Biology 2/8

Describe the structure of the cell membrane

Phospholipid bilayer with hydrophilic heads facing outwards and hydrophobic tails inwards Embedded proteins (intrinsic and extrinsic)

Contains cholesterol, glycoproteins, and glycolipids

Cell membrane function

Selectively permeable barrier

controls passage of substances in and out the cell

barrier between internal and external cell environments

Describe the structure of the nucleus

Surrounded by a double envelope with pores

contains chromatin (DNA associated with proteins)

nucleolus inside

Nucleus Function

Site of transcription & premRNA splicing - mRNA production

site of DNA replication

nucleolus makes ribosomes nuclear pores allow movement of substances (e.g. mRNA) between nucleus and cytoplasm

Mitochondria Structure

Double membrane with inner membrane folded into cristae

70S ribosomes in matrix

small, circular DNA

enzymes in matrix

Mitochondria Function

Site of aerobic respiration

produces ATP

Chloroplast structure

Thylakoid membranes stacked to form grana, linked by lamellae

stroma contains enzymes

contains starch granules, small circular DNA and 70S ribosomes

Chloroplast function

Chlorophyll absorbs light for photosynthesis to produce organic molecules such as glucose

Organisms containing chloroplasts

Plants

Algae

Golgi apparatus structure

Fluid-filled, membrane-bound sacs (horseshoe shaped) vesicles at edge

Golgi apparatus function

Modifies proteins received from RER

packages them into vesicles to transport to cell membrane for exocytosis

makes lysosomes

involved in the formation of glycoproteins

Lysosome structure

Type of Golgi vesicle containing digestive enzymes

Lysosome function

Contains digestive enzymes

e.g lysozymes to hydrolyse pathogens/cell waste products

Rough endoplasmic reticulum function

Site of protein synthesis

folds polypeptides to secondary & tertiary structures

packaging into vesicles to transport to Golgi

Smooth endoplasmic reticulum function

Synthesises and processes lipids

Cell wall function

Provides structural strength, rigidity and support to cell

helps resist osmotic pressures

Ribosome structure

Small and large subunit

made of protein and rRNA free floating in cytoplasm & bound to RER

70S in prokaryotes, mitochondria and chloroplasts

80S in eukaryotes

Ribosome function

Site of translation in protein synthesis

Rough endoplasmic reticulum structure

System of membranes with bound ribosomes

continuous with nucleus

Smooth endoplasmic reticulum structure

System of membranes with no bound ribosomes

Cell wall structure

In plant, fungal and bacterial cells

plants - made of microfibrils of cellulose

fungi - made of chitin

bacteria - murein

Cell vacuole structure

Fluid-filled

surrounded by a single membrane called a tonoplast

Contrast prokaryotic & eukaryotic cells

Prokaryotic cells are smaller

prokaryotes have no membrane bound organelles

prokaryotes have smaller 70S ribosomes

prokaryotes have no nucleus - circular DNA not associated with histones

prokaryotic cell wall made of murein instead of cellulose/chitin

Occasional features of prokaryotes

Plasmids - loops of DNA

capsule surrounding cell wall - helps agglutination + adds protection

flagella for movement

Cell vacuole function

Makes cells turgid - structural support

temporary store of sugars, amino acids

coloured pigments attract pollinators

Protein carriers

Bind with a molecule, e.g. glucose, which causes a change in the shape of the protein

this change in shape enables the molecule to be released to the other side of the membrane

Protein channels

Tubes filled with water enabling water-soluble ions to pass through the membrane

selective

channel proteins only open in the presence of certain ions when they bind to the protein

How are specialised cells organised in multicellular organisms?

