2.1.5 BIOLOGICAL MEMBRANES SUPERSTACK

What is the main function of membranes?

To separate the contents of cells from their environment and create distinct compartments within cells.

What is compartmentalisation?

The formation of separate membrane-bound areas in a cell to allow specific conditions for reactions and protect vital components.

Why is compartmentalisation vital?

It allows incompatible metabolic reactions to occur simultaneously in different parts of the cell.

What is the plasma membrane?

The cell surface membrane that separates the cell from its external environment.

What is the basic structure of all membranes?

A phospholipid bilayer.

Describe the phospholipid bilayer.

Hydrophilic phosphate heads face outwards;

hydrophobic fatty acid tails face inwards, forming a hydrophobic core.

Why is the phospholipid bilayer suited to its function?

Because both sides of the membrane can interact with the aqueous environments inside and outside cells.

What is the hydrophilic part of a phospholipid?

The phosphate head.

What is the hydrophobic part of a phospholipid?

The fatty acid tail.

Describe the fluid mosaic model.

Phospholipids can move within the layer (fluid), and proteins of various shapes and sizes are embedded in the bilayer (mosaic).

Why is membrane fluidity important?

It allows flexibility and movement of proteins within the membrane.

Name the main components of the plasma membrane.

Phospholipids

cholesterol

glycoproteins

intrinsic and extrinsic proteins.

What are intrinsic proteins?

Transmembrane proteins embedded through both layers of the bilayer.

How are intrinsic proteins held in place?

By interactions between hydrophobic R-groups and the hydrophobic core of the membrane.

What are channel proteins?

Intrinsic proteins that form hydrophilic channels for passive transport of ions and polar molecules down a concentration gradient.

What are carrier proteins?

Intrinsic proteins that change shape to transport molecules, used in both passive and active transport.

What are glycoproteins?

Intrinsic proteins with attached carbohydrate chains.

Functions of glycoproteins?

Act as receptors for chemical signals and in cell adhesion (forming tight junctions).

What is cell signalling?

The process by which cells communicate using chemical signals binding to receptors to trigger a response.

Give an example of 2 types glycoprotein receptor action.

Neurotransmitter receptors (e.g. acetylcholine)

peptide hormone receptors (insulin, glucagon).

How do some drugs work on receptors?

By binding to them, altering cell responses

What are glycolipids?

Lipids with attached carbohydrate chains acting as cell markers or antigens.

What is the function of glycolipids?

They enable immune system recognition of self and non-self cells.

What are extrinsic proteins?

Peripheral proteins on one side of the bilayer.

How do extrinsic proteins interact with the membrane?

Through hydrophilic R-groups interacting with phospholipid heads or intrinsic proteins.

What is cholesterol?

A lipid molecule with hydrophilic and hydrophobic ends found between phospholipids.

What is the role of cholesterol in membranes?

It regulates membrane fluidity and stability.

How does cholesterol stabilise membranes?

It binds to phospholipid heads and tails, pulling them together, preventing them from becoming too fluid or too rigid.

How does cholesterol prevent membranes from becoming too solid?

It stops phospholipids from grouping too closely and crystallising.

What are the three main roles of membranes at the cell surface?

1) Partially permeable barrier

2) Site of cell signalling/communication,

3) Site of chemical reactions.

What are the roles of membranes within cells?

To compartmentalise organelles, act as partially permeable barriers, and provide sites for metabolic reactions.

Give an example of a membrane’s role in photosynthesis.

Chloroplast thylakoid membranes contain enzymes for photosynthesis.

Why do alcohol, caffeine, and nicotine act quickly on the body?

They are lipid-soluble and diffuse rapidly through membranes.

Why are some organelle membranes highly folded?

It increases surface area for chemical reactions and enzyme attachment.

Define partially permeable membrane.

A membrane that allows certain molecules to pass through but not others.

Name the two main types of membrane protein.

Intrinsic and extrinsic proteins.

What happens to membrane phospholipids as temperature increases?

They gain kinetic energy, move more, and the membrane becomes more fluid and loses structure.

What happens to membranes at very high temperatures?

They are disrupted or destroyed, and permeability increases.

What happens to carrier and channel proteins at high temperatures?

They denature, affecting transport across the membrane.

Why are membranes described as disrupted rather than denatured?

Only proteins denature; membranes as a whole are disrupted or destroyed.

Why does water not disrupt membranes?

Water is polar and stabilises the bilayer

hydrophilic heads interact with water while hydrophobic tails stay protected inside.

How do organic solvents affect membranes?

They dissolve lipids, disrupting the bilayer and increasing permeability.

Why are alcohols used in antiseptic wipes?

They dissolve bacterial cell membranes, killing the cells.

What is the effect of high-concentration alcohols on membranes?

They destroy cells completely.

What is the effect of low-concentration alcohols on membranes?

They increase membrane fluidity and permeability by inserting between phospholipids.

How does alcohol affect neurones?

It disrupts their membranes, stopping normal nerve impulse transmission, causing behavioural changes.

Why is beetroot used to investigate membrane permeability?

It contains betalain pigment that leaks out when membranes are damaged.

Why are beetroot pieces washed before the experiment?

