4 Cell membranes and transport

Question 1

How can the fluid mosaic model be described?

Answer 1

*Fluid- because the individual phospholipid and protein molecules can move relative to one another and this gives the membrane a flexible structure that is constantly changing in shape
*Mosaic- because the proteins that occur in the phospholipid bilayer vary in shape, size and pattern (scattered) in the same way as the tiles in a mosaic

Question 2

What makes up a phospholipid?

Answer 2

*A hydrophilic phosphate head – attracted to water but not lipid
*2 fatty acid hydrophobic tails- repelled by water but mix readily with lipid

Question 3

How do the hydrophobic and hydrophilic interactions account for the formation of the phospholipid bilayer and the arrangement of proteins?

Answer 3

-One layer of phospholipids with their hydrophilic heads pointing inwards (attracted to water in the cell cytoplasm)
-The other layer of phospholipids with their hydrophilic heads pointing outwards (attracted to watery environment which surrounds the cell)
-The hydrophobic tails of both layers pointing to the center of the membrane (repelled by water on both sides and forms hydrophobic core)

Question 4

Can phospholipids move within their own layer?

Answer 4

Yes

Question 5

What are the components of the cell surface membrane?

Answer 5

*Phospholipids
*Glycoproteins
*Glycolipids
*Cholesterol
*Protein

Question 6

What is the arrangement of proteins, glycoproteins, glycolipids and cholesterol in the cell surface membrane?

Answer 6

*Proteins- arranged more randomly (scattered) in the bilayer and some move within the bilayer while others are anchored in one location, held by fibers in the cytoplasm.
- They are associated with the phospholipid bilayer in 2 ways: peripheral and integral proteins

Question 7

Extrinsic (peripheral) proteins

Answer 7

-Occur on both surfaces of the bilayer, but never extend completely across it

Question 8

Intrinsic (integral) proteins

Answer 8

-May have regions embedded in the bilayer from one side to the other
-Some are channel proteins, while others are carrier proteins
-Proteins have regions of amino acids with hydrophobic R-groups and these are repelled by the aqueous environment and face the internal hydrophobic core of the bilayer, regions that face the aqueous internal/external environment have amino acids with hydrophilic R-groups

Question 9

Channel proteins

Answer 9

• form open water-filled tubes to allow water-soluble ions to diffuse across the membrane
• down the concentration, facilitated diffusion

Question 10

Carrier proteins

Answer 10

• bind to molecules such as glucose and amino acids, then change shape to move these molecules across the membrane
• against a concentration gradient using energy from ATP=active transport

Question 11

Glycoproteins

Answer 11

carbohydrate chains are attached to many proteins on the outer surface of the cell surface membrane

Question 12

Glycolipids

Answer 12

• made up of a carbohydrate portion, that is hydrophilic and covalently bonded with a lipid tail that is hydrophobic and located in hydrophobic core of the bilayer.
• The carbohydrate portion extends from the bilayer into the watery environment outside the cell

Question 13

Cholesterol

Answer 13

complex lipid that has a hydrophilic and hydrophobic portion (which is embedded in the hydrophobic region of the bilayer) and cholesterol molecules interact with the fatty acid tails of the phospholipid molecules

Question 14

Role of phospholipids (permeability)

Answer 14

• the hydrophobic core that is created by the fatty acid tails of the bilayer which is permeable allows many substances (small, lipid soluble/non polar, hydrophobic, respiratory gases, water) to cross the membrane
• however the bilayer is impermeable to small polar molecules and ions as the hydrophobic interactions in the hydrophobic core prevent water-soluble substances from crossing the bilayer directly

Question 15

Role of phospholipids (membrane fluidity)

Answer 15

-the saturated and unsaturated fatty acid tails of phospholipids produce kinks in the tails and these help to keep the membrane fluid as they prevent phospholipids form packing very closely to one another
-Hydrocarbon chains of fatty acids interact with each other. The shorter chains interact less which makes the phospholipids less closely packed, making the membrane more fluid. Hydrophobic interactions of the fatty acid tails contribute to membrane stability
-Membranes that high a proportion of phospholipids with short fatty acid chains and that have unsaturated fatty acids with a high degree of bending (due to kinks) will be very fluid and flexible

Question 16

What parts of the phospholipid bilayer contribute to membrane fluidity?

