Photosynthesis – Lesson # 1

Introduction

Why study photosynthesis?• Practical reasons:• Photosynthetic organisms are autotrophs

(they make their own food)• Without autotrophs, heterotrophs (organisms

that eat other organisms) would not have a source of food/energy

Photosynthesis – What is it?

• The conversion of light energy, carbon dioxide and water into high energy compounds (e.g. Glucose).

• The general equation for photosynthesis is: CO2(g) + H2O(l) + light energy [CH2O](aq) + O2(g)

• Photosynthesis can be thought of as the reverse of cellular respiration. Why ?

Where in a plant does photosynthesis take place?

(Cross section of a leaf)

Leaves – Getting the Raw Materials• Light passes through the upper epidermis to reach the mesophyll layers.

• Water is absorbed by plant roots, and is delivered to the leaf via the vascular bundle.

• Carbon Dioxide gets into the leaf through the stoma (see picture below)

Guard Cells and Stomata

• Guard cells control the diameter of the stoma • The stoma opens during the day and closes at night

(usually) • Sunlight activates a proton pump in the guard cells

causing H+ to exit the cells. • The electrical potential of the cells becomes negative• K+ ions flow in to correct this difference• The influx of K+ ions draws water into the guard cell

making them turgid• The swelling of the guard cells opens the stoma

Guard Cells and Stomata

• See animation: • http://academic.kellogg.cc.mi.us/

herbrandsonc/bio111/animations/0021.swf

• So what?

Photosynthesis - Overview

Photosynthesis takes place in the chloroplast.

The following structures inside the chloroplast are important:• thylakoids• stroma

Why are these structures important?Photosynthesis is a 2 step process. Step 1 (the light reactions) occurs in the thylakoids. Step 2 (the Calvin Cycle) occurs in the stroma

Photosynthesis - Overview

Light Reactions - Overview• Light Reactions: light is absorbed by

chlorophyll which drives the transfer of electrons to (NADP+). NADP+ is reduced to form NADPH.

• ATP is also produced during the light reactions in a process known as photophosphorylation

Take home point: Light reactions produce ATP and NADPH which are used in the Calvin Cycle to make sugar. Oxygen is also produced (as a result of the water splitting) as a by-product.

Calvin Cycle - Overview

• Calvin Cycle: begins with carbon fixation. The Calvin Cycle uses NADPH and ATP from the light reactions to reduce carbon dioxide to carbohydrates (e.g. Glucose).

• The Calvin Cycle is sometimes referred to as the “light independent reactions”. Why?

Tomorrow: • More detail on light reactions and Calvin Cycle• You need to work on a photosynthesis cheat

sheet (4 pages) and have a format like this:

Light Reactions CAM Plants

Calvin Cycle C4 Plants

Homework

• Read section 3.1 in your text (pp 138-144)

• Try questions 1, 2a, 4, and 5 on page 145 of your textbook

• Wednesday: Quiz on Cellular Respiration (Can use cheat sheet)