Plant Physiology: Transpiration, Transport and Respiration
Notes for competitive exams on water absorption, ascent of sap, transpiration, stomata, phloem translocation and the stages of respiration in plants.
By GK24 Editorial Team· Published · 4 min read
A plant has no heart, yet water travels from a root hair in the soil to a leaf a hundred feet above the ground, and sugar made in that leaf reaches a growing root tip the same day. This chapter explains how: absorption of water by roots, the rise of sap through the xylem, the loss of water vapour called transpiration, the movement of food through the phloem, and respiration, by which the plant releases the energy stored in that food. Examiners ask it as a set of one line facts, so learn the tissue, the organelle, the theory and the name attached to each process.
Absorption of water and the ascent of sap
Water enters the root hairs by osmosis, the movement of water across a semipermeable membrane from a dilute solution to a concentrated one. Minerals enter mostly by active transport, which spends energy. The water then crosses the cortex and reaches the xylem, from where it must climb. Three explanations are offered. Root pressure, a positive push developed in the roots, can raise water only a few metres. Capillarity in the narrow xylem tubes also accounts for a short rise. The accepted explanation is the cohesion tension theory of Dixon and Joly: water molecules hold together strongly by cohesion, stick to the xylem walls by adhesion, and form an unbroken column, which is pulled upward by the suction created when water evaporates from the leaf. This suction is the transpiration pull, and it is strong enough to raise sap to the top of the tallest tree. Xylem carries water and minerals in one direction only, upward, while the phloem carries food both up and down.
Transpiration
Transpiration is the loss of water as vapour from the aerial parts of a plant. About nine tenths of it takes place through the stomata, the rest through the cuticle and, in woody stems, through lenticels. A stoma is bounded by two kidney shaped guard cells; when potassium ions move into the guard cells, water follows by osmosis, the cells become turgid and the pore opens, and the reverse closes it. The rate of transpiration rises with light, temperature and wind speed and falls as the humidity of the air rises. The rate is measured with a potometer, such as Ganong's potometer, which records the water taken up by a cut shoot.
Transpiration cools the leaf, creates the pull that lifts water and distributes dissolved minerals, but it also costs the plant most of the water it absorbs, which is why it is often called a necessary evil. It must be separated from two related phenomena. Guttation is the loss of liquid water, with dissolved salts, from special pores called hydathodes at the leaf margin, seen in the morning on Colocasia and grasses. Bleeding is the loss of sap from a cut or injured part.
Translocation of food
Food, mostly sucrose, moves through the sieve tubes of the phloem from a source, such as a mature leaf, to a sink, such as a fruit, a root or a bud. The accepted explanation is the pressure flow or mass flow hypothesis of Munch: loading sugar at the source makes that end concentrated, water enters it from the xylem by osmosis, and the resulting pressure drives the solution to the sink, where sugar is unloaded. The classic proof is the girdling or ringing experiment, in which removal of a ring of bark, and with it the phloem, stops downward movement of food and swells the stem above the ring.
Respiration in plants
Respiration breaks down food to release energy as ATP. Aerobic respiration has three stages. Glycolysis, also called the Embden Meyerhof Parnas pathway, splits one molecule of glucose into two of pyruvic acid in the cytoplasm and gives a net gain of two ATP. The Krebs cycle, also called the citric acid or tricarboxylic acid cycle and named after Hans Krebs, oxidises pyruvate completely in the matrix of the mitochondrion, releasing carbon dioxide. The electron transport chain on the inner mitochondrial membrane passes electrons to oxygen, forming water, and most of the ATP is made here by oxidative phosphorylation. Anaerobic respiration, or fermentation, goes only as far as ethanol and carbon dioxide in yeast, or lactic acid in muscle and in some bacteria, and yields only two ATP per glucose, which is why it is so wasteful.
The respiratory quotient, the ratio of carbon dioxide released to oxygen consumed, identifies the substrate: it is one for carbohydrates, less than one for fats, slightly below one for proteins and more than one for organic acids. It is zero in fermentation, where no oxygen is used. The compensation point is the light intensity at which photosynthesis just balances respiration, so there is no net exchange of gases.
