Photosynthesis — Combined Science 0653 - Biology Revision Notes & Topical Questions

Section 1 of 26What photosynthesis is

Photosynthesis

Photosynthesis the process by which plants manufacture carbohydrates from raw materials using energy from light.

Raw materials

• Raw materials carbon dioxide and water.
• Light is energy, not a substance, so it is not a raw material.

Products

• Products glucose, with oxygen released as a waste product.

Section 2 of 26Word equation

• Equation
  • carbon dioxide + water → glucose + oxygen.
  • The reaction needs light and chlorophyll.

Section 3 of 26Chlorophyll

Chlorophyll

Chlorophyll

• Chlorophyll a green pigment found in the chloroplasts of plant cells.

Colour

• Colour it reflects green light, which gives plants their green colour.

Function

• Function it absorbs light energy and transfers it into energy in chemicals for making carbohydrates such as glucose.

Requirement

• Requirement photosynthesis cannot happen without chlorophyll.

Section 4 of 26Uses and storage of carbohydrates

Starch

• Starch glucose is converted into starch, an effective energy store.

Cellulose

• Cellulose glucose is converted into cellulose to build cell walls.

Respiration

• Respiration glucose is used in respiration to release energy.

Sucrose

• Sucrose glucose is converted into sucrose for transport in the phloem.

Nectar

• Nectar sugar is made into nectar to attract insects for pollination.

Lipids

• Lipids carbohydrates are converted into lipids as an energy store in seeds.

Amino acids

• Amino acids glucose combines with nitrogen and other mineral ions from the roots to make amino acids for proteins.

Section 5 of 26Why plants need mineral ions

Own molecules

• Own molecules plants do not eat, so they must make proteins, lipids and DNA themselves.

Other elements

• Other elements proteins contain nitrogen, and chlorophyll contains magnesium and nitrogen.

Uptake

• Uptake mineral ions are actively absorbed from the soil by the root hair cells.

Section 6 of 26Mineral deficiencies

Mineral ion	Function	Deficiency symptom
Magnesium	Needed to make chlorophyll	Yellowing between the veins of the leaves
Nitrate	Source of nitrogen for amino acids and proteins	Stunted growth and yellow leaves

Section 7 of 26Testing a leaf for starch

• Reason
  • glucose is quickly used or stored, so a leaf is tested for starch to show where photosynthesis has happened.
  • Starch is stored in the chloroplasts where photosynthesis takes place.

1

Drop the leaf in boiling water to kill the cells and break down the membranes.

2

Heat the leaf in hot ethanol for 5 to 10 minutes to remove the chlorophyll.

3

Dip the leaf in boiling water to soften it.

4

Spread the leaf on a white tile and cover it with iodine solution.

5

Starch is present if the leaf turns blue-black.

• Safety
  • ethanol is extremely flammable, so turn off the Bunsen burner and heat it in an electric water bath.

Section 8 of 26More from the specification

More from the specification

Identifying the Primary Limiting Factor

• On a multi-variable rate-of-photosynthesis graph,
  • when increasing light intensity no longer boosts oxygen production, the plateau indicates that carbon dioxide availability or ambient temperature has become limiting.

Experimental Design for Aquatic Plants

• Submerging Cabomba or Elodea in sodium hydrogencarbonate solution under an inverted funnel allows precise counting of evolved oxygen bubbles per minute as distance from a light source varies.

Leaf Mesophyll Structure and Gas Diffusion

• Loosely arranged spongy mesophyll cells provide large moist surface areas and extensive intercellular air spaces for rapid carbon dioxide diffusion towards photosynthetic palisade mesophyll cells.

Light intensity

• The amount of light.

Limiting factor

• A factor in such short supply that it restricts the rate of a process.

Temperature

• Average energy of the particles in an object.

Mesophyll

• The leaf tissue made of palisade and spongy cells.

Concentration

• Amount of solute dissolved in a volume of solution.

Indicator

• Substance that changes colour in acids and alkalis.

Section 9 of 26Investigating the need for chlorophyll

Variegated leaf

• Variegated leaf a leaf that is partly green and partly white.

Method

• Method test the leaf for starch.
• Only the green areas containing chlorophyll turn blue-black.
• The white areas stay orange-brown because there is no photosynthesis and no starch.

Section 10 of 26Investigating the need for light

1

Destarch the plant by keeping it in a dark cupboard for 24 hours.

2

Cover part of a leaf with aluminium foil.

3

Place the plant in sunlight for a day.

4

Remove the leaf and test it for starch using iodine.

• Result
  • the covered area stays orange-brown and the exposed area turns blue-black, so light is needed.

