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Bio FactsheetSeptember 1997 Number 4

Structure to function in eukaryotic cellsThe cell surface membrane and the membranes which form organelles ineukaryotic cells all have the same basic structure, known as the fluidmosaic model. Such membranes provide control of the entry and exit ofsubstances into cells and organelles and such control is a result of thephospholipid bilayer and membrane proteins.

In eukaryotic cells, such membranes divide the cytoplasm into multiplecompartments (organelles). Organelles allow different functions to occurefficiently and simultaneously in different parts of the cell. For example,the outer double membrane of the mitochondrion separates out thosereactions which occur in mitochondria from those in the general cytoplasm.Furthermore, the internal membranes of the mitochondria allow the enzymicreactions of the Kreb’s cycle to be kept quite separate from the electrontransfer chain reactions (ETC). This is essential since both sets of reactionshave different enzymes, hence different pH optima. By splitting up thecytoplasm of the mitochondria, the membranes which form the crista allow

enzymes and substrates to be concentrated and pH to be optimised.Membranes can therefore be said to compartmentalise the interior ofeukaryotic cells.

The relationship between structure and function can be described in termsof the whole cell or in terms of the individual organelles of the eukaryoticcell.

Table 1 describes the structure and function of a motor neuron cell which iscommonly featured in examination questions. The structure and functionof eukaryotic organelles is described overleaf.

Cell Structure Function

Motor Neuron

Long axons

Axon contains axoplasm

For rapid transmission of nerve impulse. Synapses, wheretwo nerves join, is the slowest part of transmission, sothe longer the axon, the fewer the synapses and the fasterthe impulses transmitted.Allows transport between cell body and axon

Nodes of Ranvier betweenSchwann cells

High phospholipid content inmembrane of Schwann cell

Allows Na+ /K+ pump to operate which sets up restingpotential. Schwann cells of myelin sheath speed up theimpulse because they increase the surface area fortransmission of current.

Provides electrical insulation.

Synaptic knob at end ofdendrite contains:(i) many mitochondria

(ii) numerous vesicles

To provide ATP for active refilling of synaptic vesicles.

For modification and release of chemical transmittersacross the synapse.

Many dendrites To allow communication with other neurons.

Cell body contains:

(i) Nucleus

(ii) dense groups of ribosomesand endoplasmic reticulumcalled Nissl granules

Provides the genetic code for the production ofneurotransmitter substances, e.g. acetylcholine andenzymes, eg. cholinesterase.

For production and transport of proteins andneurotransmitters.

axon may be a metre or more in length

nodes ofRanvier

axon

directionof nerveimpulse

Table 1.

Exam Hint - Structure to function questions are very commonly seton all A level Biology syllabuses. They are one of the syllabus areaswhere all candidates should be capable of gaining the highest marks.Once the functions of organelles have been memorised, candidatesshould become confident at interpreting the function of unknown cells.

Nisslgranules

nucleus

dendrites

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Structure and Function of Organelles Structure Function

Double nuclear envelope To enclose and protect DNA (normally visible aschromatin granules).

Nuclear pores.

Normally, the nuclear pores areplugged by an RNA/proteincomplex.

Allow entry of substances such as nucleotides for DNAreplication and exit of molecules such as mRNA duringprotein synthesis.

Small molecules pass through the pores by diffusion,whereas large molecules such as partly completedribosomes pass through actively.

Nucleoplasm containschromatin granules made ofDNA and associated proteins.

Nucleoplasm also containsnucleoli

It is these which, during cell division, condense to formchromosomes.

Produces partly-completed ribosomes, coenzymes,nucleotides, proteins (including enzymes for nucleic acidsynthesis) and RNA molecules.

The outer membrane of thenuclear envelope is continuouswith the rough endoplasmicreticulum membranes.

This makes the perinuclear space continuous with thelumen of the endoplasmic reticulum, thus allowing easytransport of substances.

