B4.3 Photosynthesis

Front 1

Recall the structure of a plant cell

Back 1

A peroxisome contains enzyme catalyse for the breakdown of H2O2 and other enzymes

Plasmodesmata are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them.

Tonoplast - a membrane which bounds the chief vacuole of a plant cell.

Front 2

Recall the structure of a leaf

Back 2

Front 3

Recall the structure of the chloroplast

Back 3

Front 4

State 9 ways in which the plant is adapted for photosynthesis

Back 4

1. Large surface area

2. Arrangement of leaves minimises overlapping

3. Thin, so light is absorbed in the first few millimetres of the leaf, so diffusion distance is kept short

4. Transparent cuticle

5. Long, narrow upper mesophyll packed with chloroplasts to collect sunlight

6. Numerous stomata

7. Stomata open and close in response to light intensity

8. Air spaces in the mesophyll to allow diffusion of CO₂ and O₂

9. Network of xylem that brings water to leaf cells and phloem that carries away the sugars produced in photosynthesis

Front 5

How many thylakoids in a granum?

Back 5

100-ish

Front 6

What are the dimensions of the chloroplast?

Back 6

1µm in diameter and 2-10µm long

Front 7

Recall light dependant reaction

Back 7

Front 8

What happens in cyclic photo-phosphorylation?

Back 8

The electron goes back to plastoquinone, so it produces ATP but no NADPH

Front 9

State 4 ways in which the thylakoid is adapted to its function

Back 9

- Large surface area for attachment of chlorophyll, electron carriers and enzymes

- Network of proteins in the grana hold the chlorophyll in a fixed position to optimise the absorption of light

-Granal membranes have enzymes attached to them that help synthesise ATP

- Chloroplasts have DNA and ribosomes so they can quickly manufacture proteins needed for light dependant reaction

Front 10

Recall the light independent reaction (Calvin cycle)

Back 10

Front 11

In which cells in the leaf does the calvin cycle normally take place?

Back 11

Mesophyll cells

Front 12

What does RuBisCO stand for?

Back 12

Ribulose 1,5-bisphosphate carboxylase oxygenase

Front 13

What is inefficient about RuBisCO? Give mechanism

Back 13

It doesn't necessarily react with CO₂ - it can also react with oxygen:

5RuBP (5C) + O₂ → 5× 3-phosphoglycerate (3C) + 5× phosphoglycolate (2C)

This is called photorespiration

This means that some carbons cannot continue the cycle, as only 3-phosphoglycerate can continue in the calvin cycle. The phosphoglycolate must leave the chloroplast and get processed by peroxisome

Front 14

How do plants avoid photorespiration?

Back 14

1. Operate the calvin cycle in low oxygen environments - the stomata allows oxygen and carbon dioxide to enter, but cells deeper inside the leaf - cells that surround the vascular bundle - called bundle sheath cell. CO2 instead of reacting with RuBP and RuBisCO, it reacts with phosphenol pyruvate (AKA PEP) (3C) and Phosphenol pyruvate carboxylase. It forms 4C molecule. PEP carboxylase can only fix CO₂. It then produces oxaloacetate - from Krebs! Can then form malate or aspartate. It then reacts again, closing the cycle reforming PEP and also CO₂ as biproduct. The malate travels through plasmodesmata into bundle sheath cell (no contact with oxygen). The malate then react with CO2 to form PEP again. This ensures no oxygen is allowed.

Front 15

Which plants perform C4 photosynthesis

Back 15

Sugar cane, corn, crabgrass

Front 16

What are the issues with living in the desert and wanting to photosynthesise?

Back 16

-Don't want to lose water, but must open stomata to let in CO₂ during the day (even though it is hottest during the day - highest water evaporation) as light dependant reaction needs to happen during the day

Front 17

What is CAM photosynthesis? Who performs CAM?

Back 17

They open stomata at night, and able to fix CO₂ (the same way as C4). The malate is then stored in vacuoles. Malate acts as CO₂ store during day time, so the stomata can close, and they still can access to CO₂ through malate store. So they then can perform Calvin cycle as usual!

Front 18

How is the chloroplast adapted to the light independent reaction?

Back 18

-Fluid in stroma contains all the enzymes needed

- The stroma fluid surrounds grana si the products of the light dependant reaction in the grana can readily diffuse out

- It contains both DNA and ribosomes so it can quickly and easily manufacture proteins

Front 19

Draw a labelled diagram for Calvin's lollipop experiment

Back 19

Compounds in the algae are separated by 2-way chromatography, and the radioactive compounds are identified.

Front 20

In Calvin's lollipop experiment, what was the use of radioactive HCO₃⁻ (¹⁴C)?

Back 20

To supply radioactive CO₂ that can be traced

Front 21

What was the use of the hot methanol in Calvin's Lollipop experiment?

Back 21

To instantly quench the photosynthesis reaction and immediately kill the algae.

Front 22

What is the use of NAD⁺ and NADP⁺?

Back 22

Oxidising agents NADP⁺ is the final electron acceptor

Front 23

What is the use of NADH and NADPH?

Back 23

Reducing agents - NADH is the electron and hydrogen donor for electron transport chain

Front 24

Draw a graph for GP and RuBP levels as light intensity suddenly drops to zero. Explain the trends.

Back 24

As the lights are switched off, the light dependant stops, as the electrons cannot be excited by the photons and progress down the chain. As a result, ATP and NADPH is not produced. The RuBP in the Calvin cycle is converted to GP as there is CO₂ available, but the GP cannot be converted into G3P and back to RuBP as there is no NADPH. This results in a decrease in RuBP, mirrored by and equal increase in GP. As the light is restored, there is a lag phase as the light dependent reaction starts producing NADPH again to convert GP back into G3P and RuBP, restoring the balance.

Front 25

Draw a graph for GP and RuBP levels as CO₂ levels drops to zero. Explain the trends.

Back 25

RuBP levels increase as it cannot be converted into GP without CO₂, and GP is converted into G3P and then RuBP. Consequently, GP levels decrease as much as RuBP levels increase. As external CO₂ is diminished, RuBisCO begins reacting RuBP with O₂ forming phosphoglycolate and 3-phosphoglycerate (photorespiration), which results in a slight decrease in RuBP near the end of the graph.

Front 26

What is the law of limiting factors?

Back 26

At any given the moment, the rate of a physiological process is limited by the factor that is at it's least favourable value

Front 27

Define light compensation point

Back 27

When the amount of CO₂ released during respiration is the same as that taken up during photosynthesis

Front 28

What is the % composition of gases in the Earth's atmosphere?

Back 28

Front 29

Draw the apparatus for measuring the rate of photosynthesis with light intensity

Back 29

After 30 minutes oxygen is drawn up capillary tube using syringe and measured on the scale to the mm³. Then apparatus is reset and experiment repeated 4-5 times

Front 30

What 5 things must be done to ensure valid results? Why?

Back 30

Make sure everything is airtight - will affect volume of gas measured
Water bath maintains constant temperature - so only light intensity changes rate of photosynthesis
KHCO₃ solution bathes Elodea - external source of CO₂ - ensures there is sufficient CO₂ and light intensity is limiting factor
Kept in dark room with only 1 artificial source of light - can control light intensity
In dark room for 2 hours before measuring begins - to prevent photosynthesis and to allow any oxygen produced before the experiment begins can disperse

Front 31

Why is this experiment not reliable?

Back 31

Measuring O₂ produced is always less than actually produced by photosynthesis as some is dissolved in solution and used in respiration.