Cells

Structure of a eukaryotic cell

Structure of a prokaryotic cell

Structure of a virus

Cell surface membrane

- Found on inside surface of animal cells, inside cell wall of other cells
- Made mainly of lipids and proteins
- Regulates movement of substances, into and out of cell
- Has receptor molecules, so can respond to chemicals, e.g. hormones

Nucleus

- A large organelle surrounded by a nuclear envelope, contains pores
- Contains chromosomes (made from protein bound linear DNA)
- Contains nucleotides
- Controls cell's activity (controls transcription of DNA)
- Nucleolus makes ribosomes

Mitochondrion

- Has a double membrane
- Inner membrane is folded to form cristae, inside this is the matrix, which contains enzymes involved in respiration
- Site of aerobic respiration to produce ATP

Chloroplast

- Small, flattened structure found in plant and algal cells
- Surrounded by a double membrane, has inner thylakoid membranes which are stacked up to form grant, grant are linked by lamellae, then, flat pieces of thylakoid membrane
- Site where photosynthesis takes place - some in grant and some in stroma (fluid)

Golgi apparatus

- Group of fluid-filled, membrane-bound flattened sacs
- Vesicles are often seen at the edges of sacs
- Modifies, packages and transports new lipids and proteins, also makes lysosomes

Golgi vesicle

- Small, fluid-filled sac in cytoplasm, surrounded by membrane
- Stores lipids and proteins made by golgi apparatus, then transports out of cell

Lysosome

- Round organelle surrounded by membrane
- Contains hydrolytic digestive enzymes, to digest invading cells or break down old components of the cell

Ribosome

- Small, free floating in cytoplasm, or attached to rough ER
- Made up of proteins and RNA
- Site where proteins are synthesised

Rough endoplasmic reticulum

- System of membranes enclosing a fluid-filled space - covered with ribosomes
- Folds and processes proteins that have been made at ribosomes

Smooth endoplasmic reticulum

- Similar to RER, but no ribosomes
- Synthesises and processes lipids

Cell wall

- Rigid structure that surrounds cells in plants, algae, fungi
- Made of cellulose / chitin (in fungi)
- Supports cell and prevent it form changing shape

Cell vacuole (plant cells)

- Found in cytoplasm, contains cell sap-solution of sugar and salts
- Surrounding membrane is called tonoplast
- Helps maintain pressure inside cell and keep cell rigid
- Also involved in isolation of unwanted chemicals in the cell

Why are epithelial cells described as specialised cells?

- In small intestine, adapted to absorb efficiently
- Villi on walls increase surface area
- Villi have folds in cell surface membrane called microvilli to increase surface area
- Lots of mitochondria, to provide energy for transport of digested food molecules into cell

Why are red blood cells described as specialised cells?

- No nucleus, so more room for O2
- Concave, increase surface area

Why are sperm cells described as specialised cells?

- Many mitochondria, to provide energy
- Flagellum to aid movement forward

Magnification

How much bigger an image is than specimen

Resolution

How well microscope distinguishes between 2 points

What is the equation for image size?

image size = magnification x actual size

How many micrometres in 1 millimetre?

1000

How many nanometres in 1 micrometre?

1000

How do optical (light) microscopes work?

Use light to form an image, focused by glass lenses, air

Advantages of optical microscopes

- Cheap
- Easy to maintain
- View living organisms

Disadvantages of optical microscopes

- Low resolution
- Low magnification

How do transmission electron microscopes work?

Use electromagnets to focus a beam of electrons, transmitted through the specimen. Denser parts absorb more electrons, so appear darker in image, in vacuum

Advantages of TEM

- High resolution

Disadvantages of TEM

- Only for thin specimen
- Only for non-living specimen

How do scanning electron microscopes work?

Scans a beam of electrons across specimen to knock off electron from specimen, which are gathered in a cathode tube to form an image

Advantages of SEM

- Used on thick specimens
- 3D image

Disadvantages of SEM

- Lower resolution
- Only non-living specimens

How do you prepare an optical microscope slide?

- Put specimen on glass slide, using temporary mount
- Pipette drop of water onto mount
- Use tweezers to put thin specimen on top
- Add stain and cover slip

What are artefacts?

Things that you can see that aren't part of the specimen

Stages of cell fractionation

1. Homogenisation - break plasma membrane and release cells into solution
2. Filtration - separate large cell/tissue debris
3. Ultracentrifugation - Pour cell fragments into tube, centrifuge at low speed (heaviest organelles, e.g. nucleus, form a sediment pellet), supernatant drained off, poured into another tube, spun at a higher speed until all organelles separated out

In cell fractionation, why is solution ice cold?

To reduce enzyme activity

In cell fractionation, why is solution isotonic?

To prevent osmotic effects

In cell fractionation, why is solution buffered?

