3.2.3 Transport across membranes

Question 1

What are the functions of cell membranes ?

Answer 1

• Control what substances can enter and exit
• Form a barrier between cell/organelle and the external environment
  
  • separation of organelles so metabolic reactions can occur independantlyn (isolation of enzymes that may damage cells)
  • form internal transport systems (rough endoplasmic reticulum)
  • form surfaces for reactions (ribosomes and protein synthesis on the rough endoplasmic reticulum)

Question 2

What is the basic structure of all cell membranes ?

Answer 2

Fluid-mosaic model
- Phospolipid molecules form a continuous bilayer (with some proteins and carbohydrates throughout)

Question 3

Why the the membrane known as a fluid-mosaic ?

Answer 3

Fluid - phospholipids always move so shape is always shifting
Mosaic - scattered pattern produced by the proteins looks like a mosaic

Question 4

Give the structure and function of a phospholipid

Answer 4

Hydrophillic heads point outwards and hydrophobic hydrocarbon tails point inwards
- Forms a barrier as the **centre of the bilayer is hydrophobic **so doesn’t allow water soluble substances to pass through

Question 5

Define intrinsic (integral) protein

Give some examples

Answer 5

embedded in the cell membrane and control transport of molecules and ions across the membrane

Carrier and channel proteins

Question 6

Define extrinsic (peripheral) proteins

Give some examples

Answer 6

on the outer or inner surface of the membrane and provide mechanical support and act as receptors

Receptors, antigens (These can connect to carbohydrates to make glycoproteins)

Question 7

Give the function of channel proteins

Answer 7

enable water soluble ions to diffuse across the membrane

in order to do simple diffusion substances must be LIPID SOLUBLE

Question 8

Give the function of carrier proteins

Answer 8

enable large molecules (amino acids, glucose) and ions to diffuse across the membrane

Question 9

Give the five general functions of proteins

Answer 9

1. provide structural support
2. act as channels to transport water soluble substances across the membrane
3. allow active transport across the membrane via carrier proteins
4. cell surface receptors for recognition of other cells and hormones
5. help cells adhere

Question 10

Give the structure and function of cholesterol

Answer 10

• fits between phospholipids molecules and binds to the hydrocarbon tails
• causes phosholipids to pack close together which restricts movement - less fluid
• so reduces lateral movement of the membrane, even at increased temperatures - This also prevents water and ion leakage

Present in ALL cell membranes (except prokaryotic)
-important in cells which arent supported by other cells - eg red blood cells

Question 11

What two factors can increase membrane fluidity ?

Answer 11

1. increase in unsaturated fatty acid chains - means chains are less tightly packed so intermolecular forces are weaker
2. higher temperatures - molecules have more energy and move more freely which increases membrane fluidity

Question 12

Give the structure and function of glycoproteins

Answer 12

• Proteins with a polysaccharide attached
  1. form recognition sites for hormones and neurotransmitters
  2. form cell antigens for cell-cell recognition (allows body to recognise its own vs foreign cells in immuse response)
  3. involved in cell adhesion (carbohydrate forms a bond with water molecules around other receptors) - This allows cells to attach and form tissues
  4. increase membrane stability as glycoproteins can form hydrogen bonds with water molecules

Question 13

Give the structure and function of glycolipids

Answer 13

• Lipids with a polysaccharide chain attached
  1. same receptor functions as a glycoprotein
  2. increase **membrane stability **by forming hydrogen bonds with water molecules

Question 14

What substances are able to pass through the semi-permeable phospholipid bilayer ?

Answer 14

• lipid-soluble substances (hormones, vitamins)
• very small molecules - gases (oxygen and carbon dioxide)
• uncharged molecules
• water as it is very small

substances such as glucose and amino acids are too large to pass through the phospholipid bilayer

Question 15

What substances can’t pass through the phospholipid bilayer?

Answer 15

• not lipid soluble
• too large to pass through the channels
• same charge as the charge on the channel proteins (causes repulsion)
• polar (electrically charged) so can’t pass the non-polar hydrophobic tails

Question 16

Define diffusion

Answer 16

net movement of molecules or ions from an area of high concentration to an area of low concentration till an equilibrium is achieved
- down the concentration gradient
- through a partially permeable membrane

Question 17

Why is diffusion a passive process ?

Answer 17

particles are in constant random motion so are able to distribute themselves until an equilibrium is reached

Question 18

What is simple diffusion ?

Answer 18

when molecules diffuse directly through the cell membrane

Question 19

What is facilitated diffusion ?

Answer 19

diffusion of large or polar molecules with the assistance of** highly specific membrane proteins**

Question 20

What is the role of carrier proteins and how do they do this ?

Answer 20

movement of **large molecules **
1. molecule binds to the protein
2. this causes the protein to change shape
3. so it can release the molecule on one side while closing off the other to prevent unwanted molecules passing

Question 21

What is the role of channel proteins and how do they do this ?

Answer 21

movement of charged ions (allows then to avoid the non-polar central layer)
1. water filled pores
2. they are gated so can open and close when required to control ion exhange

Question 22

What five factors affect rate of diffusion ?

