# Biology Unit 4

?

## Estimating population sizes

A population is all the organisms of one species in a habitat. Populations of different species in a habitat make up a community.

A quadrat is a sqaure frame enclosing a known area. You can study the small area within a quadrat and scale up your findings to make estimates for larger areas. You have to count all the organisms in a 1m2 quatrat and multiply the number of organisms by the total area.

To estimate population size you can also use the capture recapture method. You capture a sample of the population and mark the animals in a harmless way. You then release them back into the environment and recapture another sample. Count how many of this sample are already marked. Then use the equation; NUMBER IN FIRST SAMPLE x NUMBER IN SECOND SAMPLE / NUMBER IN SECOND SAMPLE PREIVOUSLY MARKED to find the population size.

The sample size effects the accuracy of the estimate. the bigger the sample, the more accurate the estimate is likely to be. When using capture recapture method, you must assume the there have been no deaths, immigration or emigration effecting the population size, the sampling methods were identical and the marking methods haven't affected the individuals chance of survival.

1 of 19

## Ecosystems and distributions of organisms

An ecosystem is all the organisms living in an area along with the non living (abiotic) conditions, eg temperature, soil quality and salinity. An ecosystem is not the same as a habitat - a habitat is just where organisms live. An ecosystem is self supporting - it contains almost everything it needs to maintain itself. Water, nutrients and essential elements such as carbon all get recycled within the ecosystem. An energy source is needed from the outside however - it is usually the sun.

Distribution is where organisms are found within a particular area.You can investigate distribution using lines called transects. You mark out a line using a tape measure and place quadrats next to eachother all the way along the line. You then count and record the organisms found in all the quadrats. If it is difficult to count all the individual organisms, you can calculate the percentage covers. This means estimating the percentage area of the quadrat covered by a particular organism judging by the number of little squares covered by the organisms. You can plot these results on a kite diagram

Kite diagrams show the abundance and distribution of organisms. The abundance of each organism is shown by the thickness of the kite shape. The abundance is plotted above and below a central line to make the shape symmetrical. The x-axis shows the distance along the transect line.

2 of 19

## Zonation

The distribution of organisms is affected by abiotic factors. Abiotic factos are all the non living, physical factors in an environment, for example light, water, temperature, oxygen, salinity (salt level) and soil quality. The distribution of organisms is affected by abiotic factors because organisms are adapted to live in certain physical conditions. This means they're more likely to survive and reproduce in areas with these conditions. Many organisms can onlu survive in the conditions they are adapted to - for example mosquitoes are adapted to live in warm conditions.

Zonation is the gradual change in the distribution of a species across a habitat. A gradual change in abiotic factors can lead to the zonation of organisms in a habitat. For example, in a coastal habitat, changes in salinity and soil depth result in zones where different types of plants grow:
Few plants can grow closer to the sea as salinity is very high - marram grass can grow as its adapted to salty conditions.

A bit further away, lichens and mosses grow as they are better adapted to less saline conditions and out compete marram grass.

Abit further away, shrubs such as heather grow as they are better adapted to deeper soil that lichens and mosses.

3 of 19

## Biodiversity

Biodiversity is a measure of the variety of life in an area and includes the amount of variation between individuals of the same species in an area, the number of different species in an area and the number of different habitats in an area. Ecosystems with higher levels of biodiversity are healthier than those without. This is because theyre better able to cope with changes in the environment.

Natural ecosystems maintain themselves without major interference from humans. Artificial ecosystems are created and maintained by humans. Native woodlands (natural) have a higher biodiversity than forestry plantations (artificial). Native woodlands: have a variety of tree species, have trees of different sizes and ages, a variety of plant species, a variety of habitats and a variety of animal species. Forestry plantations have one species of a tree, blocks of trees which are planted at the same time meaning theyre the same age, fewer plant species as trees are densely planted, fewer habitats as there arent enough plant species to create them and fewer animal species.

Lakes have many fish species, a variety of plant species and a variety of animal species however fish farms have one species of fish which is farmed for food, fewer plant species as food waste can cause algae blooms killing plants and fewer animal species so predators cannot kill the fish.

