Cells - AQA A-level biology (AS)

Magnification

the number of times bigger the image or drawing is compared to the object to its real size

resolution

the minimum distance needed to differentiate between 2 adjacent objects

Micrometer

1x10-6

Nanometer

1x10-9

Light microscope

A type of microscope which has a condenser, objective lens and eyepiece lens and light is passed through the thin specimen an up through the objective and eyepiece lenses to the eye.

electron microscope

Beams of electrons are used to visualize structures in a vacuum. Electrons have a smaller wavelength than light so electron microscopes have a higher resolution than light microscopes.

Scanning electron microscope (SEM)

A type of electron microscope which bounces beams of electrons off the surface of an object to develop a 3D image of the specimen (no need therefore for thin sections).

Transmission electron microscope (TEM)

A type of electron microscope which asses a beam of electrons through a very thin section of specimen (which often has been stained with heavy metals to show up the fine internal structures).

Cell fractionation

the process where cells are broken up and the different organelles they contain are separated out

Homogenation

the first stage of cell fractionation when cells are broken up by a homogenizer or blender and organelles are released from the cell.

Ultracentrifugation

the second stage of cell fractionation when the fragments in filtered homogenate are separated in a machine called a centrifuge

Graticule

a series of lines on a microscope which can be used to calculate the size of objects

Eukaryote

A cell that has a membrane-bound nucleus and chromosomes. The cell also possesses a variety of other membrane-bound organelles, such as mitochondria and endoplasmic reticulum.

Organelle

a part of a cell

nucleus

organelle which contains the hereditary material (DNA) which codes for the synthesis of proteins in cytoplasm

mitochondria

organelle where energy is released in aerobic respiration

chloroplast

organelle where energy is released in aerobic respiration

chloroplast

organelle that is the site of photosynthesis

rough endoplasmic reticulum (RER)

organelle which provides a large surface area for the synthesis of proteins and glycoproteins and where proteins are transported that are syntheised on ribosomes

Smooth endoplasmic reticulum (SER)

organelle where lipids and carbohydrates are synthesized, stored and transported

Golgi

Organelle that contains a stack of membranes that make up flattened sacs (cisternae) where glycoproteins are synthesized, lipids are transported, modified and stores, proteins are packaged and lysosomes are formed.

Lyosome

Organelle formed when the vesicles produced by the Golgi apparatus contain enzymes. They are where unwanted materials & worn-out organelles are digested.

ribosome

organelle made of rRNA and protein where protein synthesis occurs

cell wall

Organelle which provides mechanical strength to prevent the cell bursting under the pressure created by the osmotic entry of water, to give mechanical strength. Made of cellulose in plants and chitin in fungi.

vacuole

Organelle which contains a solution of mineral salts, sugars, amino acids, wastes and sometimes pigments such as anthocyanins. They support herbaceous plants by making cells turgid, may provide a temporary food store and if they contain pigments they may colour petals to attract pollinating insects.

tissue

a collection of similar cells that perform a specific function

organ

a combination of different tissue that are coordinated to perform a variety of functions

organ system

many organs work together in an organ system to perform a particular function

prokaryote

a cell of an organism belonging to the kingdom prokaryote that is characterized by lacking a nucleus and membrane-bound organelles such as bacteria

plasmid

small cirucular loops of DNA which contain genes that bacteria can have

flagellum

part of a bacterial cell which helps the bacteria to move

virus

acellular , non-living pacticles that are smaller than bacteria. They contain DNA or RNA but can only multiply inside living host cells

capsid

a protein coat which encloses the nucleic acid in a virus

attachment protein

the capsid can have these which are essential to allow the virus to identify and attach to a host cell

mitosis

the type of nuclear division in which the daughter cells have the same number of chromosomes as the parent cell.

daughter cell

the cells that are produced by cell division

chromatid

one of the two stands of a chromosome that are joined together by a single centromere prior to cell division

centromere

the place where the two copies of DNA after replication are joined together

interphase

the period of the cell cycle when the cell is not dividing

prophase

The first stage of mitosis when the chromosomes become visible and when spindle fibres develop. The nucleolus disappears and the nuclear envelope breaks down.

metaphase

the second stage of mitosis when the chromosomes arrange themselves across the equator of the cell

anaphase

The third stage of mitosis when the centromeres divide into two and the spindle fibres pull the individual chromatids making up the chromosome apart to their respective, opposite poles of the cell.

telophase

The fourth stage of mitosis when chromosomes reach their respective poles and become longer and thinner, finally disappearing altogether, leaving only widely spread chromatin. The spindle fibres disintegrate and the nuclear envelope and nucleolus re-form.

cytokinesis

when the cytoplasm divides

spindle fibers

these form the spindle apparatus which are responsible for pulling the chromatids to separate ends of the cell

centrioles

where the spindle fibers develop from in animal cells

Equator

where the chromosomes arrange themselves during metaphase

binary fission

the process by which cell division occurs in prokaryotic cells

cancer

A group of diseases caused by a growth disorder of cells as a result of damage to the genes that regulate mitosis and the cell cycle which results in uncontrolled growth and division of cells.

tumor

a group of abnormal cells which develops and constantly expand in size

plasma membrane

membranes consisting of a phospholipid bilayer found around and within all cells. The cell-surface membrane is the plasma membrane that surronds cells.

phosopholipid

Triglyceride in which one of the three fatty acid molecules is replaced by a phosphate molecule. Phospholipids are important in the structure an functioning of plasma membranes.

