OCR Gateway B Module B4: It's a Green World

Ecosystems:

An ecosystem (e.g. a garden) comprises of all the animals/plants living there
A habitat is where a plant or animal lives
All animals and plants living in this ecosystem are called the community and the number of a particular plant/animal in the community is the population
Distributions of organisms can be mapped with a transect line; string is laid across a path, and at regular intervals, the organisms in a square frame (quadrat) are counted/assessed for percentage cover, with data plotted on a kite diagram:

Abiotic changes (not biological), such as trampling cause zonation, different numbers of species in different locations across a habitat
In an artificial ecosystem (such as a fish farm), only one species is often kept, to avoid competition and increase yield
An ecosystem is self supporting as energy is passed on through the food chain/web. There is a balance of gases, with respiration and photosynthesis

Population size and diversity:

Collection and counting methods include pooters, nets, pitfall traps and quadrats
Biodiversity is the variety of different species living in a habitat
The population size is often too large to measure, so it can be estimated by obtaining data from a small area and scaling up
A more accurate method for animals is capture-recapture. A sample (of any size) is taken, and all animals marked, then released. At another time, another sample is taken, with the total number recorded and those that are already marked. The population size is calculated as follows:
population size = (number in 1^s^t sample x number in 2ⁿ^d sample) ÷ number in 2ⁿ^d sample previously marked
The accuracy would be increased with more samples. Additionally, this method assumes that there are no deaths, migration or reproduction, and the markings do not affect survival of the animals

Photosynthesis:

The balanced symbol equation for photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (carbon dioxide + water → glucose + oxygen), which requires light energy and happens in the chlorophyll. Oxygen is a waste product and is not needed by the plant
The product, glucose, and other simple sugars can be used in respiration; converted into cellulose to make cell walls; converted into starch, fats and oils for storage; or converted into proteins for growth and repair. They are transported as soluble sugars
Starch is used for storage because it is insoluble and does not move from storage areas. It does not affect the water concentration of cells and cause osmosis, unlike glucose
The required carbon dioxide enters through stomata and water enters through root hairs. Oxygen, the product, also exits through stomata

How photosynthesis was discovered:

Greek scientists thought that plants only took minerals out of the soil to grow and gain mass
Van Helmont concluded that plant growth could not only be due to minerals in the soil, through an experiment with a willow tree
Priestley showed that plants produce oxygen
Experiments with alga chlorella and an isotope of oxygen ¹⁸O as part of a water molecule have shown that light energy is used to split water rather than carbon dioxide as previously thought - the oxygen produced comes from the water, not the carbon dioxide
We now know that photosynthesis works as follows:
- Light energy splits the water, releasing hydrogen ions and oxygen gas
- The hydrogen ions react with carbon dioxide to produce glucose (C₆H₁₂O₆)

Rate of photosynthesis:

The rate of photosynthesis is increased with more carbon dioxide, more light, or a higher temperature (to increase enzyme action). A lack of one of these factors will limit the rate, called a limiting factor
Photosynthesis only occurs during the day if no artificial light is provided, although respiration provides the plant with energy at all times
In respiration, oxygen and glucose react to form carbon dioxide, water and energy, the opposite to photosynthesis. Hoewever, more oxygen is produced by photosynthesis than is used in respiration
Plants respire and photosynthesise - photosynthesis is used to create glucose, used for storage and the other uses listed above. This can then be used to produce energy when it is needed. They respire less because some of the glucose from photosynthesis is used for things other than energy storage. Therefore, they take in more oxygen than they produce at night

Leaf structure:

Stomata allow for gases to diffuse into the leaf
Green leaves have many specialised cells, listed below from top (sunlight-facing) to bottom:
- Wax cuticle
- Upper epidermis
- Palisade mesophyll
- Spongy mesophyll
- Lower epidermis, containing guard cells and stoma
- Wax cuticle
The outer epidermis is transparent to allow entry of light
The palisade layer contains most chloroplasts - they are not needed in cells that don't get much light
The spongy mesophyll cells are loosely spaced to allow for diffusion between photosynthesising cells and the stomata. They are arranged for a large surface area
The guard cells allow for gases to enter the leaf
The cuticles made from wax help protect the leaf without blocking light from entering

Leaf shape adaptations:

Leaves are broad to allow for a large surface area to get lots of light
They are thin to enable diffusion
Chloroplasts contain chlorophyll a and b, and other pigments (carotene and xanthophyll) to use light from a variety of wavelengths
They have vascular bundles (veins) for transport of chemicals and support
They have guard cells to regulate the flow of carbon dioxide, oxygen, and to minimise water loss

Diffusion:

Diffusion is the movement of particles in a gas/liquid from an area of high concentration to an area of low concentration, with random movements of the particles
This is how molecules of water, oxygen and carbon dioxide can enter/leave cells through the cell membrane
For example, if a cell is using lots of carbon dioxide, there is very little inside (low concentration), so more carbon dioxide will enter
Leaves are specialised for diffusion with large surface areas, openings between guard cells (stomata), which are spread out, and gaps between spongy mesophyll cells
Rate of diffusion can be increased by reducing the distance for molecules to travel, having a steeper concentration gradient (higher difference in concentration between the two areas), or having a larger surface area for diffusion