To maintain pH

Describe interphase

G1 - cell grows, new organelles and proteins made
S - DNA unravels and replicated, ready to divide
G2 - cell grows, proteins needed for mitosis made, ATP content increased

Describe prophase

- Chromosomes shorten and thicken (condense)
- Centrioles move to opposite points, forming spindle fibres
- Nuclear envelope breaks down, chromosomes free in cytoplasm

Describe metaphase

- Chromosomes line up on equator of cell, attach to spindle fibres by centromeres

Describe anaphase

- Centromeres divide, separating each pair of sister chromatids
- Spindles contract, pulling them to opposite poles

Describe telophase

- Chromatids uncoil, become long and thin = chromosomes
- Nuclear envelope forms around each group = 2 nuclei

Describe cytokinesis

Cytoplasm divides, creating 2 identical daughter cells

What is cancer?

The uncontrollable division of cells, leading to the formation of tumours and cancers

Chemotherapy

Prevents synthesis of enzymes needed for DNA replication, so cell is unable to live G1 stage to S, so cell cycle is interrupted and cell kills itself

Radiotherapy

Damages DNA, this damage is detected at check-points, and the cell kills itself

Hormone therapy

Stops growth of cancers dependent on hormones, e.g. prostate cancer

Surgery

Removes tumour

What are the stages of binary fission?

1. Circular DNA and plasmids replicate
2. Cell gets bigger, DNA loops move to opposite poles
3. Cytoplasm begins to divide (cell wall forms)
4. Cytoplasm divides and 2 daughter cells formed, each with 1 copy of loop of DNA, but variable plasmids

What are the stages of virus replication?

1. Viruses attach to host cell's receptor proteins
2. Genetic material (nucleic acid) injected into host cell
3. Host cell replicates injected material
4. Viral components assemble (viruses formed)
5. Replicated viruses 'burst' out, causing death of host cell

Describe the fluid mosaic model of the cell membrane

- Phopholipid molecules form a bilayer which is fluid (molecules constantly moving)
- Heads = hydrophilic, tails = hydrophobic
- Once emerged in water, the orientate themselves to form a bilayer, meaning they are amphipathic
- Bilayer is not bonded, stable and enables molecules to move freely
- Small, non-polar molecules can pass bilayer easily, and H2O even through it is polar
Channel and carrier proteins present

What are intrinsic proteins?

Span both lengths of phospholipid bilayer

What are extrinsic proteins?

Span one length of phospholipid bilayer

What are glycolipids and glycoproteins?

Lipids and proteins with short carbohydrate chains attached to them

How does temperature affect the phospholipid bilayer?

- Below 0 - little energy, tightly packed, channel and carrier proteins denature
- 0 to 45 - more energy, highly permeable
- 45+ - bilayer starts to break down, more permeable, channel and carrier proteins denature

Describe simple diffusion

- Passive
- Net movement of particles from high to low concentration
- Steeper gradient = higher rate of diffusion
- Small, non-polar and H2O can pass easily

What factors affect diffusion?

- Length of diffusion pathway
- Surface area
- Concentration gradient

Describe facilitated diffusion

- Passive
- Large or charged particles diffuse through carrier or channel proteins
- Down concentration gradient (high to low)
- Uses carrier and channel proteins

Carrier proteins

Attached to molecule, protein changes shape to allow molecule to pass through membrane

Channel proteins

Form pores in membrane for particles to diffuse through

What factors affect facilitated diffusion?

- Concentration gradient
- Number of proteins

Describe osmosis

- Passive
- Diffusion of water molecules across a partially permeable membrane from area of high water potential to low water potential
- More negative the water potential = more solutes

Describe isotonic solution and its effect on animal and plant cells

- Same water potential in and out of cell
- No net movement of water
- No change, plant may be flaccid

Describe hypOtonic solution and its effect on animal and plant cells

- Higher water potential outside cell
- Animal = lysis
- Plant = turgid

Describe hypERtonic solution and its effect on animal and plant cells

- Higher water potential inside cell
- Animal = crenation
- Plant = cytoplasm shrinks, plasmolysis

Factors affecting osmosis

- Water potential gradient
- Thickness of exchange surface
- Surface area

Describe active transport

- Active
- Against concentration gradient
- Carrier proteins - a molecule attaches, protein changes shape which moves molecule across membrane
- ATP hydrolysed to form ADP and Pi, which releases energy so solutes can be transported

Co-transport of sodium and glucose

1. Na+ ions are pumped out of epithelial cel to blood by active transport, through sodium-potassium pump
2. This creates a concentration gradient, as there is a higher conc of Na+ in lumen than cell
3. Na+ ions enter epithelial cell from lumen, through symports via co-tranport with glucose
4. Concentration of glucose inside cell increases
5. Glucose moves down conc gradient into blood by facilitated diffusion

Factors affecting co-tranport

- Speed of carrier proteins
- Number of carrier proteins
- Rate of respiration and availability of ATP

What is an antigen?

Molecules that generate an immune response

What is a pathogen?

Organisms that cause disease, have foreign antigens on their surface

What are abnormal body cells?

Cancerous or pathogen infected cells with abnormal antigens on surface

What is a toxin?

Poisonous molecules (no antigens)

Why are cells from other individuals of same species issues?

e.g. organ transplant or blood transfusion - different antigens

What is antigenic variation?

