edexcel biology unit 6

What are photosynthetic organisms?

Photosynthetic organisms as the main producers of food and therefore biomass

Describe photosynthesis in plants and algae

Endothermic reaction that uses light energy to react carbon dioxide and water to produce glucose and oxygen

Explain the effect of temperature as a limiting factor on the rate of photosynthesis

Temperature affects enzyme activity in photosynthesis; increasing temperature increases kinetic energy and collision rate, so rate rises to an optimum, beyond optimum, enzymes denature (shape changes, active site destroyed), rate falls sharply to zero

Explain the effect of light intensity as a limiting factor on the rate of photosynthesis  (1)

Light provides energy for the light-dependent stage; increasing light intensity increases the rate of photosynthesis proportionally at low levels, rate plateaus when another factor (e.g., CO₂ or temperature) becomes limiting

Explain the effect of carbon dioxide concentration as a limiting factor on the rate of photosynthesis

CO₂ is a raw material for the light-independent stage; increasing CO₂ concentration increases the rate as more substrate available for enzymes; rate plateaus when another factor limits it

What are the direct and inverse proportions of the rate of photosynthesis?

Rate of photosynthesis is directly proportional to light intensity and inversely proportional to the distance from a light source

What is the inverse square law relationship in photosynthesis?

1/distance^2

What are the adaptations of root hair cells?

elongated projections, thin cell wall and membrane, large permanent vacuole and mitochondria

Explain how the structure of root hair cells is adapted to absorb water and minerals ions: root hair cells

elongated projections (root hairs) massively increasing surface area for osmosis and diffusion/active transport (ions)

Explain how the structure of root hair cells is adapted to absorb water and minerals ions: thin cell wall and membrane

thin cell wall and membrane for short diffusion distance

Explain how the structure of root hair cells is adapted to absorb water and minerals ions: large permanent vacuole

Large permanent vacuole speeds water movement

Explain how the structure of root hair cells is adapted to absorb water and minerals ions: mitochondria

many mitochondria provide ATP for active transport of mineral ions against concentration gradient from soil

what are the structures of xylem cells?

lignified dead cells, lignin deposited in walls, hollow continuous tubes

lignified dead cells

transport water and minerals through the plant, cells die and lose contents upon lignification

lignin deposited in walls

Lignin deposited in walls strengthens against collapse under tension

hollow continuous tubes

allow uninterrupted upward flow of water/minerals by transpiration pull

what are the structures of phloem cells?

sieve tubes, sieve plates, companion cells

sieve tubes

form continuous, vertical tubes running up and down the plant, enabling the bidirectional movement of sap, known as translocation.

sieve plates

pores that connect the cytoplasm of adjacent cells, allowing rapid, unobstructed flow of sucrose and amino acids.

companion cells

provide ATP for active unloading of sucrose via bidirectional mass flow from source to sink

explain how water and mineral ions are transported through the plant by transpiration

water enters roots by osmosis and minerals by active transport to move up xylem as continuous column, water then evaporates from mesophyll cells into air spaces and diffuses out via stomata as vapour, creating tension pulling water up; root hair cells absorb water by osmosis and mineral ions via active transport from the soil, the transpiration stream moves water and dissolved minerals upward through dead, hollow xylem vessels

describe how sucrose is transported around the plant by translocation

sucrose produced in leaves and loaded into phloem sieve tubes, a high sucrose concentration lowers water potential so water enters by osmosis creating high water pressure, mass flows of sap and sinks where sucrose unloaded creating pressure gradient which drives flow

What are the structures of a leaf?

Broad/flat shape, thin cross section, palisade mesophyll, spongy mesophyll, stomata

structure of a leaf: shape

Large surface area for light absorption

structure of a leaf: cross section

Short diffusion distance for gases

structure of a leaf: palisade mesophyll

Tall cells packed with chloroplasts near upper surface for max light capture

structure of a leaf: spongy mesophyll

Irregular cells with large air spaces for gas diffusion

Explain how the structure of a leaf is adapted for photosynthesis and gas exchange: stomata

stomata mainly on lower surface for gas exchange while minimising water loss

What are the environmental factors that affect the rate of water uptake?

Light intensity, air movement and temperature

Explain the effect of environmental factors on the rate of water uptake by a plant: light intensity

Higher light intensity increases photosynthesis rate, more stomata open for CO₂, more transpiration, faster water loss from leaves, faster water uptake from roots (to replace lost water via transpiration pull)

Explain the effect of environmental factors on the rate of water uptake by a plant: air movement

removes water from leaf surfaces, leading to more water uptake from roots to maintain the balance

Explain the effect of environmental factors on the rate of water uptake by a plant: temperature

higher temperature increases evaporation rate from mesophyll and enzyme activity also increases diffusion rate causing overall faster water loss and increased uptake (until extreme heat closes stomata or denatures enzymes)

How are plants adapted to survive in extreme environments?

Leaf size and shape, the cuticle, stomata

Explain how plants are adapted to survive in extreme environments: leaf size and shape

needle-shaped leaves or spines minimise surface area, which reduces transpiration, some plants have no leaves and photosynthesise in stems

Explain how plants are adapted to survive in extreme environments: the cuticle

thick waxy cuticle on stems reduces evaporation through epidermis while allowing light through for photosynthesis

Explain how plants are adapted to survive in extreme environments: stomata

stomata close during the hottest part of the day to prevent excessive water loss, even though this limits photosynthesis

Explain how plant hormones control and coordinate plant growth and development,

plants hormones are chemical messengers produced in meristems and diffuse to target cells to control responses, promoting cell elongation in shoots and inhibit in roots

What is phototropism?

Response to light

Explain how plant hormones control and coordinate plant growth and development (phototropisms)

Auxin produced in shoot tip redistributes to shaded side causing more elongation on shaded side, shoot bends towards light

What is gravitropism?

Response to gravity

Explain how plant hormones control and coordinate plant growth and development (negative gravitropism)

In shoots, auxin accumulates lower side which promotes elongation and shoot bends upwards

Explain how plant hormones control and coordinate plant growth and development (positive gravitropism)

in roots, auxin accumulates lower side,  inhibits elongation,  root bends downwards

what are the commercial uses of auxins

weedkillers and rooting powders

Describe the commercial uses of auxins in weedkillers

Synthetic high concentrations cause uncontrolled rapid growth in broad-leaved weeds which outgrow resources/die;

Describe the commercial uses of auxins in rooting powders

Applied to cuttings to stimulate root formation/growth at cut end causing faster establishment of new plants

what are the commercial uses of gibberelins

germination, fruit and flower formation and the production of seedless fruit

Describe the commercial uses of gibberellins in germination

Gibberellins break seed dormancy,  promote germination and stimulate enzyme production to break down starch reserves into sugars for embryo growth

Describe the commercial uses of gibberellins in fruit and flower
formation

Gibberellins increase fruit size and flower production applied to promote elongation and development

Describe the commercial use of in gibberellins in the production of seedless fruit

Gibberellins induce fruit development without fertilisation and seedless varieties for better eating quality/market value

Describe the commercial use of ethene in fruit ripening

used to manage the timing of ripening, allowing fruit to be transported while firm and unripe, then ripened just before sale