Answer 16

• Cholesterol
• Phospholipoids (saturated and unsaturated fatty acid tails, hydrophobic interactions of fatty acid tails and composition of phospholipid)

Question 17

Role of cholesterol (permeability)

Answer 17

-Cholesterol contains a small hydrophilic portion located in the phosphate head region of the bilayer and a longer hydrophobic tail that interacts with the fatty acid tails of the phospholipid molecules
-Therefore, plays a role in preventing the movement of dissolved ions and polar molecules across the bilayer

Question 18

Role of cholesterol (membrane fluidity)

Answer 18

-Regulates fluidity and adds stability to membrane
-Prevents close packing of phospholipids when temps decrease to help keep membrane fluid (increase fluidity in cold temps)
-When temps increases they can prevent excessive movement of phospholipids therefore acts to decrease fluidity

Question 19

Temperature (to do with fluidity)- An increase in the proportion of cholesterol molecules makes the membrane less fluid

Answer 19

-An increase in temp increases the fluidity of membranes
-The increase in kinetic energy will increase the movement of molecules
-A high temp will affect tertiary and quaternary protein stricture and if too high can denature them and stop them from functioning
-Microorganisms that live in high-temp environments have heat-stable proteins in their membranes

Question 20

Why is temp is not a main factor when considering membrane fluidity in mammalian cells

Answer 20

they are in a constant temperature environment, however unicellular organisms can vary their membrane composition in response to a change in the external environment

Question 21

Role of glycolipids

Answer 21

-Act as antigens for cell-to-cell recognition
-Helps maintain stability of membrane
-Helps with cell-to-cell adhesion and therefore forms tissues

Question 22

Role of proteins

Answer 22

-Provides structural support
-Acts as carrier proteins (transporting water-soluble substances such as small polar substances and ions)
-Forms channel proteins and aquaporins (transports water-soluble substances such as small polar substances and ions)
-Functions as enzymes
-Helps with cell-to-adhesion and therefore forms tissues
-Acts as cell surface receptors for cell signalling molecules

Question 23

Role of glycoproteins

Answer 23

-Acts as recognition sites for hormones and neurotransmitters
-Helps with cell-to-cell adhesion and therefore forms tissues
-Acts as antigens for cell-to-cell recognition (proteins can also act as antigens)

Question 24

What is cell signalling?

Answer 24

Process by which cells interact with their environment and with the cells around them

Question 25

What is the process of cell signalling?

Answer 25

*Synthesis and secretion of specific chemicals (ligands) from cells
-Particular cells synthesize and secrete specific cell signalling molecules (ligands which bind to other biological molecules, but can be glycoproteins/proteins

*Transport of ligands to target cells
-Cell signalling molecules are transported in the capillary network and pass into tissue fluid
-Only target cells respond to the presence of cell signalling molecules
-In plants, cell signalling molecules can be transported within the phloem sap/move form cell to cell via plasmodesmata/cell walls

*Binding of ligands to cell surface to cell surface receptors on target cells
-Target cell have membrane proteins that act as receptors for cell signalling molecules
-Each type of cell signalling molecule only binds to a receptor that has a complementary shape (cell signalling is specific)

*Response by target cells
-The binding of a ligand to its specific receptor triggers events to occur within the target cell that eventually leads to the desired response

Question 26

What is the process of cell recognition?

Answer 26

-Proteins, glycoproteins and glycolipids act as markers on the surface of cell so that cell of the immune system can identify the cells as “self”
-In this way self-antigens (immune system cells) do not respond to their presence
-If the cells are transferred into other organisms that has its own set of self-antigens, then the antigens will be considered foreign, and the cells will be destroyed in an immune system response

Question 27

Glycocalyx (external surface)

Answer 27

• in some animal cells, glycolipids and glycoproteins form part of the glycocalyx (external surface)
• has a role in cell recognition and cell-to-cell adhesion, can strengthen, protect and provide stability to the cell surface membrane

Question 28

What processes are responsible for movement into and out of cells?

Answer 28

• Simple diffusion
• Facilitated diffusion
• Osmosis
• Active transport

Question 29

What is the difference between passive and active transport?

Answer 29

*Passive- energy comes from natural, inbuilt motion (kinetic energy) of particles, rather than ATP from respiration (simple and facilitated diffusion)
*Active- energy comes from ATP from respiration (active transport)

Question 30

Simple diffusion

Answer 30

• the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower
• Lipid soluble molecules vitamins (A and D)

Question 31

What factors affect the rate of diffusion?

