Cells topic 2

Structure of Nucleus

1. Nuclear envelope: double membrane surrounding nucleus, outer membrane continuous with the (R)ER of the cell.
2. Nuclear pores: allow the passage of larger molecules, such as mRNA, out of the nucleus.
3. Nucleoplasm: granular, jelly-like material making up the bulk of the nucleus.
4. Chromosomes: protein-bound, linear DNA.
5. Nucleolus: small spherical region(s) in nucleoplasm. Manufactures ribosomal RNA and assembles ribosomes.

Function of the Nucleus

1. Controls cell's activities - produces mRNA and tRNA - protein synthesis. Controls entry and exit of materials, and contains nuclear reactions.
2. Retains genetic material in the form of DNA and chromosomes.
3. Manufactures ribosomal RNA and ribosomes.

Structure of Mitochondria

1. Double membrane surrounding organelle - controls entry and exit of material.
2. Cristae - extensions of the inner membrane, providing a large surface area for the attachment of enzymes and other proteins during respiration.
3. Matrix - makes up the remainder - contains proteins, lipids, ribosomes and DNA (allows mitochondria to produce own proteins) and some respiratory enzymes.

Function of the Mitochondria

1. Sites of Krebs Cycle and oxidative phosphorylation pathway in aerobic respiration - responsible for ATP production.

Structure of Chloroplasts

Found in plants and algae.
1. Chloroplast envelope - double plasma membrane, highly selective, surrounds the organelle.
2. Grana - stacks of disc-shaped thylakoid membrane.
3. Thylakoids - contain chlorophyll used in photosynthesis, can be linked by lamellae to other grana.
4. Stroma - fluid-filled matrix where Calvin Cycle takes place. Also contains starch grains.

Functions of Chloroplasts

Site of Photosynthesis:
LDR in thylakoid membranes.LIR in stroma.
1. Granal membranes provide a large SA for LDR - photosystems, e- carriers and enzymes etc.
2. Chloroplasts contain DNA and ribosomes - can quickly and easily manufacture some of the proteins needed for photosynthesis.

Structure of Endoplasmic Reticulum

1. 3D system of sheet-like membranes - continuous with the outer membrane of the nuclear double membrane.
2. Membrane contains a network of tubules and flattened sacs called cisternae.
3. RER - ribosomes on the outer surface of the membranes.
4. SER - lacks ribosomes on its surface and is often more tubular in its appearance.

Function of Endoplasmic Reticulum

RER
1. Large SA for protein/glycoprotein synthesis.
2. Provides a pathway for material transport throughout the cell, especially for proteins.
SER
1. Synthesises, stores and transports lipids and carbohydrates.

Structure of Golgi Apparatus

1. Compact system of flattened sacs and stacked membranes (cisternae).
2. Vesicles - modified proteins and lipids transported to cell membrane where they fuse with it, and then egest contents to the outside.

Function of Golgi Apparatus

1. Form glycoproteins by adding carbs to proteins.
2. Produce secretory enzymes, such as those secreted by the pancreas - apparatus is developed in secretory cells, especially those in the small intestine.
3. Secrete carbs, such as cellulose for plant cell walls.
4. Transports, modifies and stores lipids.
5. Forms lysosomes.

Structure of Lysosomes

Golgi vesicles with proteases, lipase and lysozymes.

Functions of Lysosomes

1. Hydrolyse foreign material ingested by phagocytes.
2. Exocytosis of enzymes to destroy extra-cellular material.
3. Apoptosis - programmed cell death.Autolysis - breaking down cells after death.
4. Digest worn out organelles - can recycle chemicals.

Structure of Ribosomes

1. Small cytoplasmic granules found in all cells, free-floating or associated with RER.
2. 80S - found in eukaryotic cells, slightly larger.
3. 70S - in prokaryotic cells, slightly smaller.
4. 2 Subunits - large and small - contain ribosomal RNA and proteins.

Functions of Ribosomes

Carry out translation stage of protein synthesis to produce polypeptides.

Structure of Cell Wall

Found in plants, algae and fungi.
1. Cellulose microfibrils embedded in a matrix - contribute to overall cell wall strength are considerably strong.and other polysaccharides.
2. Middle lamella - marks the boundary between adjacent cell walls and cements adjacent cells together.

