Leaf structure — Combined Science 0653 - Biology Revision Notes & Topical Questions

Section 1 of 25Structures to identify

Structures to identify

Cuticle

• Cuticle the waxy layer on the upper surface.

Epidermis

• Epidermis the upper epidermis and the lower epidermis.

Mesophyll

• Mesophyll the palisade mesophyll and the spongy mesophyll, with air spaces.

Stomata and guard cells

• Stomata and guard cells pores and the cells that control them.

Vascular bundle

• Vascular bundle xylem and phloem.

Chloroplasts

• Chloroplasts in the mesophyll cells.

Section 2 of 25Functions of the layers

Structure	Description	Function
Wax cuticle	Waterproof layer on top of the leaf	Reduces water loss by evaporation
Upper epidermis	Thin and transparent	Lets light pass to the palisade mesophyll
Palisade mesophyll	Column-shaped cells packed with chloroplasts	Absorbs as much light as possible for photosynthesis
Spongy mesophyll	Large air spaces between cells	Gives a large surface for gas exchange
Lower epidermis	Contains most of the stomata	Allows gas exchange and reduces water loss
Vascular bundle	Xylem and phloem	Transports substances to and from the leaf

Section 3 of 25Palisade mesophyll

Position

• Position a layer just under the upper epidermis.

Shape

• Shape long, narrow cells packed tightly together.

Chloroplasts

• Chloroplasts they contain many chloroplasts, which absorb light.

Section 4 of 25Spongy mesophyll

Spongy mesophyll

Air spaces

• Air spaces large spaces between the rounded cells.

Function

• Function carbon dioxide diffuses through the air spaces to the palisade cells and oxygen diffuses out.

Section 5 of 25Vascular bundles

Xylem

• Xylem carries water and dissolved mineral ions up to the mesophyll cells.

Phloem

• Phloem carries sucrose and amino acids away from the leaf.

Position

• Position in the leaf the xylem is on the inside and the phloem is on the outside.

Section 6 of 25Stomata and guard cells

Stoma (plural stomata)

• Stoma (plural stomata) a small pore that allows gas exchange between the leaf and the air.

Guard cells

• Guard cells two cells around each stoma that change shape to open or close it.

Number

• Number in most plants stomata are more numerous on the lower epidermis.

Section 7 of 25Pathway of carbon dioxide

1

Carbon dioxide diffuses from the atmosphere through the stomata.

2

It passes into the air spaces around the spongy mesophyll cells.

3

It diffuses into the mesophyll cells.

4

It reaches the chloroplasts.

Section 8 of 25More from the specification

1Balanced Photosynthesis Chemical Equation

• Photosynthesis is an endothermic photochemical reaction summarized by: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, requiring light absorption by chlorophyll molecules.

2Investigating Leaf Starch Accumulation

• De-starching a potted plant in darkness, exposing part of a leaf to light while shielding the rest,
  • and testing with iodine reveals blue-black starch only where chlorophyll absorbed radiant energy.

3Chloroplast Distribution Across Leaf Tissues

• Palisade mesophyll cells packed with abundant chloroplasts are positioned directly beneath transparent epidermal cells to capture maximum photon energy for glucose synthesis.

4Indicator

• Substance that changes colour in acids and alkalis.

5Light intensity

• The amount of light.

6Limiting factor

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

7Chlorophyll

• Green pigment that absorbs energy from sunlight.

8Temperature

• Average energy of the particles in an object.

9Glucose

• Sugar that is the fuel for respiration.

10Starch

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

11Carbohydrate

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

12Concentration

• Amount of solute dissolved in a volume of solution.

Section 9 of 25Adaptations of the whole leaf

Feature	Adaptation
Large surface area	Increases the area for diffusion of carbon dioxide and absorption of light
Thin	Lets carbon dioxide diffuse quickly to the palisade cells
Chlorophyll	Absorbs light energy so photosynthesis can take place
Network of veins	Brings water to the leaf cells and takes carbohydrates away
Stomata	Allow carbon dioxide to diffuse in and oxygen to diffuse out

Section 10 of 25Adaptations of the layers

Layer	Adaptation
Thin transparent cuticle	Protects the leaf without blocking sunlight
Transparent upper epidermis	Lets light reach the palisade cells
Palisade layer at the top	Light hits the chloroplasts directly, which maximises absorption
Spongy layer	Air spaces let carbon dioxide diffuse through the leaf and increase the surface area
Vascular bundles	Thick cell walls help to support the stem and leaf

Section 11 of 25Gas exchange for photosynthesis

Carbon dioxide in

• Carbon dioxide in it diffuses from the air through the stomata into the air spaces and on to the mesophyll cells.

