Biology shell revision


Nature + variety of living organisms: characterist

Characteristics of living organisms: MRSGREN

Movement -  they move

Respiration - they respire

Sensitivity - they espond to their surroundings

Growth - they grow and develop.

Reproduction - they reproduce

Excretion - they excrete their waste

Nutrients - they require nutrients to survive

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Nature + variety of living organisms: features of

Plants: These are multicellular organisms; they contain  chloroplasts and are able to carry out photosynthesis; they have cellulose cell walls; they store carbohydrates as starch or sucrose. Examples include flowering plants, such as a cereal (for example maize) and a herbaceous legume (for example peas or beans).

Animals: These are multicellular organisms; they do not contain chloroplasts and are not able to carry out photosynthesis; they have no cell walls; they usually have nervous system and are able to move from one place to another; they often store carbohydrates as glycogen.

Organelle Animal Cell Plant Cell Chloroplast no yes Cell Wall no yes Sap Vacuole no yes Chlorophyll no Found in chloroplast


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Nature + variety of living organisms: features of

Fungi: These are organisms that are not able to carry out photosynthesis; their body is usually organised into a mycelium made from thread like structures called hyphae, which contain many nuclei; some examples are single-celled; they have cell walls made of chitin; they feed by extracellular secretion of digestive enzymes on to food material and absorption of the organic products; this is known as saprotrophic nutrition; they may store carbohydrate as glycogen. Examples include Mucor, which has the typical fungal hyphal structure, and yeast which is single-celled.

Bacteria: These are microscopic single-celled organisms; they have a cell wall, cell membrane, cytoplasm and plasmids; they lack a nucleus but contain a circular chromosome of DNA; some bacteria can carry out photosynthesis but most feed off other living or dead organisms. Examples include Lactobacillus bulgaricus, a rod-shaped bacterium used in the production of yoghurt from milk, and Pneumococcus, a spherical bacterium that acts as the pathogen causing pneumonia.

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Nature + variety of living organisms: features of

Protoctists: These are microscopic single-celled organisms. Some, like Amoeba, that live in pond water, have features like an animal cell, while others, like Chlorella, have chloroplasts and are more like plants. A pathogenic example is Plasmodium, responsible for causing malaria.

Viruses: These are small particles, smaller than bacteria; they are parasitic and can reproduce only inside living cells; they infect every type of living organism. They have a wide variety of shapes and sizes; they have no cellular structure but have a protein coat and contain one type of nucleic acid, either DNA or RNA. Examples include the tobacco mosaic virus that causes discolouring of the leaves of tobacco plants by preventing the formation of chloroplasts, the influenza virus that causes ‘flu’ and the HIV virus that causes AIDS.

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Nature + variety of living organisms: pathogens

Definition of pathogens: Pathogens are organisms that cause disease. They include microorganisms such as bacteria, viruses and fungi

The immune system

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Levels of Organisation/ Cells

Organisms are made from organizations of smaller structures:

Organelles - intracellular structures that carry out specific functions within a cell
Cells - the basic structural and functional unit from which all biological organisms are made
Tissues - a group of specialized cells, which are adapted to carry out a specific function
Organs - a collection of two or more tissues, which carries out a specific function or functions
Organ Systems - a group of two or more organs

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Levels of Organisation: organelles

Functions of the Organelles:

Cytoplasm - site of chemical reactions in the cell

Cell Membrane - controls what enters / leaves the cell (selectively permeable)

Nucleus - contains nucleic acids, which code for the synthesis of specific proteins. These proteins control all activity in the cell

Mitochondrion - site of respiration

Chloroplast - site of photosynthesis

Cell Wall - made from cellulose. Strengthens the cell and allows it to be turgid

Sap Vacuole - contains the cell sap. Acts as a store of water, or of sugars or, in some cases, of waste products the cell needs to excrete.

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Movement in and out of cells: diffusion and osmosi

Diffusion – the movement of molecules from high concentration to low concentration, down a concentration gradient.


