2.1.5 Biological membranes

Question 1

What does the term fluid-mosaic model refer to in cell membranes?

Answer 1

• Fluid - Phospholipids form a bilayer in which the phospholipid molecules are constantly moving
• Mosaic - Proteins of different sizes and shapes embedded in the phospholipid bilayer, resembling a mosaic pattern.

Question 2

Name the key components of a plamsa membrane.

Answer 2

Phosphlopid bilayer
Cholesterol
Proteins - can be intrinsic or extrinsic
Glycoproteins
Glycolipids

Question 3

How are phospholipids arranged in the bilayer and why?

Answer 3

Each phospholipid has one hydrophilic head and two hydrophobic tails.
They arrange into a bilayer with heads facing outward (towards water) and tails inward (away from water).

Question 4

Which substances can pass directly through the bilayer and which cannot?

Answer 4

Can: Lipid-soluble substances (they dissolve in the bilayer).

Cannot: Water-soluble substances, ions and large molecules (due to the hydrophobic core).

Question 5

What does cholesterol do in the cell membrane?

Answer 5

Cholesterol adds stability.
Its hydrophobic regions bind with phospholipid fatty acid tails, causing them to pack more closely together.
Reducing membrane fluidity

Question 6

What is the difference between intrinsic and extrinsic membrane proteins?

Answer 6

• Intrinsic - Embedded through both sides of the bilayer, e.g. channel and carrier proteins which transport large molecules and ions.
  Extrinsic - Present on only one side og the bilayer, they provide support or involved in cell signalling

Question 7

What are glycoproteins and glycolipids and their functions?

Answer 7

Glycoproteins = Proteins + carbohydrates
Glycolipids = Lipids + carbohydrates
Functions:

• Cell adhesion
• Cell recognition
• Cell signalling

Question 8

What are the two main roles of cell membranes?

Answer 8

1. Cell surface membrane - surround cells to acts as a barrier between a cell and its environment
2. Organelle membranes - act as a barrier between the organelle and cytoplasm = Compartmentalisation.

Question 9

What does it mean when a membrane is partially permeable?

Answer 9

It allows some molecules to pass through, but blocks others.

Question 10

What happens to membrane permeability at temperatures below 0°C?

Answer 10

Phospholipids have very low kinetic energy
They are packed closely together to form a rigid cell membrane, decreasing permeability
So cholesterol inserts itself between the phospholipids, preventing them from packing too closely - helping maintain fluidity and flexibility.

Question 11

How does temperature between 0–40°C affect membrane permeability?

Answer 11

Phospholipids gain kinetic energy and move more.
They become less tightly packed.
Membrane becomes more permeable.

Question 12

What are the effects of high temperatures (above 40°C) on the membrane?

Answer 12

The phosphlipid bilay breaks down
Channel & carrier proteins denature - cannot control what enters or leaves cell
Very high permeability

Question 13

What effect does increasing solvent concentration have?

Answer 13

Higher solvent concentration = more disruption to structure of cell membrane = greater membrane permeability.

Question 14

What is the definition of diffusion?

Answer 14

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.
(It continues until equilibrium is reached).

Question 15

Does diffusion require energy?

Answer 15

No - diffusion is a passive process.

Question 16

What kind of molecules use simple diffusion to cross the membrane?

Answer 16

• Small molecules (e.g. oxygen, carbon dioxide)
• Non-polar molecules - Can dissolve in the hydrophobic core of plasma membrane

Question 17

**

What kind of molecules require facilitated diffusion?

Answer 17

• Large molecules (e.g. glucose)
• Charged or polar particles (e.g. ions)

Question 18

What is the role of carrier proteins in facilitated diffusion?

Answer 18

1. A large molecule attaches to a carrier protein.
2. This causes the carrier protein to change shape
3. The carrier protein then releases the molecule on the opposite end of the membrane.

Question 19

What is the role of channel proteins?

Answer 19

Channel proteins form pores in the cell membrane, which ions can travel through.

Question 20

Describe 5 factors that affect the rate of diffusion.

Answer 20

1. Temperature - Higher temp = more kinetic energy = faster diffusion
2. Concentration gradient – Steeper gradient = faster rate
3. Thickness of membrane - Thinner = shorter distance for particles = faster diffusion
4. Surface area - Larger SA = more particles can cross membrane at once = faster diffusion
5. Number of carrier or channel proteins - More proteins = faster facilitated diffusion

Question 21

What is active transport and how is it different from diffusion?

Answer 21

Active transport is the movement of particles from a lower concentration to a higher concentration (against the concentration gradient).
It is an active process, requiring energy in the form of ATP from respiration.

Question 22

What are the steps in carrier protein function during active transport?

Answer 22

1. Molecule or ion binds to the carrier protein.
2. ATP binds to the protein and is hydrolysed to ADP + Pi.
3. This causes the protein to change shape and move the molecule across.
4. The phosphate (Pi) detaches, and the protein returns to its original shape.

