Unit 3 - Photosynthesis

Question 1

What is photosynthesis? Where does it occur? Explain the products of photosynthesis.

Answer 1

Photosynthesis occurs in plants during the day, in the chloroplasts. They contain mitochondria for ATP generating purposes.

The overall equation for photosynthesis is: 12H₂O + 6CO₂ + light energy —-> 6O₂ +C₆H₁₂O₆

The glucose is mainly used to create sucrose, starch, and other carbohydrates. These are the true end products.

Question 2

Draw and state the parts of the chloroplast. Also outline their function (7)

Answer 2

• Chloroplasts consist of a double outer membrane (2 lipid bilayers) enclosing the fluid inside called stroma that contains lots of enzymes for photosynthesis
• Embedded in the stroma are the disc-shaped thylakoids. One stack of thylakoids is called a granum
• Highways in between the thylakoids are called intergranum or lamallae
• The chloroplast also contains starch grains (food storage) and droplets of oil (used in lipid metabolism)
• DNA of the chloroplast is also embedded in the structure

Question 3

Draw and state the parts of the leaf cell. Also outline their function where applicable (12)

Answer 3

• A waxy cuticle that protects against excessive water loss.
• An upper epidermis layer that below exists a layer of palisade mesophyll cells. These have the most chloroplasts because they are closer to the sun.
• A lower epidermis layer that above exists a layer of spongy mesophyll cells.
  
  • Also contains stoma and guard cells, creating a substomatal air chamber. The stoma allows for water take in and O₂ release, which are opened and closed by the guard cells. Stomata exist at the bottom of the cell to avoid water loss by evaporation.
• A vascular bundle that contains a xylem (carries water from roots upwards) and a phloem (transports sugars down to roots). It is surrounded by bundle sheath cells

Question 4

State the parts of the palisade and spongy mesophyll cells (4)

Answer 4

Palisade cells contain:

• A cellulose wall
• Chloroplasts
• A sap vacuole (storage)
• A nucleus
• Spongy mesophyll cells contain these as well, but have less chloroplasts and are smaller

Question 5

Explain C3 (PGA) plants

Answer 5

Ex., rice, beans

On hot, dry days the stomata in all plants are closed in order to conserve water. As a result, O₂ cannot leave the plant, increasing the concentration of O₂:CO₂.

In C₃ plants, this leads to rubisco picking up O₂ for RuBP instead of CO₂, resulting in an oxidised 5-carbon unstable molecule which breaks into 1 PGA and 1 phosphoglycolate (waste).

This process is hard on the plant and also results in a slower C-B cycle. This is called photorespiration (rubisco oxygenates RuBP)

Question 6

Explain C4 plants

Answer 6

Ex., corn, sugarcane

Unlike in C₃ plants, when the stomata close, photorespiration does not occur (to a large extent).

C₄ (occurs regardless of temp) plants have large mesophyll cells that surround the vascular bundle. In these cells, CO₂ is fixed to PEP which only fishes for CO₂. This produces oxaloacetate via PEP carboxylase, which is shuttled by malate acid to and released in the sheath cells. It then releases CO₂, becomes pyruvate, and is used again to recreate PEP.

The CO₂ is now fixed by rubisco. The C-B cycle now occurs in the sheath cell.

While C₃ plants slow their growth during the heat, C₄ plants can continue to grow normally.

Question 7

Draw C3 and C4 plants

Question 8

Explain CAM plants

Answer 8

Ex., cactus

These are succulent plants (water-storing). Unlike other plants, they open their stoma at night and close during the day, due to perpetual extreme heat. At night, they take up CO₂ and fix them to various organic acids (called crassulacean acid metabolism – CAM), which are made in mesophyll cells and stored in vacuoles.

During the day, when the light reactions can supply ATP and NADPH, the plants can keep their stomata closed and use their stored CO₂ to activate the C-B cycle.

Question 9

What are pigments, where are they found and what are they used for? What colours of light are best for photosynthesis?

Answer 9

Pigments are photon absorbing molecules located within the thylakoid. These are used in photosynthesis to capture light.

Blue and red colours are best for photosynthesis.

Question 10

Describe the structure of the pigment chlorophyll

Answer 10

Chlorophyll is found in all photosynthetic organisms. They contain a porphyrin ring (head) and phytol chain (tail). Magnesium is the central atom

Chlorophyll has alternating double bonds. The double bonds are what is struck by the photons, which boots an electron in the double bond to an excited state. This is what occurs in the photosystems.

Question 11

Chlorophyll molecules are located in ______ of the cell membrane of ______ (bacteria that use photosynthesis) only

Answer 11

Chlorophyll molecules are located in infoldings of the cell membrane of cyanobacteria (bacteria that use photosynthesis) only

Question 12

State the 6 types of pigments and their colour. What colours of light do chlorophyll a and b like?

Answer 12

Chlorophylls (a, b) – green; a likes methly red, b likes aldehyde blue

Carotenoids – yellow-orange

Phycocyanin (accessory)

Phycoerythrin (accessory)

Anthocyanin – red, dark blue

Xanthophyll – yellow

Question 13

State the 3 factors that affect photosynthesis. Why do they rely on each other?

