Plant Structure
Le
arn
in
g
O
b
je
ctive
(s
)
After studying this section, you will be able to:
Recall the typical ultrastructure of animal cells and contrast this with the ultrastructure of typical plant cells (presence of cell wall, chloroplasts, amyloplasts, vacuole, tonoplast, plasmodesmata, pits and middle lamellae)
Compare the structure and function of the polysaccharides starch and cellulose including the role of hydrogen bonds between β-glucose molecules in the formation of cellulose microfibrils.
Explain how the arrangement of cellulose microfibrils in plant cell walls and secondary thickening contribute to the properties of plant fibres, which can be exploited by humans.
Describe/compare the structure and location in the plant stem of sclerenchyma fibres and xylem vessels. Describe how their physical properties enable them to be used for human benefit
Explain the relationship between structure and function in sclerenchyma fibres (support) and in xylem vessels (support and transport of water and mineral ions through the stem)
Explain how the use of plant fibres and plant polysaccharides may contribute to sustainability.
A. Plant Cell Ultrastructure
We have already reviewed the ultrastructure of animal cells and seen the structure in relation to function the following organelles: 1-nucleus, 2-nucleolus, 3-ribosomes, 4-rough and smooth endoplasmic reticulum, 5-mitochondria, 6 centrioles, 7-lysosomes, and 8-Golgi apparatus. Plant cells like animal cells are eukaryotic cells and they share these same organelles. Plant cells do not have centrioles. In addition plant cells have some organelles not found in animal cells:
Cell wall – found outside the cell membrane and made of cellulose microfibrils embedded in a matrix of pectin and other substances. Two kinds: primary cell wall is made mainly of cellulose
Plastids – these are surrounded by an envelope (double membrane) and contain their own DNA. There are several types of plastids including chloroplasts and amyloplasts.
Chloroplasts contain the
green pigment
chlorophyll which is used in photosynthesis. Amyloplasts store starch in the form of amylopectin.
Permanent vacuole – surrounded by the tonoplast (single membrane) and containing cell sap
The Chloroplast
Small, flattened & surrounded by double membrane. Chlorophyll found on internal membranes called thylakoids which are arranged in some areas into stacks called grana. Grana are linked together by intergranal lamellae.
The chloroplasts membranes are embedded in a fluid called stroma. Some of the reactions of photosynthesis occur on thylakoids whilst others occur in the stroma. Chloroplasts (and mitochondria) have their own genetic material (DNA) and ribosomes.
Amyloplasts
Like chloroplasts, amyloplasts are plastids – surrounded by a double membrane, but they do not have a pigment and are colourless. Glucose from photosynthesis is polymerised into amylopectin (branched form of starch) and stored in amyloplasts. When the cell needs energy, the amylopectin is hydrolysed back to glucose. They are especially plentiful in storage organs
like potato and cassava tubers. Sometimes when potato tubers are exposed to sunlight, they go green as amyloplasts change into chloroplasts.
Permanent Vacuole
These are surrounded by a single membrane called tonoplast and containing cell sap. The vacuole is very important in keeping plant cells rigid, or turgid.
The plant cell wall
All plant cells have a primary cell wall composed mostly of cellulose and which surround growing cells or cells capable of growth. Secondary walls may be laid later when the cell stops growing and they are thickened structures containing lignin (a rigid polymer) and surrounding specialized cells such as xylem vessels or fibre cells. Lignin or lignified cellulose is the main component of wood.
Other structures associated with the plant cell wall are:
Pits:
regions of thin cell wall – only primary
allows transport of substances between cells
Plasmodesmata:
channels in cell wall that link adjacent cells together – cytoplasm of one cell is continuous with another through the plasmodesmata
allows transport and communication between cells
Middle Lamella:
is an adhesive sticking adjacent plant cells together gives plant stability
contains pectin’s (calcium pectate)
B. Plant Polysaccharides
Polysaccharides are formed when several monosaccharides are linked together (condensation reaction). Starch and cellulose are two common polysaccharides made from glucose monomers. (We have already described monosaccharides like glucose and disaccharides like maltose in Unit 1)
Energy (calories) in most of our foods is provided by polysaccharides, especially starch. Cellulose is the most abundant naturally occurring molecule. It provides structure and support in plant cell walls.
STARCH
Made of glucose molecules linked by glycosidic bonds. Used as an energy store in plants & not soluble.
