What you'll learn
This revision guide covers the structures of flowering plants and how these are adapted to carry out essential life processes. You'll learn how roots, stems, and leaves are organised and adapted for photosynthesis, water and mineral uptake, and transport. Understanding these adaptations is crucial for answering application questions where you analyse unfamiliar plant structures.
Key terms and definitions
Photosynthesis — the process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen
Chloroplast — the organelle containing chlorophyll where photosynthesis occurs
Stomata (singular: stoma) — pores in the leaf surface that allow gas exchange; each pore is surrounded by two guard cells
Xylem — vascular tissue that transports water and mineral ions from roots to leaves
Phloem — vascular tissue that transports dissolved sugars from leaves to other parts of the plant
Transpiration — the loss of water vapour from plant leaves through stomata
Guard cells — specialised cells that surround stomata and control their opening and closing
Root hair cells — specialised cells on root surfaces with extensions that increase surface area for water and mineral absorption
Core concepts
Structure of the leaf
The leaf is the main photosynthetic organ in most plants. Its structure is specifically adapted to maximise photosynthesis while minimising water loss.
Waxy cuticle
- Transparent, waterproof layer covering the upper epidermis
- Prevents excessive water loss by evaporation
- Transparent to allow light penetration to photosynthetic tissues
Upper epidermis
- Single layer of cells with no chloroplasts
- Transparent to allow maximum light transmission
- Covered by the waxy cuticle
Palisade mesophyll layer
- Located beneath the upper epidermis
- Contains densely packed cells with numerous chloroplasts
- Chloroplasts can move within cells to maximise light absorption
- Main site of photosynthesis due to high chloroplast concentration
- Cells are cylindrical and arranged in columns to increase light capture
Spongy mesophyll layer
- Located beneath the palisade layer
- Contains irregularly shaped cells with air spaces between them
- Fewer chloroplasts than palisade cells
- Air spaces allow efficient gas exchange
- Large internal surface area for carbon dioxide absorption from air spaces
Lower epidermis
- Contains numerous stomata (more than upper epidermis in most plants)
- Guard cells surrounding each stoma control opening and closing
- Fewer stomata on upper surface reduces water loss in direct sunlight
Vascular bundles (veins)
- Contain xylem vessels and phloem tubes
- Xylem transports water and minerals to photosynthesising cells
- Phloem transports sugars (sucrose) away from the leaf
- Network of veins ensures all cells are close to transport tissues
Adaptations of the leaf for photosynthesis
The leaf structure maximises the rate of photosynthesis through several key adaptations:
Large surface area
- Broad, flat shape captures maximum sunlight
- Thin structure ensures short diffusion distances for gases
- Carbon dioxide can reach all mesophyll cells quickly
Arrangement of tissues
- Palisade cells positioned near upper surface where light intensity is highest
- Transparent upper epidermis and cuticle allow light penetration
- Chloroplasts arranged to absorb maximum light
Gas exchange system
- Stomata allow carbon dioxide to diffuse into the leaf
- Spongy mesophyll air spaces provide large surface area for gas exchange
- Oxygen produced during photosynthesis diffuses out through stomata
Water and mineral supply
- Extensive network of xylem vessels delivers water for photosynthesis
- Water also maintains turgidity, keeping the leaf flat and exposed to light
- Minerals such as magnesium (needed for chlorophyll) and nitrates (for protein synthesis) are transported via xylem
Stomata and gas exchange
Stomata are essential for gas exchange but also represent the main route of water loss from plants.
Location and distribution
- Most abundant on the lower epidermis (shaded from direct sunlight)
- Fewer on upper epidermis (some plants, especially aquatic ones, have none on upper surface)
- Distribution varies between plant species based on habitat
Guard cell structure and function
- Kidney-shaped cells surrounding each stoma
- Contain chloroplasts (unlike other epidermal cells)
- Cell wall is thicker on the inner side facing the stoma
- Changes in turgidity cause stomata to open or close
Opening mechanism
- Light triggers active transport of ions into guard cells
- Water follows by osmosis, increasing turgor pressure
- Thicker inner wall causes guard cells to bend, opening the stoma
- Allows carbon dioxide entry and oxygen exit during photosynthesis
Closing mechanism
- In darkness or water stress, guard cells lose water by osmosis
- Guard cells become flaccid
- Stoma closes, reducing water loss through transpiration
- Essential for plant survival during drought or at night
Root structure and function
Roots anchor the plant and absorb water and mineral ions from the soil.