In complex multicellular organisms:

Eukaryotic cells become specialised for specific functions

Specialised cells form tissues

Tissues group into organs

Organs work together in systems to carry out life processes

Features of viruses

Non living and acellular

contain genetic material, capsid and attachment proteins

some (HIV) contain a lipid envelope + enzymes (reverse transcriptase)

3 types of microscopes

Optical (light) microscopes

Scanning electron microscopes (SEM)

Transmission electron microscopes (TEM)

Magnification

How many times larger the image is compared to the object calculated by equation:

Magnification = Image size / Actual Size

Resolution

The minimum distance between two objects in which they can still be viewed as separate Resolution is limited by the wavelength of light or electrons, depending on the microscope type

Optical microscopes

Beam of light used to create image

glass lens used for focusing

2D coloured image produced

Evaluate optical microscopes

Poorer resolution as long wavelength of light - small organelles not visible

lower magnification

can view living samples

simple staining method

vacuum not required

Transmission electron microscopes

Beam of electrons passes through the sample used to create an image

focused using electromagnets

2D, black & white image produced

can see internal ultrastructure of cell

structures absorb electrons and appear dark

Evaluation TEMs

Highest resolving power

high magnification

extremely thin specimens required

complex staining method

specimen must be dead

vacuum required

Scanning electron microscopes

Beam of electrons pass across sample used to create image

focused using electromagnets

3D, black and white image produced

electrons scattered across specimen producing image

Evaluation SEMs

High resolving power

high magnification

thick specimens usable

complex staining method

specimen must be dead

vacuum required

Why calibrate eyepiece graticule?

Calibration of the eyepiece is required each time the objective lens is changed

calibrate to work out the distance between each division at that magnification

Purpose of cell fractionation

Break open cells & remove cell debris

so organelles can be studied

Homogenisation

Process by which cells are broken open so organelles are free to be separated

done using homogeniser (blender)

Homogenisation conditions

Cold reduces enzyme activity preventing organelle digestion

Isotonic prevents movement of water by osmosis - no bursting / shrivelling of organelles

Buffered resists pH changes preventing organelle + enzyme damage

Ultracentrifugation

Homogenate solution filtered to remove cell debris

solution placed in a centrifuge which spins at a low speed initially

then increasingly faster speeds to separate organelles according to their density

Differential centrifugation

Pellet contains the most dense organelles (e.g. nuclei) at lowest speed; supernatant is re-spun to isolate less dense organelles

spun at higher speeds

chloroplasts -> mitochondria -> lysosomes -> RER/SER -> ribosomes (least dense)

Binary Fission

Involves circular DNA & plasmids replicating

cytokinesis creates two daughter cells

each daughter cell has one copy of circular DNA and a variable number of plasmids

Cell cycle

1) Interphase (G1, S, G2)

2) nuclear division - mitosis or meiosis

3) cytokinesis

Interphase

Longest stage in the cell cycle

when DNA replicates (S-phase) and organelles duplicate while cell grows (G1&G2-phase)

DNA replicates and appears as two sister chromatids held by centromere

Mitosis

One round of cell division

two diploid, genetically identical daughter cells

growth and repair (e.g. clonal expansion)

comprised of prophase, metaphase, anaphase and telophase

Prophase

Chromosomes condense and become visible

nuclear envelope disintegrates

in animals - centrioles separate & spindle fibre structure forms

Metaphase

Chromosomes align along equator of cell

spindle fibres released from poles attach to centromere and chromatid

Anaphase

Spindle fibres contract using ATP to pull chromatids apart

centromere divides in two

Telophase

Chromosomes at each pole become longer and thinner again

spindle fibres disintegrate + nucleus reforms

Mitotic index

Used to determine the proportion of cells undergoing mitosis

Mitotic index = Total number of cells in mitosis / total number of cells

Fluid mosaic model

Describes the structure and fluidity of the membrane with proteins embedded in a phospholipid bilayer.

with scattered embedded intrinsic and extrinsic proteins

membrane contains glycoproteins, glycolipids, phospholipids and cholesterol

Phospholipids in membranes

Phospholipids align as a bilayer

hydrophilic heads are attracted to water

hydrophobic tails repelled by water

Cholesterol

Present in eukaryotic organisms to restrict the movement of other molecules making up the membranes

adds rigidity to the membrane resistant to high temperatures & prevents water + dissolved ions leaking out

