The Nature and Variety of Living Organisms

KLO 1.2: describe the common features shown by eukaryotic organisms: plants, animals, fungi, protoctists. 

Eukaryotic = organisms that contain a nucleus, their cells contain nucleus surrounded by a membrane along with other membrane bound organelles. 

• They can be multicellular or unicellular, must have a nucleus with a distinct membrane

Plants

• Are multicellular organisms consisting of many different types of cells

• Plants carry out the process of photosynthesis through its chloroplast, this is called autotroph which is when an organism makes its own food, and it feeds on this. 

• Stores carbohydrates (energy) as starch or sucrose

  • Starch is a complex carbohydrate/sugar stored within various cells 

• Contains cell walls made of cellulose

Examples: Herbaceous Legumes, Cereal, Flower Plants

Contains Extra Organelles Than Animal Cells

Animals

• Are multicellular organisms

• Cells do not contain cell walls, hence cells can change shape

• Cells do not contain chloroplast, hence are unable to carry out photosynthesis

  • Feed on organic material/substance made by other living things: heterotroph

• Stores carbohydrates as glycogen, commonly stored in liver of most animals

• Has nervous coordination

• Are able to move from place to place

Examples: Mammals, Insects, Amphibian, Reptiles

Contains Organelles In Both Animal and Plant Cells

Fungi

• Usually multicellular but some are unicellular (yeasts) 

• These organisms can not carry out photosynthesis as they do not contain chloroplasts

• Bodies are usually organised into networks called mycelium

  • make of thread like structure known as hyphae which contain many nuclei

  • Contain fruiting body of mushroom/toadstool

• Cells have cell walls made of chitin

• Feed by secreting extracellular digestive enzymes onto food material, usually decaying organic matter, then absorbing digested molecules - Saprotrophic nutrition

  • Some are parasitic and feed in living material

• Stores carbohydrates as glycogen

• Do not have nervous coordination

Examples: Moulds, Mushroom, Yeasts

Protoctists

• Diverse group of organisms that don’t belong to other eukaryotic kingdoms

• Mainly extremely microscopic and single celled but some aggregate into large forms (colonies/chains of cells)

• Some have features that make them look like animal cells, protozoa

  • E.g Plasmodium - pathogenic protist that causes malaria

• Some have plant cell like feature, algae

  • E.g Chlorella

Meaning some protoctista photosynthesise and some feed on organic material made by other organisms

Examples: Amoeba, Paramecium, Plasmodium, Chlorella

(Should be able to recognize, draw, and interpret images of cells, so practice drawing & labelling in revision)

KLO 1.3: describe the common features shown by eukaryotic organisms such as bacteria

Prokaryotic = organisms are always unicellular and do not contain nucleus, instead nuclear material of prokaryotic cells are found in found in the cytoplasm

Bacteria

• Microscopic unicellular organisms that are the largest group of prokaryotes

• Contain cell walls made of peptidoglycan, cell membranes, cytoplasm, and plasmids ( small circular loops of DNA that contain genes)

• lack a nucleus, but contains a nucleoid (circular chromosome of DNA)

• Does not have a mitochondria and other membrane bound organelles like eukaryotic cells

• Some water bound bacteria contain flagellums, a tail like projection allowing flagella movement

• Some can carry out photosynthesis (even without chloroplast as they can posses chlorophyll and enzymes needed to synthesise sugars from carbon dioxide

Examples: 

• Lactobacillus: a rod shaped bacterium used in the production of yoghourt from milk

• Pneumococcus - a spherical bacteria that acts as the pathogen causing pneumonia

Eukaryotic Cells & Prokaryotic Cells

KLO 1.4: understand the term pathogen and know that pathogens may include fungi, bacteria, protoctista, and viruses

Pathogens = any microorganisms that causes disease in other organisms 

• Not all species of bacteria, protoctists, fungi, and pathogens

• But all viruses are pathogens as they can only exist by living inside living cells of other organisms or by using these cells to create more viruses

• Pathogenic fungi, Pathogenic bacteria, Pathogenic Protoctista

Virus

• Are not alive as they do not carry out the 8 life’s processes

• Are parasitic and can only reproduce in living cells by taking over host cell’s metabolic pathways to make multiple copies of itself

• Share biological characteristics

  • Wide variety of shapes and sizes

  • Microscopic particles that are much smaller than bacteria

  • Infect every type of living organism

  • Has no cellular structure

  • Contain a protein coat called capsid

  • Contains 1 type of nucleic acid, either DNA or RNA

  • Contains a viral envelope, similar to the cell membrane

Examples: 

• Tobacco Mosaic Virus (TMV): Causes discoloration on leaves of tobacco plants by preventing the formation of chloroplast

• HIV Virus: Causes Aids

• Influenza Virus: Causes flue

Ecology and the Environment

KLO 4.1: understand the terms population, community, habitat, ecosystem

Organism/Individual

• Individual living things, all have same basic needs

Population

• A group of organisms of the same species living in the same place at the same time

Community

• All of the different populations living in the same area at the same time

  • In a community, species depend on other species for food, shelter, pollination, seed dispersal, etc

  • If 1 is removed, it impact the whole community, this shows interdependence

Habitat

• The place where an organism lives

Ecosystem

• All the biotic and abiotic factors in a community that interact with each other in an area at the same time

  • Biotic factors include all the living component 

    • Plants, animals, etc

  • Abiotic factors include all the non living components

    • Light intensity, mineral/nutrient concentration, water, etc

• Ecosystems can vary greatly in size and scale

  • An ecosystem can be as small as a garden pond and as large as the whole of antarctica

KLO 4.2: practical: investigate population size of an organism in 2 different areas using quadrat

Ecology is a branch of biology that studies the distribution and abundance of species along with interactions between species and species interactions within abiotic environment

Ecologist study these interactions by investigation ecosystems

1 Piece of equipment that might be used to investigate population size is a quadrat

Quadrat

• Square frames made of wood/wire that can be a variety of size (0.2 m2 to 1 m2

  • Are placed on the ground and organism within are recorded

• Plant species are commonly studied using quadrats to identify abundance

• Abundance can be measured by recording

  • Number of an individual species: total number of individuals of a single species

  • Species richness: total number of different species

  • Percentage cover: approximate percentage of quadrat area in which an individual species covers (used when it is difficult to count individually)

