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Bio Factsheet
September 1997 Number 4
Structure to function in eukaryotic cells
The cell surface membrane and the membranes which form organelles in eukaryotic cells all have the same basic structure, known as the fluid mosaic model. Such membranes provide control of the entry and exit of substances into cells and organelles and such control is a result of the phospholipid bilayer and membrane proteins.
In eukaryotic cells, such membranes divide the cytoplasm into multiple compartments (organelles). Organelles allow different functions to occur efficiently and simultaneously in different parts of the cell. For example, the outer double membrane of the mitochondrion separates out those reactions which occur in mitochondria from those in the general cytoplasm. Furthermore, the internal membranes of the mitochondria allow the enzymic reactions of the Kreb’s cycle to be kept quite separate from the electron transfer chain reactions (ETC). This is essential since both sets of reactions have different enzymes, hence different pH optima. By splitting up the cytoplasm 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 of eukaryotic cells.
The relationship between structure and function can be described in terms of the whole cell or in terms of the individual organelles of the eukaryotic cell.
Table 1 describes the structure and function of a motor neuron cell which is commonly featured in examination questions. The structure and function of eukaryotic organelles is described overleaf.
Cell Structure Function
Motor Neuron
Long axons
Axon contains axoplasm
For rapid transmission of nerve impulse. Synapses, where two nerves join, is the slowest part of transmission, so the longer the axon, the fewer the synapses and the faster the impulses transmitted. Allows transport between cell body and axon
Nodes of Ranvier between Schwann cells
High phospholipid content in membrane of Schwann cell
Allows Na +^ /K +^ pump to operate which sets up resting potential. Schwann cells of myelin sheath speed up the impulse because they increase the surface area for transmission of current.
Provides electrical insulation.
Synaptic knob at end of dendrite contains: (i) many mitochondria
(ii) numerous vesicles
To provide ATP for active refilling of synaptic vesicles.
For modification and release of chemical transmitters across the synapse.
Many dendrites To allow communication with other neurons.
Cell body contains:
(i) Nucleus
(ii) dense groups of ribosomes and endoplasmic reticulum called Nissl granules
Provides the genetic code for the production of neurotransmitter substances, e.g. acetylcholine and enzymes, eg. cholinesterase.
For production and transport of proteins and neurotransmitters.
axon may be a metre or more in length
nodes of Ranvier
axon
direction of nerve impulse
Table 1.
Exam Hint - Structure to function questions are very commonly set on all A level Biology syllabuses. They are one of the syllabus areas where all candidates should be capable of gaining the highest marks. Once the functions of organelles have been memorised, candidates should become confident at interpreting the function of unknown cells.
Nissl granules
nucleus
dendrites
Structure and Function of Organelles Structure Function
Double nuclear envelope To enclose and protect DNA (normally visible as chromatin granules).
Nuclear pores.
Normally, the nuclear pores are plugged by an RNA/protein complex.
Allow entry of substances such as nucleotides for DNA replication and exit of molecules such as mRNA during protein synthesis.
Small molecules pass through the pores by diffusion, whereas large molecules such as partly completed ribosomes pass through actively.
Nucleoplasm contains chromatin granules made of DNA and associated proteins.
Nucleoplasm also contains nucleoli
It is these which, during cell division, condense to form chromosomes.
Produces partly-completed ribosomes, coenzymes, nucleotides, proteins (including enzymes for nucleic acid synthesis) and RNA molecules.
The outer membrane of the nuclear envelope is continuous with the rough endoplasmic reticulum membranes.
This makes the perinuclear space continuous with the lumen of the endoplasmic reticulum, thus allowing easy transport of substances.
Mitochondrion Double membrane Isolates reactions of the Kreb’s cycle and electron transfer chain from the general cytoplasm. Such compartmentalisation allows high concentrations of enzymes and substrates to be maintained which increases the rate of respiratory reactions.
