. The protoplast is not exerting pressure against the cell wall.

In higher plants, the materials are transported by the vascular tissues. Which are of two types:-

The xylem and
The phloem.

(i) The xylem tissue

This is a plant vascular tissue which is mainly concerned with transportation of water and dissolved mineral salts through the plant.

Structure of the xylem

The histology of the xylem tissue reveals the presence of four types of cells.

Note: The only conducting cells are the vessels and tracheids.

1. TRACHEIDS

Structural features:

-They are more or less elongated cells with tapering ends.

-They have secondary thickened or lignified walls with a variety of pits (simple or bordered).

-They are not perforated.

-They are dead at maturity ie: they lose all the protoplasmic contents leaving an empty lumen.

NOTE:

Tracheids are present in all vascular plants, but in the coniferophytes they are only xylem conducting cells

Diagram

2. VESSEL MEMBERS

– These are perforated elements that aggregate into files of cells connected to one another by means of perforations.

-The vessel members are more specialised than the tracheids and they are characterized by the following features:-

They have secondary thickened wall.
They are dead at maturity.
They are shorter and wider.
They have bordered pits along their sides.
They have perforated plates.

Diagram:

BIOLOGY NOTES ADVANCED LEVEL (FORM 6) - ALL TOPICS

Role of vessels and tracheids

The vessels and tracheids conduct water and dissolved mineral salts through the plant ie: from the roots to the shoots.

Adaptations of the Xylem (vessels and tracheids) to transport

Both have long cells joined end to end. This allows the flow of water and dissolved mineral salts in a continuous column.
The end walls of the xylem vessels have been broken down forming uninterrupted flow of water from the roots to the leaves. Even in the tracheids where the end walls are present, larger bordered pits reduce the resistance of flow due to the presence of end walls. Absence of end walls in the vessels and presence of bordered pits in tracheids facilitate easy flow of water as resistance to flow is reduced.
There are pits at particular parts in the lignified walls. These allow lateral movement of water and mineral salts where this is necessary.
Narrowness of the lumen of vessels and tracheids increases the capillarity force.
The walls are lignified (for strength) making them especially rigid to prevent them from collapsing due to large tension force set up by the transpiration pull.
Impregnation of the walls with lignin material increases the adhesion of water molecules which helps the water to rise up the plant by capillarity.
Loss of protoplast in the vessels and tracheids leaves an empty lumen which forms a continuous tube as one cell rests on top of the other.
Since the conducting cells of the xylem tissues are dead, their materials are transported through them passively and this minimizes energy consumption.

Question: Describe the histology of the xylem conducting cells and show how they structurally relate to their function.

3. THE XYLEM FIBRES

-These are elongated, slender, thick walled and non conducting cells.

-They are thought to have been evolved together with the tracheids and they function as supporting elements.

-They also facilitate lateral movement of materials and they sometimes store food.

4. THE XYLEM PARENCHYMA

-These consist of simple undifferentiated living cells.

-They have lignified pitted walls and they are frequently arranged in radial sheets to form rays.

-They function as pathways for lateral movement of materials and they sometime store food.

(ii) The phloem
tissue

It is chiefly concerned with translocation of photosynthetically manufactured food from the autotropic parts (sources) to the heterotropic parts (sinks) of the plant.

Structure of phloem

The histology of the phloem tissue reveals the presence of the following structural cells:-

Sieve element.
Companion cells.
Phloem parenchyma.
Phloem fibres and sclereids

1. SIEVE ELEMENTS

-These include the sieve cells and sieve tube members.

2. COMPANION CELLS

These are associated with the sieve tube members only in the Angiospermophytes. They are highly specialized parenchyma cells.
They arise from the same meristematic initial with the sieve tube cells.
They contain nucleated dense cytoplasm in communication with the cytoplasm of sieve tube member by means of plasmodesmata in the pitted areas of the thin dividing walls.

3 .PHLOEM PARENCYMA

These contain stored carbohydrates and accumulation of tannins and resins.
The phloem parenchyma is always in communication with the sieve elements and companion cells by the adjacent sieve areas.

4. PHLOEM FIBRES AND SCLEREIDS

These occur in both primary and secondary phloems.
The walls may be lignified or not but generally pitted with simple or bordered pits thus facilitating lateral movements of food substances.

Phloem:

Summary:
1. The sieve tube elements when mature do not have the nucleus, no ribosomes, no golgi bodies, no tonoplast. There are no mitochondria and there is a very little peripheral cytoplasm. However, the cells remain living since they

are connected to the companion cells

2. The companion cells have a dense cytoplasm with a nucleus, mitochondria and ribosome. The cells are very metabolically active.

3. The sieve tube members and the companion cells are in communication with one another by means of a large plasmodesmata… Click here to read more