Investing population size in 2 different areas using quadrats

Method

1. Use 2 tape measures to lay out survey area (ie 10 m by 10 m) in chosen habitat

2. Use random number generator to create a set or coordinates to place 1st quadrat (ie. (4,5) -> 4 m along x axis and 5 m along y axis)

3. Count number of individual of chosen species (plant) that are found in quadrat

4.  Repeat step 2 and 3 to a total of 10 times and take average of results for accuracy

5. Move survey area number 2 and repeat steps 2 - 4

6. Calculate population of species using: estimated population = total area/ area sample x total number of species countred

   1. Ie. survey area 1 = 100(total area was 10 m x 10 m) / 10 (each quadrat square was 10 ) x 21 (total number of individuals counted) 

Results

Once averages are calculated, abundance of study species in each area can be calculated

Species abundance is affected by

• Biotic Factors like: competition, predator-prey relationships, interactions with other organisms

• Abiotic factors like: light intensity, mineral availability, water availability, pH, temperature, salinity

Limitation

• It can be easy to miss individuals in quadrat, especially is covered by leaves

  • Use pencil/stick to move leaves out of the way

• Identifying species may be tricky

  • Use a species key to identify species

Apply CORMS

Change —----> What is being changed in the investigation (Independent Variable)

Organism —----> A control relating to the organism being used

Repeat —----> Repeats must be carried out for reliable results

Measurement 1 —----> How will you measure the measurement (Dependent Variable)

Measurement 2 —----> What time scale is used (Dependant Variable)

Same —----> What will be controlled/ kept the same  in the investigation/experiment (Control Variable)

Independent variable – the variable that is altered during a scientific experiment.

Dependent variable – the variable being tested or measured during a scientific experiment.

Controlled variable – a variable that is kept the same during a scientific experiment. Any change in a controlled variable would invalidate the results.

In this Experiment

C —----> changing the study area in which data was collected

O—----> count the same species of organism in each quadrat

R —----> repeated a total of 10 times for reliable results

M1 —----> counting the the total number of individual from designated species found across quadrat

M2 —----> isn’t relevant in this scenario

S —---->  control size of quadrat, random way quadrats are placed on the study area and the day the results were collected

KLO 4.5: understand how biotic and abiotic factors affect the population size and distribution of organisms

Biotic = Living 

Abiotic = Non - Living

Note: when answering questions regarding charts/graphs, should always give not only analysis but specific examples of where it is happening in the chart and why it is connected to and contributes to the point

KLO 4.6: understand different trophic level names including producers, primary, secondary, tertiary consumers, and decomposer

Trophic Levels

• Describes feeding relationships between organisms

• Energy flows from sun into producers in the form of light (sunlight)

  • Produces convert light energy into chemical energy, this chemical energy gets passed on through the trophic levels

• Eventually all energy is transferred to the environment as being passed on is lost or used up by the organism

  • Is a non cyclic process as it is “lost” to the environment

  • Is a direct contrast to chemical elements an organism 

Producers: produce their own organic nutrients through energy from sunlights, organism that makes its own energy rich food compounds

Primary Consumer: Feed on producers/Consumer producers

Secondary Consumer: Feed/Consume primary consumers

Tertiary,Quaternary, quinnary …. Apex predator/top carnivores

Decomposers: Organisms that break down/consume dead materials helping recycle it

KLO 4.10: describe stages in the carbon cycle including respiration, photosynthesis, decomposition, combustion

Carbon Cycle

• Carbon is taken out of the atmosphere in the form of carbon dioxide by plants to be used in photosynthesis

• Passed onto animals and microorganisms by feeding/consuming

• Returned/Released back into the atmosphere in the form of carbon dioxide by plant, animals, and decomposers through respiration

• If animals and plants die in conditions where decomposing organisms aren’t present, carbon in their body can be converted over millions of year of significant pressure, fossilising into fossil fuels

• When fossil fuels are burned, carbon combines with oxygen and carbon dioxide is released

  • Increased use of fossil fuels contributes to the increase CO2 content in the atmosphere

Stages

Photosynthesis: “fixes” carbon atoms into organic compounds like starch and glucose

Respiration: produces inorganic CO2 from organic compound (mainly carbohydrates) as they are broken down to release energy

Decomposition: Carbon in dead decaying matter is broken down and once in decomposer, is respired into carbon reserves

Combustion: releases carbon dioxide into atmosphere as fossil fuels are burned

KLO 4.16: understand biological consequences of water by sewage

Bioaccumulation

Concentration of a substance, ie pesticides/herbicides, in an organism by not breaking down. Building up/accumulating into the tissues of the organism over time

Biomagnification

Increase in concentration of toxic substances as it moves through the food chain

Water Pollution by Sewage

• Aerobic bacteria in the water, polluted by the sewage, use up the majority of the dissolved oxygen in the water as it decomposes the organic material, reduction in oxygen levels leads to death of larger aquatic organisms

  • Only species like anaerobic bacteria are able to survive

  • Water becomes more oxygenated again as it moves towards and mixes with cleaner waters.

• Untreated sewage contains pathogenic bacteria which is a danger to human health

KLO 4.17: understand biological consequences of eutrophication caused by leached minerals from fertilisers

Eutrophication

Leaching: process of nutrients removed from soil as excess water passes through

Eutrophication

• Excess minerals/nutrients enter a water body in 2 ways

  • Artificial nitrate/phosphate fertilisers-causes algal blooms

  • Sewage - increased decomposing done by decomposing bacteria leads to decreasing levels of dissolved oxygen

  • Common misconception but sewage does not cause algal blooms

Structure & Function in Living Things

KLO 2.1: describe levels of organisation in organisms: organelle, cell, tissue, organs, organ systems

In a multicellular organism…

Organelle

• A component within a cell that carries out a specific task

Cell

• Basic functional and structural unit in a living organism

Tissues

• A group of cells of similar structure working together to perform a particular function

Organ

• Made from groups of tissues working together to perform a particular function

Organ Systems

• Several different organs with related functions working together to perform body functions within the organisms

Examples

• Plant: chloroplast - palisade cell - epidermis mesophyll - leaf - shoot system

• Animal/Human:  mitochondria - muscle cell - muscle - heart - circulatory system

KLO 2.2: describe cell structures including nucleus, cytoplasm, cell membrane, cell wall, mitochondria, ribosomes, vacuole

Cell structure

• Cells are separated from surrounding environment by cell membrane

• Within cell membranes is the cytoplasm

• Eukaryotic cells have organelles contained within their cytoplasm

  • Organelles are subcellular compartments where specific processes take place within the cell

