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- CO2 from intercellular air space dissolves in water film
- dissolved CO2 diffuses across cell wall & plasma membrane into cell contents

USING UP CO2 DURING PHOTOSYNTHESIS:
- co2 is absorbed from air into surrounding air spaces via stomata
- the concentration of co2 is lower in the outside air than in the air spaces (thus, a concentration gradient exists)
- co2 diffuses down the gradient throughout the air spaces

GIVING OUT O2 DURING PHOTOSYNTHESIS:
- o2 is given out into the outside air from the surrounding air spaces
- there is now a higher concentration of o2 in the outside air than in the surrounding air spaces (concentration gradient exists)
- o2 diffuses down gradient, out of plant through stomata

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1) ALLOW MAXIMUM ABSORPTION OF SUNLIGHT
- leaves are arranged so that there is no overlap between them, so they don't block each other from receiving sunlight
- palisade cells have the largest number of chloroplasts
- palisade cells are arranged in column to pack more cells into the layer
- transparent upper epidermis allows light to penetrate into layers beneath
- spongy mesophyll cells also contain chloroplasts to capture light energy that passes through the mesophyll layer

2) ALLOW MAXIMUM ABSORPTION OF CO2 AND O2
- leaf is held out from the stem and is completely exposed to surrounding air
- numerous stomata on lower epidermis to facilitate uptake of CO2 and giving out of O2
- extensive network of intercellular air spaces in mesophyll layer, especially in the spongy mesophyll layer, to facilitate rapid diffusion of gases
- lamina is very thin so that the diffusion distance is very short

3) FOR CELLS TO HAVE A READY SUPPLY OF WATER & MINIMISE WATER LOSS
- extensive network of veins containing xylem vessels so that every mesophyll cell is close to a water supply
- presence of cuticle on upper epidermis to reduce water evaporation from the upper surface
- most stomata are found on the lower epidermis to reduce escape of water vapour
- guard cells are able to close/open stomata which helps to reduce water vapour evaporation from the air spaces

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Guard cells:
- pair of bean-shaped cells flanking a stoma
- contains chloroplasts
- helps to open/close stoma

Stoma:
- controls water loss and entry/exit of gases in and out of plant
- on underside of leaf to prevent excessive water loss

*on a hot day, guard cells become turgid so the stoma is closed

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- connects to external via stoma
- allows air to diffuse to cells inside leaf
- pore present on lower epidermis
- allows gaseous exchange to take place between inside of leaf and external environment

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XYLEM:
- transports water and mineral salts from roots to leaf

PHLOEM:
- transports organic nutrients produced from leaf to other parts of plant

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Palisade mesophyll:
- 1 or 2 layers of tightly packed cells
- found below upper epidermis
- cells are rectangular in shape
- cells contain large number of chloroplasts

Spongy mesophyll:
- found below palisade layer
- cells are rounder and smaller
- less tightly packed with more and larger air spaces between them
- cells also contain chloroplasts

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Upper epidermis:
- layer of very tightly packed cells
- cells do not contain chloroplasts
- forms a protective layer around cells inside leaf

Wax cuticle:
- waxy substance secreted by upper epidermal cells
- present as a layer over upper epidermis
- helps to reduce water evaporation from leaf surface

**epidermal cells do not contain chloroplasts because the absence makes them transparent, which allows sunlight to penetrate through the epidermis and reach into the mesophyll cells below so photosynthesis can take place

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Anatomy, cross structure & cross section

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Anatomy, cross structure & cross section

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anatomy of plant:
- stem
- leaves
- roots

cross structure of a leaf:
- lamina (leaf blade): flat layer forming bulk of leaf
- veins: contain vascular bundles that branch repeatedly to spread throughout the lamina
- petiole (leaf stalk): connects leaf to rest of plant)
- midrib: continues from petiole into the lamina

cross section of leaf:

- upper epidermis (with wax cuticle on top)
- palisade mesophyll
- spongy mesophyll
- lower epidermis (with stoma and guard cells)

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- to supplement the nitrogen content in soil (in farms)
- increase crop plants yield by farmers

HOWEVER, farmers tend to overuse such fertilizers, which leads to:
1) AQUATIC ENVIRONMENTS
- excess nitrates may be washed out of soil by rain into rivers/streams/lakes/seas = leaching
- this upsets the ecological balance in aquatic environments
- eutrophification: excessive salts leach into soil and wash into water bodies
- algae thrives and blocks water plants from receiving sunlight
- thus, the plants are unable to photosynthesis and die
- amount of oxygen in water decreases
- aquatic creatures die due to lack of oxygen
- plants decompose and release carbon dioxide, and  bacteria thus thrive

2) DRINKING WATER SYSTEM
- excess nitrates may enter groundwater and enter the drinking water system
- health risks in humans (may cause cancer)
- health risks of farm animals

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NATURAL (erosion of rocks, decomposition)
ARTIFICIAL (fertilizers)

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MAGNESIUM DEFICIENCY:
- foliage turns yellow
- older leaves turn yellow/brown prematurely

NITROGEN DEFICIENCY:
- plant is severely stunted
- few stems
- older leaves turn yellow

PHOSPHORUS DEFICIENCY:
- stem is very thin
- few stems
- older leaves are shed
- leaves are dull green; curling occurs at margin of leaf

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SULPHUR: formation of amino acids for protein synthesis

NITROGEN: formation of amino acids for protein synthesis and synthesis of DNA

PHOSPHORUS: synthesis of DNA

MAGNESIUM: formation of chlorophyll

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Glucose is used to make:

- phosphates by combining it with phosphorus
- amino acids and then proteins by combining it with nitrogen/sulfur
- starch
- nucleic acids (DNA)
- cellulose in cell walls
- sucrose
- oil (lipids)

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- The reactions in photosynthesis are catalysed by enzymes which have an optimum temperature
- As the temperature increases, the enzyme activity increases, so the rate of photosynthesis increases
- Beyond the optimum temperature, the enzymes denature and the rate of photosynthesis decreases

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Factors affecting rate of photosynthesis

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Factors affecting rate of photosynthesis

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- carbon dioxide concentration
- amount of water present
- light intensity
- temperature
- amount of chlorophyll

A factor is a limiting factor when it directly influences the rate of photosynthesis.
If a factor is increasing/decreasing and the rate of photosynthesis remains unchanged, then the factor is no longer the limiting factor.

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It takes place in the absence of oxygen. Glucose is converted to pyrase during glycolysis. Only carbon dioxide, ethanol and a small amount of energy is produced.

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The plant uses oxygen and the sugars produced during photosynthesis to produce carbon dioxide, water and energy for plant growth. It takes place 24 hours a day, in the mitochondria of all cells

glucose + oxygen -> carbon dioxide + water + ATP

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The process of losing water vapour by evaporation through the stoma of the plant.

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The process by which green plants, in
the presence of chlorophyll (in chloroplasts), take in light
energy and produce glucose and oxygen from carbon dioxide and water. It only takes place in the daytime.

6 CO2 + 6 H2O --> C6H12O6 + 6 O2
light energy

The light energy is converted to chemical energy, which is used to:
- split water molecules into hydrogen and oxygen (HYDROLYSIS)
- combine hydrogen molecules with carbon dioxide to form glucose

Plants are autotrophs that make their own food from inorganic materials.
Animals are heterotrophs that feed on other organisms to obtain nutrients.