CAPE 2 - Photosynthesis: Structure of the leaf, Photophosphorylation & The Calvin Cycle

The structure of leaf

¢In flowering plants, the major photosynthetic organ is the leaf.

¢ The functions of a leaf are best achieved by containing chlorophyll, absorbing carbon dioxide (and disposing of oxygen) and have a water and solute supply/transport route.

¢has a large surface area and arrangement such that it can absorb as much light as possible.

Shape and position

¢Large surface area of the lamina

¢Large surface area-to-volume ratio for maximum exposure to light and efficient gas exchange

¢Arrangement of leaves (leaf mosaic) helps the plant to absorb as much light as possible

¢Blade held at right angles to incident light

¢Thinness minimises diffusion pathway for gaseous exchange

Stomata

¢many stomata in the lower epidermis, which are pores in the epidermis through which gaseous exchange occurs.

¢Each stomata is bounded by two guard cells, and

¢changes in the turgidity of theses guard cells cause them to change shape

¢so that they open and close the pore. If the guard cells gain water, the pore is open, and vice-versa.

Mesophyll

¢main site of photosynthesis - have many more chloroplasts than spongy mesophylls, and also have several adaptions to maximise photosynthetic efficiency;

¢Large Vacuole - Restricts chloroplasts to a layer near the outside of the cell where they can be reached by light more easily.

¢Cylindrical Arrangement - They are arranged at right angles to the upper epidermis, reducing the number of light-absorbing cross walls preventing light from reaching the chloroplasts. This also allows long-narrow air spaces between them, providing a large surface area for gaseous exchange.

¢Thin cell walls - to allow gases to more easily diffuse through them.

Vascular System

¢Supplies water and mineral salts (xylem)

¢Removes products of photosynthesis (phloem)

¢As supporting skeleton together with lignified collenchyma and sclerenchyma

Chloroplasts: The Sites of Photosynthesis in Plants

¢In eukaryotes, photosynthesis takes place in organelles called chloroplasts.

¢Approximately 3 – 10 µm in diameter and are visible with a light microscope

¢Surrounded by two membranes, which form the chloroplast envelope.

¢Contain chlorophyll and other photosynthetic pigments located on a system of membranes

¢The membranes run through a ground substance called stroma.

¢The membrane system is the site of the light-dependent reactions in photosynthesis.

¢The membranes are covered with chlorophyll and other pigments, enzymes and electron carriers.

¢The system contains of many flattened, fluid-filled sacs called thylakoids which form stacks called grana.

¢The stroma is the site of the light independent reactions of photosynthesis.

¢The structure is gel-like containing soluble enzymes for the Calvin cycle and other chemicals such as sugars and organic acids.

Trapping Light Energy

¢Light energy is trapped by photosynthetic pigments

¢Different pigments absorb different wavelengths of light.

¢The photosynthetic pigments of higher plants form two groups: chlorophylls and carotenoids.

A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes

¢ A photosystem

—Is composed of a reaction center surrounded by a number of light-harvesting complexes

¢ The light-harvesting complexes

—Consist of pigment molecules bound to particular proteins

—Funnel the energy of photons of light to the reaction center

¢ When a reaction-center chlorophyll molecule absorbs energy

—One of its electrons gets bumped up to a primary electron acceptor

Light-dependent reactions - Photophosphorylation

CLICK HERE FOR: Cyclic and Noncyclic Photophosphorylation animation

¢Include ATP synthesis in photophosphorylation and photolysis to give hydrogen ions

¢The hydrogen ions combine with a carrier molecule NADP to make reduced NADP

¢Photophosphorylation of ADP to ATP can be cyclic or non-cyclic depending on the pattern of electron flow.

¢NADP - nicotinamide adenine dinucleotide phosphate.

—It is a coenzyme that serves as an electron carrier in a number of reactions, being alternately oxidised.

¢NADPH – reduced nicotinamide adenine dinucleotide phosphate (NADP) carrying electrons and bonded with a hydrogen (H) ion; the reduced form of NADP.

Non-cyclic Photophosphorylation

Chemiosmosis

CLICK HERE FOR: Chemiosmosis animation

The Light Independent Reaction: Cyclic Photophophorylation - Cyclic Electron Flow

¢Occurs under certain conditions

—. Photoexcited electrons take an alternative path

—. Only photosystem I is used

—. Only ATP is produced

The Calvin cycle uses ATP and NADPH to convert CO2 to sugar

CLICK HERE FOR: Calvin Cycle animation

¢The Calvin cycle

—Is similar to the citric acid cycle

—Occurs in the stroma

¢The Calvin cycle has three phases

—Carbon fixation

—Reduction

—Regeneration of the CO2 acceptor

Photorespiration:

¢Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates

¢On hot, dry days, plants close their stomata

—Conserving water but limiting access to CO2

—Causing oxygen to build up

¢In photorespiration

—O2 substitutes for CO2 in the active site of the enzyme rubisco

—The photosynthetic rate is reduced

C4 & CAM Plants

¢C4 plants minimize the cost of photorespiration

—By incorporating CO2 into four carbon compounds in mesophyll cells

¢These four carbon compounds

—Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle

¢CAM plants

—Open their stomata at night, incorporating CO2 into organic acids

¢During the day, the stomata close

—And the CO2 is released from the organic acids for use in the Calvin cycle