  • Following organelles are present

    • Nucleus

    • Mitochondria

    • Ribosomes

  • Plant cells contain a few additional organelles

    • Cell walls

    • Chloroplasts

    • permanent vacuole

KLO 2.3: describe the functions of nucleus, cytoplasm, cell membrane, cell wall, mitochondria, ribosomes, vacuole

Organelle Functions

KLO 2.4: know the similarities and differences between plant and animal cell structures

KLO 2.7 identify the chemical elements present in carbohydrates, proteins and lipids (fats and oils) 

• Most of the molecules in living organisms fall into three categories: carbohydrates, proteins and lipids

• These all contain carbon and so are described as organic molecules

Chemical Elements

KLO 2.8 describe the structure of carbohydrates, proteins and lipids as large molecules made up from smaller basic units: starch and glycogen from simple sugars, protein from amino acids, and lipid from fatty acids and glycerol

Carbohydrates

• Molecules made of carbon (carbo) hydrogen (hydr) oxygen (ate)

• Used by the body for respiration to release energy

• Humans get most of their carbohydrates in the form of starch

  • Potatoes, rice, pasta

• Starch is a large insoluble carbohydrate molecule made by plants

  • Animals store carbohydrates as glycogen

Insoluble: Large carbohydrates like starch need to be broken down by enzymes in digestion to release simple sugars needed for respirations. Ie. glucose

• Glycosidic bonds join/bond simple monomer sugars.

• A monosaccharide is a simple sugar e.g. glucose (C6H12O6) or fructose

  • Glucose molecules contain lots of energy which can be released in respiration by breaking the bonds between the carbon atoms

  • Glucose = hexagon simple sugars bonded together with glycosidic bonds into carbohydrate molecules

  • Fructose = pentagon simple sugars bonded together with glycosidic bonds into carbohydrate molecules

• A disaccharide is made when two monosaccharides join together

  • Maltose is formed from two glucose molecules

  • Sucrose is formed from one glucose and one fructose molecule

• A polysaccharide/polypeptide is formed when lots of monosaccharides join together

  • Starch, glycogen or cellulose are all formed when lots of glucose molecules join together

  • Polysaccharides are insoluble and therefore useful as storage molecules

Lipids

• Molecules made of carbon, hydrogen, oxygen

• Lipids are divided into fats (solids at room temperature) and oils (liquids at room temperature)

  • Food high in animal fats: meat, butter, cheese, milk, eggs, fish

  • Foods high in plant oils: sunflower oil, olive oil, rapeseed oil, margarine

• Uses

  • Makes cell membrane

  • Provides insulation

  • Protects organs

  • Stores energy

• Most fats (lipids) are made up of triglycerides

  • One glycerol molecule chemically bonded by ester bonds to three fatty acid chains

  • The fatty acids vary in size and structures

• Bonded by ester bonds

Proteins

• Molecules made of carbon, hydrogen, oxygen, and nitrogen

• All cells contain protein

  • Needed for tissue growth and repair

• Animal foods high in protein

  • Meat

  • Fish

  • Eggs

  • Cheese

• Plants generally contain less protein that animals, but some have more

  • Beans

  • Peas

  • Nuts

• Proteins are large molecules made up of amino acids

  • 20 different types of amino acids used to make up 1000s of different proteins

  • Different proteins have different amino acid sequences resulting in them being different shapes

  • Even a small difference in the amino acid sequence will result in a completely different protein being formed

  • The different sequences of amino acids cause the polypeptide chains to fold in different ways and this gives rise to the different shapes of proteins

  • In this way, every protein has a unique 3-D shape that enables it to carry out its function

  • The shape of a protein determines its function

• Peptide bonds are used to bond amino acids

KLO 2.9 practical: investigate food samples for the presence of glucose, starch, protein and fat

• There are various chemical tests which can detect which molecules are present

• Before you can carry out any of the food tests described below, you may need to prepare a food sample first (especially for solid foods to be tested)

• To do this:

  • Break up the food using a pestle and mortar

  • Transfer to a test tube and add distilled water

  • Mix the food with the water by stirring with a glass rod

  • Filter the mixture using a funnel and filter paper, collecting the solution

  • Proceed with the food tests

Iodine - Starch

1. Add a small amount of food sample into a test tube

2. Add a few drops of iodine solution to the food sample

3. A sample containing starch  will show a colour change from orange-brown to blue-black

Benedict’s Solutions - Glucose (a reducing sugar) 

1. Take a food sample and add it into a test tube

2. Add Benedict's solution to the sample solution in a test tube

3. Heat the tube in a hot water bath for 5 minutes

4. Remove from heat and observe the colour

5. A positive test will show a colour change from blue to orange to brick red

Biuret Solution - Protein 

1. Take a food sample (must be in liquid form)  and add it into a test tube

2. Add drops of Biuret solution to the food sample

3. A positive test will show a colour change from blue to violet / purple

Emulsion Test - Lipids

1. Take a food sample and add it into a test tube

2. Add a few drops of ethanol to the food solution

3. Shake the test tube with a bung on the top and leave for one minute

4. Pour the ethanol into another test tube of water

5. If the solution turns cloudy, the food contains lipids. This cloudiness is called an emulsion.

Results Table

CORMS Evaluation

C: Changing the type of food in the sample

O: isn’t relevant in this experiment

R: Repeat the investigation several times for each food sample to ensure a reliable result

M1: presence of the specific biological molecule in each food type by noting the colour change

M2: ....after testing with each specific testing agent

S: control the volume of each testing agent used, the quantity of the food sample, the concentration of the testing agents, the temperature of the water bath for the Benedict's test.

Tips

• When describing food tests in exam answers, make sure you give the starting colour of the solution and the colour it changes to for a positive result.

