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A-level cells summary

Edward Ward`

on 8 December 2012

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Transcript of Cells

By Edward Ward Cells Summary New Cells Cell cycle-Describes events that take place as one parent cell splits produce 2 new genetically identical daughter cells Interphase
G1 Organelles replicated
Proteins for DNA replication made
S Synthesis of new DNA (chromosomes replicated, 2 replicas of the chromosomes are produced, held together at the centromere)

G2 Cell growth Mitosis - comes after interphase
4 Stages
Telophase Prophase
Nuclear envelope disappears
Chromosomes condense
Centrioles move to opposite poles of cell
Spindle fibres form Metphase
Chromosomes (made up of a pair of sister chromatids) attach to spindle fibres by centromere
Chromosomes moved to centre of cell Anaphase
Spindle fibres shorten
Centromere splits, sister chromatids move apart
Sister chromatids are now classed as individual chromosomes
Chromosomes pulled towards pole in V shape as centromere leads Telophase
Spindle fibres dissolve
Nuclei reform around the chromosomes at opposite poles
Chromosomes relax and uncoil so they can no longer be seen
Cell starts nipping in at sides as cytokinesis begins Cytokinesis
Cell starts nipping in down the sides along a cleavage furrow
Eventually furrow reaches across the entire cell and 2 cells split apart Plant cells are different
Only meristem cells can undergo mitosis
Plants do not have centrioles so they can't produce spindles, instead the tubulin protein threads are made in the cytoplasm
Cytokinesis begins with a cell plate forming where the equator was, then new material for cell membrane and cell wall is laid down along the cell plate Mitosis - Produces 2 genetically identical daughter cells Meiosis - produces 4 hapliod daughter cells Meiosis is a 2 stage process
1st stage is exactly like mitosis
During the 2nd stage Homologous chromosomes cross over genes and are held together by the crossed over genes
Then they are pulled apart and put into 2 separate cells, which means each daughter cell only has half the correct number of chromosomes and is haploid Differences between meiosis and mitosis
The products of meiosis are haploid, products of mitosis are diploid
The products of meiosis are not genetically identical, the products of mitosis are
Meiosis only occurs in the sex organs, mitosis can occur anywhere Microscopy Light Microscopes Electron Microscopes Chromatin - material staining dark red in nucleus during interphase of mitosis. consists of nucleic acid and proteins. Chromatin condenses into chromosomes during prophase Magnification - The degree to which an image is larger than the actual object itself. Resolution - The degree to which it is possible to see 2 different points that are very close together. The higher the resolution, the greater the detail you can see. Light microscopes use multiple lenses to produce am image that can be viewed directly at the eyepiece lens
Light is passed though a condenser lens, then through the sample
Beam of light is focused through the objective lens and then the eyepiece lens Light microscope fact file
Magnification - 1500x
Resolution - 200nm Types of Electron Microscope
Transmission electron microscope (TEM)
Scanning electron microscope (SEM) Transmission Electron microscope
Electron beam passes through very thin sample
Electrons pass through denser parts of the sample less easily, giving contrast
Final image is 2D
Magnification possible 500 000x Scanning Electron Microscope
Electron beam directed onto sample and bounce off it
Image produced is projected onto screen or photographic paper
Final image is 3D
Magnification possible is 100 000x Electron microscopes generate beam of electrons
Beam of electrons has a wavelength of 0.004nm, 100 000x shorter than a light wavelength
Magnets are used to focus the beam of electrons onto a specimen
Electrons aren't visible to the human eye so the image produced is projected onto a screen or photographic paper - image is called an electron micrograph Advantages of an Electron microscope
High magnification 500 000x
High resolution 0.2nm
Can produce 3D images so you can look at the outside of cells Disadvantages of an Electron microscope
They're very large and expensive
Samples have to be placed in a vacuum so the air particles don't deflect the electrons
Preparing samples, and using electron microscopes both require a high degree of skill and training Advantages of a light microscope
Cheap so schools can acquire them
Easy to use
Can look at a lot of things using them
Portable so useful for field work if transporting samples is difficult Disadvantages of a light microscope
Low magnification 1500x
Low resolution 200nm - limited by the wavelength of light, 2 objects can only be seen if light waves can pass between them Cell Size and Magnification Actual Size X Magnification Image Size If you have 2 pieces shown in the triangle you can calculate the other one by using the equation shown on the left. Just remember to convert mm to um by multiplying by 1000. Stem cells and clones Stem cells Stem cells - Undifferentiated cells capable of becoming a number of possible cell types Stem cells are produced by mitosis Stem cells can omnipotent (totipotent) or pluripotent Omnipotent (totipotent) stem cells can become any type of cell by differentiation
Pluripotent stem cells can become most types of cells Stem cells occur naturally in adults in small numbers
These stem cells are called adult stem cells and they are pluripotent