Forms solid grains inside plant cells (often inside amyloplasts & chloroplasts). Mixture of 2 polysaccharides - amylose and amylopectin.
Amylopectin is branched, amylose is not. Both molecules are 1, 4 linked (link
between carbon atoms 1 and 4 of successive α-glucose units).
The chains coil up into a basic spiral or helical shape making the molecules compact.
Hydrogen bonds inside the compact spiral shape hold the polysaccharide chain.
The branches in amylopectin are formed by other 1, 4 linked chains joining the main polysaccharide by 1,6 linkages.
CELLULOSE
Most abundant organic molecule - found in plant cell walls, constituting on average 20-40% of the plant cell wall.
Made of glucose units. Every other glucose is rotated through 180 - this makes the chains straight, not coiled.
Hydrogen bonding between monosaccharide molecules of adjacent chains gives strength. Cellulose molecules arranged into bundles called microfibrils. Microfibrils held together in fibres.
A cell wall will have several layers of fibres running in different directions - gives great strength almost equal to steel.
Provides support in plants and stops plant cells bursting when they absorb water. Freely permeable to water and solutes.
It is very slow to decompose and not easily digested. Enzyme cellulase can break down cellulose, but it is relatively rare in nature.
Ruminants (like cows) and other herbivores like termites have bacteria in the gut capable of breaking down cellulose. Carnivores and omnivores cannot digest cellulose, and in humans it is referred to as fibre.
The bond is flexible so starch molecules can coil up, but the bond is rigid, so cellulose molecules form straight chains.
Hundreds of these chains are linked together by hydrogen bonds between the chains to form cellulose microfibrils. These microfibrils are very strong and rigid, and give strength to plant cells, and therefore to young plants and also to materials such as paper, cotton and cello tape.
Comparisons of structure/function
Starch
Cellulose
Made of α-glucose; links are α-glycosidic Coiled into α-helix (amylose) or branched flexible (amylopectin) molecules so can be packed into small space
H-bonds within each chain forming helix Insoluble (because of its large size), does not affect osmosis – but they can form H-bonds with water at ends of molecules, so can be hydrolysed when needed.
Easy to digest (side chains makes it easy for enzymes to get to glycosidic linkages
Reacts with iodine to form blue-black colour Provides store of energy in cells. Compact and lots can be stored. Can easily by mobilised (dissolved to form soluble/mobile products)
Made of β-glucose; links are β-glycosidic Straight, unbranched chains
H-bonds between chains, forming microfibrils
Cannot form H-bonds with water therefore insoluble
Difficult to digest (fibre or roughage in food), only few bacteria have cellulose enzymes Does not react with iodine
Provides structural support (e.g. in plant cell walls) it is very strong due to microfibrils (bundle of cellulose molecules linked by H-bonds) and insoluble
C. Plant Stem Structure
Plants have fewer types of tissues than animals. The tissues of a plant are organized into three tissue systems: the dermal tissue system, the ground tissue system, and the vascular tissue system.
1. Dermal tissue system - protects the soft tissues of plants and controls interactions with the plants' surroundings. Called Epidermis but in older plants it is replaced by the periderm. May be covered by cuticle or may produce hairs or hooks.
2. Ground tissue system – made of parenchyma, collenchyma and sclerenchyma cells. Its function for photosynthesis, storage, regeneration, support, and protection.
3. Vascular tissue – made of xylem and phloem, which function to transport water and dissolved substances. Vascular tissue may also contain meristematic tissue called cambium which divides to make secondary xylem and phloem during growth or repair.
Tissue System (+ components)
Tissue Functions Location of Tissue Systems Dermal Tissue System
Epidermis
Periderm (older stems + roots)
• protection
• prevents water loss
Ground Tissue System Parenchyma tissue Collenchyma tissue Sclerenchyma tissue photosynthesis • food storage • regeneration • support • protection Vascular Tissue System
Xylem tissue Phloem tissue
• transports water & minerals • transports food
For AS Biology we are only going to look at the tissues involved in transport and
support:
Vascular bundles: contain xylem, phloem and cambium tissue. Their function is support and transport of water, mineral ions and manufactured carbohydrates. In roots they are in the centre to provide support as the root burrows into the soil. In stems they are found near the outside to withstand bending forces.