Answer 31

*Concentration gradient- the greater the difference in concentration between 2 regions of molecules/ions, the faster the rate of reaction.
*Area over which diffusion takes place- the larger the area, the faster the rate of diffusion
*The distance over which diffusion occurs- the shorter the distance, the faster the rate of diffusion

Question 32

Facilitated diffusion

Answer 32

• passive process where molecules move from a region of where they are highly concentrated to one of lower concentration, using only kinetic energy of the molecules themselves, along a concentration gradient.
• Occurs at specific locations in the membrane where there are channel and carrier proteins.
• Water soluble vitamins (B and C)

Question 33

Process of facilitated diffusion

Answer 33

-These proteins allow the diffusion of water-soluble substances (including ions) across the membrane, avoiding the hydrophobic core of the phospholipid bilayer
-Channel proteins have ion channels which are selective and have a specific shape which allows one specific ion through.
- Some ion channels remain open, most are closed until ion is present at which point they open and allow diffusion to occur

Question 34

Alternative form of facilitated diffusion:

Answer 34

-Involves carrier proteins and when a particular molecule specific to the protein is present, it binds with the protein at a specific binding point which causes the protein to change shape in such a way that the molecule is released to the other side of the membrane
-Energy from ATP is not used
-Molecules move from a region of where they are highly concentrated to one of lower concentration, using only kinetic energy of the molecules themselves

Question 35

Osmosis

Answer 35

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 partially (selective) permeable membrane (passive), down the water potential gradient

Question 36

Solute

Answer 36

any substance that is dissolved in a solvent

Question 37

Solution

Answer 37

the solute and solvent together

Question 38

What is water potential?

Answer 38

• the pressure created by water molecules and is represented by the Greek letter psi (Ψ) and is measured in kPa. Under standard conditions of 25 degrees Celsius and 100kPa, pure water have a water potential of zero (highest water potential)

Question 39

Features of water potential

Answer 39

-The addition of a solute to pure water lowers its water potential
-The water potential of a solution must always be less than zero
-The more solute that is added, the lower (more negative) its water potential
-Water moves by osmosis from a region of higher (less negative) water potential to one of lower (more negative) water potential

Question 40

Hypotonic solution

Answer 40

a solution with a higher concentration of water (higher water potential therefore more dilute)

Question 41

Hypertonic solution

Answer 41

a solution with a lower concentration of water (lower water potential therefore more concentrated)

Question 42

Isotonic

Answer 42

solutions with the same concentration of water and solute (no net movement of water)

Question 43

How is osmosis carried out in an animal cell? (common example is red blood cell)

Answer 43

in notes

Question 44

How is osmosis carried out in a plant cell?

Answer 44

in notes

Question 45

Why don’t the cells burst in a plant cell?

Answer 45

-The cellulose fibers of the cell wall have great tensile strength and when these fibers are laid down in layers and angles they contribute to the overall strength and rigidity of the cell wall
-Water entering a plant cell by osmosis enters the vacuole, which increases in size and causes the protoplast to swell and push against the cellulose cell wall
-Cell wall is capable of only very limited extension therefore pressure builds up on it that resists the further entry of water (this increases the water potential of the cell)
-The protoplast of the cell is pushed against the cell wall and the cell is said to be turgid

Question 46

Active transport

Answer 46

• the movement of molecules or ions across the cell surface membrane into or out of a cell from a region of lower concentration to a region of higher concentration using energy and carrier molecules

Question 47

What are the main features of active transport?

Answer 47

-Energy is supplied by ATP
- Against a concentration gradient
-Carrier proteins act as “pumps”
-The protein molecules undergo a change in shape
-Process is selective

Question 48

What are some examples of active transport?

Answer 48

*Root hair cell- the concentration of mineral ions in the soil solution is so low that root hair cells of plants need to use active transport
*In the lumen of the intestine- glucose is actively transported into intestinal epithelial cells by active transport as it would be wasteful to take up glucose by facilitated diffusion as after one meal only part of the glucose produced because of digestion would be absorbed
*Sodium potassium pump

Question 49

How is the process of active transport carried out (ion being transported)?

Answer 49

-Carrier proteins in the cell surface membrane accept the ions to be transported on one side of it
-The ions bind to the specific binding site on the inside of the carrier protein
-On the cytosol side of the cell surface membrane, ATP binds to the carrier protein which causes ATP to split into ADP and Pi, causes the protein molecule to change shape and opens to the opposite side of the membrane
-The ions are then released to the other side of the membrane
-Pi is released from protein (ADP and Pi can be reused to produce ATP molecules in respiration) which causes the protein to return to its original shape, ready for the process to be repeated

Question 50

Pi

Answer 50

inorganic phosphate

Question 51

Cytosol side

Answer 51

refers to the side of the cell surface membrane that faces the cytosol, which is the fluid component of the cytoplasm

Question 52

What is a sodium-potassium pump?

