Biological membranes

Question 1

Describe the basic make up of the fluid mosaic model (phospholipid bilayer)

Answer 1

OUTSIDE- watery/aqueous due to extracellular fluid
-outer most part of the cell is the plasma membrane
-phosphate head is polar and hydrophilic
-acts as a barrier to polar molecules and ions- e.g. water, Na+
INSIDE- cytoplasm (aqueous)
-2x fatty acid tails that point inwards due to non-polar and hydrophobic

Question 2

What are peripheral proteins

Answer 2

Also known as integral proteins, they go all the way through the phospholipid bilayer

Question 3

Describe the channels and how they work

Answer 3

-Aquaporin - specific channel for moving water
-Channel Proteins - transports molecules and ions such as Na+, water, glucose. Move by facilitated diffusion, a type of passive transport; go down the concentration gradient (high to low)
-Carrier Proteins - require ATP to work, can move molecules or ions against a concentration gradient by active transport (low to high)

Question 4

What are glycolipids and glycoproteins

Answer 4

GLYCOLIPID - lipid with a chain of carbohydrate molecules attached
GLYCOPROTEINS - protein with a chain of carbohydrate molecules attached

Question 5

How are non polar molecules transported

Answer 5

Non polar molecules such as oxygen can diffuse straight through the phospholipid bilayer

Question 6

What provides support

Answer 6

-Cytoskeletal proteins below the phospholipid bilayer provide support
-also cholesterol provides stability and fluidity as well as reducing permeability

Question 7

Structure and function of the glycocalyx

Answer 7

-made up of glycoproteins and glycolipids
-allows for cell to cell attachment
-cell surface receptor
-binds to hormones
-cell signalling
-antigens ‘self’ - means immune system won’t attack our cells as they recognise them

Question 8

What is the role of surface membranes

Answer 8

-cell signalling
-contain channel and carrier proteins
-may have enzymes for chemical reactions
-antigens to identify as self
-site of chemical reactions

Question 9

What is the role of membranes in organelles

Answer 9

-lysosome - keep hydrolytic enzymes away from other organelles
-mitochondria - cristae have large surface area allowing for efficient metabolic processes - enzymes held in right place

Question 10

What is meant by polar and non polar

Answer 10

-Polar molecule has an uneven distribution of charge - therefore attracted to water
-Non polar molecules are repelled from water as they have evenly spread charges

Question 11

Define diffusion

Answer 11

the net movement of particles down a concentration gradient from high concentration to low concentration

Question 12

What factors affect rate of diffusion

Answer 12

-temperature - the more kinetic energy means faster rate
-concentration gradient - steeper means faster rate
-diffusion distance - the shorter it is means a faster rate
-surface area - larger surface area means faster rate - e.g. microvilli, alveoli
-size of molecule - smaller means faster rate

Question 13

What can and cannot diffuse across membranes

Answer 13

CAN
-any non polar
-small molecules
-fat soluble membranes - some hormones e.g. testosterone, progesterone, oestrogen as all made of lipid
CANNOT
-any polar molecules
-large molecules
-ions

Question 14

How are concentration gradients maintained

Answer 14

1. Removing molecules from one side of the membrane -e.g. oxygen used for respiration
2. Adding molecules to the other side - e.g. oxygen delivered to cells from blood

Question 15

Define osmosis

Answer 15

Water moves from high water potential to a low water potential gradient across a partially permeable membrane

Question 16

Define water potential

Answer 16

-measure of the tendency of water molecules to diffuse from one region to another
-little solute=less concentration=higher water potential
-lots of solute=higher concentration=lower water potential
-pure water has the highest water potential

Question 17

Describe water potential in animal and plant cells when placed in high water potential (hypotonic)

Answer 17

-water potential inside cells is lower than that of pure water water as there are solutes in solution in the cytoplasm and inside the large vacuole of plant cells
-when cells placed inside solution of higher water potential water molecules move by osmosis, down water potential gradient into the cell
-in animal cells, if lot of water molecules enter, cell will swell and burst due to plasma membrane break - called cytolysis
-in plant cells, the rigid and strong cellulose cell wall will prevent bursting; the cell will become turgid which supports the plants

Question 18

Describe water potential in animal and plant cells when placed in low water potential (hypertonic)

Answer 18

-water leaves cells by osmosis across a partially permeable membrane
-in animal cells, water leaves, the cell shrinks and appears wrinkled - it is crenated
-in plant cells, the membrane pulls away from cell wall therefore the tissue is described as flaccid and the cell is plasmolysed
-cells that are plasmolysed suffer a degree of dehydration and their metabolism cannot proceed

