- Module 2.1.5 Biological Membranes

Question 1

What are the 6 main membranes in a plant cell?

Answer 1

1- Plasma membrane
2- Tonoplast membrane
3- Outer Mitochondrial Membrane
4- Inner Mitochondrial Membrane
5- Outer Chloroplast membrane
6- Nuclear envelope
Membranes are flexible and able to break and fuse together easily.

Question 2

What is compartmentalisation?

Answer 2

Formation of separate membrane-bound areas in a cell. Vital to a cell as metabolism includes many incompatible reactions. Eg a lysosome may carry digestive enzymes at pH 4.8 and the rest of the cell may be at 7.2.

Question 3

Role of the plasma membrane

Answer 3

Separates the contents of the cell from the outside environment.
Partially permeable, only some substances can cross.
Site of cell communication (cell signalling).
Attachment to the cytoskeleton in some organisms - can help support the cell and maintaining it’s shape.
Site of some chemical reactions.

Question 4

What is cell signalling?

Answer 4

Communication between cells and cell recognition. A molecule released by one cell, attaches to another and causes a response.

Question 5

Structure of the plasma membrane.

Answer 5

Membranes are formed from a phospholipid bi-layer. Hydrophobic fatty acid tails point inwards and form a hydrophobic core, hydrophilic phosphate heads point outwards.
The phosphate group is negatively charged, making the head polar hydrophilic. The fatty acid tails are uncharged and non-polar and hydrophobic.

Question 6

What happens when a phospholipid is submerged in water?

Answer 6

It forms a micelle.

Question 7

How do phospholipids control movement of substances?

Answer 7

They act as a barrier to most substances.
Small, non-polar molecules like oxygen and carbon dioxide diffuse rapidly across the membrane.
Small, polar molecules such as water and urea struggle to diffuse across- it would be very slow.
Charged particles (ions) are unlikely to diffuse across a membrane at all.

Question 8

What are the components of a cell membrane?

Answer 8

Various proteins and lipids particular to each cell type.
Phospholipid bi-layer.
Glycoprotein
Glycolipid
Cholesterol
Protein molecules (intrinsic and extrinsic)

Question 9

What is cholesterol?

Answer 9

It is a lipid that stabilises the membrane. It binds to the fatty acid tails and makes them more compact, reducing the fluidity of the membrane.
It also reduces the permeability of the membrane to charged particles and prevents crystalisation of the phospholipids.

Question 10

What are glycolipids and glycoproteins?

Answer 10

Glycoproteins - cell membrane proteins that have a carbohydrate chain attached on the extracellular side.
Glycolipids are lipids that have carbohydrate chains attached, also on the outer surface of the membrane.

Question 11

What is the function of glycolipids and glycoproteins?

Answer 11

Carbohydrate chain allows them to act as receptor molecules - this allows them to bind with substances at the cell surface.
Some act as cell markers or antigens, for cell identification (allows immune system to determine self or non self)

Question 12

What different receptor types are there?

Answer 12

Signalling receptors, bind to hormones and neurotransmitters.
Receptors in endocytosis
Receptors involved in cell adhesion and stabilisation.(cell adhesion allows cells to group and form tissues)

Question 13

What are intrinsic proteins?

Answer 13

Span the width of the membrane (embedded through both layers)
Transport water soluble charged particles (carrier and channel proteins)

Question 14

What are protein channels?

Answer 14

A hydrophilic passage-way for charged substances (and polar molecules) to passively diffuse down the concentration gradient.

Question 15

What are carrier proteins?

Answer 15

Change shape to move substances from one side of the membrane to the other. This can be down the concentration gradient (passive) and against it (active).

Question 16

What are extrinsic proteins?

Answer 16

Aka peripheral proteins,
Can be either side of the bilayer
Some can move between layers
Normally have hydrophilic R groups on their surface to interact with phospholipid heads

Question 17

What is the relationship between increasing temperatures and membranes?

Answer 17

If you increase temperature the kinetic energy increases. The membrane becomes more fluid and breaks down/denatures at high temps. Loss of structure increases membrane permeability. Channel and carrier proteins can become denatured.

Question 18

What happens to the permeability of membranes at low temperatures?

Answer 18

Membrane permeability decreases.
Phospholipids vibrate much less, pack together tightly and and rarely provide pathways for molecules to pass through.

Question 19

What happens to membrane proteins at low temperatures?

Answer 19

Protein channels remain in place, but transporter proteins may not work as well bc low temps make it difficult for the cells to provide ATP for active transport. (respiration is controlled by enzymes)

Question 20

What happens to the ions and molecules at a low temp that stops them from passing through the membrane as much?

Answer 20

Molecules and ions move around less, so fewer hit the membrane and pass through.

Question 21

What can determine the membrane’s permeability at lower temps?

Answer 21

The proportions of saturated and unsaturated acids. Some organisms, like fish, plants and microorganisms can change the proportions to adjust.

Question 22

What solvents are less polar than water, or non polar? What do they do?

Answer 22

Alcohol is less polar, benzene is non-polar.
Organic solvents dissolve the phospholipid bi-layer and the cholesterol which disrupts their function and causes them to become more permeable. This is why alcohol is used in antiseptic wipes, to dissolve the bacteria membranes.

Question 23

What are the factors affecting diffusion?

