• Revision:GCSE Biology - Plant Biology

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The problem with the way plants are grown is that they use up all the goodness in the soil. There are several ways to avoid this

  • Changing the land every few years
  • Crop rotation
  • Fallowing
  • Adding a crop of clover or legumes, because they have nitrogen fixing bacteria in the roots which add nitrates to the soil by turning nitrogen in the air into nitrates.
  • Fertilisers can be used. Organic fertilisers (e.g. manure or compost) give out nutrients as they decay. The only problem is they take a long time, but they can be made on the farm.
  • Inorganic fertilisers such as NPK (nitrogen, phosphorus and potassium) are manufactured in special factories

A normal plant is made up of

  • A flower – attracts insects which then carry pollen and pollinate other plants to allow reproduction
  • The leaf – Produces the food (Plants do not get food from soil)
  • Root hairs – Absorb minerals (ions) and water from the ground
  • Roots – Holds the plant down and absorbs water and ions from the soil
  • Stem – Keeps the plant upright and allows food and water to be transported through the plant



Leaves are designed for making food.


  • The waxy cuticle stops it losing water and drying out
  • The epidermal cells are a protective layer (and contain no chloroplasts). They make the wax.
  • The palisade cells contain the most chloroplasts. They are long to maximize the chance of the light hitting them and although you cant see it on this diagram they have spaces to allow gas diffusion (Carbon dioxide in, oxygen out) (also called the palisade mesophyll)
  • The spongy layer contains a few chloroplasts which catch what the palisade cells miss. They also have lots of air spaces to allow air to circulate better. (Also called the spongy mesophyll)
  • The vascular bundle (also know as the leaf vein) contain the xylem and phloem (Phloem = Food). These cover all the leaf and bring water, while taking away the glucose created in photosynthesis.
  • The stomata are tiny pores which let carbon dioxide in, and oxygen and water vapour out. However, if the water supply they close automatically so the plant doesn’t dry up. The guard cells actually close the stomata. Open = Turgid --- Closed = Flaccid. While this stops carbon dioxide loss, it also stops photosynthesis.
  • The leaf is very thin which minimises the distances which carbon dioxide has to travel
  • They are also arranged in the best way for catching the maximum amount of light without blocking out the light for other leaves

Plants stay upright because of packing cells. These cells lie in the stem and are kept turgid. This keeps them firm and keeps the plant supported. If the plant should run out of water then these cells become flaccid and the plant will droop. There are also strands of cellulose which are tough and rubbery


Transpiration – The loss of water from a plant and is caused by water evaporating inside a plant.

  • This creates a slight shortage of water in the leaf which draws more water up from the root
  • This has two benefits – It draws minerals up and it cools the plant down (like sweating).

The table below shows the best and worst conditions for transpiration (I.E. best conditions means it happens the most)

Factors Best conditions Worst conditions
Light (amount of) Sunny Overcast and miserable (foggy)
Temperature Hot Cool
Air movement (amount of) Windy Still
Humidity Dry Wet

This constant stream transports vital minerals and ions from the soil into the plant. Also, it should be obvious that when the plant is in the hottest conditions the stream will be at its fullest, cooling the plant down to its fullest potential. Almost all the water is got from the root hairs.

During the night the stomata close to avoid unnecessary water loss (No photosynthesis at night)

When a plant is well watered the cells are turgid and therefore the plant stands tall and strong. However, during a drought the cells are flaccid and the plant droops because the cells have lost their turgor pressure.

Plants transport food and water around inside them using two different sets of tubing; xylem and phloem. They usually run along side each other and are called the leaf vein or vascular bundle

The phloem tubes (or sieve tubes) are made up of living cells with a filters running through, and carries food from the leaves (where it is made) to everywhere else. They carry sugars, fats, proteins and other things like that to growing regions in shot and root tips, and to/from storage organs in the roots. They need a good supply of oxygen, otherwise they will die

Xylem tubes (or vessels) are made of dead cells joined end to end. The walls of dead cells are stiff and contain lignin, which gives the plant support. They carry water and minerals from the roots to the leaves using the pressure of the transpiration stream and root pressure. Minerals are also transported with the water. In the roots, the xylem takes in water using root hairs to increase the surface area. The concentration of salt in the root hair is higher than in the soil, so water is drawn in by osmosis NB both extend down to the roots.


Features of photosynthesis include - The flat leaves of plants have a large surface area, the palisade layer means there are more chloroplasts near the top and the guard cells control the movement of gasses coming in and out of the leave.

