Photosynthesis

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Lesson notes

What is Photosynthesis?

**Photosynthesis** is the process by which plants, algae, and some bacteria convert **light energy** (usually from the sun) into **chemical energy** stored in sugars.
It is carried out by **photoautotrophs**—organisms that make their own food using light, carbon dioxide, and water.
The overall chemical equation is: **6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂**.
Photosynthesis is essential for life on Earth because it produces **oxygen** and is the foundation of most food chains.

Where Does Photosynthesis Happen?

In plants, photosynthesis occurs in **chloroplasts**, organelles found mainly in leaf cells.
Chloroplasts contain **chlorophyll**, a green pigment that absorbs **red and blue light** and reflects green light.
Leaves are adapted for photosynthesis: they are broad and thin to capture light, have **stomata** for gas exchange, and contain **palisade mesophyll** cells packed with chloroplasts.

Light-Dependent Reactions

These reactions occur in the **thylakoid membranes** of chloroplasts and require **light**.
Light energy splits water molecules (**photolysis**), releasing **oxygen** as a byproduct.
The energy is used to produce **ATP** and **NADPH**, two energy-carrying molecules.

Light-Independent Reactions (Calvin Cycle)

These reactions take place in the **stroma** of chloroplasts and do not need light directly.
They use **ATP** and **NADPH** from the light-dependent reactions to convert **CO₂** into glucose.
Carbon dioxide is **fixed** into organic molecules through a series of steps called the **Calvin cycle**.

Inputs and Outputs

**Inputs**: carbon dioxide (CO₂), water (H₂O), and sunlight.
**Outputs**: glucose (C₆H₁₂O₆) and oxygen (O₂).
Glucose can be used for energy (via cellular respiration) or stored as starch.

Importance of Photosynthesis

Photosynthesis produces **oxygen** that most living things need to breathe.
It is the primary source of **energy** for nearly all ecosystems.
It removes **carbon dioxide** from the atmosphere, helping regulate Earth's climate.

Cross-section of a leaf showing tissues involved in photosynthesis: cuticle, epidermis, palisade mesophyll (with chloroplasts), spongy mesophyll, air spaces, stoma, and guard cells.

Internal structure of a chloroplast: outer and inner membrane (envelope), stroma, grana (stacks of thylakoids), and starch grains.

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Practice questions

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1.Which gas is released as a byproduct of oxygenic photosynthesis?
Easy
- ACarbon dioxide
- BOxygen
- CNitrogen
- DHydrogen
2.What is the main pigment that absorbs light energy in plants?
Easy
- AChlorophyll
- BCarotenoid
- CPhycocyanin
- DRetinal
3.In which organelle does photosynthesis occur in plant cells?
Medium
- AMitochondrion
- BNucleus
- CChloroplast
- DRibosome
4.What are the two main products of the light-dependent reactions?
Medium
- AGlucose and oxygen
- BATP and NADPH
- CCarbon dioxide and water
- DATP and glucose
5.What is the primary role of the Calvin cycle in photosynthesis?
Hard
- ATo split water molecules
- BTo produce ATP
- CTo fix carbon dioxide into organic compounds
- DTo release oxygen
6.Which of the following is a reactant in photosynthesis?
Easy
- AGlucose
- BOxygen
- CCarbon dioxide
- DATP
7.A plant is placed in a sealed jar with a sensor that measures carbon dioxide levels. The jar is kept in the dark. What will happen to the carbon dioxide concentration over time?
Medium
- AIt will decrease because photosynthesis uses CO₂.
- BIt will increase because respiration releases CO₂.
- CIt will stay the same.
- DIt will decrease because respiration uses CO₂.
8.In an experiment, algae are grown in water containing dissolved carbon dioxide labeled with radioactive carbon (¹⁴C). After a short time in the light, where is the radioactivity most likely to be found?
Hard
- AIn oxygen gas
- BIn glucose
- CIn chlorophyll
- DIn water