Answer 23

Membrane thickness
Surface area
Temperature
Concentration gradient.

Question 24

What is simple diffusion?

Answer 24

Diffusion where there is no membrane involved.

Question 25

What is diffusion across a membrane?

Answer 25

Diffusion when a membrane is involved.

Question 26

What are the factors affecting simple diffusion?

Answer 26

Temperature
Concentration gradient/difference

Question 27

What are the factors affecting diffusion across a membrane?

Answer 27

Temperature
Concentration gradient
Surface area
Membrane thickness

Question 28

What are the key three adaptations of alveoli?

Answer 28

Large surface area
Lining that is one cell thick
Very good blood supply

Question 29

What are villi?

Answer 29

Tiny structures that increase the surface area of the small intestine.

Question 30

What are the important features of the villi?

Answer 30

Wall is one cell thick, ensure only a short distance for absorption to happen by diffusion and active transport.
Network of blood capillaries
Internal structure called lacteal

Question 31

What is the lacteal in the villi?

Answer 31

Transports fatty acids and glycerol away from the small intestine in the lymph.

Question 32

What kind of flow do fish have over their gills?

Answer 32

Countercurrent flow, blood and water travel in opposite directions. To maintain the concentration gradient. Means that they can extract oxygen at almost 3x the rate of humans.

Question 33

Describe countercurrent flow.

Answer 33

As blood flows in the opposite direction to water it always has a lower oxygen concentration than the water. This ensures that diffusion happens at every point in the gills.

Question 34

What is facilitated diffusion?

Answer 34

Large or polar molecules pass through the membrane with the help of a protein.
Proteins are specific to the molecule that they carry.
It’s a passive process.
Either channel proteins or Carrier proteins.

Question 35

How are channel proteins used in facilitated diffusion?

Answer 35

Ions and small molecules.
Act like a pore
Often can be opened and closed.
Most common type in facilitated diffusion.

Question 36

How are carrier proteins used in facilitated diffusion

Answer 36

Larger molecules.
Molecules attach to the protein and it changes shape to transfer the molecule across the membrane.
Some are used in facilitated diffusion, some for active transport.

Question 37

What is partially permeable vs selectively permeable?

Answer 37

Partially permeable = membranes that allow some molecules through.
Selectively permeable = membranes that allow specific molecules through.

Generally membranes are partially permeable but membranes with proteins are selectively.

Question 38

Define active transport.

Answer 38

Movement against the concentration gradient.
Uses a carrier protein and ATP.
Each carrier protein is specific.

Question 39

What are the 6 steps of active transport?

Answer 39

1- Ion to be transported binds to the receptor on the carrier protein.
2- Inside the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate.
3- Phosphate binding to the protein causes it to change shape, opening up to the inside of the cell
4- Ion then released to the inside of the cell
5- Phosphate molecule is released from the protein and reforms ATP.
6- Carrier protein returns to it’s original shape.

Question 40

What is bulk transport?

Answer 40

Form of active transport.
Large molecules like enzymes, hormones and whole cells (bacteria) are too large to move through proteins so are moved in and out of cells by bulk transport.

Question 41

What types of bulk transport are there?

Answer 41

Exocytosis
Endocytosis (pinocytosis - liquids, phagocytosis - solids)

Question 42

What is the definition of osmosis?

Answer 42

The net movement of water molecules across a partially permeable membrane, from a region of high water potential to low water potential.

Question 43

What is water potential?

Answer 43

The tendency of water molecules in a system to move.

Question 44

What is the value of pure water’s water potential? What is this like compared to solutions?

Answer 44

Pure water has the highest water potential at 0kPa. Solutions have lower water potential and it has a negative value.

Question 45

What are the two things that water potential determines?

Answer 45

The direction and rate of movement of water.

Question 46

What are the units of hydrostatic pressure?

Answer 46

kPa

Question 47

What happens in animal cells when in a solution with higher water potential?

Answer 47

Cell surface membranes cannot stretch to withstand increased pressure. So the cell bursts, cytolysis.

Question 48

What happens in ANIMAL CELLS when in a solution with lower water potential?

Answer 48

They lose water and undergo crenation.

Question 49

What are hypertonic, isotonic and hypotonic solutions?

Answer 49

Hypertonic is one that has lower water potential than the cell, so there is net movement of water out of the cell.
Isotonic is when water potential in the solution and the cell are the same, there is no net movement of water.
Hypotonic is when the solution has a higher water potential than the cell, so there is net movement of water out of the cell.

Question 50

What is turgor?

Answer 50

Pressure against the cell walls.

Question 51

What happens when turgor pressure increases? (plant cell)

Answer 51

It resists further entry of water into the cell, the cell is turgid.

Question 52

What happens when a PLANT CELL is placed in a solution with a lower water potential than it’s own?

Answer 52

Water is lost by osmosis, the cell becomes plasmolysed.

Question 53

What are plasmolysed, flaccid and turgid cells?

Answer 53

Plasmolysed cells are cells that have lost water and shrunk.
Flaccid cells are cells with normal levels of water.
Turgid cells are cells that have had more water enter the cell so are swollen.

Question 54

How is the kidney and osmosis related?

Answer 54

In mammals, maintaining a constant blood solute potential is a major function of the kidney that relies on the process of osmosis.