Answer 13

There are three main factors that affect the rate of photosynthesis. More of these is typically more effective for the plant:

1. Light intensity (photon amount)
2. CO₂ concentration
3. Temperature

Additionally, wavelength of light (colour) can affect the rate of photosynthesis as well.

These factors are all dependent on each other. For example, as light intensity increases, the rate of photosynthesis will increase so long as other factors are in adequate supply. As the rate increases, eventually another nonincreasing factor will become insufficient to support a higher rate of photosynthesis despite the high light intensities, so it plateaus.

Question 14

State some factors that can inhibit photosynthesis

Answer 14

• High concentrations of oxygen
• Lack of water, CO₂
• Lack of light
• Enzyme denaturing (pH, temperature)
• Air pollution

Question 15

Compare photosynthesis and cell respiration:

• Chemical Equation
• Energy
• Electron Carries
• Electron source
• ETS Products
• Final Electron Acceptor
• Proton Reservoir
• Occurs In…

Answer 15

Question 16

Compare cyclic and noncyclic phosphorylation

Answer 16

Typical light reactions in leafy plants are noncyclic. Electrons move through 2 photosystems, are removed by NADP⁺, and replaced by photolysis. This results in ATP and NADPH production and produces lots of energy.

Cyclic light reactions occur in small photosynthetic bacteria. Here, the electrons move through photosystem in a cycle, and never need replenishing. After photosystem I’s ETS, they simply return to photosystem I and are carried back to the ETS by ferredoxin. This results in ATP production, but in lesser amounts.

Question 17

Compare the action spectrum and the absorption spectrum

Answer 17

Action Spectrum: a graph illustrating the effectiveness with which different wavelengths of light promote photosynthesis

Absorption Spectrum: a graph illustrating the wavelengths of light absorbed by a pigment

Question 18

Explain the connection of endosymbiosis between eubacteria and cyanobacteria

Answer 18

In the past, it is believed that eukaryotic cells provided protection from harsh conditions for the bacteria, and the bacteria provided food (glucose) in return.

There are two pieces of evidence that support the endosymbiosis between eukaryotes and cyanobacteria

1. Both cyanobacteria and plant chloroplasts contain chlorophyll a
2. Both have many similar gene sequences.

Question 19

Explain the function of β-carotene in plants and humans

Answer 19

In plants, β-carotene absorbs light that would normally damage chlorophyll. In humans, β-carotene is split into two molecules of vitamin A, which is oxidized into retinal, and protects the eye from photon damage.

Question 20

Why are leaves red/yellow in the fall? What prompts this change?

Answer 20

Leaves are green in the summer due to the high levels of chlorophyll pigments in them (they reflect green) and are yellow, orange, or red in the fall because chlorophyll pigments are degraded, which normally masks the other pigments.

In autumn, because of changes in the length of daylight and changes in temperature, the leaves stop their food-making process. The chlorophyll breaks down, the green color disappears, and the yellow to orange colors become visible and give the leaves part of their fall splendor.

Question 21

Define anthocynanins and explain their function. Where are they located?

Answer 21

Anthocyanins: pigments in vacuoles that give rise to the red colour in autumn leaves. These are synthesized in the fall in ripening tomatoes or turning maple leaves.

The colors can help attract pollinating animals to flowers and animals that will help spread seeds.

Question 22

Describe how the stomata open and close

Answer 22

Stomata open when guard cells are turgid (swollen) and close when guard cells are flaccid (limp).

As K+ levels increase in the guard cells, the water potential of the guard cells drops, and water enters the guard cells, swelling them and opening the stomata.

When the guard cells have lost water, they become flaccid and close the stomata. This may occur when the plant has lost an excessive amount of water. They are also often closed at night since photosynthesis is not occuring.

Question 23

State 5 factors affecting the opening and closure of the stomata

Answer 23

1. Light - When present, light plays an important role in the movement of guard cells. Light causes movement of K+ into the guard cells, causing them to take in water and become tugid, opening the stomata
2. Temperature - Severe heat causes the stomata to close to prevent water loss
3. Water - Under conditions of water scarcity also, the stomata close due to flaccid guard cells.
4. Abscissic acid - Abscissic acid accumulates in the leaves when the plants experience water stress or water deficit. It has been observed, that ABA (Abscissic acid) stimulates closure of stomata under these conditions.
5. Low CO₂

Question 24

What happens to plants at night?

Answer 24

Photosynthesis cannot occur due to the lack of light energy. Instead, they use their mitochondrion to undergo cell respiration. They use the glucose produced during the day.

Question 25

What is transpiration? Explain the 2 ways that transpiration assists photosyntheis

Answer 25

Transpiration is the process by which plants absorb and distribute water through their roots and release water vapor through their leaves.

1. It creates a “transpiration pull” that moves water, minerals, and other substances from roots, where they are absorbed, to leaves where they are used.
2. It produces an evaporative cooling effect that prevents leaves from heating to temperatures that could inhibit or denature the enzymes that catalyze the reactions of photosynthesis.

Question 26

Describe the layout of spirogyra bacteria

Answer 26

They have helical or spiral arrangement of the chloroplasts that is diagnostic of the genus.

Question 27

Another name for ATPase enzyme

Answer 27

H+ pore

Question 28

Define photosynthetic

Answer 28

Photosynthetic: used in photosynthesis