Phloem Transport: Flow from Source to Sink Questions in English

(B) The term $Phloem$ was coined by $Nageli$.
Its primary function is the translocation of organic food materials (carbohydrates) from the leaves to the stem and roots in a downward direction.

(D) The correct answer is $D$.
When a strip of bark is removed from around the tree trunk,a process known as girdling occurs.
This process removes the phloem,which is responsible for the translocation of food (sucrose) from the leaves to the roots.
While the xylem remains intact initially,the roots are deprived of the food supply produced by photosynthesis.
As the roots cannot replenish their food reserves,they eventually die,which leads to the death of the entire tree.

(A) In plants,food synthesized during photosynthesis (primarily glucose) is converted into sucrose for transport through the phloem.
Sucrose is a non-reducing sugar,which makes it chemically stable and less reactive during transport compared to glucose.
Therefore,sucrose is the primary form of sugar transported from source to sink in plants.

(B) When a ring of bark is removed from a stem (a process known as girdling),the tissues external to the xylem are removed.
This includes the periderm (cork and cork cambium),cortex,and the phloem.
Since the phloem is responsible for the translocation of food (sucrose) from leaves to other parts of the plant,its removal disrupts the downward transport of nutrients.
Therefore,the vascular tissue removed is the phloem.

(B) Xylem sap primarily consists of water and dissolved minerals absorbed from the soil,which results in a relatively low solute concentration.
In contrast,phloem sap is rich in organic solutes,primarily sucrose,which are transported from source to sink.
Therefore,the concentration of solutes in xylem sap is significantly lower than that in phloem sap.

(D) Plants transport carbohydrates from source (leaves) to sink (storage organs like potato tubers) in the form of sucrose through the phloem.
Once sucrose reaches the potato tuber,it is converted into starch,which is an insoluble polysaccharide.
This polymerization of sucrose into starch is advantageous because it creates an osmotic gradient that allows more sucrose to flow into the tuber,and it serves as an efficient,compact form of energy storage.

(A) Swanson and Whitney $(1953)$ reported that the translocation of organic solutes out of the leaf is highly sensitive to temperature.
They observed that at low temperatures, the rate of translocation is significantly reduced or inhibited.
The optimum temperature range for the translocation process is between $20^{\circ}C$ and $30^{\circ}C$.

(D) $Hartt$ and his coworkers $(1964)$ proposed that the movement of assimilates from a leaf depends upon radiant energy.
This study demonstrated that light intensity influences the rate of translocation of organic solutes in plants.

(A) The bimodal theory of translocation,which suggests that phloem transport involves both active and passive processes,was proposed by $D.S. Fenson$ in $1957$. This theory attempts to explain the movement of organic solutes through the sieve tubes.

(C) The concept of active mass flow, which suggests that the translocation of solutes in the phloem requires metabolic energy (in the form of $ATP$ derived from oxygen respiration), was proposed by $Mason$ and $Phillis$ in $1936$. They demonstrated that the movement of organic solutes is an active process dependent on the metabolic activity of the sieve tube elements.

(B) Marshall and Wardlaw proposed the $ \text{Volume flow} $ theory to explain the translocation of organic solutes in plants.
This theory suggests that the movement of sap occurs due to the pressure gradient created by the difference in osmotic pressure between the source and the sink.

(A) The pressure flow hypothesis,also known as the mass flow hypothesis,was proposed by $Munch$ in $1930$.
To demonstrate this,he used an apparatus consisting of two osmometers (bulbs of semipermeable membrane) connected by a tube.
One bulb contained a concentrated sugar solution (representing the source),and the other contained a dilute sugar solution (representing the sink).
When placed in water,water enters the concentrated bulb due to osmosis,creating turgor pressure that pushes the solution towards the dilute bulb,simulating the translocation of organic solutes in the phloem.

(B) According to $Munch$,the translocation of solutes occurs in mass along with cell sap through the sieve tubes from a region of higher turgor pressure to a region of lower turgor pressure (i.e.,along a turgor pressure gradient). This is known as the $Pressure-Flow$ or $Mass-Flow$ hypothesis.

(D) The mass flow hypothesis,also known as the pressure-flow hypothesis,was first proposed by Ernst $M$ünch in $1930$.
This hypothesis explains the mechanism of translocation of sugars (sucrose) from the source (leaves) to the sink (roots,fruits,etc.) through the phloem.
According to this theory,a pressure gradient is created due to the osmotic movement of water into the phloem,which drives the bulk flow of sap.