Oxygen out

• Oxygen out oxygen made in photosynthesis leaves by the reverse route.

Air spaces

• Air spaces they give carbon dioxide access to a large surface area of cells.

Section 12 of 25Why the leaf is thin and broad

Large area

• Large area it catches as much light as possible.

Short distance

• Short distance a thin leaf gives a short diffusion distance for carbon dioxide to reach the palisade cells.

Section 13 of 25Water for the leaf

Water supply

• Water supply xylem brings water to the leaf cells.

Cuticle

• Cuticle the waxy layer reduces water loss so water is kept for photosynthesis.

Stomata position

• Stomata position in most plants more stomata are on the lower epidermis,
  • where there is less exposure to direct sunlight and heat,
  • which reduces water loss.

Section 14 of 25Removing the products

Phloem

• Phloem carries sucrose away from the leaf to where it is needed or stored.

Translocation

• Translocation this movement is the transport of sucrose and amino acids.

Section 15 of 25More from the specification

1Photosynthesis balanced equation

• Photosynthesis is the process by which green plants synthesize glucose and oxygen from carbon dioxide and water using light energy: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

2Investigating limiting factors

• When investigating the effect of changing light intensity, carbon dioxide concentration,
• or temperature on the rate of photosynthesis, one factor is varied while others are kept constant.

3Photosynthetic rate graph interpretation

• On a rate-intensity graph, the initial linear slope indicates that light intensity is the limiting factor; once the curve plateaus horizontally, another factor has become limiting.
  • (e.g. temperature or CO₂)

4Storage of glucose as starch

• Excess glucose produced by leaf mesophyll cells is converted into insoluble starch for storage, preventing osmotic disruption to plant cells.

5Photosynthesis balanced equation

• Photosynthesis is the process by which green plants synthesize glucose and oxygen from carbon dioxide and water using light energy: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

6Investigating limiting factors

• When investigating the effect of changing light intensity, carbon dioxide concentration,
• or temperature on the rate of photosynthesis, one factor is varied while others are kept constant.

7Photosynthetic rate graph interpretation

• On a rate-intensity graph, the initial linear slope indicates that light intensity is the limiting factor; once the curve plateaus horizontally, another factor has become limiting.
  • (e.g. temperature or CO₂)

8Storage of glucose as starch

• Excess glucose produced by leaf mesophyll cells is converted into insoluble starch for storage, preventing osmotic disruption to plant cells.

9Indicator

• Substance that changes colour in acids and alkalis.

10Light intensity

• The amount of light.

11Limiting factor

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

12Temperature

• Average energy of the particles in an object.

13Glucose

• Sugar that is the fuel for respiration.

14Starch

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

15Concentration

• Amount of solute dissolved in a volume of solution.

16Mineral

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

Section 16 of 25Stomata

Stomata

• Stomata small pores through which carbon dioxide enters and oxygen and water vapour leave.

Opening

• Opening stomata are usually open in the day and closed at night, which matches when photosynthesis occurs.

Transpiration

• Transpiration the stomata are the main site of water loss from the leaf.

Section 17 of 25Guard cells

Shape change

• Shape change a guard cell changes shape as it absorbs or loses water.

Opening

• Opening when the guard cells take in water they become turgid and bend apart, so the stoma opens.

Closing

• Closing when they lose water they become flaccid and the stoma closes.

Control

• Control this controls diffusion of carbon dioxide and oxygen and helps to regulate water loss.

Section 18 of 25Veins in the leaf

Vascular bundles

• Vascular bundles they contain xylem and phloem and form the veins that reach every part of the leaf.

Xylem

• Xylem carries water and mineral ions up to the mesophyll cells for photosynthesis and to replace water lost by transpiration.

Phloem

• Phloem carries sucrose and amino acids away from the leaf, in translocation.

Section 19 of 25Xylem and phloem in the leaf

Tissue	What is moved	Direction	Cells
Xylem	Water and mineral ions	Into the leaf	Dead
Phloem	Sucrose and amino acids	Away from the leaf	Living

Section 20 of 25Water vapour loss

Evaporation

• Evaporation water evaporates from the surfaces of the spongy mesophyll cells into the air spaces.

Diffusion

• Diffusion the vapour then diffuses out through the open stomata.

Large surface area

• Large surface area the many interconnected air spaces and the stomata give a large area, so evaporation is rapid when stomata are open.

Section 21 of 25Gas exchange at different times

Time	Processes	Net gas exchange
Bright light	Photosynthesis faster than respiration	Carbon dioxide in, oxygen out
Night	Respiration only	Oxygen in, carbon dioxide out

Section 22 of 25Why leaves control water loss

Cuticle

• Cuticle the waxy layer reduces evaporation from the surface.

