Topic 4 Cell Membranes and Transport.

Question 1

Cell surface membrane sometimes referred to as the plasma membrane:
- How thick is the cell surface membrane?
- What cells are surrounded by a very thin cell surface membrane?
- What is the function of the cell surface membrane?

Answer 1

• About 7 nm thick.
• All cells, including those of both eukaryotes and prokaryotes, are surrounded by a very thin cell surface membrane.
• It is partially permeable and controls the exchange of materials between the cell and its environment.
• Contains proteins and important molecules for cell signalling and recognition.
• The membrane contains various proteins that facilitate the transport of molecules across the bilayer.

Question 2

Describe the structure of a cell surface membrane.

Answer 2

The cell surface membrane is composed of the phospholipid bilayer which is composed of two layers of phospholipids, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward, proteins embedded in or attached to the bilayer, and carbohydrates often attached to proteins or lipids.

Question 3

Define fluid mosaic model.
Describe the fluid mosaic model.

Answer 3

The fluid mosaic model describes cell membranes as flexible layers of phospholipids with embedded proteins.

“Fluid” refers to the ability of lipids and proteins within the membrane to move laterally between monolayers.
“Mosaic” highlights the diverse arrangement of proteins embedded in the lipid bilayer, resembling a mosaic pattern.

Question 4

What are the components of the plasma membrane?

Answer 4

• Phospholipids.
• Cholesterol.
• Proteins.
• Glycolipids (Carbohydrate chains attached to phospholipids).
• Glycoproteins (Carbohydrate chains attached to proteins).

Glyco means carbohydrate chains.

Question 5

Describe the structure of a phospholipid.

Answer 5

Phospholipids consist of a glycerol backbone, two fatty acid tails, and a phosphate group (PO₄³⁻).
The glycerol connects to the phosphate group, which is often linked to additional functional groups.
The fatty acid tails are hydrophobic (water-repelling), while the phosphate group is hydrophilic (water-attracting).

Question 6

What is the role of the hydrophilic head?

Answer 6

The hydrophilic heads of phospholipids are polar due to the presence of the phosphate group and capable of forming hydrogen bonds with water. These interactions help stabilize the structure of the cell membrane.

Question 7

• What is the role of the fatty acid hydrophobic tails?
• What position do the hydrophobic tails face in a phospholipid bilayer?
• What substances can pass through the hydrophobic core?
• What is the role of the fatty acids?

Answer 7

• The hydrophobic tails of phospholipids are non-polar and repel water.
• In a phospholipid bilayer, the hydrophobic tails face inward, away from the water and forming a hydrophobic core while the hydrophilic heads face outward.
• Small, non-polar molecules (like oxygen and carbon dioxide) and some lipid-soluble substances can pass through the hydrophobic core of the membrane.
• The fatty acids help maintain the fluidity of the membrane.

Question 8

What is membrane fluidity and what factors affect membrane fluidity?

Answer 8

Membrane fluidity refers to the flexibility and movement of lipids and proteins within the cell membrane. It describes how easily these components can move laterally and how the membrane can adapt to changes in temperature and environmental conditions. High fluidity allows for better membrane function while low fluidity can make the membrane more rigid and less permeable.

Membrane fluidity is influenced by several factors:
- Temperature.
- Fatty acid tail length.
- Cholesterol.
- Fatty acid composition.

Question 9

Membrane fluidity is influenced by several factors:
- Temperature.

Answer 9

Higher temperatures increase kinetic energy, leading to greater movement of the tails and increased fluidity. Conversely, lower temperatures can cause the tails to pack more tightly, reducing fluidity.

Question 10

Membrane fluidity is influenced by several factors:
- Fatty acid tail length.

Answer 10

Shorter tails enhance fluidity, while longer tails pack more closely, reducing fluidity.

Question 11

Membrane fluidity is influenced by several factors:
- Cholesterol.

Answer 11

Cholesterol molecules insert themselves between phospholipids, stabilizing the membrane and maintaining fluidity across varying temperatures by preventing tight packing of the phospholipid tails.

