Leaves and Photosynthesis

Leaf Structure and Function

Leaves Primarily light harvesting organs Primary location of photosynthesis Responsible for photon collection and carbon fixation Usually thin, flat, which increases surface area Increased surface area and stomatal openings lead to water transport through tree Requires adaptations to prevent unnecessary water loss

Leaf Organization Epidermis Cuticle Usually clear, nonphotosynthetic, outer cell wall thick compared to inner cell wall Upper Covers the upper surface Lower Covers lower surface Cuticle Made of waxy layer called cutin Reduces water loss Thicker in upper epidermis

The Mesophyll Major mass of photosynthetic tissue Located between the upper and lower epidermis Often two sublayers Near upper epidermis, see palisade mesophyll Longer parallel cells Can be several layers, related to environment Major site of photosynthesis Lower cells called spongy mesophyll Allow for gas exchange and gas movement within leaf Also photosynthetic

Structure/Function in Leaves Leaves are the site of photosynthesis – this is main function Harvest light energy Produce glucose and other sugars Upper epidermis is clear to allow light to pass; is waxy to prevent excessive water loss Mesophyll contains bulk of chloroplasts Air space between mesophyll cells open to the open air through the stomata; CO2 diffuses into thin water layer on mesophyll cells

Water passes out of the leaf through stomata, drives the movement of water through the plant, bringing nutrients to the plant cells via the xylem; similarly water flowing back to the stem and roots carries sugar via the phloem

Mesophyll contains bulk of chloroplasts Air space between mesophyll cells open to the open air through the stomata; CO2 diffuses into thin water layer on mesophyll cells Water passes out of the leaf through stomata, drives the movement of water through the plant, bringing nutrients to the plant cells via the xylem; similarly water flowing back to the stem and roots carries sugar via the phloem

Photosynthesis: Capturing Energy

Autotrophs and Chemotrophs Carbon fixation is the process of building complex carbon compounds from simple carbon compounds. Organisms that fix carbon are autotrophs – they use carbon dioxide as a carbon source, and combine it with water Photoautotrophs provide nearly all the energy used by living systems on Earth

Photoautotrophs: Organisms that gain energy from light in order to carry out carbon fixation Photosynthetic plants, algae and bacteria Use light energy to make ATP and carbohydrate Chemoautotrophs: Organisms that use chemical energy only to cause carbon fixation and to build structure Certain bacteria Heterotrophs: Organisms that gain energy by eating other organisms, including autotrophs Animals, nonphotosynthetic plants, nonphotosynthetic unicellular organisms (such as protists), bacteria, and fungi

The Chloroplast The site of light harvesting The site of the start of carbohydrate synthesis

Chlorophyll, Part 1 Chlorophyll collects light energy (absorbs it) in a resonant porphyrin group that hangs out like a kite on the surface of the thylakoid Chlorophyll a initiates the light-dependent reactions Chlorophyll b is an accessory pigment Carotenoids are yellow and orange pigments that capture light energy and pass electrons to chlorophyll

Partition of Function in the Chloroplast The light-dependent reactions (the harvesting of light) occur on thylakoid membranes The carbon fixation reactions (formation of carbohydrate) occur in the stroma

Overview of Photosynthesis Photosynthesis is a redox reaction: Carbon dioxide is reduced to sugar Water is oxidized to molecular oxygen

How Photosynthesis Starts hn absorbed by chlorophyll Energy (as an electron) “falls” from one chlorophyll to the next. Eventually falls into the reaction center; a special chlorophyll molecule Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy

2 major parts to photosynthesis Light Dependent Reactions Take place on grana on the thalakoid membranes. A photon of light excites the chlorophyll. This electron is passed through an electron transport system. During this process water is broken apart. Oxygen is produced H+ ions build up and are picked up by NADP ATP is produced

Protons Build up Inside Thylakoids The activity of the ETC causes a gradient of protons across the thylakoid membrane

The Chloroplast ATP Synthase Protons fall back through the chloroplast ATPsynthase Makes ATP by combining ADP and phosphate in a process called chemiosmosis Much like the mitochondrial IM ATPsynthase

