3.3.2 Cells

Structure and function of Eukaryotic cells

Cell surface membrane

Structure:

  • main components are phospholipids (glycerol + phosphate head, 2 fatty acids as tail) and proteins

  • phospholipids form a bilayer (hydrophobic tails point towards each other and hydrophilic heads on outside)

  • proteins embedded between phospholipids (intrinsic proteins extend through bilayer, extrinsic proteins exits on outer layer)

  • cholesterol within bilayer stabilises and reduces permeability of membrane

Function:

  • forms a barrier between cells and surroundings

  • strong = offers structural support

  • flexible = allows cell to grow and divide

  • self sealing - cell can divide without bursting

  • selectively permeable - controls passage of material in and out of cells

Nucleus

Structure:

  • nucleolus produces RNA used to make ribosomes

  • endoplasmic reticulum on surface with nuclear pores between them

  • chromatin - form of DNA when cell not in mitosis

Function:

  • control cell via DNA and protein synthesis

  • contain genetic material

  • synthesise RNA and ribosomes

Mitochondria

Structure:

  • double membrane forms mitochondrial envelope

  • inner membrane folded into cristae (increased surface area for enzymes in respiration)

Function:

  • location of anaerobic respiration

  • outer membrane controls exit/entry

Chloroplasts

Structure:

  • Thylakoids contains chlorophyll

  • Stacks of thylakoids = granum

Function:

  • grana provide large surface area for chlorophyll

  • stroma contains enzymes for stage 2 photosynthesis

  • absorb light energy for photosynthesis

Golgi apparatus/vesicles

Structure:

  • compact stack of membranes

  • consists of cisternae

Function:

  • assembles, modifies and packages lipids

  • forms lysosomes

Lysosomes

Structure:

  • contain lysozymes

Function:

  • store hydrolytic enzymes

  • digest and recycle worn out organelles

  • hydrolyse ingested bacteria

  • autolysis of cell after death

Ribosomes

Structure:

  • larger 80S ribosomes in cytoplasm and RER

  • smaller 70S ribosomes in mitochondria

  • composed of 2 subunits

Function:

  • protein synthesis (location of condensation reactions that join amino acids)

Rough/smooth endoplasmic reticulum

Structure:

  • Rough ER = ribosomes attached

  • Smooth ER = no ribosomes

Function:

  • transport system

  • RER = transport of proteins

  • SER = synthesis, storage, modification and transport of lipids and carbohydrates

Cell wall

Structure:

  • made from cellulose

  • plasmodesmata between plant cells

Function:

  • cell walls allow plant cells to become full of water without bursting

  • cell wall is fully permeable

  • plasmodesmata allow water and minerals to pass through cells easier

  • provides strength and structure for cell/plant

Cell vacuole

Function:

  • holds solutions

Structure and function of Prokaryotic cells

Prokaryotic cells are much smaller than Eukaryotic cells

  • Cytoplasm

    • Lacks membrane bound organelles

  • Ribosomes

    • smaller 70S ribosomes

  • Free DNA

    • circular chromosome not enclosed in nucleus and not associated with proteins

  • Cell wall

    • contains murein (glycoprotein)

  • Plasmids



    • small circular loops of DNA

  • Cell capsule

  • Flagella









Microscopy

formula: magnification = size of image/size of real object

resolution = ability to distinguish between two objects that are close together and see greater detail. Determined by wavelength

magnification = increase in apparent size of object

Light microscope

Uses lenses

Lower resolution due to longer wavelength = less detailed

lower magnification = can only see cells not organelles

living specimen on slide to see active processes e.g. mitosis

Produces colour images

Electron microscope

Transmission EM - used to study fine internal structure of cells

Scanning EM - used to study surface structure of cells and to obtain 3D images

In this type of microscope specimens are dead because they are in vacuum (need oxygen) and sections must be extremely thin to allow electrons to pass through

Uses magnets

Higher resolution due to shorter wavelength = more detailed

higher magnification = can see smaller organelles

Only black and white images produced

Risk of artefacts - visible details that aren’t part of specimen being observed e.g. finger prints

Requires more complex preparation

Cell fractionation

Cell fractionation = isolating organelles from cells for autolysis to obtain knowledge about their function

Organelles separated due to differences in density

Homogenisation:

  • chopped tissues homogenised in an isotonic, buffered, cold solution

    • COLD = slows metabolic activity and prevents autolysis

    • ISOTONIC = salt concentration same as cell so water will not enter/leave cell via osmosis so organelles will not burst/shrivel and can function as usual

    • BUFFERED = maintains specific pH which prevents damage to enzymes and other proteins in organelles (no denaturing)

  • homogenate transferred into centrifuge tubes

Centrifugation:

  • first sample centrifuged at low speed for short period of time

  • cell debris collects in pellet at the base below supernatant, which contains organelles

  • supernatant centrifuged at high speed for long time

  • separated by densities - heavier on bottom

Densities: nucleus, chloroplast, mitochondria, lysosomes, ER, ribosomes

Cell replication

Mitosis

Interphase - cell carries out normal functions:

  • Growth 1 - increase in rate of synthesis

  • Synthesis - DNA is replicated

  • Growth 2 - energy stores increase

MITOSIS:

  • Prophase

    • chromosomes condense and become visible as 2 sister chromatids joined by a centromere

