Chloroplast structure

Introduction

Chloroplast structure -Chloroplasts are the most common and essential plastids in green plants as they are the site of photosynthesis. They possess the ability to convert radiant energy into energy stored in food material, which is directly and indirectly utilized by living organisms. Chloroplasts were first observed by Antonie van Leeuwenhoek in algae in 1800. Their primary role is to facilitate photosynthesis, which not only sustains plant life but also forms the basis of the food chain for heterotrophic organisms.

Morphology

The shape, size, and number of chloroplasts may vary among different species. However, within a single tissue type, their characteristics remain consistent.

Shape

In lower plants, chloroplasts exhibit variable shapes, including filamentous, discoid, spheroid, ovoid, or club-shaped.
In most algae, they appear reticulate, spiral, or stellate.
In higher plants, chloroplasts are biconcave or plano-convex in shape.
Some algae, such as Spirogyra, have spirally arranged chloroplasts that increase surface area for photosynthesis.
In Zygnema, chloroplasts appear stellate, while Chlamydomonas has cup-shaped chloroplasts.

Size

The size of chloroplasts is influenced by chromosome number and habitat.
Polyploid plants have larger chloroplasts than diploid plants.
Plants grown in the shade have larger chloroplasts and contain more chlorophyll compared to those exposed to direct sunlight.
The average size in higher plants ranges from 2-3 µm in thickness and 4-10 µm in length.
In algae, chloroplasts can reach up to 40 µm in size.

Number

The number of chloroplasts varies from cell to cell but remains constant within a particular cell type.
In algae, a single chloroplast per cell is common.
In higher plants, each cell typically contains 20 to 40 chloroplasts.
Some plant cells, such as mesophyll cells, may contain up to 100 chloroplasts.

Structural Organization

1. Chloroplast Envelope

The double-membrane system of the chloroplast serves as a barrier, separating the cytoplasm from the internal photosynthetic region.

Each lipoprotein-based membrane is 40-60 Å thick.
The outer membrane is permeable to small molecules, allowing free movement between the cytosol and the peri-plastid space (100-250 Å wide).
The inner membrane is selectively permeable, restricting the movement of larger molecules such as sugars. Transport of these molecules requires ATP and reducing power.
The inner membrane contains transport proteins, which regulate the exchange of metabolites such as phosphates, sugars, and ATP.

2. Stroma (Matrix)

The stroma is a watery, protein-rich, transparent substance that houses:

DNA, RNA, and ribosomes, enabling chloroplasts to synthesize some of their own proteins.
50% soluble proteins.
Starch grains for carbohydrate storage.
Plastoglobuli (lipid droplets) that increase in number during chloroplast inactivity.
Phytoferritin granules, which store iron-protein complexes.
Pyrenoids in algae, containing ribulose bisphosphate carboxylase (RuBisCO), a key enzyme in carbon fixation.
In higher plants, RuBisCO is found in stroma centers.
Grana and intergrana (stroma lamellae) are embedded in the stroma.
The stroma also contains enzymes necessary for the Calvin cycle, which converts CO₂ into organic compounds.
Chaperone proteins, which assist in the folding of newly synthesized proteins, are present in the stroma.

3. Grana and Fretwork System (Lamellar System)

Electron microscope studies reveal that the stroma contains grana, composed of stacks of thylakoids.

Each granum consists of 40-60 thylakoids.
Thylakoid size ranges from 0.3-1.7 µm.
Grana are interconnected by stroma lamellae (frets/intergrana), facilitating communication between them.
The thylakoid membrane contains chlorophyll and photosynthetic pigments, supporting the light-dependent reactions.
The thylakoid lumen provides an internal space where water photolysis occurs.
The stroma is the site of the Calvin cycle (light-independent reactions), where CO₂ fixation synthesizes sugars, starch, proteins, and fatty acids.