KLO 2.10 understand the role of enzymes as biological catalysts in metabolic reactions

Biological Catalysts

Biological Catalyst: A substance that increase the rate of a chemical reaction without being changed or consumer in the reaction

Enzymes

• proteins that act as biological catalysts to speed up the rate of a chemical reaction without being changed or used up in the reaction

  • Amino acid chains are folded, each enzyme is folded in a different shape help by peptide bonds between amino acids

  • They are biological because they are made in living cells

• Necessary to all living organisms as they maintain reaction speeds of all metabolic reactions at a rate that can sustain life

  • Metabolic Reaction: All the chemical or mechanical reactions that happen within a cell or organism to keep it alive

  • Ie.  if we did not produce digestive enzymes, it would take around 2 - 3 weeks to digest one meal; with enzymes, it takes around 4 hours

• Activation energy: amount of energy needed for a reaction

  • Lowering activation energy means not as much energy is needed as before. Reactions can happen sooner as energy for reaction is smaller

• Controls all reactions that happens in a cell

How they Work

Active Site: Part of the enzyme that has the same shape as the substrate

Substrate: Molecule enzyme wants to change, correctly fits into the active site

• Specific to one particular substrate(s) as the active site of the enzyme, where the substrate attaches, is a unique shape to the substrate

• When the substrate moves into the enzyme’s active site they become known as the enzyme-substrate complex

• After the reaction has occurred, the products leave the enzyme’s active site as they no longer fit it and it is free to take up another substrate

1. Enzymes and substrates randomly move about in solution

2. When an enzyme and its complementary substrate randomly collide an enzyme-substrate complex forms, and the reaction occurs

3. A product (or products) forms from the substrate(s) which are then released from the active site. The enzyme is unchanged and will go on to catalyse further reactions

KLO 2.11 understand how temperature changes can affect enzyme function, including changes to the shape of active site

Temperature

• Enzymes are proteins and have a specific shape, determined by the amino acids that make the enzyme and held in place by bonds

• This is extremely important around the active site as the specific shape is what ensures the substrate will fit into the active site and enable the reaction to proceed

• Enzymes work fastest at their ‘optimum temperature’

  • Optimum temperature: temperature in which the maximum number of successful collisions occur

  • In the human body, the optimum temperature is 37⁰C

• Increasing the temperature towards the optimum increases the activity of enzymes as the more kinetic energy the molecules have the faster they move and the number of collisions with the substrate molecules increases, leading to a faster rate of reaction

• This means that low temperatures do not denature enzymes, they just make them work more slowly due to a lack of kinetic energy

• Heating to high temperatures (beyond the optimum) will break the bonds that hold the enzyme together and it will lose its shape

  • This is known as denaturation

  • Substrates cannot fit into denatured enzymes as the shape of their active site has been lost

  • Denaturation is irreversible - once enzymes are denatured they cannot regain their proper shape and activity will stop

KLO 2.12 practical: investigate how enzyme activity can be affected by changes in temperature

Equipement

• Amylase

• Starch

• Iodine

• Thermometer

• Small testing tubes

• Beaker filled with water

• Dropper

• Stopwatch

• Spotting tile

Method

1. Place around 5 cm cubed of starch into a testing tube and 1 cm cubed of amylase in a testing tube into a water bath of around 22 degrees celsius

2. Add drops of iodine into the spotting tile

3. Pour amylase into the  starch tube

4. Immediately take an put a couple of drops in one tile 

5. Take one minute intervals to add drops to the next tile till the tile remains the same colour as the amylase has broken down the starch

6. Heat up the water bath to around 47 degrees celsius

7. Mix together a new amount of starch and amylase

8. Immediately take an put a couple of drops in one tile 

9. Take one minute intervals to add drops to the next tile until the  tile remains the same colour as the amylase has broken down the starch, should be quicker as amylase is heated

10. Use various temperatures to identify the reactions

Result

• Amylase is an enzyme which breaks down starch

• The quicker the reaction is completed, the faster the enzyme is working

• This investigation shows:

  • At the optimum temperature, the iodine stopped turning blue-black the fastest

    • This is because the enzyme is working at its fastest rate and has digested all the starch

• At colder temperatures (below optimum), the iodine took a longer time to stop turning blue-black

  • This is because the amylase enzyme is working slowly due to low kinetic energy and few collisions between the amylase and the starch

• At hotter temperatures (above optimum) the iodine turned blue-black throughout the whole investigation

  • This is because the amylase enzyme has become denatured and so can no longer bind with the starch or break it down

CORMMS Evaluation

C: Changing the temperature of each round, impacting rate of reaction

O: isn’t relevant in this experiment

R: Repeat the investigation several times for each temperature to ensure a reliable result

M1: measure the time taken for the reaction

M2: the iodine to remain the same colour

S: control the concentration and volume of starch solution, iodine and amylase used in the investigation

Tips

• Describing and explaining experimental results for enzyme experiments is a common type of exam question so make sure you understand what is happening and can relate this to changes in the active site of the enzyme when it has denatured, or if it is a low temperature, relate it to the amount of kinetic energy the molecules have.

KLO 2.13 understand how enzyme function can be affected by changes in pH altering the active site

pH

• An enzyme is denatured once it is placed beyond its optimum pH, the shape get denatured

• Acids or alkali interfere with the enzyme’s peptide bonds holding the enzyme together

• The optimum pH for most enzymes is 7

  • Some enzymes that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2)

  • Some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)

• If the pH is too high or too low, the bonds that hold the amino acid chain together to make up the protein can be disrupted/destroyed

• This will change the shape of the active site, so the substrate can no longer fit into it, reducing the rate of activity

• Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop

Tip:

• Remember the terminology when writing about enzymes is very important. Make sure you refer to an enzyme becoming 'denatured' not 'dying'.Being able to describe AND explain the effect of each environmental condition on enzyme action is key.Practise describing and explaining using the graphs and then check your descriptions against your notes.

KLO 2.15 understand the processes of diffusion, osmosis and active transport by which substances move into and out of cells

In order for cells to carry out the chemical reactions it needs to, substance must enter and leave the cell. This happens in the following 3 ways

• Diffusion

• Osmosis

• Active Transport

Diffusion

• Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration

  • Concentration gradient: high to low concentration

• Diffusion happens when a substance is more concentrated in one area than another. This difference is concentration is the concentration gradient

  • Molecules move randomly due to kinetic energy, however net movement follows the concentration gradient. 

• Diffusion in living organisms

  • For living cells, the principle of the movement down a concentration gradient is the same, but the cell is surrounded by a cell membrane, which can restrict the free movement of the molecules

  • The cell membrane is a partially permeable membrane - this means it allows some molecules to cross easily, but others with difficulty or not at all

    • The simplest sort of selection is based on the size of the molecules (i.e. smaller molecules can diffuse across the membrane but larger molecules cannot)

• Diffusion helps living organisms to:

  • Obtain many of their requirements

  • Get rid of many of their waste products

  • Carry out gas exchange for respiration

Tip:

• Remember that diffusion is a passive process, so when it occurs in a living organism, the cells of that organism do not provide the particles involved with energy to diffuse. The particles that are moving about randomly have their own kinetic energy.