Stem cells also form in the uterus during fertilisation as an embryo
Embryonic stem cells are omnipotent Clones Clones are genetically identical cells or organisms from one parent Cells produced during mitosis are clones All bacteria in a colony are clones, produced by binary fission Binary fission has 2 stages:
DNA replicates
Cell divides in 2, no mitosis involved Many plants reproduce asexually during vegetative propogation
Main plant sends out runners
Buds appear along runner
Bud grows into clone and runner to main plant splits Bacteria are prokaryotes (have no nuclear envelope)
Bacteria have a single, naked (not associated with hisonte proteins)
Bacterial also may have plasmids of DNA
Plasmids may have genes for antibiotic resistance
Plasmids can be swapped, so plasmids are useful in genetic engineering Artificial Cloning Natural cloning Cuttings taken from plants are genetically identical to the parent plant Placing a nucleus into an empty egg cell and reinserting it into the womb produces an organism genetically identical to the nucleus donator Artificial cloning involves the intervention or guidance of man The use of egg cells is an area of science which raises many ethical issues Epithelial tissues – form linings
Connective tissue – holds structures together and provides support (this includes blood)
Muscle tissue – contract to allow movement
Nervous tissue – create electrical impulses from stimuli and allows us to conduct these impulses Types of animal tissues Plants need water, minerals and carbohydrates in the entirety of the plant
Xylem and Phloem move these things around the plant
Xylem and phloem come from dividing meristem cells such as cambium Transport tissues Cells can differentiate by:
Changing the numbers of a particular organelle
Changing the shape of the cell
Changing the contents of the cell

In some cases differentiation may involve all 3 types Cells can differentiate in a number of ways Upper epidermis
See through to allow all the light through to the lower levels of the plant full of chloroplasts Cuticle
See through to allow all the sun light through to the lower levels
Waterproof to prevent water loss Cuticle and upper epidermis Column shaped
Exposed surface is covered in cilia
The cilia move at the same time moving what’s on their surface
Cilia use ATP to move
Ciliated epithelial cells found in trachea with goblet cells
Goblet cells produce mucus
Mucus traps dust and microorganisms, preventing them from reaching the lungs
Cilia move mucus up to throat to be swallowed
Also found in fallopian tubes where they move eggs Ciliated epithelial tissue Meristem cells produce cells that elongate and end of cells break down – continuous tube
Walls of cells reinforced with lignin deposits – also kills contents of cells
Xylem made up of xylem vessels and parenchyma cells and fibres
Carry water UP the plant Xylem Large projection increases surface area of cell
More room for diffusion to take place
Large vacuole pushes cytoplasm against cell wall and plasma membrane, keeping cell turgid and projection stiff
No chloroplasts as cell is found underground Root Hair cell Cells keep nucleus
Cytoplasm looks granular because enormous numbers of lysosomes are produced
Lysosomes contain powerful digestive enzymes
Neutrophil designed to ingest and break down invading organisms Neutrophils (type of white blood cell) Cells lose nucleus, Golgi apparatus, Rough Endoplasmic Reticulum and mitochondria
Cells are packed full of protein (polypeptide) haemoglobin
Shape of cell changes so they become biconcave discs Erythrocytes (Red bloods cells) Single celled organisms have a large surface-area-to-volume ratio so they can receive oxygen and remove carbon dioxide through their membrane

