Biology revision - Plant reactions

note - all based on my notes so please do not used word-for-word and give credit. Thank you.

Reactions in plants

There are 5 kingdoms of organisms:

fungi
plants
prokaryotes
protoctists
animals

Photosynthesis is the process which allows plants and algae to make their own food. It occurs in the chloroplasts where light is absorbed by chlorophyll (a green pigment found in the leaf). It is an endothermic reaction.

Chemical reaction for photosynthesis: carbon dioxide + water → glucose + oxygen

Note: chlorophyll and light energy are not part of the actual equation. They go on the arrow as they are only conditions

To photosynthesise, a plant needs water, carbon dioxide, chlorophyll and light energy. Water comes in through the roots from the soil, whereas carbon dioxide comes from the air. The products of photosynthesis are glucose and oxygen. Glucose is stored as starch as it is used for energy but oxygen is released into the air through stomata.

Limiting Factor: any factor that slows down the rate of photosynthesis when there is not enough of it.

The limiting factors of photosynthesis are temperature, amount of carbon dioxide, light and water.

Light is a condition for photosynthesis which means it can only occur when there is light. This is usually in the day-time so photosynthesis stops at night.

Aerobic respiration

Aerobic respiration is a series of chemical reactions that release energy from glucose. It is an exothermic reaction with its goal to release energy for use. This is the equation:

Glucose + oxygen → carbon dioxide + water OR C6H₁₂O6+ O₂ →6CO₂ + 6H₂O

Glucose is a product of photosynthesis. Oxygen comes from the air. Carbon dioxide is a waste product as it is poisonous so it is exhaled. Water vapour is also exhaled as a waste product. Energy is what is useful.

Difference between photosynthesis and respiration: Photosynthesis can only happen when there is light but respiration happens all the time. During the day, a plant produces more oxygen from photosynthesis than it needs for respiration so oxygen is given off. When it is dark, photosynthesis is decreased so the plant uses less carbon dioxide. The plant continues to respire throughout the night so more carbon dioxide is produced.

Water is important to plants because it…

Carries dissolved mineral ions (a charged substance)
Keeps cells rigid so the plant doesn’t wilt
Cools the leaves through evaporation
Used in photosynthesis

Glucose is also important to plants. A special vessel – the phloem – transports glucose around the plant in the form of sugars dissolved in water.

Plant adaptations

Root Hair Cells

Roots take up mineral salts and water from the soil. They are well adapted as they are branched out to get more water from a large volume of soil.
The surface of the roots have root hair cells, well adapted to increase the uptake of water and salts:

Adaptations of root hair cells:

1. The shape increases its surface area, allowing more water and mineral absorption

2. The cell walls are thin so that it is easy for minerals and water to pass through

(no chloroplasts as no light energy can be absorbed underground)

Plant Vessels

The xylem transports (physical process) water and minerals from the roots up the plant stem and into the leaves. The phloem moves (requires energy) food substances produced by photosynthesis to where they are needed, such as developing seeds.

In simple terms – The phloem transports glucose whereas the xylem transports water.

The Leaf

There are more chloroplasts near the top of the leaf so more light energy is absorbed.

The cuticle is a waxy, waterproof layer on the outside of a leaf that stops them from losing too much water.

The actual leaf: leaves are broad and flat, giving it a large surface area, meaning more light absorption. Pores called stomata allow gas exchange to occur:

Stomata

These are small holes in a leaf that open and close to allow gas exchange.

Stomata open and close throughout the day due to water flowing in and out.

Stomata isn’t an actual part of the plant but is just the name of the hole.

Stomata shut at night (due to there being no light) to prevent water loss.

Note regarding chloroplasts: chloroplasts can move to the surface of the leaf in dim light and move the opposite direction in bright light to stop them from getting damaged.

Plant products

Plants make different lipids (a group of insoluble molecules like fats and oils). The cuticle of the leaf contains lipids, making it waterproof. Plants use lipids to make parts of cells like the cell membrane. Glucose is a product of photosynthesis which is stored in starch. Test for starch:

Prepare your food sample in a test tube.
Use a pipette to drop iodine solution (orange-brown in colour) into the test tube.
If the solution turns to a blue-black colour, starch is present.

The starch stays in the chloroplasts until photosynthesis stops when the plant is no longer producing glucose. It is then broken down into smaller sugar molecules called sucrose which is transported around the plant via the phloem.

Sucrose can make:

Starch (in a storage organ like a potato)
Glucose for respiration
Other molecules in the plant (like cellulose)

Proteins contain carbon, hydrogen, oxygen and nitrogen and are made from amino acids. The first 3 substances are found in glucose but nitrogen is found in nitrates, a mineral salt. All enzymes that carry out processes such as photosynthesis and respiration are proteins so a good supply of nitrates is essential. Nitrates are found in the soil and are used to make the amino acids which are used to make the proteins. The order of amino acids is determined by the DNA.