(D) Curtis proposed that the upward and downward movement of materials (specifically food) takes place through the phloem.
This conclusion was drawn based on his ringing experiments,which demonstrated that the translocation of organic solutes occurs in both directions within the phloem tissue.

(D) The phloem sap is primarily composed of water and sucrose,but it also contains other sugars,hormones,and amino acids.
Chemical analysis of phloem sap reveals that it contains a high concentration of carbohydrates (mainly sucrose) and various organic nitrogenous compounds (such as amino acids and amides).
Therefore,both carbohydrates and organic nitrogenous compounds are present in significant amounts in the phloem.

(A) The translocation of minerals and organic nutrients in plants occurs through the $Phloem$.
Unlike $Xylem$,which primarily transports water and minerals in an upward (unidirectional) direction from roots to leaves,$Phloem$ is responsible for the bidirectional transport of food (sucrose) and minerals from the source (leaves) to the sink (roots,fruits,or growing regions).
Therefore,the correct answer is $Phloem$.

(B) The transverse walls of sieve tube elements are perforated by numerous pores,which are known as sieve pores.
These pores allow the continuity of the cytoplasm between adjacent sieve tube elements,facilitating the translocation of sugars and other organic nutrients from one cell to another.

(D) The cytoplasmic flow hypothesis,also known as the protoplasmic streaming hypothesis,was supported by Curtis in $1935$.
This hypothesis suggests that the movement of food materials (solutes) through the sieve tube elements of the phloem occurs due to the streaming movement of the cytoplasm (protoplasmic streaming) within these cells.
Therefore,the correct option is $D$.

(B) Thaine $(1964)$ proposed the theory of transcellular streaming.
He defined transcellular streaming as the movement of particulate and fluid constituents of the cytoplasm through linear files of longitudinally oriented plant cells.
This theory suggests that cytoplasmic strands can move through the sieve pores of sieve elements,facilitating the transport of substances.

(A) Fensom and Williams $(1974)$ proposed the contractile protein hypothesis for phloem transport.
They observed a network of interlinked microfibrils in the sieve tube elements.
These microfibrils were composed of contractile threads of $P$-protein,which are believed to facilitate the movement of sap through the sieve tubes via rhythmic contractions.

(D) The pressure flow hypothesis,also known as the Munch hypothesis,explains the translocation of sugars from source to sink in plants.
It suggests that the movement of phloem sap occurs due to a pressure gradient.
This mechanism is often described as a $Volume$ $flow$ or $Solution$ $flow$ because the entire sap (containing water and dissolved solutes like sucrose) moves together as a bulk flow.
Since this process is driven by osmotic pressure gradients created by active loading and unloading of sugars,it is also referred to as an $Active$ $solution$ $flow$.
Therefore,all these terms are used to describe the improvements or characteristics proposed in the pressure flow mechanism.

(D) The transport of organic solutes (food) in plants occurs through the phloem.
This movement is bidirectional and multidirectional.
$1$. Upward flow: From storage organs like seeds to developing leaves or from leaves to the shoot apex.
$2$. Downward flow: From mature leaves (source) to roots (sink).
$3$. Lateral flow: From the phloem to the cortex and epidermis.
Therefore,all the mentioned directions are possible.

(B) The Munch hypothesis,also known as the Pressure-Flow Hypothesis,explains the translocation of sugars in plants.
According to this theory,the flow of soluble substances (sucrose) occurs due to a pressure gradient.
Photosynthesis in the leaves creates a high concentration of sucrose,which leads to high osmotic pressure and high turgor pressure.
In contrast,the sink (roots or storage organs) has a lower concentration of sucrose,leading to lower turgor pressure.
This difference in turgor pressure between the source and the sink causes the mass flow of sap through the phloem.

(C) Girdling or ringing involves the removal of a ring of bark (including phloem) from the stem of a woody plant.
Since the phloem is responsible for the translocation of food (sucrose) from the leaves (source) to the roots (sink),removing this ring blocks the downward movement of food.
As a result,the synthesized food accumulates in the parts of the plant above the cut,such as the fruits and branches,leading to their enhanced growth.
Therefore,the correct option is $C$.

(B) The translocation of sucrose (a form of sugar) from the source (leaves) to the sink (roots,fruits,or storage organs) is primarily carried out by the phloem tissue.
Phloem consists of sieve tube elements and companion cells that facilitate the transport of organic solutes.
In contrast,xylem is primarily responsible for the conduction of water and dissolved minerals from the roots to the aerial parts of the plant.
Therefore,phloem is the correct tissue for the translocation of sucrose.