More questions

9.Which part of the chloroplast is the site of the light-dependent reactions?
Medium
- AStroma
- BThylakoid membrane
- CInner membrane
- DOuter membrane
10.What is the main energy source for photosynthesis?
Easy
- AWind
- BSunlight
- CHeat from soil
- DChemical energy from water
11.Why do most leaves appear green?
Hard
- AThey absorb green light and use it for photosynthesis.
- BThey reflect green light and absorb other colors.
- CThey contain a green pigment that absorbs all colors equally.
- DThey produce green-colored oxygen.
12.A student places a water plant in a beaker of water under a bright light and counts the number of bubbles released per minute. Which variable is the student most likely measuring?
Medium
- ARate of photosynthesis
- BAmount of carbon dioxide used
- CAmount of glucose produced
- DRate of respiration
13.In an experiment, a student places a pondweed in a beaker of water at 20°C and counts the number of bubbles released per minute under different light intensities. The results are shown in the table: Light intensity (arbitrary units) | Bubbles per minute 0 | 0 10 | 8 20 | 15 30 | 21 40 | 25 50 | 26 60 | 26 What is the most likely reason that the bubble count stops increasing after 40 units of light intensity?
Medium
- AAnother factor (e.g. carbon dioxide concentration) is limiting the rate of photosynthesis.
- BThe light intensity is too high and damages the chloroplasts.
- CThe plant has run out of water.
- DThe bubbles are too small to count accurately at high light intensities.
14.A farmer wants to increase the rate of photosynthesis in his greenhouse. Which combination of changes would most likely increase the rate of photosynthesis?
Medium
- AIncrease carbon dioxide concentration and increase light intensity.
- BDecrease temperature and decrease light intensity.
- CIncrease oxygen concentration and decrease carbon dioxide concentration.
- DDecrease water supply and increase light intensity.
15.A plant is placed in a sealed container with a sensor that measures the concentration of carbon dioxide. The container is exposed to light for 6 hours, then placed in darkness for 6 hours. Which graph best shows the expected change in carbon dioxide concentration over the 12-hour period?
Hard
- AA steady decrease in the light, then a steady increase in the dark.
- BA steady increase in the light, then a steady decrease in the dark.
- CNo change in the light, then a steady increase in the dark.
- DA steady decrease in the light, then no change in the dark.
16.Which of the following is a product of the light-dependent reactions of photosynthesis?
Easy
- AGlucose
- BCarbon dioxide
- COxygen
- DWater
17.A student investigates the effect of light color on the rate of photosynthesis in algae. She places algae in three flasks and shines red, green, and blue light on them respectively. She measures the oxygen production after 30 minutes. Which flask is expected to produce the least oxygen?
Medium
- AThe flask with red light.
- BThe flask with green light.
- CThe flask with blue light.
- DAll flasks produce the same amount of oxygen.
18.In an experiment to measure the rate of photosynthesis, a student uses a pH indicator that changes color as carbon dioxide concentration changes. The indicator is red when carbon dioxide concentration is high and purple when it is low. The student places a leaf in a sealed tube with the indicator and exposes it to light. What color change would you expect to see after 30 minutes?
Hard
- ARed to purple.
- BPurple to red.
- CNo change.
- DRed to yellow.
19.The diagram shows a cross-section of a leaf. Which layer contains the most chloroplasts and is the main site of photosynthesis?
Medium
- AUpper epidermis
- BPalisade mesophyll
- CSpongy mesophyll
- DLower epidermis
20.The diagram shows a chloroplast. Which part is the site of the light-dependent reactions?
Medium
- AThe stroma
- BThe grana (thylakoid membranes)
- CThe outer membrane
- DThe starch grain
21.A student wants to compare the rate of photosynthesis in two different species of plant. She places each plant in a separate sealed container with the same initial carbon dioxide concentration and light intensity. After 10 minutes, she measures the carbon dioxide concentration in each container. Which of the following is the dependent variable in this experiment?
Hard
- AThe species of plant.
- BThe light intensity.
- CThe carbon dioxide concentration after 10 minutes.
- DThe initial carbon dioxide concentration.
22.What is the main purpose of photosynthesis for a plant?
Easy
- ATo produce oxygen for the atmosphere.
- BTo convert light energy into chemical energy stored in glucose.
- CTo absorb water from the soil.
- DTo release carbon dioxide into the air.
23.A plant is grown in a sealed jar with a fixed amount of carbon dioxide and water. The jar is placed in sunlight. After several days, the plant has grown and produced new leaves. The total mass of the plant plus the jar and its contents is measured before and after. What would you expect to happen to the total mass?
Medium
- AIt increases because the plant has absorbed carbon dioxide from the air and converted it into organic matter.
- BIt decreases because the plant has used up water and carbon dioxide.
- CIt stays the same because matter cannot be created or destroyed.
- DIt increases because the plant has absorbed minerals from the soil.
24.A student sets up an experiment to measure the rate of photosynthesis in a water plant by counting bubbles of oxygen released per minute. She places the plant in a beaker of water at 25°C and uses a lamp at different distances. The results are shown: Distance from lamp (cm) | Bubbles per minute 10 | 30 20 | 20 30 | 13 40 | 8 50 | 5 What is the most appropriate conclusion from these data?
Hard
- AThe rate of photosynthesis is inversely proportional to the distance from the lamp.
- BThe rate of photosynthesis is directly proportional to the light intensity.