Root hair cells
- Specialised epidermal cells with long extensions (root hairs)
- Extensions are typically 1-2 mm long
- Massively increase surface area for absorption
- Short lifespan (few days) but constantly replaced
- No chloroplasts as they are underground
Water absorption mechanism
- Soil water has higher water potential than root hair cell cytoplasm
- Water moves into root hair cells by osmosis
- Water then moves across the root to xylem vessels
- Transported up the plant in the transpiration stream
Mineral ion absorption
- Mineral ions (e.g., nitrates, magnesium, phosphates) are often at lower concentration in soil than in root cells
- Active transport moves ions against the concentration gradient
- Requires energy from respiration
- Root hair cells contain many mitochondria to provide ATP for active transport
Root structure
- Outer layer (epidermis) includes root hair cells
- Cortex stores starch and allows water passage
- Central vascular cylinder contains xylem and phloem
- Root cap protects growing tip as it pushes through soil
Stem structure and function
The stem supports the plant and connects roots to leaves, containing vascular tissues for transport.
Support function
- Holds leaves in position to maximise light absorption
- Supports flowers for pollination
- Contains strengthening tissues (lignified xylem, sometimes collenchyma or sclerenchyma)
Transport tissues arrangement
- Vascular bundles arranged in a ring (dicot stems) or scattered (monocot stems)
- Xylem located towards the inside of each vascular bundle
- Phloem located towards the outside
- Cambium (in some plants) between xylem and phloem allows growth
Xylem vessels
- Dead cells with no cytoplasm when functional
- Cell walls strengthened with lignin
- End walls broken down to form continuous tubes
- Lignin in spiral or ring patterns provides strength while allowing flexibility
- Transport water and dissolved mineral ions upwards
Phloem tubes
- Made of sieve tube elements joined end-to-end
- Sieve plates between cells have pores allowing flow
- Living cells but no nucleus
- Each sieve tube element has companion cell providing metabolic support
- Companion cells have many mitochondria for active processes
- Transport sugars (mainly sucrose) and amino acids in solution (sap)
- Movement called translocation — can occur in any direction
Transpiration and water movement
Transpiration is the loss of water vapour from leaves, creating a pull that draws water up through xylem vessels.
The transpiration stream
- Water evaporates from mesophyll cells into air spaces
- Water vapour diffuses out through stomata
- Creates a water deficit in mesophyll cells
- Water drawn from xylem in veins by osmosis
- Cohesion between water molecules creates continuous column in xylem
- Water pulled up from roots to replace water lost from leaves
- Continuous flow of water from roots through stem to leaves
Factors affecting transpiration rate
Light intensity
- Increased light causes stomata to open
- More stomata open increases water loss
- Transpiration rate increases
Temperature
- Higher temperature increases kinetic energy of water molecules
- Faster evaporation and diffusion
- Transpiration rate increases
Humidity
- High humidity reduces concentration gradient between leaf and air
- Slower diffusion of water vapour out of leaf
- Transpiration rate decreases
Air movement (wind)
- Wind removes water vapour from around stomata
- Maintains steep concentration gradient
- Transpiration rate increases
Plant mineral requirements
Plants require various mineral ions for healthy growth, absorbed from soil through roots.
Nitrate ions (NO₃⁻)
- Required for amino acid and protein synthesis
- Needed for DNA synthesis
- Essential for making chlorophyll
- Deficiency symptoms: stunted growth, older leaves turn yellow (chlorosis)
Magnesium ions (Mg²⁺)
- Required component of chlorophyll molecules
- Essential for photosynthesis
- Deficiency symptoms: chlorosis (yellowing) of leaves, starting with older leaves
Phosphate ions (PO₄³⁻)
- Required for DNA synthesis and ATP production
- Important for cell membranes (phospholipids)
- Needed for healthy root growth
- Deficiency symptoms: poor root growth, purple-tinged leaves
Potassium ions (K⁺)
- Required for enzyme activation
- Necessary for stomatal opening and closing
- Deficiency symptoms: yellow/brown leaf edges, poor flower and fruit development
Worked examples
Example 1: Leaf adaptation (4 marks)
Question: Explain how the structure of palisade mesophyll cells is adapted for photosynthesis.
