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Explain why large molecules need to be broken down into small molecules in the
digestive system.

Molecules need to be soluble in order to be absorbed pass the villi wall and into the blood. However, large molecules are insoluble. Thus, we have to break down these large molecules into smaller soluble molecules , so they can be transported pass the villi wall and abosrbed into the bloodstream.


Explain why a leaf is described as an organ, not a tissue.

A leaf is composed of more than one type of cell, therefore is an organ (tissues only contain one type of cell.) For example, epidermal cells, spongey mesophyll cells etc. A leaf also performs various functions (e.g. transpiration, photosynthesis, protein synthesis) while a tissue performs only a specific function.


With reference to the water potential gradient, explain why plants may die when
grown in salty soil.

Salty soil has a lower water potential compared to the cells in the plant. This difference in water potential sets up a water potential gradient, thus water osmoses out of the plant into the soil. Consequently, the plant cells will lose their turgor and become flaccid. Thus the plant wilts and most likely dies due to lack of water.


State two reasons why the source and sink for translocation in a plant may change at different stages in the growth of a plant.

During germination, the cotyledons is the source and the growing leaves are the sinks since they cannot photosynthesise yet. However when they start to photosynthesise, they become the sources and the rest of the plant becomes the sink.
Also in decidious trees in autumn, they shed their leaves (which used to be the sources), now the storage organs in the roots become the sources and the branches and the stem become the sink.


Describe how the stigmas of wind-pollinated flowers differ from the stigmas of insect pollinated

Stigmas of wind pollenated flowers are usually fearthery, and stick out from the flower (exposed) to increase the liklyhood of 'catching' the pollen.
Stigmas of insect pollentated flowers usually are smaller, sticky and placed inside the flower in stratiegic locations so the insect will 'brush' the pollen onto the stigma.


Discuss the implications to a species of self-pollination.

Disadv: Almost no genetic variation and weaknesses or diseases of the parent will be directly inherited by the offspring, therefore if there is a sudden change in enviroment all the offspring will be affected (less chance to adapt to changing conditions.)
adv: Dreater chance of pollenation, less wastage of pollen.


Describe how bacteria are used to produce enzymes for biological washing powders.

bacteria is cultured in a container where they are provided with nutrients for respiration, optimum temperature and pH. The contents is stirrued so the mixes the bacteria with the nutrients and prevents settling. The bacteria secrete enzymes which are collected from the mixture through filtration and cruching the mixture.


Explain how enzymes in biological washing powders act to remove food and blood
stains from clothes.

The enzymes contain carbohydrases and proteases that turn the complex insoluble food molecules and blood protiens into soluble small molecules (amino acids, sugars, fatty acids and glycerol.) that can be washed away be water.


What is a Limiting factor?

A limiting factor is an external factor of a process, such as photosynthesis, that is in the shortest supply and will start torestrict the rate of reaction. Only one factor can limit the rate of reaction at a single moment in time, it depends on which one is in shortest supply.


How does light intensity affect photosynthesis?

As light intensity increases so does rate of photosynthesis, but only up to a certain extent. Beyond this point, any increase in L.I will have little effect on the rate of photosynthesis due to another factor becoming the limiting factor.


how does carbon dioxide concentration affect photosynthesis?

As CO₂ concentration increases so does rate of photosynthesis. because the higher the CO₂ concentration outside the leaf the bigger the the concentration gradient between the inside of the leaf and the atmosphere, thus rate of photosynthesis will increase. However, eventually, the rate will level out due to another factor becoming the limiting factor.


After glucose is made, what is it stored as? Why is it stored is this substance?

After glucose is produced, it is converted into starch: a polysaccharide made from joining hundreds of glucose monomers together.
It is stored as starch due to its relative insolubility. If stored as glucose (which is soluble in water) it would change the water potential in the cells, thereby upsetting the osmotic balance between the cells and cause changes in water movement throughout the plant. However starch is osmotically inactive, thus, this would not disturb the water potential of the cell contents.
Therefore, starch can be stored in large quantities for long terms.


What are the uses of glucose in the plant?

1. respiration - the energy created is used to drive many metabolic reactions.
2. Making cellulose - which is found in all cell walls
3. Protein synthesis - glucose is combined with nitrates to form amino acids. Which are then made into protiens
4. Active transport - energy from cellular respiration is used to actively transport mineral salts -which are in very dilute solution- into the root hair cells.
5. Storage - excess carbohydrates are stored in tubersand the stem. Surcrose and fructose is stored in fruits.


Why do you think that palisade mesophyll cells are located on the upper side of the leaf and contain many chloroplasts?

