• Active transport is a mode of transportation in plants, which uses stored energy to move the particles against the concentration gradient. In a plant cell, it takes place in the root cells by absorbing water and minerals. • The intake of mineral ions by the root system along the concentration gradient by diffusion without the expenditure of energy is called passive absorption.

1. ACTIVE & PASSIVE TRANSPORT OF IONS

2. TRANSPORT
Transportation is the process of movement or distribution of different materials from one place to another.
Conducting tissues – Xylem and Phloem play an important role.
 Based on the distance travelled by sap or solute they are classified as
- Short distance (cell to cell transport)
- Long distance transport
 Based on energy expenditure during transport, they are classified as
- Passive transport
- Active transport

3. TRANSPORT BASED ON ENERGY EXPENDITURE
 PASSIVE TRANSPORT
 Biological process of movements of ions
within the cell across cell membrane along
the concentration gradient without any
external energy.
 It is natural phenomenon. It is downhill
process.
 It includes diffusion, facilitated diffusion,
imbibition, filtration and osmosis.
 ACTIVE TRANSPORT
 Biological process involves movement of
molecules from a region of lower
concentration to a region of higher
concentration against a concentration
gradient with the use of external energy.
 It is an uphill process. Energy obtained
from respiration.
 It includes primary active transport and
secondary active transport.

4. PASSIVE TRANSPORT
DIFFUSION
 The movement of particles or molecules from a region
higher concentration to a region of lower concentration
along a concentration gradient.
 It is a spontaneous dispersion of solutes in the dispersion
phase.
 In diffusion, the movement of molecules is continuous and
random in order in all directions.
 Ex: If a drop of ink is poured into a glass of water, the ink
particles diffuse in the water gradually. Simple Diffusion

5. FACILIATED DIFFUSION
 It is passive transportation of ions or molecules across the cell
membrane through specific transmembrane integral proteins.
 The molecules, which are large and insoluble require a carrier
substance for their transportation through the plasma membrane.
 The cell membrane is permeable only to a few molecules that are
smaller in size and non-polar.
 Transport proteins – Channel proteins – channel protein forms a
channel in the cell membrane for easy passage of molecules to
enter the cell. They may open up for specific molecules.
 Porin – large transporter protein found in the outer membrane of
plastids which facilitate small molecules to pass through the
membrane.
 Aquaporin – water channel protein embedded in the plasma
membrane. It regulates the massive amount of water transport
across the membrane.

6. FILTRATION
 In this process, the cell membrane permits only those substances which
are soluble and could easily pass through its pores. There is selective
absorption of nutrients.
 This process does not require any energy and takes place along the
concentration gradient.
OSMOSIS
 The movement of water or solvent molecule from the solution of lower
concentration to the solution of higher concentration to the place of its
lower concentration through a semi – permeable membrane.
 Endosmosis – Movement of water into the cell
 Exosmosis – Movement of water out of the cell
 There are three types of osmotic solution - Hypertonic solution – highly
concentrated solution.
 Hypotonic solution – less concentrated solution
 Isotonic solution – when the two solutions are equally concentrated Process of osmosis

7. IMBIBITION
 The absorption of water by the solid particles of an adsorbent
without forming a solution is called Imbibition.
 Solid substance or adsorbent which take part in imbibition are
called Imbibants. The liquid which is imbibed is known as Imbibate.
 Imbibition of water of increases the volume of the imbibant due to
which pressure is created known as Imbibitional Pressure (IP).
SIGNIFICANCE OF IMBIBITION
 The germination of seeds begins with imbibition.
 Breaking of seed coat.
 Imbibition of water by cell walls of root hairs results absorption of
water.
 It entrain the genetic clock that controls circadian rhythm in
Arabidopsis and other plants.