Question 23

Describe four factors that influence how fast active transport occurs.

Answer 23

1. Temperature - Increases kinetic energy and respiration rate (too high = denatured proteins).
2. Membrane thickness - Thinner = faster transport.
3. Number of carrier proteins - More proteins = faster rate.
4. Rate of respiration - More ATP available = more active transport.

Question 24

What is bulk transport and when is it used?

Answer 24

Bulk transport moves large molecules or many molecules at once across the membrane. It is an active process that uses ATP.
Includes Endocytosis and Exocytosis

Question 25

Describe **endocytosis** and its two forms.

Answer 25

In endocytosis, the membrane **engulfs** materials to form a **vesicle** that **enters** the cell. * **Phagocytosis** - uptake of solid materials * **Pinocytosis** - uptake of liquid materials

Question 26

How does **exocytosis** work?

Answer 26

**Vesicles** (mostly formed by Golgi body) move towards and **fuse** with cell-suraface membrane Materials are then **released outside** cell.

Question 27

What are the components of a **solution**?

Answer 27

A **solute** (e.g glucose), dissolved in a **solvent** (e.g. water)

Question 28

Define **water potential** and state its units.

Answer 28

Water potential (Ψ) is the **pressure exerted by water** molecules against the **membrane** or container. It is measured in **kiloPascals (kPa)**.

Question 29

What is the difference between high and low water potential?

Answer 29

* High water potential = High concentration of water (low solute) & **Less negative kPa** * Low water potential = Low concentration of water (high solute) & **More negative kPa** **Pure water** has the highest water potential: Ψ = 0 kPa

Question 30

Define osmosis.

Answer 30

The diffusion of **water molecules** across a **partially permeable membrane** from an area of **higher water potential** to an area of **lower water potential**.

Question 31

What happens to animal cells in different solutions💧?

Answer 31

* **Hypotonic** solution = Solution has a **higher** water potential than the cell - Water enters → Cell **bursts** * **Isotonic** solution = Solution has **same** water potential as the cell - No net movement → Cell remains **same**. * **Hypertonic** solution = Solution has a **lower** water potential than the cell - Water leaves → Cell **shrinks**

Question 32

What happens to **plant cells** in different solutions💧?

Answer 32

* **Hypotonic** solutions = Solution has **higher Ψ** than the cell - Water move into cell → Cell **swell** and becomes **turgid**. * **Isotonic** solutions = Solution has the **same Ψ** than the cell - No net movement of water → Cell stays **same size**. * **Hypertonic** solutions = Solution has **lower Ψ** than the cell - Water moves out of cell → Cell **shrinks** and becomes **flaccid/plamolysed**

Question 33

Why do plant cells behave differently to animal cells when placed in different solutions?

Answer 33

Plant cells have a **rigid cell wall**, made of cellulose, outside the cell membrane. Providing **structural support** and prevents the cell from bursting when water enters.

Question 34

Describe four factors that affect the rate of osmosis.

Answer 34

1. **Temperature** - Higher temp = molecules have more kinetic energy = faster osmotic rate 2. **Water potential gradient** - Steeper gradient = faster osmosis 3. **Thickness of membrane** - Water travels shorter distances with thinner = faster osmotic rate 4. **Surface area** - Larger SA = more molecules cross at once = making osmosis faster.

Question 35

What is ATP and why is it important for cells?

Answer 35

ATP (adenosine triphosphate) is involved in energy transfer within cells. It is an example of a phosphorylated nucleotide.

Question 36

What are the three parts of the ATP structure?

Answer 36

1. 3 Phosphate groups 2. Ribose - 5 carbon sugar 3. Adenine - a nitrogenous base

Question 37

What are some of the key uses of ATP in the body?

Answer 37

* Movement, e.g. muscle contraction * Active transport of molecules against the concentration gradient * Synthesis of large molecules, e.g DNA and proteins * Secretion of substances from cells (exocytosis), e.g. releasing hormones from glands.

Question 38

What happens during ATP hydrolysis?

Answer 38

ATP is broken down into ADP (adenosine diphosphate) and inorganic phosphate (Pi) **using water**. This **releases energy**. The reaction is catalysed by the enzyme **ATP hydrolase**

Question 39

What happens during ATP condensation?

Answer 39

ADP and Pi join to form ATP, with **water being released**. This **requires energy** and traps chemical energy in the bond. The reaction is catalysed by the enzyme **ATP synthase**.

Question 40

Why is ATP well-suited for its role as an energy source?

Answer 40

* Hydrolysis of ATP releases **small amoun**t of energy = little energy lost as heat * Breaks down in **one step**, allowing rapid energy release. * **Quickly re-synthesised**, ensuring a constant supply. * The inorganic phosphate can phosphorylate other compounds, making them more reactive. * The phosphate bonds are **unstable and easily broken**, releasing energy. * It is **soluble**, allowing easy transport around the cell.