Question 19

Describe active transport

Answer 19

-substances move against the concentration gradient - needs more energy than the kinetic energy of molecules and therefore is provided by hydrolysis of ATP
-cells or organelles may need to accumulate more of a particular ion than they could do by simple or facilitated diffusion alone - e.g. root hair cells use active transport to absorb ion from soil

Question 20

Describe the role of carrier proteins in active transport

Answer 20

-these membrane proteins have specific regions or sites that combine reversibly with only certain solute molecules or ions
-also have a region that binds to and allows the hydrolysis of molecule of ATP to release energy - this way they act as enzymes
-this energy helps carrier protein change its conformation and in doing so, it carries the ion from ones side of the cell membrane to the other

Question 21

Describe bulk transport

Answer 21

-some cells need to transport large molecules and particles that are too large to pass through plasma membrane in and out
-they do this by bulk transport a process that requires ATP
-either exocytosis or endocytosis

Question 22

Describe endocytosis

Answer 22

-this is how large particles may be brought into a cell - they do not pass through the plasma membrane
-instead a segment of the plasma membrane surrounds and encloses the particle and brings it into the cell, enclosed in a vesicle
-phagocytosis is a type of endocytosis - means ‘eating by cells’ and refers to type of intake of solids
-pinoendocytosis is a type of endocytosis where liquids are ingested
-ATP is needed to provide energy to form the vesicles and move them, using molecule motor proteins, along cytoskeleton threads into the cell interior

Question 23

Describe exocytosis

Answer 23

-this is how large molecules may be exported out of cells - they do not pass through plasma membrane, instead a vesicle containing them is moved towards and then fuses with the plasma membrane
-in all cases, ATP is needed to fuse the membranes together as well as for moving the vesicles
-a molecule of ATP is hydrolysed for every step that a motor protein takes along the cytoskeletal thread as it drags it cargo

Question 24

FACTORS AFFECTING MEMBRANE STRUCTURE AND PERMEABILITY : temperature and kinetic energy

Answer 24

-increasing the temperature gives all molecules more kinetic energy and as a result these molecules move faster
-decreasing temperature lowers kinetic energy of molecules causing them to move slower

Question 25

FACTORS AFFECTING MEMBRANE STRUCTURE AND PERMEABILITY : phospholipids and changing temperature

Answer 25

-many organisms do not generate heat to maintain their body temperature and so their temperature varies with their environment

Question 26

Describe what happens when temperature drops

Answer 26

-saturated fatty acids become compressed
-however there are many unsaturated fatty acids making up the cell membrane phospholipid bilayer - as they become compressed the kinks in their tail push adjacent phospholipid molecules away which maintains membrane fluidity
-therefore proportions of unsaturated and saturated fatty acids within cell membrane determine membranes fluidity in cold
-cholesterol in membrane also buffers effect of lowered temperature to prevent reduction in fluidity - prevents phospholipid molecules packing together too tightly
-some organisms, such as fish, can change their composition of fatty acids in response to lower temperatures

Question 27

Describe what happens when temperature increases

Answer 27

-phospholipids acquire more kinetic energy and move around randomly more - this increases membrane fluidity and permeability increases
-affects the way membrane embedded proteins are position and how they function - e.g. proteins act as enzymes could alter rate of reactions they catalyse
-increase in membrane fluidity may affect the infolding of plasma membrane during phagocytosis
-may also change ability of cells to signal to other cells by releasing chemicals, often by exocytosis
-presence of cholesterol molecules buffers, to some extent the effect of increasing heat as it reduces the increase in membrane fluidity

Question 28

How are proteins affected by temperature

Answer 28

-proteins are not as stable as lipids
-high temperatures cause atoms within their large molecules to vibrate- this breaks hydrogen and ionic bonds that hold structure together therefore unfold
-their tertiary structure changes and cannot change back once they cool - they have denatured
-underneath plasma membrane are cytoskeletal threads made of protein - if both membrane embedded proteins and cytoskeletal threads become denatured, plasma membrane will begin to fall apart and become more permeable
-membrane embedded enzymes will cease to function if they denature as shape of active site will change and therefore rate of reactions catalysed will be lowered

Question 29

What is the effect of solvents on phospholipids

Answer 29

-organic solvents, such as acetone and ethanol, will damage cell membranes as they dissolve lipids