(B) The $Munch$ theory,also known as the $Pressure$ $Flow$ $Hypothesis$,is the most widely accepted theory to explain the translocation of organic solutes (sugars) in plants.
According to this theory,the movement of food occurs from the source (where sugar is produced) to the sink (where sugar is consumed or stored) due to a pressure gradient created by osmotic differences.
This mechanism facilitates the long-distance transport of solutes through the phloem sieve tubes.

(D) The $Pressure \ flow \ hypothesis$ (also known as the $Munch \ hypothesis$ or $Mass \ flow \ hypothesis$) is the most widely accepted theory to explain the translocation of organic solutes (sugars) from the source (leaves) to the sink (roots,fruits,or storage organs) over long distances in plants.
According to this theory,glucose produced in the leaves is converted into sucrose,which is then loaded into the phloem sieve tube cells. This creates a high osmotic pressure,causing water to enter from the adjacent xylem,resulting in a high turgor pressure. The sap then moves towards the sink,where the pressure is lower,facilitating the bulk flow of nutrients.

(A) The protoplasmic streaming theory, proposed by $de \text{ } Vries$, suggests that the movement of solutes occurs through the streaming movement of the cytoplasm within the sieve tube elements.
This cytoplasmic streaming facilitates the transport of sugars from one sieve tube element to the next through the sieve pores.
However, it is important to note that the currently accepted mechanism for sugar translocation in plants is the $Munch$ Pressure Flow Hypothesis, which relies on active transport and osmosis.

(D) The hypothesis regarding transcellular strands in the context of phloem transport was proposed by $R. Thaine$.
He suggested that these strands are composed of proteinaceous material and are involved in the translocation of solutes through the sieve elements.

(A) The electro-osmotic hypothesis was proposed independently by $Fensom$ $(1957)$ and $Spanner$ $(1958)$.
According to this hypothesis,the translocation of solutes through the phloem occurs due to the flow of water driven by an electrical gradient.
Specifically,the movement of $K^+$ ions creates an electrical potential difference,and water moves along with these ions,carrying the solutes in the positive direction of the electrical gradient.

(C) The correct answer is $C$.
Protoplasmic streaming theory suggests that the translocation of food materials in two directions (bidirectional) occurs due to the streaming movement of the protoplasm within the sieve tube elements.
This movement allows for the simultaneous transport of solutes to different parts of the plant.

(B) The concept of translocation of solutes in plants, specifically the Pressure Flow Hypothesis (also known as the Mass Flow Hypothesis), was proposed by $Ernst$ $Munch$ in $1930$. This hypothesis suggests that solutes move from the source (where they are produced) to the sink (where they are utilized or stored) through the phloem, driven by a pressure gradient, which is analogous to the circulatory system in animals.

(C) The Munch theory (Pressure Flow Hypothesis) suggests that translocation is a passive physical process driven by turgor pressure gradients. However,a major criticism is that it fails to account for the active transport components. Specifically,Ziegler $(1956)$ pointed out that the movement of sugars from mesophyll cells into the sieve tubes often occurs against a concentration gradient,which requires metabolic energy and cannot be explained solely by physical diffusion.

(B) The correct answer is $B$. The transport of sugar in plants occurs through the phloem via the pressure-flow hypothesis. $A$ high concentration of potassium ions $(K^+)$ is maintained in the sieve tube solution. $A$ current of $K^+$ ions passes through the sieve pores by electro-osmosis,and sugar and water molecules that are adhered tightly to $K^+$ ions are carried along with them through the phloem.

(B) The translocation of sugar (phloem transport) is an energy-dependent process that requires $ATP$.
Photosynthesis is the process of synthesizing food (sugars) using light energy.
While light is essential for photosynthesis to produce sugars,the translocation of these sugars from the source to the sink requires significantly less light intensity compared to the light intensity required for the optimal rate of photosynthesis.
Therefore,the light intensity required for translocation is less than that required for photosynthesis.

(D) Translocation refers to the movement of organic solutes (primarily sucrose) from the source (leaves) to the sink (roots,fruits,or growing regions) through the phloem.
$(1)$ Light: It influences the rate of photosynthesis,which determines the amount of sucrose produced at the source.
$(2)$ Oxygen: Phloem loading and unloading are active processes that require $ATP$. Oxygen is essential for cellular respiration to produce $ATP$.
$(3)$ Metabolic control: The metabolic activity of the source and sink cells regulates the pressure gradient required for the mass flow of sap.
Since all these factors influence the process,the correct answer is $D$.

