Chapter 17 - Energy for Biological Processes (Photosynthesis)

Define autotrophs and give examples

Organisms which produce their own food from organic molecules, e.g. algae, green plants

Define heterotroph and give examples

Organisms which derive energy from other living organisms, e.g. humans

What are the two parts of photosynthesis and where do they occur

Light dependent - in grana of chloroplast, require transport proteins and must be fixed hence why it happens on a membrane

Light independent - in stroma of chloroplast, involves a cycle and must have movement hence why it happens in the fluid

Describe the two distinct regions of chloroplasts

• Stroma - site of light independent reactions. Made of substance containing: enzymes which take part in Calvin cycle, DNA/ribosomes involved in protein synthesis, starch grains and lipid droplets formed from products of photosynthesis

• Granum - site of light dependent reactions. Consists of flattened, fluid filled sacs called thylakoids, formed into stacks called grana (pl) or granum (s). Grana connected by lamellae/intergranal lamellae. Membranes covered with pigments used in light dependent reactions (Hill reaction)

Describe Lamellae

Double membrane connecting grana

Describe thylakoid

Membranous sac. Chlorophyll and electron carriers embedded in these membranes. Where light reactions of photosynthesis take place

Describe intermembrane space

Space formed by folded thylakoid membranes. Allow a reservoir of hydrogen ions to build up when they are pumped from the stroma by electron transport proteins.

Describe granum

Stack of thylakoid membranes. Plural is grana

Describe stroma

Fluid filling the interior of chloroplast. Site of light independent reactions and Calvin cycle

Describe plasmids

Small circular DNA coding for some chloroplast proteins

Describe 70s ribosomes

Smaller than ribosomes in the cytoplasm. Site of synthesis for proteins and enzymes necessary for photosynthesis

How do pigments work

Different pigments absorb different wavelengths of light and reflect the rest.

Chlorophyll is the main pigment in plants and this pigment absorbs red and blue light whilst reflecting green light making plants look green.

Different combinations of pigments are the reason for different shades and colours

What is a primary pigment and accessory pigment

Primary pigment - Chlorophyll a

Accessory pigment - Chlorophyll b, carotene

What is an antennae complex/reaction centre and how does it work

A group of pigments arranged as a complex of pigments and proteins.

They are arranged as a funnel shape with accessory pigments and a primary pigment.

The primary pigment is chlorophyll an and is at the heart of all antennae complexes.

Role of antenna complex is to harvest light energy and transfer this to the reaction centre, where chlorophyll a is located.

What do the black bands on an absorption spectrum represent

The wavelengths of light which are absorbed come up as a black band whilst the colours which were reflected do not

Define photophosphorylation

The synthesis of ATP from ADP and phosphate that occurs in a plant using radiant energy absorbed during photosynthesis.

Describe chemiosmosis

ATP produced by photosynthesis and respiration is synthesised by process called chemiosmosis

It involves the diffusion of protons from high conc to low conc through partially permeable membrane.

Movement of protons as they flow down conc gradient releases energy that is used in attachment of an inorganic phosphate to ADP, forming ATP

This process depends on creation of a proton concentration gradient. Energy to do this comes from high-energy electrons (excited electrons)

Describe structure and function of electron transport chain

• Electron transport chain made of series of electron carriers, each with progressively lower energy levels

• As electrons move from carrier to carrier, energy is released, which is used to pump protons across a membrane, creating conc difference and therefore proton gradient

• Proton gradient maintained due to impermeability of membrane to hydrogen ions

How does ATP synthase work

• The only way protons can move back down the conc gradient is through hydrophilic membrane channels linked to the enzyme ATP synthase.

• Flow of protons through these channels provides energy used to synthesise ATP from ADP and Pi

Describe photolysis

Light energy from the sun causes water molecules to split up into hydrogen ions, electrons and oxygen molecules.

The electrons released replace the electrons lost from the reaction centre of PSII.

Protons released into the lumen of the thylakoids, increasing proton concentration across the membrane and as they move back through the membrane down electrochemical gradient, they drive formation of ATP.

Once hydrogen ions return to stroma, they combine with NADP and an electron from PSI to form reduced NADP. This is used in light-independent reaction.

Process removes hydrogen ions from the stroma so maintains the proton gradient across the thylakoid membranes.

Describe non-cyclic photophosphorylation (Light dependent)

• Light energy of wavelength 680nm hits PSII and is absorbed

• This excites 2 electrons up to the electron transport chain

• Protons are pumped across the thylakoid membrane into the thylakoid space, creating a proton gradient

• Protons flow down gradient, through channels associated with ATP synthase

• Kinetic energy from movement of protons allow ADP and Pi to join forming ATP

• ATP is produced by chemiosmosis

• Electrons lost from PSII are replaced from electrons released by photolysis

• Light of wavelength 700nm hits PSI

• 2 excited electrons pass to another electron transport chain and ATP again produced by chemiosmosis

• Electrons lost from PSI are replaced by the electrons coming from first electron transport chain

• Electrons leaving electron transport chain and a hydrogen ion are accepted by NADP forming reduced NADP

• Reduced NADP provides the hydrogen or reducing power in the production of organic molecules (e.g. glucose) in light-independent stage

Describe cyclic photophosphorylation (Light dependent)

Electrons leaving the electron transport chain after PSI are then returned to PSI instead of being used to form reduced NADP.

This means PSI can still lead to production of ATP without any electrons from PSII.

Reduced NADP is not produced when this happens.

Describe the calvin cycle (light independent)

• CO2 enters intercellular spaces, diffuses into stroma where it joins (fixes CO2) 5C molecule, ribulose biphosphate (RuBP)

• Ribulose biphosphate carboxylase (RuBisCO) catalyses the reaction and unstable 6C intermediate formed.

• Unstable 6C compounds breaks down forming two 3C glycerate 3-phosphate (GP) molecules

• Each GP molecule converted to another 3C molecule, triose phosphate (TP)

• This happens because of 2 things: 2ATP is hydrolysed to 2ADP releasing energy, 2NADPH is oxidised to 2NADP+. The hydrogen and energy released are used to reduce GP to TP

• TP is then used to regenerate RuBP. 5 carbons are needed for RuBP to be regenerated. Hence, ATP is hydrolysed to release energy which is used to convert TP to RuBP, leaving one carbon to be used to produced molecules like glucose, starch, cellulose, lipids, amino acids, nucleic acids

Name some limiting factors which could reduce the rate of photosynthesis

• Light intensity

• Carbon dioxide concentration

• Temperature

Effect of light intensity on Calvin Cycle

• Reducing light intensity will reduce rate of light-dependent stage of photosynthesis

• This will reduce quantity of ATP and reduced NADP produced

• ATP and reduced NADP are needed to convert GP to TP

• Therefore, concentration of GP will increase whilst TP decreases (as GP isn’t being converted)

• As there is less TP to regenerate RuBP, concentration of RuBP will also decrease

• Reverse will happen when light intensity increased

Effect of carbon dioxide concentration and temperature on Calvin Cycle

• All reactions in Calvin cycle are catalysed by enzymes

• At lower temperatures, enzymes have less kinetic energy so fewer successful collisions and reduced rate of reaction

• Hence, decreasing temperature results in lower concentrations of GP, TP and RuBP. Increasing temperatures will have the same effect as enzymes will be denatured (irreversible)

• Carbon dioxide is essential in Calvin cycle. Low concentrations of CO2 lead to reduced concentrations of GP and TP

• Concentration of RuBP will increase as it is still being formed from TP but not being used to fix carbon dioxide