Osmosis

• Movement of water from a dilute solution to a more concentrated one across the partially permeable cell membrane

  • Water moves from an area of high water potential to an area of low water potential

  • Partially permeable: this membrane has pores through which very small molecules, including water, can pass, but not larger molecules

• Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane

• All cells are surrounded by a cell membrane which is partially permeable

• Water can move in and out of cells by osmosis

• In doing this, water is moving down its concentration gradient

• The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)

• a dilute solution has a high water potential (the right-hand side of the diagram below) and a concentrated solution has a low water potential (the left-hand side of the diagram below)

Osmosis in Animal Cells

• Animal cells lose and gain water as a result of osmosis

• As animal cells do not have a supporting cell wall, the results of osmosis can be severe

• If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up)

• If an animal cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis as it has no cell wall to create turgor pressure

• It will continue to gain water until the cell membrane is stretched too far and it bursts

• It is important that osmosis is carefully controlled in organisms to avoid damage to cells through lysis

  • the disintegration of a cell by rupture of the cell wall or membrane

• The human body is adapted to maintain the optimum osmotic balance using processes such as sweating or increasing and decreasing urine concentration

  • This is all part of osmoregulation

Osmosis in Plant Cells

• Once a plant cell is placed in pure water, water enters the cell through osmosis and the cell becomes turgid 

• Plant cells lose or gain water as a result of osmosis

• As plant cells have a supporting cell wall, they are protected from cell lysis

• If a plant cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis

  • The vacuole gets smaller and the cell membrane shrivels away from the cell wall

  • It becomes flaccid or plasmolysed (shrivelled up)

• If a plant cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis

  • The vacuole gets bigger, pushing the cell membrane against the cell wall

  • The plant cell is described as being turgid or as containing a high turgor pressure (the pressure of the cytoplasm pushing against the cell wall)

• Water entering the cell by osmosis makes the cell rigid and firm

• This is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant - making the plant stand upright with its leaves held out to catch sunlight

• If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts

Tips:

• Osmosis refers only to the movement of water molecules, so if in an exam you are talking about the movement of water, make sure you mention osmosis as this will often earn you a mark. The best explanations to do with osmosis will refer to water potential, so if you are aiming for a 7, 8 or 9 you will need to understand the concept and use it in your explanations.

Active Transport

The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration

• Energy is needed because particles are being moved against a concentration gradient, in the opposite direction from which they would naturally move (by diffusion)

• Active transport across the cell membrane involves protein carrier molecules embedded in the cell membrane

• Movement of molecules from a low concentration to a high concentration against the concentration gradient

• Energy is required for movement to occur, energy from respiration

• Molecules here are being transported against the concentration gradient, from a region of lower concentration (outside the cell) to a region of higher concentration (inside the cell)

Active Transport in Animals

• Food molecules (such as the sugar glucose) can be absorbed across the wall of the small intestine by diffusion, but this is dependent on a concentration gradient existing between the lumen of the intestine and the bloodstream

• Active transport allows molecules such as glucose to be transported into the bloodstream from the lumen of the small intestine (the gut) when the concentration of sugar molecules in the blood is higher

• The active uptake of glucose by epithelial cells in kidney tubules in the kidney nephron allows for the reabsorption of glucose back into the blood so that none is lost in the urine

• Sugar molecules are used in respiration to release energy for cells to function

Active Transport in Plants

• Root hair cells lining the surface of plant roots need to move minerals such as magnesium ions from a region of lower concentration (the very dilute solution of minerals in the soil surrounding the roots) to a region of higher concentration (inside the cytoplasm of the cell)

• Mineral ions are needed by plants to function

  • Magnesium ions are required to make chlorophyll

  • Nitrate ions are needed to make amino acids for protein synthesis (and subsequently growth)

KLO 2.16 understand how factors affect the rate of movement of substances into and out of cells, including the effects of surface area to volume ratio, distance, temperature and concentration gradient

Factors influencing Diffusion

• Surface area to volume ratio

  • The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface

  • Many cells which are adapted for diffusion have increased surface area in some way 

    •  e.g. root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion)

• You should be able to calculate and compare surface area to volume ratios

• Diffusion distance

  • The smaller the distance molecules have to travel the faster transport will occur

  • This is why blood capillaries and alveoli have walls which are only one cell thick, ensure the rate of diffusion across them is as fast as possible

• Temperature

  • The higher the temperature, the faster molecules move as they have more kinetic energy (movement)

  • This results in more collisions against the cell membrane and therefore a faster rate of movement across them

• Concentration gradient

  • The greater the difference in concentration on either side of the membrane, the faster movement across it will occur, faster overall diffusion

  • This is because on the side with the higher concentration, more random collisions against the membrane will occur

Tips:

• You should have carried out investigations into the factors that influence the rate of diffusion and as so should be able to use the information above to explain experimental results in an exam. You should also be able to plan and carry out an experiment which can investigate the effect of one of these factors.

KLO 2.17 practical: investigate diffusion and osmosis using living and non-living systems

Equipment/Apparatus

• Potatoes

• Knife

• Sucrose solutions (from 0 Mol/dm3 to 1 mol/dm3)

• Test tubes

• Balance

• Paper towels

• Ruler

• Test tube rack

Method

1. Make up a dilution series of sugar solutions in the range of 0.2M – 1.0M as follows:

2. Set up 6 labelled test tubes with 10cm3 of each of the sucrose solutions

3. Using the knife and ruler, cut 6 equally-sized cylinders of potato

4. Blot each one with a paper towel and weigh on the balance to the nearest 0.1g

5. Put 1 piece into each concentration of sucrose solution

6. After 40 mins, remove them, blot with paper towels and reweigh them

7. Calculate the percentage change in mass.  

8. Plot a graph of the percentage change in mass against the concentration of the sugar solution 

Results

• The potato cylinder in the distilled water will have increased its mass the most as there is a greater concentration gradient in this tube between the distilled water (high water potential) and the potato cells (lower water potential)

• This means more water molecules will move into the potato cells by osmosis, pushing the cell membrane against the cell wall and so increasing the turgor pressure in the cells which makes them turgid - the potato cylinders will feel hard

• The potato cylinder in the strongest sucrose concentration will have decreased its mass the most as there is a greater concentration gradient in this tube between the potato cells (higher water potential) and the sucrose solution (lower water potential)

• This means more water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the cylinder - the potato cylinders will feel floppy

• If looked at underneath the microscope, cells from this potato cylinder might be plasmolysed, meaning the cell membrane has pulled away from the cell wall

CORRMS Evaluation

• C: Mass of the potato cylinders and the concentration of sucrose in the solutions

• O: The potato cylinders will all be taken from the same potato or potatoes of the same age

• R: We will repeat the investigation several times to ensure our results are reliable

• M1: difference in mass of potato cylinder before and after sugar solution

• M2: potatoes are left for 40 mins in sugar solution

• S: control the volume of sucrose solution, size/dimensions of potatoes, type of potatoes, amount of time left in the solution. 