Multicellular organisms have a smaller surface-area-to-volume ratio and not all cells are in contact with the external medium
Therefore multicellular organisms need specialised cells, to form tissues and organs, for specific functions Size matters Differentiation refers to the changes occurring in cells of a multicellular organism so that each different type of cell become specialised to perform a specific function Differentiation Spongy mesophyll layer
Loosely packed cells
Gases can move around in this layer
CO₂ can get to palisade layer for photosynthesis
O₂ can get to stomata to leave leaf Palisade mesophyll layer
Tightly packed cells
Long and thin to get as many in as possible
Full of chloroplasts to absorb as much light as possible Mesophyll layers A leaf is made up of several different tissues all working together to maximise photosynthesis Leaves – The organs of photosynthesis Cells flatten and become thin
Form thin, flat and smooth tissues
Allows liquids to pass over them easily
They also form thin walls
Short diffusion pathway (alveoli)

Held in place by basement membrane
Made of collagen and glycoproteins
Attaches epithelial tissue to connective tissue Squamous epithelial tissue Meristem cells produce cells that elongate and line up
Ends of cells don not fully break down to make sieve plates
Phloem made up of sieve tubes and companion cells
Companion cell found next to sieve tube
Companion cells have dense cytoplasm and are very metabolically active
Play an important role in moving carbohydrates and amino acids UP and DOWN the plant Phloem Sperm Cell Cell is small, thin and long so it’s more hydrodynamic
Nucleus only contains 23 chromosomes so it produces a genetically different organism when it fuses with the egg nucleus
Lysosome in head contain acrosome to break down wall of egg so nucleus can fuse with egg nucleus
Lots of mitochondria produce lots of ATP so the undulipodium on sperm cell can move propelling it around Guard cells
Cells which open and close stomata
Inner cell wall contains a thick spiral of cellulose
When cell becomes turgid only the outer cell wall stretches
Guard cells bulge at edges and stoma opens Stomata
Pores in bottom of leaf
Can be opened or closed by guard cells
Allows exchange of gases in leaf
CO₂ into plant
O₂ out of plant Stomata and Guard cells Clearly the larger object has a lower surface-area-to-volume ratio. This means it’s more difficult for the larger object to get the right amount of oxygen and nutrients to survive Surface area: 54
Volume: 27
Ratio: 2:1 Surface area: 6
Volume: 1
Ratio: 6:1 3 1 3 1 3 1 There is a physical limit to the maximum size of a cell
This limit is governed by the need to support structures within the cell and the surface-area-to-volume ratio Size matters Structure inside a cell Organelles Found only in plant cells and bacteria
In plant cells cell wall is made of cellulose
In bacteria cell wall is made of peptidoglycan (murein)
Found outside plasma membrane
Cell wall is very strong so when cell absorbs a lot of water it doesn’t burst, it instead becomes turgid Cell walls Small tubes of protein fibres (microtubules)
Pair of them next to nucleus in animal cells
May be found in protoctists
Not found in plant cells
Centrioles take part in cell division
Form spindle fibres which move the chromosomes during mitosis Centrioles May be spherical or sausage shaped
Made of 2 membranes separated by a fluid filled space
Inner membrane highly folded to form cristae
Cristae increase surface area
Central part of mitochondria, matrix, (area in inner membrane) contains mitochondria DNA
Site of aerobic respiration, where ATP produced Mitochondria Only found in plant cells and protoctists
Site of photosynthesis
2 membranes separated by a fluid filled space
Continuous inner membrane with an elaborate network of flattened membrane sacs called thylakoids
Stack of thylakoids is a granum
Chlorophyll molecules are present on the thylakoid membranes
Stroma is like the cytoplasm of a cell
Stroma lamellae connect granum and keep them apart
Chlorophyll absorbs light, splits water molecules into hydrogen ions in granum
Hydrogen ions made into ATP and reduced NADP (NADPH₂)
ATP and reduced NADP made into carbohydrates in stroma
May contain a starch grain Chloroplasts Vesicles containing powerful digestive enzymes
Role is to break down materials
Neutrophils contain lots of lysosomes for breaking down invading microorganisms
Acrosome in head of sperm cell helps it break down the material surrounding the egg Lysosomes Cells contain a network of fibres made of protein
Protein fibres keep cell’s shape stable by providing an internal framework
Some fibres (actin filaments) are like muscles fibres
They can move against one another
This causes the movement seen in some white blood cells
Also move organelles around in cell
Microtubules (made of tubulin) may be used to move a microorganism, move a substance past the cell or to move organelles through the cell