Osmosis:

Osmosis is a type of diffusion, with a partially-permeable membrane that allows for the passage of smaller water molecules but not larger ones like glucose
It is how water moves in and out of plants cells through the cell membrane

Water in cells:

The plant cell wall provides support
The entry of water into plant cells increases pressure pushing on the cell wall, known as turgor pressure. This pressure supports the cell, stopping it from collapsing, but when too much water loses the cell, the cell collapses (plasmolysis), becoming flaccid, and the plant droops (wilts)
Animal cells, however, lack a cell wall, so when too much water enters, they swell up and burst (lysis). When too much leaves, it shows crenation by shrinking

Xylem and phloem cells:

Xylem and phloem are transport tissue which contain specialised plant cells. They span from the root to the leaf, through the stem
Xylem cells consist of dead cells and have a thick, strengthened cellulose cell wall with a hollow lumen
Phloem cells contain columns of living cells
They form vascular bundles in dicotyledonous plants
Xylem cells carry water and minerals from the soil (absorbed through root hair cells), up the stem, to the leaves, where the moisture may evaporate (transpire)
Phloem cells carry food substances (e.g. sugars) up and down stems. Transport of food substances is called translocation

Transpiration:

Transpiration is the evaporation and diffusion of water from inside leaves to the atmosphere. This water loss helps create a constant flow from the roots to the leaves
Root hairs have a large surface area for water uptake (through osmosis)
Higher temperature (which increases evaporation), more air movement (which blows away air containing water vapour), a decrease in humidity (allowing more water to evaporate) or a higher light intensity (making the stomata open) will increase the rate of transpiration
Wilting is caused when there is too much water loss in the leaves. This is reduced with stomatal openings on the bottom of the leaves
Spongy mesophyll cells are covered with a film of water where gases can dissolve. This water can then escape. This improves the speed of photosynthesis
Guard cells work by containing chloroplasts, which photosynthesise, producing sugars which cause the cells to swell by increasing turgor pressure. This will cause them to curve, opening the stoma

Minerals:

Minerals are taken up by root hair by active transport, rather than diffusion, because the root hairs already contain them in high concentrations. Active transport requires energy from respiration
Plants need minerals (which are present in the soil in low concentrations), such as:
Nitrates for cell growth (it is needed to produce amino acids). Without enough nitrates, the leaves will turn yellow and growth will slow
Phosphates are used to create DNA and cell membranes, contributing to growth. Without them, leaves will discolour and root growth will be poor
Potassium compounds aid enzyme action, speeding up respiration and photosynthesis. A lack of potassium causes poor flower/root growth and discoloured leaves
Magnesium compounds, which are used to create chlorophyll for photosynthesis. If the plant does not receive enough, leaves will turn yellow
In an experiment to show the effects on plants of mineral deficiencies, a soil-less culture should be used with each trial missing one mineral to get accurate results

Decay:

Detritivores, such as earthworms, maggots and woodlice feed on decaying material (detritus). They increase rate of decay by breaking up the detritus, which increases surface area for further breakdown by microbes. Decay can also be sped up by increasing temperature, oxygen and water
37 °C is optimum for bacteria and 25 °C is optimum for fungi, at these temperatures, rate of respiration is increased. Higher temperatures will denature enzymes
If more oxygen is available, it can be used for aerobic respiration to grow/reproduce faster
Saprophytes (e.g. fungi) are organisms that feed on dead/decaying material
Fungi use extracellular digestion; they produce enzymes to digest food outside cells and reabsorb the simple soluble substances.
Food preservation methods:
- When canning, foods are heated to kill bacteria, then sealed in a vacuum to prevent bacteria entering again
- Cooling foods reduces bacterial/fungal growth
- Freezing kills some bacteria/fungi
- Drying removes water so bacteria cannot feed
- Adding salt/sugar kills some bacteria/fungi, as they will absorb the high concentration, removing water from them
- Vinegar (an acid) can be added to kill most bacteria/fungi

Pesticides:

Pesticides (e.g. insecticides, fungicides and herbicides) can enter food chains and be passed on into predators, creating a lethal dose
They can harm organisms that aren't pests
Some are persistent and take a long time to break down
Organic farming is preferred by many, who believe that it produces tastier, healthier crops. It doesn't use pesticides or artificial fertilisers, instead it often uses:
- Animal manure and compost as natural fertilisers
- Crop rotation to avoid build-up of soil pests and maintain minerals in the soil
- Varying seed planting times to avoid certain life cycles of pests

Biological control:

Biological control uses living organisms to control pests, such as using ladybirds to eat aphids which damage plants
It can, however, cause rapid increases in the population of the introduced organism, affecting native species. This has happened with cane toads in Australia

Intensive farming:

Intensive farming uses artificial pesticides, fertilisers and limited living conditions for animals to produce high yields in small spaces and cheaply
It raises concerns about animal cruelty
Hydroponics is growing plants without soil, using a regulated flow of minerals in water
It increases efficiency of energy transfer in food chains by removing competing organisms, such as weeds which will consume minerals
By keeping animals warm inside a barn in small spaces, they can focus more energy on growth