Functions of Cell Wall

1. (Cellulose) - to provide mechanical strength to prevent cell wall bursting under pressure created by osmotic entry of water.
2. To provide mechanical strength to the cell as a whole.
3. Allows water to pass along it - contributes to the movement of water through the plant.

Structure of Vacuoles

1. Fluid-filled sac bounded by a single membrane.
2. Single membrane around it called tonoplast.
3. Solution of mineral salts, sugars, amino acids, wastes and sometimes pigments such as anthocyanins.

Functions of Vacuoles

1. Support herbaceous plants and herbaceous parts of woody plants by making cells turgid.
2. The sugars and amino acids can act as a temporary food source.
3. Pigments - may attract pollinating insects due to colour.

Can you explain that eukaryotic cells become specialised for specific functions in complex multicellular organisms?

In complex multicellular organisms, eukaryotic cells become specialised for specific functions. Specialised cells are organised into tissues, tissues into organs and organs into systems.

Can you describe how specialised cells are organised into tissues, organs and organ systems?

In multicellular eukaryotic organisms, specialised cells are grouped to form tissues. Different tissues join together to form organs and different organs make up and organ system.
E.g.Epithelial cells make up epithelial tissue. Epithelial, muscular and glandular tissues all work together to form the stomach - an organ. The stomach is part of the digestive system - an organ system made up of all the organs involved in digestion.

Define Tissue
Give an example

A collection of similar cells that are aggregated together and work together to perform a specific function.
Example = epithelial tissue - consists of sheets of cells, lining the surfaces of organs, often having a protective or secretory function.

Define organ.
Comment on the difference between capillaries, veins and arteries, relative to the term organ.

A combination of aggregated tissues that are co-ordinated together to perform a variety of functions, one of which is the predominant major function.
While capillaries, veins and arteries all have the same major function, i.e.e carrying blood, capillaries are not organs, unlike veins and arteries as they are made up of only one tissue - epithelium.

Define organ system.
Give examples

Organs working together as a single unit - systems may be grouped together to perform particular functions more efficiently.
Digestive, Respiratory, Circulatory systems etc.

Define eukaryotic cell

A larger cell with a true nucleus that is bounded by a nuclear membrane/nuclear envelope.

Can you name and explain an example of a eukaryotic cell with specific adaptations?

A sperm cell:- Many mitochondria for ATP production which to allow movement- Undulipodium (tail) for movement towards egg- Acrosome (specialised lysosome) to help the sperm penetrate the egg- Diploid nucleus, as it is a gamete

Define prokaryotic cell

A smaller cell which has no true nucleus or nuclear envelope.

Can you describe how prokaryotic cells differ from eukaryotic cells?

Prokaryotic cells:- have no nucleus- smaller 70S ribosomes- cytoplasm lacks membrane-bound organelles- much smaller cell- cell wall contains murein (a glycoprotein)- plasmids may be found in prokaryotic cells
Eukaryotic Cells:- have nucleus- 80S ribosomes- membrane-bound organelles- larger cell- cell wall made of cellulose or chitin- no plasmids

Can you list other features of prokaryotic cells?

Prokaryotic cells can have:- one or more plasmids- a slime capsule surrounding the cell- one or more flagella

Principles of Optical Microscopes

Simple convex glass lenses used in pairs in a compound light microscope to focus an object at a short distance by 1st lens, then magnified by 2nd lens.

Can you describe the pros and cons of optical microscopes?

Pros:- cheap- images in colour- no training required- live specimens
Cons:- low magnification x1500- low resolution- 2D images

Principles of Transmission Electron Microscopes

1. Electron gun produces e- beam, focused onto the specimen by a condenser electromagnet.
2. Beam passes through a thin section of the specimen from below. Parts absorb e- and appear dark; others let e- pass through and appear bright - produces image on screen - photomicrograph.

Can you describe the pros and cons of transmission electron microscopes?