Osmosis – the movement of water molecules from high concentration to low concentration through a partially permeable membrane


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Movement in and out of cells: active transport

Active Transport – the movement of molecules from low concentration to high concentration against the concentration gradient. Energy is required for movement to occur. As is a protein

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Biological molecules: food tests

Food Tests:

Lipids are tested for using the Emulsion test - add ethanol --->cloudy white precipitate = positive

Proteins are tested for using the Biuret test - add Biuret solution ---> blue to purple = positive

Starch is tested for using Iodine solution - add iodine ---> orange to blue/black = positive

Glucose is tested for using Benedict’s test - add Benedict's solution, heat up in test tube ---> blue to brick red/orange = positive

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Biological molecules: components of food groups

Components of the main Food Groups:

The main food groups are carbohydrates, lipids and proteins. All three groups are made from smaller molecules.

Carbohydrates are large molecules made from one or more sugars (e.g. both Starch and Glycogen are both polymers of Glucose)

Proteins are polymers of Amino Acids

Lipids are made from one glycerol molecule and three fatty acid molecules joined together.

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Biological molecules: food gruops and functions

Food Group and their Function

Lipids (fats & oils) - Used as a long-term energy store. Also have a role in protection and insulation.

Carbohydrates - Made from single sugars or chains of sugars. They are used in respiration to provide energy.

Proteins - Broken down into amino acids, which our body absorbs then proteins are used for growth and repair.

Fibre - Regulates bowel movement. Sloughs off old lining of intestine.

Water - Essential as a solvent for chemical reactions (e.g. cytoplasm).

Vitamins and Minerals - Essential for the normal function of some enzymes and proteins e.g. Fe2+ is an essential part of Haemoglobin and Mg2+ is part of Chlorophyll

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Biological molecules: enzymes


- Are proteins
- Are biological catalysts (speed up chemical reactions)
- Are specific to one particular Substrate
- Are affected by temperature and pH
- Are not used up in the reaction they catalyze

enzyme prods.GIF

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Biological molecules: Temperature and pH in enzyme

Temperatue effecting enzyme activity: ( pic. 1 )

1. Initially, raising the temperature increases the rate of reaction.
2. However, after the optimum temperature is reached the enzyme begins to change shape and the active site stops being able to bind to the substrate.
3. The enzyme becomes denatured and stop working (the rate of reaction is zero at this point).

enzyme temp.GIFenzyme ph.GIF

pH efffecting enzyme activity: (pic. 2)

1. Initially, increasing the pH increases the rate of reaction.
2. However, after the optimum pH is reached the enzyme begins to change shape and the active site stops being able to bind to the substrate.
3. The enzyme becomes denatured and stops working (the rate of reaction is zero at this point).

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Nutrition in humans: measuring energy in food

Humans need to eat a balanced diet. This really means some of every food group, but not too much or too little of a particular one. The two groups that provide energy (through respiration) are lipids and carbohydrates. Per mass lipids have about 10x more energy in them than carbohydrates.

an experiment that can show how much energy there is in food:

burn a sample of food and use it to heat a fixed volume of water. If you record the change in temperature of the water you can use the equation below to find out the energy the food gave to the water;

Energy = change in temp. x volume of water x 4.2J/g/oC

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Nutrition in humans: Vitamins and minerals

Vitamin - Mineral Function

Vitamin A - Present in fish, cheese and eggs. It forms an essential part of the pigment in rods and cones that detects light. Lack of Vitamin A can lead to blindness.

Vitamin C - Present in citrus fruit. It forms an essential part of collagen protein, which makes up skin, hair, gums and bones. Lack of Vitamin C causes scurvy.

Vitamin D - Present in fish, but made naturally by our body when sunlight shines on the skin. It is essential for regulating the growth of bones. Lack of Vitamin D can cause rickets.

Calcium - Present in milk, cheese & dairy foods. It is essential for bone growth and muscles. Lack of calcium can lead to osteoporosis.

Iron - Present in red meat and some vegetables (e.g. spinach). Is part of haemoglobin. Lack of iron causes anaemia.

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The digestive system: diagram

dig sys.GIF

The purpose of digestion is to break food into molecules that are small enough to be absorbed into the bloodstream.
There are two types of digestion

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The digestive system: chemical and mechanical

Mechanical Digestion: digestion by physically breaking food into smaller pieces (i.e. not using enzymes). Carried out by;
- mouth and teeth chewing food
- stomach churning food

Chemical Digestion: digestion using enzymes:

enzyme table.GIF

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The digestive system: key ideas

Ingestion - taking food into the digestive system

Digestion - breaking food down into molecules small enough to be absorbed into the bloodstream.

