2.84 Understand that the central nervous system consists of the brain and spinal cord and is linked to sense organs by nerves

The central nervous system consists of:

• brain
• spinal cord

and is linked to sense organs (i.e. eyes, tongue, touch receptors) by nerves

2.83 Describe how responses can be controlled by nervous or by hormonal communication and understand the differences between the two systems

Endocrine (hormonal) system

• Long lasting response
• Widespread effect
• Hormones produced all over the body, carried by plasma
• Pituitary gland and hypothalamus are key players

Nervous

• Short lived but very quick
• Localised; specific to an area
• Often involves the contraction of a muscle

2.56 Describe experiments to investigate the role of environmental factors in determining the rate of transpiration from a leafy shoot

• Weigh the plant at the start
• Put it in a test tube with only its roots / end in the water
• Put a layer of oil over water
• Leave for x amount of time
• Weigh plant again
• Record results
• Repeat, changing one variable (i.e temperature)

2.55 Explain how the rate of transpiration is affected by changes in humidity, wind speed, temperature and light intensity

Humidity
Decreases amount of transpiration because the concentration of H₂O in the air will equal that of the plant, so it can't move from high to low concentration as per usual 

Wind
Increases transpiration because water is blown away, which means there is a greater difference in H₂O concentration

Temperature
Increases transpiration: the heat makes the water evaporate more quickly.

Light intensity
Stronger light means more transpiration because leaves absorb more heat from the light, so water evaporates quicker

2.54 understand that transpiration is the evaporation of water from the surface of a plant

Transpiration is when water evaporates from the surface of a plant; typically below the leaf because of the stomata being opened by guard cells

2.53 Explain how water is absorbed by root hair cells

• Water is absorbed by osmosis
• (Minerals absorbed by active transport)
• Hair cells are long in shape, giving it a large surface area (also, they branch)
• Epidermal cells increase surface area further
• Makes osmosis faster and more efficient
• Substances are transported by the xylem

4.5 Understand the concepts of food chains, food webs, pyramids of number, pyramids of biomass and pyramids of energy transfer

Food chain
Shows energy transfer between producers all the way to tertiary consumers. Only 1 line
eg: grass > zebra > lion

Food web
Shows links between various food chains within the same habitat or environment
eg:

Pyramid of numbers
Shows number of organisms in each trophic level and isn't always a nice pyramid shape
eg:

Pyramid of biomass
Shows mass of organism in each trophic level, and is almost always a pyramid shape
eg:

Energy transfer pyramid / chain
Shows energy transfer between organisms, typically in a food chain
eg

4.4 Explain the names given to different trophic levels to include producers, primary, secondary and tertiary consumers and decomposers

Producers
Produce their own 'food' (glucose) from the sun's light energy

Primary consumer
Eats (consumes) producer and gains its energy; often a herbivore

Secondary consumer
Eats (consumes) primary consumer and gains its energy; often an omnivore

Tertiary consumer
Eats secondary consumer and gains its energy; top of the food chain; often a carnivore

When these die, they're broken down by fungi and bacteria. Nitrogen and carbon compounds are returned to the earth (see cycles)

4.3 Explain how quadrats can be used to sample the distribution of organisms in their habitats.

• Quadrats placed in different areas at random
• One is chosen
• Population counted (must be either plants or VERY slow moving animals i.e. slug)
• Repeated in different areas
• Population counted
• You can compare results and see where population is more dense (i.e. there might be more flowers near a river bed)

4.2 Explain how quadrats can be used to estimate the population size of an organism in two different areas

Sample area is taken by quadrat, can be repeated many many times to make it more accurate, then multiplied to 'cover' desired area of land. 

4.1 Understand the terms population, community, habitat and ecosystem

• Population: number of a certain type or types or organism(s) living in a particular area at a particular time
• Community: Interacting group of various species in a common location: typically relying on one another
• Habitat: physical location of an organism, including what is available to it
• Environment: the natural biological factors that affect a species or particular organism (i.e. humans)

2.67 Understand the origin of carbon dioxide and oxygen as waste products of metabolism and their loss from the stomata of a leaf

Both CO₂ and O₂ are waste products of metabolic reactions:

• CO₂ + H₂O > C₆H₁₂O₆ + O₂ (photosynthesis)
• C₆H₁₂O₆ + O₂>  CO₂ + H₂O (respiration)

which are carried out by plants. 

2.52 Describe the role of xylem in transporting water and mineral salts

• Hollow, dead cells
• Transports water and mineral salts
• Side by side with phloem
• EG Nitrates, phosphates, etc.