Pros:- high resolution images- high magnification- visible internal structures
Cons:- expensive- training is required- no colour images- 2D images- only thin specimens

Principles of Scanning Electron Microscopes

1. Beam of e- directed onto surface of specimen - passed back and forth across specimen.
2. e- scattered by specimen - scattering pattern analysis allows us to get a 3D image.

Can you describe the pros and cons of scanning electron microscopes?

Pros:- 3D images- high magnification- high resolution- thick specimens
Cons:- expensive- training is required- no colour images

How do you prepare a slide for an optical microscope?

1. Pipette a drop of water onto the slide2. Use tweezers to place a thin section of your specimen on top of the droplet3. Add a drop of a stain4. Add a cover slip - remove all air bubbles

What is the difference between magnification and resolution?

MAGNIFICATION - increasing the size of an image. Up until the limit of resolution, an increase in magnification = an increase in detail.
RESOLUTION = minimum distance apart that two objects can be for them to appear as separate items.

What is a graticule? Why do we need it?

GRATICULE = a glass disc with an etched scale placed in the eyepiece of a microscope.
Needed to measure the size of objects under objective lensesNeed to calibrate the eyepiece graticule - each objective lens will magnify to a different degree.

Describe how to calibrate the eyepiece graticule

1. Use a stage micrometer = special microscope slide with an etched scale - line up scales on graticule and micrometer.
2. Once lined up, can calculate length of divisions on eyepiece graticule.
x40 mag gives 25microm per graticule unit
therefore, x400 mag gives 25/10microm per graticule unit.
NB = Only need to calibrate once, providing the same objective lens is used.

What is the formula to calculate Magnification?

Conversions:
km to mm to mmm to mmicrometre to mnanometre to m

km to m - x1000m to m - x1mm to m - /1000micrometre to m - /1,000,000 (x10^6)nanometre to m - /1,000,000,000 (x10^9)

Can you describe the principles of cell fractionation and ultracentrifugation in separating cell components?

1. Homogenisation- tissue is broken up in a cold, isotonic buffer solution to release the organelles into a solution
2. Filtration- the homogenised cell solution is filtered through a gauze- this separates any large cell debris
3. Ultracentrifugation- the cell fragments are poured into a test tube and placed in a centrifuge and spun at a low speed- a thick sediment - the pellet - is at the bottom of the tube and the fluid above is the supernatant- the supernatant is drained into a new tube and spun again at a higher speed- a new pellet forms and again, the supernatant is drained off and spun again at an even higher speed- this process is repeated at higher speeds each time until all the organelles are separated out

Why is a cold, isotonic buffer needed?

COLD - to reduce enzyme activity that could break down organelles.
ISOTONIC - same water potential as tissue sample - to prevent water moving in or out of the cells by osmosis, causing lysis.
BUFFERED - to prevent changes in pH which could affect/denature enzymes.

How are the organelles separated out during centrifugation?

They are separated in order of mass and the order is usually:- nuclei- mitochondria- lysosomes- endoplasmic reticulum- ribosomes

Define Virus

Acellular, non-living particles. Smaller than bacteria.

Can you explain that viruses are acellular and non-living?

Viruses are nucleic acids surrounded by protein so they are not living.

Can you describe the structure of virus particules?

- no plasma membrane, cytoplasm, ribosomes- a core of genetic material : DNA or RNA- capsid surrounding core- attachment proteins around edge of capsid

Outline the differences between mitosis and meiosis.

Mitosis results in 2 genetically identical diploid daughter cells.
Meiosis results in 4 genetically different haploid daughter cells.

List the stages of Mitosis

InterphaseProphaseMetaphaseAnaphaseTelophase

Outline Interphase

Precedes Mitosis
1. Cell is not dividing.2. Considerable cellular activity - replication of DNA, two copies on centromere.

Outline Prophase

1. Chromosomes become more visible, thicken.2. Centrioles move to opposite ends of the cell (poles).3. Spindle fibres develop from each of the centrioles (spindle apparatus).

Outline Metaphase

1. Chromosomes seen to be made up of two chromatids.2. Microtubules attach to centromere - chromosomes pulled to the cell equator where they line up.