To remove any pigment released from cells cut during preparation.

Why are samples taken after five minutes at each temperature?

To allow time for pigment to diffuse out once the temperature has stabilised.

Why is the experiment repeated three times?

To obtain reliable results and calculate a mean.

Why is a blue filter used in the colorimeter?

Betalain absorbs blue light best, so absorbance readings are most accurate.

How does absorbance relate to membrane permeability?

Higher absorbance = more pigment released = more membrane damage.

At what temperature is the beetroot membrane most disrupted?

Where absorbance increases sharply (around 50–60 °C).

How could you investigate the effect of solvents on membrane permeability?

Soak beetroot pieces in increasing concentrations of alcohol and measure pigment absorbance with a colorimeter.

Define diffusion.

The net movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient.

Is diffusion an active or passive process?

Passive – it does not require metabolic energy (ATP).

What causes diffusion to occur?

The random movement of particles due to their kinetic energy.

When does diffusion stop?

When equilibrium is reached – the concentrations are equal on both sides.

Explain what equilibrium means in diffusion.

The point at which there is no net movement of particles, although particles still move randomly in both directions.

Why is diffusion rapid over short distances but slow over long ones?

Because particles collide more frequently over longer distances, reducing the rate of diffusion.

State two factors that affect the rate of diffusion.

Temperature and concentration gradient.

Explain how temperature affects diffusion rate.

Higher temperature increases kinetic energy of particles, causing them to move and diffuse faster.

Explain how concentration difference affects diffusion rate.

A larger concentration gradient increases the net movement of particles down the gradient.

What is meant by the term ‘concentration gradient’?

The difference in concentration between two regions.

What is simple diffusion?

The diffusion of small, non-polar molecules directly through the phospholipid bilayer.

Which molecules can diffuse directly through the membrane?

Non-polar molecules such as oxygen (O₂) and carbon dioxide (CO₂).

Why can’t ions diffuse through the phospholipid bilayer?

Because the hydrophobic core of the membrane repels charged particles.

Why can polar molecules only diffuse slowly through membranes?

They interact with the hydrophobic core; smaller polar molecules pass more easily than larger ones.

What does ‘partially permeable membrane’ mean?

A membrane that allows some substances to cross but not others.

Name three factors affecting diffusion across a membrane.

• Surface area

• Thickness of the exchange surface

• Concentration gradient

Define facilitated diffusion.

The passive movement of polar molecules or ions across a membrane through channel or carrier proteins, down a concentration gradient.

Why are membranes with channel proteins selectively permeable?

Because most channel proteins are specific to a single molecule or ion.

Does facilitated diffusion require ATP?

No – it is a passive process.

Explain how carrier proteins work in facilitated diffusion.

A specific molecule binds to the carrier protein, causing it to change shape and allow the molecule to pass through.

List five factors that affect the rate of facilitated diffusion.

Temperature

concentration gradient

surface area

membrane thickness

number of channel proteins.

What is active transport?

The movement of molecules or ions from a region of lower concentration to a region of higher concentration using energy and carrier proteins.

Why does active transport require energy?

Because particles are moved against their concentration gradient

What supplies energy for active transport?

ATP (adenosine triphosphate).

What type of membrane protein is used in active transport?

Carrier proteins that act as pumps.

What binds to the carrier protein on the outside of the cell?

The molecule or ion to be transported.

What happens on the inside of the cell during active transport?

ATP binds to the carrier protein and is hydrolysed into ADP and phosphate.

What effect does phosphate binding have on the carrier protein?

It changes the protein’s shape, opening it to the inside of the cell.

What happens after the molecule enters the cell?

The phosphate group detaches and recombines with ADP to form ATP; the carrier protein returns to its original shape.

Why is active transport selective?

Each carrier protein is specific to a particular substance.

What is bulk transport?

The movement of large molecules or whole cells into or out of a cell using vesicles, requiring ATP.

What types of substances are moved by bulk transport?

Large molecules such as enzymes, hormones, or whole cells like bacteria.

What are the two types of bulk transport/ processess?

Endocytosis and exocytosis.

What is endocytosis?

The process by which the cell membrane engulfs material to bring it into the cell.

What is exocytosis?

The process by which vesicles fuse with the cell membrane to release materials out of the cell.

Why do endocytosis and exocytosis require ATP?

ATP provides energy for moving vesicles and changing membrane shape.

Define osmosis.

The diffusion of water across a partially permeable membrane from a higher water potential to a lower water potential.

Is osmosis active or passive?

Passive.

What is water potential (Ψ)?

The pressure exerted by water molecules; measured in kPa.

What is the water potential of pure water?

0 kPa.

What happens when solute is added to water?

Water potential becomes more negative.

Direction of water movement in osmosis?

From higher Ψ to lower Ψ.

What happens to an animal cell in a higher Ψ solution?

Water enters → cell swells and may burst (cytolysis).

What happens to an animal cell in a lower Ψ solution?

Water leaves → cell shrinks (crenation).

What does isotonic mean?

Same water potential inside and outside the cell → no net water movement.

What happens to a plant cell in a higher Ψ solution?

Water enters → cell becomes turgid.