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Number 198
Chloroplasts – Structure and Function
The lamellae:
• Provide a large surface area for the attachment of the enzymes
and co-enzymes involved in the electron transfer chain of the
light dependent reactions.
Chloroplasts are the site of photosynthesis in green plants. This is
the process by which carbon dioxide and water are converted into
organic chemicals using light as a source of energy. Oxygen is
produced as a very useful by-product of the reaction.
•
This Factsheet explains how the structure of a chloroplast is
designed to enable it to carry out its functions effectively and reviews
recent exam questions on this topic
Exam Hint:– Whenever a structure or cell possesses extra
membranes it increases the surface area available, in this
case for the attachment of extra pigment molecules, but in
other cells it can increase the rate of absorption (e.g. microvilli
of kidney tubule cells), cristae in mitochondria for attachment
of extra stalked particles).
Fig 1. The structure of a chloroplast
outer membrane
inner membrane
Partition the stroma so that different chemical reactions can
take place without interfering with each other.
chloroplast
envelope
The biconvex shape of the chloroplast is yet another way of
increasing surface area to maximise absorption of light energy
ribosomes
(70S)
Sometimes in the exam you will be presented with an electron
micrograph of a chloroplast. Usually, the first question simply asks
you to label it.
lipid
droplet
starch grain
intergranal
lamella
stroma
(matrix)
Typical Exam Question
Label parts A B & C
one granum
(stack of
disk-like
thylakoids)
A
C
Exam Hint: – You may be asked to identify the different
structures in a chloroplast so make sure you learn the diagram.
Chloroplasts are organelles bounded by a double membrane – this
separates the contents of the chloroplast from the rest of the cell,
allowing the photosynthetic reactions to take place without
interference from chemicals in the cytoplasm.
Answer
B
A - stroma;
B - granum / grana / thylakoids;
C - starch (grain);
Extract from Chief Examiner’s report
Many candidates appear to believe that chlorophyll and
chloroplasts are tissues.
Pigment molecules absorb light energy which ultimately becomes
converted to chemical energy incorporated into the glucose and
other compounds produced as a result of photosynthesis.
Chlorophyll a and chlorophyll b absorb only certain wavelengths
of light and therefore only a certain amount of the energy falling
on the chloroplast. Xanthophyll, carotene and phaeophytin are
accessory pigments that absorb light of other wavelengths,
ensuring more energy is trapped by the chloroplast.
Plate-like membranes, called lamellae (thylakoids) run along the
length of the chloroplast from end to end – these are the site of the
light-dependent reactions.
Chlorophyll molecules (and other pigment molecules) that absorb
light energy are situated on the lamellae. At intervals, extra lamellae
are inserted to form structures called grana (singular:granum). A
chloroplast contains approximately 3000 lamellae -they increase the
surface area available for the attachment of extra pigment molecules
to trap more light energy. The pigment molecules are arranged in
such a way that they do not shade each other so that the maximum
amount of light energy can be absorbed.
The pigment molecules form clusters called photosystems. Each
photosystem consists of a central chlorophyll molecule called
the primary pigment molecule. Located around this are several
hundred accessory pigment molecules – light energy absorbed
by all these molecules is transferred to the central primary pigment
molecule.
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Bio Factsheet
198. Chloroplasts – Structure and Function
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Isolating chloroplasts
In green plants most of the chloroplasts are found in the leaf
mesophyll, mainly in the palisade cells. These are located near
the top of the leaf where most light falls.
Typical Exam Question
Starting with some lettuce leaves, describe how you would
obtain a sample of undamaged chloroplasts. Use your
knowledge of cell fractionation and ultracentrifugation to
answer this question.
Exam Hint:– a synoptic question may ask you about the
structure of a chloroplast and then go on to the chemical
reactions involved, so make sure you understand the lightdependent and light-independent reactions.
Markscheme
1. Chop up (accept any reference to crude breaking up);
2. Cold;
3. Buffer solution;
4. Isotonic / same water potential;
5. Filter and centrifuge filtrate;
6. Centrifuge supernatant;
7. At higher speed;
8. Chloroplasts in (second) pellet;
The light-dependent reactions produce ATP and NADPH which
are required for the subsequent synthesis of carbohydrates in
the light-independent reactions.
Summary
Chloroplasts are highly specialised cell organelles, their structure
reflecting their function.