Tips:

• Questions involving osmosis experiments are common and you should be able to use your knowledge of these processes to explain the results.Don’t worry if it is an experiment you haven’t done – simply figure out where the higher concentration of water molecules is – this is the solution with the higher water potential - and explain which way the molecules move due to the differences in water potential.

KLO 2.18 understand the process of photosynthesis and its importance in the conversion of light energy to chemical energy

Photosynthesis

• Photosynthesis happens in the chloroplast of a plant cell

• Chloroplasts contain chlorophyll which traps energy from the sun and uses it to turn water and carbon dioxide into glucose for the plant

• Photosynthesis is an endothermic reaction in which energy from sunlight is transferred to the chloroplasts in green plants

  • Energy from sunlight is absorbed by chlorophyll, a green pigment found inside chloroplasts

  • Green plants use this energy to make the carbohydrate glucose from the raw materials carbon dioxide and water

  • At the same time, oxygen is made and released as a waste product

• Photosynthesis can be defined as the process by which plants manufacture carbohydrates from raw materials using energy from light

• Plants are

  • Autotrophs – they can make complex molecules (glucose) from simple molecules (carbon dioxide and water)

  • Producers - they can make their own food and so are the first organism at the start of all food chains

The products of photosynthesis

• Plants use the glucose they make as a source of energy in respiration

• They can also use it to

  • Produce starch for storage

  • Synthesise lipids for an energy source in seeds

  • To form cellulose to make cell walls

  • Produce amino acids (used to make proteins) when combined with nitrogen and other mineral ions absorbed by roots

  • Stored in fruit as sucrose

Tips:

• If asked for the raw materials required for photosynthesis, the answer is carbon dioxide and water.Although required for the reaction to take place, light energy is not a substance and therefore cannot be a raw material.

KLO 2.19 know the word equation and the balanced chemical symbol equation for photosynthesis

Word Equation

      light

Carbon dioxide + water —---------->  Glucose + Oxygen

    Chlorophyll

Carbon Dioxide

• Diffuses into leaves through stomata

Water

• Taken up by the roots and transported through the xylem into the leaves

Glucose

• Used to make substance needed in the plant

• Used for respiration

Oxygen

• Diffuses out of the leaf through the stomata

• Used in respiration

Balanced Chemical Equation

     light

6CO2 + 6H20 —---------->  C6H1206 + 6O2

Chlorophyll

• Six carbon dioxide molecules combine with six water molecules to make one glucose molecule and six oxygen molecules

Tips:

• The photosynthesis equation is the exact reverse of the aerobic respiration equation so if you have learned one you also know the other one! You will usually get more marks for providing the balanced chemical equation than the word equation.

KLO 2.20 understand how varying carbon dioxide concentration, light intensity and temperature affect the rate of photosynthesis

• Plants do not have unlimited supplies of their raw materials so their rate of photosynthesis is limited by whatever factor is the lowest at that time

• So a limiting factor can be defined as something present in the environment in such short supply that it restricts life processes

• There are three main factors that limit the rate of photosynthesis:

  • Temperature

  • Light intensity

  • Carbon dioxide concentration

• Although water is necessary for photosynthesis, it is not considered a limiting factor as the amount needed is relatively small compared to the amount of water transpired from a plant so there is hardly ever a situation where there is not enough water for photosynthesis

• The number of chloroplasts or the amount of chlorophyll in the chloroplasts can also affect the rate of photosynthesis

Temperature

• The temperature of the environment affects how much kinetic energy all particles have – so temperature affects the speed at which carbon dioxide and water move through a plant

• The lower the temperature, the less kinetic energy particles have, resulting in fewer successful collisions occurring over a period of time

• Increasing temperature increases the kinetic energy of particles, increasing the likelihood of collisions between reactants and enzymes which results in the formation of products

• At higher temperatures, however, enzymes that control the processes of photosynthesis can be denatured (where the active site changes shape and is no longer complementary to its substrate) – this reduces the overall rate of photosynthesis

Light Intensity

• The intensity of the light available to the plant will affect the amount of energy that it has to carry out photosynthesis

• The more light a plant receives, the faster the rate of photosynthesis

• This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

Carbon dioxide concentration

• Carbon dioxide is one of the raw materials required for photosynthesis

• This means the more carbon dioxide that is present, the faster the reaction can occur

• This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply

Chlorophyll

• The number of chloroplasts (as they contain the pigment chlorophyll which absorbs light energy for photosynthesis) will affect the rate of photosynthesis

• The more chloroplasts a plant has, the faster the rate of photosynthesis

• The amount of chlorophyll can be affected by:

  • Diseases (such as tobacco mosaic virus)

  • Lack of nutrients (such as magnesium)

  • Loss of leaves (fewer leaves means fewer chloroplasts)

Tips:

• Interpreting graphs of limiting factors can be confusing for many students, but it’s quite simple. In the section of the graph where the rate is increasing (the line is going up), the limiting factor is whatever the label on the x axis (the bottom axis) of the graph is. In the section of the graph where the rate is not increasing (the line is horizontal), the limiting factor will be something other than what is on the x axis – choose from temperature, light intensity or carbon dioxide concentration.