Proteins on microtubules move organelles and other things along the microtubule like its a track
These proteins are called microtubule motors, they use ATP to move Cytoskeleton Tiny organelles – even smaller in bacteria
Purpose is protein synthesis
mRNA is used to assemble proteins from amino acids
Some ribosomes are found in cytoplasm, the others are bound to endoplasmic reticulum
Each ribosome is made up of 2 subunits
Small subunit – where mRNA bonds and is decoded
Large subunit – where amino acids added to growing protein chain Ribosomes Vesicles
A small sac which is filled with a fluid or gas
Transport things around the cell
Arrive and merge with Golgi apparatus at the cis face
Leave and bud off the Golgi apparatus at the trans face Stack of membrane bound, flattened sacs
Receives proteins from endoplasmic reticulum and modifies them
May add sugar molecules
Packages modified proteins into vesicles to be transported
Some proteins may go to the cell surface to be secreted
Doesn’t have a fixed shape as it is constantly changing shape Golgi Apparatus Smooth Endoplasmic Reticulum Studded with ribosomes
Consists of a series of flattened, membrane-bound sacs called cisternae
Continuous with outer nuclear membrane
Produces proteins which are stored in the cisternae until they’re packaged into vesicles Rough Endoplasmic Reticulum Endoplasmic reticulum For more information visit
www.biology-innovation.co.uk/pages/plant-biology-ecology/photosynthesis/ The light dependent stage
Takes place in granum
During the light dependent stage light energy is used to split water molecules into hydrogen ions (protons) and oxygen, the waste product of photosynthesis
In the process of splitting water molecules by moving electrons around ADP is converted into ATP
The hydrogen ions then reduce NADP to NADPH₂ (reduced NADP) The light independent stage
CO₂, H₂, ATP and reduced NADP are used in this process to make an intermediate hexose sugar
Hexose sugar can be polymerised to from lipids, amino acids, sugars and starch
This stage does not require light so it may take place at night Photosynthesis is a 2 stage process
The light dependent stage
The light independent stage Photosynthesis – A 2 stage process Chromatin
DNA in its active form
Stores genes
Consists of nucleic acids and histone proteins
Condense into chromosomes during prophase
Stains dark red Nucleoplasm (nuclear sap/karyoplasm)
Consists primarily of water, dissolved ions and a complex mixture of molecules
Acts as a suspension medium for organelles of nucleus
Keep nucleus’ shape and structure
Helps transport ions, molecules and other substances Nucleolus
Spherical ‘knot’ of chromatin
Visible when cell isn’t dividing
Manufactures RNA and ribosomes Nuclear pores
Regulate passage of molecules between nucleus and cytoplasm
Large enough for large molecules to pass through Inner membrane
Selective permeable membrane Outer membrane
Regulates exchange of materials between nucleus and cytoplasm Nuclear envelope
Surrounds nucleus
Made of 2 membranes with a fluid between them
Contain nuclear pores Largest organelle
Contains most of genetic information
When stained shows dark patches (chromatin) Nucleus Some bacteria have a flagella for moving around Division
Bacterium cannot undergo mitosis, mitosis refers to nuclear division. As bacteria have no true nucleus they cannot undergo mitosis Genetic information
Have no nucleus
DNA found as loop in cytoplasm
‘naked’ DNA – not associated with histone proteins
Also contain small discs of DNA called plasmids
Can be swapped between bacteria
Code for antibiotic resistance
Shared through pilus
Can pass on resistance to antibiotics
Plasmids also helpful with genetic engineering Mesosomes
Site of ATP production
Folded regions of plasma membrane Prokaryotes have ribosomes that function the same, they’re just smaller than in eukaryotes Eukaryotes – organisms with cells that have a nucleus and membrane bound organelles
Prokaryotes – Organisms with cells that do not have a true nucleus or membrane bound organelles Bacterium organelles Not studded with ribosomes
Consists of a series of flattened. membrane bound sacs called cisternae
Function varies according to cell
Liver cell - enzymes produced to break down harmful toxins
Steroid cells - produce steroid hormones (oestrogen and testosterone) Made up of a cylinder containing 9 pairs of microtubules in a circle surrounding 2 in the middle
Shorter than undulipodia
Usually found in large numbers
Move substance across cell
Made of tubulin
Dynein motor protein try push one microtubule past the other using ATP
Cilia bends because microtubules fixed at base
They move in rhythm to move substances across cell surface Cilia Made up of a cylinder containing 9 pairs of microtubules in a circle surrounding 2 in the middle
Longer than cilia
Usually found in 1s or 2s
Move entirety of cell
Made of tubulin
Dynein motor protein try push one microtubule past the other using ATP
Undulipodia bends because microtubules fixed at base