8. ACTIVE TRANSPORT
 Active transport is a mode of transportation in plants, which uses stored energy to move the particles
against the concentration gradient. In a plant cell, it takes place in the root cells by absorbing water and
minerals.
 Active transport always leads to accumulation of molecules are ions towards one side of the membrane.
 This mode of transportation in plants is carried out by membrane proteins and transports the substance
from the lower concentration to higher concentration.
Active Transport

9. PRIMARY ACTIVE TRANSPORT
 One type of active transport channel in the cell membrane will bind to
the molecule it is supposed to transport – such as a sodium ion – and
hold onto it until a molecule of ATP comes along and binds to the
protein.
 The energy stored in ATP then allows the channel to change shape,
spitting the sodium ion out on the opposite side of the cell
membrane.
 This type of active transport directly uses ATP and is called primary
active transport.
SECONDARY ACTIVE TRANSPORT
 In this type of active transport, the protein pump does not use ATP
itself, but the cell must utilize ATP in order to keep it functioning.
 Secondary active transport is a kind of active transport that uses
electrochemical energy.

10. ANTIPORT PUMPS
 Antiport pumps are a type of transmembrane co-transporter protein. They pump
one substance in one direction, while transporting another substance in the
opposite direction.
 These pumps are extremely efficient because many of them can use one ATP
molecule to fuel these two different tasks.
 Sodium – potassium pump is an example of antiport pump.
SYMPORT PUMPS
 Symport pumps take advantage of diffusion gradients to move substances.
Diffusion gradients are differences in concentration that cause substances to
naturally move from areas of high to low concentration.
 Substance that wants to move from an area of high concentration to low
concentration down its concentration gradient is used to carry another substance
against its concentration gradient. Example of symport pump – sodium – glucose
transport protein.

11. ENDOCYTOSIS
 Endocytosis is the process of capturing a substance or
particle from outside the cell by engulfing it with the cell
membrane.
 The membrane folds over the substance and it becomes
completely enclosed by the membrane.
 At this point a membrane-bound sac, or vesicle, pinches off
and moves the substance into the cytosol. There are two
main kinds of endocytosis:
 Phagocytosis or cellular eating, occurs when the dissolved
materials enter the cell. The plasma membrane engulfs
the solid material, forming a phagocytic vesicle.
 Pinocytosis or cellular drinking, occurs when
the plasma membrane folds inward to form a channel
allowing dissolved substances to enter the cell. When the
channel is closed, the liquid is encircled within a pinocytic
vesicle.

12. EXOCYTOSIS
 Exocytosis describes the process of vesicles fusing with the plasma membrane and releasing their contents
to the outside of the cell.
 Exocytosis occurs when a cell produces substances for export, such as a protein, or when the cell is getting
rid of a waste product or a toxin.
 Newly made membrane proteins and membrane lipids are moved on top the plasma membrane by
exocytosis.

13. MECHANISM OF MINERAL ABSORPTION

14. MINERAL ABSORPTION
 Plants absorb minerals from the soil and translocate them to other parts. Minerals are absorbed in the form
of soil solution contained in the pore spaces between the soil particles and the root hair.
 The soil solution contains the mineral salts in the dissolved state.
 Penetration and accumulation of ions into living cells or tissues from surrounding medium by crossing
membrane is called Mineral absorption.
 Movement of ions into and out of cells or tissues is termed as transport or flux.
 Entry of the ion into cell is called influx and exit is called efflux.
 Several theories have been put forth to explain the mechanism of translocation of mineral salts. These
theories can be placed under,
 Passive Absorption
 Active Absorption

15. PASSIVE ABSORPTION OF MINERAL IONS
 The intake of mineral ions by the root system along the concentration gradient by diffusion without the
expenditure of energy is called passive absorption.
 It is a non - selective process. There is a ion exchange. Equilibrium is attained in passive absorption.
Passive absorption is explained by three theories namely,
MASS FLOW
THEORY
ION
EXCHANGE
THEORY
DONNAN
EQUILIBRIUM