Longitudinal section of stem showing structure of cell types in vascular bundles
Individual cell types of the xylem (left) and phloem (right) as seen from the outside
Xylem Tissue consists of Xylem Vessels, tracheids, fibres and Parenchyma Cells. Xylem Vessels are made vessel members, dead cells that have become elongated and reinforced and waterproofed with deposits of Lignin and have end walls so that successive cells form a tubes with wide Lumen. They conduct water and mineral ions from the roots upward through the stem to the leaves.
Phloem Tissue is made up of Sieve Tubes, Companion Cells (parenchyma) and fibres. Sieve tubes line up and their perforated ends form Sieve Plates through which substances can move. Companion Cells lie next to Sieve Tube Cells and allow them to stay alive. Phloem transports sap (sugars manufactured by photosynthesis and dissolved in water) from the leaf to other parts of the plant.
The structures labelled ‘fibres’ in the diagram on the previous page refer to sclerenchyma fibres.
sclerenchyma, is made of various kinds of hard, woody cells that serve the function of support in plants. Mature sclerenchyma cells are dead cells that have heavily thickened walls containing lignin. They may occur in different shapes and sizes, but are often greatly elongated cells with long, tapering ends which overlap/interlock to form fibres. The presence of the thick cellulose walls strengthened with lignin provides maximum support to a plant. They can be found almost anywhere in the plant body, including the stem, the roots, and the vascular bundles in leaves. In vascular bundles, sclerenchyma fibres are often found just outside the bundle but sometimes they can completely encircle the vascular bundle with sclerenchyma fibres scattered in between xylem and phloem cells.
Sclerenchyma fibres have end walls that are closed and they do not conduct any materials. Lignin is deposited on the walls of sclerenchyma fibres as rings or spirals making them strong but flexible. The tensile strength of sclerenchyma depends on how much lignin is in the wall and also on the length of the fibres.
1. The roots of two groups of pea plants were placed in solutions containing radioactive potassium
ions. For the experimental plants a respiratory inhibitor was added to the solution. At regular intervals the solutions surrounding the roots were tested for radioactive potassium ions. The table shows the results of this investigation.
Time from placing roots in solution/minutes
Concentration of radioactive potassium ions in the solutions surrounding the roots/arbitrary units
Experimental plants Control plants
0 7.5 7.5 15 6.6 3.3 30 6.4 2.9 60 6.3 2.4 120 6.3 1.2 240 6.3 0.6 a)
(i) The rate of uptake of potassium by the experimental plants in the first 15 minutes was 0.06
units per minute.
Calculate the rate of uptake of potassium by the control plants over the same time period.
(ii) Suggest an explanation for the difference between the rates of uptake of the experimental and
control plants in the first 15 minutes.
(iii) The rate of potassium ion uptake in the control plants in the first hour was faster than in the
second hour. Suggest why.
b)
At the end of the investigation sections were cut across the stems of the pea plants and the amount of radioactivity measured. The diagram shows a section across the stem of a pea plant.
(i) Give one feature by which this section can be
recognised as a stem.
(ii) Using a guideline, label and name the tissue
in which you would expect to find the greatest amount of radioactivity.
(Marks available: 7)
Answer outline and marking scheme for question: 1
Give yourself marks for mentioning any of the points below:
a)
(i) 0.28 (units per minute)
(ii) uptake in (control plants) by active transport;/ Use energy/ATP from respiration;/Amount absorbed by experimental plants is due to diffusion. (iii) Concentration falls therefore rate of diffusion falls;/Active transport involves
carrier/membrane proteins;/More potassium ions so more chance of collision with carriers.
(5 marks)
b)
(i) Cylindrical arrangement of vascular bundles/vascular tissues in bundles; (ii) Correct label to Xylem.
(2 marks)
(Marks available: 7)
Summary of differences between xylem vessels and sclerenchyma fibres (their adaptations for their different functions)
Xylem Vessels (Sclerenchyma) Fibres
Bundles of dead cells Hollow Lumen Columns
Long cylinders with no end walls (open ends) Transport water + minerals up the plant and provide support
Walls thickened with lignin (strength, waterproof)
Pits in the walls allow transport of water + ions out of xylem
Bundles of dead cells Hollow Lumen Columns
Short overlapping structures with ends closed Provide support, do not transport any materials Walls thickened with lignin (strength, waterproof) and contain more cellulose
E. Plants as Renewable Resources
Humans utilise many different components of plants for various functions. These include use of wood for construction and fuel wood, cotton and cellulose fibres for textile; plant material for making paper, fibreboards and pulp; use of starch in industry to make adhesives, resins and other polymers. Many of these uses augment or replace synthetic products many of which are less environmentally friend that ‘natural’ materials from plants.