4 of 19

## Photosynthesis

Photosynthesis uses energy from the sun to change carbon dioxide and water into glucose and oxygen. It takes place in chloroplasts in plant cells - they contain pigments like chlorophyll that absorb light energy. The balanced symbol equation is:                                                                                                                                        6CO2 + 6H20 -----> C6H12O6 + 6O2                                                                                                  Carbon dioxide + water ---(light energy)--> glucose + oxygen                                                             First, the light energy is used to split water into oxygen gas and hydrogen ions. Carbon dioxide gas then combines with hydrogen ions to make glucose and water.

Glucose is converted into other substances. Plants use some of the glucose for respiration. This releases energy so they can convert the rest of the glucose into various other useful substances. It is also converted into cellulose for making cell walls, especially in a rapidly growing plant. It can be turned into lipids (fats and oils) for storing in seeds. It is also turned into starch and stored in roots stems and leaves ready for use when photosynthesis isn't happening, like at night. Finally it can be combined with nitrates to make amino acids which are then made into proteins.

Starch is insoluble which makes it good for storing. It can/t dissolve in water and move away from storage areas in a solution. It doesn't affect the water concentration inside cells.

5 of 19

## Understanding photosynthesis

In 350 BC greek scientists concluded that the only thing touching plants was soil, so they decided that plants must grow and gain mass by taking in minerals from the soil.  In 1638 Van Helmont dired some soil, weighed it and put it in a pot. He planted a willow tree which he had weighed in the soil. He added rainwater to the pot whenever it was dry. 5 years later he removed the tree from the pot. It had gained about 74kg in mass however the soils mass had not changed a lot. Van helmont concluded that the tree must have gained mass from something other than the soil. Because he only added soil he concluded that the tree had gained mass from it.                                                In the 1770s, Priestly placed a burning candle in a sealed container and observed that the flame went out after a short time. The candle could not be relit whilst in the container. He then placed a burning candle and a living plant in the container. The flame went out after a short time but a few weeks later the candle could be re lit. Priestly decided that the burning candle used up something in the container and that it made the flame go out. He decided that the plant restored the air. He then filled a container with exhaled air and put a mouse in it. The mouse only survived for a few seconds. He repeated this ahain but put a plant in the container for a few days before adding the mouse and it survived for a few minutes. Priestly decided that breathing took something out of the air. He concluded that plants restored whatever was in the air that breathing and burning had taken out.

6 of 19

## More on photosynthesis

Plants release oxygen during photosynthesis. To find out where this oxygen came from, scientists supplied plants with water containing an isotope of oxygen called oxygen-18. The carbon dioxide the plants received contained ordinary oxygen-16. When plants photosynthesise, they release oxygen-18. This showed that the oxygen came from the water that was supplied, not the carbon dioxide.

There are three limiting factors which affect the rate of photosynthesis. Not enough light slows down the rate of photosynthesis as it provides the energy needed. If the light level is raised, the rate of photosynthesis will increase up to a certain point. Beyond this point it wont make any difference as then it will be eitherr the temperature or CO2 level which is now the limiting factor.

CO2 is one of the raw materials needed for photosynthesis. The amount of CO2 will only increase the rate of photosynthesis up to a certain point. After this the graph flattens out showing that CO2 is no longer the limiting factor. As long as the light and CO2 are in plentiful supply the factor limiting photosynthesis will be temperature.

As the temperature increases so does the rate of photosynthesis however if its too high the enzymes will denature so the rate rapidly decreases. This happens at about 45 degrees. Usually the temperature isnt the limiting factor as its usually too low if anything.

7 of 19

## diffusion

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. Diffusion occurs in both liquids and gases as the individual particles are free to move about randomly. The simplest kind is when gases diffuse through each other. This is what happens when perfumme diffuses through the air in a room.

Cell membranes hold a cell together however they let things in and out too. Only very small molecules can diffuse through the cell membrane such as simple sugars, water or ions. Big molecules like starch or proteins can't pass through. Particles flow through the cell membrane from where there's a higher concentration gradient to a lower concentration gradient. They move about randomly so they go both ways but if there are a lot more particles on one side of the wall, generally there will be more overall movement from that side. The rate of diffusion depends on:                                                                                                                                       Distance - the shorter the distance the quicker the rate of diffusion.                                                                                                                              Concentration difference - substances diffuse more quickly if there is a big difference in concentration.                                                                                                                                        surface area - the more surface area available for molecules to move across,t the faster they can get from one side to the other.

