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Chloroplasts – Structure and FunctionChloroplasts are the site of photosynthesis in green plants. This isthe process by which carbon dioxide and water are converted intoorganic chemicals using light as a source of energy. Oxygen isproduced as a very useful by-product of the reaction.

This Factsheet explains how the structure of a chloroplast isdesigned to enable it to carry out its functions effectively and reviewsrecent exam questions on this topic

Fig 1. The structure of a chloroplast

Exam Hint: – You may be asked to identify the differentstructures in a chloroplast so make sure you learn the diagram.

Chloroplasts are organelles bounded by a double membrane – thisseparates the contents of the chloroplast from the rest of the cell,allowing the photosynthetic reactions to take place withoutinterference from chemicals in the cytoplasm.

Extract from Chief Examiner’s reportMany candidates appear to believe that chlorophyll andchloroplasts are tissues.

Plate-like membranes, called lamellae (thylakoids) run along thelength of the chloroplast from end to end – these are the site of thelight-dependent reactions.

Chlorophyll molecules (and other pigment molecules) that absorblight energy are situated on the lamellae. At intervals, extra lamellaeare inserted to form structures called grana (singular:granum). Achloroplast contains approximately 3000 lamellae -they increase thesurface area available for the attachment of extra pigment moleculesto trap more light energy. The pigment molecules are arranged insuch a way that they do not shade each other so that the maximumamount of light energy can be absorbed.

one granum(stack ofdisk-likethylakoids)

stroma(matrix)intergranal

lamella

starch grain

lipiddroplet

ribosomes(70S)

outer membrane

inner membrane

chloroplastenvelope

The lamellae:• Provide a large surface area for the attachment of the enzymes

and co-enzymes involved in the electron transfer chain of thelight dependent reactions.

• Partition the stroma so that different chemical reactions cantake place without interfering with each other.

Exam Hint:– Whenever a structure or cell possesses extramembranes it increases the surface area available, in thiscase for the attachment of extra pigment molecules, but inother cells it can increase the rate of absorption (e.g. microvilliof kidney tubule cells), cristae in mitochondria for attachmentof extra stalked particles).

The biconvex shape of the chloroplast is yet another way ofincreasing surface area to maximise absorption of light energy

Sometimes in the exam you will be presented with an electronmicrograph of a chloroplast. Usually, the first question simply asksyou to label it.

Typical Exam QuestionLabel parts A B & C

A

B

C

AnswerA - stroma;B - granum / grana / thylakoids;C - starch (grain);

Pigment molecules absorb light energy which ultimately becomesconverted to chemical energy incorporated into the glucose andother compounds produced as a result of photosynthesis.Chlorophyll a and chlorophyll b absorb only certain wavelengthsof light and therefore only a certain amount of the energy fallingon the chloroplast. Xanthophyll, carotene and phaeophytin areaccessory pigments that absorb light of other wavelengths,ensuring more energy is trapped by the chloroplast.

The pigment molecules form clusters called photosystems. Eachphotosystem consists of a central chlorophyll molecule calledthe primary pigment molecule. Located around this are severalhundred accessory pigment molecules – light energy absorbedby all these molecules is transferred to the central primary pigmentmolecule.

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Isolating chloroplasts

Typical Exam QuestionStarting with some lettuce leaves, describe how you wouldobtain a sample of undamaged chloroplasts. Use yourknowledge of cell fractionation and ultracentrifugation toanswer this question.

Markscheme1. 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 lamellae are embedded in a watery matrix called the stroma. Thestroma contains numerous starch grains, lipid droplets, enzymes,dissolved salts, ribosomes and a circular strand of DNA. It is in thestroma that the light-independent reactions take place.

The DNA enables chloroplasts to replicate themselves when thecell divides, ensuring new cells receive the correct number ofchloroplasts.

The possession of DNA and ribosomes by individual organellesis unusual. The only other structures where this occurs aremitochondria.

Ribosomes produce proteins for use by the chloroplasts, particularlythe enzymes required for the photosynthetic reactions.Theribosomes occur in the stroma, attached to the lamellae,or in groups called polyribosomes.

Exam Hint:– a synoptic question may ask you about thestructure of a chloroplast and then go on to the chemicalreactions involved, so make sure you understand the light-dependent and light-independent reactions.

The light-dependent reactions produce ATP and NADPH whichare required for the subsequent synthesis of carbohydrates inthe light-independent reactions.

Individual palisade cells contain numerous chloroplasts to trap lightenergy. The chloroplasts are arranged around the edge of the cell sothat light can reach them without having to pass through thecytoplasm.

Palisade cell

vacuole

chloroplast

cell wall

nucleus

chloroplast

cuticle

stomata

air space

palisademesophyll

spongymesophyll

Cross section of leaf

Fig 2. Structure of leaf

In green plants most of the chloroplasts are found in the leafmesophyll, mainly in the palisade cells. These are located nearthe top of the leaf where most light falls.

SummaryChloroplasts are highly specialised cell organelles, their structurereflecting their function.

Make sure you understand how the following structures aredesigned to enable photosynthesis to be carried out as efficientlyas possible

• 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 tokeep calm and work them out.

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Untypical ( but real) Exam QuestionThe light-dependent stage of photosynthesis takes place onthylakoid membranes in chloroplasts. These membranes arearranged into stacks known as grana. The diagram shows thearrangement of photosytems in the thylakoid membrane, andsummarises the processes that take place there.