KLO 2.21 describe the structure of the leaf and explain how it is adapted for photosynthesis

Plant leaves have complex structures with layers of different tissues containing specially adapted cells

Leaf Structures

What we typically think of a plant cell is actually the palisade mesophyll cell

Leaf’s Adaptation to Photosynthesis

• The specialised cells in leaves have adaptive features which allow them to carry out a particular function in the plant

KLO 2.22 understand that plants require mineral ions for growth, and that magnesium ions are needed for chlorophyll and nitrate ions are needed for amino acids

Mineral Ions

• Photosynthesis provides a source of carbohydrates, but plants contain and require many other types of biological molecule; such as proteins, lipids and nucleic acid (DNA)

• As plants do not eat, they need to make these substances themselves

• Carbohydrates contain the elements carbon, hydrogen and oxygen but proteins, for example, contain nitrogen as well (and certain amino acids contain other elements too)

• Two fundamental mineral ions required by plants are nitrogen and magnesium, without a source of these elements, plants cannot photosynthesise or grow properly

• Plants obtain these elements in the form of mineral ions actively absorbed from the soil by root hair cells

• ‘Mineral’ is a term used to describe any naturally occurring inorganic substance

Obtaining Minerals:

• Moved into root cells in a water solution. 

• Often in law concentration (soil) compared to high concentration of root hair cell 

•  Minerals need to be actively transported against concentration gradient

• This is quite energy consuming

KLO 2.23 practical: investigate photosynthesis, showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll

• There are some key experiments that you should know about in this topic. 

  • Show that starch is produced in photosynthesis

  • Demonstrate that oxygen is given off by a water plant in photosynthesis

  • You need to be able to adapt these experiments to prove the requirement for light, chlorophyll and carbon dioxide.

• Plants make glucose during photosynthesis and store it in their cells as starch!

  • You can test a leaf with iodine to show it contains starch.

Investigating Light & Photosynthesis

• Although plants synthesise glucose during photosynthesis, their leaves cannot be tested for its presence as the glucose produced is quickly used up, converted into other substances and transported or stored as starch.

• Starch is stored in the chloroplasts where photosynthesis occurs so testing a leaf for starch is a reliable indicator of which parts of the leaf are photosynthesising

Equipment/Apparatus

• Beakers

• Leaf tissue

• Bunsen burner

• Tripod

• Gauze platform

• Prongs

• Iodine solution

• White tile

• Apron

• Safety goggles

• Gloves

Method 1 

1. First remove a leaf from the plant.

2. Place it in a beaker of boiling water for 30 seconds to kill it.

   1. This kills the tissue and breaks down the cell walls

3. Then place it in a boiling tube of ethanol, inside the beaker of boiling water for 5 to 10 minutes, and make sure the Bunsen flame is switched off. 

   1. This removes the green pigment called chlorophyll from the leaf so you can see the results clearly.

4. Remove the leaf after a minute using forceps and rinse with cold water under the tap. 

   1. This is done to soften the leaf tissue after being in ethanol

5. Spread the leaf out on a white tile and cover/drip it with iodine solution

6. Iodine will turn blue black if starch is present. 

Method 2

1. Destarch the plant by placing it in a dark cupboard for 24 hours

   1. This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment

2. Following de-starching, partially cover a leaf of the plant with aluminium foil and place the plant in sunlight for a day

3. Remove the covered leaf and test for starch using iodine using the method above

Results

• The leaf from method 1 will turn black/blue, a positive iodine test because there is starch present

• The leaf from method 2 will not turn black/blue, a negative iodine tests, it contains no starch

  • This is because the leaf has been destarched. 

  • It has been left in a dark cupboard for a few days, the plant has not been able to photosynthesise and has therefore used up its store of starch for energy instead. 

• Some leaves are variegated, this means they have some green and some white parts, if tested for starch

  • The white parts will just turn yellow/brown, the same colour as the iodine as there is no starch made here due to the absence of chlorophyll

  • The parts of the leaf with chlorophyll turn blue/black as starch has been produced here by photosynthesis

  • This means that chlorophyll is needed for photosynthesis to occur 

• In a green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all areas of the leaf

• The area of the leaf that didn’t receive light will remain orange-brown as it did not receive any sunlight and could not photosynthesise, while the area exposed to sunlight will turn blue-black

• This proves that light is necessary for photosynthesis and the production of starch

CORRMS

• C: appearance of light on the leaves

• O: leaves taken from same plant or plants of similar age

• R: We will repeat the investigation several times to ensure our results are reliable

• M1: observe the colour change on the leaf when iodine is applied

• M2: leaves are boiled in ethanol for 5 - 10 minutes

• S: control the temperature of the room, the volume of water, the volume of ethanol, the size of the flame

Investigating Carbon Dioxide & Photosynthesis

• A iodine starch test can be used to determine carbon dioxide

• The soda lime absorbs CO2 so if you leave the plant in the sealed bell jar for a while it will stop photosynthesising and use up its starch reserves for energy. 

• Therefore if you tested a leaf for starch it wont give a positive result.

Photosynthesis proof: CO2 is required

CORRMS

• C - We are changing whether there is carbon dioxide or no carbon dioxide

• O - The leaves will be taken from the same plant or same species, age and size of plant

• R - We will repeat the investigation several times to ensure our results are reliable

• M1 - We will observe the colour change of the leaf when iodine is applied

• M2 - ...after a while

• S - We will control the temperature of the room and the light intensity

Investigating Oxygen Gas & Photosynthesis

• You can use an aquatic plant such as Elodea (pondweed) to measure the rate of photosynthesis under various conditions.

• As the plant photosynthesises it produces oxygen gas as bubbles. 

• You can measure the volume produced in a given time to calculate the rate. 

• You could also count the number of bubbles in a given time

  • You could change the distance the lamp is from the plant to investigate the effect of light intensity on the rate of photosynthesis

  • Or sodium hydrogen-carbonate can be added to increase the CO2 in the water, or the temperature of the water could be changed.

KLO 2.24 understand that a balanced diet should include appropriate proportions of carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre

Balanced Diet

Why is Food needed

• It supplies us with fuel for energy

• Helps fight disease

• Keeps bodies healthy

• Provides materials for growth and repair of tissues

Balanced Diet

• A balanced diet consists of all of the food groups in the correct proportions

• The necessary key food groups are:

  • Carbohydrates

  • Proteins

  • Lipids

  • Dietary Fibre

  • Vitamins

  • Minerals (mineral ions)

  • Water

Malnutrition

• Having an unbalanced diet can lead to malnutrition

• Malnutrition can cause a variety of different health problems in humans

KLO 2.25 identify the sources and describe the functions of carbohydrate, protein, lipid (fats and oils), vitamins A, C and D, the mineral ions calcium and iron, water and dietary fibre as components of the diet

Sources & Functions of Dietary Elements

Vitamins & Minerals

KLO 2.26 understand how energy requirements vary with activity levels, age and pregnancy

Variations in Energy Requirements

• The nutritional requirements for individuals will vary throughout their lifetime

