topic 2 - cell structure

What is the cell vacuole?

A membrane-bound organelle found in the cytoplasm of plant cells. It contains cell sap - a weak solution of sugar and salts. The surrounding membrane is called the tonoplast. It helps mantain pressure in the cell keeping it rigid

What is the difference in size between prokaryotic and eukaryotic cells?

Prokaryotic - less than 2 micrometres
Eykaryotic - can be up to 50x bigger

What is an organ system?

When organs work together to perform particular functions more efficiently.

What is the structure of the nucleus?

The nucleus is

• surrounded by a nuclear envelope/ double membrane which is continuous with the ER.

• It also contains large pores for the transfer of materials (in protein synthesis etc)

• Inside the nucleus there are chromosomes

• contains a nucleolus where ribosomal RNA is produced & so production of ribosome

• contains a nucleoplasm which is a granular jelly material

What is the rough endoplasmic reticulum?

Cristernae (folded membranes) enclosing a fluid-filled space with ribosomes on its surface.
It synthesizes proteins through the ribosomes.
It also provides a pathway for materials.

What is the function of the golgi apparatus?

Modify proteins into glycoproteins.Transport store & modify lipids Secrete carbohydrates
The golgi apparatus also make lysosomes. Produce secretory enzymes.

What is the structure of a chloroplast?

A small flattened structure surrounded by a chloroplast enevelope (double plasma membrane) with inner membranes called thylakoid membrane.

Some of them stack to form grana which are linked by lamellae - thin flat pieces of thylakoid membrane - and all contain chlorophyll.

The thylakoids are surrounded by stroma - fluid filled matrix which contain starch grains & enzymes needed for photosynthesis.

What is the cell wall?

surrounds the cell membrane of the plant cell; provides support and shape and is found in plant, algae and fungi cells. It supports the cell and prevents it from changing shape.

What is the structure of a (retro) virus?

- Attachment proteins on the outside - to identify & attach to host cells
- Lipid envelope 'instead' of a cell membrane as it isnt a cell
- Matrix
- Capsin - protein coat that surround the nucleic acid
- Reverse transcriptase enzyme (only in retro viruses)

What is a tissue?

Cells that are grouped together for working efficiency/ a collection of similar cells that perform a specific function

What is the magnification of an optical microscope vs an electron microscope?

Optical = max x1,500
Electron = max x1,500,000

What is the resolution of an optical microscope vs an electron microscope?

Optical = max 0.2 micrometres
Electron = max 0.0002 micrometres

How do transmission electron microscopes form an image?

A beam of electrons is focused by electromagents onto a specimen creating a black and white image as denser or more stained parts of the specimen absorb more electrons

How are scanning electron microscopes and transmission electron microscopes different?

In SEMs the electrons are scattered across a specimen knocking off electrons depending on the shape into a cathode ray tube which can form a 3D image. TEMs can't create 3D images.

What is cell fractionation?

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

Why are the tissues in cell fractionation placed in placed in cold isotonic buffer solution?

Cold - to limit enzyme activity and the breaking down of the organelles
Buffer - keeps the pH constant to limit any damage to the organelles
Isotonic - prevents osmosis in order to keep the organelles the same shape

What are the steps of cell fractionation?

1. Homogenisation

2. Filtration

3. Storage

4. Ultracentrifugation

What is homogenisation?

Breaking up of cells in a blender or grinding them. This releases the organelles into the homogenate by breaking open the cell membranes.

What is filtration?

The homogenate is filtered through a gauze to separte any large debris e.g connective tissue from the organelles. The organelles are too small to get trapped in the gauze.

What is ultracentrifugation?

The process by which the fragments in the filtered homogenate are separated in a machine called a centrifuge.

What happens in ultracentrifugation?

the tube of homogenate is placed in a centrifugue and spun
the heaviest organelle is forced to the bottom where a pellet forms and is collected
the fluid at the top of the tube (supernatant) is removed and transferred to another tube to be spun again at a faster speed
this is repeated until you have the organelle(s) you want

In which order would the organelles be removed by centrifugation?

increasing speed --------->

• Nuclei

• Chloroplasts (in plants)

• Mitochondria

• Lysosomes

• Endoplasmic reticulum

• Ribosomes

What are the properties of a membrane made out of a phosholipid bilayer?

- allows lipid soluble substances to enter and leave the cell freely
-prevents water soluble substances from leaving & entering
- makes the membrane flexible and self-sealing
- small non-polar (CO2) molecules and water (which is polar) can get through the membrane

What is the plasma membrane?

outer boundary of the cell and organelles

What is the fluid mosaic model?

Model used to describe the plasma membrane that its a fluid structure caused by the phospholipid bilayer with a "mosaic" of various proteins embedded in it

What is the difference between intrinsic and extrinsic proteins?

