Cell membranes and transport

Why is it called fluid

both phospholipids and proteins can move by diffusion

Why is it called mosaic

Because the proteins are in a scattered pattern

Describe the structure of the phospholipid bilayer

Has hydrophilic polar heads that interact with water and has non-polar fatty acid tails that are hydrophobic

What do the unsaturated tails allow for

They allow the tails to bend and fit closer together

How is the bilayer formed

The hydrophobic interactions from the tails drive the tails inwards and the hydrophilic interactions drive the heads out, creating a bilayer

What is the arrangement of proteins on the bilayer

Extrinsic: are on the inner and outer surface but do not penetrate the hydrophobic core

Intrinsic: span the whole width of the bilayer

What are the roles of intrinsic proteins on the membrane

1) carrier molecules or channel protein —> allowing for the transportation of ions, sugars and AA’s 2) receptors for hormones and neurotransmitters

What are the roles of extrinsic proteins on the membrane

1) receptors for hormones and neurotransmitters 2) involved in cell signalling on the cytosolic side 3) can act as enzymes

Functions of membranes

1) keep cellular components in cell 2) allowing selected molecules in and out 3) isolating organelles from rest of cytoplasm 4) site for biochemical reactions 5) allows cell to change shape

What are the two things phospholipids can form when they are mixed with water

1) Micellars 2) Bilayers

Describe the structure of micelles

Ball like structures that, hydrophilic heads face out into water and shield the hydrophobic tails

Describe the structure of bilayers

Sheet like structure where hydrophilic heads also shield tails, around 7nm wide. With phosphate hydrophilic heads facing out and the hydrophobic fatty acid tails facing inwards

What is the arrangement of cholesterol in the membrane

They fit between the phospholipid molecules, with rest of structure going into the fatty acid tail region

What is the arrangement of glycolipids in the membrane

The lipid portion sits in the hydrophobic layer with the carbohydrate chain extending out from membrane surface

What is the arrangement of glycoproteins in the membrane

is dependant on where the extrinsic and intrinsic proteins are sitting

Role of glycolipids on the membrane

1) Act as a receptor molecule 2) Cell recognition, acting as antigens 3) used when cells come into contact with each other —> allowing recognition 4) can sometimes play role in cell signalling by acting as receptor binding

Roles of glycoproteins

1) Play a role in cell recognition, presenting as antigens to T cells, 2) Act as receptors for hormones as well as binding with substances at the cell surface 3) Allows cells to attach to each other to form tissue 4) Can also play role in cell signalling —> receptors for signalling molecules

Roles of phospholipids

1) Creates a barrier to substances 2) Permeability- makes it difficult for polar molecules or ions to pass through 3) provides fluidity 4) allows non-polar molecules like O2 to diffuse quickly

Roles of cholesterol

1) Maintains the correct fluidity for membranes, decreasing fluidity at high temps, increasing fluidity at low temps 2) Increases stability by increasing strength by being between phospholipid molecules 3) prevents phospholipid molecules from packing close together 4) hydrophobic regions prevent entry of polar molecules

Steps of the cell signalling cascade

1) Signal si ligand chemical which is secreted by signalling cell

2) Ligand binds to receptor which changes shape allowing signal transduction

3) The changed receptor interacts with the G protein

4) G protein activates the specific enzyme which goes on to cause a second message to be released

5) Small and soluble second messenger amplifies signal across cell

6) results in cells response —> metabolic change

When does the process of transduction happen in the cell signalling cascade

When the signal is passed along and amplified inside the cell through a chain of molecules

What is a ligand

is a signalling molecule that is relayed through a chain of chemical messengers inside the cell

Define diffusion

The net movement of a particle down a concentration gradient from a region of high concentration to a region of low concentration

Define facilitated diffusion

Transport of substances across a biological membrane from an area of high concentration to an area of low concentration with the help of a transport molecule

Define osmosis

The net movement of water from an area of high water potential to an area of low water potential across a partially permeable membrane

Define active transport

the movement of molecules and ions through a cell membrane from a region of low concentration to a region of high concentration using energy from respiration

Define endocytosis

When the cell surface membrane engulfs a material to form a small sac

Define exocytosis

The process by which materials are removed from cells

What is the rate of diffusion proportional to

(surface area x difference in conc.) divided by length of diffusion path

What does Fick’s law help to explain

It helps to explain how diffusion is altered and how certain factors can cause it to be faster or slower

Why is surface area important for diffusion

1) cells rely on diffusion for internal transport 2) with increased surface area, this increases diffusion

Why can hydrophobic molecules cross membranes

Because the interior of the membrane is hydrophobic

Why is the steepness of the concentration gradient important for diffusion

1) increased steepness = increased diffusion rate as there are more net molecules on one side of membrane than the other, forcing net movement

Why is temp. important for diffusion

Increased temp results in molecules and ions having more kinetic energy, increasing movement —> increasing diffusion rate

Why is the nature of molecules and ions important for diffusion rate

1) large molecules require more energy 2) non-polar molecules diffuse easier because they are soluble in the non-polar phospholipid tails