Biology for University Study - Topic 7: Photosynthesis and transport in multicellular plants
Transport in Multicellular Plants
This topic covers:
The need for transport systems in multicellular organisms.
Transport of water.
Transport in multicellular plants.
Translocation.
Differences between sieve tubes and xylem vessels.
Photosynthesis
This topic covers:
An energy transfer process
The light-dependent and independent reactions of photosynthesis
Leaf structure and function
Chloroplast structure and function
Factors necessary for photosynthesis
Trapping light energy
Learning Outcomes
Understand the need for transport systems in multicellular organisms.
Understand the distribution of xylem and phloem in stems, roots, and leaves of plants.
Understand the transport mechanism in plants.
Understand photosynthesis as an energy transfer process in plants.
Understand the structure and function of chloroplasts.
Investigate the limiting factors of photosynthesis.
Unit Roadmap
This module covers 12 topics:
Cell structure, biological molecules, and enzymes
Cell, nuclear division, and genetic control
Cell membranes, the mammalian transport system, and mammalian heart
Gas exchange, and lung and cardiovascular diseases
Infectious diseases and immunity
Energy and respiration
Photosynthesis and transport in multicellular plants
Homeostasis and regulation
Control and coordination
Inherited change and inheritance
Selection, evolution, biodiversity, and conservation
Gene technology and biotechnology
Previous topic: The need for energy in living organisms, ATP, detailed understanding of aerobic and anaerobic respiration, and respiratory substrates.
Today’s topic: Transport in multicellular plants and photosynthesis.
Next topic: Homeostasis and regulation in mammals and plants.
Structure of Transport Tissues
Unicellular organisms:
Have a short diffusion distance due to a large surface area to volume ratio.
Are one cell thick and metabolically inactive.
Obtain nutrients and oxygen and remove waste products and carbon dioxide via the cell surface membrane through diffusion.
Do not require a specialized transport system.
Multicellular organisms:
Have a decreased surface area to volume ratio.
Have a relatively long distance between the surface and the center of the organism.
Have increased metabolic activity with increased size.
Require a specialized transport system to supplement nutrients and oxygen and to remove waste products like carbon dioxide and water.
Seed: Monocot VS Dicot
Monocot
1 cotyledon
Fibrous roots
Have petals in multiples of 3
Narrow, parallel veins
Scattered vascular bundles
Dicot
2 cotyledons
Tap roots
Have 4 or 5 petals
Oval or palmate, net-like veins
Ringed vascular bundles
Herbaceous Dicotyledonous Plants
Have seeds, a network of veins, broad blades leaves, and a tap root with lateral branches.
Leaf structures:
Upper epidermis
Lower epidermis
Mesophyll
Vascular bundles:
Xylem
Phloem
Cross-Section of a Dicotyledonous Leaf
Waxy cuticle
Upper epidermis
Palisade mesophyll cell
Bundle sheath
Vascular bundle
Phloem
Xylem
Spongy mesophyll cell
Air space
Lower epidermis
Stoma with two guard cells
Transverse Section of the Stem
Stem parts:
Epidermis
Cortex
Endodermis
Vascular bundles (xylem and phloem)
Pith or medulla
Other Labelled parts:
Cambium
Pericycle (Parenchyma or Sclerenchyma)
Medullary rays (Parenchyma between vascular bundles)
Sclerenchyma and Collenchyma
Parenchyma
Transverse Section of a Dicot Plant Root
Root parts
Epidermis
Endodermis
Cortex
Vascular bundles (xylem and phloem)
Pericycle
Other labeld parts:
Stele
Root Hair
Distribution of Xylem and Phloem
Xylem:
Found along with phloem in the vascular bundle.
Carries dissolved minerals and water up the plant.
Provides structural support to the plant.
Acts as food storage.
Distributed in the root, stem, and leaves.
Phloem:
Found along with xylem in the vascular bundle.
Carries organic compounds, particularly sucrose, from the leaf to the root.
Is a complex tissue made up of sieve tube elements and companion cells.
Distributed in the root, stem, and leaves.
Structure of Phloem Tissue
Microscope slide image and drawing of a sieve tube element and companion cell in transverse section (TS).
Photomicrograph image and drawing of a sieve tube element and companion cell in longitudinal section (LS).
Transmission electron micrograph image and drawing of a sieve tube element and companion cell in transverse section (TS).
Structure of Xylem Tissue
Images of xylem vessel elements:
A = photomicrograph (drawing) in longitudinal section (lignin is stained red).
B = scanning electron micrograph (drawing) in transverse section.
C = microscope image (drawing) in transverse section (lignin is stained purple).