Mitochondrion Double membrane Isolates reactions of the Kreb’s cycle and electrontransfer chain from the general cytoplasm. Suchcompartmentalisation allows high concentrations ofenzymes and substrates to be maintained whichincreases the rate of respiratory reactions.

Whereas the outer membrane is permeable to smallmolecules such as sugars, salts and nucleotides, the innermembrane is selectively permeable. This enables themitochondrion to control the chemical composition of thematrix, thus optimising conditions for enzyme activity.

The inner membrane isspanned by proteins (porins)

Allows entry of pyrovic acid and oxygen and the exit ofATP and carbon dioxide.

The inner membrane is foldedto form cristae

Greatly increases the surface area for the attachment ofenzymes and co-enzymes involved in the electron transferchain and allows the sequential attachment of electroncarriers in the ETC.

nuclearpore

nucleolus

nucleoplasmcontainingchromatin

nuclearenvelope

loop ofDNA

matrix

fluid-filledspace inner

membrane

outermembrane

ribosomes

(i) The matrix contains 70Sribosomes

(ii) DNA

(iii) enzymes eg. decarboxylase

For protein manufacture eg. enzymes.

codes for proteins.

eg. in Kreb's cycle

Nucleus

crista

Ribosome Ribosomes provide:(i) Binding sites for the binding of mRNA which

allows translation of the DNA code.

(ii) Two sites for the binding of 2 tRNA molecules.

(iii) The enzymes necessary for (i) and (ii).

Ribosomes recognise the initiation and termination codonson mRNA.

Ribosomes are capable of moving along the mRNA strand.This allows decoding of the mRNA and synthesis of apolypeptide chain.

largesubunit

smallsubunit

Ribosomes consist of two sub-units both made of rRNA andprotein. The rRNA part of theribosome is formed in thenucleus and moves out of thenucleus via the pores. Theprotein part is then assembledin the cytoplasm. Ribosomesmay occur in dense clusters inthe cytoplasm where they areknown as polysomes or mayoccur on the membranes of theendoplasmic reticulum.

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Cell Structure Function

Endoplasmic Reticulum Endoplasmic reticulum is asystem of hollow tubes andsacs.

Rough endoplasmic reticulum(rer) is covered with ribosomesand consists of aninterconnected system offlattened sacs,

Smooth endoplasmic reticulum(ser) - which lacks ribosomes -is a system of interconnectedtubules.

Allows transport of substances within the cell.

The ribosomes on the rough endoplasmic reticulum maysynthesise proteins which can then be transported throughthe cell in the cavities of the endoplasmic reticulum. Thepercentage of rer is high in cells which produce proteins forexport e.g. - digestive enzymes, growth factors and serumproteins. The smooth endoplasmic reticulum can give riseto Golgi bodies and this allows the packaging of newlyproduced proteins in Golgi vesicles which can then moveto the cell surface membrane for secretion.

The smooth endoplasmic reticulum is the site ofcarbohydrate and lipid metabolism eg. it synthesisestriglycerides, cholesterol and phospholipids which becomepart of the cell surface membrane and is also involved in themodification of substances such as steroid hormones whichwill then be secreted.

The percentage of smooth endoplasmic reticulum is highin cells which are involved in the metabolism of lipidsand drugs.

Golgi body Allows internal transport. Vesicles contain materials to besecreted. Vesicles protect the molecules as they aretransported across the cytoplasm to the cell surfacemembrane.

Proteins from the rer are modified before secretion. Forexample, carbohydrates may be added to proteins to formglycoproteins such as mucus which can then be enclosed invesicles for secretion out of the cell. Golgi vesicles mayalso fuse with primary lysosomes which will then formsecondary lysosomes capable of digesting food particles.

Golgi body consists offlattened cisternae (membranebound cavities) which may bestacked on top of each otherand which may invaginate andfuse to form vesicles

The Golgi body is connected tothe rer

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Cell Structure Function

Chloroplasts. Double Membrane

The stroma contains a series ofmembrane-bound flattened sacscalled thylakoid membranes.Thylakoid membranes may bestacked into grana.

