B2.1.5 Biological membranes

What are the 5 functions of a membrane?

1. Keeping all cellular components in the cell.

2. Allowing selected molecules to move in and out of the cell.

3. Isolating organelles from the rest of the cytoplasm, allowing cellular processes to occur.

4. A site for chemical reactions.

5. Allowing a cell to change shape.

What is the fluid mosaic model?

Suggests that proteins are found within the phospholipid bilayer in the fluid of the membrane.

What are phospholipids made of?

• A charged (hydrophilic) head which is attracted to water- a phosphate group.

• A non-polar (hydrophobic) tail that is repelled by water and mixes with fats- a fatty acid.

What is a micelle?

The phospholipids form a circle with the head on the outside and the tails in the middle. They form when exposed to water to protects the tails from water.

What is a bilayer?

The phospholipids form a double layer with the heads on both sides and all tails in the middle. They form when exposed to water to protects the tails from water.

What is a cell membrane made of?

A sea of phospholipids in a bilayer with protein molecules in between the phospholipid molecules. It also contains receptors for other molecules such as hormones and toxins and cholesterol for stability.

Explain why phospholipids form a bilayer in plasma membranes.

Phospholipids have a polar phosphate group which are hydrophilic and will face the aqueous solution. The fatty acid tails are non-polar and will move away from an aqueous environment. As both tissue fluid and cytoplasm is aqueous, phospholipids form two layers with the hydrophobic tails facing inwards and the phosphate group hydrophilic heads facing outwards, interacting with the aqueous environment.

Name 5 components of a cell membrane (other than phospholipids)

1. Intrinsic proteins- integral, channel and carrier

2. Glycoproteins

3. Glycolipids

4. Extrinsic proteins- peripheral

5. Cholesterol

What is the function of integral proteins?

Intrinsic proteins that are embedded through both layers of a membrane. They have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place. They cover the whole width of the membrane.

What is the function of channel proteins?

Transport- provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes. This allows ions, sugars and amino acids to be transported because they can’t diffuse across a membrane.

What is a carrier protein?

Transport- an important role in passive (down concentration gradient) and active (against concentration gradient) transport into cells, often involving the shape of the protein changing. This allows ions, sugars and amino acids to be transported because they can’t diffuse across a membrane.

What is the function of a glycoprotein?

• Intrinsic proteins that are embedded in the cell-surface membrane with attached carbohydrate chains.

• Cell adhesion- help to join cells together.

• Cell communication- act as receptors for chemical signals. When the chemical binds to the receptor, it triggers a response from the cell that causes events inside the cell, e.g. for neurotransmitters at nerve cell synapses or for peptide hormones such as insulin.

What is the function of a glycolipid?

Lipids with attached carbohydrate chains. These molecules are called antigens and can be recognised by immune system cells as self or non-self.

What is the function of peripheral proteins?

• Present in one side of the bilayer.

• They normally have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or with the intrinsic proteins.

• They act as receptors for hormones such as neurotransmitters or are involved in cell recognition.

• Extrinsic proteins on the cytosolic side of the membrane are involved in cell signalling or chemical reactions. They can dissociate from the membrane and move into the cytoplasm.

What is the function of cholesterol?

• A lipid with a hydrophilic end and a hydrophobic end, like a phospholipid.

• It regulates the fluidity of the membrane.

• Cholesterol molecules are positioned between phospholipids in a membrane bilayer.

• This is how cholesterol adds stability to membranes without making them to rigid.

• The cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising.

What molecules can and cannot diffuse across membranes?

• Small, non-polar molecules (O2, CO2) diffuse rapidly across membranes.

• Small, polar molecules (H2O, urea) diffuse slowly across membranes.

• Charged particles (ions) cannot diffuse across.

What is diffusion?

The net movement of particles from an area of high concentration to low concentration, down the concentration gradient.

Is diffusion active or passive?

Passive as no metabolic energy is expended.

What is Fick’s law?

Rate of diffusion is proportional to: surface area x difference in concentration / length of diffusion path (membrane thickness)

What is simple diffusion?

Small non-polar molecules in high concentration on one side of the membrane can easily pass through the bilayer.

What is facilitated diffusion?

Polar or large molecules cannot pass straight through the bilayer. Carrier proteins or protein channels allow the molecules to pass through. This is still passive as no energy is required.

What is active transport?

• Movement of particles against the concentration gradient.

• Cell uses carrier proteins to pump substances across the membrane.

• Requires expenditure of energy in the form of ATP.

• Cells that undergo active transport have lots of mitochondria to produce energy.

What is active transport used for?

• Uptake of glucose and amino acids in the small intestine.

• Absorption of mineral ions by plant roots.

• Excretion of hydrogen ions and urea by kidneys.

• Exchange of sodium and potassium ions in neurons and muscle cells.

How does active transport work?

• Molecule binds to carrier protein.

• ATP attaches to the membrane of the protein on the inside of the cell.

• The binding of the phosphate ion to the protein causes the protein to change shape.

• This makes access to the inside of the cell open for the molecule but closed to the outside.

What is bulk transport?

When extremely large substances need to be moved across a cell membrane. Exocytosis and endocytosis.

What is endocytosis?

Bulk transport of material into the cell. 3 processes- phagocytosis, pinocytosis, receptor-mediated endocytosis.

This involves changes to membrane shape.

What is exocytosis?

Bulk transport of material out of the cell- essentially the reverse of endocytosis.

What are vesicles function?

To transport substances around the cell as they can bind with the cell plasma membrane to take in or expel content.

What is osmosis?

Net movement of solvent molecules from a region of their high concentration to one of low concentration across a partially permeable membrane.

How is the net movement of water by osmosis determined?

By differences in water potential between two solutions connected by a partially permeable membrane.

What is water potential, how is it measured, what is its symbol and what is the range of value?

• The tendency of water molecules in a system to move.

• It is determined by the symbol shown in the picture and is measured in kiloPascals (kPa).

• It is always less than or equal to 0.

What substance has the highest and lowest water potential?

Pure water- highest- 0kPa

Solutions- lowest- negative

What way do particles move in regards to water potential?

From high to low water potential.

What effect does a greater amount of solutes have on water potential?

Lower water potential.

What is solute potential?

The contribution of solutes to water solution. Its always negative.

What effect does pressure have on water potential?

Greater pressure means higher water potential. It’s always positive.

Equation for water potential?

Water potential = solute potential + pressure potential

What is osmotic concentration?

The amount of dissolved solutes in a solution. Insoluble molecules don’t affect it.

What is the osmotic concentration of an isotonic solution?

The same as the cytoplasm of the cell.

What is the osmotic concentration of a hypertonic solution?

Higher than the cytoplasm of the cell.

What is the osmotic concentration of a hypotonic solution?

Lower than the cytoplasm of the cell.

Why do cell membranes need to be fluid? (3 reason)

• Diffusion

• Membrane fusing (e.g. vesicle fusing with membrane in exocytosis)

• Cells to move and change shape

What does heat do to membrane structure?

Increases kinetic energy of phospholipids which disrupts membrane structure, creating gaps in the bilayer making it more permeable.

What effect does solvents have on membrane structure?

Water is polar molecule which is important in creating membrane stability with the phospholipids.

Alcohols are less polar so disrupt the structure of the membrane by moving into the bilayer.