Structure of the Xylem Tissue
Xylem vessels consist of dead cells.
They have a thick, strengthened cellulose cell wall with a hollow lumen.
The end walls of the cells have disappeared, so a long, open tube is formed.
The walls of the xylem vessel contain holes called pits through which water enters.
Xylem Vessel Elements: Structure and Functions
Lignified cell walls:
Impermeable to water and adds strength to withstand hydrostatic pressure, preventing vessel collapse.
No end plates:
Allows unimpeded cohesive and adhesive forces, facilitating mass flow of water and dissolved solutes.
No protoplasm:
Does not impede the mass flow of water and dissolved solutes.
Pits in wall:
Allows lateral movement of water, enabling continuous flow and preventing air bubble formation.
Vessels of small diameter:
Assists with capillary action and helps prevent the water column from breaking.
Structure of the Phloem Tissue
The function of the phloem tissue is to transport food nutrients such as sucrose and amino acids from the leaves to all other cells of the plant; this is called translocation.
Unlike the xylem, the phloem tissue is made of columns of living cells, which contain a cytoplasm but no nucleus, and its activities are controlled by a companion cell next to it, which has a nucleus, but companion cells have no function in translocation.
Phloem Sieve Tube Elements: Structure and Functions
Cellulose cell wall:
Strengthens the wall to withstand the hydrostatic pressure that moves the assimilates.
Sieve plates with sieve pores:
Allows for the continuous movement of organic compounds.
No nucleus, vacuole, or ribosomes in mature cells:
Maximizes space for the translocation of assimilates.
Thin cytoplasm:
Reduces friction to facilitate the movement of assimilates.
Phloem Companion Cells: Structure and Functions
Nucleus and other organelles present:
Provides metabolic support to sieve tube elements and helps with the loading and unloading of assimilates.
Large number of mitochondria:
Provides ATP for the active transport of assimilates into and out of the companion cells.
Transport proteins in plasma membrane:
Moves assimilates in and out of the sieve tube elements.
Plasmodesmata:
Allows for the movement of organic compounds from the companion cells to the sieve tube elements.
Stomata
Stomata are tiny pores on the surface of plant leaves and stems, primarily responsible for gas exchange between the plant and the atmosphere.
They allow carbon dioxide to enter for photosynthesis and oxygen and water vapor to exit during transpiration.
Stomata are generally more numerous on the underside of leaves.
Transpiration
When the plant opens its stomata to let in carbon dioxide, water that is on the surface of the cells of the spongy mesophyll and palisade mesophyll, evaporates followed by diffusion of water vapor out of the leaf.
This process is called transpiration.
Water and Mineral Ion Transport
Mineral ions and organic compounds are transported within a plant while dissolved in water.
The mineral ions dissolved are transported in the xylem tissue, whereas the dissolved organic compounds are transported in the phloem tissue.
The plant roots are responsible for the uptake of water and mineral ions.
To compensate for the continuous loss of water via transpiration in the leaves, plants must take in a constant supply of water and dissolved minerals.
Apoplast & Symplast Pathways
Water is transported from the soil to the xylem through the apoplast and symplast pathways.
Apoplast pathway:
When transpiration rates are high, most water travels via the apoplast pathway.
It is a series of spaces running through the cellular cell walls and the hollow tubes of the xylem.
When the water reaches the endodermis, the waxy band of suberin within the cell wall blocks this pathway.
This band, called the Casparian strip, forms a barrier for the water and dissolved minerals.
Hence the water and dissolved mineral ions then take the symplast pathway.
Symplast pathway:
A smaller volume of water flows via the symplast pathway, which includes the cytoplasm and plasmodesmata.
Symplast Pathway
A smaller volume of water flows via the symplast pathway.
The water moves into the cell by osmosis.
The movement of water in the symplast pathway is much slower than the apoplast pathway.
Water travels from a high water potential (soil) to a low water potential (xylem), via the apoplast or symplast.
Water and the Transpiration Pull
The mass flow of water in plants is helped by the polar nature of water.
The formation of hydrogen bonds between the water molecules results in cohesion between water molecules and adhesion between the cellulose in the cell walls and the water molecules.
The difference in the water potential between the top and bottom of the plant causes the water to move from the roots to the leaves.
Cohesion-Tension Theory
The gradient is maintained due to the constant loss of water from the leaves by transpiration and the constant uptake of water at the roots by osmosis.
This mechanism is called cohesion-tension theory, which results in a continuous column of water because of the cohesive nature of the water molecules.
