3.2 cells

Question 1

what does a light microscope have

Answer 1

Light microscopes use a pair of convex glass lenses that can resolve images that are 0.2um apart.

Question 2

how do electron microscopes work

Answer 2

Electron microscopes work in a similar way to light microscopes, but instead use a beam of electrons that are focused by electromagnets inside a vacuum environment, which is needed so that particles in the air do not deflect the electrons out of the beam alignment.

Question 3

how do transmission electron microscopes work

Answer 3

has an electron gun that focuses a beam of electrons that is focused onto the specimen by a condenser electromagnet. It passes through a thin section. Parts of the specimen absorb electrons and therefore appear dark. An image is produced on a screen and can be photographed by a micrograph

Question 4

how do scanning electron microscopes work

Answer 4

A beam of electrons is directed onto the surface of the specimen from above. The beam is then passed back and forth across a section of specimen in a regular pattern. The electrons are scattered by the specimen and the pattern of this scattering depends on the contours of the surface. A £D image can be built by computer analysis of the pattern of scattered electrons

Question 5

what are the limitations of electron microscopes

Answer 5

• system must be in a vacuum so living specimens cannot be observed.
• A complex staining process is required which may introduce artefacts into the image.
• Specimens have to be very thin, particularly for TEM so that the electrons can pass through.
• SEM has a lower resolving power than TEM, but both have greater resolving power than a light microscope.

Question 6

what is cell fractionation

Answer 6

Cell fractionation is the process in which different parts and organelles of a cell a separated so that they can be studied in detail.

Question 7

what is the homogenate put in with cell fractionation

Answer 7

The homogenate at the beginning is placed in a cold, buffered solution of the same water potential as the cells. This is to prevent the organelles from bursting under osmotic pressure, to inactivate any enzymes from breaking down organelles and so that the pH does not fluctuate.

Question 8

what is the cell fractionation process

Answer 8

1. The cells are first blended in an homogeniser forming the resultant fluid called the homogenate.
2. This tube of homogenate is then placed in a centrifuge and spun at a slow speed.
3. The heaviest organelles, the nuclei, are forced to the bottom of the tube where a thin sediment or pellet forms.
4. The fluid at the top, called the supernatant, is removed which leaves just the sediment of the nuclei. 5. The supernatant is then transferred to another tube and spun at a slightly faster speed. This time the pellet that forms contains the next heaviest organelle, the mitochondria.
5. This process continues so that each time the speed is increased the next heaviest organelle is sedimented and separated out.

Question 9

what is the nucleus

Answer 9

• Large organelle surrounded by a nuclear envelope (double membrane)
• Contains chromosomes and 1 or more nucleolus
• Nucleoplasm
• Controls cells activities
• DNA contains code to make proteins
• Nucleus pores allow substances to move in or out
• Nucleolus makes ribosomes

Question 10

what is the cell wall

Answer 10

Rigid structure that surrounds the plasma membrane
In plants and algae- made of mainly cellulose
Fungi- made of chitin
Supports cell and keeps it’s rigid shape
Where two plant cell walls come together, it forms a middle lamella and strands of cytoplasm called plasmodesmata reach across to that cell
The plasmodesmata communicates between cells.

Question 11

what is mitochondria

Answer 11

Double membrane
Inner membrane folds to form cristae
Matrix inside contains enzymes involved in respiration
Site of aerobic respiration – produces ATP (energy source)
Large number in active cells

Question 12

what is rough endoplasmic reticulum

Answer 12

System of membranes
Surface is covered in membranes
folds and processes proteins made at the ribosomes

Question 13

what is smooth endoplasmic reticulum

Answer 13

similar to RER but has no ribosomes
Synthesizes and processes lipids
Transport system

Question 14

what is lysosomes

Answer 14

Type of Golgi vesicles with no clear internal structure
Contains lysozymes( digestive enzymes)
can be used to digest invading cells and break down old cells components

Question 15

what is golgi apparatus

Answer 15

Group of fluid-filled sac in cytoplasm-bound flattened sacs
Processes and packages new lipids and proteins
Makes lysosomes

Question 16

what is golgi vesicles

Answer 16

Small fluid-filledsac in cytoplasm, surrounded by a membrane
Produced by the Golgi apparatus
Stores lipids and proteins make by the Golgi apparatus and transports it out of cells

Question 17

what is chloroplast

Answer 17

Found in plants and algal cells
Surrounded by a double membrane
Membrane inside is called thylakoid membranes and are stacked to form grana and linked together by lamellae
Site of photosynthesis

Question 18

what is cell vacuole

Answer 18

Contains sap-sugar and salts
Surrounded by a membrane called a tonoplast
Maintains pressure inside cells and keeps them rigid
Isolates unwanted chemicals
When full its turgid
When empty its plasmolyzed
Vacuoles are very important for osmotic activity

