Transport across Membranes

Question 1

What is the structure of a cell membrane?

Answer 1

A phospholipid bilayer (two phospholipid molecules thick)

Question 2

What is a phospholipid?

Answer 2

Two fatty acid chains linked to two of the three carbons of a glycerol molecule

Question 3

Properties of fatty acid chains and the head (glycerol + phosphate)

Answer 3

2 fatty acid chains: non-polar, hydrophobic (avoids water, faces inwards)

Head: polar, hydrophilic (likes water, faces outward)

Question 4

Why are phospholipids amphipathic?

Answer 4

Due to their distinct hydrophobic (fatty acid chains) and hydrophilic (glycerol + phosphate head) regions

Question 5

What is the fluid mosaic model

Answer 5

It describes the structure of the cell membrane
Fluid: phospholipids move freely and laterally within the phospholipid bilayer
Mosaic: protein molecules randomly scattered and embedded within the phospholipid bilayer

Question 6

What are glycolipids and glycoproteins?

Answer 6

Glycolipids: carbohydrate molecules covalently bonded to lipids
Glycoproteins: carbohydrate molecules covalently bonded to proteins

Question 7

What is membrane fluidity influenced by?

Answer 7

Unsaturated fatty acids increase membrane fluidity and decreases the temperature the membrane solidifies at
Saturated fatty acids decreases membrane fluidity and increase the temperature the membrane solidifies at
Cholesterol (only in animal but not plant cell membranes) stabilise membrane fluidity, increasing fluidity at low temperature and decreasing fluidity at high temperature

Question 8

Describe cell membranes

Answer 8

They are partially permeable and allow some but not all substances to pass through them

Question 9

What passes through the cell membrane via diffusion?

Answer 9

Small molecules and lipid-soluble substances are able to pass directly through the hydrophobic interior of the phospholipid bilayer via simple diffusion.
Non-polar molecules that can pass: oxygen, carbon dioxide, nitrogen
Polar but small: water, glycerol
Large hydrophobic molecules: hydrocarbons

Polar molecules have a slight positive and negative charge and cannot pass directly across the lipid bilayer

Question 10

What are transport proteins?

Answer 10

(Carrier and channel proteins) allow useful molecules like ions, amino acids, sugars (like glucose) a and other water-soluble substances to move across membranes, moving down a concentration gradient. This process is facilitated diffusion, a passive process.

Question 11

How do water molecules move across?

Answer 11

Either by osmosis or with the aid of channel proteins known as aquaporins, which create hydrophilic pathways.

Question 12

Define diffusion

Answer 12

Diffusion is the net movement of molecules from an area of higher concentration to an area of lower concentration, down a concentration gradient.

It is a passive process.
Facilitated diffusion requires the aid of transport proteins, while simple diffusion does not

Question 13

Factors affecting diffusion rate

Answer 13

Steepness of concentration gradient (increase)
Temperature (increase)
Diffusion distance (decrease)
Size of molecules (decrease) 
Surface area to volume ratio (increase)

(Change to increase diffusion rate)

Question 14

What is equilibrium?

Answer 14

When there is no NET movement of molecules, molecules are uniformly distributed

Question 15

Define osmosis

Answer 15

Osmosis is the net movement of water molecules from an area of less negative water potential to an area of more negative water potential, across a partially permeable membrane.
It is a passive process

When water molecules enter the cell via aquaporins, it is still osmosis (with the cell membrane being the partially permeable membrane)

Question 16

What is water potential?

Answer 16

The tendency of water to move from one place to another
Pure water’s water potential = 0
Less negative means less concentrated, more negative means more concentrated

Question 17

What are isotonic, hypotonic, hypertonic solutions?

Answer 17

Isotonic: same water potential as the cell
Hypotonic: less negative water potential (more water)
Hypertonic: more negative water potential (less water)

Question 18

Animal cells in isotonic, hypotonic, hypertonic solutions

Answer 18

Isotonic: no change, no net movement of water molecules since water potential is the same

Hypotonic: cell wall will swell and burst
Due to absence of cell wall and net movement of water into the cell

Hypertonic: cell will shrink and crenate
Due to net movement of water out of the cell

Question 19

Plant cells in isotonic, hypotonic, hypertonic solutions

Answer 19

Isotonic: no change, no net movement of water molecules since water potential is the same

Hypotonic: cell increases in size and becomes turgid
Due to absence of cell wall and net movement of water into the cell

Hypertonic: cell will plasmolyse, where cell membrane pulls away from the cell wall
Due to net movement of water out of the cell

Question 20

Definition of active transport

Answer 20

Active transport is the net movement of molecules from an area of lower concentration to an area of higher concentration, up a concentration gradient, which is an active process requiring energy.

Question 21

Traits of cells that do active transport

Answer 21

High energy demand, large numbers of mitochondria. Protein transporters are involved.

Question 22

How are water and mineral salts absorbed at root hair cells?

Answer 22

Concentrated cell sap has more negative water potential than soil. Water molecules are absorbed via osmosis, and mineral ions are absorbed via active transport

Question 23

What are exocytosis and endocytosis?

Answer 23

The bulk transport of substances (large molecules) across membranes in transport vesicles. Energy is required

The amount of cell membrane is kept relatively constant (in non-growing cells), by allowing the addition and loss of membranes to be approximately equal.

Question 24

What is exocytosis?

Answer 24

When a cell removes substances by the fusion of a vesicle with the cell membrane
The contents of the vesicle exit the cell, and the vesicle becomes a part of the cell membrane

Example: secretion of substances like enzymes or hormones out of secretory cells

Question 25

What is endocytosis?

Answer 25

When substances are taken into the cell by forming new vesicles in the cell membrane
Small area of the cell membrane sinks inwards to form a pocket, which deepens and pinches to form a vesicle

Example: phagocytosis

Question 26

How does a virus enter a cell?

Answer 26

Through endocytosis