Water Potential Gradient Illustration
Leaf:
Evaporation from stomata
Xylem:
Cohesion and adhesion
Adhesion to cellulose in xylem vessel wall
Roots:
Osmosis into root hairs
Movement of water
Cohesion between water molecules
Leaves from Xerophytic Plants
Xerophytes have physiological and structural adaptations to maximize water conservation.
Xerophytic adaptations of leaves:
Leaves reduced to scales, spines, or needles
Leaves curled or rolled or folded
Stomata closed during light and open in dark
All these adaptations reduce transpiration by reducing the surface area available and daytime water loss.
Translocation
Organic molecules such as sucrose and amino acids move from the source to the sinks via phloem sieve tubes in plants, called translocation.
Vascular plants produce nutrients like sucrose in their leaves, which is the source.
These nutrients are transported to the rest of the shoot and to the root tips, called the sink.
Translocation: Source and Sink
Leaf (Source)
Flower, Lateral Meristem, Seed, Tuber, Fruit, Apical Meristem (Sink)
Transport Mechanisms: Companion Cells
Companion cells transfer assimilates to phloem sieve tubes.
When the sucrose molecules take the apoplastic pathway, the modified companion cells pump hydrogen ions out of the cytoplasm into their cell walls via a proton pump.
As a result, a large concentration of hydrogen ions in the cell wall of the companion cells moves down the concentration gradient back to the cytoplasm of the companion cells.
The cotransporter protein facilitates the movement of the hydrogen ions and sucrose into the companion cell against the concentration gradient of sucrose.
The sucrose molecules finally move into the sieve tubes from the companion cells via the plasmodesmata.
The assimilates are amino acids and sugars that are translocated between the sources and sinks in phloem sieve tubes.
Transport Mechanisms: Hydrostatic Pressure Gradient
Energy is required in the phloem tissue to create pressure differences for the mass flow of organic solutes.
The pressure difference is created by actively loading sucrose into the sieve elements at the source, which lowers the water potential in the sap.
Due to this, the water moves into the sieve elements by traveling down the water potential gradient by osmosis, which then increases the hydrostatic pressure at the source.
This results in the solutes being removed from the sieve elements, causing water to follow at the sink by osmosis, which then creates a hydrostatic pressure gradient.
This hydrostatic pressure difference between the source and the sink results in the mass flow of water containing dissolved organic solutes.
Structure of Chloroplasts
Chloroplasts are the site of photosynthesis.
They have an outer membrane and an inner membrane.
Thylakoids are fluid-filled sacs in chloroplasts and are the site of the light-dependent stage of photosynthesis.
Thylakoids are stacked into structures called a granum, which are held together with lamellae, which make up bits of thylakoid membrane.
The fluid inside chloroplasts is called stroma, the site of light-independent stage of photosynthesis.
Structure of Chloroplast
Outer membrane
Inner membrane
Membrane envelope
Ribosomes
Stroma lamellae
Granum
Genetic material
Starch grains
Thylakoids
Photosynthetic Pigments
Photosynthetic pigments absorb light energy needed for photosynthesis.
They are found in the thylakoid membrane of chloroplasts, attached to proteins.
The protein and pigment together are called a photosystem.
There are different types of photosynthetic pigments that absorb different wavelengths.
Main Pigments
The 4 main pigments include chlorophyll a, chlorophyll b, carotene, and xanthophyll.
Photosystems containing chlorophyll a are called photosystem II (PSII) and absorb light best at wavelength 680 nm.
Photosystems containing chlorophyll b are called photosystem I (PSI) and absorb light best at wavelength 700 nm.
Additional Pigments
Carotenoids are another group of pigments that absorb violet and blue-green light, which help capture light but also have an important role in getting rid of excess light energy.
Xanthophyll is a yellow pigment that belongs to the carotenoid group. It is found in plants and algae, which helps to capture light energy for photosynthesis.
Photosynthesis
Photosynthesis produces glucose from carbon dioxide and water and light energy.
There are 2 stages to it: the light-dependent reaction and the light-independent reaction.
The light-dependent reaction needs light, and the light-independent reaction doesn’t.
The light-dependent stage uses light energy to excite electrons in the chlorophyll, causing them to be released from the chlorophyll in a process called photoionization.
Energy from this electron is used to add a phosphate group to ADP to form ATP (photophosphorylation) and form reduced NADP (NADPH) from NADP.
In the process, gets turned into ions and (Photolysis).
The light-independent stage (also known as the Calvin cycle) produces glucose using the products of the light-dependent stage and recycles them back to their original reactants.
Non-Cyclic Phosphorylation
There are 2 different ways photophosphorylation in the light-dependent reaction can be carried out: non-cyclic photophosphorylation and cyclic photophosphorylation.