• An individual will still require the same key food groups, but in different quantities depending on a number of factors such as age, height, sex, activity levels, pregnancy and breastfeeding

KLO 2.27 describe the structure and function of the human alimentary canal, including the mouth, oesophagus, stomach, small intestine (duodenum and ileum), large intestine (colon and rectum) and pancreas

Digestion 

The chemical and mechanical breakdown of food where relatively large insoluble molecules are broken into smaller soluble molecules that can be absorbed into the bloodstream

• The digestive system is an example of an organ system in which several organs work together to digest and absorb food

• Digestion is a process in which relatively large, insoluble molecules in food (such as starch, proteins) are broken down into smaller, soluble molecules that can be absorbed into the bloodstream and delivered to cells in the body

• These small soluble molecules (such as glucose and amino acids) are used either to provide cells with energy (via respiration), or with materials with which they can build other molecules to grow, repair and function

• The human digestive system is made up of the organs that form the alimentary canal and accessory organs

  • The alimentary canal is the channel or passage through which food flows through the body, starting at the mouth and ending at the anus

  • Digestion occurs within the alimentary canal

  • Accessory organs produce substances that are needed for digestion to occur (such as enzymes and bile) but food does not pass directly through these organs

Digestive System

The Stages of Food Breakdown

1. Ingestion - the taking in of substances, e.g. food and drink, into the body through the mouth

2. Mechanical digestion - the breakdown of food into smaller pieces without chemical change to the food molecules

3. Chemical digestion - the breakdown of large, insoluble molecules into small, soluble molecules

4. Absorption - the movement of small food molecules and ions through the wall of the intestine into the blood

5. Assimilation - the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells

6. Egestion - the passing out of food that has not been digested or absorbed (as faeces) through the anus

KLO 2.28 understand how food is moved through the gut by peristalsis

Peristalsis

• Peristalsis is a mechanism that helps moves food along the alimentary canal

  • Muscles in oesophagus create waves of contraction forcing movement

  • In the stomach, it is churned into a less solid form called chyme

• Peristalsis is controlled by circular and longitudinal muscles

  • Circular muscles contract to reduce the diameter of the lumen of the oesophagus or small intestine

  • Longitudinal muscles contract to reduce the length of that section the oesophagus or the small intestine

• Mucus is produced to continually lubricate the food mass and reduce friction

• Dietary fibre provides the roughage required for the muscles to push against during peristalsis

Circular and longitudinal muscles in the alimentary canal contract rhythmically to move the partially digested food mass along in a wave-like action

KLO 2.29 understand the role of digestive enzymes, including the digestion of starch to glucose by amylase and maltase, the digestion of proteins to amino acids by proteases and the digestion of lipids to fatty acids and glycerol by lipases

Digestive Enzymes

• Food is partially digested mechanically (by chewing, churning and emulsification) in order to break large pieces of food into smaller pieces of food which increases the surface area for enzymes to work on

• Digestion mainly takes place chemically, where bonds holding the large molecules together are broken to make smaller and smaller molecules

• Chemical digestion is controlled by enzymes which are produced in different areas of the digestive system

• Enzymes are biological catalysts – they speed up chemical reactions without themselves being used up or changed in the reaction

• There are three main types of digestive enzymes – carbohydrases, proteases and lipases

Carbohydrases

• Carbohydrases are enzymes that break down carbohydrates to simple sugars such as glucose

  • Amylase is a carbohydrase which is made in the salivary glands, the pancreas and the small intestine

  • Amylase breaks down starch into maltose

  • Maltase then breaks down maltose into glucose

Proteases

• Proteases are a group of enzymes that break down proteins into amino acids

  • Pepsin is an enzyme made in the stomach which breaks down proteins into smaller polypeptide chains

  • Proteases made in the pancreas and small intestine break the peptides into amino acids

Lipases

• Lipases are enzymes that break down lipids (fats) to glycerol and fatty acids

  • Lipase enzymes are produced in the pancreas and secreted into the small intestine

Note: The pancreas is an accessory organ in the digestive system. Food does not pass directly through it, but it has a key role in producing digestive enzymes as well as the hormones that regulate blood sugar (insulin and glucagon).

KLO 2.30 understand that bile is produced by the liver and stored in the gallbladder

Bile

• Bile is an alkaline substance produced by cells in the liver

• Before being released into the small intestine bile is stored in the gallbladder

• Is Not an Enzyme

KLO 2.31 understand the role of bile in neutralising stomach acid and emulsifying lipids

Role of Bile

• Neutralising the hydrochloric acid from the stomach

  • The alkaline properties of bile allow for this to occur

  • This neutralisation is essential as enzymes in the small intestine have a higher (more alkaline) optimum pH than those in the stomach

• Breaking apart large drops of fat into smaller ones (and so increasing their surface area)

  • This is known as emulsification

Bile salts break large lipid droplets into smaller ones with a larger surface area

NOTE: Emulsification is the equivalent of tearing a large piece of paper into smaller pieces of paper.This is an example of mechanical digestion, not chemical digestion – breaking something into smaller pieces does not break bonds or change the chemical structure of the molecules which make it up, which is the definition of chemical digestion.

KLO 2.32 understand how the small intestine is adapted for absorption, including the structure of a villus

Adaptations of Small intestine

• The small intestine is adapted for absorption as it is very long and has a highly folded surface with millions of villi (tiny, finger-like projections)

  • These adaptations massively increase the surface area of the small intestine, allowing absorption to take place faster and more efficiently

• Peristalsis helps by mixing together food and enzymes and by keeping things moving along the alimentary canal

Villi of Small intestine

• Villi have several specific adaptations which allow for the rapid absorption of substances

  • A large surface area

    • Microvilli on the surface of the villus further increase the surface available for absorption

  • A short diffusion distance

    • The wall of a villus is only one cell thick of intestinal epithelial cells

  • A steep concentration gradient

    • The villi are well supplied with a network of blood capillaries that transport glucose and amino acids away from the small intestine in the blood

    • A lacteal (lymph vessel) runs through the centre of the villus to transport fatty acids and glycerol away from the small intestine in the lymph

    • Enzymes produced in the walls of the villi assist with chemical digestion

    • The movement of villi helps to move food along and mix it with the enzymes present

NOTE: The way in which the structure of a villus is related to its function comes up frequently in exam questions so it is worth ensuring you have learned these adaptations and how they influence the rate of absorption.