Introduction to plant, animal and bacteria TYPES of CELL STRUCTURE and the FUNCTIONS of their components

including sub-cellular structures and differences between plant, animal and bacterial cells and unicellular organisms

 What features do animal cells and plant cells have in common?   In what way do plant cells differ from animal cells?   Can you correctly draw and label an animal cell and a plant cell?  What are subcellular structures? What is their function in cells?

Sub-index for this page

(a) Introduction to cells - prokaryotes and eukaryotes

(b) Animal cells including humans! (eukaryotes, eukaryota)

(c) Plant and algal cells (eukaryotes, eukaryota)

(d) Bacteria are prokaryotes - prokaryotic cells, prokaryota

(e) Fungal cells (eukaryotes)

(f) Yeast cells (eukaryotes)

(g) A note on the structure of viruses (NOT classified as living organisms)

(h) The scale of things and orders of magnitude

(i) Protists

(j) Some more examples of unicellular organisms

(a) Introduction to CELLS and types of cells

The majority of living things are made up of cells, the building blocks of life.

You can think of most cells as a unit of life surrounded and enclosed by an outer membrane of some sort.

Appendix 1. Viruses can be considered a 'non-cellular' form of life.

Why is cell biology important?

Cell biology is very important t understand the structure and function of cells e.g. what do the various sub-cellular structures do? and how does a cell function as a living unit.

Knowledge of cells helps us understand how organisms develop and interact with other organisms e.g. sexual reproduction or our bodies own defences in fighting bacteria.

Knowledge of cell chemistry is also important in the diagnosis of disease and developing drugs to counteract adverse medical conditions e.g. anti-cancer drugs.

What is a cell?

A cell is the smallest unit of life able to control its own activities, BUT, it relies on the rest of the organism (if multicellular) or the surroundings (if unicellular) to provide it with raw materials i.e. nutrients and removal of waste material.

The different parts of a cell are referred to as subcellular structures.

The term organelle refers to specialized sub-cellular structure within a cell that perform a specific function (e.g., mitochondria, ribosomes, chloroplasts).

Organelles in unicellular organisms are the equivalent of organs in multicellular organisms.

You should know and understand  that the structures of different types of cells are related to their functions.

 You should know and understand the similarities and differences between animal cells, bacteria and plant cells.

See Appendix 2. for the scale of things and order of magnitude

The two main groups of cells

Cells can be either eukaryotic or prokaryotic in character.

Eukaryotes are organisms made of eukaryotic cells, which are complex cells, and all plants and animals are made up of such cells.

Eukaryotes are usually multi-cellular organisms, but can consist of one cell e.g. yeast or algae (unicellular).

Plant and animal cells (eukaryotic cells) all have a cell membrane, cytoplasm and genetic material enclosed in a true nucleus in the cytoplasm (compared to prokaryotic cells described below) and other sub-cellular structures called organelles e.g. mitochondria, chloroplasts, ribosomes etc.

However there are significant differences between eukaryotic plant and animal cells (see later).

Watch out for adaptations in both unicellular and multicellular organisms that help them survive and reproduce.

See also Classification - domain, kingdom, phylum, class, order, family, genus, species, Linnaeus naming of organisms

Prokaryotes, are smaller and simpler single celled organisms (unicellular prokaryotic) eg bacteria and archaea are prokaryotic cells.

Prokaryotic cells do NOT have a true nucleus containing the DNA - the DNA is in free floating loops/strands in the cytoplasm.

Archaea are now considered as a separate domain of single celled organisms (see diagram below) even though they are like bacteria

See classification and evolutionary trees gcse biology revision notes

Prokaryotic cells are more primitive than eukaryotic cells and the oldest fossil evidence suggests that bacteria were evolving as much as 3.5 billion years ago.

Planet Earth is reckoned to be about 4.5 billion years old (4.5 x 10⁹ years from radiometric data - using the half-lives of elements in rocks - the calculations are VERY complex).

The early bacteria probably existed as thin purple or green coverings on shorelines.

These early bacteria used photosynthesis but produced sulfur instead of oxygen as a waste product.

See also Classification - domain, kingdom, phylum, class, order, family, genus, species, Linnaeus naming of organisms

including the three domain system

Sections (b) and (c) give a detailed comparison of animal cells, plant cells and bacteria cells

All plant and animal cells have similarities in basic structure, BUT, there are important differences between them.