Growing crops

Germination

Water and oxygen enters.
The entry of water allows molecules to move around so that reactions can occur. It also triggers the release of enzymes.
The enzymes break down the starch to glucose. Enzymes work faster when warmer to a certain degree.
Glucose enters the embryo, allowing it to respire and grow.

Substances found in a seed

Starch – used as an energy store
Lipids – used as an energy store
Proteins – store of amino acids
Cellulose – in cell walls of the embryo
Enzymes – used to break down starch

Yield: the amount of useful product that is obtained from a crop

Variety: when organisms have different qualities/forms/types.

Cross-breeding: different varieties are bred together to maintain specific characteristics.

Farmers can increase their yield through greenhouses, machines which can harvest faster than humans, hedgerows and cutting down forests for more agricultural land.

Pesticides: These are chemicals which kill pests, organisms that damage crops. Insecticides kill insect pests and fungicides kill fungi that cause plant disease. Herbicides kill weeds which compete with crop plants for water, light and mineral salts. Modern herbicides are selective: they kill weeds with broad leaves but not the crop plants with the narrow leaves.

Farming problems

What happens?	Problems this causes	Possible solutions
Insecticides get sprayed onto hedges	This kills insects that are not harmful to the crops and are useful for other animals.	The farmer leaves a strip around the edge of the field that is not sprayed with chemicals.
Farmers run over the nests of birds that make nests in fields in some areas of the country.	This kills the chicks.	Encourage the farmer to leave an area in the middle of the field where machinery doesn’t go.
Fertilisers wash into rivers and lakes and pollute them.	This causes water animals such as fish to die.	The farmer makes sure that only the right amounts of fertilisers are applied and fertiliser is not applied if it is about to rain.
Hedges and trees are removed to make it easier for machinery to move around.	Animals need to run across large areas of open country in order to move from wood to wood to find food and mates. This makes them easy targets for predators.	The farmer is encouraged or paid to plant hedges to reconnect areas of woodland back together.

Eutrophication – when fertilisers are added to farmland in large quantities, plants and animals in nearby rivers and lakes can sometimes be killed (but not by poisoning):

Excessive nutrients from fertilisers are flushed from the land into rivers and lakes by rainwater.
These pollutants cause excessive aquatic plant growth of algae, duckweed, etc.
Algae covers the surface, preventing sunlight reaching over plants. The plants die and oxygen in the water is depleted (used up).
Dead plants are broken down by bacteria decomposers, using up even more oxygen in the water.
Oxygen levels reach a point where no life is possible so fish and other organisms die.

Biodiversity: the range of different species of organisms in a given area. It is important to preserve biodiversity as it reduces the reliance of organisms on another.

The carbon cycle

Carbon Cycle: the movement of carbon around an ecosystem.

When plants photosynthesise, they take in carbon dioxide from the air.

Organisms, including decomposers, release carbon dioxide into the atmosphere.
When fuels are burnt during combustion, carbon dioxide is released into the air.
Carbon is ‘stored’ in plants so when animals eat them, they are transferred into and out of the animal.

The main processes in the carbon cycle are:

Combustion
Feeding
Photosynthesis
Respiration
Decomposition

Advantages and disadvantages of farming strategies

What is it?	Advantages	Disadvantages
Insecticides	Kills insects which harm the crops.	Predators in a food chain may eat many animals containing small amounts of insecticides. This means the top predator of the food chain gets a large amount of the substance, harming it.
Fertilisers	Gives plants important minerals such as nitrates.	If fertilisers wash up into rivers, sea plants (such as algae or duckweed) absorb the fertilisers. This causes them to grow excessively, covering the body of water and blocking the sunlight. Plants below die because of their inability to photosynthesise and decomposers break down the dead material. The decomposers respire, using up the oxygen in the water, causing the fish to die. This process is called eutrophication.
Herbicides	Kills weeds which compete with the crop plants for water, light, minerals, etc.	If they are not selective herbicides, they may harm some crops.
Clearing land	Forests are cut down to make farmland. Hedgerows are moved to create space for big machines which help to plant and harvest crops. It can also make space for greenhouses.	Animals need to move across large areas of land to hunt for food, look for mates, etc. but without the hedgerows, they are left in the open so predators can spot them easily and kill them. The habitats of many animals are lost. This can cause endangerment or extinction.

Note regarding leaf adaptations: leaves are thin for a short diffusion distance (for carbon dioxide and oxygen). The endothermic layer protects the leaf, reduces water loss, allows gas exchange and lets light reach photosynthetic cells.

Terms to remember:

transpiration is the evaporation of water from the leaf of a plant.
turgid means firm and flaccid means floppy.