8 of 19

## Leaves and diffusion

The leaf consists of: the waxy cuticle, the upper epidermis, the palisade mesophyll layer, the chloroplast, the spongy mesophyll layer, the vascular bundle, air spaces, the lower epidermis, guard cells and the stoma.

Photosynthesis and respiration are opposite processes. Photosynthesis:                                         Carbon dioxide + water ---> glucose + oxygen (requires energy).                                                       Respiration however = glucose + oxygen ---> carbon dioxide + water. (releases energy)Photosynthesis only happens during the day but plants must respire all the time to get all the energy they need to live. During the day, plants make more oxygen by photosynthesis than they use in respiration. So in daylight they release oxygen and take in carbon dioxide. At night, plants only respire so they take in oxygen and release carbon dioxide.

When a plant photosynthesises it uses up lots of CO2 meaning theres hardly any in the leaf. This means more CO2 moves into the leaf by diffusion. At the same time oxygen is being made as a waste product of photosynthesis. Some is used in respiration and the rest diffuses out of the leaf.

At night there is no photosynthesis going on as there is no light. Lots of CO3 is made in respiration and a lot of oxygen is used up. There is lots of CO2 in the leaf and not enough oxygen so CO2 diffuses out and O2 diffuses in.

9 of 19

## leaves and photosynthesis

Leaves are adapted for diffusion. They are broad for a larger surface area. Theyre also thin which means carbon dioxide and water only have to diffuse a short distance to reach the photosynthesising cells where they are needed. There are stomata on the lower surface which let gases like CO2 and O2 in and out. They also allow water to escape - transpiration. Leaves have guard cells which control when the stomata open and close, controlling gas exchange. There are air spaces in the spongy mesophyll layer which allow gases to move between the stomata and photosynthesising cells. This means there is also a larger surface area.

Leaves are adapted to absorb light. Theyre broad so there is a large surface area exposed to light. They contain chloroplasts which contain chlorophyll pigments which absord light energy. Different pigments absorb different wavelengths of light. Chlorophyll A, Chlorophyll B, Carotene and Xanthophyll are all pigments in the chloroplast. The cells that contain the most chloroplasts are arranged in the palisade layer near the top of the leaf where they can get the most light. The upper epidermis is transparent so that light can pass through to the palisade layer.

The vascular bundles are the transport vessels. These are the xylem and phloem. They deliver water and other nutrients to every part of the leaf and take away the glucose produced by photosynthesis. They also help support the leaf structure.

10 of 19

## Osmosis

Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher concentration to a region of lower water concentration. A partially permeable membrane is one with very small holes in it. These holes are so small that only tiny molecules can pass through them and bigger molecules (ie sucrose) cant. The water molecules can pass both ways through the membrane during osmosis. This happens because water molecules move randomly. As there are more water molecules on one side than the other, there is a steady net flow of water into the region with fewer water molecules, ie the stronger sucrose solution. This means the concentrated sucrose solution becomes more dilute. Osmosis is in effect a form of diffusion.

When a plant is well watered its cells become plump and swollen due to its drawing water in by osmosis. These cells are described as turgid. The content of the cells push against the inelastic cell wall - this is turgor pressure. If there is no water in the soil the plant will wilt. This is because the cells start to lose water and therefore lose their turgor pressure. They become flaccid. If the plant is short of water the cytoplasm inside its cells shrinks and the membrane pulls away from the cell. The cell is said to be plasmolysed. The plant droops.

Animal cells dont have a cell wall so of it takes in too much water it bursts (lysis) and if it loses too much water it shrivels up (crenation).

11 of 19

## Transport systems in plants

The phloem tubes transport food - theyre made of columns of living cells with perforated end plates to allow stuff to flow through. They transport food substances (mainly sugars) both up and down the stem to growing and storage tissues. This movement of food substances around the plant is known as translocation.