Both eukaryotic and prokaryotic cells contain various subcellular structures, of some are referred to as organelles - a subcellular structure performing a particular function, on a larger scale, rather a like an organ in an animal.

There are also important differences between prokaryotic cells (single celled archaea and bacteria) and (usually) multicellular) eukaryotic cells (plants and animals)

Using diagrams and explanatory notes, their similarities and differences in sub-cellular structures will be described and their functions explained.

Note that an organelle is a specialised sub-cellular unit that performs a specific function. It has its own membrane and examples include mitochondria (organelle of energy production molecules) in both plants and animals which have their own DNA and in plant cells, chloroplasts, the organelle of photosynthesis..

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(b) ANIMAL CELLS including humans! (eukaryotes, eukaryota)

Most animal cells have the following five parts in these eukaryotic cells - the so called subcellular structures, and, remember, plants cells usually have the same five components too (1. to 5. below).

The diagram shows the principal subcellular structures of an animal cell.

1. Cell membrane

Usually from/to blood vessels (in animals), the cell membrane controls what enters the cell (oxygen, glucose, other nutrients) and controls what leaves the cell (carbon dioxide and other waste products).

Transport in plants is via the xylem and phloem systems.

The cell contents i.e. the sub-cellular structures like cytoplasm, nucleus, (small vacuoles in animal cells), mitochondria etc. are all held together and enclosed, by the very thin soft cell membrane which controls the passage of substances in and out of the cell.

Because not everything can pass through the membrane, it is described as a semi-permeable or a partially permeable membrane.

The cell membrane allows the free passage of water and gases but may act as a selective barrier to other chemicals.

The cell membrane also contain receptor molecules that are used in cell communication e.g. by hormones.

Through the cell membranes, specific substances can diffuse out of one cell and diffuse into another neighbouring cell, from a cell of higher concentration to a cell of lower concentration.

Reminder: Diffusion is due to the random movement of ions and molecules in the fluid, from a region of higher concentration to a region of lower concentration, in this case the regions are separated by the semi-permeable cell membrane.

e.g. usually, for an animal ...

outside a cell the concentrations of oxygen and glucose are higher in the blood, so both will diffuse into a cell,

and inside the cell the concentration of carbon dioxide and other waste products are higher, so these will diffuse out of a cell through the cell membrane into the blood for disposal via lungs or kidney.

2. Mitochondria (an example of an organelle - a subcellular structure performing a particular function)

A mitochondria organelle has a double membrane, the inner one is folded in a complex way

Most of the aerobic energy releasing chemistry of respiration occurs in the mitochondria, which is where most energy is released in respiration - eg the aerobic 'burning' of glucose to release energy.

e.g. glucose + oxygen == via enzymes ==> carbon dioxide + water + energy

The equation of aerobic respiration, an exothermic chemical reaction and catalysed by the appropriate enzymes.

glucose  +  oxygen  ===>  carbon dioxide  +  water

C₆H₁₂O_6(aq)  +  6O_2(g)  ===>  6CO_2(g)  +  6H₂O_(l)  +  energy

RESPIRATION - aerobic and anaerobic in plants, fungi and animals, conditions, substrates etc.  gcse biology revision notes

Mitochondria are the power house of the cells and contain all the enzymes needed for the chemical reactions that provide the chemical energy for any of the cells functions.

Liver cells carry out lots of metabolic reactions so lots of energy needed, so they contain a lot more mitochondria. The energy releasing molecule ATP is made in the mitochondria.

Similarly, muscle cells need lots of energy eg to contract, so again, they have a lot more mitochondria than other cells to supply the energy for the physical work animals perform.

Mitochondria have their own DNA in the form of circular chromosomes, which is inherited from the mother only.

It is believed that mitochondria where once separate microorganism - an ancient purple bacteria, that somehow became incorporated and used by another unicellular organism.

3. Cytoplasm

Cytoplasm is a jelly like fluid (gel-like) in which most of the cells chemical reactions take place and most of these reactions are catalysed by enzymes (biological catalysts) which facilitate and control the rate of these reactions.

Anaerobic respiration (glycoyslis, fermentation) take place in the cytoplasm, but most aerobic respiration takes place in the mitochondria, another specialised organelle.