Absorption - taking molecules into the bloodstream. This happens almost entirely in the small intestine(ileum)

Assimilation - using food molecules to build new molecules in our bodies. i.e. the food molecule physically becomes part of our body.

Egestion - Removing unwanted food from the digestive system (having a poo!). This is not excretion, because the unwanted food has never, technically, been inside the body.

Peristalsis - the contraction of muscle in the intestine wall behind a bolus of food (ball of food). This pushes the bolus through the intestine.

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The digestive system: Small intestine adaptations

SI adapt.GIF

Adaptation - Explanation

Thin wall - The intestine wall in thin, which speeds the rate of diffusion of molecules into the blood
Rich blood supply - This helps carry absorbed molecules away from the intestine quickly. This means there is always a low concentration of food molecules in the blood, which maintains a high concentration gradient
Intestine length - Roughly 7m long, which increases the surface area
Surface Area - Villi and microvilli increase the surface area of the small intestine

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Aerobic: glucose + oxygencarbon dioxide + water (+ energy)

C6H12O6 + 6O2 → 6CO2 + 6H2O

Energy is shown in brackets because it is not a substance. Notice that:

  • Glucose and oxygen are used up
  • Carbon dioxide and water are produced as waste products

Aerobic respiration happens all the time in the cells of animals and plants. Most of the reactions involved happen inside mitochondria, tiny objects inside the cytoplasm of the cell. The reactions are controlled by enzymes.

Anaerobic: Not enough oxygen may reach the muscles during exercise. When this happens, they use anaerobic respiration to obtain energy.Anaerobic respiration involves the incomplete breakdown of glucose. The waste product is lactic acid rather than carbon dioxide and water:

glucoselactic acid (+ little energy)  or C6H12O6 + 6O2 ---> 6CO2 + 6H2O (+ Energy Released)

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The breathing system

resp sys.GIF (

 Breathing in (inhaling)                          

1. Intercostal muscles contract, pulling the ribcage up and out
2. Diaphragm contracts moving down
3. The volume of the Thoracic Cavity increases
4. The pressure in the Thoracic Cavity decreases
5. Air is drawn into the lungs to equalize the pressure

Inhaling is an active process, i.e. it requires energy for muscle contraction

Breathing out (exhaling)

1. Intercostal muscles relax, the ribcage moves inwards and down
2. Diaphragm relaxes moving up
3. The volume of the Thoracic Cavity decreases
4. The pressure in the Thoracic Cavity increases
5. Air leaves the lungs to equalize the pressure

The entire process is passive, i.e. no energy is required as there is no muscle contraction

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The breathing system: Alveoli

Alveoli and their adaptations for gas exchange:

alv.GIF (

- Alveolus is one cell thick
- Capillary wall is one cell thick
- Many alveoli produce a huge surface area
- Alveoli wall is moist
- Breathing maintains a high concentration gradient for O2 and CO2
- Blood movement maintains a high concentration gradient for O2 and CO2

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The breathing system: smoking effects

Cigarette smoke contains tar, nicotine, carcinogens, CO and poisons

Chemical and Effect

Tar - Blocks up alveoli, making gas exchange more difficult. Also clogs up cilia (little hairs lining the lungs, whose job is to “wave” and remove mucus and trapped bacteria out of the lungs).

Nicotine - Speeds heart rate and damages arteries, causing furring of artery walls (atherosclerosis). This leads to heart disease and vascular diseases. It is also addictive.

Carcinogens - Damages the DNA of alveoli cells. This can lead to them reproducing faster than normal, which will cause a tumour to form. The tumour is the start of cancer.

Carbon Monoxide - Attaches permanently to haemoglobin, reducing the ability of the blood to carry O2.