Section 11 of 26Investigating the need for carbon dioxide

1

Destarch two plants by keeping them in the dark for a long time.

2

Put one plant in a bell jar with a beaker of sodium hydroxide, which absorbs carbon dioxide.

3

Put the other plant in a bell jar with a beaker of water as the control.

4

Leave both in bright light for several hours.

5

Test a leaf from each plant for starch.

• Result
  • the leaf without carbon dioxide stays orange-brown and the control leaf turns blue-black.

Section 12 of 26Investigating the rate of photosynthesis

Pondweed

• Pondweed an aquatic plant such as Elodea is used.
• Oxygen from photosynthesis leaves the cut end as bubbles.

Counting bubbles

• Counting bubbles more bubbles per minute means a faster rate.

Collecting gas

• Collecting gas a more accurate method is to collect the oxygen in an inverted test tube, measuring cylinder or gas syringe and measure its volume.

Section 13 of 26Changing one factor at a time

Factor	How it is changed
Light intensity	Move a lamp different distances from the beaker
Temperature	Change the temperature of the water in the beaker
Carbon dioxide concentration	Dissolve different amounts of sodium hydrogencarbonate in the water

• Fair test
  • keep all other variables constant.
  • A glass tank between the lamp and the beaker absorbs heat so that the temperature does not change with the light intensity.

Section 14 of 26Gas exchange in plants

Respiration

• Respiration plants respire all the time, taking in oxygen and giving out carbon dioxide.

Photosynthesis

• Photosynthesis in daylight plants take in carbon dioxide and release oxygen.

Night

• Night plants do not photosynthesise but still respire, so they take in oxygen and give out carbon dioxide.

Bright light

• Bright light photosynthesis is faster than respiration, so there is a net intake of carbon dioxide and a net output of oxygen.

Section 15 of 26Hydrogencarbonate indicator

Carbon dioxide

• Carbon dioxide it is an acidic gas in water, so the indicator shows its concentration.

Method

• Method place leaves from the same plant in stoppered tubes with indicator and compare light and dark conditions over a few hours.

Tube	Conditions	Indicator colour	Conclusion
Leaf in light	Light	Purple	Net intake of carbon dioxide
Leaf in dark	Dark	Yellow	Net intake of oxygen and release of carbon dioxide
No leaf	Light	Red	Control for comparison

Section 16 of 26More from the specification

More from the specification

Photosynthetic synthesis of amino acids

• Photosynthetic synthesis of amino acids plants combine triose phosphate sugars with absorbed soil nitrates to synthesize essential amino acids for growth.

Photosynthesis Chemical Equation

• Chlorophyll absorbs light energy to convert carbon dioxide and water into glucose and oxygen.
  • (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂)

Leaf Structure and Mesophyll Cells

• Photosynthesis primarily occurs within palisade mesophyll cells of leaves,
• which are densely packed with chloroplasts to maximize light absorption.

Limiting Factors on Photosynthesis

• Light intensity, carbon dioxide concentration,
• and temperature act as limiting factors; on a rate graph,
• increasing a limiting factor increases rate until another factor limits it.

Synthesis and Storage of Carbohydrates

• The glucose produced during photosynthesis is converted into starch for storage, cellulose for cell walls,
• or sucrose for phloem translocation.

Limiting factor

• A factor in such short supply that it restricts the rate of a process.

Amino acid

• The unit from which proteins are made.

Glucose

• Sugar that is the fuel for respiration.

Carbohydrate

• Nutrient, such as starch and sugar, that gives energy.

Mesophyll

• The leaf tissue made of palisade and spongy cells.

Chloroplast

• A structure with chlorophyll where photosynthesis happens.

Mineral

• Substance plants take from the soil in small amounts for healthy growth.

Section 17 of 26Balanced equation

Equation

• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in the presence of light and chlorophyll.

Energy change

• Energy change light energy is converted into chemical energy in the bonds of glucose.

Respiration link

• Respiration link the equation is the exact reverse of the equation for aerobic respiration.

Section 18 of 26Limiting factors

Limiting factor

Limiting factor something present in the environment in such short supply that it restricts life processes.

Unlimited supplies

• Unlimited supplies the rate would be limited only by the plant's own ability to absorb the materials and make them react.

Usual case

• Usual case the rate is limited by whichever factor is in shortest supply at that time.

Section 19 of 26The three main limiting factors

Temperature

Light intensity

Carbon dioxide concentration

Water

• Water it is not a limiting factor, because the amount needed is small compared with the amount lost in transpiration.

Section 20 of 26Temperature

Increasing temperature

• Increasing temperature the rate rises because the reaction is controlled by enzymes.

High temperature

• High temperature beyond a certain point the enzymes denature and the rate falls.

