AQA Categories Archives: 3.2 Cells

Cell recognition and the immune system

Cells (AQA AS Biology) PART 6 of 6 TOPICS



TOPICS: Structure of eukaryotic cells  Structure of prokaryotic cells and viruses  Methods of studying cells  All cells arise from other cells  Transport across cell membs  Cell recognition and the immune system

Cell recognition and the immune system:

Each cell in our body has an antigen which tells the body whether it is foreign (not part of the body) or self (is part of the body). An antigen is a protein on the surface that triggers an immune response. Certain influenza viruses keep changing their antigens to avoid an attack by antibodies and T cells.

Phagocytosis should be known: Receptors on the phagocyte (a white blood cell) attach to the antigens on the microorganism which is called opsonisation (NB: Opsonisation does not need to be known). The microorganism is then engulfed into a vesicle called a phagosome. A lysosome fuses all its hydrolytic enzymes such as lysozymes into the phagosome to make it a phagolysosome. Once the microorganism has been hydrolysed it will be eliminated by exocytosis where it leaves via a vesicle. (NB: You have to say hydrolysed and not broken down as you will not get a mark). The antigens that were on the antigen are used by the phagocyte on the surface to become an antigen-presenting cell.

This phagocyte interacts with a T-Helper cell that can recognise the complementary antigen. Once it has binded it causes the number of cytotoxic T cells and B cells to increase. Normal body cells can become antigen-presenting cell when infected. Complementary cytotoxic T cells bind to the antigen to produce chemicals which kill the infected cell. With B cells, if a T-Helper cell binds to a B cell it will cause it to differentiate into many plasma cells that make the same antibodies but in bigger numbers. One the pathogens have been eliminated most of the plasma cells are destructed with a few plasma cells becoming memory cells so that a secondary response is faster and better when a second invasion happens. These memory cells last for several decades.

An antigen is a protein that forms antigen-antibody complexes when complementary paired with an antigen.

Agglutination is where antibodies called agglutinins link bacterial cells together to form a clump called an agglutinate. Some viruses react to the surface of red blood cells (the haemaglutinins) to cause haemagglutination.

Herd immunity is where a significant amount of people in a population are vaccinated to stop the spread of infections. If only a few people are vaccinated then herd immunity will fail.

Active immunity is where the body makes its own antibodies against pathogens whereas passive immunity is where antibodies are made by someone else.

Replication of HIV is needed to be known: HIV has glycoproteins as its receptors on a lipid envelope. It has a capsid in the middle which contains an RNA molecule and reverse transcriptase, integrase and protease. It attaches its receptors onto a T-Helper cell by complementary shapes. The HIV molecule enters the cell by endocytosis leaving all its glycoproteins on the surface of the cell and fusing its RNA molecule and three enzymes. Reverse transcriptase makes a DNA molecule using the RNA strand and degrades the RNA strand. A second DNA molecule is made and becomes a circular DNA molecule. This is then incorporated into the host’s DNA by the enzyme integrase. This has now become proviral DNA and this can remain dormant. RNA polymerase uses this to make mRNA and the viral genome RNA. The mRNA is translated to make viral enzymes and viral protein structures. Protease breaks some of the poly-proteins up which is used as a core round the viral RNA and is released by budding out of the cell where the glycoproteins were on the membrane. This is continued to be made without the damaging the body until the cell is ruptured. The same HIV molecules can target many more T-Helper cells leading to AIDS when he body’s immune system has weakened.

Some monoclonal antibodies deliver chemotherapy direct to the cancer cells which then are destroyed.

Pregnancy test kits have monoclonal antibodies which bind to a hormone called HCG which is passed in the urine. The monoclonal antibodies have a molecule attached to them which is blue so if positive a blue line appears.

The ELISA test is where antibodies are attached at the bottom of a container. A mixture of HCG (for example) linked to an enzyme ae added to the test mixture. If no HCG without the enzyme is present then the HCG with the enzyme will bind to the antibodies leading to a colour change in solution when a complementary substrate is added. Vice versa means that there will be less binding of HCG with the enzyme to the antibodies.

Monoclonal antibodies have disadvantages too. They do not just target diseased tissue but healthy tissue as well which is why chemotherapy treatment is once in a while. Access to this kind of treatment or the use of monoclonal antibodies in general circumstances depends on race and socioeconomic status.