- CAs the distance increases, light intensity decreases, causing a decrease in the rate of photosynthesis.
- DThe plant produces more bubbles when it is closer to the lamp because it is warmer.
25.In an experiment to measure the effect of light intensity on the rate of photosynthesis, a student places a pondweed in a beaker of water and counts the number of bubbles released per minute. The student changes the distance of the lamp from the pondweed. Which of the following is the dependent variable in this experiment?
Medium
- AThe number of bubbles per minute
- BThe distance of the lamp from the pondweed
- CThe temperature of the water
- DThe size of the pondweed
26.A student sets up an experiment to investigate the effect of carbon dioxide concentration on the rate of photosynthesis in algae. She uses a pH indicator that changes color as carbon dioxide concentration changes. She places equal amounts of algae in three flasks with different concentrations of sodium hydrogencarbonate (a source of carbon dioxide) and measures the time taken for the indicator to change color. Which of the following is the most appropriate control for this experiment?
Hard
- AA flask with algae but no sodium hydrogencarbonate
- BA flask with sodium hydrogencarbonate but no algae
- CA flask with boiled algae and sodium hydrogencarbonate
- DA flask with distilled water only
27.A scientist uses a carbon dioxide sensor to measure the rate of photosynthesis in a plant inside a sealed chamber. The chamber is initially in the dark, then exposed to light. Which graph best shows the change in carbon dioxide concentration over time?
Hard
- AConcentration increases in dark, then decreases in light
- BConcentration decreases in dark, then increases in light
- CConcentration remains constant in dark, then decreases in light
- DConcentration decreases in dark, then remains constant in light
28.A student wants to investigate the effect of temperature on the rate of photosynthesis. She places pondweed in water at different temperatures and counts the number of bubbles released per minute under constant light. Which of the following variables should she keep constant?
Medium
- ALight intensity and carbon dioxide concentration
- BTemperature and light intensity
- CNumber of bubbles and temperature
- DCarbon dioxide concentration and number of bubbles
29.In an experiment, a student places a water plant in a beaker of water and counts the number of bubbles released per minute under a bright light. What is the most likely gas in the bubbles?
Easy
- AOxygen
- BCarbon dioxide
- CNitrogen
- DWater vapor
30.A student investigates the effect of light color on photosynthesis using algae. She places algae in three flasks and shines red, green, and blue light on them. After 24 hours, she measures the amount of oxygen produced. Which flask would produce the least oxygen?
Hard
- AGreen light
- BRed light
- CBlue light
- DAll produce the same amount
31.A student sets up an experiment to measure the rate of photosynthesis in a water plant by counting bubbles of oxygen released per minute. She places the plant in water at 20°C and uses a lamp at a fixed distance. She repeats the experiment with the lamp at different distances. Which of the following is a valid conclusion if the number of bubbles increases as the lamp gets closer?
Medium
- AIncreasing light intensity increases the rate of photosynthesis
- BIncreasing temperature increases the rate of photosynthesis
- CDecreasing carbon dioxide concentration increases the rate of photosynthesis
- DIncreasing distance increases the rate of photosynthesis
32.A student wants to compare the rate of photosynthesis in two different species of plant. She places each plant in a separate sealed container with the same initial amount of carbon dioxide and water. The containers are placed in identical light conditions. After 24 hours, she measures the final concentration of carbon dioxide in each container. Which plant has a higher rate of photosynthesis?
Hard
- AThe plant that has a lower final carbon dioxide concentration
- BThe plant that has a higher final carbon dioxide concentration
- CThe plant that has a higher final oxygen concentration
- DCannot be determined from carbon dioxide concentration alone
33.In an experiment, a student places a water plant in a beaker of water and adds a small amount of sodium hydrogencarbonate to increase carbon dioxide concentration. She counts the number of bubbles released per minute under a bright light. She then repeats the experiment using boiled (cooled) water. What is the most likely effect on the number of bubbles?
Hard
- AFewer bubbles because boiling removes dissolved carbon dioxide
- BMore bubbles because boiling adds oxygen to the water
- CNo change because boiling does not affect photosynthesis
- DFewer bubbles because boiling kills the plant
34.What is the main purpose of photosynthesis?
Easy
- ATo convert light energy into chemical energy stored in glucose
- BTo release energy from glucose for cellular activities
- CTo absorb water and minerals from the soil
- DTo produce carbon dioxide and water
35.Which of the following is a product of photosynthesis?
Easy
- ACarbon dioxide
- BWater
- COxygen
- DATP
36.In which organelle does photosynthesis occur?
Easy
- AMitochondrion
- BChloroplast
- CNucleus
- DRibosome
37.What is the role of chlorophyll in photosynthesis?
Medium
- ATo absorb light energy and convert it to chemical energy
- BTo split water molecules into oxygen and hydrogen
- CTo fix carbon dioxide into glucose
- DTo transport electrons in the thylakoid membrane
38.Which gas do plants take in during photosynthesis?
Easy
- AOxygen
- BCarbon dioxide
- CNitrogen
- DHydrogen
39.In the light-dependent reactions, what is split to release oxygen?
Medium
- ACarbon dioxide
- BGlucose
- CWater
- DATP
40.What are the products of the light-dependent reactions that are used in the Calvin cycle?
Medium
- AATP and NADPH
- BGlucose and oxygen
- CCarbon dioxide and water
- DChlorophyll and light