4. Nucleus

The cell nucleus contains all the genetic material, the deoxyribonucleic acid (DNA codes) of the genes in the chromosomes which control the cells functions and the cell division in replication.

The nucleus controls the activities of the cell by sending instructions to the cytoplasm.

The genetic material is organised into chromosomes and the chromosomal DNA contains the instructions for making proteins e.g. that make up tissue or enzymes.

Therefore the nucleus is where RNA is made from the double helix DNA template.

5. Ribosomes

Ribosomes are involved in the translation of the genetic material from the chromosomes, they can decode the DNA to carry out various chemical synthesis e.g. ribosomes are where protein synthesis takes place - from amino acids in the cytoplasm of the cell - the protein 'factory'!

This organelle is a tiny structure and can just be seen as a dot with a light microscope.

They can be free to move in the cytoplasm or attached to an internal network of channels in the cell.

6. Other features

Glycogen granules

Stored food for respiration.

Small vacuoles - much smaller than in plant cells

Some animal cells may have several small vacuoles of water containing various dissolved substances - might be food or waste products.

7. Some differences between animal, plant and bacteria cells

Animal cells are much larger than bacterial cells, with important differences from plant cells.

Animal cells, unlike plant cells, do not have

(i) an outer rigid cell wall for strength, (ii) a large permanent vacuole and (iii) chloroplasts (for photosynthesis).

Reminder note: What is an organelle?

An organelle is a specialized part of a cell having some specific function, a sort of cell organ. Organelles are only found in eukaryotes (plant and animal cells). The nucleus, mitochondria, ribosomes and chloroplasts are examples of organelles.

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(c) PLANT and algal CELLS (eukaryotes, eukaryota)

Plant cells are much larger than bacterial cells, with important differences from animal cells.

The diagram shows the principal subcellular structures of a plant cell.

Like animal cells, plants cells have (1) a cell membrane, (2) mitochondria, (3) cytoplasm, (4) nucleus and (5) ribosomes, all of which perform the same functions as in the animal cells.

All of these five are described in section (b) animal cells.

The three extra principal different sub-cellular structures that plant cells have, and animal cells do not are: (i) a rigid cell wall, (ii) chloroplasts and (iii) a large permanent vacuole - animal cells do NOT have these three features, but some have small vacuoles.

You need to be able to describe the function of the components of a plant cell including chloroplast, large vacuole, cell wall, cell membrane, mitochondria, cytoplasm and nucleus (see diagram and notes below) and know the differences between plant and animal cells.

(i) Plant and algal cells have a more rigid cell wall made of cellulose, which strengthens the cell, supports it and therefore the plant's structure as a whole.

The plant cell wall is effectively an additional layer outside of the cell's inner membrane.

It is made from cellulose fibres that provide strength to the cell and collectively the strength of a whole multicellular plant.

Unlike the cell membrane, the cell wall does not control what materials can enter or leave the cell.

(ii) Chloroplasts the sites of photosynthesis

Chloroplasts (an organelle) has a complex internal membrane structure.

Chloroplasts can absorb light energy to make food via chlorophyll in photosynthesis

The chloroplasts contain green chlorophyll molecules which are involved in the energy absorbing process of photosynthesis. The chlorophyll molecules absorb the light energy from the sun to promote the endothermic reaction below. The chloroplasts must also contain all the enzymes to catalyse the whole series of complex reactions to make sugars - the equation below is a greatly simplified summary!

sunlight energy + carbon dioxide + water ==> sugars (e.g. glucose) + oxygen

6H₂O_(l) + 6CO_2(g) ====> C₆H₁₂O_6(aq) + 6O_2(g)

Therefore chloroplasts are the site of food production for the plant. The sugars may be used directly as a source of energy or converted to starch grains - the plant's food store (and part of our food store as well!).

Chlorophyll absorbs mainly in the violet-blue and orange-red regions of the visible spectrum, hence it appears green, the light NOT absorbed.

(iii) Large permanent vacuole

Most plant cells have a single large permanent vacuole surrounded by a membrane containing cell sap, a dilute solution of mineral salts and sugars. It maintains the internal pressure to support the cell.

The central vacuole is a cellular organelle found in plant cells. It is often the largest organelle in the plant cell.

The central vacuole's membrane's functions is to hold useful materials and wastes.