Selectively permeable membrane

Molecules must have specific properties to pass through the plasma membrane

• lipid soluble (hormones e.g. oestrogen)

• very small molecules

• non-polar molecules (oxygen)

Simple diffusion

• Net movement of molecules from an area of higher concentration to an area of lower concentration

• until equilibrium is reached

• passive

Facilitated diffusion

• Passive process using protein channels/carriers

• down the concentration gradient

• used for ions and polar molecules e.g sodium ions

• and large molecules e.g. glucose

Osmosis

• Net movement of water

• from an area of higher water potential to an area of lower (more negative) water potential

• across a partially permeable membrane

Water potential

• The pressure created by water molecules

• measured in kPa and represented by symbol ψ

• pure water has a water potential of 0kPa the more negative

• the water potential, the more solute must be dissolved

Hypertonic solution

• When the water potential of a solution is more negative than the the cytoplasm of the cell

• water moves out of the cell by osmosis

• both animal and plant cells will shrink and shrivel

Hypotonic solution

• When the water potential of a solution is more positive (closer to zero) than the cytoplasm of the cell

• water moves into the cell by osmosis

• animal cells will lyse (burst)

• plant cells will become turgid

Isotonic

• When the water potential of the surrounding solution is the same as the water potential inside the cell

• no net movement in water

• cells would remain the same mass

Active transport

• The movement of ions and molecules from an area of lower concentration to an area of higher concentration using ATP and

• carrier proteins carrier proteins act as selective pumps to move substances

Role of carrier protein in active transport

• When molecules bind to the receptor - ATP will bind to protein on inside of membrane and is hydrolysed to ATP / Pi

• protein changes shape and opens inside membrane

Co-transport

• The movement of two substances across a membrane together, when one is unable to cross the membrane itself

• involves a cotransport protein

• involves active transport

• e.g. absorption of glucose/amino acids from lumen of intestines

Molecules lymphocytes identify

• Pathogens (bacteria, fungi, viruses)

• cells from other organisms of same species (transplants)

• abnormal body cells (tumour cells)

• toxins (released from bacteria)

Antigens

• Foreign proteins on the cellsurface membrane

• trigger an immune response when detected by lymphocytes

Antigenic variability

• When pathogenic DNA mutates, causing a change in shape to the antigen

• previous immunity is no longer effective as memory B cells do not recognise the altered shape of the antigen.

• specific antibody no longer binds to new antigen

Physical barriers

Anatomical barriers to pathogens

• skin

• stomach acid

• lysozymes in tears

What is the role of phagocytes in the immune response?

• Part of the non-specific immune response

• Engulf and digest pathogens using lysozyme in lysosomes (phagocytosis)

• After digestion, they present the pathogen’s antigens on their surface

• → becoming antigen-presenting cells (APCs) to activate T cells

T lymphocytes

• Made in bone marrow and mature in thymus gland

• involved in cell-mediated immune

• response respond to antigen-presenting cells

Antigenpresenting cells

Any cell that presents a non-self antigen on their surface

• infected body cells

• macrophage after phagocytosis

• cells of transplanted organ

• cancer cells

Role of helper T cells

• Have receptors on their membrane that attach to antigens on APCs

• become activated - clonal selection

• Activate B cells to divide and produce antibodies

• Stimulate cytotoxic T cells

Role of cloned helper T cells

• activate B lymphocytes

• stimulate macrophages for phagocytosis

• become cytotoxic killer T cells

Cytotoxic T cells

• Destroy abnormal/infected cells by releasing perforin

• which forms pores in the target cell membrane, leading to cell lysis

B lymphocytes

• Made in bone marrow and mature in bone marrow

• involved in humoral immune response

• involves antibodies

Humoral response

• B cells are activated by helper T cells (clonal selection)

• B cells divide by mitosis (clonal expansion)

• Differentiate into: → Plasma cells: produce specific antibodies

• → Memory B cells: provide long-term immunity

B memory cells

• derived from B lymphocytes

• remember specific antibody for particular antigen

• will rapidly divide by mitosis and differentiate in plasma cells upon secondary encounter

• resulting in large numbers of antibodies rapidly

Antibodies

• Quaternary structure proteins made of four polypeptide chains

• different shaped binding site = variable region

• complementary to a specific antigen

Agglutination

• Antibodies have two binding sites and are flexible

• antibody binds to multiple antigens, clumping pathogens together (agglutination), making them easier to destroy.