Intrinsic - completely spans the bilayer
Extrinsic - only on the surface of the bilayer or partially embedded

What are the roles of intrinsic proteins?

- some act as ion channels for water soluble molecules
- some are enzymes
- provide structural support
- form carrier proteins for active transport

- form carrier proteins for molecules which are not ions etc for facilitated diffusion

What are the roles of extrinsic proteins?

- provide mechanical support
- when bound to glycolipids they act as cell receptors and antigens
- form recognition sites for cells by identifying them
- helps cells adhere together

What does cholesterol do in the cell membrane?

stabilizes the membrane by holding nearby phosholipids hydrophopic tails together reducing the flexibility of the membrane by making them more tightly packed together

Why is cholesterol needed in some cell membranes?

To provide more stability for cells which aren't supported by other cells e.g RBCs
They also have hydrophobic areas so its able to provide a further barrier to polar substances moving through the membrane.

What are glycoprotiens and their function in the cell membrane?

Proteins with carbohydrates attached.
Involved in cell recognition and acting as receptors.

What are glycolipids and their function in the cell membrane?

Lipids with proteins attached.
Can act to maintain the stability of the membrane but also as receptors on the surface.

What is simple diffusion?

molecules pass directly through the plasma membrane without the assistance of another molecule

What is facilitated diffusion?

The passive diffusion of a particle across a cell membrane via a channel protein.
There are 2 types - ion channels for water soluble proteins and carrier proteins for larger molecules

What are ion channels?

Intrinsic proteins which open for specific ions to be transported across the membrane.
The opening and closing can only happen when there a conc gradient and ions are present

What are carrier proteins?

Intrinsic proteins which can change shape when a particular molecule is present. (the protein &molecule must be complementary)
The two bind together forcing the to carrier protein to change shape in order to release the molecule on the other side passively.

Once the molecule is released the carrier protein returns to its original shape.

What are the factors which affect facilitated diffusion?

- Concentration gradient - higher conc = more diffusion
- Number of channels & carrier proteins - more equals = more diffusion

What is active transport?

the movement of ions or molecules across a cell membrane into a region of higher concentration from a region of lower concentration, assisted by carrier proteins and requiring ATP

What is the definition of a prokaryotic cell?

A prokaryotic cell is characterized by a lack of any membrane bound organelles e.g. nucleus, mitochondria. An organism made out of these cells belongs to the kingdom prokaryote.

What is the definition of a eukaryotic cell?

A eukaryotic cell has membrane bound organelles like the nucleus, chromosomes as well as the ER and mitochondria

What is the function of the nucleus?

stores DNA and controls the cells activities by controlling the transcription of proteins.
- the nucleolus makes ribosomes

What is the structure of the mitochondria?

• A double membrane

• The inner one is called the cristae.

• A matrix in the mitochondria - fluid in the centre - which has the enzymes required for respiration.

• Mitochondrial DNA in a loop which codes for the proteins/enzyme.

What is the function of the mitochondria?

To produce ATP from aerobic respiration

What is the smooth endoplasmic reticulum?

An organelle which is like the RER but has no ribosomes and instead synthesizes and processes lipids not proteins.

What is the structure of the RER and the SER?

They both are made of membranes continuous to the nucleus that fold into cristernae.
However the RER has ribosomes on its surface where as the SER doesn't.

What does a eukaryotic cell look like?

contains:
- cell-surface membrane
- nucleus
- mitochondria
- chloroplasts (plants & algae)
- golgi apparatus and golgi vesicles
- lysosomes
- ribosomes
- rough endoplasmic recticulum
- smooth endoplasmic recticulum
- cell wall (plants, algae & fungi)
- cell vacuole

What is the structure of a golgi apparatus & vesicles?

Made up of cristernae which are flattened fluid-filled membrane bound sacs and vesicles which are small hollow rounded structures

What is the function of the golgi vesicles?

To sort and store the modified lipids and proteins and to transport them so they can be released by being cut off at the end of the cristernae and fusing with the cell membrane so they can be released

What is the function of chloroplasts?

To do photosynthesis - this happens in the grana and the stroma.

What are lysosomes?

Round organelles surrounded by a membrane with no clear internal structure - formed when the vesicles from the golgi appaartus form enzymes.

They contain lysozymes which are digestive enzymes like protease etc which is protected from the rest of the cell by the membrane.

What is the function of a lysosome?

- to break down material ingested by phagocytic cells
- to destroy material around the cell - exocytosis
- to digest worn down organelles for useful chemicals which can be recyled
- to do autolysis - break down cells after their death

What are ribosomes?

Small granules found in all cells made up of proteins & RNA which contain a small&large subunit one containing proteins & the other rRNA

They come in two types 80S which are larger and found in eukaryotic cells and 70S which are smaller and found in prokaryotic cells.