The palisade mesophyll layer is located on the upper surface of the leaf so they can recieve light at a higher light intensity -due to the light rays not being absorbed by as many cell walls.- Thus rate of photosynthesis is increased.
The palisade mesophyll conatins many chloroplasts so it can maximise the amount of light it absorbs, thereby increasing the rate of photosynthesis.


Why are waxy cuticles waxy?

To help water loss by evaporation on the surface of the leaves.


Explain the function of phloem tubes.

Is to transport surcrose, amino acids, waterand other nutrients from the sources -where the material is synthesised- to the sinks -where the material is in need.-


Explain the function of Xylem vessels.

To transport water and mineral salts from the roots, up the stem and into the leaves where its used for photosynthesis and transpiration.


Describe the structure of xylem vessels.

Xylem vessels are composed of dead cells. The end cell walls of these dead cells have been removed to form a fine, continuous, tube.
The walls of the xylem vessels are thickened and impregnated with a substance called ligin. Ligin is a very strong, stiff and waterproof material, it provides the plant with support; keeps the stem upright and prevents water leaking out of the xylem vessels.
Xylem vessels only transport water and minerals UP.


Describe the structure of phloem tubes.

Phloem tubes consist of living cells called seive tube members with perforated seive plates at the end of each cell to allow surcrose and amino acids dissolved in water to flow through.
Seive tubes transport materials in BOTH DIRECTIONS.


What are the benefits of transpiration?

1. To obtain minerals from the soil - A rapid flow of water into the roots may be needed to obtain sufficient mineral salts which are in very dilute solution.
2. It cools the plant - this is important as leaves exposed to direct sunlight will absorb much of the I.R radiation and the temperature level may rise to a level which could denature many enzymes and 'kill' the cytoplasm of the cells.
3. Water for photosynthesis- self explanatory


What is special about the structure of root hair cells?
How does it help their function?

Root hair cells have a thin, elongated cytoplasmic extensions increase the total surface area by which water and mineral salts can be absorbed through.


Describe the mechanism for the uptake of water.

Water is absorbed into the roots by osmosis. The water in the soil has a higher water potential than the cell sap in the root hair cells; this creates a water potential gradient and water passes into the root hair cells. This raises the turgor pressure and the water potential of the root hair cells. Thus, the water is forced out of the cells and into the cortex cells behind it by osmosis due to the higher water potential of the root hair cells. Continuing this way, an osmotic gradient is formed across the cortex to the xylem where the water is 'sucked' up into the transpiration stream.


How does water move up the stem?

Water in the leaf evaporates and diffuses out of the leaf. this creates a low concentration of water in the leaves. This shortage of water draws water from the xylem to replace the water lost in the leaves. Due to the strong cohesion between water molecules, water molecules further below are 'pulled' up to replace the ones lost into the leaf. Water from the roots move into the stem to replace the ones lost up the stem. This transpiration 'pull' is how water moves up the stem.


What causes stomata to open and close?

During the day, the guard cells photosynthesize producing glucose. This creates a low water potential in the cell sap of the guard cells. Due to the water potential gradient, water passes into the guard cells forcing the vacuole to exert turgor pressure onto the cell wall, thus the guard cells turn turgid and the stomata opens allowing gas exchange to occur.
At night, the guard cells don't photosynthesize, instead their rate of cellular respiration increases, using up the glucose and increasing the water potential of the guard cells. Thus, water moves out of the guard cells due to the water potential gradient, turning the guard cells flaccid, thereby closing the stomatal pore.
The guard cells control the amount of water leaving the plant.


How do stomata control the loss of water?

When supplies of water are plentiful, the whole plant becomes turgid. When the guard cells become turgid they open the stomata. when the uptake of water is serverly reduced, the plant tissues become flaccid. When the guard cells become flaccid they close the stomata, automatically preventing further water loss.
Everything works perfectly!!!!


What is wilting?

When transpiration takes place faster than water is being taken into the plant by the roots. Consequently, the plant loses its turgor and 'limps', the stem becomes unsupported and drops, the leaves shrivel, and the stomata closes.


What is the function of nitrates?
Describe the deficiency symptoms.

Nitrate ions are essential for protein synthesis, as they are combined with carbohydrates to form amino acids.
A plant with a lack of nitrates will have stunted growth and yellow older leaves.


What are the functions of phosphates?
Describe the deficiency symptoms.

Used for DNA and plays an important role in many chemical reactions
A plant with a lack of phosphates will have poor root growth and purple younger leaves.


what are the functions of potassium?
Describe the deficiency symptoms.

Helps the enzymes involved in photosynthesis and respiration to work.
A plant with a lack of phosphates will have yellow leaves with dead spots and stunted growth.


What are the functions of magnesium?
Describe the deficiency symptoms.

Is a crucial element for chlorophyll synthesis
A plant with a lack of magnesium will have stunted growth and pale yellow leaves.