Passive immunity

• Antibodies introduced into body

• plasma and memory cells not made as no interaction with antigen

• short-term immunity

• fast acting

Active immunity

• Immunity created by own immune system - antibodies made

• exposure to an antigen

• plasma and memory B cells made

• long-term immunity

• slower acting

Natural active immunity

• After direct contact with pathogen through infection

• body creates antibodies and memory B cells

Artificial active immunity

• Creation of antibodies and memory cells following introduction of an attenuated pathogen or antigens

• vaccination

Vaccinations

• Small amounts of dead or attenuated pathogens or antigens are injected/ingested

• activating the primary immune response, leading to the production of plasma and

• memory cells. memory cells are able to divide rapidly into plasma cells when reinfected

Primary vs Secondary response

• Primary = first exposure to the pathogen

• longer time for plasma cell secretion & memory cell production

• for the secondary response, memory cells divide rapidly into plasma cells

• so a large number of antibodies made rapidly upon reinfection

Herd immunity

• When a large proportion of the population is vaccinated,

• → the spread of the pathogen is reduced

• → providing protection for individuals who are not immune (e.g. unvaccinated or immunocompromised)

Monoclonal antibodies

• A single type of antibody that can be isolated and cloned

• antibodies that are identical - from one type of B lymphocyte

• complementary to only one antigen

Uses of monoclonal antibodies

• Medical diagnosis: e.g. pregnancy tests, detecting specific antigens (e.g. HIV, cancer markers)

• Targeted drug treatment: antibodies deliver drugs directly to specific cells (e.g. tumour cells)

• ELISA tests: detect presence and quantity of an antigen

Pregnancy test

ELISA test which uses 3 monoclonal antibodies and enzymes to test for hCG

Purpose of ELISA test

• Detect the presence and quantity of an antigen

• used for medical diagnosis. Eg., HIV

Ethical issues with monoclonal antibodies

• Use of animals (e.g. mice) in antibody production raises welfare concerns

• Involves creating tumour cells to produce hybridomas

• Potential for serious side effects in some patients during treatment

• Requires informed consent for patients in clinical trials

HIV structure

• Core = RNA and reverse transcriptase

• capsid = protein coat

• lipid envelope taken from hosts cell membrane

• attachment proteins so it can attach to Helper T cells

HIV replication

• HIV binds to the CD4 receptor on a T helper cell

• Viral RNA & reverse transcriptase enter the cell

• Reverse transcriptase converts viral RNA into DNA

• Viral DNA is inserted into the host’s DNA

• Host cell machinery makes viral proteins & RNA

• New HIV particles assemble and leave the cell

Auto Immunodeficiency Syndrome (AIDs)

• When HIV has destroyed too many T helper cells, host is unable to produce adequate immune response to other pathogens

• host susceptible to opportunistic infections and cancer

Role of antibodies in ELISA

• First antibody added is complementary to antigen in well - attaches

• second antibody with enzyme added which attaches to first antibody as complementary.

• when substrate solution added enzyme can produce colour change

Why vaccines may be unsafe

• Inactive virus may become active - viral transformation

• non-pathogenic virus can mutate and harm cells

• side effects of immune response

• people may test positive for disease

How is mitosis linked to cancer and its treatment?

• Mitosis is normally a controlled process

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

• Many cancer treatments target and disrupt the cell cycle to slow or stop cell division (e.g. by preventing DNA replication or spindle formation)