3.2: Photosynthesis

Cuticle

A transparent layer on both surfaces of a leaf.

Epidermis

A transparent layer on both surfaces of a leaf, the lower also having stomatal pores.

Palisade

A type of mesophyll which are cylindrical and elongated at right angles so leaves can accommodate a large number, have a large vacuole to form a single periphery layer and avoid shading each other, have spaces for carbon dioxide diffusion and thin cell walls.

Spongy

Type of mesophyll with air spaces to allow for carbon dioxide diffusion and thin cell walls.

Chlorophyll A

Type of pigment that absorbs light at both ends of the absorption spectrum, is in all plants, and has a yellow-green pigment. The primary pigment.

Chlorophyll B

Type of pigment that absorbs light at both ends of the absorption spectrum, is only in more complex plants and has a blue-green pigment. An accessory pigment.

Carotenoids

Comprises β-carotene and xanthophyll pigments, absorbing light in the blue-green region.

B-carotene

Type of pigment that absorbs light in the blue-green region, is in all plants, and has an orange pigment. An accessory pigment.

Xanthophylls

Type of pigment that absorbs light in the blue-green region, is in most plants, and has a yellow pigment.

Photosystems

Collections of pigments that collect light energy to pass onto chlorophyll A. They lie in the plain of the thylakoid membrane and are anchored by its phospholipids and held together by protein molecules in clusters of up to 400. Has two parts, the antenna complex and reaction centre.

Antenna complex

Part of the photosystem, comprised of accessory pigments and passes light to the reaction centre.

Reaction centre

Centre of the photosystem, containing two molecules of chlorophyll A, known as the primary pigment. These molecules absorb light, become excited and release an electron to an electron carrier.

PS1

Arranged around a chlorophyll with an absorption peak of 700nm. It is used in cyclic photophosphorylation. It receives electrons from PS(x), and either feeds its electrons into the electron carrier chain or uses them to reduce NADP.

PS2

Arranged around a chlorophyll with an absorption peak of 680 nm. It is involved in cyclic and non-cyclic photophosphorylation. It gets its electrons from photolysis, and they are used in the electron carrier chain before giving them to PS(x).

Accessory

In the antenna complex, the pigments that transfer light to the primary pigment in the reaction centre.

Primary

Pigment in the reaction centre. The light is passed here by accessory pigments in the antenna complex.

Photoionisation

Process where chlorophyll A is oxidised (releases an electron) by light, a reducing agent.

Light dependent

Stage of photosynthesis that requires light and produces oxygen, ATP and reduced NADP. Occurs in the thylakoid membrane.

Non cyclic

Type of photophosphorylation where the electron pathways are linear. Involves both photosystems. Electrons are produced via PS2, passed along electron carriers, and then given to PS1. Occurs in some organisms.

Cyclic

Type of photophosphorylation where the electron pathways are cyclical. Involves PS1. Electrons are passed along electron carriers and recycled back into PS1. Occurs in all photosynthesising organisms.

Photolysis

Dissociation of water into electrons, hydrogen and oxygen due to light. Oxygen is a waste product, protons are used to reduce NADP + maintain proton gradient, and electrons are used to reduce PS2. This is enhanced by a protein complex in PS2. Also known as the Hill reaction, as he proved its occurence.

NADP

Coenzyme used in light-dependent phosphorylation. Is reduced by hydrogen (from photolysis) and electrons (from non-cyclic photophosphorylation) in the stoma, maintaining the electro chemical gradient. The final electron/hydrogen acceptor.

Light independent

Stage of photosynthesis that requires the products of light dependent (ATP and NADP reduced) and is responsible for the synthesis of all products necessary to the cell. Occurs in the thylakoid membrane.

Calvin

Cycle where carbon molecules are created for product synthesis. The light independent stage.

Ribulose bisphosphate (RuBP)

A five carbon acceptor molecule which combines with CO2 from the air using the enzyme rubisco.

Ribulose bisphosphate carboxylase (rubisco)

Catalyses the combination of ribulose bisphosphate with CO2 to create an unstable 6 carbon compound, which splits.

Glycerate-3-phosphate (GP)

Formed by the splitting of the unstable 6 carbon compound. Can be synthesised into AcCoA for fat production, and amino acids for protein synthesis.

Triose phosphate

Created after the reduction (via NADP reduced) and phosphorylation (via ATP) of glycerate-3-phosphate. Used to recycle ribulose bisphosphate (5/6) or product synthesis (1/6).

Glucose phosphate

Triose phosphate is converted to this from the Calvin cycle, and then it is condensated into starch.

Fructose phosphate

First hexose sugar created, which can be converted to glucose and then combined with glucose to make sucrose.

Glycerol

A fat triose phosphate can be converted to.

Triglycerides

AcCoA and triose phosphate can be condensated to form these.

Chromatography

Technique used to separate liquids using a solvent. Used to separate pigments and photosynthesis products (Calvin).

Autoradiograph

Used to detect radiation.

Rate limiting

Step that controls the rate of photosynthesis and therefore determines the overall rate. Is the least optimal factor.

Light compensation

Point where light intensity is so low that all CO2 necessary is provided by respiration, so no gas exchange occurs.

Nitrogen

Absorbed as NH4+ or NO3-, and converted to NH4+ to form the amino group of amino acids. Necessary for growth in the plant, and an absense causes chlorosis.

Magnesium

Absorbed as Mg2+. Forms part of the chlorophyll molecule, and a lack causes chlorosis. Also an enzyme activator for ATPase, DNA polymerase, etc.

Chlorosis

Occurs when chlorophyll can not be made in the cell due to nutrient deficiencies. Leaves begin to yellow. Starts first in older leaves and nutrients are transported to newer ones.

5

How many, out of 6, triose phosphates are used to recycle ribulose bisphosphate.

Interior - stroma

Movement of the protons through ATP synthetase - from the area with high proton concentration to low.