Xylem vessels take water up - theyre made of dead cells joined end to end with no cell walls between them and a lumen down the middle. The thick side walls are made of cellulose. Theyre strong and stiff which gives the plant good support. They carry water and minerals from the roots up the shoot to the leaves in the transpiration stream.

The xylem and phloem run alongside eachother in vascular bundles. Where they are found in each type of plant structure is related to the xylem's other function: support.

Root cross section - the roots have to resist crushing as they push through the soil. The xylem is in the center to give it strength.

Stem cross section - stems need to resist bending. The xylem forms a sort of scaffholding. The phloem os always around the outside of the stem.

Leaf cross section - the xylem and phloem together make a network of veins for support.

12 of 19

## Water flow through plants

Root hairs take in water by osmosis - the cells on plant roots grow into long hairs which stick out into the soil. Each branch of a root will be covered in millions of these. This gives the plant a big surface area for absorbing water from the soil. There is usually a higher concentration of water in the soil so the water is drawn into the root hair cell by osmosis.

Transpiration is the loss of water in a plant. Its caused by evaportation and diffusion of water vapour inside the leaves. This creates a shortage of water in the leaves so more water is drawn up from the rest of the plant through the xylem vessels. This means that more water is drawn up from the roots, so there is a constant transpiration stream of water through the plant. The transpiration stream benefits the plant in some ways:
The constant stream of water from the ground helps keep the plant cool.

It provides the plant with a constant supply of water for photosynthesis.

The water creates turgor pressure in the plant cells which helps support the plant and stops it wilting.

Minerals needed by the plant can be brought in from the soil along with the water.

13 of 19

## Water flow through plants

an increase in light intensity increases the transpiration rate. Stomata close as it gets darker. Photosynthesis cannot occur in the dark so they dont need to be open to let CO2 in. When the stomata are closed, water can't escape.                                                                                      An increase in temperature means tranpiration occurs more quickly. When its warm the particles have more energy to evaportate and diffuse out of the stomata.                                                   If there is a lot of air movement around the leaf transpiration happens more quickly as any water vapour around the leaf is swept away maintaining a low concentration of water outside the leaf meaning diffusion can occur quickly.                                                                         If the air around the leaf is very dry transipartion occurs more quickly.                                                   Plants have adaptions to reduce water loss from their leaves. Leaves have a waxy cuticle covering the upper epidermis. This helps make the surface of the leaf waterproof. Most stomata are found on the lower surface where its cooler and darker. This helps slow down diffusion of water out of the leaf. The bigger the stomata and the more stomata the leaf has, the more water the plant will lose. Plants in hot climates need to conserve water so they have fewer and smaller stomata.

Stomata close automatically when supplies of water dry up. Guard cells open and close the stomata as they go turgid or flaccid. Thin outer walls thickened inner walls make this opening and closing function work properly.

14 of 19

## Minerals needed for healthy growth

Nitrates - contain nitrogen for making amino acids and proteins. These are needed for cell growth. If a plant cant get enough nitrates cell growth will be poor and the older leaves will be yellow.         Phosphates - needed for respiration and growth. Contain phosphorus for making DNA and cell membranes. Plants without enough phosphate have poor root growth and discoloured older leaves.                                                                                                                                                Potassium - is needed to help the enzymes needed for photosynthesis and respiration. If there isnt enough potassium in the soil plants have poor flower and fruit growth and discoloured leaves.

Magnesium is needed in smaller amounts and is required for making chlorophyll needed for photosynthesis. Plants with little or no magnesium have yellow leaves.

Root hairs take in minerals using active transport. However the concentration of minerals in the soil is usually less than the concentration in the root hair cells. Normal diffusion therefore doesnt occur. Active transport therefore occurs. This uses energy from respiration to help the plant pull minerals into the root hair against the concentration gradient.

