Transport across the cell membrane

What is the function of the cell-surface membrane

to control the movement of substances in and out of the cell

why is the structure of the membrane known as the fluid-mosaic model

It is fluid because individual phospholipids can move relative to one another, which makes the membrane flexible. It is like a mosaic because the proteins embedded in it vary shapes and sizes.

structure of phospholipids

hydrophilic head and hydrophobic tail

function of the phospholipids

to allow lipid-soluble substances to enter, and prevent water-soluble substances from entering and leaving the cell, to make the membrane flexible and self-sealing

extrinsic proteins

on the surface of the membrane. They provide mechanical support and also act as cell receptors for molecules (hormones)

intrinsic proteins

span the length of the bilayer. Protein channels and carrier proteins are intrinsic proteins.

function of proteins

provide structural support, act as channels for transporting water-soluble substances, allow active transport through carrier proteins, cell-surface receptors for hormones, and help cells join together

cholesterol

pulls together fatty acid tails of the phospholipid bilayer and makes the membrane less fluid. It reduces lateral movement, prevents the loss the water and dissolved ions, and makes the membrane less fluid at high temps

glycolipids

carbohydrate bonded with a lipid with the carbohydrate portions extends outwards. They act as cell-surface receptors for the ABO system, and help cells form tissues

glycoproteins

carbohydrate chain bonded to extrinsic proteins. Act as cell-surface receptors for hormones and neurotransmitters. They act as recognition sites, help cells form tissues, and help cells recognise each other

what is diffusion

diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration until equilibrium is reached, down a concentration gradient

is diffusions passive?

Diffusion is passive because it does not require energy in the form of ATP

What is facilitated diffusion?

Facilitated diffusion uses protein channels and carrier proteins to diffuse substances that are too big or not-lipid soluble, so were unable to pass via simple diffusion.

what are protein channels

Protein channels are water-filled hydrophilic channels that only allow water-soluble ions to pass. However, it only allows specific ions to pass

What are carrier proteins

Carrier proteins bins with complementary molecules, which causes the protein to change shape, so the molecule is released inside the membrane

what is osmosis

Osmosis is the movement of water from an area of higher water potential to an area of lower water potential through a selectively permeable membrane, down a concentration gradient

what is water potential

the pressure created by water molecules. Pure water's water potential is always 0. The more solute in water, the lower (more negative) the water potential is

external solution has a higher water potential than the cell solution

water will move into the cell and it will swell and burst

external solution has a lower water potential than the cell solution

water will leave the cell and it will shrink

external solution has the same water potential as the cell solution

Water will neither enter nor leave the cell and there is no change to the state of the cell

water potential change in RBC

If a RBC is placed in pure water, water will enter the cell, causing it to burst and release its contents (It comes haemolysed). Animal cells live in a solution with a similar water potential to its own to prevent this.

what is active transport

active transport is the movement of substances in or out of the cell from an area of lower concentration to an area of higher concentration, using energy in the form of ATP

How does active transport work

The molecule binds to the recognition site on the carrier protein. ATP binds on the other side of the protein on the inside. ATP splits into ADP and a phosphate molecule. The protein changes shape and the molecule is released inside the cell. The protein reverts to its original shape and ADP+P forms ATP in respiration

sodium-potassium pump

a carrier protein that uses ATP to actively transport sodium ions out of a cell and potassium ions into the cell.

what factors increase transport across the membrane

microvilli provide increase in surface area for diffusion, increased number of protein channels and carrier proteins

what is co-transport

A type of facilitated diffusion. Sodium ions are transported out into the blood plasma through the sodium-potassium pump. This makes the concentration of sodium inside the epithelial cell lower than outside in the lumen of the intestine. Sodium ions diffuse inside the cell, carrying a glucose molecule with them. The glucose moves against the concentration and the sodium moves down its concentration gradient

co-transport is an indirect form of active transport

This is because the movement of sodium going down the concentration gradient powers the movement of glucose into the cell, rather than ATP