16. I. MASS FLOW THEORY
 According to this theory, ions are absorbed by the roots along with the mass flow of water under the
influence of transpiration.
 This theory was proposed by Hylmo in 1953 and was supported by Russel (1960), Kramer (1965) and others.
 Because of this reason, rapidly transpiring plants absorb more minerals from the soil and the rate of mineral
absorption increases with increase in the rate of transpiration. It is a non – selective process.
II. ION - EXCHANGE THEORY
 According to ion exchange theory, root cells intake soil ions for the exchange of ions of same sign and
equivalent charge.
 Ion absorbed to the cell wall or membrane of root surface cells and those present in intercellular spaces are
readily exchanged with ions in the external soil solution.
 During this process, cations are exchanged for cations and anions are exchanged for anions. This is called
Ion exchange mechanism.
 Ion exchange between plant roots and soil solution occurs via contact exchange and carbonic acid
exchange.

17. A. CONTACT EXCHANGE THEORY
 This was put forward by Jenny and Overstreet (1939).
 According to this theory, the ions absorbed on the surface of root cells and clay micelle are not held tightly
but oscillate within a small volume of space called oscillation volume.
 When the oscillation volume of ion absorbed on the root overlaps the oscillation volume of ion absorbed
on the clay, the ion of the clay is taken by the root surface.
 In order to meet the ionic charge, the ion of root surface goes to the clay.

18. B. CARBONIC ACID EXCHANGE THEORY
 According to this theory, soil solution plays an important role by acting as
a medium for ion exchange.
 The CO2 released during respiration of root cells combines with water to
form carbonic acid (H2CO3).
 Carbonic acid dissociates into H+ and HCO3- in the soil solution.
 H+ ions exchange with cation (K+) absorbed on clay micelle and the
anions from micelles get released into soil solution.
 This anion (K+) combines with HCO3- to form K+HCO3. The K+HCO3 is then
exchanged for H+ on the root surface.
 The mineral ions may diffuse across the lipid bilayer of plasma membrane
or through ion transport channels in the membrane. Here, the movement
of ions takes place along the concentration gradient.

19. III. DONNAN EQUILIBRIUM
 Donnan equilibrium is a diffusion equilibrium at which the net charge of inorganic (mineral) ions outside the
cell membrane is equal to the net charge of free mineral ions and fixed ions inside the cell.
 Fixed ions do not move across the membrane but provide some electrical charge to absorb mineral ions from
outside the cell. Electrical charge of fixed ions of Donnan phase is called Donnan potential.
 Inorganic ions present in and outside the cell are freely diffusible across the membrane. In the presence of
fixed ions, some more inorganic ions diffuse into the cell to neutralise the electric potential on both sides of
the membrane.
 If the root cell contains fixed anions, more cations will diffuse into the cell. If there are fixed cations inside the
cell, more anions will diffuse into the cell.

20. ACTIVE ABSORPTION OF MINERAL IONS
 The intake of mineral ions by the root system against concentration gradient with the expenditure of energy
is called active absorption.
 It is a selective process. There is no ion exchange and no equilibrium is attained in active absorption.
 Active absorption may take place through carrier molecules or transporters in the plasma membrane.
CARRIER CONCEPT
 It was proposed by Van den Honert in 1937. According to the concept, ions move through the biological
membrane along with carrier molecules in that membrane.
 Carrier acts as a vehicle which is transmembrane protein that binds with the ion and transports it from
outside to inside of the cell or vice versa.
 As the carrier has sites to bind with specific ions, it binds with ion to form Carrier - ion complex at the outer
surface of the membrane.

