4.1.structure of plants

7/30/2019 4.1.Structure of Plants

1/14

Unit 2: Developments, Plants & the Environment Topic 4: Biodiversity & Natural Resources

Edexcel AS Biology Mo Idriss1

Plant Structure

LearningObjective(s)

After studying this section, you will be able to:

Recall the typical ultrastructure of animal cells and contrast this with theultrastructure 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 -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


2/14



Plastids these are

surrounded by an

envelope (double

membrane) and contain

their own DNA. Thereare 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 containingcell 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 areembedded 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


3/14



like potato and cassava tubers. Sometimes when potato tubers are exposed to sunlight, they go

green as amyloplasts change into chloroplasts.

Permanent VacuoleThese 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 iscontinuous with another through the plasmodesmata

allows transport and communication between cells


4/14



Middle Lamella:

is an adhesive sticking adjacent plant cells together gives plant stability contains pectins (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 -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.


5/14



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.

Part of a cellulose molecule


6/14



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.


7/14



Comparisons of structure/function

Starch CelluloseMade of -glucose; links are -glycosidic

Coiled into -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 -glucose; links are -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.


8/14



Tissue System (+

components)

Tissue Functions Location of Tissue Systems

Dermal Tissue SystemEpidermis

Periderm (older stems +

roots)

protection

prevents water loss

Ground Tissue SystemParenchyma tissue

Collenchyma tissueSclerenchyma tissue

photosynthesis

food storage

regeneration support

protection

Vascular Tissue SystemXylem 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 theoutside 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


9/14



Xylem Tissue consists ofXylem Vessels, tracheids, fibres and Parenchyma Cells. Xylem Vessels

are made vessel members, dead cells that have become elongated and reinforced and

waterproofed with deposits ofLignin 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. Maturesclerenchyma cellsare 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 formfibres. 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.

Section of Dicot and Monocot plants to show the position of sclerenchyma and vascular tissues.
http://www.britannica.com/EBchecked/topic/529124/sclerenchyma-cellhttp://www.britannica.com/EBchecked/topic/529124/sclerenchyma-cellhttp://www.britannica.com/EBchecked/topic/529124/sclerenchyma-cellhttp://www.britannica.com/EBchecked/topic/205799/fibrehttp://www.britannica.com/EBchecked/topic/205799/fibrehttp://www.britannica.com/EBchecked/topic/205799/fibrehttp://www.britannica.com/EBchecked/topic/205799/fibrehttp://www.britannica.com/EBchecked/topic/529124/sclerenchyma-cell


10/14



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 insolution/minutes Concentration of radioactive potassium ions in the solutionssurrounding the roots/arbitrary units

Experimental plants Control plants0 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 thesecond 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 tissuein which you would expect to find the greatestamount of radioactivity.

(Marks available: 7)

Marking scheme overleaf


11/14



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

No pits in the side walls


12/14



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 towater, 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.


13/14



F. Transport of Water

Lea

rning

Objective(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


14/14



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 ofadhesion 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

G. Plants as Renewable Resources

4.1.structure of plants

Documents