Question 19

what is ribosomes

Answer 19

Very small (70S in prokaryotes and 80S in eukaryotes)
Floats free in the cytoplasm or attached to the rough endoplasmic reticulum
Made up of proteins and rRNA
Made up of two sub-units
not surrounded by a membrane
Site of protein synthesis

Question 20

what is bacteria

Answer 20

Versatile, adaptive and successful
Success is due to small size( 0.1-10µm)
Cellular structure is relatively simple
Cell wall is made up of murein
Murein is a polymer of polysaccharides and peptides
Many protect themselves by secreting a capsule of mucilaginous slime
Ribosomes are smaller- 70S
Store food reserves as glycogen granules and oil droplets
Genetic material is in the form of circular strand of DNA. Sperate from this, are smaller circular pieces of DNA called plasmids
Can reproduce independently and give bacterium resistance to harmful chemicals such as antibiotics
Plasmids are used extensively as vectors( carrier of genetic material) in genetic engineering.

Question 21

what is the characteristics of viruses

Answer 21

A-cellular, non-living particles
Smaller than bacteria (20-300nm)
Contain nucleic acids such as DNA and RNA as genetic material but can only multiply in living cells
Nucleic acid is enclosed within a protein coat called the capsid.
Some viruses (such as the human immunodeficiency virus) are further surrounded by a lipid envelope, if it isn’t present, the capsid
Have attachment proteins which are essential to allow the virus to identify and attach to a host cell

Question 22

how things move in/out of the membrane

Answer 22

Some are passive- requires no ATP/energy
Diffusion
Facilitateddiffusion ( makes it happen)
Osmosis
Active- needs transport
Active transport
A combination may be used to give:
Co-transport

Question 23

what are phospholipids

Answer 23

Form a bilayer. Are important components of cell-surface membranes for:
The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attached by water on both sides
The hydrophilic tails of both phospholipid layers point to the centre of the cell membrane, repelled by the water on both sides
Lipid-soluble material moves through the membrane via the phospholipid portion. Phospholipids functions are to
Allow lipid-soluble substances to enter and leave the cells
Prevent water-soluble substances entering and leaving the cell
Make the membrane flexible and self-sealing

Question 24

proteins in transport

Answer 24

Are interspersed throughout the cell surface membrane
Embedded in the phospholipid bilayer in 2 main ways:
Some proteins occur in the surface of the bilayer and never extends completely across it. Act either as mechanical supports to the membrane or, in conjunction with glycolipids, as cell receptors
Others completely span the phospholipid bilayer from one side to the other. Some are protein channels, which form water-filled tubes to allow water-soluble ions to diffuse across the membrane.

Question 25

What is the purpose of proteins in the membrane

Answer 25

Functions of proteins in the membrane are to:
Provide structural support
Act as channels transporting water-soluble substances across the membrane
Allow active transport across the membrane through carrier proteins
Helps cells adhere together
Act as receptors

Question 26

what is cholesterol

Answer 26

Occur within the phospholipid bilayer of the cell-surface membrane. Add strength to the membranes.
Very hydrophobic and therefore play an important role in preventing loss of water and dissolved ions from the cell. They also pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules but without making the membrane too rigid.

Question 27

what is cholesterols role in the membraneq

Answer 27

Functions in the membrane are to:
Reduce lateral movement of other molecules including phospholipids
Makes the membrane less fluid at high temperatures
Prevents leakage of water and dissolved ions from the cell

Question 28

what are glycoproteins

Answer 28

Carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane. Thee glycoproteins also act as cell-surface receptors, more specifically for hormones and neurotransmitters
Functions of glycoproteins in the membrane are to:
Act as recognition sites
Helps cells to attach to one another and form tissues
Allows cells to recognise one another, for example lymphocytes can recognise an organisms own cells

Question 29

what are glycolipids

Answer 29

Made up of a carbohydrate covalently bonded with a lipid. The carbohydrate portion extends from the phospholipid bilayer into the receptor for specific chemicals

Question 30

what are glycolipids role in the membrane

Answer 30

Functions in the membrane are to:
Act as recognition sites
Helps cells attach to one another and so form tissues
Help maintain the stability of the membrane

Question 31

what is the fluid mosiac model of membrane structure

Answer 31

Fluid- individual phospholipid can move relative to one another. This gives the membrane a flexible structure that is constantly changing in shape
Mosaic- proteins embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as stones or tiles of a mosaic.
[ see physical flashcards for diagram]

Question 32

what is membrane structure

Answer 32

Fluidmosaicmodel
Phospholipids
Globular proteins
Outside isintrinsic
Span membranes –intrinsic

Question 33

what is permeability of cell surface membrane

Answer 33

Controls the movement of substances in and out of the cell. In general, most molecules do not freely diffuse across it because many are:
Not soluble in lipids and therefore cannot pass through the phospholipid bilayer
Too large to pass through the channels
Of the same charge as the charge on protein channels and so, even if they are small enough to pass, they are repelled.
Electrically charged/ are polar and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer

Question 34

what is diffusion

Answer 34

Diffusion- net movement ofmolecules or ions from an area of higher concentration to one where their concentration is lower until it is evenly distributed
All particles are constantly in motion due to kinetic energy .
The motion is random, with no set pattern
Particles are constantly bouncing off one another and other objects
Particles that are concentrated together in a part of a closed vessel will distributed themselves evenly.