Outline Anaphase

1. Centromeres divide, separating each pair of sister chromatids.2. Chromatids pulled to their respective poles as spindles contract, centromeres first - v-shaped.

Outline Telophase

1. Chromosomes reach their respective poles and then uncoil, become long and thin again - chromosomes again.2. Spindle fibres disintegrate; nuclear envelope and nucleolus reform.3. Cytoplasm divides in cytokinesis.

Why is mitosis so important?

GROWTH - ensures that all cells growing from original cell of an organism are genetically identical.
REPAIR - Important that replacement cells produced have an identical structure and function to lost cells.
REPRODUCTION - Single-celled organisms divide by mitosis to give 2 new organisms - each new organism is genetically identical to the parent organism.

How does cancer arise? Difference between benign and malignant?

Result of gene damage controlling mitosis and cell cycle.
Mutant cells are structurally and functionally different - most mutant cells die but surviving mutant cells become tumours.
Malignant - grow rapidly, less compact, more likely to be life-threatening.Benign - grow more slowly, more compact, less likely to be life-threatening.

How can cancer be treated?

How can cancer be treated?Involves killing dividing cells by blocking a part of the cell cycle - cell division and therefore cancer growth ceases.
Chemo disrupts cell cycle by preventing DNA replication or by inhibiting the metaphase stage of mitosis by interfering with spindle formation.

Outline the process of Binary Fission

How prokaryotic cells divide:
1. Circular DNA molecule replicates, and both copies attach to the cell membrane.2. Plasmids also replicate.3. Cell membrane begins to grow between the two DNA molecules and begins to pinch inwards - dividing cytoplasm in two.4. New cell wall forms between the DNA molecules, dividing the original cell into two identical daughter cells - each with a copy of the circular DNA and a variable number of copies of the plasmids.

Outline how viruses replicate

Acellular, non-living, so do not undergo cell division.
1. Attach to host cell with the attachment proteins on their surface.2. Inject nucleic acid into host cell.3. Viral nucleic acid then hijacks the cell's machinery and codes for metabolic processes to produce viral components - nucleic acid, enzymes, structural proteins, which are then assembled into new viruses.

Outline role of phospholipids in cell-surface membrane structure.

Their hydrophilic/hydrophobic interactions lead to the formation of a phospholipid bilayer.
1. Allow lipid-soluble substances to enter/exit cell.
2. Prevent water-soluble substances entering and leaving cell.
3. Make the membrane flexible and self-sealing.

Outline role of proteins in cell-surface membrane structure.

Some in surface of bilayer:
1. Act to provide mechanical support to membrane.
2. Along with glycolipids, act as cell receptors for molecules such as hormones.
Some span the entire membrane:
3. Protein channels - water filled tubes allowing water-soluble ions to diffuse across the membrane.
4. Protein carriers - bind to ions or molecules like glucose/amino acids - then change shape to move these molecules across the membrane.
5. Help cells adhere together.
6. Form cell-surface receptors for identifying cells.

Outline the role of cholesterol in cell-surface membrane structure.

Within the phospholipid bilayer of cell-surface membranes:
1. Reduce lateral movement of other molecules (including phospholipids) - pulls together the fatty acid tails, limiting movement without making the membrane too rigid.
2. Make the membrane less fluid at high temperatures.
3. Prevent leakage of water and dissolved ions from the cell, as cholesterol molecules are very hydrophobic.

Outline role of glycolipids in cell-surface membrane structure.

Carbohydrate covalently bonded to a lipid in the membrane:
1. Carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell - there acts as a cell-surface receptor for specific chemicals (ABO blood system)
2. Help maintain the stability of the membrane.
3. Help cells to attach to one another and so form tissues.

Outline the role of glycoproteins in cell-surface membrane structure

Carbohydrate chains attached to many proteins on the outer surface of the cell-surface membrane.
1. Cell-surface receptors for hormones and neurotransmitters.
2. Help cells to attach to one another and so form tissues.
3. Allow cells to recognise one another - lymphocytes can recognise an organism's own cells.

Comment on the permeability of the cell-surface membrane.

