B2a- Cells, Organs and Populations

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  • Created by: Miahtd13
  • Created on: 06-05-14 17:36


Animal & Plants Cells

Nucleus: contains genetic material that controls the activities of the cell

Cytoplasm: a gel-like substance where most of the chemical reactions happen. It contains    enzymes that control these reactions

Cell Membrane: holds the cell together and controls what goes in and out

Mitochondria: these are where most of the reactions for respiration take place. Respiration releases energy that the cell needs to work

Ribosomes: these are where proteins are made in the cell

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Plant Cell Only 

Rigid cell wall: made of cellulose. It supports the cell and strengthens it 

Permanent Vacuole: contains cell sap, a weak solution of sugar and salts

Chloroplasts: these are where photosynthesis occurs, which makes food for the plant. They                              contain a green substance called chlorophyll 

Yeast  It is a single-celled microorganism. A yeast cell has a nucleus, cytoplasm, and a cell membrane surrounded by a cell wall.

Bacterial Cells They are singe-celled microorganisms that have no nucleus. They contain cytoplasm and a cell membrane surrounded by a cell wall. The genetic material floats in the cytoplasm because bacterial cells don't have a nucleus

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DIFFUSION: the spreading out of particles from an area of high concentration to an area of low concentration

Diffusion occurs in solutions and gases only because the particles are free to move about randomly

An example is when gases diffuse through eachother e.g. perfume particles diffusing through air

The bigger the difference in concentration, the faster the diffusion rate.

Cell Membranes: they hold the cell together but allow dissolved substances to pass in and out by diffusion. Only very small molecules can diffuse through cell membranes though e.g. oxygen, glucose, amino acids and water. Big molecules like starch and proteins can't fit through the membrane.

Just like with diffusion in air, particles flow through the cell membrane from where there's a high concentration to where there's a low concentration. They're only moving about randomly so they go both ways but if there's a lot more particles on one side, there's a net movement from that side.

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Specialised Cells

Palisade Leaf Cells- Adapted for Photosynthesis

Packed with chloroplasts for photosynthesis. More of them are crammed at the top of the cell so they're nearer the light. Their tall shape means a lot of surface area is exposed down the side for absorbing carbon dioxide, from the air in the leaf. Their thin shape means a lot can be packed at the top of a leaf. Palisade leaf cells are grouped together at the top of the leaf where most of the photosynthesis occurs. 

Guard Cells- Adapted to Open and Close Stomata

They have a special kidney shape which opens and closes the stomata in a leaf. When the plant has lots of water the guard cells fill with it, become plump and turgid. This makes the stomata open so gases can be exchanged for photosynthesis. 

When the plant is short of water, the guard cells lose water and become flaccid making the stomata close. This helps stop too much water vapour from escaping. Thin outer walls and thickened inner walls make the opening and closing work. They're also sensitive to light and close at night to save water without losing out on too much photosynthesis. Guard cells are adapted to their function of allowing gas exchange and controlling water loss in a leaf.

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Specialised Cells.2

Red Blood Cells-Adapted to Carry Oxygen

The concave shape gives a big surface area for absorbing oxygen. It also helps them pass smoothly through capillaries to reach body cells. They're packed with haemoglobin- the pigment that absorbs the oxygen. They have no nucleus, to leave even more room for the haemoglobin. They're an important part of the blood. 

Sperm & Egg Cell- Specialised for Reproduction

The main functions of an egg cell are to carry the female DNA and to nourish the developing embryo in the early stages. The egg cell contains huge food reserves to feed the embryo. When a sperm fuses with the egg, the egg's membrane instantly changes its structure to stop any more sperm getting in. This makes sure the offspring end up with the right amount of DNA.

The function of a sperm is to get the male DNA to the female DNA. It has a long tail and streamlined head to help it swim to the egg. There are a lot of mitochondria in the cell to provide the energy needed. Sperm also carry enzymes in their heads to digest through the egg cell membrane. Sperm and eggs are very important cells in reproduction. 

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Cell Organisation

Large Multicellular Organisms are made up of Organ Systems

The process by which cells become specialised for a particular function is called differentiation. 

Differentiation occurs during the development of a multicellular organism. These specialised cells form tissues, which form organs, which form organ systems. Large multicellular organisms have different systems inside them for exchanging and exporting materials.

