2.1.5 Biological Membranes

Describe the structure of the phospholipid bilayer

Hydrophilic phosphate heads, with a hydrophobic core of fatty acid tails. The phospholipids are able to slide past each other.

What types of molecules can pass through the phospholipid bilayer?

Non-polar, small molecules as they are able to dissolve

Give 3 key roles of cell membranes at the surface of cells

They control what enters or leaves the cell, they contain antigens which are important in cell recognition and they are a site for enzymes

Give 2 key roles of cell membranes within cells

They compartmentalise into organs and they permit concentration gradients

Give an example of an organelle in which it is important that they are compartmentalised?

Lysosomes contain hydrolytic enzymes which can break down many different kinds of biomolecules. These need to be compartmentalised otherwise they would break down other cell components too

Why do we use the fluid mosaic model to describe cell membrane structure?

It explains how biological molecules are arranged to form cell membranes

Why does the fluid mosaic model describe the membrane as fluid?

The phospholipids and some of the proteins are free to move within the membrane

Why does the fluid mosaic model describe the membrane as a mosaic?

They are many different protein, glycoprotein and cholesterol molecules interspersed within the phospholipid bilayer

What are the four main components within the fluid mosaic model?

Glycolipids & glycoproteins, cholesterol, phospholipids and transport proteins

Describe the role and location of the phospholipids within the membrane

Phospholipids form the basic structure of the membrane. Their tails form a hydrophobic core, allowing the bilayer to act as a barrier to polar molecules or ions. This ensures water-soluble molecules cannot leak out of the cell and also unwanted molecules cannot get in. They can also act as signaling molecules

Describe the role and location of cholesterol within the cell surface membrane

Cholesterol maintains the fluidity of the membrane - it keeps fluidity down at high temps by stopping phospholipids moving too much, but also stops the membrane freezing by stopping them packing too close. It also increases the mechanical strength and stability of membranes. Cholesterol molecules bind to the hydrophobic tails of the phospholipids.

Describe the role and location of glycolipids and glycoproteins within the cell surface membrane

Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface. They act as receptor molecules by binding with certain substances. Some also act as antigens for cell to cell recognition

What are the two types of transport proteins?

Channel proteins and carrier proteins

What do transport proteins do (general)?

They create hydrophilic channels to allow ions and polar molecules to travel through the membrane

How do channel proteins work

They act like a pore in the membrane that lets water molecules or small ions through quickly

How do carrier proteins work?

They change shape to transport a substance across the protein

What disrupts the membrane structure?

Less polar solvents e.g ethanol disrupt the membrane structure by getting in between the phospholipids. This makes it more fluid and more permeable, and at high concentrations can cause the membrane to disintegrate

Define conformational change

When there is a change in the shape of a protein - e.g when a non-competitive inhibitor binds to an enzyme and the shape changes

Define diffusion

The net movement of a molecule from a region of high concentration to a region of low concentration, down the concentration gradient

Does diffusion require energy?

No - it is a passive process which relies on the KE of the molecules alone

When do we use simple diffusion?

Small, uncharged molecules e.g oxygen and steroid based hormones

Where is oxygen at its lowest concentration?

In the mitochondria

When do we use facilitated diffusion?

For large polar molecules (e.g glucose/amino acids), or ions such as sodium and chloride

When are channel proteins used within facilitated diffusion?

They allow charged substances e.g ions to diffuse through the cell membrane. They provide a water filled pore through the centre of the protein

When are carrier proteins used within facilitated diffusion?

They allow larger polar molecules to diffuse through the cell membrane - they bind to the molecule diffusing and change shape, allowing the molecule to pass through to the other side of the membrane

What factors affect the rate of diffusion?

Temperature, distance, surface area, size of the molecule and gradient

Define active transport

The transport of substances across a membrane, against the concentration gradient using energy from respiration

What type of proteins perform active transport?

Carrier proteins

Why is active transport different to facilitated diffusion?

Although they both use carrier proteins, active transport requires energy

Why does active transport require energy?

Molecules are being moved against the concentration gradient

How is the energy for active transport released?

Hydrolysis of ATP

How will cells that perform high levels of active transport be adapted?

They will have lots of mitochondria

Define osmosis

The net movement of water from an area of high water potential to an area of low water potential, down the water potential gradient

What does water potential mean?

The tendency of water to move out of a solution ie a concentrated solution has a low water potential

What is the water potential of pure water?

0kPa

What are the two components of water potential in plant cells?

Solute potential from the sap vacuole, and pressure potential from the cell wall

What is it called when a plant cell is in a hypotonic solution?

Turgid

What is it called when a plant cell is in a hypertonic solution?

Plasmolysed - the cell membrane pulls away from the cell wall

What is a hypertonic solution?

A solution which has a lower water potential

What is endocytosis

Bulk transport into the cell

What is exocytosis?

Bulk transport out of the cell

What is bulk transport and briefly describe how it works?

The movement of large amounts of molecules across a membrane. Vesicles move and fuse with the plasma membrane, or vesicles are pinched and move away from the plasma membrane. It requires energy.

What are the two types of endocytosis?

phagocytosis and pinocytosis

What happens during phagocytosis?

This is the bulk intake of solid material by phagocytes

What happens during pinocytosis?

This is the bulk intake of liquids

How do cold temperatures affect transport?

The membrane will become more rigid, so there will be restricted conformational changes, no bulk transport due to lack of fluidity and less diffusion as KE decreases

How doe hot temperatures affect transport?

The transmembrane proteins may denature, the membrane itself will become more fluid which increases permeability