Controls the movement of substances into/out of the cell.
Most molecules don't freely diffuse across it because many are:
1. Not lipid-soluble2. Too large to pass through3. Same charge as protein channel charges - repelled even if small4. Charged/polar - can't pass through the non-polar hydrophobic tails in the phospholipid bilayer.

Explain the "fluid-mosaic" model of the cell-surface membrane structure.

= arrangement of all the various molecules combined into the structure.
Fluid = membrane is flexible and can constantly change in shape as individual phospholipid molecules can move relative to one another.
Mosaic = Proteins embedded in the phospholipid bilayer vary in shape, size

Define Diffusion

The net movement of molecules/ions/particles from a region of higher concentration to a region of lower concentration.

State Fick's Law

diffusion rate = (SA x gradient x permeability) / (membrane thickness).

Define Facilitated Diffusion

The movement of larger/charged/polar species made easier by protein channels/carriers that span the membrane.
It is a passive process, only difference is that fd occurs at specific points on the plasma membrane where there are protein molecules.

Comment on the roles of the proteins involved in facilitated diffusion.

CHANNEL Proteins = Water-filled hydrophilic channels allowing specific water-soluble ions to pass through- selective channels- only open when specific ion binds to protein causing it to change shape -\> open and closed on different sides of the membrane.
CARRIER Proteins = Molecule specific to the protein binds, changes shape of protein -\> molecule released to the inside of the membrane.

Define Osmosis

The net passage of water molecules from a region of higher water potential to a region of lower water potential across a selectively permeable membrane.

Comment on the water potential of a cell

The water potential is the pressure created by water molecules.
The addition of water will lower the wp.wp = 0 when pure water at 298K, standard conditions.
Water will move towards the area of lowest wp by osmosis, until an equilibrium is established.

The water potential of a plant cell is -400kPa. The cell is put in a solution of water potential of -650kPa. Describe and explain what will happen to the cell.

The cell membrane will shrink away from the cell wall and become flaccid. The water will move down the wp gradient via osmosis.

Comment on osmosis in plant cells when water moves in/out.

Protoplast (cell parts within cell wall) pushes on cell wall when swells - turgid.
If water leaves by osmosis, a point is reached where protoplast no longer presses on cellulose cell wall - incipient plasmolysis.
Further loss of water by osmosis -\> cell contents shrink further and protoplast pulls away from cell wall - cell is said to be plasmolysed.

Define Active Transport

The movement of particles into/out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.

Outline how active transport takes place

1. Molecules bind to a carrier protein and ATP attaches to the membrane protein on the inside of cell/organelle.
2. Binding of phosphate ion to protein changes shape, allowing access for molecules to inside of the membrane but closed to the outside.

Outline the process of Co-transport in the ileum

1. Na

Comment on the relationship between Fick's Law and the adaptations of specialised cells.

Specialised cells will have different surface areas, numbers of channel/carrier proteins, differences in concentration gradients/ wp ---- all to increase or decrease the rate of movement across cell membranes.
--\> think how permeability affected by numbers of proteins in a given area etc. - relate to fick's law.

Describe and explain the effect of an increase in temperature on the rate of diffusion

The rate of diffusion increases as the temperature rises because the molecules have more KE so move faster. There are more successful collisions.

List factors which result in a high rate of diffusion of molecules of a gas through a membrane

- increases difference in concentration- high temperature- high pressure- large surface area- short diffusion pathway

Comment on the significance of proteins on the cell surface in regards to recognition.

Protein molecules on the cell-surface membrane enable the immune system to identify:
1. Pathogens.2. Cells from other organisms of the same species - non-self material (different genes).3. Toxins (produced by pathogens - leads to chemotaxis)4. Abnormal body cells - such as cancer cells.

Define antigen

A molecule that triggers an immune response by lymphocytes.

Describe the effect of antigen variability on disease and disease prevention

Memory Cells from the primary immune response do not recognise the different antigens as they are immunologically distinct.=\> no secondary response - so primary response occurs again

Give an example of a disease with antigen variability and how it is prevented

Influenza virus
Influenza vaccine changes yearly -\> new immunologically distinct strains not recognised by memory cells circulate in population.=\> most suitable vaccine implemented in a vaccination programme.