The chloroplast stroma contains:

(i) Starch grains .

(ii) Lipid stores - otherwiseknown as plastoglobuli.

(iii) Pyrenoids - crystallisedRuBPC.

(iv) DNA RNA andribosomes.

Allows the isolation of photosynthetic reactions.

Grana allow a huge surface area for the assembly ofchlorophyll molecules for light absorption and also allowthe sequential attachment of enzymes and co-enzymesinvolved in the electron transfer chain of the light-dependentstage. Such membranes also allow quite different chemicalreactions to occur in different parts of the chloroplast.

(i) Which act as a carbohydrate store.

(ii) Accumulate when membranes have beenbroken down, for example during senescence.

(iii) The enzyme which fixes carbon dioxide.

(iv) All involved in nucleic acid and protein synthesis.

loopof DNA

innermembrane stroma

lamella

lipidstore starch

grain

thylakoids

Lysosomes When released, these enzymes can break down oldorganelles, storage molecules or, indeed, the whole cell,when it dies.

Lysosomes are vesicles whichcontain hydrolytic enzyme,collectively known aslysozymes.

Movement of substances within the cell.The organelles within eukaryotic cells workclosely together. For example, in the productionof a secretary protein such as a digestive enzyme:

1. The genetic code for the protein lies inthe chromatin granules in thenucleoplasm of the nucleus.

2. This code, now in the form of mRNA,moves through the nuclear pores.

3. The mRNA attaches itself to ribosomeson the rough endoplasmic reticulumwhich is continuous with the outermembrane of the nuclear envelope.

4. The code is translated into apolypeptide chain.

5. The polypeptide pass into the lumen ofthe endoplasmic reticulum.

6. The polypeptide is transferred to thegolgi body and packaged in a vesicle.

7. The vesicle merges with the cell surfacemembranes and the protein is released.

rernucleus golgi vesicle

ribosome

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Acknowledgements;This Factsheet was written and researched by Kevin Byrne

Curriculum Press, Unit 305B, The Big Peg,120 Vyse Street, Birmingham. B18 6NF

Biopress Factsheets may be copied free of charge by teachingstaff or students, provided that their school is a registeredsubscriber.ISSN 1351-5136

Practice Questions

1. Outline the similarities between chloroplasts and mitochondria.

2. The diagram shows a generalised eukaryotic cell.

Answers

Marking points are shown by semicolons

1. Both organelles are surrounded by two membranes;Both show internal compartmentalisation - i.e. internal membraneswhich allow different reactions to occur in different parts of theorganelle;Both have DNA;Both have ribosomes;Both are therefore capable of enzyme synthesis;Both possess a readily permeable outer membrane and a selectivelypermeable inner membrane;In both cases, permeability is brought about by proteins (porins)which span the membrane;

2. (i) Nuclear membrane;(ii) Mitochondrion;(iii) Golgi body;

3. See text

4. (i) Golgi body;(ii) Mitochondrion;(iii) Nucleus;(iv) Electron transfer chain reactions/enzymes;(v) Control of entry/exit of substance;(vi) Increases surface area for chlorophyll to absorb light/allows

sequential arrangement of electron carriers;(vii) Ribosomes synthesise protein which can be transported

through the endoplasmic reticulum

Identify structure:

(i) A(ii) B(iii) C

3. Explain how the structure of each of the followingorganelles aids its function:

(i) chloroplast(ii) mitochondrion

4. Complete the table below by filling in the blanks:

B

A

C

Organelle Structure/Features Function

.....(i)............ Flattened cisternae Carbohydrate andlipid metabolism

.....(ii)............ Internal membranes greatly Increases surface areafolded into cristae for .....(iv)...........

.....(iii)............ Pores normally blocked by an .....(v)............RNA/protein complex

Chloroplast Thylakoid membranes stacked .....(vi)............into grana

Rough endoplasmic Flattened interconnecting sacs .....(vii)............reticulum covered in ribosomes

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