Whereas the outer membrane is permeable to small molecules such as sugars, salts and nucleotides, the inner membrane is selectively permeable. This enables the mitochondrion to control the chemical composition of the matrix, thus optimising conditions for enzyme activity.
The inner membrane is spanned by proteins (porins)
Allows entry of pyrovic acid and oxygen and the exit of ATP and carbon dioxide.
The inner membrane is folded to form cristae
Greatly increases the surface area for the attachment of enzymes and co-enzymes involved in the electron transfer chain and allows the sequential attachment of electron carriers in the ETC.
nuclear pore
nucleolus
nucleoplasm containing chromatin
nuclear envelope
loop of DNA
matrix
fluid-filled space (^) inner membrane
outer membrane
ribosomes
(i) The matrix contains 70S ribosomes
(ii) DNA
(iii) enzymes eg. decarboxylase
For protein manufacture eg. enzymes.
codes for proteins.
eg. in Kreb's cycle
Nucleus
crista
Cell Structure Function
Chloroplasts. Double Membrane
The stroma contains a series of membrane-bound flattened sacs called thylakoid membranes. Thylakoid membranes may be stacked into grana.
The chloroplast stroma contains:
(i) Starch grains.
(ii) Lipid stores - otherwise known as plastoglobuli.
(iii) Pyrenoids - crystallised RuBPC.
(iv) DNA RNA and ribosomes.
Allows the isolation of photosynthetic reactions.
Grana allow a huge surface area for the assembly of chlorophyll molecules for light absorption and also allow the sequential attachment of enzymes and co-enzymes involved in the electron transfer chain of the light-dependent stage. Such membranes also allow quite different chemical reactions to occur in different parts of the chloroplast.
(i) Which act as a carbohydrate store.
(ii) Accumulate when membranes have been broken down, for example during senescence.
(iii) The enzyme which fixes carbon dioxide.
(iv) All involved in nucleic acid and protein synthesis.
loop of DNA inner membrane (^) stroma
lamella lipid store (^) starch grain
thylakoids
Lysosomes When released, these enzymes can break down old organelles, storage molecules or, indeed, the whole cell, when it dies.
Lysosomes are vesicles which contain hydrolytic enzyme, collectively known as lysozymes.
Movement of substances within the cell.
The organelles within eukaryotic cells work closely together. For example, in the production of a secretary protein such as a digestive enzyme:
- The genetic code for the protein lies in the chromatin granules in the nucleoplasm of the nucleus.
- This code, now in the form of mRNA, moves through the nuclear pores.
- The mRNA attaches itself to ribosomes on the rough endoplasmic reticulum which is continuous with the outer membrane of the nuclear envelope.
- The code is translated into a polypeptide chain.
- The polypeptide pass into the lumen of the endoplasmic reticulum.
- The polypeptide is transferred to the golgi body and packaged in a vesicle.
- The vesicle merges with the cell surface membranes and the protein is released.
rer nucleus golgi vesicle
ribosome
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 teaching staff or students, provided that their school is a registered subscriber.
ISSN 1351-
Practice Questions
- Outline the similarities between chloroplasts and mitochondria.
- The diagram shows a generalised eukaryotic cell.
Answers
Marking points are shown by semicolons
- Both organelles are surrounded by two membranes; Both show internal compartmentalisation - i.e. internal membranes which allow different reactions to occur in different parts of the organelle; Both have DNA; Both have ribosomes; Both are therefore capable of enzyme synthesis; Both possess a readily permeable outer membrane and a selectively permeable inner membrane; In both cases, permeability is brought about by proteins (porins) which span the membrane;
- (i) Nuclear membrane; (ii) Mitochondrion; (iii) Golgi body;
- See text
- (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
- Explain how the structure of each of the following organelles aids its function:
(i) chloroplast (ii) mitochondrion
- Complete the table below by filling in the blanks:
B
A
C
Organelle Structure/Features Function
.....(i)............ Flattened cisternae Carbohydrate and lipid metabolism
.....(ii)............ Internal membranes greatly Increases surface area folded 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