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Nutrition in flowering plants:Photosynthesis

Nutrition in Flowering Plants:

Plants are photoautrophic (i.e. they generate their own “food” using energy from the Sun.) They do this through photosynthesis Carbon Dioxide + Water ---> Oxygen + Glucose

6CO2 + 6H2O ---> 6O2 + C6H12O6

Through photosynthesis light energy is converted into chemical energy in the bonds in glucose. Plants use glucose for the following;

- Respiration
- Stored as Starch
- Turned into Cellulose (cellulose is a polymer of glucose)
- Used to make fats and oils

At any point the rate of photosynthesis can be increased by adding more CO2, more water, more light or heating towards optimum temperature (photosynthesis is catalyzed by enzymes). However, at a certain point the addition of more e.g. light will not increase the rate of photosynthesis any further. This is because a second factor is limiting the rate of photosynthesis.

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Nutrition in flowering plants: adaptations

You need to know the parts of the leaf and their adaptations.

leaf structure.GIF (

In addition to water and CO2 plants also need specific minerals;

Nitrate – used to make amino acids for use in plant proteins
Magnesium – forms part of the chlorophyll molecule
Potassium - essential for cell membranes
Phosphate - essential part of DNA and cell membranes

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Leaf Structure and adaptation for photosynthesis

Cuticle - Stops the leaf from losing water (remember, water is used in photosynthesis)

Epidermis - Transparent protective layer. Protects the leaf without inhibiting photosynthesis.

Palisade cells - Are packed full of chloroplasts. Are long and thin so light has to pass through as many chloroplasts as possible.

Air Spaces - Increase the surface area inside the leaf to maximise gas exchange across the surface of the Spongy Mesophyll cells Stoma Allow exchange of CO2 and O2

Guard Cells - Allow the stoma to open and close to stop the leaf losing too much water

Vein (containing Xylem) - Brings a steady supply of water to the leaf.

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Tropism in plants:stimuli

Plants also respond to stimuli. As plants don't have nerves their responses are limited to hormones only. Plants respond to the following stimuli;

- Gravity. Roots grow towards gravitational pull and stems grow away. This is Geotropism.
- Water. Roots grow towards water. This is Hydrotropism.
- Light. Shoots grow towards light. This is Phototropism.

phototrop.GIF - phototropism

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Organisms in the Environment

Population: all the individuals of a particular species within a defined area

Community: a group of different populations living in the same area

Habitat: the physical, chemical and biological environment in which an organism lives

Ecosystem: a community of living things and the environment in which they livequad.GIF

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Feeding relationships

Food chains are used to show the relationships between species in a habitat. E.g.

Each level in a food chain is called a Trophic Level

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Feeding relationships: pyramids


This shows the populations (to scale) of the species in the chain.
Sometimes a Pyramid of numbers can be inverted (i.e. have a tiny base). This occurs if there is a parasitic relationship in the food chain i.e. one tree, but many caterpillars eating the leaves!

To stop this a pyramid of biomass is more frequently used. This always has a pyramidal shape.


Biomass – the mass of the organic material an organism is made from (i.e. dry it out totally and weigh it, water doesn’t count!) We can also represent the energy flow in a food chain using a Pyramid of Energy Transfer.


This gives an indication of the huge amount of energy that is not passed on to the next trophic level. This is because at each level energy is wasted on;

- Respiration (most of it as waste heat)
- Undigested / egested food
- Used in movement

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Energy transfer in food chains

The amount of available energy decreases from one stage to the next.

Some of the available energy goes into growth and the production of offspring. This energy becomes available to the next stage, but most of the available energy is used up in other ways. For example:

  • energy released by respiration is used for movement and other life processes, and is eventually lost as heat to the surroundings
  • energy is lost in waste materials, such as faeces.
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Cycles within ecosystems


Key ideas – Evaporation, Condensation, Precipitation & Transpiration (rather unhelpfully not shown on this diagram)

The Carbon Cycle:


Key ideas – Respiration, Photosynthesis, Decomposition & Combustion

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The nitrogen cycle


This is not particularly easy to understand. You need to know the roles of all the different bacteria. There are 4;

- Decomposers – turn nitrogen in protein into ammonium (NH4+)
- Denitrifying Bacteria – turn ammonium (NH4+) into N2
- Nitrifying bacteria – turn ammonium (NH4+) into nitrate (NO3-)
- Nitrogen-fixing bacteria – turn N2 into ammonium (NH4+)

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