15 of 19

## Decay

When living things die and decompose/ release material as waste the elements they contain are returned to the soil or air. Decomposition is done by microorganisms like soil bacteria and fungi. The rate of decay is affected by temperature (warm temperatures speed up respiration in microorganisms), the amount of water (things decay faster when theyre moist as microorganisms need water) and the amount of oxygen (decay is faster when oxygen is available as microorganisms respire aerobically). When these factors are at optimum levels these organisms grow and reproduce more quickly meaning there is more of them to decay living things.          Detrivores feed on dead and decaying material. Earthworms maggots and woodlicea are all detrivores. They break things up into smaller bits giving a bigger surface area for smaller decomposers to work on speeding up decay.                                                                                     Saprophytes feed on decaying materials by extracellular digestion - they feed by secreting digestive enzymes on the material outside of their cells. The enzymes break down the material into smaller bitswhich can be absorbed by the saprophytes. Most saprophytes are fungi. Putting food in airtight cans keeps decomposers out. Cooling slows down decay as it slows down the reproduction rate. Drying means there is no water for  the decomposers to use. Adding salts/sugar means water is lost by osmosis. This damages decomposers. Adding vinegar kills decomposers as it is acidic.

16 of 19

## Intensive farming

Intensive farming means trying to produce as much food as possible from your land, plants or animals. This can be done in many ways which all involve reducing energy loss at each stage of the food chain: Using herbicides to kill weeds means more energy from the sun goes to the crops and not to any other competing plants. Using pesticides kills insects which eat crops meaning no energy is transferred into a different food chain. Battery farming animals is when animals are kept close toghet indoors so that theyre warm and cant move about. This saves them wasting energy and stops them from using energy to stay warm.

Hydrophonics is another method of intensive farming. It is when plants are grown in nutrient solutions instead of soil. It is often used to grow glasshouse tomatoes on a commercial scale as well as to grow plants in areas with barren soil. Advantages include that mineral levels can be controlled more accurately and diseases controlled more effectively however disadvantages are that lots of fertilisers need to be added and there is no soil to anchor roots/support plants.

Intensive farming can destroy the world we live in making it polluted, unattractive and devoid of qildlife. Removal of hedges destroys the natural habitat of wild creatures and leads to soil erosion Careless use of fertilisers can cause eutrophication and pesticides disturb food chains.

17 of 19

## Pesticides and biological control

Pesticides are sprayed onto crops to kill the creatures that damage them but they can also kill organisms that aren't pests. this can cause a shortage of food for animals further up the food chain. Some pesticides are persistent meaning they are hard to get rid of from ecosystems. There is danger of pesticides being passed along the food chain and killing animals further up.

Biological control means using living things to control a pest. You canuse a predator, a parasite or a disease. Aphids are a pest as they eat roses and vegetables. Ladybirds are aphid predators so people release them into their fields and gardens to keep aphid numbers down. Certain wasps and flies produce larvae which develop in or on a host insect killing the insect host. Myxomatosis is a disease which kills rabbits. The virus was released in australia as a biological control when the population grew and destroyed crops.

Advantages - no chemicals are used meaning less pollution, disruption of food chains and risk to people eating sprayed foods. There is no need to keep repeating the treatment.

Disadvantages - the predator introduced may not eat the pest meaning it is useless. The predator could eat useful species ie pollinators, the predators population may increase and get out of control. The predator may not stay in the area where it is needed.

18 of 19

## Alternatives to intensive farming

Use of organic fertilisers recycles the nutrients left in plant and animal waste. It doesnt work as well as artificial fertilisers but it is better for the environment.                                                                    Crop rotation is growing a cycle of different crops in a field each year. This stops the pests and diseases of one crop building up and stops nutrients running out. Most crop rotations include a nitrogen fixing crop which puts nitrates back in the soil.                                                                      Weeding is when weeds are physically removed from the soil rather than being removed with a herbicide. It is more labour intensive but doesnt require chemicals.                                                  Varying seed planting times later or earlier in the season avoids any main pests for the crop. This means farmers wont need to use any pesticides.                                                                                Biological control is also used.

Advantages include that fewer chemicals are used, it is better for the environment as there is less chance of polluting rivers with fertilisers and there is less disruption of food chains. For a farm to be classed as organic it has to follow guidelines of ethical treatment of animals.

Disadvantages: Organic farming takes up more space than intensive farming so more land has to be farmland rather than being set aside for wildlife and for other uses. It is more labour intensive providing more jobs but also making food more expensive.

19 of 19