2.4 Transport across Membranes

Question 1

What are the roles of the cell-surface membrane?

Answer 1

• separates the internal cell environment from the external environment
• controls the exchange of material across two areas
• acts as an interface for communication

Question 2

Membranes are …

Answer 2

partially/selectively permeable.

Question 3

By which main processes can can substances cross membranes?

Answer 3

• diffusion
• osmosis
• active transport

Question 4

The major component of the cell-surface membrane is the …

Answer 4

phospholipid bilayer.

Question 5

What is the fluid mosaic model?

Answer 5

The fluid mosaic model explains how different components of the cell membrane are arrange.
- “fluid” because the components can move laterally, allowing diffusion
- “mosaic” because it is made of many parts working together like a mosaic made of many tiles

Question 6

What are the main components of the cell surface membrane?

Answer 6

• phospholipids
• cholesterol
• glycolipids
• glycoproteins
• other proteins (e.g. transport proteins) → can be intrinsic or extrinsic

Question 7

Phospholipids in the cell-surface membrane.

Answer 7

• forms a bilayer
• hydrophilic phosphate heads point out
• hydrophobic fatty acid tails point inwards
• Individual phospholipid molecules can move around within their own monolayers by diffusion
• forms the basic structure of the membrane
• acts as a barrier to most water-soluble substances due to hydrophobic fatty acid tails
• ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in
• can be chemically modified to act as signalling molecules by moving within the bilayer to activate enzymes or being hydrolysed and releasing smaller water-soluble molecules that bind to specific receptors in the cytoplasm

Question 8

Cholesterol in the cell-surface membrane.

Answer 8

• regulates the fluidity of the membrane (more cholesterol = less fluid)
• sit in between the phospholipids, preventing them from packing too closely together when temperatures are low; this prevents membranes from freezing and fracturing
• stabilises the cell membrane at high temperatures
• contributes to the impermeability of the membrane to ions and increases mechanical strength and stability of membranes
• only in eukaryotic membranes

Question 9

Glycolipids and glycoproteins in the cell-surface membrane.

Answer 9

• lipids or proteins with carbohydrate chains attached
• carbohydrate chains project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid monolayer)
• location enables glycolipids and proteins to act as receptor molecules and bind to certain substances
• there are 3 main types of receptors:
  1. signalling receptors for hormones and neurotransmitters
  2. receptors involved in endocytosis
  3. receptors involved in cell adhesion and stabilisation (carbohydrates can form hydrogen bonds with water molecules)
• some act as cell markers or antigens, for cell-to-cell recognition

Question 10

Proteins in the cell-surface membrane.

Answer 10

• embedded within the membrane → intrinsic
• most commonly, they span the entire membrane → transmembrane proteins
• transport proteins (e.g. channel/carrier) → create hydrophilic channels to allow ions and polar molecules to travel through the membrane
• transport proteins are specific to a particular ion or molecule
• help control which substances enter or leave

Question 11

Describe how the movement of substances across the cell membranes is affected by membrane structure.

Answer 11

1. Phospholipid bilayer allows the movement of non-polar, lipid soluble molecules
2. Phospholipid bilayer prevents the movement of polar, water soluble molecules
3. Carrier proteins allow active transport
4. Carrier / channel proteins allow facilitated diffusion and co-transport
5. Shape and charge of channel / carrier determines which
   substances move
6. More transport proteins = more movement
7. Larger surface area = more diffusion/movement
8. Cholesterol affects fluidity / permeability

Question 12

What is diffusion?

Answer 12

The net movement of a substance from a region of its higher concentration to a region of its lower concentration.

Question 13

Which factors effect diffusion?

Answer 13

• steepness of concentration gradient
• temperature
• surface area
• properties of molecules or ions

Question 14

How does concentration gradient effect the rate of diffusion?

Answer 14

The steeper the concentration gradient, the faster the rate of diffusion.

Question 15

How does temperature effect the rate of diffusion?

Answer 15

A higher temperature means molecules have more kinetic energy so they move faster resulting in a higher rate of diffusion.

Question 16

How does surface area effect the rate of diffusion?

Answer 16

A larger surface area means more molecules can cross it at any one moment so there is a faster rate of diffusion.

Question 17

How do the properties of molecules or ions effect the rate of diffusion?

Answer 17

Smaller molecules diffuse more quickly than large molecules as they require less energy to move. Uncharged and non-polar molecules can diffuse directly across the phospholipid bilayer.

Question 18

What is facilitated diffusion?

Answer 18

A type of diffusion that occurs through transport proteins found on the plasma membrane.

Question 19

What are the two types of proteins that enable facilitated diffusion?

Answer 19

• Channel proteins
• Carrier proteins

Question 20

What is osmosis?

Answer 20

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.

Question 21

What is meant by water potential?

Answer 21

The tendency of water to move out of a solution.

Question 22

Osmosis in plant cells.

Answer 22

• If plants are placed in a solution with a very high water potential water will enter the cell, making it turgid.
• If plants are placed in a solution with a very low water potential water will leave the cell, making it plasmolysed (this process is called plasmolysis)

Question 23

Osmosis in animal cells.

Answer 23

• Hypertonic solution: water leaves the cells → shrivelled
• Isotonic solution: no net movement
• Hypotonic solution: water enters the cells → cells swell and may burst

Question 24

What is active transport?

Answer 24

Active transport is the movement of molecules and ions through a cell membrane against the concentration gradient using energy from respiration.

Question 25

What is co-transport?

Answer 25

Co-transport is the coupled movement of substances across a cell membrane via a carrier protein. (Involves both facilitated diffusion and active transport)

Question 26

How are root hair cells adapted for exchange?

Answer 26

• shape increases it’s surface area so greater rate if osmosis
• thin wall for a short diffusion distance
• permanent vacuole containing cell sap which is more concentrated than soil water so maintains a steep water potential gradient

Question 27

How are epithelial cells specialised for efficient diffusion?

Answer 27

• microvilli which increase the surface area
• each villus has a constant blood supply which maintains the concentration gradient

Question 28

How are neurones and muscle cells adapted for facilitated diffusion?

Answer 28

• cell membranes contain many channel proteins for sodium, potassium and calcium ions
• K+ ions high concentration in the neuron
• Na+ ions high concentration outside the neuron