• At low temperatures, cholesterol helps maintain membrane fluidity by preventing phospholipids from packing too tightly.
• At high temperatures, cholesterol helps stabilize membrane structure by reducing excessive fluidity. It limits the movement of phospholipids.

Question 12

Membrane fluidity is influenced by several factors:
- Fatty acid composition.

Answer 12

Saturated fatty acids have straight tails, allowing tighter packing, which decreases fluidity.
Unsaturated fatty acids have kinked tails, promoting more space and increasing fluidity.

Question 13

Describe the structure of cholesterol.

Answer 13

Cholesterol is a small molecule which features a hydroxyl (-OH) group as part of its structure. Cholesterol has a hydrophilic head (the hydroxyl group) and a hydrophobic tail (the hydrocarbon rings), which allows it to fit within the phospholipid bilayer of cell membranes.

NOTE THAT: Cholesterol is only found in eukaryotes and not prokaryotes.

Question 14

What is cholesterol’s function in membrane structure?

Answer 14

• Cholesterol helps maintain membrane fluidity by preventing phospholipids from packing too tightly.
• It provides structural stability to the membrane, making it less permeable to very small water-soluble molecules.

Question 15

What are the two types of membrane proteins, and how do they differ from each other?

Answer 15

• Extrinsic/peripheral proteins: An extrinsic protein is a type of protein that is loosely attached to the outside or inside of the cell membrane, not embedded in it.
• Intrinsic/integral proteins: An intrinsic protein is a type of protein that is embedded within the cell membrane e.g. transport proteins.

Question 16

Describe the structure of intrinsic/integral proteins.

Answer 16

• In transmembrane proteins where the protein spans the entire membrane, with regions extending both inside and outside the cell:
  - The parts of the protein that are within the lipid bilayer are hydrophobic, whereas the parts exposed to the aqueous environments inside and outside the cell are hydrophilic.

Question 17

Describe the structure of glycoproteins.

Answer 17

• These are carbohydrate chains attached to proteins.
• Glycoproteins have a protein backbone.
• One or more carbohydrate groups are attached to the protein, usually through covalent bonds.
• Glycoproteins can be embedded in the cell membrane (integral glycoproteins) or attached to the membrane surface (peripheral glycoproteins).

Question 18

What are the functions of membrane proteins?

Answer 18

• Membrane proteins play a crucial role transport by facilitating the movement of substances across the cell membrane. They can be classified as:
  - Channel proteins: These form pores that allow specific ions or molecules to pass through the membrane by faciliated diffusion.
  - Carrier proteins: These bind to specific substances and undergo a change in shape to transport them across the membrane by active transport.
• Some membrane proteins function as enzymes.
• They act as receptors for hormones and other signals, facilitating communication between cells.
• Glycoproteins aid in cell recognition by facilitating communication between cells.
• Glycoproteins assist in cell adhesion, allowing cells to stick to each other.

Question 19

Describe the structure of channel proteins.

Answer 19

Channel proteins have a highly specific structure that includes:

• Some channel proteins can be gated, meaning they can open or close in response to specific signals (like ligand binding (e.g. with sodium ions Na+) or voltage changes), controlling access to the membrane.
• The pore of channel proteins is filled with water, creating a hydrophilic environment lined with polar or charged R groups of amino acids. This structure facilitates the passage of water and specific ions through the channel.

Question 20

Describe the structure of carrier proteins.

Answer 20

Carrier proteins have a highly specific structure that includes:
- Carrier proteins have specific binding sites for the substances they transport, allowing them to recognize and bind to particular molecules.

• Conformational change which occurs upon binding, carrier proteins undergo a change in shape, moving the substance across the membrane.

For example: The sodium-potassium pump (Na⁺/K⁺ pump) is a type of carrier protein.

Question 21

Explain the active transport mechanism of the sodium-potassium pump (Na⁺/K⁺ pump).