Answer 52

• involves the ion moving into a cell at the same time as a different one is being removed
  -Sodium ions are actively removed from the cell, while potassium ions are actively taken in from the surroundings
  -Process is essential for the creation of a nerve impulse

Question 53

What are the 2 forms of bulk movement that require ATP?

Answer 53

• Endocytosis (into a cell)
• Exocytosis (out of a cell)

Question 54

What is the process of endocytosis and its 2 forms?

Answer 54

• Phagocytosis
• Pinocytosis

Question 55

Phagocytosis

Answer 55

-Involves the invagination of the cell surface membrane to form a cup-shaped depression in which large particles/whole organisms are contained
-The depression is then pinched off to the inside of the cell, forming a vesicle
-When the vesicle is pinches off, the 2 areas of cell surface membrane meet and fuse so that all the membrane remains intact
-In unicellular organisms (and a few specialized cells in higher organisms) phagocytosis is used as a method of feeding

Question 56

Pinocytosis

Answer 56

Similar to the above except the vesicles formed are smaller and is often used for the uptake of liquids

Question 57

What is the process of exocytosis?

Answer 57

-Vesicles which are budded off from Golgi body within the cell move towards the cell surface membrane along microtubules
-They fuse with the cell surface membrane and their contents are expelled into the medium outside
-In higher organisms, exocytosis is used to release hormones

Question 58

Invagination

Answer 58

folding in of itself of the cell surface membrane

Question 59

Dialysis (visking) tubing

Answer 59

an artificial membrane tubing which is manufactured with pores of a particular diameter (to allow some molecules to pass through and others not)

Question 60

Investigating simple diffusion and osmosis using ?

Answer 60

plant tissue and non-living materials, including dialysis tubing and agar

Question 61

Osmosis: set up and steps

Answer 61

-Sucrose solution containing food colouring is added to a section of dialysis tubing (the molecules in the food colouring are too large to pass through the pores of the tubing)
-The tubing is tied at one end and a syringe is used to add the sucrose solution
-The other end of the tubing is tied around a bung with a capillary tube inserted
-The outside of the tubing is rinsed with distilled water to remove any sucrose solution
-The tube is then placed in a beaker of distilled water

Question 62

Osmosis: explanation

Answer 62

-The solution in the tubing has a lower water potential than the external solution (water, with water potential of zero)
-Water moves into the tubing by osmosis down the water potential gradient and over time this causes the dialysis tubing to swell
-The increase in volume in the tubing pushes solution up the capillary tube

Question 63

Diffusion

Answer 63

Investigated using dialysis tubing for example by adding a mixture of glucose and starch to a section of tubing tied at the end, tying the top and placing it in a beaker of distilled water

Question 64

Diffusion: set up and steps:

Answer 64

-Water in the external medium enters dialysis tubing by osmosis down the water potential gradient
-Glucose molecules diffuse out of the tubing down the concentration gradient into the external medium
-Glucose and water molecules are small enough to pass through pores of the tubing, while starch is too large
-There are more water molecules entering the mixture in the tubing than glucose molecules leaving, therefore tubing swells (is of greater mass than at the start of the investigation)

Question 65

Diffusion: explanation

Answer 65

-The presence of glucose inside and outside the tubing is confirmed using samples heated with Benedicts solution or test trips specific to glucose can be used
-The presence of starch inside the tubing and absence of starch in the external solution is confirmed using iodine solution

Question 66

Investigating simple diffusion using agar blocks with different surface area to volume ratios

Answer 66

Agar blocks containing a pH indicator can be prepared to investigate diffusion

Question 67

Investigating simple diffusion using agar blocks with different surface area to volume ratios: set up and steps

Answer 67

-Cubes of different dimensions are cut from agar blocks containing a pH indicator
-The indicator changes colour as dilute HCI diffuses into the cube
-The time taken for each cube to change colour completely can be timed to give an end point
-To make comparisons, the surface area to volume ratio (SA: V) of the cubes needs to be calculated

Question 68

Cuboids

Answer 68

Surface area = 2(𝒍𝒘 + 𝒘𝒉 + 𝒍𝒉)
Volume = l × b × h

Question 69

Cylinders

Answer 69

Surface area = 2πrh + 2πr ^2
Volume= π r² h