It also functions to maintain the proper internal pressure within the plant cells to provide structure and support for the growing plant.
 

Other features

Starch grains

Stored food for respiration from the glucose made by photosynthesis.

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(d) BACTERIA (prokaryotes - prokaryotic cells, prokaryota)

The diagram shows the principal subcellular structures of an bacterial cell.

Bacterial cells, single-celled microorganisms, are much smaller than plant or animal cells with some quite distinct and different sub-cellular features.

A prokaryotic bacterial cell consists of cytoplasm within a membrane surrounded by a cell wall.

Bacteria do NOT have a real nucleus, chloroplasts or mitochondria.

Cell wall and inner membrane

The cell contents i.e. the cytoplasm, DNA etc. are all held together within the cell wall by the surface membrane which controls the passage of substances in and out of the cell.

The surrounding outer cell wall gives a bacterium extra structural support.

The cell wall can also be surrounded by a capsule.

Note, unlike plant cells, animal cells do not have a cell wall.

Cytoplasm

The jelly like fluid in which most of the cells chemical reactions take place with the aid of enzyme catalysts.

Although they do not have mitochondria, bacterial cells can still respire aerobically in the cytoplasm.

Chromosomal DNA - the genetic material is not confined in a nucleus which doesn't exist in bacteria

The genes are not in a distinct true nucleus, the genetic material is a sort of jumbled cluster comprising of one long circular strand (loop) of DNA floating free in the cytoplasm sometimes accompanied by one or more small rings of DNA called plasmids. As with any other cells the string of DNA controls the cell's activities and cell division for replication.

This single chromosome controls the cells functions and the cell division in replication.

The chromosomal DNA moves freely around in the cytoplasm and is not confined in a distinct nucleus as in plant and animal cells.

Plasmid DNA, not part of the chromosome

Plasmids are small hoops of extra DNA that are separate from the chromosomal DNA.

Plasmids contain genes that help tolerance against drugs and this drug resistance can be passed from one bacteria to another - a problem in dealing with bacterial infectious diseases.

This is how the bacteria MSRA have evolved and become so dangerous because of their antibiotic resistance.

Not all prokaryote cells contain plasmids.

Shape and Flagella (flagella plural, flagellum singular)

Bacteria come in all sorts of shapes e.g. rods, spirals etc. and some have a tail!

The flagellum is a long thin tail, a hair-like structure that projects out of the body of the cell, and can rotate to move the bacteria along.

Some bacterial cells have more than one flagella (flagellum) protruding from the outer layers of the bacterium.

The 'tail' flagellum can be driven by a tiny biochemical electric motor with moving parts, mostly made of proteins!

A rotating flagellum is quite a remarkable piece of biochemical engineering - bioengineering!

The flagellum enables a bacterium away from harmful substances (e.g. toxins) and move towards beneficial materials like nutrients or oxygen.

Ribosomes

As with other cells, the place of protein synthesis from decoding genetic material from chromosomes.

Other comments on prokaryotes like bacteria

Unlike eukaryotic cells, prokaryotic cells do not contain a defined nucleus nor do they contain mitochondria or chloroplasts.

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(e) FUNGAL CELLS (eukaryotes)

Fungal cells share some similarities with plant and animal cells, but are different to both these groups.

Fungi include yeasts and mushrooms.

In common with plant and animal cells, fungal cells have a nucleus and contain mitochondria and have a cell membrane.

Some differences are:

fungal cells have a cell wall like plant cells,

they have no chloroplasts like animal cells, but unlike plant cells, which do have chloroplasts for photosynthesis.

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(f) YEAST CELLS (eukaryotes)

Yeast is used in the production of alcoholic beverages eg beer, wine etc. and in bread making.

A yeast cell has the same organelles as a mature eukaryotic cell.

A yeast cell, a single-cell microorganism, has a nucleus, cytoplasm, mitochondria enclosed in a cell membrane which is surrounded by a cell wall.

Yeast can be regarded as a single celled fungus.

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(g) A note on the structure of viruses (which are NOT classified as living organisms)

The basic structure of a virus

Viruses are not considered to be a living organism such as a plant, animal, bacteria or archaea.

Viruses are not considered to alive because they do not fulfil the seven life processes, namely: movement, respiration, sensitivity, nutrition, excretion, reproduction and growth.