Similar Cells are organised into Tissues

A tissue is a group of similar cells that work together to carry out a particular function. It can include more than one type of cell. In mammals (e.g. humans) examples of tissues include:

Muscular Tissue: which contracts (shortens) to move whatever its attached to

Glandular Tissue: which makes and secretes chemicals like enzymes and hormones

Epithelial Tissue: which covers some parts of the body e.g. the inside of the stomach 

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Cell Organisation.2

Tissues are organised into Organs

An organ is a group of different tissues that work together to perform a certain function. For example the stomach is an organ made up of these tissues:

Muscular Tissue: which removes the stomach wall to churn up the food

Glandular Tissue: which makes and secretes digestive solutions to digest food. 

Epithelial Tissue: which covers the outside and inside of the stomach 

Organs are Organised into Organ Systems

An organ system is a group of organs working together to perform a particular function. For example, the digestive system breaks down food and is made up of these organs: Glands which produce digestive solutions (e.g. pancreas and salivary glands).  Stomach & Small Intestine           ( which digest food). Liver produces bile. Small intestine absorbs soluble food molecules. Large intestine absorbs water from undigested food leaving faeces. The digestive system exchanges materials with the environment, taking in nutrients and releasing substances such as bile.

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Plant Structure and Photosynthesis

Plants are made of organs like stems, roots and leaves. These organs are made of tissues. For example leaves are made of:

Mesophyll tissue: this is where most of photosynthesis in a plant occurs

Xylem and Phloem: they transport things like water, mineral ions and sucrose around the plant.

Epidermal tissue: this covers the whole plant

Carbon Dioxide + Water = Glucose + Oxygen

Photosynthesis Produces Glucose Using Sunlight

Photosynthesis is the process that produces "food" in plants and algae. The "food" it produces is glucose. Photosynthesis happens inside the chloroplasts. Chloroplasts contain a green substance called chlorophyll which absorbs sunlight ans uses its energy to convert carbon dioxide from the air and water from the soil into glucose. Oxygen is also produced as a by-product. Photosynthesis happens in the leaves of all green plants- this is largely what the leaves are for. 

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The Rate of Photosynthesis

The Limiting Factor Depends on the conditions

These factors stop photosynthesis from happening any faster

Environmental Conditions

  • At night, light is the limiting factor
  • In winter, temperature is the limiting factor
  • If it is warm and bright enough, carbon dioxide will become the limiting factor

You can do experiments to work out the ideal conditions for photosynthesis in a particular plant. The easiest type to use is a water plant like Canadian pondweed-you can easily measure the amount of oxygen produced in a given time to show how fast photosynthesis is happening (oxygen) is made during photosynthesis

You could either count the bubbles given off, or collect the oxygen in a gas syringe for more accurate results. 

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The Rate of Photosynthesis.2

Not Enough Light Slows down the rate of Photosynthesis

  • Light provides the energy needed for photosynthesis
  • As the light level is raised, the rate of photosynthesis increases steadily but only up to a certain point. Beyond the optimum, it won't make a difference to the rate, it will be temperature or carbon dioxide levels which will be the limiting factor
  • In the lab, you can change the light intensity by moving a lamp closer to or further away from your plant

Too Little Carbon Dioxide slows it down

  • Carbon dioxide is one of the raw materials needed for photosynthesis. As with light intensity, the amount of carbon dioxide will only increase the rate of photosynthesis up to its optimum point. After this carbon dioxide is no longer the limiting factor
  • As long as light and carbon dioxide are in plentiful supply, then the factor limiting photosynthesis must be temperature. There are many ways to control the amount of carbon dioxide. One way is to dissolve different amounts of sodium hydrocarbonate in the water which releases carbon dioxide
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The Rate of Photosynthesis.3

The Temperature has to be just right

  • Usually if the temperature is the limiting factor, its because its too low- the enzymes needed for photosynthesis work more slowly at low temperatures
  • But if the plant gets too hot, the enzymes it needs for photosynthesis and its other reactions will be damaged 
  • This happens at about 45 degrees Celsius
  • Experimentally, the best way to control the temperature of the flask is to put it in a water bath

How to make these experiments fair tests

  • Use a bench lamp to control the intensity of the light but don't block the light with anything 
  • keep the flask in a water bath to keep the temperature constant
  • You can't do much to the carbon dioxide levels- you would just have to use a large flask and do the experiment as quickly as you can so the plant doesn't use up too much of the carbon dioxide in the flask. If you use sodium hydrogencarbonate make sure the flask is changed each time.
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Artificially Creating Ideal Conditions for Farming