Photosystems are joined together by electron-carriers, which are proteins that transfer electrons. Together, they form an electron transport chain.
In non-cyclic phosphorylation, chlorophyll in PSII absorbs light energy, and an excited electron escapes and travels down the electron transport chain, losing energy as it goes.
This energy is used to move ions (protons) inside the thylakoid, which causes a proton concentration gradient.
The protons then move down their concentration gradient into the stroma via the enzyme ATP synthase, which is embedded in the thylakoid membrane.
This movement combines ADP and a phosphate group to form ATP.
The electron that escaped the chlorophyll molecule needs to be replaced and is done so by the photolysis of water. A by-product of this reaction is oxygen.
Next, light is absorbed by chlorophyll in PSI, which causes another electron to be released at an even higher energy level. This electron reacts with NADP, reducing it, forming reduced NADP (NADPH).
This electron that is released from the chlorophyll needs to be replaced. So, the electron that escaped from PSII is used to replace it.
Cyclic Photophosphorylation
Cyclic photophosphorylation only uses PSI and only produces ATP.
Here, the electron that is released from chlorophyll isn’t passed on to make reduced NADP (NADPH) but goes back into the chlorophyll.
So, light energy excites the electrons in chlorophyll. They are released, and the energy is used to bring protons into the thylakoid, which then travel down their concentration gradient via the ATP synthase enzyme, producing ATP.
The electron then goes back into the chlorophyll.
This reaction produces a small amount of ATP and nothing else.
Checkpoint Summary
The transverse section of the leaf includes upper and lower epidermidis, mesophyll, vascular bundles like xylem and phloem.
The transverse section of the stem includes epidermis, endodermis, cortex, vascular bundles, pith, or medulla.
The transverse section of the root includes epidermis, endodermis, cortex, vascular bundles, and pericycle.
The xylem tissue is found along with the phloem tissue in the vascular bundles.
The xylem is distributed in the roots, leaves, and stem and carries water and dissolved minerals up the plant.
Phloem is a complex tissue made up of sieve tube elements and companion cells and carries organic compounds like sucrose from the leaf to the root.
Water is transported from the soil to the xylem through the apoplast and symplast pathways.
Transpiration is a process that when the stomata open to let in carbon dioxide, water on the surface of the mesophyll cells diffuses out of the leaf.
Organic molecules such as sucrose and amino acids move from the source to the sinks via phloem sieve tubes in plants, called translocation.
There is mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink.
Thylakoids are fluid-filled sacs in chloroplasts and are the site of the light-dependent stage of photosynthesis.
Light-Independent Reaction
The light-independent reaction (also known as the Calvin cycle) uses the products of the light-dependent reaction and to produce organic molecules such as glucose.
The Calvin cycle has 4 main stages:
First, the is combined with ribulose bisphosphate, catalyzed by the enzyme rubisco. This forms an unstable 6-carbon compound that quickly breaks down into 2 glycerate 3-phosphate molecules.
The next stage is the formation of triose phosphate.
The hydrolysis of 2 ATP molecules into ADP and phosphate groups provides the energy for the reaction.
The reaction also needs some ions, which are provided by the reduced NADP (NADPH), which is oxidized, forming NADP.
This forms 2 triose phosphate molecules. Five out of six triose molecules continue into the next stage of the Calvin cycle to form ribulose bisphosphate, making sure there is enough for the cycle to keep going. The triose phosphate is the molecule used to make the organic molecules the plant needs.
The 2 triose phosphate molecules use energy from the rest of the ATP molecules, forming one ribose bisphosphate molecule.
Glucose is a hexose sugar, meaning it is a simple 6-carbon molecule. Hexose sugars are then joined together to form larger molecules, such as starch and cellulose.
Triose phosphate contains 3 carbons, so 2 will be needed to form glucose or any other hexose sugars.
Three turns of the Calvin cycle produce six triose phosphate molecules. Five of them will be needed to produce ribulose bisphosphate (a pentose sugar), so six turns of the Calvin cycle produce 1 glucose molecule.
This may seem a bit inefficient; however, it allows the Calvin cycle to continue.
Producing Organic Substances from Photosynthesis
The Calvin cycle is used to make the organic molecules a plant needs.
Larger carbohydrates are made from joining hexose sugars together.
Lipids are made using 1 glycerol molecule, which can be made using triose phosphate, and 3 fatty acids, which can be made using glycerate 3-phosphate.
Some amino acids are made from glycerate 3-phosphate.
