Energy Transfers in and between Organisms - Photosynthesis Flashcards
What is photosynthesis?
Photosynthesis is a reaction in which light energy is used to produce glucose in plants. The process requires water and carbon dioxide, with the products being glucose and oxygen.
There are two stages of photosynthesis, these are the light dependent stage and the light independent stage.
The rate of photosynthesis is determined by carbon dioxide concentration, light intensity and temperature.
How does energy enter an organism?
Life depends on continuous transfers of energy. How this energy enters an organism depends on its type of nutrition. In plants, energy in light is absorbed by chlorophyll and then transferred into the chemical energy of the molecules formed during photosynthesis. These molecules are used by the plant to produce ATP during respiration. Non-photosynthetic organisms feed on the molecules produced by plants and then also use them to make ATP during respiration.
What is the site of photosynthesis?
The leaf is the main photosynthetic structure in eukaryotic plants. Chloroplasts are the cellular organelles within the leaf where photosynthesis takes place.
What is the purpose of a leaf?
Photosynthesis takes place largely in the leaf. Leaves are adapted to bring together the three raw materials of photosynthesis (water, carbon dioxide, and light) and removes its products (oxygen and glucose).
What are the adaptations of a leaf?
- a large surface area that absorbs as much sunlight as possible
- an arrangement of leaves on the plant that minimises overlapping and so avoids the shadowing of one leaf by another
- thin, as most light is absorbed in the first few micrometres of the leaf and the diffusion distance for gases is kept short
- a transparent cuticle and epidermis that let light through to the photosynthetic mesophyll cells beneath
- long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight
- numerous stomata for gaseous exchange so that all mesophyll cells are only a short diffusion pathway from one
- stomata that open and close in response to changes in light intensity
- many air spaces in the lower mesophyll layer to allow rapid diffusion in the gas phase of carbon dioxide and oxygen
- a network of xylem that brings water to the leaf cells, and phloem that carries away the sugars produced during photosynthesis
What is the equation for photosynthesis?
6CO2 + 6H2O –> C6H12O6 + 6O2
What is the overall equation for photosynthesis?
6CO2 + 6H2O –> C6H12O6 + 6O2
This equation is highly simplified. Photosynthesis is a complex metabolic pathway involving many intermediate reactions. It is a process of energy transferral in which some of the energy in light is conserved in the form of chemical bonds. There are three main stages to photosynthesis.
What are the three main stages to photosynthesis?
- Capturing of light energy by chloroplast pigments such as chlorophyll.
- The light-dependent reaction, in which some of the light energy absorbed is conserved in chemical bonds. During the process an electron flow is created by the effect of light on chlorophyll, causing water to split (photolysis) into protons, electrons, and oxygen. The products are reduced NADP, ATP, and oxygen.
- The light-independent reaction, in which these protons (hydrogen ions) are used to produce sugars and other organic molecules.
What is the difference between respiration and photosynthesis in plants?
All plant cells respire all the time, while only those plant cells with chloroplasts carry out photosynthesis - and then only in the light.
What is the structure of chloroplasts?
In eukaryotic plants, photosynthesis takes place within cell organelles called chloroplasts. These vary in shape and size but are typically disc-shaped, 2-10um long, and 1um in diameter. They are surrounded by a double membrane. Inside the chloroplast membranes are two distinct regions (the grana and the stroma).
What is the grana?
The grana are stacks of up to 100 disc-like structures called thylakoids where the light-dependent stage of photosynthesis takes place. Within the thylakoids is the photosynthetic pigment called chlorophyll. Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana. These are called inter-granal lamellae.
What is the stroma?
The stroma is a fluid-filled matrix of a chloroplast where the light-independent stage of photosynthesis takes place. Within the stroma are a number of other structures such as starch grains.
What is the light-dependent reaction?
The light-dependent reaction of photosynthesis involves the capture of light whose energy is used:
- to add an inorganic phosphate (Pi) molecule to ADP, thereby making ATP
- to split water into H+ ions (protons) and OH- ions; as the splitting is caused by light, it is known as photolysis
- to produce reduced NADP (NADPH)
What is oxidation and reduction?
OXIDATION
- loss of electrons
- loss of hydrogen
- gain of oxygen
REDUCTION
- gain of electrons
- gain of hydrogen
- loss of oxygen
Oxidation results in energy being given out, whereas reduction results in it being taken in. Oxidation and reduction always take place together.
(LDR - The Making of ATP)
What happens when a chlorophyll molecule absorbs light energy?
It boosts the energy of a pair of electrons within this chlorophyll molecule, raising them to a higher energy level. These electrons are said to be in an excited state.
The energy is transferred between antenna pigment molecules, where some energy is lost as heat until it reaches chlorophyll a. This is called the photoprotective effect and ensures chlorophyll a has the right amount of energy.
(LDR - The Making of ATP)
What is photoionisation?
When the electrons reach the reaction center (chlorophyll a), they are transferred to a primary electron acceptor. As a result the chlorophyll molecule becomes ionised and so the process is called photoionisation. The charge separation drives the process of photolysis.
(LDR - The Making of ATP)
What is an electron carrier/primary electron acceptor?
The electrons that leave the chlorophyll are taken up by a molecule called an electron carrier/primary electron acceptor. Having lost a pair of electrons, the chlorophyll molecule has been oxidised. The electron carrier/primary electron acceptor, which has gained electrons, has been reduced.
(LDR - The Making of ATP)
What happens when the electrons are taken up by an electron carrier/primary electron acceptor?
The electrons are now passed along a number of electron carriers in a series of oxidation-reduction reactions. These electron carriers form a transfer chain that is located in the membranes of the thylakoids.
(LDR - The Making of ATP)
Why do the electrons lose energy at each stage?
Each new carrier is at a slightly lower energy level than the previous one in the chain, and so the electrons lose energy at each stage. Some of this energy is used to combine an inorganic phosphate molecule with an ADP molecule in order to make ATP.
(LDR - The Making of ATP)
How can the precise mechanism by which ATP is produced be explained by the chemiosmotic theory?
- Each thylakoid is an enclosed chamber into which protons (H+) are pumped from the stroma using protein carriers in the thylakoid membrane called proton pumps.
- The energy to drive this process comes from electrons released when water molecules are split by light (photolysis of water).
- The photolysis of water also produces protons which further increases their concentration inside the thylakoid space.
- Overall this creates and maintains a concentration gradient of protons across the thylakoid membrane with a high concentration inside the thylakoid space and a low concentration in the stroma.
- The protons can only cross the thylakoid membrane through ATP synthase channel proteins - the rest of the membrane is impermeable to protons. These channels form small granules on the membrane surface and so are also known as stalked granules.
- As the protons pass through these ATP synthase channels they cause changes to the structure of the enzyme which then catalyses the combination of ADP with inorganic phosphate to form ATP.
(LDR - Photolysis of Water)
Why are water molecules split?
The loss of electrons when light strikes a chlorophyll molecule leaves it short of electrons. If the chlorophyll molecule is to continue absorbing light energy, these electrons must be replaced. The replacement electrons are provided from water molecules that are split using light energy. This photolysis of water also yields protons and oxygen.
The equation for this process is:
2H2O –> 4H+ + 4e- + O2
(LDR - Photolysis of Water)
What happens to the protons yielded from the photolysis of water?
These protons pass out of the thylakoid space through the ATP synthase channels and are taken up by an electron carrier called NADP.