    • nucleoulus disappears

    • centrioles migrate to poles of cell

    • nuclear envelope starts to disappear

  • Metaphase

    • chromosomes line up along equator of cell

    • spindle fibres attach to centromere

    • nuclear envelope disappears

  • Anaphase

    • spindle fibres contract and pull sisters chromatids to opposite poles of cell

    • poles move further apart - lengthening cell

    • each chromatid now called daughter chromosome

  • Telophase

    • daughter chromosomes form groups at opposite poles and become surrounded by new nuclear envelopes

    • spindle disappears

    • nucleoli reform

Cytokinesis - cytoplasm divides to form 2 daughter cells

Mitosis is a controlled process

Uncontrolled cell division can lead to the formation of tumours and cancers

Many cancer treatments are directed at controlling the rate of cell division e.g. slowing down cell cycle, stopping spindle fibres attaching to centromere, inhibiting DNA replication

Binary fission

Occurs in prokaryotic cells:

  • replication of the circular DNA and of plasmids

  • division of the cytoplasm to produce two daughter cells, each with a single copy of the circular DNA and a variable number of copies of plasmids

Viral replication

  • virus attachment proteins bind to complementary receptor proteins on the surface of a host cell

  • the virus then injects its DNA or RNA into the host cell

  • the host cell then uses its nucleic acid and protein-building machinery (ribosomes) to produce new viral particles

  • host cell bursts and new viral particles released

Transport across cell membranes

Plasma membrane structure

Fluid mosaic:

  • fluid because individual molecules can move relative to each other

  • mosaic because its made up of phospholipids and proteins

Diffusion

The net movement of molecules from a region of high concentration to lower concentration down a concentration gradient (passive process)

Factors affecting diffusion:

  • size of concentration gradient

  • thickness of exchange surface (diffusion pathway)

  • areas over which diffusion takes place (surface area)

  • temperature

Facilitated diffusion:

  • carrier proteins move large molecules in or out of cell down a concentration gradient

  • molecules bind to specific carrier protein

  • carrier undergoes conformational shape change

  • molecule released on other side

  • passive process

Active transport

Movement of molecules in/out of a cell from region of low concentration to high concentration against a concentration gradient using energy in the form of ATP and carrier proteins

  • specific solute binds to carrier protein on one side of membrane

  • hydrolysis of ATP into ADP and Pi causes conformational change in shape of carrier protein

  • solute molecule is consequently translocated accross membrane against gradient and released

Co-transport

A form of active transport which uses ATP to transport 2 distinct molecules across plasma membrane using carrier proteins

  • sodium ions actively transported out of cell by potassium pump

  • higher concentration of sodium in intestinal lumen compared to inside cell

  • sodium diffuse down their concentration gradient through the co transport protein and carry glucose or amino acids with them

  • glucose or amino acids pass into blood by facilitated diffusion

Osmosis

Net movement of water from a high water potential to a low water potential through a selectively permeable membrane down a water potential gradient (passive)

Water travels through the membrane via aquaporins (channel proteins)

Water diffusing into cell = hypotonic

water diffusing out of cell = hypertonic

Immune system

Antigens

Antigen = any part of organism or substance (usually a protein) that is recognised as foreign. This stimulates an immune response causing production of antibodies

Antigens recognise:

  • pathogens

  • toxins

  • abnormal body cells (cancerous)

  • foreign material

Phagocytosis

  • phagocyte attracted by a substance

  • pathogen engulfed and enclosed in vesicle (phagosome)

  • vesicle fuses with lysosome to form phagolysosome

  • lysosome contains enzymes (lysozymes)

  • pathogen digested/hydrolysed

  • molecules leave by exocytosis

Humoral immunity

  • macrophages engulf pathogen and become antigen presenting cells

  • T helper cells bind through their complimentary receptor to the antigen and are activated

  • Surface antigens taken up by B cells which also display them on surface

  • Specific T helper cells attach to same antigen on the B cells

  • Specific B cells are activated by T helper cells and divide by mitosis to form clones of same B cell

  • Plasma cells (secrete antibodies) or memory B cells formed

Antibodies

Antibody diagram

4 polypeptide chains - quaternary protein structure

Complimentary to antigens to form antigen antibody complex

Function of antibodies:

  • AGGLUTINATION - leading to destruction of pathogen

  • Mark pathogens for destruction (by macrophages)

Monoclonal antibodies:

  • antibodies produced by a single clone of B cells so they all have the same tertiary structure - can only bind and will all recognise 1 antigen

  • used to target cancer cells

  • ELISA test:

    • sample added and antigen stick to well

    • mAB that is specific to antigen added

    • excess washed away to remove any unbound antibody

    • 2nd antibody that binds to 1st antibody added - has an enzyme attached to it

    • excess washed away

    • colourless substrate of enzyme is added and will change colour of solution - amount of antigen present is relative to the intensity of colour that develops

Vaccines

Herd immunity: immunising a sufficiently large number of people to protect an entire population from the spread of a particular disease

Problems with vaccines:

  • pathogen may mutate

  • some may not get vaccinated

  • some many variations of antigens that it is impossible to develop vaccines for them all

HIV

  • antibiotics are ineffective against HIV as viruses rely on host cells

  • Anti-retroviral therapy (drugs)

HIV diagram