Make sure you understand how the following structures are
designed to enable photosynthesis to be carried out as efficiently
as possible
The lamellae are embedded in a watery matrix called the stroma. The
stroma contains numerous starch grains, lipid droplets, enzymes,
dissolved salts, ribosomes and a circular strand of DNA. It is in the
stroma that the light-independent reactions take place.
•
•
•
•
•
•
The DNA enables chloroplasts to replicate themselves when the
cell divides, ensuring new cells receive the correct number of
chloroplasts.
The possession of DNA and ribosomes by individual organelles
is unusual. The only other structures where this occurs are
mitochondria.
double membrane
lamellae
pigment molecules
stroma
DNA
ribosomes
Some Chief Examiners like to set questions using unfamiliar diagrams.
This doesn’t make the questions any harder – you just need to
keep calm and work them out.
Ribosomes produce proteins for use by the chloroplasts, particularly
the enzymes required for the photosynthetic reactions.The
ribosomes occur in the stroma, attached to the lamellae,
or in groups called polyribosomes.
Fig 2. Structure of leaf
Cross section of leaf
Palisade cell
cuticle
nucleus
cell wall
palisade
mesophyll
chloroplast
chloroplast
spongy
mesophyll
vacuole
air space
Individual palisade cells contain numerous chloroplasts to trap light
energy. The chloroplasts are arranged around the edge of the cell so
that light can reach them without having to pass through the
cytoplasm.
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stomata
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198. Chloroplasts – Structure and Function
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Position of chloroplasts in leaf tissue
Untypical ( but real) Exam Question
The arrangement of chloroplast-containing cells and tissue within
leaves also increases the efficiency of light capture.
The light-dependent stage of photosynthesis takes place on
thylakoid membranes in chloroplasts. These membranes are
arranged into stacks known as grana. The diagram shows the
arrangement of photosytems in the thylakoid membrane, and
summarises the processes that take place there.
light
H+
light
NADP
stroma
ADP + Pi H+
thylakoid
membrane
C
2e B
P680
Explain how the palisade mesophyl cells within a leaf are well
adapted for photosynthesis
AT P
closely packed to absorb more of incident light;
columnar shape / arranged at right angles to surface of leaf, to
reduce number of light absorbing cross walls;
large vacuole pushes chloroplasts to edge of cell;
chloroplasts on periphery of cell, short (diffusion) path for carbon
dioxide;
chloroplasts on periphery of cell to absorb light;
large number of chloroplasts / much chlorophyll, to absorb light ;
chloroplasts can move within cells to absorb as much light as
possible;
chloroplasts can move to prevent damage (in high light intensity) ;
cylindrical cells resulting in air spaces;
air spaces (between cells) to allow circulation of gases;
large surface area for, gas exchange/diffusion;
cell walls are thin, so short diffusion pathway/(greater) light
penetration;
air spaces act as reservoir of carbon dioxide;
AVP; e.g. non pigmented vacuole to allow light penetration
AT P
Sythetase
NADPred
E
A
2e -
Typical Exam Question
2e P700
thylakoid
H+
H2 O
space
2H+ + ½O2
(lumen)
H+ + +
H+ H+ H
H
(a) What pigment is represented by P680 and P700? (1)
(b) Name the type of molecule represented by B. (1)
(c) Name the process in which water molecules are split. (1)
(d) What is the advantage of having membranes stacked into
grana? (2)
Markscheme
(a) chlorophyll
(b) electron carrier / cytochrome / protein / electron acceptor /
ferredoxin /plastoquinone;
(c) photolysis
(d) increases surface area;
for absorption of light;
Synoptic Questions
These questions require you to pull together your knowledge of
different parts of the specification – to do some joined – up Biology
thinking.
So even though the diagram looked horrific, the questions were
actually very straightforward.
Typical Synoptic Question
Chloroplasts contain both polypeptides and ribosomes. An
investigation was carried out to determine whether any of these
polypeptides were synthesised by chloroplast ribosomes.
Typical Exam Question
The diagram shows the structure of a photosynthetic bacterium.
An extract of isolated chloroplasts was obtained. The number of
different, newly synthesised polypeptides produced in this extract
was measured after incubating in the light and in the dark.
(a) Explain why isotonic saline solution was used to wash the
isolated chloroplasts
(2 marks)
(b) Radioactive amino acids were added to the extract. Explain
why the use of radioactive amino acids would enable the
investigators to find and identify the polypeptides which had
been newly synthesised.
(2 marks)
(c) It was found that more polypeptides were synthesised in the
light than in the dark. Suggest an explanation for this.