(B) The aphid stylet technique is used to collect phloem sap from the sieve tubes of plants.
Since phloem sap primarily consists of sucrose and other organic solutes transported from source to sink,the analysis of this sap helps in understanding the path and composition of the flow of organic solutes in plants.

(A) In the study of phloem transport,$H. Moose$ $(1975)$ analyzed the composition of phloem sap (exudates) using the aphid stylet technique. He specifically worked with the plant species $Robinia$ $pseudoacacia$ (Black Locust) to determine the concentration of sugars and other solutes transported through the sieve tubes.

(D) The role of phloem in the translocation of food (sucrose) can be demonstrated using several experimental techniques:
$1$. Tracer technique: Using radioactive isotopes like $^{14}C$ to track the movement of photosynthates.
$2$. Sap analysis technique: Analyzing the composition of phloem sap obtained through aphid stylets or incisions.
$3$. Chemical analysis: Examining the chemical composition of the phloem contents to confirm the presence of sugars.
Therefore,all the mentioned techniques are used to prove that phloem is the element responsible for food translocation.

(C) Vein loading (or phloem loading) is the process by which sugars (sucrose) are transported from the mesophyll cells into the sieve tube elements of the phloem.
$1$. Sucrose is actively transported into the companion cells and then into the sieve tube elements.
$2$. This high concentration of sugar in the phloem creates a hypertonic environment,causing water to move from the adjacent xylem vessels into the phloem by osmosis.
$3$. Therefore,both the movement of food into the sieve tubes and the subsequent movement of water from the xylem are integral parts of the pressure-flow hypothesis mechanism.

(C) The translocation of food occurs from organs where food is in high concentration (source),such as leaves,tubers,or rhizomes,to organs where it is in low concentration (sink or consumption end).
Roots act as a primary consumption end because they require nutrients for growth and metabolic activities but do not perform photosynthesis to produce their own food.
Therefore,the correct answer is $C$.

(B) The correct answer is $B$.
In plants,the vascular tissue responsible for the translocation of food (sucrose) from the source (leaves) to the sink (roots,fruits,and other storage organs) is the $Phloem$.
$Xylem$ is responsible for the transport of water and minerals,while $Root$ $hairs$ are involved in water absorption and $Stomata$ are involved in gaseous exchange.

(B) The translocation of organic substances like sugars through the phloem can be demonstrated by the process of 'ringing' or 'girdling' the stem.
In this experiment,a ring of bark (including the phloem) is removed from the stem of a plant,while the xylem is left intact.
Over time,it is observed that the portion of the stem above the ring swells due to the accumulation of organic solutes (sugars) that cannot move downwards because the phloem has been removed.
This confirms that the phloem is the tissue responsible for the translocation of organic food materials.

(C) In flowering plants,the products of photosynthesis are transported from the source (leaves) to the sink (roots,fruits,or storage organs) through the phloem tissue.
This process is known as translocation.
Although glucose is the primary product of photosynthesis,it is converted into a non-reducing sugar,$Sucrose$,for transport.
$Sucrose$ is chemically stable,less reactive,and highly soluble,making it the ideal form for long-distance transport through the phloem sieve tubes.
Therefore,the correct answer is $Sucrose$.

(C) The correct answer is $C$. In plants,carbohydrates are synthesized in the leaves (source) and transported to storage organs like fruits (sink) in the form of sucrose,which is a soluble sugar. This transport occurs through the phloem tissue. Once the sugar solution reaches the fruit,it is converted into starch for storage. As the fruit matures,this starch is later converted back into sucrose,which gives the ripened banana its sweet taste.

(B) Plants produce glucose during photosynthesis in the leaves. Since glucose is soluble and reactive,it is converted into sucrose for transport through the phloem. Sucrose is translocated from the leaves to the sink organs,such as potato tubers. Once it reaches the tuber,the sucrose is converted back into starch,which is an insoluble storage polysaccharide,allowing for the accumulation of large quantities of energy reserves without affecting the osmotic potential of the cell.

(C) Plants synthesize carbohydrates in the form of glucose during photosynthesis. However,glucose is highly reactive and not suitable for long-distance transport. Therefore,it is converted into a non-reducing sugar,$Sucrose$,which is chemically stable and less reactive. $Sucrose$ is then loaded into the sieve tube elements of the phloem and transported to various sink organs of the plant.

(A) Girdling is the process of removing a ring of bark,which includes the $phloem$,from the stem of a plant.
Since the $phloem$ is responsible for the translocation of food (sucrose) from the leaves (source) to the roots (sink),removing it prevents the roots from receiving essential nutrients.
As a result,the roots are starved of energy and die first,eventually leading to the death of the entire plant.