Bacteria have flagella
Look like undulipodia but have a different internal structure
Made of a spiral of protein called flagellin attached to protein disc at base
Using energy from ATP disc rotates and spins flagella Flagella (undulipodia) Flagella, Undulipodia and cilia Binds to receptors on plasma membrane
Opens channel proteins
Viruses enter cell through channel proteins
Viruses hijack cell and use it to reproduce
Cell bursts and releases all the viruses into the body Invaders – viruses Chemical messengers
Produced in tissues and released from glands
Affect target cells by binding to receptors on plasma membrane

E.g. Insulin
Produced by beta cells in islets of langerhans in pancreas
Lowers blood glucose levels
Binds to receptors on target cells (muscle and liver cells)
Makes channel proteins open
Glucose can diffuse into cells
Celll can store glucose as glycogen or use it in respiration Hormones Coenzymes Protein molecule that acts as a biological catalyst
May be bound to membranes
More membrane there is the greater the number of enzymes it can hold
Explains why mitochondria and chloroplasts have a large surface area on inner membrane Enzymes Membrane components and their roles – Enzymes and coenzymes Fluid mosaic model All membranes are permeable to water
Can diffuse through the lipid bilayer
Aquaporins (protein channels that allow water molecules through) can increase permeability to water
Cell membranes permeable to water and some solutes are partially permeable membranes
Many water soluble molecules cannot pass through
Charged (ions) /polar particles
Hydrophilic tails cannot interact with the charged/polar particles so they cannot pass through