4. Thylakoid Membrane and Quantasome Concept

The thylakoid membrane consists of protein and lipid layers, with chlorophyll molecules sandwiched between them.
Park and Pon discovered photosynthetic units within the thylakoid membrane, called Quantasomes.
- These are paracrystalline, spheroid particles, each measuring 185 Å in length, 155 Å in width, and 100 Å in thickness.
- Each quantasome contains 300 chlorophyll molecules along with carotenoids and essential enzymes.
- Quantasomes function as structural and functional units of photosystems, where photochemical reactions occur.

5. Photosystems and Light Harvesting Complexes

Chloroplasts contain two distinct photosystems:

Photosystem I (PS-I): Contains P700, a pigment absorbing light at 700 nm.
Photosystem II (PS-II): Contains P680, a pigment absorbing light at 680 nm.

The pigments of PS-II absorb light energy and transfer it to PS-I, where photochemical reactions take place. All other pigment molecules function as light-harvesting molecules, absorbing light and transmitting it rapidly to reaction centers, enabling the conversion of light energy into chemical energy.

Chloroplast MCQs

Who first observed chloroplasts in algae?
a) Robert Hooke
b) Antonie van Leeuwenhoek ✅
c) Matthias Schleiden
d) Theodor Schwann
Chloroplasts are responsible for:
a) Respiration
b) Photosynthesis ✅
c) Protein synthesis
d) DNA replication
The shape of chloroplasts in Spirogyra is:
a) Discoid
b) Reticulate
c) Spiral ✅
d) Stellate
What is the primary pigment found in chloroplasts?
a) Xanthophyll
b) Chlorophyll ✅
c) Carotene
d) Anthocyanin
The number of chloroplasts per cell in higher plants is usually:
a) 1-5
b) 10-15
c) 20-40 ✅
d) 50-100
The inner membrane of the chloroplast is:
a) Fully permeable
b) Semi-permeable ✅
c) Impermeable
d) Selectively permeable to all molecules
The space between the outer and inner membranes of a chloroplast is called:
a) Periplasmic space
b) Peri-plastid space ✅
c) Stroma
d) Thylakoid space
The fluid-filled region inside a chloroplast where the Calvin cycle occurs is called:
a) Thylakoid
b) Grana
c) Stroma ✅
d) Cristae
Chloroplasts are found in:
a) Animal cells
b) Fungal cells
c) Plant cells and algae ✅
d) All prokaryotic cells
The function of the granum in chloroplasts is to:
a) Store ATP
b) Absorb light energy for photosynthesis ✅
c) Transport nutrients
d) Store starch

The membrane-bound compartments inside the chloroplast that contain chlorophyll are called:
a) Cristae
b) Thylakoids ✅
c) Ribosomes
d) Stroma
A stack of thylakoids is known as:
a) Grana ✅
b) Fret
c) Cristae
d) Pyrenoid
The grana of a chloroplast are interconnected by:
a) Lamellae ✅
b) Vesicles
c) Ribosomes
d) Nucleosomes
In which part of the chloroplast do light reactions occur?
a) Stroma
b) Inner membrane
c) Thylakoid membrane ✅
d) Outer membrane
What is the main function of the stroma?
a) Conducting light reactions
b) Synthesizing proteins
c) Carrying out the Calvin cycle ✅
d) Producing ATP directly
The reaction center of Photosystem I contains:
a) P700 ✅
b) P680
c) ATP synthase
d) RuBisCO
The pigment responsible for capturing light in Photosystem II is:
a) P700
b) P680 ✅
c) Carotenoids
d) Xanthophyll
The role of the thylakoid membrane is to:
a) Synthesize glucose
b) Capture light energy ✅
c) Store ATP
d) Break down carbohydrates
Where are ATP and NADPH produced in the chloroplast?
a) Stroma
b) Grana ✅
c) Cytoplasm
d) Mitochondria
Which molecule acts as the primary acceptor of electrons in Photosystem II?
a) NADPH
b) Ferredoxin
c) Pheophytin ✅
d) Plastocyanin