Question 35

what factors affect diffusion

Answer 35

Factors affecting diffusion
Large surface area
Temperature
Concentration difference
Length of diffusion pathway
( shorter= faster diffusion)

Question 36

what is the role of diffusion in absorption

Answer 36

As carbohydrates and proteins are being digested continuously, there are normally a greater concentration of glucose and amino acids within the ileum than in the blood. There is therefore a concentration gradient down which glucose moves by facilitated diffusion from being inside the ileum into the blood. Given that blood is being continuously being circulated by the heart, the glucose absorbed into it is continuously being removed by cells as they use it up during respiration. Helps maintains the concentration gradient between the ileum and the blood. This means the rate of movement by facilitated diffusion across epithelial cell-surface membranes is increased

Question 37

what is the process of co-transport

Answer 37

1. Sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump, into the blood. This takes place in 1 type of protein carrier molecule found in the cell-surface membrane of the epithelial cells
2. Maintains a much higher concentration of sodium ions in the lumen of the intestine than inside of the epithelial cells
3. Sodium ions diffuse into the epithelial cells down this concentration gradient through a different type of protein carrier( so-transport protein) into the cell surface membrane. As the sodium ions diffuse in through this 2nd carrier protein, they carry either amino acids or glucose into the cell
4. The glucose/amino acids pass into the blood plasma by facilitated diffusion

Question 38

what is osmosis

Answer 38

The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane
Water potential
Measured in units of pressure, usually kiloPascals (kPa)
Is the pressure created by water molecules. Under standard conditions of temp and water (25° and 100kPa), water is said to have a water potential of 0.

Question 39

what does osmosis follow

Answer 39

Addition of a solute to pure water will lower its water potential
Water potential of a solution must always be less than 0, that is, a negative value
More solute that is added ( more concentrated), the lower (more negative) the water potential
Water will move by osmosis from a region of higher ( less negative) water potential (e.g. -20kPa) to one of lower ( more negative) water potential ( e.g. -30kPa)

Question 40

what is facilitated diffusion

Answer 40

Where diffusion is helped
Charged ions and polar molecules don’t diffuse easily due to the hydrophobic nature of the fatty acid tails of the phospholipids in the membranes
Movement of these molecules is made easier ( facilitated) by transmembrane channels and carriers that span the membrane
Is a passive process- relies on inbuilt motion (kinetic energy) of diffusing molecules

Question 41

what does facilitated diffusion occur down

Answer 41

Occurs down a concentration gradient but occurs atspecificpoints of the plasma membrane where there are special protein molecules :
Protein channels
Carrier proteins

Question 42

what is protein channels and facilitated diffusion

Answer 42

Proteins form water-filled hydrophobic channels across a membrane
Allows specific water-soluble ions to pass through
Is selective and only opens in the presence of specific ions
If these aren’t present, it stays closed
Ions bind with the protein, causing it to change the shape in a way that closes it to one side of the membrane and opens the other side.

Question 43

what is carrier proteins and facilitated diffusion

Answer 43

Span the plasma membrane
When a molecule such as glucose that is specific to the protein is present, it binds with the protein. This causes it to change shape in such a way that the molecule is released to the inside of the membrane. No external energy is needed for this. The molecule moves from a region where they are highly concentrated to one of lower concentrations, using only the kinetic energy of the molecules themselves.

Question 44

what is active transport

Answer 44

The movement of molecules or ions into or out of a cell from a region of lower concentrated to a region of higher concentrations using ATP and carrier proteins.

Question 45

how is ATP is used in active transport

Answer 45

ATP is used to:
Directly move molecules
Individually move molecules using a concentration gradient which has already been set up by (direct ) active transport. Known as co-transport.

Question 46

how does ATP differ from passive forms of transport

Answer 46

Differs from passive forms of transport :
Metabolic energy in the form of ATP is needed.
Substances are moved against a concentration gradient, that is from a lower to higher concentration
Carrier protein molecules which act as ‘pumps’ are involved.
The process is very selective, with specific substances being transported.

Question 47

what is the process of direct active transport

Answer 47

Carrier proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it.
The molecule or ion binds to receptor sites on the carrier protein
On the side of the cell/organelle, ATP binds to the protein, causing it to split into ADP and Pi. As a result the protein molecule changes shape and opens to the opposite side of the membrane.
The molecule or ion is then released to the other side of the membrane
The phosphate molecule is released from the protein which causes the protein to revert to its original shape, ready for the process to be repeated, the Pi then recombines with ADP to form ATP during respiration