P680

2e-E

P700

CA

light light

2e-

2e-B

NADPred

NADPH+ADP + P

i

AT PSythetase

H+

H+

AT P

stroma

thylakoidspace(lumen)

thylakoidmembrane

2H+ + ½O2

H2O 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;

So even though the diagram looked horrific, the questions wereactually very straightforward.

Typical Exam QuestionThe diagram shows the structure of a photosynthetic bacterium.

Describe two ways in which the structure of a chloroplast differsfrom the structure of this bacterial cell.(2)

Markschemechloroplast has no cell wall;two membranes surrounding chloroplast;grana/thylakoids;starch granules;

Position of chloroplasts in leaf tissueThe arrangement of chloroplast-containing cells and tissue withinleaves also increases the efficiency of light capture.

Typical Exam QuestionExplain how the palisade mesophyl cells within a leaf are welladapted for photosynthesis

closely packed to absorb more of incident light;columnar shape / arranged at right angles to surface of leaf, toreduce number of light absorbing cross walls;large vacuole pushes chloroplasts to edge of cell;chloroplasts on periphery of cell, short (diffusion) path for carbondioxide;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 aspossible;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) lightpenetration;air spaces act as reservoir of carbon dioxide;AVP; e.g. non pigmented vacuole to allow light penetration

Synoptic QuestionsThese questions require you to pull together your knowledge ofdifferent parts of the specification – to do some joined – up Biologythinking.

Typical Synoptic QuestionChloroplasts contain both polypeptides and ribosomes. Aninvestigation was carried out to determine whether any of thesepolypeptides were synthesised by chloroplast ribosomes.

An extract of isolated chloroplasts was obtained. The number ofdifferent, newly synthesised polypeptides produced in this extractwas measured after incubating in the light and in the dark.

(a) Explain why isotonic saline solution was used to wash theisolated chloroplasts (2 marks)

(b) Radioactive amino acids were added to the extract. Explainwhy the use of radioactive amino acids would enable theinvestigators to find and identify the polypeptides which hadbeen newly synthesised. (2 marks)

(c) It was found that more polypeptides were synthesised in thelight than in the dark. Suggest an explanation for this.

(2 marks)

Markscheme(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/ lightdependent reaction needed;

Exam Hint:- Questions about chloroplasts nearly always link them to other aspects of photosynthesis, so make sure you revise thelight-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.Bio Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registeredsubscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any otherform or by any other means, without the prior permission of the publisher. ISSN 1351-5136

Practice questions1. The diagram shows how some organelles may be distinguished

from each other.

Organelle found in prokaryoticand eukaryotic cellsOrganelle A

Organelle found only ineukaryotic cells

Organelle found inanimal cells and in plantcells. Does not containmembranes arrangedin stacks

Organelle found inplant cells. Containsinner membranesarranged in stacksOrganelle B

Larger organellesurrounded by anenvelope through whichthere are pores.Usually one per cell.Organelle C

Smaller organelle surroundedby an outer membrane. Has aninner membrane, folded toform cristae.Many in a cell.Organelle D

Which organelle is the chloroplast? (1)

2. The diagram below shows the structure of a chloroplast, asseen using the electron microscope.

B

CA

(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 takeplace (1)

(ii) the light-independent reactions take place (1)(c) Describe three ways in which structure C is adapted for

photosynthesis (3)

3. Explain how chloroplasts carry out photosynthesis. (10 marks)

4. Many biologists believe that both chloroplasts and mitochondriaevolved from prokaryotic organisms that inhabited thecytoplasm of eukaryotic host cells.

Suggest two structural features of mitochondria and chloroplaststhat are also present in prokaryotic cells (2)

Answers1.B

2.(a)A double membrane/envelope;Bstroma;C lamellae/grana/thylakoid;

(b)(i)C; (ii) B;

(c) numerous membranes for attachment of chlorophyll/pigmentmolecules;pigment molecules absorb light energy;provide large surface area for attachment of extra pigmentmolecules;provide large surface area for attachment of enzymes/co-enzymes (involved in light- dependent reactions);partition stroma to separate chemical reactions;(Any 3)

3.A = accept AVP = acceptable valid points1large surface area of chloroplast, qualified; e.g. biconvex shape;grana / thylakoid(s) (membranes), give large surface area;site of, light dependent reactions / photophosphorylation; refto energy, transduction / conversion;chlorophyll / pigments, for light absorption / lose excitedelectrons; 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 intophotosystems / light harvesting complexes (or clusters);A quantasomes;chlorophyll (a) molecule at centre of, photosystem / reactioncentre; pigments ‘funnel’ electrons to centre of photosystem(idea of antennae /);electron carriers / ETC system, in membrane;proton pumps / chemiosmosis / ref to movement of hydrogenions / protons into thylakoid space;ATP production / synthetase / ATP ase; NADP present; Calvin cycle / light independent stage, enzymesin 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 havedifferent absorption peaks

4.free / naked,DNA / DNA not surrounded by membrane / envelope have aninner folded membrane ;ribosomes, smaller than those in cytosol / similar in size toprokaryotic ribosomes ;A ref to 70S and 80S circular DNA ;absence of introns R absence of a nucleus from the chloroplastor mitochondrion R ref to membranous organelles aschloroplasts and mitochondria are these organelles