Process, place and theory at a glance
| Process | Where it happens | Key point |
|---|---|---|
| Water transport | Xylem, upward only | Cohesion tension theory of Dixon and Joly; transpiration pull |
| Food transport | Phloem sieve tubes, both ways | Pressure flow hypothesis of Munch; source to sink |
| Transpiration | Stomata, cuticle, lenticels | About ninety per cent through stomata; potassium opens the guard cells |
| Glycolysis | Cytoplasm | Glucose to two pyruvic acid; net two ATP |
| Krebs cycle | Mitochondrial matrix | Complete oxidation of pyruvate; carbon dioxide released |
| Electron transport chain | Inner mitochondrial membrane | Oxygen is the final electron acceptor; most ATP formed |
Exam Point of View
Questions come in four clusters. First, tissue and direction: xylem upward for water, phloem both ways for food. Second, theory with scientist: cohesion tension with Dixon and Joly, pressure flow with Munch, Krebs cycle with Hans Krebs. Third, place in the cell: glycolysis in the cytoplasm, Krebs cycle in the mitochondrial matrix, electron transport chain on the inner mitochondrial membrane, Calvin cycle in the chloroplast stroma. Fourth, the small numbers: net two ATP from glycolysis, four as the gross figure, about thirty six from complete oxidation, respiratory quotient one for carbohydrate and about 0.7 for fat. The common traps are confusing guttation with transpiration, potometer with auxanometer, and the gross ATP of glycolysis with the net.
Important Facts
| Water conducting tissue | Xylem, upward only; mature vessels are dead |
|---|---|
| Food conducting tissue | Phloem, through sieve tubes with companion cells |
| Accepted theory of ascent of sap | Cohesion tension theory of Dixon and Joly, driven by transpiration pull |
| Stomatal transpiration | About ninety per cent of total transpiration |
| Stomatal opening | Influx of potassium ions into guard cells, followed by water |
| Instrument for transpiration | Potometer, such as Ganong's potometer; cobalt chloride paper for a quick test |
| Guttation | Liquid water with salts through hydathodes; seen in Colocasia, grasses and tomato |
| Food translocation | Pressure flow or mass flow hypothesis of Munch, source to sink |
| Glycolysis | Cytoplasm; glucose to two pyruvic acid; net two ATP, gross four |
| Krebs cycle | Mitochondrial matrix; also called citric acid or tricarboxylic acid cycle |
| Respiratory quotient | One for carbohydrates, about 0.7 for fats, slightly below one for proteins, above one for organic acids |
| Anaerobic products | Ethanol and carbon dioxide in yeast, lactic acid in muscle and Lactobacillus |
Practice MCQs on this topic
The ascent of sap to the top of a tall tree is explained mainly by which force?
- A.Root pressure
- B.Capillarity alone
- C.Transpiration pull under the cohesion tension theory
- D.Active transport by xylem vessels
Show answer
Dixon and Joly explained that water molecules cling to one another by cohesion and to the xylem walls by adhesion, forming an unbroken column; evaporation from the leaf surface creates a suction at the top that pulls this column upward.
Read the full explanation →The loss of water in liquid form from the margins of leaves through special pores is called
- A.transpiration
- B.guttation
- C.bleeding
- D.imbibition
Show answer
It occurs through pores called hydathodes at the leaf margin or tip, usually in the early morning when the soil is moist, the air is humid and transpiration is low, so root pressure forces out drops of water with dissolved salts; Colocasia, grasses and tomato show it well.
Read the full explanation →Which tissue is responsible for the transport of prepared food in a plant?
- A.Xylem
- B.Phloem
- C.Parenchyma
- D.Sclerenchyma
Show answer
Its sieve tubes, helped by companion cells, carry sucrose and other soluble food from a source such as a mature leaf to a sink such as a root, fruit or bud, and the movement can be upward or downward.
Read the full explanation →The opening of stomata is brought about by the movement of which ion into the guard cells?
- A.Sodium
- B.Calcium
- C.Potassium
- D.Magnesium
Show answer
In light, potassium ions are actively pumped into the guard cells, their solute concentration rises, water enters by osmosis, the cells swell and because their inner walls are thicker they bend apart, opening the pore; when potassium leaves, the cells lose water and the pore closes.
Read the full explanation →Which instrument is used to measure the rate of transpiration in a cut shoot?