\displaystyle \mathsf{Carbon\ dioxide} + \mathsf{water} \longrightarrow \mathsf{Glucose} + \mathsf{oxygen}

There are four things needed for photosynthesis to occur

  • Light
  • Carbon dioxide (from the air)
  • Water (from the soil)
  • Chlorophyll – This is found in chloroplasts and is what makes green plants green. It takes water, carbon dioxide and combines it with the energy from light to create glucose. Oxygen is just a by-product

The rate of photosynthesis is affected by three different variables

  • Light
    • As the amounts of light energy increase the rate of photosynthesis increases. However, this only happens to a point, and after that light is no longer a limiting factor (e.g. after that point no matter how much light there is it wont increase the rate of photosynthesis). Chlorophyll only absorbs red and blue light, but reflects the green light which is why plants are green.
  • CO2 levels
    • Same as light, it only increases the rate up to a certain point, it makes no difference. As long as there is a lot of light and carbon dioxide then temperature must be controlling the rate of photosynthesis.
  • Temperature
    • Chlorophyll contains enzymes; so obviously, if the temperature rises above 40 degrees centigrade then these enzymes begin to denature until at 45 when they are destroyed. However, usually the temperature is too low, and needs heating up a bit.

Photosynthesis is the respiration equation backwards

(Respiration: \displaystyle \mathsf{Oxygen} + \mathsf{Glucose} \longrightarrow  \mathsf{Carbon\ Dioxide} + \mathsf{water} [energy released])

Plants change the energy from the sun into food energy (glucose) then animals eat the plants and this energy is transferred along. Without plants there would be no chemical energy and therefore no animals.

If a plant is put into an enclosed environment then the levels of carbon dioxide decrease and oxygen increase over a longer period of time. This is because plants produce oxygen during the day, but a night this stops because there is no light. Carbon dioxide is always being produced from respiration, so at night the levels of carbon dioxide increase while the levels of oxygen (being used for respiration) decrease. In the day carbon dioxide decreases as it is being used for photosynthesis at a faster rate than it is being produced. Overall, plants create more oxygen than they use, and take in more carbon dioxide than they produce, which is why the atmosphere is nice and oxygen full.

The glucose made by photosynthesis is used for many different purposes. It is initially used for respiration, which releases energy which in turn allows the plant to convert glucose into other useful things (the plant will also need to gather minerals from the soil).

  1. Respiration (see above)
  2. Chlorophyll – Glucose is used to make more chlorophyll, which makes more glucose etc
  3. Stored as fats – Glucose is turned into lipids (fats and oils) for storage in seeds.
  4. Making cell walls –glucose is converted into cellulose for making cell walls (especially in rapidly growing plants
  5. Turned into starch – Glucose is stored as starch in roots and leaves for use when photosynthesis cant happen (e.g. winter, when it is too cold and there are no leaves) Starch is insoluble, which means it doesn’t bloat the storage cells due to excess water from osmosis
  6. Turned into proteins - Glucose is combined with nitrates from the soil to form amino acids which turn into proteins

Mineral Purpose 'Lack of' symptoms
Nitrates Used for making amino acids and for synthesising proteins Stunted growth + yellow older leaves
Phosphates Have important roles in the reactions of photosynthesis and respiration Poor root growth + purple new leaves
Potassium Helps the enzymes in photosynthesis and respiration work Yellow leaves with dead spots
Iron and magnesium (in tiny amounts) Needed for making chlorophyll -

Growth hormones in plants

Auxins control the growth of new shoots and roots, and are produced in the tip of these new shoots and roots, and diffuse back into the shoot. If the tip is removed then there are no auxins being produced, and the shoot may stop growing. Auxins also contain a substance which stops side growth, but if the tip is removed then this inhibitor is no longer present, so plants will grow sideways. This is why if you cut the top of a hedge it becomes fatter.

Auxins produce completely different results in shoots and roots. In shoots auxins make the shoot grow more, in roots, they inhibit growth

In shoots In roots
When a shoot is exposed to light it produces lots of auxins on the shaded side. This speeds up growth on the shaded side, and the shoot bends towards the light.
When a shoot is growing sideways it produces lots of auxins on the bottom side (i.e. towards gravity). This means that the bottom side grows faster and the shot bends upwards.
When the root detects moisture, it produces lots of auxins on the side near the moisture. This means that that side stops growing, while the other side continues, so the root bends towards the moisture.
When a shoot is growing sideways, it produces lots of auxins on the bottom side (i.e. towards gravity). However, this stops the bottom side from growing, so the top side continues and the shoot bends towards the bottom.

Plant hormones in commerce

  1. Creating seedless fruit is possible. Normally, fruit is produced when a plant is pollinated, and the fruit contains seeds (i.e. that is the whole point of the fruit). However, if certain growth hormones are applied then he fruit will develop without any seeds
  2. Fruit can be ripened after being picked using ripening hormone. This means that the fruit can be picked when they are unripe (and therefore less likely to be damaged), transported to the supermarket and ripened either at the supermarket itself or on the way there.
  3. Clones can be produced by taking cuttings of plants, and putting the cuttings in soil which contains rooting compound. They will quickly produce roots and you have lots of little plants
  4. Hormones which disrupt the growing patterns of broad-leaved plants can be added to lawns. This kills the broad-leaved weeds, while leaving the narrow-leaved grass untouched.

Also See

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