Biological science uses the phrase 'strains' of virus and not species.

Viruses are the smallest agents of infectious disease and are exceedingly small (about 20 - 500 nm diameter) and essentially round in shape.

Viruses are consist of a relatively short length of genetic material (DNA or RNA) which is enclosed in a thin protein coat, which is sometimes surrounded by an extra thin fatty coating or envelope.

A typical size of a virus is about 1/50th of a red blood cell, but they can vary in size from 20 to 500 nm).

Within the protein shell the DNA/RNA nucleic acid can be single- or double stranded.

The entire infectious virus particles are unable to grow or reproduce without a host.

They have non of the usual sub-cellular structures seen in most plant or animal cells described above.

Viruses are different from all other infectious microorganisms because they are the only group of microorganisms that cannot replicate outside of a host cell.

Viruses do not consume food, but they obtain materials and energy from host cells by hijacking their host cell's cellular machinery.

Specific types of viruses only infect specific cells and persuades them to reproduce the invading virus.

Some scientists argue that they are more like complex molecules than living creatures.

Viruses are known to infect nearly every type of organism on Earth and some viruses, called bacteriophages, even infect bacteria - nothing is safe from some virus or other!

For more on virus infection mechanism see communicable diseases - pathogen infections

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(h) The scale of things and orders of magnitude

Object	Size		Object	Size
a common ladybird	~7 mm, 7000 µm		HIV virus	~0.1 µm, ~100 nm
diameter of a human hair	~0.1 mm, ~100 µm		thickness of cell membrane	~0.007 µm, ~7 nm
typical plant leaf cell	~0.07 mm, ~70 µm		diameter of DNA strand	~2.5 nm
diameter of red blood cell	~0.007 mm, ~7 µm, 7000 nm		diameter of a carbon atom	~0.34 nm

 When comparing the size of small objects like cells, scientists refer to differences in sizes as an order of magnitude.

This means that an object is described as being greater of smaller by a factor of 10.

For example, the leaf cell size is one order greater (10 x) in size than a red blood cell.

The common ladybird is two orders greater (100 x or 10 x 10) in size than a plant leaf cell.

The thickness of the cell membrane is three orders smaller {1/1000 or 1/(10 x 10 x10)} in size than the diameter of a red blood cell.

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(i) Protists

Protists are single celled organisms e.g. protozoan pathogens like those causing malaria, but this diverse group of organisms includes simple single celled algae and amoebas.

An amoeba is a classification of protist (single-celled eukaryotic organism that is neither plant, animal, bacteria, nor fungus) that are amorphous in shape. They move via forming 'feet-like' pseudopodia, which are also used for feeding.

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(j) Some more examples of eukaryotic unicellular organisms

The simplified diagram shows the structure of three singled cell organisms found in fresh water.

The cell membrane encloses the cytoplasm and the usual cell features like the nucleus.

The diagram also illustrates some structural adaptations to help the organism to survive e.g.

flagellum to move towards food or away from predators,

chloroplasts for photosynthesis, so the organism can make its own food.

Dinoflagellate

Dinoflagellates are protists, and in this case, it contains chloroplasts for photosynthesis and can manufacture its own food.

It also has flagellum that enables it to move at will in a forward direction with a 'swimming' action.

Other dinoflagellates do not have chloroplasts and feed on other microscopic creatures. feed on

Amoeba

The amoeba is a single celled primitive organism that does not contain chloroplasts, so cannot photosynthesize its own food and must scavenge for smaller organisms or waste decayed organic material.

The nucleus, mitochondria and vacuoles are shown.

The amoeba has a very flexible shape and can surround a food source and 'digest' it.

The amoeba can use a contractile vacuole to collect excess water and expel it out through the cell membrane.

Amoeba feed on algae and bacteria and tiny fragments of dead plant or animal material.

Euglena

Euglena are microscopic protists containing chloroplasts to photosynthesize sugars for food, but they can also ingest bacteria and small microbes for food too.

They has have flagellum to enable independent movement.

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Some typical learning objectives for this page on cell structure and function

Eukaryotes and prokaryotes : 

Know that plant and animal cells (eukaryotic cells) have a cell membrane, cytoplasm and genetic material enclosed in a nucleus.

Know that bacterial cells (prokaryotic cells) are much smaller and simpler in comparison. They have cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop and there may be one or more small rings of DNA called plasmids.