They assemble proteins from amino acids.

What is the structure of a bacterial cell?

- (sometimes) flagellum
- a circular loop of DNA present as a coiled up strand that floats free and is unattached to histone proteins
- cytoplasm
- ribosomes : 70S - smaller than in eukaryotic cells
- cell surface membrane
- cell wall made of murein - a glycoprotein
- capsule made up of a mucilaginous layer of slime
- ( sometimes ) plasmids

Why do some bacteria have capsules?

To protect the bacterium from attack by the immune system cells.

How do bacteria reproduce?

asexually in binary fission

What happens in binary fission?

- the DNA replicates - the circular DNA only once but the plasmids multiple times
- then the DNA & plasmids migrate to opposite ends of the cell
- cell enlargens
- the two cells begin to pinch off
- the new cell walls form

What is the end product of binary fission?

two daughter cells with a copy of the circular DNA but a variable number of plasmids

Why are viruses not considered living?

They cannot reproduce by themselves and have to hijack other cells and they are also acellular

How can viruses have RNA?

The reverse transcriptase enzyme can convert the RNA to DNA

How do viruses replicate?

- the virus attaches to a complementary host cells receptor proteins using their attachment proteins
- its genetic material is released into the cell
- the genetic material is replicated by the cells machinery
- the viral components assemble
- this happens many times until the number of viruses cause the cell to burst releasing the viruses

What are algal cells?

Eukaryotic cells which can be unicellular and multicellular and are similar to plant cells as they have chloroplasts.

How are algal cells different to plant cells?

Can be unicellular, and have varying shapes and sizes of chloroplasts e.g one big chloroplast

What are fungal cells?

Eukaryotic cells , can be unicellular and multicellular most similar to plant cells.

How are fungal cells different to plant cells?

Their cell walls are made of chitin, not cellulose.
They don't have chloroplasts because they don't need to photosynthesise

What are examples of specialised cells?

Liver cells - lots of mitochondria, golgi appartus, RER & SER in order to secrete substances and do lots of metabolic activities
Epithelial cells (small intestine)- microvilli on their surface and lots of mitochondria for absorption of molecules

What are organs?

A combination of tissues that are coordinated to perform a variety of functions that often have an overriding function

What is the difference between magnification & resolution?

magnification - how many times larger the image is compared to the object
resolution - how well you can distinguish two points as being separate

How do light microscopes work?

by shining light through a thinly prepped specimen

Why can't light microscopes be used to view ribosomes, endoplasmic recticulum & lysosomes?

The wavelength of light is larger so gives a low resolution so smaller organelles can’t be seen.

What are the limitations of electron microscopes?

- difficulty in preparing the specimen which limits the resolution achieved
- a vacuum has to be used so living specimens can't be viewed
- complex & toxic stains are used
- in TEMs the specimen needed to be very thin
- higher energy electrons can destroy the specimen

What are some differences between optical and electron microscopes?

Optical :
- relatively cheap & easy to use
- small in size
- gives a colour image and living specimens can be viewed

Electron :
- expensive
-require training to use
- very large
- only gives black & white images and only dead specimens can be viewed

How would you prepare a temporary mount for microscopy?

- pipette a small drop of water onto the slide
- use tweezers to place a thin section of specimen onto the slide
- add a drop of stain
- put the cover slip on carefully so there aren't any air bubbles

What is osmosis?

The diffusion/movement of water molecules from a region of higher water potential to a region of lower water potential across a partially/selectively permeable membrane.

What is water potential?

The amount of pressure enacted on the partially membrane by the water molecules.
- Measured in kilopascals as its a measure of pressure

What is a higher water potential indicative of?

Higher water potential = more water molecules = higher conc of water

What happens when the water potential is equal on both sides?

There is an equilibrium where there is a net zero movement of water particles as the same number of particles are flowing in one direction and vice versa

What is the highest water potential possible?

0kPa = pure water - so any solution is negative with the more negative solution being the lower water potential

What do isotonic, hyertonic and hypotonic mean?

All are to do with the concentration of the solute e.g sugar not the conc of water.
Isotonic - same conc of solute inside & outside the cell
Hypertonic - high conc of solute outside the cell = higher water potential inside the cell
Hypotonic - high conc of solute inside the cell = higher water potential ouside the cell

What happens to an animal cell in :
- isotonic solution
- hypertonic solution
- hypotonic solution?

Isotonic - nothing happens to the cell
Hypertonic - water moves outside the cell causing the cell to shrink/shrivel
Hypotonic - water moves inside the cell causing an increase in hydrostatic pressure leading to the cell lysising if too much water is absorbed as the cell membrane can't stand the pressure ---> for RBC this is haemolysis

What happens to an plant cell in :
- isotonic solution
- hypertonic solution
- hypotonic solution?