The Light independentReactions So-called because they do not directly need light They occur in the stroma of the chloroplast They fix carbon to make carbohydrate They are the Calvin-Benson Cycle reactions The power to run this cycle comes from the ATP generated in the first part of photosynthesis The Hydrogen is transferred from NADPH to Carbon dioxide

The Calvin Cycle:Phases 1 & 2 1. Carbon uptake Adds carbon dioxide to 5C ribulose bisphosphate (RuBP) Catalyzed by RUBISCO; ribulose bisphosphate carboxylase 2. Carbon reduction phase Citrate is made and broken to form phosphoglycerate (PGA) PGA is rearranged and phosphorylated by ATP NADPH reduces the backbone further to form glyceraldehyde-3-phosphate (PGAL or G3P)

The Calvin Cycle: Phase 3 3. Reformation of RuBP: PGAL (G3P) is rearranged, & phosphorylated With further investment of ATP… To make RuBP, a bisphosphorylated compound Alternatively, PGAL is shuttled out of the cycle to produce glucose and other carbohydrates elsewhere

When RUBISCO Doesn’t Work In high light and temperature: Photosynthesis is very active Water is easily lost Leaf stomata close (small pores on the underside of the leaf) to protect against water loss Oxygen builds up

On hot, dry days, plants close their stomata Conserving water but limiting access to CO2 Causing oxygen to build up

RUBISCO has mixed affinity for oxygen and CO2 It binds O2 when O2 is abundant PGAL is NOT produced

Photorespiration And as a result, photorespiration occurs: No carbohydrate is produced Instead, CO2 and H2O are produced NO ATP is produced, however Photosynthetic efficiency is degraded

Different enzyme is present as an adaptation in C4 and CAM plants Alternative mechanisms of carbon fixation have evolved in hot, arid climates Different enzyme is present as an adaptation in C4 and CAM plants PEP carboxylase

Evolutionary “Work-Arounds” Avoid Photorespiration 1. The C4 pathway Physically sequesters the carbon dioxide-requiring RUBISCO (carbon fixation) away from high oxygen Uses compartmentation with biological membranes (in different cells) In crabgrass, corn, and sugar cane 2. Crassulacean acid metabolism (CAM) Carries out C fixation separated in time from high temperature and high oxygen In desert plants: succulents and cacti (also lilies and orchids) Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy

C3 versus C4 Anatomy Bundle sheath cells are arranged differently in C3 and C4 plants Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy

C4 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

C4 leaf anatomy and the C4 pathway CO2 Mesophyll cell Bundle- sheath cell Vein (vascular tissue) Photosynthetic cells of C4 plant leaf Stoma Mesophyll C4 leaf anatomy PEP carboxylase Oxaloacetate (4 C) PEP (3 C) Malate (4 C) ADP ATP Sheath Pyruate (3 C) CALVIN CYCLE Sugar Vascular tissue Figure 10.19

The CAM pathway is similar to the C4 pathway Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells. (a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times. (b) Pineapple Sugarcane Bundle- sheath cell Mesophyll Cell Organic acid CALVIN CYCLE Sugar CO2 C4 CAM CO2 incorporated into four-carbon organic acids (carbon fixation) Night Day 1 2 Organic acids release CO2 to Calvin cycle Figure 10.20

CAM Plants 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

The CAM pathway Seen in xeric plants (very dry conditions; desert) Separation of function by TIME instead of location / compartmentation PEP carboxylase works at night when the stomata are open (they close during the day in desert plants) OA is made and malate derived from it is stored in the vacuole During the day, CO2 is derived from the malate and made available to the Calvin cycle. Not as efficient at supporting rapid growth as is seen in C4 plants like crabgrass and corn, is good adaptation for desert plants Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy

Calvin cycle is all-important! In C3, C4 and CAM plants, the Calvin Cycle always receives the carbon dioxide that is made into carbohydrate... . . . under all conditions! Scientists are very interested in getting C4 chemistry genetically introduced into plants to increase crop efficiency. Biology, Sixth Edition Chapter 8, Photosynthesis: Capturing Energy