Know the similarities and differences between animal, plant and bacterial cells.

You should be able to explain how the main sub-cellular structures, including the nucleus, cell membranes, mitochondria, chloroplasts in plant cells and plasmids in bacterial cells are related to their functions.

Know that most animal cells have the following parts: a nucleus, which controls the activities of the cell, cytoplasm, in which most of the chemical reactions take place, a cell membrane, which controls the passage of substances into and out of the cell, mitochondria, which is where aerobic respiration takes place, ribosomes, which are where protein synthesis occurs.

Be able to recognise, draw and interpret images of cells.

Know the parts found in animal cells (listed above)

Know that plant cells often have: chloroplasts, which absorb light to make food by photosynthesis a permanent vacuole filled with cell sap.

Know that plant and algal cells have a more rigid cell wall made of cellulose, which strengthens the cell.

The diagram above illustrates some of the basic features to recognise, without the key below!

More detailed diagrams of typical animal cells and plants cells are in sections (b) and (c).

L = mitochondria, N = chloroplasts, O = cilia hairs, P = cell wall, R = cell membrane, T = vacuole, W = nucleus, Z = cytoplasm

Cell 1: Ciliated cell - mucous membrane cell with cilia hairs to move mucous along, found in nose, throat, stomach and lungs.

Cell 2: Plant cell - nucleus, chloroplasts, cell membrane, cell wall, large vacuole, nucleus, mitochondria

Cell 3: Red blood cell - has no nucleus, but important carrier of oxygen for respiration in all cells with mitochondria.

Cell 4: Non-specific cell - just showing cytoplasm, cell membrane, nucleus and mitochondria.

Cell 5: Sperm cell - showing swimming tail and nucleus.

Cell 6: Plant root hair cell - cytoplasm, nucleus, mitochondria, cell wall, cell membrane, (NO chloroplasts!)

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General CELL BIOLOGY and GENETICS revision notes index

Introduction to plant and animal cell structure and function - comparison of subcellular structures gcse biology revision notes

Stem cells and medical uses, and introduction to cell differentiation and specialisation gcse biology revision notes

Cell division - cell cycle - mitosis, meiosis, sexual/asexual reproduction, binary fission  gcse biology revision

Microscopy - the development and use of microscopes in biology - optical and electron  gcse biology revision notes

Diffusion - including demonstration, factors & Fick's Law, osmosis investigation and active transport  gcse biology

Examples of surfaces for the exchange of substances in animal organisms   gcse biology revision notes

Respiration - aerobic/anaerobic in plants, fungi & animals, substrates/products, experimental investigations

Enzymes - structure, functions, optimum conditions, investigation experiments, digestion  gcse biology revision

See also Enzymes and Biotechnology (gcse chemistry notes)

Culturing microorganisms like bacteria - testing antibiotics and antiseptics  gcse biology revision notes

DNA and RNA structure and Protein Synthesis  gcse biology revision notes

An introduction to genetic variation and the formation and consequence of mutations  gcse biology revision notes

Introduction to the inheritance of characteristics and genetic diagrams (including Punnett squares)  including technical terms, Mendel's work and inherited genetic disorder, genetic testing gcse biology revision notes

The human GENOME project - gene expression, chromosomes, alleles, genotype, phenotype, variations, uses of genetic testing including 'pros and cons' gcse biology revision notes

Inherited characteristics and human sexual reproduction, genetic fingerprinting and its uses gcse biology

Genetic engineering: uses - making insulin, medical applications, GM crops & food security gcse biology

More complicated genetics: Sex-linked genetic disorders, inheritance of blood groups gcse biology revision

See also section on Cloning - tissue culture of plants and animals gcse biology revision notes page

IGCSE revision notes biology cell structure KS4 Science notes on biology cell structure GCSE guide notes on biology cell structure for schools colleges academies science course tutors images pictures diagrams for biology cell structure science revision notes on biology cell structure for revising module topics notes to help on understanding of biology cell structure university courses in science careers in science jobs in the industry laboratory assistant apprenticeships technical internships USA US grade 8 grade 9 grade10 AQA science GCSE notes on biology cell structure Edexcel science notes on biology cell structure for OCR 21st century science OCR GCSE Gateway science notes WJEC gcse science CCEA/CEA gcse science

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