5.6.2 Chloroplast Structure and Function Flashcards
What are chloroplasts?
Chloroplasts are the organelles in plant cells where photosynthesis occurs
These organelles are roughly 2 - 10 μm in diameter (they are larger than mitochondria)
Describe what each chloroplast is surrounded by.
Each chloroplast is surrounded by a double-membrane envelope
Each of the envelope membranes is a phospholipid bilayer
The outer membrane is permeable to a range of ions and small molecules
The inner membrane contains transport proteins that only allow certain molecules or ions to enter or leave the chloroplast
What are chloroplasts filled with?
Chloroplasts are filled with a cytosol-like fluid known as the stroma
CO2, sugars, enzymes and other molecules are dissolved in the stroma
Describe the separate system of membranes found in the stroma. (Thylakoid membranes and Grana).
A separate system of membranes is found in the stroma
This membrane system consists of a series of flattened fluid-filled sacs known as thylakoids
The thylakoid membranes contain pigments, enzymes and electron carriers
These thylakoids stack up to form structures known as grana (singular – granum)
Grana are connected by membranous channels called stroma lamellae, which ensure the stacks of sacs are connected but distanced from each other
The membranes of the grana create a large surface area
This membrane system provides a large number of pigment molecules that ensure as much light as necessary is absorbed
The pigment molecules are arranged in light-harvesting clusters known as photosystems
In a photosystem, the different pigment molecules are arranged in funnel-like structures in the thylakoid membrane (each pigment molecule passes energy down to the next pigment molecule in the cluster until it reaches the primary pigment reaction centre)
What other organelles does the stroma contain?
The stroma also contains small (70S) ribosomes, a loop of DNA and starch grains:
The loop of DNA codes for some of the chloroplast proteins (other chloroplast proteins are coded for by the DNA in the plant cell nucleus)
The proteins coded for by this loop of chloroplast DNA are produced at the 70S ribosomes
Sugars formed during photosynthesis are stored as starch inside starch grains
What two stages is photosynthesis made up of?
The process of photosynthesis is made up of two stages, the light-dependent stage and the light-independent stage
These stages take place in specific locations within the chloroplast
Where does the light-dependent stage take place?
The first stage, the light-dependent stage takes place on the thylakoid membranes of the grana
Light becomes trapped within the reaction centres of the grana.
Where does the light-independent stage take place?
The light-independent stage takes place in the stroma
What are the adaptations of choloroplasts to photosynthesis?
Stroma: The gel-like fluid contains enzymes that catalyse the reactions of the light-independent stage. The stroma surrounds the grana and membranes, making the transport of products from the light-dependent stage into the stroma rapid
Grana: The granal stacks create a large surface area for the presence of many photosystems which allows for the maximum absorption of light. It also provides more membrane space for electron carriers and ATP synthase enzymes
DNA: The chloroplast DNA (cpDNA) contains genes that code for some of the proteins and enzymes used in photosynthesis
Ribosomes: The presence of ribosomes allows for the translation of proteins coded by cpDNA
Inner membrane of chloroplast envelope: The selective transport proteins present in the inner membrane control the flow of molecules between the stroma and cytosol (the cytoplasm of the plant cell)