Pathogens change their surface antigens due to changes in DNA code

Describe phagocytosis

- Non-specific response
- A phagocyte recognises foreign antigens on pathogen, as the pathogen's chemical products attract phagocytes
- Phagocytes bind to pathogen's surface, as receptors bind to antibodies
- The membrane of phagocyte folds inwards, forming pseudopodia, so pathogen is trapped in phagosome
- Lysosomes fuse with phagosomes, and release hydrolytic digestive enzymes (e.g. lysozyme) which digest the bacterium
- Nutrients can then be absorbed in cytoplasm or exocytosed

What is a phagocyte?

A phagocyte (e.g. macrophage/neutrophil) is a type of WBC found in blood and tissues and are the first cells to respond to an immune system trigger

Describe the response of a helper T cell in the cellular response

- Specific response
1. Antigen-presenting cell comes into contact with T cell
2. Helper T cells divide by mitosis to produce clones = clonal selection
3. Release cytokines which stimulate B cells and cytotoxic T cells and macrophages to carry out phagocytosis

Describe the response of a cytotoxic T cell in the cellular response

- Specific response
1. Antigen-presenting cell comes into contact with T cell
2. Cytotoxic T cells divide by mitosis to produce clones = clonal selection
3. Each Tc cell will attach to specific antigens it has receptors for
4. Secretes toxic substances, such as hydrogen peroxide, which pierces holes in membrane
5. Causes death of cell

Describe the role of B lymphocytes in the humeral response

- Specific response
1. B lymphocyte comes into contact with antigen-presenting cell
2. Activated B cell divides by mitosis = clonal selection
3. Plasma cells have large numbers of ribosomes to manufacture and secrete lots of specific antibodies which attach to antigens to destroy cell
(antibodies produced by plasma cells are monoclonal antibodies as they are genetically identical)
4. Memory cells remember antibodies so when antigens invade again, they can be destroyed before symptoms occur

What is an antibody?

A protein produces by lymphocytes, in response to the presence of the complementary antigen

Structure of an antibody

- The shape of antigen-binding site and side chains on amino acids are different for different antibodies
- Bonds form with specific antigen to form an antigen-antibody complex

What is agglutination?

- When many antibodies attach to antigens on surface of bacteria
- Bacteria clumps together, stuck and unable to move or reproduce
- This attracts macrophages and neutrophils which destroy them by phagocytosis

Primary and secondary immune response

- At first exposure, it takes time for plasma cells to meet pathogen and clone
- At second exposure, memory cells can produce antibodies quickly as antigen is recognised

What is antigenic variability? Give an example

Caused by mutations, e.g. new flu strain each year as old vaccine will not work

Describe what happens when a vaccination is given

- Vaccine contains dead or attenuated pathogen
- Vaccine is injected which stimulates a primary response which leaves memory cells to generate a secondary response if individual is infected again
- Prevents significant harm while primary response is happening

What is active immunity?

Person is exposed to antigen and makes their own antibodies against it, retaining memory cells for long-lasting immunity

What is passive immunity?

Person does not make their own antibodies so memory cells do not retain info, short-term immunity

What is herd immunity?

- As long as most (about 95%) are vaccinated, it usually does not matter if a few are not
- It would be very difficult for bacteria/virus to move to unvaccinated individuals
- Protects young children and people with an immune system that is not working effectively

Structure of HIV

What does HIV stand for?

Human Immunodeficiency Virus

Why is HIV described as a retrovirus?

Genetic material is RNA, not DNA

Describe HIV replication

- HIV infects a particular type of helper T cell
1. Membrane contains glycoproteins which enable virus to identify, bind to and fuse with host cell
2. Once it enters, RNA is converted to DNA using enzyme reverse transcriptase
3. This codes for production of new viruses
4. DNA is incorporated into chromosomes of host cell
5. Person can live for many years without symptoms
6. Eventually, viral DNA is activated, transcribed and translated and bursts out

How is AIDS acquired?

1. Body produces anti-HIV antibodies
2. Gradually, number of functioning helper T cells decreases
3. Eventually, there are so few Th cells left that the person is unable to fight off other pathogens
4. AIDS is acquired

What does AIDS stand for?

Acquired Immunodeficiency Syndrome

Why are antibiotics ineffective against HIV?

Antibiotics destroy cell walls or interfere with metabolic pathways in cells, but viruses are acellular = no cell wall or metabolism

What are used to treat HIV and how do they work?

- Antiretrovirals (ARV) bind to revers transcriptase and stop it from copying virus' DNA
- Do not cure HIV, just prevents symptoms worsening

What are monoclonal antibodies?

Antibodies produced by large clones of plasma cells, all secrete same specific antibody

How are monoclonal antibodies useful in medicine?

- Can be produced with complementary shapes to abnormal proteins (e.g. cancer) and bind to cancer cells to deliver drug to those cells only
- Used in medical diagnosis (e.g. pregnancy tests - monoclonal antibody binds to HCG hormone which causes a colour change)
- Reduce effects of auto-immune diseases