# Chloroplast Structure & Function ## Overview of Chloroplasts - Chloroplasts are the **organelles** in plant cells where photosynthesis takes place - These organelles measure roughly **2-10 μm in diameter** - Larger than mitochondria, making them visible under light microscopy at high magnification - Found in photosynthetic organisms including plants, algae, and some protists

Chloroplasts are membrane-bound organelles that convert light energy into chemical energy through photosynthesis

## The Chloroplast Envelope - Each chloroplast is enclosed by a **double-membrane envelope** - Both membranes are phospholipid bilayers - Creates a barrier between the chloroplast contents and the cell cytoplasm ### Outer Membrane - The outer membrane is **permeable** to a range of ions and small molecules - Acts like a sieve, allowing many substances to pass through freely - Similar to having an open gate that lets most visitors through ### Inner Membrane - The inner membrane contains **transport proteins** - These proteins are highly selective - Only specific molecules or ions can enter or leave the chloroplast - Think of this as a security checkpoint that only allows certain substances through - This selective permeability allows the chloroplast to control its internal environment ## The Stroma - Chloroplasts are filled with a fluid called the **stroma** - Similar in consistency to the cytosol found in the cell cytoplasm - Acts as the background medium for many reactions ### Stroma Contents The stroma contains dissolved substances including: - $\text{CO}_2$ (carbon dioxide) - raw material for photosynthesis - Sugars - products of photosynthesis - Enzymes - biological catalysts for chemical reactions - Other organic molecules required for metabolism

The stroma is where the light-independent reactions (Calvin cycle) occur

## The Thylakoid Membrane System - Within the stroma exists a **separate system of membranes** - Distinct from the envelope membranes - Forms a complex internal architecture ### Thylakoids - The membrane system consists of **flattened fluid-filled sacs** called **thylakoids** - Imagine these as flat, disc-shaped bags stacked within the chloroplast - Each thylakoid has an internal space called the thylakoid lumen ### Thylakoid Membrane Components The thylakoid membranes contain three crucial components: 1. **Pigments** - molecules that absorb light energy 2. **Enzymes** - catalyse photosynthesis reactions 3. **Electron carriers** - transfer electrons during the light-dependent reactions ### Grana Structure - Thylakoids stack up to form structures called **grana** (singular: granum) - Like stacking coins on top of each other - Each stack is one granum - Multiple stacks throughout the chloroplast form the grana | Structure | Description | Function | |-----------|-------------|----------| | Thylakoid | Flattened membrane sac | Site of light-dependent reactions | | Granum | Stack of thylakoids | Increases surface area for light absorption | | Grana | Multiple stacks | Maximises photosynthetic capacity | ### Stroma Lamellae - **Stroma lamellae** are membranous channels connecting grana - Act like bridges between the stacks - Ensure the stacks remain connected while maintaining distance from each other - Allow communication between different grana

Stroma lamellae ensure thylakoid stacks are interconnected but spaced apart for optimal function

## Surface Area for Light Absorption - The membranes of the grana create a **large surface area** - Critical for maximising light absorption - More membrane = more space for pigments ### Pigment Arrangement - The membrane system provides space for a **large number of pigment molecules** - Ensures as much light as necessary is absorbed - Like having many solar panels to capture maximum sunlight ## Photosystems - Pigment molecules are organized into **light-harvesting clusters** called **photosystems** - Sophisticated arrangements for optimal energy capture ### Photosystem Structure - In a photosystem, different pigment molecules are arranged in **funnel-like structures** within the thylakoid membrane - Each pigment molecule captures light energy - Energy is passed down from one pigment molecule to the next - Energy transfer continues until reaching the **primary pigment reaction centre** - Similar to a relay race where energy is passed along instead of a baton

A photosystem is a light-harvesting complex where pigments are arranged to funnel energy to a central reaction centre