Stomata closing

• Stomata closing they close at night, or when the plant is short of water, to save water.

Section 23 of 25More from the specification

More from the specification

Photosynthesis Equation and Light Energy Absorption

• In green plants, light energy absorbed by chlorophyll pigments in chloroplasts drives the endothermic reaction converting carbon dioxide and water into glucose and oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

Limiting Factors in Photosynthesis

• Environmental factors that govern the rate of photosynthesis include light intensity, carbon dioxide concentration,
• and ambient temperature;
• when one factor is at its least favorable value, it becomes the limiting factor.

Storage of Carbohydrates as Starch

• Glucose produced in leaf mesophyll cells is rapidly converted and stored as insoluble starch grains to prevent osmotic swelling of plant cells.

Investigating Photosynthesis in Aquatic Plants

• When investigating the rate of photosynthesis using aquatic plants like Elodea, counting oxygen bubbles released per minute at varying distances from a light source illustrates how light intensity affects photosynthetic rate.

Photosynthesis Equation and Light Energy Absorption

• In green plants, light energy absorbed by chlorophyll pigments in chloroplasts drives the endothermic reaction converting carbon dioxide and water into glucose and oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

Limiting Factors in Photosynthesis

• Environmental factors that govern the rate of photosynthesis include light intensity, carbon dioxide concentration,
• and ambient temperature;
• when one factor is at its least favorable value, it becomes the limiting factor.

Storage of Carbohydrates as Starch

• Glucose produced in leaf mesophyll cells is rapidly converted and stored as insoluble starch grains to prevent osmotic swelling of plant cells.

Investigating Photosynthesis in Aquatic Plants

• When investigating the rate of photosynthesis using aquatic plants like Elodea, counting oxygen bubbles released per minute at varying distances from a light source illustrates how light intensity affects photosynthetic rate.

Concentration

• Amount of solute dissolved in a volume of solution.

Chloroplast

• A structure with chlorophyll where photosynthesis happens.

Indicator

• Substance that changes colour in acids and alkalis.

Light intensity

• The amount of light.

Limiting factor

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

Palisade mesophyll

• Column-shaped cells with many chloroplasts that carry out most photosynthesis.

Chlorophyll

• Green pigment that absorbs energy from sunlight.

Temperature

• Average energy of the particles in an object.

Glucose

• Sugar that is the fuel for respiration.

Starch

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

Carbohydrate

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

Section 24 of 25Key terms

Term	Meaning
Cuticle	A waterproof layer of wax on the leaf surface.
Epidermis	The thin outer layer of cells of a leaf.
Palisade mesophyll	Column-shaped cells with many chloroplasts that carry out most photosynthesis.
Spongy mesophyll	Loosely packed cells with air spaces for gas exchange.
Stoma	A pore in the leaf surface for gas exchange.
Guard cell	A cell that opens and closes a stoma.
Vascular bundle	A group of xylem and phloem.

Term	Meaning
Adaptation	A feature that suits an organism or structure to its function.
Surface area	The total area of a surface exposed to the environment.
Diffusion distance	The distance molecules travel when diffusing.
Translocation	Transport of sucrose and amino acids in the phloem.
Air space	A gap between cells that lets gas diffuse.
Transparent	Letting light pass through.

Term	Meaning
Stoma	A pore in the leaf for gas exchange.
Guard cell	One of two cells that open and close a stoma.
Turgid	Full of water and firm.
Transpiration	Loss of water vapour from the leaf.
Translocation	Transport of sucrose and amino acids in the phloem.
Vein	A vascular bundle visible in the leaf.

Section 25 of 25Exam technique

1

Saying the palisade layer is at the bottom — it is below the upper epidermis.

2

Saying the upper epidermis has chloroplasts — it is transparent so light reaches the palisade cells.

3

Confusing xylem and phloem — xylem carries water and phloem carries sucrose and amino acids.

4

Saying the cuticle is part of the cell — it is a waxy layer on the surface.

1

Saying the leaf is thick for photosynthesis — it is thin for a short diffusion path.

2

Giving only a feature without its function — link each feature to what it does.

3

Saying stomata are for water only — they let carbon dioxide in and oxygen out.

4

Saying the palisade layer is at the bottom — it is at the top where the light is strongest.

1

Saying guard cells open when flaccid — they open when turgid.

2

Saying stomata are only for water loss — they also let carbon dioxide in for photosynthesis.

3

Thinking phloem carries water up — xylem carries water and phloem carries sucrose and amino acids.

4

Saying plants respire only at night — they respire all the time.