Photosynthesis as an Energy Transfer Process
The absorbance spectra shows the absorbance of chlorophyll A and chlorophyll B and carotenoid pigments at different wavelengths of light.
Chlorophylls absorb wavelengths in the blue-violet and red regions of the light spectrum.
Carotenoids absorb wavelengths in the blue-violet region of the spectrum.
The action spectrum shows the rate of photosynthesis at different wavelengths of light.
The rate of photosynthesis is highest at the blue-violet and red regions of the light spectrum, as these are the wavelengths of light that chlorophylls and carotenoids can absorb.
Chromatography of Chloroplast Pigments
Chromatography is an experimental technique used to separate mixtures.
The mixture is dissolved in a fluid, and the dissolved mixture then passes through a static material.
Different components of the mixture then pass through the static material at different speeds.
This results in the separation of the mixture components.
The retardation factor () is calculated for each component of the mixture.
value = distance traveled by component / distance traveled by solvent
Carotenoids have the highest value, close to 1.
Chlorophyll B has a much lower value.
Chlorophyll A has an value which is between that of carotenoids and chlorophyll B.
A small value means that the pigment is less soluble and larger in size.
Investigation of Limiting Factors
Light intensity, carbon dioxide, and temperature are examples of limiting factors of photosynthesis.
Light intensity:
Light is needed to provide energy for photosynthesis; the higher the intensity of light, the more energy.
Only certain wavelengths of light are used in photosynthesis, where the photosynthetic pigments chlorophyll a & b and carotene absorb the red and blue light in sunlight.
Temperature:
As the photosynthetic reactions involve enzymes like ATP synthase and rubisco, if the temperature falls below 10°C, the enzymes become inactive.
If the temperature is more than 45°C, then the enzymes start to denature.
At high temperature also, the stomata close to avoid losing too much water; as a result, carbon dioxide does not enter the leaf, which causes photosynthesis to slow down.
Carbon dioxide:
Carbon dioxide makes up 0.04% of gases in the atmosphere, which gives a higher rate of photosynthesis.
Any higher than this would cause the stomata to close.
Investigating the Rate of Photosynthesis Using Redox Indicators
Using chloroplasts:
Crush the leaves in a medium that has an equal water potential as the leaf cells to generate a concentrated extract of intact and functional chloroplasts in tubes.
Arrange the setup with light of varying intensities or wavelengths, shining on the chloroplasts.
DCPIP or methylene blue indicator is added to each tube along with a small volume of leaf extract.
The time taken by the redox indicator to go colorless is recorded, which gives us the rate of photosynthesis.
Using whole aquatic plants:
Aerate the water very well by bubbling air through the water.
Ensure that the plant (pondweed) is illuminated sufficiently before use so that the plant has all the enzymes needed for photosynthesis.
Set up the experiment in a dark room with the plant submerged in 1% sodium bicarbonate to ensure a constant supply of carbon dioxide for the pondweed.
Neatly cut the stem of the pondweed before placing it into the boiling tube.
Measure the gas volume that is accumulated in the gas syringe for about 10 minutes.
Change the independent variables like light intensity, carbon dioxide, or temperature.
Measure the gas volume accumulated in the syringe again and record the results to plot a graph of the volume of oxygen generated in a minute against the distance from the light source.
Quiz Questions
Q1. What is the need for transport systems in multicellular organisms?
Q2. What are the two plant tissues to transport water, nutrients, and minerals?
Q3. What are chloroplasts?
Q4. What are the four photosynthetic pigments?
Q5. What are the three limiting factors in photosynthesis?
Q6. List the structural elements of the transverse sections of the plant leaf, stem, and root?
Q7. What is photosynthesis?
Q8. What are thylakoids?
Q9. Define the Calvin cycle?
Do You Know This?
Photosynthesis can continue after sunset because the light-independent reaction can continue using the products of the light-dependent reaction for a bit longer!
Discussion Session
Discuss the photosynthetic reactions.
Topic Summary
Multicellular organisms need a specialized transport system as they have a small surface to volume ratio and hence cannot rely on diffusion only to supply cells with substances and remove waste products.
Plants have tissues to transport water, nutrients, and minerals, like Xylem and phloem.
Mineral ions and organic compounds are transported within a plant while dissolved in water.
Chloroplasts are the site of photosynthesis, and they have an outer and inner membrane.
Photosynthetic pigments absorb light energy needed for photosynthesis, and the 4 main pigments include chlorophyll a, chlorophyll b, carotene, and xanthophyll.
Photosynthesis produces glucose from carbon dioxide and water and light energy.
Light intensity, carbon dioxide, and temperature are examples of limiting factors of photosynthesis.