## Additional Chloroplast Contents The stroma also houses: ### 70S Ribosomes - Small ribosomes found in chloroplasts - **70S** refers to their size (Svedberg units) - Same size as bacterial ribosomes - Smaller than the 80S ribosomes in the cell cytoplasm - Function: protein synthesis within the chloroplast ### Loop of DNA - Chloroplasts contain their own **circular DNA molecule** - Independent from the nuclear DNA in the cell - Codes for **some chloroplast proteins** - Evidence supporting the endosymbiotic theory of chloroplast origin - Other chloroplast proteins are coded by nuclear DNA and imported

Proteins coded by chloroplast DNA are produced at the 70S ribosomes within the chloroplast

### Starch Grains - **Starch grains** store the products of photosynthesis - Sugars formed during photosynthesis are converted to starch - Starch is stored inside these grains - Acts as an energy reserve for the plant ## Location of Photosynthesis Stages Photosynthesis occurs in **two distinct stages**, each in a specific location: ### Light-Dependent Stage **Location:** Thylakoid membranes of the grana - Light energy is trapped within the reaction centres of the grana - Photosynthetic pigments absorb light - Energy is converted into chemical energy (ATP and reduced NADP) - Water molecules are split (photolysis) ### Light-Independent Stage **Location:** Stroma - Uses products from the light-dependent stage (ATP and reduced NADP) - Carbon dioxide is fixed into organic molecules - The Calvin cycle operates here - Produces sugars and other organic compounds | Stage | Location | Key Events | |-------|----------|------------| | Light-dependent | Thylakoid membranes | Light absorption, electron transport, ATP synthesis, water photolysis | | Light-independent | Stroma | Carbon fixation, Calvin cycle, sugar synthesis |

Although called "light-independent," this stage still requires products from the light-dependent reactions

## Adaptations of Chloroplasts for Photosynthesis Chloroplasts are highly specialized organelles with multiple adaptations: ### Stroma Adaptations - **Contains enzymes** that catalyse light-independent stage reactions - **Surrounds the grana and membranes** - Makes transport of products from light-dependent stage into stroma rapid - Short diffusion distances increase efficiency - Like having a factory floor that surrounds all the machinery ### Grana Adaptations - **Granal stacks create large surface area** - Space for many photosystems - Allows maximum absorption of light energy - More surface area = more photosynthetic capacity - **Provides membrane space** for: - Electron carriers (electron transport chain) - ATP synthase enzymes (for ATP production) - Like having more workbenches in a workshop ### Chloroplast DNA (cpDNA) - **Contains genes** coding for some proteins and enzymes used in photosynthesis - Allows rapid protein production without waiting for nuclear transcription - Gives chloroplasts some autonomy - Speeds up response to changing light conditions ### Ribosomes - **Presence of ribosomes** allows translation of proteins coded by cpDNA - Enables on-site protein synthesis - Quick replacement of damaged proteins - Immediate production of needed enzymes ### Inner Envelope Membrane - **Selective transport proteins** control molecule flow - Regulate movement between stroma and cytosol - Maintain optimal conditions for photosynthesis - Prevent unwanted substances from entering - Allow export of photosynthetic products to the cell ## Summary Table of Chloroplast Components | Component | Location | Function | |-----------|----------|----------| | Outer membrane | Envelope | Permits passage of ions and small molecules | | Inner membrane | Envelope | Selective barrier with transport proteins | | Stroma | Interior fluid | Contains enzymes for Calvin cycle; medium for reactions | | Thylakoid | Membrane system | Site of light-dependent reactions | | Granum | Stacked thylakoids | Increases surface area for light absorption | | Stroma lamellae | Between grana | Connects grana while maintaining spacing | | Photosystem | Thylakoid membrane | Light-harvesting complex with pigments | | 70S ribosomes | Stroma | Synthesize chloroplast proteins | | cpDNA | Stroma | Codes for some chloroplast proteins | | Starch grains | Stroma | Store carbohydrate products |

Understanding chloroplast structure is key to understanding where and how photosynthesis occurs - structure directly relates to function