Continue Reading

Transport across membranes

Cells (AQA AS Biology) PART 5 of 6 TOPICS



TOPICS: Structure of eukaryotic cells  Structure of prokaryotic cells and viruses  Methods of studying cells  All cells arise from other cells  Transport across cell membs  Cell recognition and the immune system

Transport across membranes:

The cell membrane of any cell has many features:

  • phospholipid bi-layer which allows water and gases to diffuse across but not large molecules
  • Glycoproteins and glycolipids act as receptors
  • Ion channels allow molecules that are too hydrophilic to go through the lipid bi-layer
  • Carrier proteins use energy in the form of ATP to actively transport molecules
  • Cholesterol stops the movement of other types of molecules

Many types of transport occur:

Facilitated diffusion involves a carrier protein which has a special binding site for specific molecules. When the molecule is attached the carrier protein changes shape to allow the molecule to go down its concentration gradient on to the other side. As this is diffusion, this always involves certain carrier proteins so the molecules that use this carrier protein always go down their concentration gradient. Channel proteins open when a certain type of molecule binds to its receptors called acetylcholine (NB: This name does not need to be known for the exam, however it is good to know what you are talking about). This channel is called a ligand-gatted ion channel. Ions such as Na+ diffuse down its concentration gradient.

Osmosis is where water molecules move from a high water potential to a low water potential through a semi-permeable membrane (NB: It is important that you say potential and not concentration at AS AQA Level as you will not get the mark. It is important that you say semi-permeable membrane and not just membrane because it does not allow all things to go through only water molecules and gases as you will not get the mark). Solutions that have a higher concentration of solutes than the cell they are surrounding are called hypertonic. Vice versa means that they are hypotonic. If the cell and the solution both have the same concentration of solutes then they are called isotonic with respect to the solution.

Active transport involves ATP being hydrolysed into ADP and Pi. The Pi controls the transport of solutes through another type of carrier protein up its concentration gradient.

Cells may be adapted also to do rapid transport where they have many proteins as part of their cell membrane.

Continue Reading

All cells arise from other cells

Cells (AQA A2 Biology) PART 4 of 6 TOPICS



TOPICS: Structure of eukaryotic cells  Structure of prokaryotic cells and viruses  Methods of studying cells  All cells arise from other cells  Transport across cell membs  Cell recognition and the immune system

All cells arise from other cells:

In a cell that is not undergoing division, DNA is visible as an indistinct mass called chromatin and nuclear membrane as well as nucleolus are still intact. Centrioles lie closely to one another.

Mitosis should be known thoroughly to such detail:

  • Interphase: There are two growth phases where the cell makes organelles as well as enzymes to get ready for mitosis. There is also an S-phase where the DNA replicates.
  • Prophase: DNA undergoes spiralization where it gets shorter and fatter. Two identical chromatids are held together at the centromere. Nucleolus and nuclear membrane disintegrate and centrioles move to opposite poles of the cell and form a spindle. NB: It is important that you say poles and not ends as you will not get a mark.
  • Metaphase: Chromosomes lies at the equator of the cell where the kinetochore of the centromere is attached to the spindle fibres. NB: It is important that you say equator of the cell and not the middle of the cell as you will not get a mark. Also you may not include kinetochore as it does need to be needed.
  • Anaphase: Centromeres divide when spindle fibres contract pulling each sister chromatid to opposite poles of the cell. Nuclear membrane reforms and the spindle disintegrates. NB: It is important that you say sister chromatid and not just chromatid as you will not get a mark.
  • Telophase (cytokinesis): The cell divides into two daughter cells. The end product being the two daughter cells have equal amount of DNA as each other as well as being compared to the parent cell.

Cell division can also be lethal as uncontrolled cell division can lead to cancers and tumours.

Binary fission involves a bacterial cell increasing in length and doubling its parts. It forms a cross septum and divides into two. The time involved to double a population is called the generation time. Two copies are separated by the growth of cell membrane.

Viruses also replicate. They inject DNA and RNA into the host cell leaving the protein coat or can enter by endocytosis. Retroviruses have an enzyme called reverse transcriptase which causes viral DNA to be made on a template of viral RNA/DNA depending on what the virus has injected. This is called reverse transcription. It acts as a gene and is passed onto daughter cells and when activated it causes the cell to burst (lysis) because of a large amount of viruses replicating quickly.

