B4. Transport Across Cell Membranes

Question 1

Why are cell membranes important?

Answer 1

Separate contents of cells from their external environments; Control exchange between cell and its environments; Enables special compartments

Question 2

What’s a phospholipid bilayer?

Answer 2

Phospholipids arranged in 2 molecule thick layer; Hydrophobic tail inwards away from water; Hydrophilic head outwards facing water

Question 3

Why is the membrane described as a fluid mosaic model?

Answer 3

Fluid: Phospholipids & proteins move around via diffusion;
Mosaic: Scattered pattern produced by proteins look like a mosaic from above

Question 4

Describe the structure of the cell membrane.

Answer 4

Phospholipid bilayer makes up membrane, contains proteins; Intrinsic proteins, Extrinsic protein

Question 5

What molecules does the cell membrane contain?

Answer 5

Lipids: Phospholipid, cholesterol, glycolipids
Proteins: Glycoproteins, other proteins eg. transport

Question 6

How are the lipid molecules in the cell membrane positioned?

Answer 6

Phospholipid - forms bilayer;
Cholesterol - fits between phospholipid molecules, orients same way, absent in prokaryotes, has hydrophobic tail & hydrophilic head
Glycolipids - Lipids with carbohydrate chains attached; Found on outer phospholipid monolayer

Question 7

How are the protein molecules in the cell membrane positioned?

Answer 7

Glycoproteins - Proteins w/ carbohydrate chains attached; Found on outer phospholipid monolayer
Intrinsic proteins - embedded in membrane, arrangement determined by hydrophilic & hydrophobic regions; Most commonly span entire membrane (transmembrane protein) eg. transport protein;
Extrinsic proteins - found on outer/inner surface of membrane

Question 8

What is the purpose of phospholipids in a cell membrane?

Answer 8

Form basic bilayer structure; Barrier against most water soluble substances (non polar fatty acid tail prevents passage of polar molecules); Prevents leakage of water soluble molecules; Sugars, amino acids, proteins; Can be chemically modified to act as signalling molecule

Question 9

What is the purpose of cholestrol in a cell membrane?

Answer 9

Regulates fluidity of membrane; Prevents phospholipids packing too close when temp low (stops membrane from freezing & fracturing); Stabilises membrane at higher temps w/ phospholipid tail; Increases permeability of membrane to ions; Increases mechanical strength and stability

Question 10

What is the purpose of glycoproteins & glycolipids in a cell membrane?

Answer 10

Receptor molecules; Binds w/ certain substances at cell surface; Signalling receptors for hormones & neurotransmitters; Receptors involved in endocytosis; Receptors involved in cell adhesion & stabilisation

Question 11

What is the purpose of proteins in a cell membrane?

Answer 11

Transport proteins, hydrophilic channels allow ions & polar molecules cross cell membrane, channel proteins & carrier proteins; Each transport protein is specific to particular ions or molecules; Transport proteins control which substances enter or leave

Question 12

What is the definition of diffusion?

Answer 12

Net movement, as a result of random motion of molecules or ions, of a substance from a region of higher concentration to lower concentration

Question 13

What are the factors that increase rate of diffusion?

Answer 13

Steeper concentration gradient; Higher temperature, more kinetic energy; Higher surface area; Smaller molecules; Non-polar molecules

Question 14

How do large polar molecules diffuse through the phospholipid bilayer membrane?

Answer 14

Facilitated diffusion; Channel proteins or carrier proteins

Question 15

Describe channel proteins.

Answer 15

Water-filled pores; Allow charged substances to diffuse through cell membrane; Gated channels;

Question 16

Describe carrier proteins.

Answer 16

Switches between 2 shapes; Binding site changes depending on shape; Diffusion down concentration gradient

Question 17

What is the definition of osmosis?

Answer 17

Ned movement of water molecules from a region of higher water potential to a region of low water potential through a partially permeable membrane

Question 18

What is the water potential of pure water at atmospheric pressure?

Answer 18

0kPa

Question 19

What is plasmolysis?

Answer 19

Contraction of protoplast of a plant cell due to loss of water

Question 20

What is a protoplast?

Answer 20

Entire cell excluding the cell wall

Question 21

What is cytolysis?

Answer 21

Cell bursts due to osmotic imbalance that causes extra water to diffuse into the cell

Question 22

Describe the investigation of water potential.

Answer 22

Potato cylinders are cut, same length and blotted dry; Initial mass recorded; Leave in solution for set time; Remove and dry excess liquid; Final length and mass recorded

Question 23

What is the definition of active transport?

Answer 23

Movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using ATP

Question 24

What structure does active transport use across the phospholipid bilayer?

Answer 24

Carrier protein, ATP used to change the shape

Question 25

What is co-transport?

Answer 25

Coupled movement of substances across a cell membrane via carrier protein; Combination of facilitated diffusion and active transport

Question 26

Describe how the active transport of glucose occurs across the ileum.

Answer 26

Na+/K+ pump, pumps Na+ from epithelial cell to blood; Na+ glucose pump, pumps Na+ & glucose, against conc gradient, from lumen of ileum into epithelial cell; Energy is given indirectly from conc gradient of sodium ions; Glucose facilitated diffusion through carrier protein into blood; Glucose is also transported by facilitated diffusion through carrier proteins

Question 27

What are the factors that increase rate of facilitated diffusion?

Answer 27

Steep concentration gradient across exchange surface; High count of channels, carrier proteins