Mark scheme answer:
- Palisade cells contain many chloroplasts (1 mark)
- Chloroplasts contain chlorophyll which absorbs light (energy) (1 mark)
- Cells are positioned near the top/upper surface of the leaf (1 mark)
- Where light intensity is highest / they receive most light (1 mark)
Examiner note: Use precise language — "many" or "numerous" rather than "lots of". Link structure to function clearly.
Example 2: Transpiration calculation (3 marks)
Question: A plant loses 250 cm³ of water in 5 hours through transpiration. Calculate the rate of water loss per hour. Show your working.
Mark scheme answer:
- Rate = volume ÷ time (1 mark)
- Rate = 250 ÷ 5 (1 mark)
- Rate = 50 cm³ per hour (1 mark)
Examiner note: Always show your working. Include units in your final answer.
Example 3: Root hair cells (6 marks)
Question: Root hair cells are adapted for absorbing water and mineral ions from soil. Describe and explain these adaptations.
Mark scheme answer:
- Long extensions/root hairs increase surface area (1 mark)
- For absorption of water and minerals (1 mark)
- Thin cell wall/membrane for short diffusion distance (1 mark)
- Large vacuole maintains water potential gradient (1 mark)
- Many mitochondria present (1 mark)
- To provide energy/ATP for active transport of mineral ions (1 mark)
Examiner note: Questions asking you to "describe and explain" require both structural features AND their functional significance.
Common mistakes and how to avoid them
Confusing xylem and phloem — Remember: xylem transports water UP (both have 'u'), phloem transports food/sugars. Xylem cells are dead; phloem cells are alive.
Thinking all plant cells photosynthesise — Only cells containing chloroplasts can photosynthesise. Root hair cells, guard cells in some definitions, and many internal cells lack chloroplasts.
Confusing diffusion and osmosis in water uptake — Water moves by osmosis (special case of diffusion for water only), while mineral ions move by active transport (not diffusion) when absorbed against a concentration gradient.
Stating stomata are only on the lower surface — Most leaves have MORE stomata on the lower epidermis, but many plants have some on the upper surface too. Floating aquatic plants may have stomata only on the upper surface.
Forgetting units in calculations — Always include appropriate units (cm³/hour, g/minute) in your final answer, especially for transpiration rate questions.
Vague explanations of adaptations — Don't just list features. Explain HOW each structural adaptation helps the function. Use "which allows..." or "this enables..." to link structure to function.
Exam technique for "Plant Structures and their Functions"
Command word awareness — "Describe" requires you to state features; "Explain" requires reasons/mechanisms; "Suggest" means apply your knowledge to unfamiliar contexts. A 4-mark "explain" question needs four separate points, each linking cause and effect.
Use annotated diagrams strategically — If a question is worth 4+ marks and asks about structure, a labelled diagram with annotations can secure marks quickly. Ensure labels have leader lines touching the correct structures.
Link structure to function explicitly — Never assume the examiner will make the connection. Always write "which allows/enables..." to show how a structural feature supports a particular function.
For transpiration questions — Remember that factors can increase OR decrease the rate. Think about concentration gradients and how each factor affects the rate of evaporation or diffusion.
Quick revision summary
Plant leaves are adapted for photosynthesis with palisade cells containing numerous chloroplasts near the upper surface. Stomata in the epidermis allow gas exchange, controlled by guard cells. Root hair cells have extensions increasing surface area for water absorption by osmosis and mineral uptake by active transport. Xylem transports water upwards; phloem translocates sugars in any direction. Transpiration is water loss through stomata, affected by light, temperature, humidity, and wind. Plants require nitrates for protein synthesis, magnesium for chlorophyll, and other minerals for healthy growth.