Answer 21

It has specific sites for binding sodium (Na⁺) and potassium (K⁺) ions. The pump uses 1 molecule of ATP to change its shape, transporting 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell against their concentration gradients.

The carrier protein also acts as enzyme (ATPase) which catalyses the reaction of ATP → ADP + free phosphate ion.

This is an example of active transport.

Question 22

Describe the structure of glycolipids.

Answer 22

• These are carbohydrate chains attached to phospholipids.
• Glycolipids have a lipid backbone anchoring them in the membrane.
• They feature one or two hydrophobic fatty acid tails that integrate the glycolipid into the lipid bilayer and a hydrophilic carbohydrate group that extends outward, facilitating cell recognition and signaling.

Question 23

Describe the function of glycolipids.

Answer 23

• Glycolipids assist in cell adhesion, allowing cells to stick to surfaces and to each other.
• Glycolipids facilitate cell recognition by signaling between cells, such as helping the immune system identify foreign cells.
• Glycolipids contribute to membrane stability by interacting with water molecules.

Question 24

What is cell signalling?

Answer 24

Cell signaling is the process by which cells communicate with each other and detect and respond to stimuli. It involves ligands which are the signaling molecules that bind to specific receptors on target cells to initiate communication.

Question 25

Describe the process of cell signalling.

Answer 25

The process of cell signaling involves several key steps:

• A signaling cell releases ligands (signaling molecules), such as hormones or neurotransmitters.
• Ligands are transported via the bloodstream to target cells.
• Ligands bind to specific receptors on the cell surface of target cells:
  - Receptor is specific and complementary in shape to the ligand.
• When a ligand binds to a receptor, it triggers a change in the receptor’s shape, activating the G protein.
• The activated G protein initiates a series of molecular events inside the cell called signal transduction which results the production of secondary messengers.
• Secondary messengers trigger an enzyme cascade which is catalysed by enzyme kinases and phosphatases amplify the signal, allowing for a fast and widespread response within the cell.
• The signaling cascade results in specific responses.
• The signaling is turned off by breaking down the ligand or secondary messengers, allowing the cell to reset and respond to new signals.

Question 26

Apart from second messengers, what are the three other basic ways in which a receptor can alter the activity of a cell?

Answer 26

• Opening an ion channel, resulting in a change of membrane potential.
• Acting directly as a membrane-bound enzyme.
• Acting as an intracellular receptor when the initial signal passes straight through the cell surface membrane,

Question 27

What are the two types of ligands?

Answer 27

• Water soluble ligands e.g. adrenaline and glucagon.
• Lipid soluble ligands e.g. steriods and oestrogen.

Question 28

What is a water-soluble ligand?

Answer 28

A water-soluble ligand is a signaling molecule that can dissolve in water and cannot cross the cell membrane. Water-soluble ligands are recognized by receptors located on the plasma membrane of target cells.

Question 29

What is a lipid-soluble ligand?

Answer 29

A lipid-soluble ligand is a signaling molecule that can dissolve in lipids and easily pass through the cell membrane. These ligands bind to intracellular receptors located in the cytoplasm or nucleus of target cells.

Question 30

Define passive transport and name the types of passive transport.

Answer 30

Passive transport is the movement of substances across a cell membrane without the use of energy (ATP). This process relies on molecules naturally moving from areas of higher concentration to areas of lower concentration, following their concentration gradient.

There are several types of passive transport, including:
- Simple diffusion.
- Facilitated diffusion.
- Osmosis.

Question 31

Define simple diffusion.

Answer 31

Simple diffusion is the net movement of molecules from a region of its higher concentration to a region of its lower concentration down a concentration gradient as a result of their random movement until equilibrium is reached.

In cells, this occurs across a phospholipid bilayer.

This is an example of passive transport.

Question 32

In simple diffusion, substances that can pass through the phospholipid bilayer include:

Answer 32

• Non-polar or lipid-soluble molecules: Examples are oxygen (O₂) and carbon dioxide (CO₂).
• Uncharged molecules: Small, uncharged molecules like water can also pass through.
• Small molecules: Molecules that are small enough can diffuse directly through the bilayer.