- A.Potometer
- B.Auxanometer
- C.Clinostat
- D.Respirometer
Show answer
Ganong's potometer and the simpler bubble potometer measure the water absorbed by a cut shoot in a given time, and since almost all of that water is lost as vapour, the reading is taken as the rate of transpiration.
Read the full explanation →Glycolysis, the first step in the breakdown of glucose, takes place in which part of the cell?
- A.Cytoplasm
- B.Mitochondrial matrix
- C.Inner mitochondrial membrane
- D.Chloroplast stroma
Show answer
Glycolysis, also called the Embden Meyerhof Parnas pathway, splits one molecule of glucose into two molecules of pyruvic acid in the cytoplasm, needs no oxygen and gives a net gain of two ATP along with two molecules of reduced NAD.
Read the full explanation →The Krebs cycle takes place in which part of the cell?
- A.Ribosome
- B.Mitochondrial matrix
- C.Golgi apparatus
- D.Nucleus
Show answer
Pyruvic acid formed in glycolysis enters the mitochondrion, is converted to acetyl coenzyme A and is oxidised completely in the matrix through the cycle named after Hans Krebs, who received the Nobel Prize for the discovery; the cycle releases carbon dioxide and reduced coenzymes.
Read the full explanation →The respiratory quotient of a germinating seed using carbohydrate as the substrate is
- A.zero
- B.less than one
- C.exactly one
- D.more than one
Show answer
The respiratory quotient is the volume of carbon dioxide released divided by the volume of oxygen used. When glucose is oxidised completely, six molecules of oxygen are used and six of carbon dioxide are released, so the ratio is one.
Read the full explanation →In anaerobic respiration in yeast, glucose is finally converted into
- A.lactic acid and water
- B.ethanol and carbon dioxide
- C.pyruvic acid and oxygen
- D.acetic acid and hydrogen
Show answer
In yeast the pyruvic acid formed by glycolysis is decarboxylated and then reduced to ethyl alcohol, a process called alcoholic fermentation, which is the basis of brewing and of the rising of dough.
Read the full explanation →The translocation of sugars through the phloem is best explained by which hypothesis?
- A.Cohesion tension hypothesis
- B.Pressure flow hypothesis of Munch
- C.Root pressure theory
- D.Imbibition theory
Show answer
Sugar is actively loaded into the sieve tubes at the source, water follows from the xylem by osmosis and raises the pressure there, and the resulting gradient pushes the whole solution to the sink, where sugar is unloaded and water returns to the xylem.
Read the full explanation →How many molecules of ATP are gained, on balance, from one molecule of glucose during glycolysis?
- A.Two
- B.Four
- C.Six
- D.Thirty six
Show answer
Glycolysis spends two molecules of ATP in the preparatory steps that phosphorylate glucose and fructose, and produces four by substrate level phosphorylation in the later steps, so the net gain is two ATP along with two molecules of reduced NAD.
Read the full explanation →Frequently Asked Questions
Why is transpiration called a necessary evil?
Because a plant loses a very large share of the water it absorbs as vapour, which looks wasteful, yet the same loss creates the transpiration pull that lifts water and minerals to the leaves and cools the leaf surface in strong sunlight. The cost is unavoidable, since the stomata that let carbon dioxide in must also let water vapour out.
What is the difference between transpiration and guttation?
Transpiration is the loss of water as vapour, mainly through the stomata, and it happens during the day when the air is dry. Guttation is the loss of liquid water, with dissolved salts, through hydathodes at the leaf margin, and it happens in the early morning when the soil is wet and transpiration is low.
Which force actually carries water to the top of a tall tree?
Transpiration pull. Evaporation from the leaf creates a tension that is transmitted down the continuous water column held together by cohesion and held to the xylem walls by adhesion. Root pressure and capillarity help only over a few metres.
Where in the cell is most of the ATP of respiration formed?
On the inner membrane of the mitochondrion, by the electron transport chain and oxidative phosphorylation, where oxygen is the final acceptor of electrons and water is formed. Glycolysis in the cytoplasm contributes only a net two ATP per glucose.
What does the respiratory quotient tell us?
It tells us which food is being respired. A value of one means carbohydrate, a value near 0.7 means fat, a value slightly below one means protein, and a value above one means an organic acid. In fermentation the value is zero, because carbon dioxide is released without any oxygen being used.