21.  Due to association with molecules to be transported, structure of
carrier protein gets modified until dissociation of the molecule.
 This complex moves across the membrane and releases the ion on
the inner side of the membrane.
 On releasing the ion, the carrier regains its original conformation
and goes back to the outer surface to pick up another ion.
 There are 3 types of carrier proteins classified on basis of handling
of molecules and direction of transport.
 UNIPORT – In this molecule of single type move across a
membrane independent of other molecules in one direction.
 SYMPORT OR CO - TRANSPORT – Symport denote an integral
membrane protein that simultaneously transports two types of
molecules across the membrane in the same direction.
 ANTIPORT OR COUNTER TRANSPORT – Antiport is an integral
membrane transport protein that simultaneously transports two
different molecules in opposite directions, across the membrane.

22. CYTOCHROME PUMP THEORY / ELECTRON TRANSPORT THEORY
 According to this theory, the anions of minerals are carried into the cell by cytochrome present in the
plasma membrane by active process while cations are carried into the cell passively.
 Lundegardh and Burstrom (1933) observed a correlation between respiration and anion absorption.
When a plant is transferred from water to a salt solution the rate of respiration increases which is called
as anion respiration or salt respiration.
 Cytochrome pump theory based on his observations on salt respiration. It is also known as anion
respiration theory.
This theory explains the absorption of minerals as follows:
 On the outer surface of plasma membrane, the iron of cytochrome is oxidized by oxygen, releasing an
electron ( e− ). The oxidized iron takes an anion.
 The electron thus released reduces a proton (H+ ) which in turn combines with oxygen to form water. This
water moves to the outer surface of the membrane.

23.  The anion travels over the cytochrome chain towards inside
via redox reactions in the cytochrome. Thus the Fe2+ (ferrous
ion) of the cytochrome becomes reduced to ferric ion (Fe3+ )
and vice versa.
 Dehydrogenase reaction occurs on the inner side of the
membrane and gives rise to a proton (H+ ) and an electron
( e− ).
 The oxidized cytochrome becomes reduced by accepting the
electron produced through the dehydrogenase reactions and
the anion is released in the cytoplasm.
 As a result of anion pumping across the membrane, anion
concentration increases in the cytoplasm. So that cations (M+
) move passively from outside to inside to balance the ionic
concentration.

24. PROTEIN - LECITHIN THEORY
 This theory was proposed by Bennet clark in 1956.
 According to this theory, anions and cations in the soil solution are carried into the cell by a carrier
molecule called lecithin present in the plasma membrane.
 Lecithin is a phospholipid formed of a phosphatidic acid and choline.
 The Phosphatidic acid tends to combine with a cation while choline tends to combine with an anion.
 Thus lecithin picks up both cations and anions at a time from the soil solution. It involves the following
steps:
 At the outer surface of the membrane , lecithin accepts an anion and cation to form a lecithin ion
complex.
 The lecithin ion complex moves to the inner side of plasma membrane because of the fluidity of the lipid
bilayer.

25.  At the inner surface of the membrane , the enzyme lecithinase
acts on the lecithin ion complex and splits it into phosphatidic
acid and choline.
 The ions are released free in the cytoplasm.
 After releasing the ions, the choline reacts with Acetyl co - A to
form Acetylcholine.
 During this reaction, one ATP is hydrolysed into ADP and
inorganic phosphate.
 The acetylcholine combines with a phosphatidic acid to
regenerate lecithin molecule at the outer surface of the
membrane.
 Now, lecithin is ready to carry another set of ions from the
external solution.

26. CONCLUSION
 Ion transport is extremely important in the vital activity of all organisms.
 Ion transporters are essential for proper cell function.
 A cell continually exchanges molecules and ions with its environment.
 When a cell transports a substance against its concentration gradient, the cell must expend energy.
 It permits the maintenance of optimum concentrations of K+, Na+, H+, Ca2+, and other ions, concentrations
that usually differ from those in the surrounding media.
 The gradient in the concentrations of K+, Na+, and Ca2+ ions inside and outside the cell is the basis
for the transmission of excitation in organisms
 Gradients of H+ ions on the membranes of mitochondrion and chloroplast organelles provide a supply of e
nergy in the cell during biological oxidation and photosynthesis.