Which enzyme is responsible for carbon fixation in the Calvin cycle?
a) ATP synthase
b) RuBisCO ✅
c) NADP reductase
d) PEP carboxylase
What is the first stable product of carbon fixation in C3 plants?
a) Phosphoglycerate (PGA) ✅
b) Malate
c) Pyruvate
d) Oxaloacetate
What is the energy currency produced during photosynthesis?
a) NADPH
b) ATP ✅
c) Glucose
d) FADH2
Oxygen is released during:
a) Light-dependent reactions ✅
b) Light-independent reactions
c) Glycolysis
d) Krebs cycle
What molecule provides electrons for the light-dependent reactions?
a) NADH
b) Water ✅
c) Glucose
d) Oxygen
The dark reaction of photosynthesis occurs in the:
a) Thylakoid
b) Grana
c) Stroma ✅
d) Periplasmic space
Which molecule is reduced in the Calvin cycle?
a) CO₂ ✅
b) O₂
c) Glucose
d) ATP
The function of NADP+ in photosynthesis is to:
a) Transfer protons
b) Act as an electron carrier ✅
c) Produce ATP
d) Release oxygen
What is the final product of the Calvin cycle?
a) ATP
b) Glucose ✅
c) Oxygen
d) NADPH
The splitting of water in photosynthesis is called:
a) Hydrolysis
b) Photolysis ✅
c) Glycolysis
d) Fermentation

The enzyme ATP synthase is located in the:
a) Outer membrane
b) Thylakoid membrane ✅
c) Stroma
d) Cytoplasm
How many ATP molecules are consumed in the Calvin cycle to synthesize one glucose molecule?
a) 12
b) 18 ✅
c) 24
d) 36
The Calvin cycle is also known as:
a) C4 cycle
b) Light reaction
c) C3 cycle ✅
d) CAM pathway
What is the primary function of carotenoids in chloroplasts?
a) Provide ATP
b) Absorb excess light and protect chlorophyll ✅
c) Transport electrons
d) Fix carbon
The photosystems in chloroplasts are located in:
a) Stroma
b) Thylakoid membrane ✅
c) Outer membrane
d) Cristae
What is the first step in the light reaction?
a) Electron transport
b) Photolysis of water ✅
c) ATP production
d) CO₂ fixation
Chloroplast DNA is:
a) Linear
b) Circular ✅
c) Double-stranded linear
d) Single-stranded
Which compound is regenerated in the Calvin cycle?
a) ATP
b) RuBP ✅
c) Pyruvate
d) NADPH
The chloroplast genome contains genes for:
a) Photosynthesis-related proteins ✅
b) RNA synthesis only
c) Only pigments
d) DNA replication
The pH of the thylakoid lumen during photosynthesis:
a) Increases
b) Decreases ✅
c) Remains constant
d) Becomes neutral

Chloroplasts are believed to have originated from:
a) Mitochondria
b) Symbiotic cyanobacteria ✅
c) Fungi
d) Prokaryotic ribosomes
Which of the following is NOT a function of chloroplasts?
a) Photosynthesis
b) Fatty acid synthesis
c) Glycolysis ✅
d) Nitrogen metabolism
What is the main function of plastoglobuli in chloroplasts?
a) Store carbohydrates
b) Store lipids and assist in thylakoid maintenance ✅
c) Capture light energy
d) Transport protons
The pH of the chloroplast stroma during active photosynthesis is:
a) 4.5
b) 5.5
c) 6.8
d) 8.0 ✅
Which of the following molecules is NOT directly involved in the Calvin cycle?
a) ATP
b) NADPH
c) Oxygen ✅
d) RuBP
What happens when chloroplasts are exposed to intense light?
a) Photosynthesis stops
b) ATP production increases ✅
c) Chlorophyll breaks down
d) Chloroplasts become inactive
The Z-scheme in photosynthesis represents:
a) Electron transport pathway ✅
b) Carbon fixation
c) ATP production
d) Water splitting
In C4 plants, carbon fixation occurs initially in the:
a) Thylakoid lumen
b) Mesophyll cells ✅
c) Guard cells
d) Phloem tissue
The final electron acceptor in the light-dependent reactions of photosynthesis is:
a) NADP+ ✅
b) Oxygen
c) Water
d) Cytochrome
The enzyme responsible for ATP synthesis in the chloroplast is called:
a) RuBisCO
b) ATPase
c) ATP synthase ✅
d) Cytochrome c