  • The most common method for artificially creating the ideal conditions for plants is to grow them in a greenhouse
  • Greenhouses help to trap the sun's heat, and make sure that the temperature doesn't become limiting. In winter a farmer/gardener might use a heater as well to keep the temperature at the ideal level. In summer it could get too hot, so they might use shades and ventilation to cool things down.
  • Light is always needed for photosynthesis, so commercial farmers often supply artificial light after the Sun goes down to allow their plants to photosynthesise for longer
  • Farmers and gardeners can also increase the level of carbon dioxide in the greenhouse. A fairly common way is to use a paraffin heater to heat the greenhouse. As the paraffin burns, it makes carbon dioxide as a by-product
  • Keeping plants enclosed in a greenhouse also keeps pests and diseases to a minimum. The farmer can add fertilisers to the soil as well to provide all the minerals needed for healthy growth.
  • Doing all of this is very expensive, but if the farmer can keep the conditions jut right for photosynthesis, the plants will grow much faster and a decent crop can be harvested more often and more can be sold. The right amount of heat, light etc has to supplied so that there isn't more than the plant needs as this would be wasting money.
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How Plants Use Glucose

For Respiration

  • Plants make glucose in their leaves
  • They then use some of the glucose for respiration
  • This releases energy which enables them to convert the rest of the glucose into various other useful substances, which they can use to build new cells and grow.
  • To produce some of these substances, they also need to gather a few minerals from soil

Making Cell Walls    

  • Glucose is converted into cellulose for making strong cell walls especially in a rapidly growing plant

Making Proteins

  • Glucose is combined with nitrate ions absorbed from the soil to make amino acids, which are then made into proteins
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How Plants Use Glucose.2

Stored In Seeds

  • Glucose is turned into lipids (fats and oils) for storing in seeds
  • Sunflower seeds, for example contain a lot of oil- we get cooking oil and margarine from them

Stored as Starch

  • Glucose is turned into starch and stored in roots, stems and leaves, ready for use when photosynthesis isn't happening like during the winter.
  • Starch is insoluble which makes it much better fir storing than glucose- a cell with a lot of glucose in it would draw in a lot of water and well up becoming turgid. 
  • Potato and parsnip plants store a lot of starch underground over the winter so a new plant can grow from it the following spring. We eat the swollen storage organs.
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Distribution of Organisms

A habitat is where an organism lives

The distribution of an organism is where an organism is found. 

Distribution is affected by environmental factors such as:

  • Temperature, availability of water
  • Availability of oxygen and carbon dioxide
  • Availability of nutrients and amount of light

An organism might be more common in one area than another due to differences in environmental factors between the two areas. E.G. in a field, you may find that daisies are more common in the open than under trees because there's more light available in the open

There are a couple of ways to study the distribution of an organism:

You can measure how common an organism is in two sample areas using quadrats and compare them. Or you can study how the distribution changes across an areas e.g. by placing the quadrat along a transect.

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Distribution of Organisms.2

Using Quadrats to Study the Distribution of Small Organisms

  • To compare how common an organism is in two sample areas you:
  • Place a 1 m squared quadrat on the ground at a random point within the first sample area e.g. divide the area into a grid and use a random number generator to pick coordinates
  • Count all the organisms within the quadrat
  • Repeat the first steps as many times as you can
  • Work out the mean of organisms per quadrat within the first sample area by dividing the total no. of organisms by the number of quadrats 
  • Repeat these steps in the second sample area
  • Finally compare the two means

Work Out Population Size in One Sample Area

  • Work out the mean no. of organisms per metre squared
  • Then multiply the mean by the total area in metres squared of the habitat
  • e.g. if the area of an open field is 800 metres squared and there are 22 daisies per metre squared then the size of the daisy population is 22 x 800=17 600
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Distribution of Organisms.3

Use Transects to Study the Distribution of Organisms Along a Line 

  • Mark out a line in the area you want to study using tape measure
  • Then collect data along the line by counting all the organisms you're looking at that touch the line

Things to think about when collecting environmental data:


  • You have to make sure the results are reliable which means they are repeatable and reproducilbe 
  • To make results more reliable you need to take a large sample size as bigger samples are more representative of the whole population
  • Use random samples. If all the samples are taken in one spot and everywhere else is different, the results won't be reproducible 
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Collecting Environmental Data


  • For your results to be valid they must be reliable and answer the original question
  • To answer the original question, you need to control all the variables
  • The question you want to answer is whether a difference in distribution between two sample areas is due to a difference in one environmental factor
  • If all the other variables that could be affecting the distribution have been controlled you'll know whether a difference is caused by the environmental factor or not
  • If you don't control the other variables you won't know whether any correlation you've found is because of chance, because of the environmental factor you're looking at or because of a different variable- the study won't give you valid data
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