bilayer

a membrane consisting of two layers of phospholipids

protein channel

a protein completely spanning the phosopholipid bilayer which form water-filled tubes to allow water soluble ions to diffuse across the membrane

carrier protein

A protein completely spanning the phospholipid bilayer which bind to ions or molecules then change shape in order to move these molecules across the membrane.

glycoprotein

Carbohydrate chains attached to a protein (often extrinsic) which are part of the cell surface membrane. They act as recognition sites, help cells to attach to one another and allows cells to recognise one another.

glycolipid

A carbohydrate covalently bonded with a lipid. They act as recognition sites, help maintain stability of the membrane and help cells attach to one another

cholesterol

lipid that is an important component of cell surface membranes because it adds strength. Excess in the blood can lead to atheroma

permeablility

How permeable a substance is depends on the size, polarity and charge of the molecule. If it is small, non-polar and fat soluble it is very permeable and can pass through the cell membrane.

fluid mosaic model

The arrangement of the various molecules of the cell-surface membrane. Fluid because the individual phospholipid molecules can move relative to one another and mosaic because the proteins vary in shape, size and pattern.

diffusion

the net movement of molecules or ions from a region of high concentration to a region of low concentration. It is passive.

facilitated diffusion

Diffusion involving the presence of protein carrier molecules to allow the passive movement of substances (normally large, polar or charged molecules) across plasma membrane.

osmosis

The passage of water from a region where there is a higher water potential to a region where there is a lower water potential through a partially permeable membrane.

water potential

The pressure created by water molecules. It is the measure of the extent to which a solution gives out water. The greater the number of water molecules present, the higher (less negative) the water potential. Pure water has a water potential of zero.

isotonic

a solution which has the same water potential as the cell within it

active transport

Movement of a substance from a region where it is in a low concentration to a region where it is in a high concentration. The process requires the expenditure of metabolic energy in the form of ATP.

co-transport

The transport of one substance coupled with the transport of another substance across a plasma membrane in the same direction through the same protein carrier

pathogen

a microorganism that causes disease

self

the body’s own cells and molecules

foriegn

not your own body’s cells and molecules

antigen

an molecule that triggers an immune response by lymphocytes

lymphocyte

type of white blood cell responsible for the immune response. They become activated in the presence of antigens. There are two types: B lymphocytes and T lymphocytes

Phagocyte

type of white blood cell which caries out a non-specific immune response and ingests and breaks down pathogens by phagocytosis.

phagocytosis

mechanism by which phagocytes engulf particles to form a vesicle or a vacuole

lysosome

contain enzymes called lysozymes which they release into the phagosome which hydrolze the bacterium

phagosome

a vesicle formed as the bacterium is engulfed by the phagocyte. The lysosome release their lysozymes into the phagosome

antigen-presentation

when ann antigen presenting cell- phagocyte displays foreign antigens on their own cell-surface membrane

cell mediated immunity

the type of response when t lymphocytes respond to antigens that are presented on a body cell

t cells

cells which mature in the thymus and are associated with cell mediated immunity

clonal selection

As the receptor on a helper T cell attaches to the antigen this activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells. These cloned T cells stimulate B cells to divide and form a clone of identical B cells all of which produce the antibody that is specific to the foreign antigen.

helper t cells

contain receptors which respond to a single antigen. Many different types of T cells, each one respond to a different antigen

cytotoxic t cells

kill abnormal cells and body cells that are infected by pathogens by producing a protein called perforin which makes holes in the cell-surface membrane

humoral immunity

the type of response which involves b lymphocytes and antibodies

b cells

each type of b cell produces a specific antibody that responds to one specific antigen

b cells

each type of b cell produces a specific antibody that responds to one specific antigen

plasma b cells

When the B cell is activated to divide by mitosis it gives a clone of plasma cells which produce and secrete the specific antibody that exactly fits the antigen on the pathogen’s surface.

antibody

a protein with specific binding sites produced by b cells in response to the presence of appropriate antigen

memory b cells

Some B cells develop into memory cells which can respond to future infections by the same pathogen by dividing rapidly and developing into plasma cells that produce antibodies as part of the secondary response.

monoclonal antibodies

antibodies produced by a single clone of cells

primary response

caused by initial response to the antigen which involves the production of antibodies and memory cells.

secondary response

Caused by second exposure to the antigen. Memory cells are responsible for this response – they divide rapidly and develop into plasma cells (which secrete many antibodies quickly) and more memory cells

passive immunity

Resistance to disease that is acquired from the introduction of antibodies from another individual, rather than an individual’s own immune system e.g. across the placenta or in the mother’s milk. It is usually short lived

active immunity

restitance to disease resulting from the activites of an individual’s own immune system whereby an antigen induces plasma cells to produce antibodies

natural immunity

a type of active immunity resulting from an individual becoming infected with a disease under normal cirucumstances

artifical immunity

a type of active immunity resulting from vaccination. It involves inducing an immune response in an individual without them suffering symtoms of the disease

vaccination

the introduction of a vaccine containing appropriate disease antigens into the body, by injection or mouth, in order to induce artificial immunity

herd immunity

arises when a sufficiently large proportion of the population has been vaccinated which makes it difficult for a pathogen to spread within the population

antigenic variablility

Pathogen may mutate frequently so that its antigens change suddenly rather than gradually which means vaccines become ineffective because the new antigens on the pathogen are no longer recognized by the immune system.

HIV

Human immunodeficiency virus which causes the disease acquired immune deficiency syndrome (AIDS). It has a lipid envelope, attachment proteins, a capsid and two single strands or RNA and enzymes.

reverse transcriptase

an enzyme in HIV which enables the production of DNA from RNA