Thursday, April 30, 2020

Unit 4: Photosynthesis Title: Overview of photosynthesis Learning Objectives:

We are learning….

• How is the plant leaf adapted to carry out photosynthesis?

• What are the main stages of photosynthesis?

Keywords: • cyclosis • photolysis • thylakoids • granum

Starter: Complete the exam question.

Talk for one minute about this diagram.

How are the leaves of plants adapted for photosynthesis?

Shape of leaves – broad, flat shape offers large surface area for absorption of sunlight and carbon dioxide.

Leaves are thin providing a shorter distance for carbon dioxide to diffuse into inner cells.

Large spaces between cells inside leaves provide an easy passage through which carbon dioxide can diffuse.

Many stomata in the lower surface of the leaf. These allow the exchange of carbon dioxide and oxygen with the air outside.

Many chloroplasts are found in the upper (palisade) cells than in the lower (spongy mesophyll) cells. These palisade cells receive most sunlight which reaches chloroplasts.

Branching network of veins provides a good supply of water to photosynthesising cells.

The process of photosynthesis

With the person next to you, outline the process of photosynthesis using the following key terms: Leaf, chlorophyll, carbon dioxide, soil, roots, oxygen, xylem, pigment, water, hydrogen, absorb, split, sugar, pigment, osmosis, stem, stomata, energy, sunlight, palisade, chloroplasts

The process of photosynthesis

The process of photosynthesis takes place mainly in the cells of the leaves . In land plants, water is absorbed from the soil by the roots and carried in the xylem up the stem to the leaf .

Carbon dioxide is absorbed from the air through the stomata (pores). In the leaf cells, the carbon dioxide combined to make sugar ; the energy and water are for this reaction comes from sunlight which has been absorbed chlorophyll . The chlorophyll by the green pigment is present in chloroplasts , found in the palisade cells towards the surface of the leaf .

Chlorophyll is a photosynthetic pigment which gives leaves their green colour. It is able to absorb sunlight and use it to split water molecules into hydrogen and oxygen (the ‘light’ reaction).

The oxygen added to escapes from the carbon dioxide leaf and the hydrogen molecules to form sugar molecules are (the ‘dark’ reaction).

Main stages of photosynthesis

There are three main stages of photosynthesis: 1. Capturing of light energy – chlorophyll in chloroplasts absorbs sunlight 2. The light-dependent reaction – light energy is converted to chemical energy. Water is split by a process of electron flow. The splitting of water by light is known as photolysis. Water is split into protons (hydrogen ions), electrons and oxygen. The products are reduced NADP, ATP and oxygen.

3. The light-independent reaction – protons (hydrogen ions) are used to reduce carbon dioxide to produce sugars and other organic molecules.

Upper epidermis single cell layer contains few chloroplasts. Waxy lipid layer prevents water loss.

Palisade mesophyll tissue – packed full of chloroplasts - site of the biochemical machinery of photosynthesis.

Spongy mesophyll tissue – cells contain fewer chloroplasts – air spaces between cells for gas exchange

This video shows the movement of chloroplasts in cells from a leaf of Canadian Pondweed (

Elodea

sp.).

Chloroplasts move in a cell to become orientated close to the source of light. They may, for example, all gather at one end of a cell. This process is called cyclosis (streaming of cytoplasm)

Chloroplast through electron microscope

Photosynthetic pigments

• • •

Absorb certain wavelengths of light Reflect other wavelengths (these are the colours we see) Arranged in photosystems in thylakoid membranes

Hexose is a product of the light-independent reaction which happens in the stroma .

Hexose molecules join together by condensation reactions (forming glycosidic bonds) to form starch .

Chlorophyll absorbs light in the blue-violet the visible spectrum.

and the red regions of Carotenoids absorb light in the blue-violet range of the spectrum.

Quick quiz

1. Which of the following leads to the production of sugar during photosynthesis?

a) Hydrogen ions combining with carbon dioxide b) Electrons excited by sunlight combining with oxygen c) d) Protons combining with electrons Water being split into carbon and oxygen 2. What is photolysis?

Splitting of water using light 3. Where in the chloroplast does the following occur; a) The light-dependent reaction?

b) Grana The light independent reaction?

Stroma c) The production of ATP Grana 4. What are the products of the light- dependent reaction?

Protons (Hydrogen ions), electrons and oxygen, ATP 5. Where are the photosynthetic pigments found in chloroplasts?

Photosystems in thylakoid membranes 6. What is the energy conversion which takes place during the light dependent reaction?

Light to chemical

Plenary: Hotseat …..

Who is brave enough to step up to the hotseat and answer questions about today’s lesson?

How successful were we this lesson?

Learning Objective

We were learning…..