Revision notes

## Photosynthesis: Night-Before Exam Notes ### What is Photosynthesis? - Process where plants, algae, and cyanobacteria convert light energy (sunlight) into chemical energy stored in glucose. - Releases oxygen as a byproduct (oxygenic photosynthesis). - Opposite of cellular respiration: photosynthesis uses CO₂ and H₂O to make glucose; respiration breaks glucose to release energy. ### Key Equation (simplified) 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ (glucose) + 6O₂ ### Where Does It Happen? - In plant cells: **chloroplasts** (contain chlorophyll, the green pigment that absorbs red and blue light, reflects green). - Chlorophyll is found in leaf cells. ### Two Main Stages 1. **Light-Dependent Reactions** (need light) - Light energy splits water (H₂O) → releases O₂, H⁺, and electrons. - Energy is used to make ATP and NADPH (energy carriers). 2. **Light-Independent Reactions (Calvin Cycle)** (do not need light directly) - Uses ATP and NADPH from stage 1. - Fixes CO₂ from air into organic molecules (carbon fixation). - Produces glucose and other carbohydrates. ### Why Is Photosynthesis Important? - Provides food (glucose) for plants and, indirectly, for animals. - Produces most of the oxygen in Earth's atmosphere. - Stores energy from the sun that powers nearly all life. - Removes CO₂ from the atmosphere. ### Key Facts to Remember - Inputs: carbon dioxide, water, sunlight. - Outputs: glucose (sugar), oxygen. - Plants are **photoautotrophs** – they make their own food using light. - Photosynthesis is an **endothermic** reaction (needs energy input). - Global photosynthesis captures ~130 terawatts of energy and converts ~100 billion tons of carbon into biomass per year. ### Quick Comparison | Process | Inputs | Outputs | |---------|--------|--------| | Photosynthesis | CO₂, H₂O, light | Glucose, O₂ | | Cellular Respiration | Glucose, O₂ | CO₂, H₂O, energy |