Answer 22

1. temperature - more kinetic energy = increased diffusion
2. concentration gradient - increased gradient means more random movement across the membrane so increased diffusion
3. surface area - increased SA means more molecules can diffuse at a given time, increasing rate of difffusion
4. membrane thickness - thinner = shorter diffusion distance
5. number of channel/carrier proteins
6. size of molecule - large molecules move more slowly as more energy is required to move them

Question 23

Define osmosis

Answer 23

diffusion of water molecules from an area of high water potential (dilute solution) to an area of low water potential (concentration solution)
- across a partially permeable membrane
- down the concentration gradient

Question 24

Define water potential

What are the units ?

Answer 24

the tendancy of water molecules to diffuse out of a solution (due to the pressure created by the water molecules)

Kilopascals

Question 25

What is the water potential of pure water, what happens if you add solute ?

Answer 25

Water potential = **zero** - adding solute **decreases the water potential **so it takes on a negative value

Question 26

Define isotonic

Answer 26

same water potential inside and outside of the cell (No net movement)

Question 27

Describe the mechanism for osmosis

Answer 27

* solute and water molecules are in** random motion** due to kinetic energy * **selectively permeable membrane** allows water molecules through * water molecules diffuse from an area of high to low water potential * osmosis continues until dynamic equilibrium is achieved (No net movement)

Question 28

In a plant cell describe what happens in an external solution of: - higher water potential - lower water potential - Equal water potential ## Footnote Define protoplast Define plasmolysis

Answer 28

Higher - **water enters** the cell via osmosis - the Protoplast swells - cell becomes** turgid **and supports the cell (Inelastic cell wall prevents bursting) Lower - **water leaves** the cell - the Protoplast shrinks - cell is **plasmolysed ** Equal -** no net movement of water** (incipient plasmolysis) ## Footnote Protoplast - elastic living part of the cell Plasmolysis - contraction of the protoplast of the PLANT CELL due to loss of water

Question 29

In an animal cell describe what happens in an external solution of: * higher water potential * lower water potential * equal water potential ## Footnote Define cytolysis

Answer 29

Higher -water enters cell (due to increased solute in cell) -cell swells and may burst (lyse) - as there is no inelastic cell wall to stop bursting Lower - water leaves cell - cell shrivels Equal - no net movement of water - normal cells ## Footnote when a cell bursts due to an osmotic imbalance

Question 30

What five factors affect the rate of osmosis ?

Answer 30

1. Water potential gradient - increased gradient = increased rate 2. Thickness of exchange surface - thinner = faster rate 3. Surface area - larger = faster rate 4. Presence of aquaporins 5. surface area

Question 31

Define active transport

Answer 31

the movement of molecules or ions in or out of the cell from a **region of low to high concentration** (against the concentration gradient) - using **energy** from respiration (ATP and carrier proteins)

Question 32

Describe the use of ATP in active transport

Answer 32

* Energy is needed as molecules are** moving against the concentration gradient** * hydrolysis reaction** releases energy from ATP** * this makes the carrier molecules lose shape * for the** transfer of molecules across the membrane**

Question 33

Describe the use of carrier proteins in active transport

Answer 33

* **act as pumps** to help transport materials against the concentration gradient, across the membrane * **specific tertiary structure** - specific binding sites mean they will only transport some substances

Question 34

Define direct active transport

Answer 34

directly uses chemical energy from ATP to transport molecules

Question 35

Describe the process of direct active transport

Answer 35

1. molecule or ion** binds to receptor site **on the carrier protein 2. inside the cell **ATP binds to the protein** - splits into ADP and a phosphate (energy) 3.** protein molecule changes shape **and **molecule/ion is released** on the other side of the membrane 4. phosphate is released so **protein reverts to original shape** (phosphate recombines with ADP)

Question 36

Define co-transport Define co-transporter

Answer 36

Co-transport: **coupled movement of substances** across a cell membrane via a **carrier protein** Co-transporter: type of **carrier protein** which binds to two molecules at the same time

Question 37

Describe the first stage of the co-transport of glucose

Answer 37

1. sodium ions are transported out of the **epithelial** into the blood 2. via **active transport** 3. in the **sodium potassium pump**

Question 38

Describe the second stage of the co-transport of glucose

Answer 38

* sodium ions **diffuse** from the lumen into the epithelial cell * down the **concentration gradient ** * via **sodium-glucose transporter proteins**

Question 39

Describe the third stage of the co-transport of glucose

Answer 39

* glucose is carried into the cell with sodium * via the **sodium glucose transporter proteins** * this causes the **glucose concentration in the cell to increase**

Question 40

Descrive the fourth stage of the co-transport of glucose

Answer 40

*** Glucose diffuses into the blood** (due to the increased concentration in the cell) * down the concentration gradient * via a protein channel * This is **facilitated diffusion**

Question 41

What four factors affect the rate of active transport ?

Answer 41

1. Speed of carrier proteins 2. Number of carrier proteins 3. Rate of respiration and availability of ATP

Question 42

How does temperature affect membrane fluidity ?

Answer 42

* increased temperature * gives the phospholipids more kinetic energy so membrane is more fluid * increasing the permeability of the membrane

Question 43

How does solvent concentration affect membrane permeability?

Answer 43

* stronger concentration means the lipid bilayer is more easily dissolved * increasing the permeability as the membrane begins to break down

Question 44

How does pH affect membrane fluidity ?

Answer 44

* pH affects protein structure and charge distribution * too high/too low affects the membrane permeability