The properties of the plant materials usually determine the uses they are put to as a renewable resource. Some of the plant resources we use include:
Cellulose – cellulose molecules are held together by H-bonds to form microfibrils. The arrangement of microfibrils in layers which run at right angles to the layers below and above also gives them added strength and flexibility. This makes cellulose very tough but yet still flexible. Cellulose is therefore useful as a component of ropes – they are strong, do not stretch but can bend. Cellulose is not easily digested (very few organisms have cellulase enzymes) but it is good in human diet for helping with the movement of food along the bowel.
These same properties of (strength and flexibility) cellulose make it good in making garments and paper as well as jute bags and sisal ropes.
Plant cell walls undergo secondary growth - additional strengthening and thickening of the primary wall. During secondary growth/thickening, hemicelluloses and lignin are added to the cellulose in the primary wall to form ‘wood’. This makes the material impermeable to water, and even harder/stronger and more resistant to chemical or enzyme breakdown. Wood (contains lignified plant fibres) is therefore a good material for building and making furniture. Xylem vessel members and sclerenchyma fibres are often heavily lignified.
Starch is the main source of energy in our foods (bread, rice, wheat, cassava, potato, pasta etc). In addition starch has many other uses including as an adhesive and a thickener in some foods. In the last decade however, increasing use has been made of starch (+ other plant biomass) as a renewable resource for generating energy. Starch can be fermented to make ethanol which can be used as a fuel.
Starch can also be used to make biodegradable ‘plastic’ to replace non-renewable oil-based plastics.
Although some synthetic materials may be ‘cheaper’ to produce than plant based materials, there production/disposal often involves addition of carbon dioxide to the atmosphere. Plant biomass on the other hand is carbon-neutral.
F. Transport of Water
Le
arn
in
g
O
b
jecti
ve(
s)
After studying this section, you will be able to:
Explain
Explain
Describe
How to prepare
Explain
Why do Plants Need Water and Mineral Ions Plants Need water for:
Making cells turgid and this helps to keep the whole plant erect especially in young plants where walls are not thickened
Water is needed to make sugars by photosynthesis (H2O + CO2 C6H12O6 + O2) All the biochemical reactions in cells do so dissolved in water
Metabolites are moved from one part of the plant body to another by being dissolved in water – for mass transport.
Some of the most important mineral ions are:
Mineral ion Role/Importance Deficiency Symptoms
Nitrogen
Absorbed as nitrate
(Although abundant in air, plants are not able to utilize it directly. They have to absorb N2 which
has been converted to nitrate in the soil)
Found in chlorophyll. also the basic element of plant and animal proteins, including DNA and RNA, and is important in periods of rapid plant growth
Poor growth; Small leaves; Weak stems
Magnesium
Absorbed as Mg2+
Component of chlorophyll, activator of certain plant enzymes,
Less photosynthesis, Small leaves, Yellow leaves, Weak stem
Calcium
Absb as Calcium ions
Involved in membrane permeability;
A component of pectin (Calcium pectate) which holds cell walls together.
Needed for growth
Iron
Absorbed as iron ions
Less photosynthesis, Small leaves, Yellow leaves, Weak stem
Transpiration: water evaporated from the surface of spongy mesophyll cells and diffuses down the diffusion gradient through stomata of leaves
Water in the spongy mesophyll leaves is replaced from the xylem, lowering hydrostatic pressure at the top of the vessel, resulting in water being drawn up from below- transpiration stream.
Hydrogen bonding between water molecules allows cohesion between water molecules; this keeps water as a continuous column in the xylem vessel – Cohesion-Tension Theory
Forces of adhesion occur between water molecules and the xylem cell walls.
Root Pressure: minerals and ions moving into roots via active transport creating a concentration gradient for osmosis (water into roots)
The movement of water through xylem vessels provides a mass flow system for the transport of inorganic ions.
Nitrate ions (form of nitrogen) are needed by plants in order to make amino acids. Plants make their own amino acids from scratch using inorganic materials by a sequence of enzyme controlled reactions the nitrogen transported in the xylem is combined with organic molecules from photosynthesis to make all 20 amino acids. Plants cannot grow without nitrate ions as they are needed in chlorophyll, nucleic acids, ATP and some growth substances.
Magnesium is needed for chlorophyll
Calcium is required for a structural role in the cell wall and permeability of the cell membrane