Question 33

Five factors that affect the rate of simple diffusion and therefore the movement of molecules through membranes:

Answer 33

• Temperature.
• Surface area to volume ratio.
• Concentration gradients.
• Nature of molecules/ions.

Question 34

How does temperature affect diffusion?

Answer 34

As temperature increases, the rate of diffusion increases. This is because the molecules gain kinetic energy and thus move faster.

Question 35

How does surface area to volume ratio affect diffusion?

Answer 35

The surface area to volume ratio compares a cell’s surface area to its volume:

• At a higher surface area compared to volume it is easier for materials to move in and out because of shorter diffusion distances.
• At a lower surface area compared to volume it is harder for materials to move in and out because of longer diffusion distances.

Question 36

How does the concentration gradient affect the rate of diffusion?

Answer 36

As the concentration gradient increases, rate of diffusion increases.

Question 37

How does the nature of molecules/ions affect the rate of diffusion?

Answer 37

Smaller molecules diffuse more easily and quickly than larger ones.

Question 38

Define facilitated diffusion.

Give examples of molecules that transported by facilitated diffusion.

Answer 38

Facilitated diffusion is the movement of larger or polar molecules from a region of high concentration to a region of lower concentration down a concentration gradient across the cell membrane with the help of specific transport proteins (channel proteins and carrier proteins).

Glucose and amino acids or ions like Na⁺, K⁺ and Cl⁻.

This process is still passive and does not require energy. Facilitated diffusion can become saturated if all transport proteins are occupied.

Question 39

Four factors that affect the rate of facilitated diffusion and therefore the movement of molecules through membranes:

Answer 39

• Concentration gradients.
• Temperature.
• Number of transport proteins: More available transport proteins (channels or carriers) can enhance the rate at which substances move across the membrane.
• Surface area of the membrane: Large surface area able to fit more transport proteins.

Question 40

Define osmosis.

Answer 40

Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane, down the water potential gradient until equilibrium is reached.

ψ (the symbol for water potential): The tendency of water molecules to move from one area to another.

Question 41

Describe osmosis in animal cells.

Answer 41

• In a solution of a higher water potential, water enters the cell, which can cause it to swell and burst (lysis).
• In a solution of the same water potential, water moves in and out at equal rates, keeping the cell stable.
• In a solution of a lower water potential, water leaves the cell, leading to shrinkage (crenation).

NOTE THAT: Animal cells do not have a cell wall.

Question 42

Describe osmosis in plant cells.

Answer 42

• In a solution of a higher water potential, water enters the cell, causing it to swell and become turgid, which is essential for maintaining structural support.
• In a solution of the same water potential, water movement is balanced, resulting in a flaccid state where the cell does not have pressure against the cell wall.
• In a solution of a lower water potential, water leaves the cell, leading to plasmolysis, where the cell membrane pulls away from the cell wall.

NOTE THAT: Plant cells do have a cell wall.

Question 43

Define active transport.

Answer 43

Active transport is the movement of molecules or ions against their concentration gradient from a region of lower to a region of high concentration using energy in the form of ATP.

An example of active transport is the Na⁺/K⁺ pump.

The ATP is needed for the conformational change of carrier proteins.

Question 44

What is the role of sodium - potassium pumps in cells?

Answer 44

• Important in nerve impulses.
• Transport of ions from soil via root hairs.
• Hydrogen pumps in cells for the translocation of sucrose into the phloem.
• Reabsorption in the kidneys.
• Absorption in the intestines.

Question 45

Define endocytosis.

Answer 45

Endocytosis is the process by which cells engulf substances from the external environment, bringing them into the cell in vesicles e.g. phagocytosis.

Question 46

Define exocytosis.

Answer 46

Exocytosis is the process by which cells release substances to the external environment by packaging them into secretory vesicles that fuse with the plasma membrane.