2.51 Describe the role of phloem in transporting sucrose and amino acids between the leaves and other parts of the plant

• Phloem has living cells in form of a column
• Has companion cells and a cytoplasm + cellulose cell wall
• Companion cells very metabolically active
• Carries dissolved substances such as sucrose and amino acids within the plant

2.50 Understand the need for a transport system in multicellular organisms

Multicellular organisms have a small surface area to volume ratio so diffusion is insufficient and would be too slow anyway. Transport systems speed up the process of getting the molecules required (i.e circulatory)

2.49 Understand why simple, unicellular organisms can rely on diffusion for movement of substances in and out of the cell

They have a very high surface area to volume ratio, so things diffuse in and out very quickly. Also, they are so small they don't use up that much in terms of energy, food, etc. so diffusion is sufficient.

2.43 Describe experiments to investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator

As light increases, so does photosynthesis. A hydrogen carbonate indicator, like limewater, is an indicator for CO₂. 

• Normal: orange
• Increase: yellow
• Decrease: purple

• Fill a test tube 1/4 full with HCl. 
• Attach leaf to bung and leave it in there
• Vary light intensity every x minutes
• (Change distance y from leaf)
• Record results
• done

2.22 Describe experiments to investigate photosynthesis, showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll

Pond weed experiment

• Pond weed in large beaker with water
• Count bubbles released in 1 minute
• Record results and repeat 3x
• Repeat, changing either temperature, concentration of CO2, light intensity and / or plant species

Image of apparatus

2.42 Describe the role of stomata in gas exchange

Stomata is just the plural form of 'stoma', which are the tiny pores on the underside of the leaf. Guard cells control whether they open or close, and therefore also control the exchanging of carbon dioxide and oxygen. I'm pretty sure I already covered how they work in another post, but I'll just write it again ... (bear with me)

If there are a lot of potassium ions present in the fluid in the guard cells, the cells will absorb a lot of water via osmosis, and will become more turgid. This opens the stoma, allowing diffusion to take place. If there aren't a lot of potassium ions present, the cells will lose water, become flaccid and close the stoma. The opening and closing of the stoma also depends on whether it is day or night. In the light, the guard cells absorb water, in the dark they lose it.

2.41 Explain how the structure of the leaf is adapted for gas exchange

• The cells in the spongy mesophyll (lower layer) are loosely packed, which increases the surface area of the leaf, allowing for more carbon dioxide to be absorbed
• Guard cells (stomata) on the lower part of the leaf controls the diffusion of gases in to the leaf.
• Leaves are thin, making it easier and quicker for gas to diffuse quickly through it.

Let me explain how the stomata work: 

- During the day, the guard cells are absorbing water (via osmosis), which makes them turgid, opening the stoma and therefore allowing gas into the plant

- At nighttime the opposite happens. The guard cells lose water, becoming flaccid and closing the stoma.

2.22 Describe experiments to investigate photosynthesis, showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll

The most common one to use involves pond weed. Basically, it's placed under water and various different factors are varied (NOT at the same time)
Different variables you could change:

• The distance between the pond weed and a lamp (which would be it's source of light), is varied
• Baking powder is added to the water (increasing the amount of CO2)
• The amount of chlorophyll in a plant (a plant with white leaves could be compared to one with green leaves, as the green one will contain more chlorophyll)

Then you would measure the amount of gas given off (by counting bubbles or by downwards displacement). If there is more gas produced the rate of photosynthesis will be quicker. The production of starch can also be tested by adding iodine.

2.21 Understand that plants require mineral ions for growth and that magnesium ions are needed for chlorophyll and nitrate ions are needed for amino acids

This means that plants need mineral ions from the soil as well as sunlight and water for growth. Different mineral ions are absorbed for different reasons, and the two you need to learn are:

- Magnesium ions, needed for chlorophyll
- Nitrate ions, used for amino acids

To Learn: Plants

List of things that should take priority in revision:

• Transpiration is important to plants because it allows a steady stream of water to happen - when water moves out through transpiration, more water can be pulled in through the roots. This is possible because water is cohesive (molecules of water are attracted to each other). If you put little blobs of water on a desk that is slippery enough and they are close, the two little blobs will form one bigger blob.
• In intense light, guard cells become turgid, which causes the stomata to open, so rate of transpiration increases
• When the stomata is open, carbon dioxide, oxygen and water vapour can diffuse in (so photosynthesis can occur) and excess water vapour can diffuse out
• Using this apparatus, rate of water loss can be measured, because the bubble moves up with the intake of water. The faster the intake, the faster the rate of transpiration.
• To investigate the effect of wind speed, temp, etc. on the experiment above, just modify the environment; i.e. for wind, add a hairdryer, etc.