Photosynthesis: The process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose, using carbon dioxide and water, and releasing oxygen.
Chloroplast: Organelle in plant cells where photosynthesis occurs; contains chlorophyll.
Chlorophyll: Green pigment in chloroplasts that absorbs light energy (mainly red and blue) for photosynthesis.
Glucose: A simple sugar (C₆H₁₂O₆) produced by photosynthesis; used as an energy source by organisms.
Light-dependent reactions: First stage of photosynthesis; requires light, splits water, produces ATP, NADPH, and oxygen.
Calvin cycle: Light-independent reactions of photosynthesis; uses ATP and NADPH to convert CO₂ into glucose.
Carbon fixation: The process of incorporating carbon dioxide into organic molecules, such as during the Calvin cycle.
Photoautotroph: An organism that uses light energy to synthesize food from carbon dioxide and water (e.g., plants, algae).
Cellular respiration: The process that breaks down glucose to release energy, using oxygen and producing carbon dioxide and water; opposite of photosynthesis.

Flashcards

What is photosynthesis?

Photosynthesis is a process by which plants, algae, and some bacteria convert light energy from the sun into chemical energy stored in sugars like glucose.

What are the inputs (reactants) of photosynthesis?

The inputs are carbon dioxide (CO₂), water (H₂O), and light energy (usually from sunlight).

What are the outputs (products) of photosynthesis?

The outputs are glucose (a sugar) and oxygen (O₂).

Write the balanced chemical equation for photosynthesis.

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

In which organelle does photosynthesis occur in plants?

Photosynthesis occurs in the chloroplasts, which contain chlorophyll.

What is chlorophyll and what does it do?

Chlorophyll is a green pigment in chloroplasts that absorbs light energy, mainly from the red and blue parts of the spectrum, and reflects green light.

Why do leaves appear green?

Leaves appear green because chlorophyll reflects green light and absorbs red and blue light.

What are the two main stages of photosynthesis?

The two stages are the light-dependent reactions and the light-independent reactions (Calvin cycle).

What happens during the light-dependent reactions?

Light energy is absorbed by chlorophyll, water is split (releasing oxygen), and energy is stored in ATP and NADPH.

What happens during the Calvin cycle (light-independent reactions)?

Carbon dioxide is fixed into organic molecules using ATP and NADPH, producing glucose.

What is the role of water in photosynthesis?

Water is split during the light-dependent reactions, providing electrons and hydrogen ions, and releasing oxygen as a byproduct.

What is the role of carbon dioxide in photosynthesis?

Carbon dioxide is used in the Calvin cycle to build glucose molecules through carbon fixation.

What is the energy currency molecule produced during the light-dependent reactions?

ATP (adenosine triphosphate) is produced and used to power the Calvin cycle.

What is NADPH and why is it important?

NADPH is a carrier molecule that stores high-energy electrons and hydrogen; it is used in the Calvin cycle to help build glucose.

How does photosynthesis relate to cellular respiration?

Photosynthesis produces glucose and oxygen, which are used in cellular respiration to release energy, producing carbon dioxide and water.

Why is photosynthesis important for life on Earth?

It produces oxygen for the atmosphere and provides the organic compounds that serve as food for nearly all living things.

What is a photoautotroph?

A photoautotroph is an organism that uses light energy to synthesize its own food from carbon dioxide and water (e.g., plants, algae, cyanobacteria).

What is oxygenic photosynthesis?

Oxygenic photosynthesis is the type of photosynthesis that produces oxygen as a byproduct; it is performed by plants, algae, and cyanobacteria.

What is anoxygenic photosynthesis?

Anoxygenic photosynthesis does not produce oxygen; some bacteria use hydrogen sulfide instead of water, releasing sulfur.

What is carbon fixation?

Carbon fixation is the process of converting inorganic carbon dioxide into organic compounds like glucose during the Calvin cycle.

Where do the light-dependent reactions take place?

They take place in the thylakoid membranes inside the chloroplast.

Where does the Calvin cycle occur?

The Calvin cycle occurs in the stroma, the fluid-filled space around the thylakoids in the chloroplast.

What is the main pigment that captures light energy in plants?

Chlorophyll a is the primary pigment, but accessory pigments like chlorophyll b and carotenoids also help capture light.

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