Isotonic - cell is fine but not as turgid as it wants to be
Hypotonic - water moves into the cell causing the hydrostatic pressure to increase so water is moved into the vacuole making it push up against the membrane & wall ( at some point enough water is taken in that no more osmosis can happen into the vacuole
- the cell is turgid
Hypertonic - water moves out of the cell causing the protoplast to shrink

What happens in active transport?

• A molecule attaches to a carrier protein complementary to it from the outside

• On the inside of the membrane ATP binds to the carrier protien

• ATP is hydrolysed forming ADP & P

• This causes the carrier protein to change shape so the molecule can now be released in the cell

• After this the P inorganic ion is also released so the carrier protein returns to its original shape

What happens in co-transport and how is it different to active transport?

Co-transport uses 2 molecules - 1 going along the conc gradient to move the other molecule going against the conc gradient where as active transport only needs 1.

In co-transport both the molecules bind to a co -transporter which is a type of carrier protein causing it to change its shape releasing the molecules on the other side of the membrane.

How is co - transport involved absorption of glucose/amino acids in the small intestine?

1. Sodium ions (Na⁺) are actively transported out of the epithelial cells and into the blood using a carrier protein called the sodium-potassium pump. This requires ATP.

2. This establishes a concentration gradient for sodium ions whereby their concentration is lower inside the epithelial cell than the lumen of the ileum.

3. Sodium ions will then move into the epithelial cell from the lumen by facilitated diffusion, down their concentration gradient. As they move in, they carry with them a molecule of glucose or an amino acid, along their concentration gradient. This uses a co-transport protein.

4. The concentration of glucose/ amino acids is now higher in the epithelial cell than the blood in the capillary. They are therefore able to move via facilitated diffusion from the epithelial cell into the blood. 

What are the 3 (general) parts of the cell cycle?

Interphase - Mitosis - Cytokenisis

What are the different parts of interphase?

G1, S, G2

What happens in the G1 phase?

G1 - The Growth Phase:
- The organelles (- DNA) are replicating
- The cell is also making cytoplasm so is getting bigger
- Enzymes for DNA replication are being made
- The cell can stay in G1 for a long time as having too many organelles isn't too disruptive for it but the cell can also move into a G0 phase

What happens in the G0 phase?

In G0 the cells aren't preparing for separation so there's no change in organelle number. Instead the cell:
- becomes dormant
- dies (apoptsis)
- goes through senenscence (aging)
The cell generally just does it's functions.

What happens in the S phase?

S Phase - DNA Synthesis:
- The DNA replicates ready for cell division forming 2 identical copies of a chromosome - chromatids
- The DNA replication occurs at multiple DNA replication forks so the most important DNA segments are replicated first
- Once the cell enters this stage it has to go into mitosis etc because having too much DNA is a problem for the cell so it can't be in this stage indefinitely like it can in the G1 phase

What happens in the G2 phase?

G2 - The Second Growth Phase:

• The cell becomes much bigger due to the replication of DNA & organelles

• More proteins/enzymes are produced to prepare for division

• Double DNA stays the same in an uncondensed form

• cell DNA is checked- if there is too much damage the cell will kill itself

What is an antigen?

Markers molecules (usually proteins) found on the surface of the cell that alert an immune response if they're detected as being foreign.

What do antigens allow the immune system to identify?

- Pathogens - bacteria, viral molecules
- other cells with different tissue types from the same species
- pollen
- toxins
- abnormal body cells (e.g. cancer cells which have different antigens due to mutations etc)

What are the antigens on viruses?

Protein spikes on their surface.

What are the antigens on bacteria?

Proteins and carbohydrate antigens.

What are pathogens?

microorganisms that cause disease

What is phagocytosis?

The ingestion of cells by the immune system as a non-specific response to a foreign body - could be a pathogen , debris etc

What is a phagocyte?

A type of white blood cell that carries out phagocytosis, found in the blood and are the first cells to act as part of an immune response.

What is a macrophage?

type of phagocyte

What happens in phagocytosis?

• the pathogens chemical products (toxins) & antigens act as attractants causing the phagocytes to move towards it

• the cytoplasm of the phagocyte warps around the pathogen engulfing it

• the pathogen is contained in the cell in a phagosome

• a lysosome then fuses with the phagosome releasing its lysozymes

• the lysozymes digest the pathogen breaking it down into soluble products which can be absorbed by the cytoplasm

• the phagocyte can then recycle any useful components and present the pathogens antigens to activate the T-cells

What are the different types of white blood cells/ lymphocytes?

-Phagocytes
- T-cells : Th-cells and Tc-cells
- B-cells

Where are the T-cells and B-cells produced?

T cells: thymus
B cells: bone marrow