Define Immunity

The ability of organisms to resist infection by protecting against disease-causing microorganisms or their toxins, that invade their bodies.

Outline the process of phagocytosis

1. Chemotaxis of phagocyte to pathogen as pathogen produces toxins and other materials. Phagocyte moves towards pathogen down concentration gradient.
2. Receptors on phagocyte surface attach to chemicals on pathogen surface.
3. Vesicle/Phagosome forms, engulfing the pathogen, and lysosomes migrate towards the vesicle.
4. Lysosomes release lysozymes into the vesicle, where they destroy the pathogen by hydrolysis.
5. Hydrolysis products of the bacterium/pathogen are absorbed by the phagocyte.

Define Antigen-presenting cell

A cell that displays foreign antigens on their surface.

Comment on the specificity of T-cells.

Receptors on each T-cell are complementary to and respond to only one antigen.
There are a vast number of different T-cells with each one responding to a different antigen.

Describe how T-lymphocytes can distinguish antigen-presenting cells from normal cells

1. Phagocytes that have hydrolysed pathogen present its antigens on their own cell-surface membrane.
2. Body cells invaded by a virus present some of the viral antigens on their own cell-surface membrane.
3. Transplanted cells have different antigens on their cell-surface membranes.
4. Cancer cells are different from normal body cells and present antigens on their own cell-surface membranes.

Define Cell-Mediated Immunity

T-lymphocytes respond only to antigens that are presented on a body cell, rather than antigens in the body's humour (fluids).

Outline the process of Cell-Mediated Immunity

1. Pathogens invade body cells/phagocytes.
2. Phagocyte places pathogen's antigens on its own cell-surface membrane.
3. Receptors on specific T-Cell complementary to this antigen.
4. Attachment activates rapid T-cell division by mitosis, to form clones of genetically identical cells.
5. Cloned T-Cells:
a) stimulate phagocytes to engulf pathogens by phagocytosis.
b) activate Tc-Cells which produce perforin protein which make pathogen membrane freely permeable.
c) develop into memory cells, enabling a rapid secondary immune response to future infections by the same pathogen.
d) stimulate clonal selection of B-Cells and stimulate them to secrete their antibody.

What are plasma cells?

Cells which secrete antibodies (usually) into blood plasma. They only survive for a few days - lead to destruction of pathogen.

What are memory cells?

Cells which circulate in blood and tissue fluid - divide rapidly into plasma cells and more memory cells -\> more antibodies at a faster rate (faster response).

Define humoral immunity

This involves B-cells and antibodies, soluble in blood and tissue fluids of body. The B-cells produce specific antibodies, specific to one antigen.

Outline the process of humoral immunity

1. Surface antigens of an invading pathogen are taken up by a B-cell.
2. B-cell processes antigens and presents them on its surface.
3. Th-cells (activated earlier) attach to the processed antigens on the B-cell and activate it.
4. B-cell is now activated to divide by mitosis to give a clone of plasma cells.
5. Cloned plasma cells produce monoclonal antibody and secrete it; antibody exactly complementary to antigen on pathogen's surface.
6. Antibody attaches to antigens on the pathogen

What is the primary immune response?

Production of antibodies and memory cells from new B-cells, after antigen enters body for the first time.

What is the secondary immune response?

Quicker, stronger immune response due to memory cells -\> clonal selection happens faster, B-memory cells activated and divide into plasma cells, producing necessary antibody.
Memory T-cells activated and divide into correct T-cells to kill cells presenting the antigen.

Define antibody

A protein produced by lymphocytes in response to the presence of the appropriate antigen

The structure of an antibody

- 4 polypeptide chains- antigen-binding sites.- constant and variable regions (variable region dependant on antibody).- receptor binding site.

Outline how antibodies help to destroy pathogens

Their 2 binding sites mean that antibodies can cause agglutination of the bacterial cells.
This makes it easier for phagocytes to locate them, as not spread out throughout the body.
They then serve as markers, stimulating phagocytes to engulf and destroy the bacterial cells to which the antibodies are attached.

Define Monoclonal Antibodies

Antibodies produced from a single group of genetically identical B-cells. Monoclonal antibodies are identical in structure.