Section 21 of 26Light intensity

More light

• More light the rate of photosynthesis increases.

Plateau

• Plateau at a certain point the rate stops rising and becomes constant because another factor is in short supply.
• The limiting factor on the plateau could be a temperature that is too low or too little carbon dioxide.

Section 22 of 26Carbon dioxide concentration

More carbon dioxide

• More carbon dioxide it is a raw material, so the rate increases.

Plateau

• Plateau the rate stops rising when another factor becomes limiting.
• The factor could be a temperature that is too low or too little light.

Section 23 of 26Reading limiting factor graphs

Reading limiting factor graphs

Split the graph into sections.

In the section where the line rises, the limiting factor is the quantity labelled on the x axis.

In the section where the line is horizontal, the limiting factor is another factor,

• such as temperature, light intensity or carbon dioxide concentration.

Section 24 of 26More from the specification

More from the specification

Leaf Internal Structure and Gas Exchange

• The leaf mesophyll layer consists of palisade cells packed with chloroplasts for maximum light absorption and spongy mesophyll cells separated by air spaces that facilitate rapid gas exchange of CO₂ and oxygen.

Synthesis and Storage of Carbohydrates in Plants

• Photosynthesis converts light energy into chemical energy stored in carbohydrates: glucose synthesized during photosynthesis is rapidly polymerized into insoluble starch grains for storage within leaf cells.

Testing Leaves for Starch Presence

• In practical investigations into photosynthesis, leaves are boiled in water to break cell membranes, bleached in warm ethanol to remove green chlorophyll,
• and stained with iodine solution to test for starch accumulation.

Investigating the Effect of Light and Carbon Dioxide

• Controlled experiments with aquatic or terrestrial plants demonstrate that photosynthesis requires both light and carbon dioxide to produce oxygen and synthesize organic carbohydrates.

Amino acid

• The unit from which proteins are made.

Oxygen

• Gas that relights a glowing splint.

Starch

• The storage carbohydrate in plants, which turns iodine blue-black.

Carbohydrate

• Nutrient, such as starch and sugar, that gives energy.

Mesophyll

• The leaf tissue made of palisade and spongy cells.

Chloroplast

• A structure with chlorophyll where photosynthesis happens.

Mineral

• Substance plants take from the soil in small amounts for healthy growth.

Indicator

• Substance that changes colour in acids and alkalis.

Section 25 of 26Key terms

Term	Meaning
Photosynthesis	The process by which plants make carbohydrates from carbon dioxide and water using light energy.
Chlorophyll	The green pigment in chloroplasts that absorbs light energy.
Chloroplast	The part of a plant cell where photosynthesis happens.
Glucose	The sugar made in photosynthesis.
Starch	The storage carbohydrate in plants, which turns iodine blue-black.
Mineral ion	An inorganic ion absorbed from the soil.
Destarch	To remove stored starch from a plant by keeping it in the dark.

Term	Meaning
Destarch	To use up stored starch by keeping a plant in the dark.
Variegated leaf	A leaf with green and white areas.
Control	A set-up kept the same except for the factor being tested.
Elodea	An aquatic pondweed used to count oxygen bubbles.
Hydrogencarbonate indicator	An indicator whose colour shows carbon dioxide concentration.
Net gas exchange	The overall difference between gas taken in and given out.
Sodium hydroxide	A substance that absorbs carbon dioxide.

Term	Meaning
Limiting factor	A factor in such short supply that it restricts the rate of a process.
Denature	To lose shape permanently, which stops an enzyme working.
Light intensity	The brightness of the light reaching the plant.
Carbon dioxide concentration	The amount of carbon dioxide available.
Plateau	The flat part of a graph where the rate no longer increases.
Rate	How fast something happens.

Section 26 of 26Exam technique

1

Giving light as a raw material — the raw materials are carbon dioxide and water.

2

Saying plants make oxygen as the main product — glucose is the product and oxygen is a by-product.

3

Saying chlorophyll absorbs green light — it reflects it.

4

Testing a leaf for glucose to show photosynthesis — the leaf is tested for starch.

1

Not destarching the plant first — stored starch would affect the result.

2

Putting the Bunsen burner on while heating ethanol — it is extremely flammable.

3

Leaving out a control — the plant with water in the jar is the control.

4

Changing two factors at once — change only one and keep the others constant.

1

Saying water is the main limiting factor — it is rarely limiting.

2

Saying a plateau means photosynthesis has stopped — it means another factor is limiting.

3

Saying high temperature always speeds up photosynthesis — enzymes denature above the optimum.

4

Writing the equation the wrong way round — carbon dioxide and water are the reactants.