The Mitotic Index should be known:

  • n/N x 100 where
  • n = number of cells in mitosis
  • N = number of cells observed


Continue Reading

Structure of eukaryotic cells

Cells (AQA AS Biology) PART 1 of 6 TOPICS



TOPICS: Structure of eukaryotic cells  Structure of prokaryotic cells and viruses  Methods of studying cells  All cells arise from other cells  Transport across cell membs  Cell recognition and the immune system

Structure of eukaryotic cells:

The ultra cellular structure of a eukaryotic cell (animals and plants)must be known with the functions of organelles:

  • Cell surface membrane is selectively permeable to control the exchange and is mainly made up of lipids and proteins.
  • Nucleus is enclosed of a membrane which is made of two membranes itself and is perforated by nuclear pores which controls the exchange between the cytoplasm and the nucleus. The nucleus contains chromatin which is the state of DNA when the cell is not dividing. A nucleolus is present in the nucleus which manufactures ribosomes.
  • Mitochondria are cigar-shaped organelles and generate ATP by aerobic respiration. It has two membranes where the inner membrane is deeply folded to make cristae which maximises surface area. Between this membrane and the outer membrane there is a space called intermembrane space. The watery matrix in the middle contains enzymes, a circular loop of DNA and granules.
  • Chloroplasts have an envelope of two membranes with a gel-like gelatinous matrix called the stroma. The stroma contains a system of membranes that contain photosynthetic pigments called thylakoid membranes are stacks of grana and lamellae as well having its own loop of DNA, enzymes and ribosomes.
  • Sacs of the golgi apparatus are called cisternae. Newly formed membranes created on the ends of these sacs are called vesicles. Golgi apparatus is a processing and packaging structure and allows enzymes to be secreted via vesicles and also makes lysosomes.
  • Lysosomes are membrane bound vesicles containing hydrolytic enzymes called lysozymes. These enzymes hydrolyse proteins, nucleic acids, lipids and carbohydrates and are made in the rough endoplasmic reticulum. Transport vesicles transport these enzymes to the golgi apparatus.
  • Rough endoplasmic reticulum is involved in folding, modification and transport of proteins. It has ribosomes on its endoplasmic reticulum.
  • Ribosomes are small organelles where protein synthesis occurs.
  • Smooth endoplasmic reticulum is involved in lipid synthesis.
  • Cell wall (only in plants) is made up of cellulose which is rigid and supports the cell.
  • Vacuoles (only in plants) are full of fluid which is used to make the cell rigid, used as a storage and waste matter can be collected of decomposing complex molecules.
  • Plasmodesmata (only in plants) are pores between two plant cells which help to transfer substances between cells

Eukaryotic cells have membrane bound organelles.

Continue Reading

Structure of prokaryotic cells

Cells (AQA AS Biology) PART 2 of 6 TOPICS



TOPICS: Structure of eukaryotic cells  Structure of prokaryotic cells and viruses  Methods of studying cells  All cells arise from other cells  Transport across cell membs  Cell recognition and the immune system

Structure of prokaryotic cells:

Prokaryotic cells are different to eukaryotic cells in many ways:

  • They are smaller to eukaryotes
  • They have no membrane-bound organelles
  • They have no nuclei but a loop of DNA
  • They have murein (a glycoprotein) in their cell walls

In addition prokaryotes have structures such as having more than one plasmids, a capsule surrounding the cell and more than one flagella.


Viruses are a-cellular and are non-living. They have a protein coat with a nucleic acid in the middle (DNA). It has an attachment protein which attaches to a cell.

NB: As part of AQA, only the illustration of complex and spherical viruses need to be known. Above is a description of a complex virus and spherical is where HIV is under

which will be covered in later topics.

] That is all for the structure of prokaryotes and viruses [

Continue Reading

Methods of studying cells

Cells (AQA AS Biology) PART 3 of 6 TOPICS



TOPICS: Structure of eukaryotic cells  Structure of prokaryotic cells and viruses  Methods of studying cells  All cells arise from other cells  Transport across cell membs  Cell recognition and the immune system

Methods of studying cells:

Optical microscopes


  • Cheaper than electron microscopes


  • Magnification and resolution is not as electron microscopes


Transmission electron microscopes


  • Smaller wavelength of electron beam allows organelles smaller than mitochondria to be seen



  • Specimen has to be dead


Scanning electron microscopes


  • Gives 3D structure of organelles


  • It is very expensive



Cell fractionation has three processes:

  1. Homogenisation is where the cell membrane is broken to release the organelles into an ice cold (preventing enzyme activity), isotonic (preventing shrivelling and bursting of organelles) buffer (to keep the pH constant to prevent the enzymes and organelles from denaturing).
  2. Filtration is where the cell membrane is separated from the organelles as well as large debris using a gauze.
  3. Ultracentrifugation is where the organelles are separated from each other. All the organelles are poured into a tube and centrifuged where it is spun at high speed at an angle. Heaviest organelles such as nuclei form a pellet at the bottom with all the lighter organelles at the top called the supernatant. The supernatant is drained off into another tube where it is spun at a higher speed at an angle to get the next heaviest organelles (mitochondria) and the process is repeated until all the organelles are separated.
Continue Reading