Which type of microscopy is best for studying the ultrastructure of chloroplasts?
a) Light microscope
b) Transmission electron microscope (TEM) ✅
c) Fluorescence microscope
d) Compound microscope
Which method is used to isolate chloroplasts from plant cells?
a) Centrifugation ✅
b) Chromatography
c) Spectroscopy
d) Electrophoresis
What happens to chloroplasts in plants grown in the dark?
a) They turn into etioplasts ✅
b) They increase in number
c) They stop functioning permanently
d) They turn into mitochondria
Which of the following is NOT found in a chloroplast?
a) DNA
b) RNA
c) Ribosomes
d) Lysosomes ✅
Which plant adaptation reduces photorespiration?
a) CAM metabolism
b) C4 metabolism ✅
c) Increasing chlorophyll concentration
d) Closing stomata permanently
Genetic material in chloroplasts is:
a) Double-stranded linear DNA
b) Single-stranded circular DNA
c) Double-stranded circular DNA ✅
d) RNA only
What is the role of phytoferritin granules in chloroplasts?
a) Store iron ✅
b) Fix carbon
c) Store lipids
d) Act as energy reserves
In chloroplast DNA engineering, genes are usually inserted into the:
a) Stroma
b) Thylakoid membrane
c) Chloroplast genome ✅
d) Outer membrane
Chloroplast transformation is often used for:
a) Producing transgenic plants ✅
b) Inducing mutations
c) Increasing respiration
d) Stopping photosynthesis
Which of the following increases the efficiency of photosynthesis?
a) High oxygen concentration
b) Increasing CO₂ levels ✅
c) Low light intensity
d) High temperature

Which molecule acts as an electron carrier in photosynthesis?
a) ATP
b) NADPH ✅
c) FADH2
d) Oxygen
The process of cyclic photophosphorylation produces:
a) NADPH only
b) Oxygen
c) ATP only ✅
d) Glucose
Which part of the chloroplast is responsible for the light-dependent reactions?
a) Stroma
b) Inner membrane
c) Thylakoid membrane ✅
d) Outer membrane
During the light-dependent reactions, oxygen is produced by the:
a) Photolysis of water ✅
b) Reduction of CO₂
c) Formation of ATP
d) Absorption of light
Which wavelength of light is least effective for photosynthesis?
a) Blue
b) Red
c) Green ✅
d) Violet
Which enzyme is primarily responsible for ATP production in photosynthesis?
a) RuBisCO
b) ATP synthase ✅
c) NADP reductase
d) Phosphofructokinase
How many molecules of CO₂ are needed to synthesize one molecule of glucose?
a) 2
b) 3
c) 6 ✅
d) 12
The main purpose of cyclic photophosphorylation is to:
a) Generate oxygen
b) Produce ATP without making NADPH ✅
c) Produce NADPH
d) Fix CO₂
Which pigment absorbs blue and green light and passes energy to chlorophyll?
a) Carotenoids ✅
b) RuBisCO
c) Xanthophyll
d) Phycobilins
What is the fate of G3P (glyceraldehyde-3-phosphate) produced in the Calvin cycle?
a) It is converted into glucose ✅
b) It forms NADPH
c) It generates oxygen
d) It is used for photolysis