Specialised proteins solve this problem Permeability Binds to receptors on cell surface of immune system cells
Enter cell and stops the immune system from working
Person becomes vulnerable to other diseases and illnesses as they have no immune system to protect them HIV (human immunodeficiency disease) Similar to hormones but not released from glands
Released by white blood cells in response to foreign anitgens
Bind to other white blood cells
Cause different white blood cells to produce anitbodies Cytokines (unrelated to cytokinesis) Cellular communication by signals – different molecules act as signals
Molecules released by cells
Cytokines and hormones
Molecules bind to ‘receptors’ (proteins) on cell surface membranes of target cells
Shape is complementary – they match each other Cell signalling Glycoproteins and glycolipids allow recognition by immune system
Gylcoproteins can also bind cells together in tissues
Extrinsic proteins can also act as a receptor site Receptor sites allow some hormones to bind with cell
Cell ‘response’ can be carried out
Cells can only respond to hormones they have a receptor for (they have complementary shapes)
Cell membrane receptors are also important in allowing drugs to bind, can affect metabolism Membrane components and their roles – Receptor sites Gives membrane mechanical stability
Steroid molecule fits between fatty acid tails
Helps make barrier more complete
Water molecules and ions cannot easily pass directly through the membrane Membrane components and their roles - Cholesterol Glycolipids Carbohydrate attached to a protein
Helps with cell signalling and cell recognition
Receptors found on target cells Glycoproteins Membrane components and their roles Used to describe molecular arrangement in membranes
Main features are:
Phospholipid bilayer
Various protein molecules floating in the phospholipid bilayer, some free, some bound to other components or structures within the cell
Extrinsic proteins embedded in bilayer on the inside or outside
Intrinsic proteins completely span the bilayer Fluid Mosaic model Consists of a phosphate group attached to glycerol, which is attached to fatty acids
Bonds are phosphoester bonds (phosphate to glycerol) and ester bonds (glycerol to fatty acids)
Polar molecule (uneven distribution of charge)
Phosphate head is slightly positive (Hydrophilic - attracted to water)
Fatty acid tails have no charge (Hydrophobic – not attracted to water)
Non-polar molecules repel water
Polar molecules can interact with water Phospholipids Separate cell contents from outside environment
Separate cell components from the cytoplasm (nucleus/mitochondria/chloroplasts)
Cell recognition and cell signalling
Holding the components of some metabolic pathways in place
Regulating the transport of materials into or out of cells Roles of membranes Carrier proteins Allow movement of some substances across membrane
Molecules too large and too hydrophilic enter and leave the cell through channel proteins
Because they can’t cross the phospholipid bilayer directly
(Too big/polar/charged) Channel proteins Membrane components and their roles Phospholipid bilayer is the basic structural component of all biological membranes
Hydrophobic layer creates a barrier to many molecules
Separates cell contents from outside world
The simple phospholipid bilayer cannot perform all the functions of all biological membranes
Other components are needed in order to make a functioning biological membrane
The number and type of these components vary according to the function of the membrane
This specialisation is part of differentiation
Plasma membrane of cells in a growing shoot contain receptors that allow them to detect the molecules that regulate growth
Muscle cell membranes contain a large number of channel proteins for rapid uptake of glucose
Internal membranes of chloroplasts contain chlorophyll and other molecules needed for photosynthesis
Plasma membrane of white blood cells contain special proteins enabling the cell to recognise foreign cells and particles All membranes are basically the same Bilayer Micelle If submerged in water a bilayer can form
A micelle can also form where phospholipids form a sphere with the tails facing inwards
In bilayers the phosphate head group faces the water and the tails are kept away from it
The phospholipids are free to move around as they are not bonded together
Hence name ‘fluid membrane’ If mixed with water, phospholipids form a layer at the surface
Hydphilic heads interact water
Hydrophobic tails stick out of the water Phospholipids and water Carbohydrate attached to a lipid
Helps with cell signalling and cell recognition
Receptors found on target cells Actively moves substances across the membrane
Substances moved (using ATP) across the cell surface membrane
Actively transports the sunstance against the concentration gradient Organic non-protein molecule that temporarily binds with substrate to enzyme active site
Essential for enzyme activity
May be bound to membranes
More membrane there is the greater the number of enzymes it can hold
Explains why mitochondria and chloroplasts have a large surface area on inner membrane Membranes in cells
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