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)
• when temperatures are low; this prevents membranes from freezing and fracturing
• stabilises the cell membrane at high temperatures
• prevents water and dissolved ions leaking out the cell
• only in eukaryotic membranes
• sit in between the phospholipids, preventing them from packing too closely together

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

Question 10

How do glycoproteins and glycolipids act as receptors?

Answer 10

• 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 11

Proteins in the cell-surface membrane.

Answer 11

extrinsic proteins :
- dont extend across the membrane
- provide mechanical support
- can be connected to carbohydrates/ lipids to make glycoproteins/ glycolipids

Intrinsic proteins :
- involved in the transport of molecules across membrane
- span the entire membrane

• ** carrier proteins** :
  will bind with larger molecules and change shape to transport them to other side of membrane
• protein channels:
  create hydrophilic channels to allow ** ions and polar molecules to travel through the membrane**
• help control which substances enter or leave

Question 12

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

Answer 12

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 13

What is diffusion?

Answer 13

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

Question 14

Which factors effect diffusion?

Answer 14

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

Question 15

How does concentration gradient effect the rate of diffusion?

Answer 15

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

Question 16

How does temperature effect the rate of diffusion?

Answer 16

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

Question 17

How does surface area effect the rate of diffusion?

Answer 17

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

Question 18

How does a large SA: V ratio effect the rate of diffusion?

Answer 18

slower diffusion as distance becomes too big

Question 19

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

Answer 19

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 20

What is facilitated diffusion?

Answer 20

movement of ions and polar molecules down a concentration gradient that occurs through protein channels/ carrier proteins found on the plasma membrane.

Question 21

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

Answer 21

• Channel proteins
• Carrier proteins

Question 22

What is osmosis?

Answer 22

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 23

What is meant by water potential?

Answer 23

how concentrated a solution is w water
Pure water = 0 kPa
- high water potential = closer to 0
- low water potential= further from 0 (less diluted there are many solvents in the solution)

Question 24

Osmosis in plant cells.

Answer 24

• 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 25

Osmosis in animal cells.

Answer 25

- **Hypertonic** solution: water **leaves** the cells as Ψ is **more negative** in solution than cell → **shrivelled** - **Isotonic** solution: **no net movement** as Ψ is same in solution and cell - **Hypotonic** solution: water **enters** the cells as Ψ is more positive in solution than cell → cells **swell** and may **burst**

Question 26

What is active transport?

Answer 26

Active transport is the **movement of molecules and ions** through a cell membrane **against the concentration gradient** using **ATP** and a **carrier protein**.

Question 27

What is co-transport?

Answer 27

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

Question 28

How are root hair cells adapted for exchange?

Answer 28

- 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 29

How are epithelial cells specialised for efficient diffusion?

Answer 29

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

Question 30

How are neurones and muscle cells adapted for facilitated diffusion?

Answer 30

- 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

Question 31

Describe the process of the Na+/K+

Answer 31

1. molecule/ ion **binds** to **complementary to receptor site** on carrier protein 2. **ATP** binds to carrier protein & is **hydrolysed** into ADP +Pi, energy is released. (phosphorylation) 3.causing **carrier protein to change shape** & **release molecule** to the other side 4. Phosphate ion is released and protein returns to original shape

Question 32

Describe the co- transport of glucose and Na+ in the ileum

Answer 32

2. Na+ use facilitated diffusion to diffuse into epithelial cell from lumen via co- transport protein 2. glucose molecule is brought into epithelial cell by co- transport protein along with Na+ --> bc of this con of glucose in epithelial cell rises 3. glucoses uses facilitaed diffusion to diffuse down its conc gradient ( as conc of glucose in blood stream is lower than epithelial cell bc blood flows and carries away absorbed glucose) 3. Na+ ions are **actively transported** out of epithelial cell into **bloodstream** by Na/K+ pump creating high conc grad of Na+ in lumen than epithelial cell so Na+ moves in

Question 33

adv of microvilli having large SA in terms of transport?

Answer 33

- allows for more co- transport proteins