(2 marks)
Markscheme
Describe two ways in which the structure of a chloroplast differs
from the structure of this bacterial cell.(2)
(a) prevent excess water entering or leaving the chloroplasts/
same water potential;
prevent lysis / shrinkage of chloroplasts;
(b) amino acids would be used to synthesise polypeptides;
use autoradiography / UV / X- rays / photographic film;
(c) ATP from photophosphorylation / photosynthesis/ light
dependent reaction needed;
Markscheme
chloroplast has no cell wall;
two membranes surrounding chloroplast;
grana/thylakoids;
starch granules;
Exam Hint:- Questions about chloroplasts nearly always link them to other aspects of photosynthesis, so make sure you revise the
light-dependent and light -independent reactions, action and absorption spectra and factors that affect the rate of photosynthesis.
Data interpretation questions involving photosynthesis experiments and limiting factors are also common.
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Acknowledgements:
This Factsheet was researched and written by Phill Hargrave.
Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.
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form or by any other means, without the prior permission of the publisher. ISSN 1351-5136
Suggest two structural features of mitochondria and chloroplasts
that are also present in prokaryotic cells (2)
Answers
1. B
2. (a) A double membrane/envelope;
B stroma;
C lamellae/grana/thylakoid;
(b) (i) C;
(ii) B;
4. Many biologists believe that both chloroplasts and mitochondria
evolved from prokaryotic organisms that inhabited the
cytoplasm of eukaryotic host cells.
(c) numerous membranes for attachment of chlorophyll/pigment
molecules;
pigment molecules absorb light energy;
provide large surface area for attachment of extra pigment
molecules;
provide large surface area for attachment of enzymes/coenzymes (involved in light- dependent reactions);
partition stroma to separate chemical reactions; (Any 3)
3. Explain how chloroplasts carry out photosynthesis. (10 marks)
(a) Name the parts labelled A, B and C.(3)
(b) Give the letter of the structure where
(i) the light-dependent reactions of photosynthesis take
place (1)
(ii) the light-independent reactions take place (1)
(c) Describe three ways in which structure C is adapted for
photosynthesis (3)
3. A = accept AVP = acceptable valid points
1large surface area of chloroplast, qualified; e.g. biconvex shape;
grana / thylakoid(s) (membranes), give large surface area;
site of, light dependent reactions / photophosphorylation; ref
to energy, transduction / conversion;
chlorophyll / pigments, for light absorption / lose excited
electrons; ref to wavelengths absorbed by chlorophyll (blue +
red or 450 + 680 nm);
ref to (orientation of) chlorophyll in membrane;
other photosynthetic pigments / named pigment(s);
absorb different wavelengths of light; arranged into
photosystems / light harvesting complexes (or clusters);
A quantasomes;
chlorophyll (a) molecule at centre of, photosystem / reaction
centre; pigments ‘funnel’ electrons to centre of photosystem
(idea of antennae /);
electron carriers / ETC system, in membrane;
proton pumps / chemiosmosis / ref to movement of hydrogen
ions / protons into thylakoid space;
ATP production / synthetase / ATP ase;
NADP present; Calvin cycle / light independent stage, enzymes
in stroma;
ref to rubisco;
ref to storage of starch or lipid;
ref DNA / ribosomes, making proteins;
AVP; e.g. double membrane qualified photosystem 1 and 2 have
different absorption peaks
A
C
B
2. The diagram below shows the structure of a chloroplast, as
seen using the electron microscope.
Which organelle is the chloroplast? (1)
Smaller organelle surrounded
by an outer membrane. Has an
inner membrane, folded to
form cristae.
Many in a cell.
Organelle D
Organelle found in
plant cells. Contains
inner membranes
arranged in stacks
Organelle B
4. free / naked,
DNA / DNA not surrounded by membrane / envelope have an
inner folded membrane ;
ribosomes, smaller than those in cytosol / similar in size to
prokaryotic ribosomes ;
A ref to 70S and 80S circular DNA ;
absence of introns R absence of a nucleus from the chloroplast
or mitochondrion R ref to membranous organelles as
chloroplasts and mitochondria are these organelles
Larger organelle
surrounded by an
envelope through which
there are pores.
Usually one per cell.
Organelle C
Organelle found in
animal cells and in plant
cells. Does not contain
membranes arranged
in stacks
Organelle found in prokaryotic
and eukaryotic cells
Organelle A
Organelle found only in
eukaryotic cells
1. The diagram shows how some organelles may be distinguished
from each other.
Practice questions
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Bio Factsheet
198. Chloroplasts – Structure and Function