OCR A Level Biology Movement across Membranes

What type of lipid forms a bilayer in cell surface membranes?

Phospholipid

How are phospholipids arranged in the phospholipid bilayer?

With their polar heads facing outwards and their non-polar tails facing inwards

What is the function of the phospholipid bilayer?

To prevent the movement of polar substances through the membrane

Where is cholesterol found in the cell surface membrane?

Between, and bound to, the tails of the phospholipids

What is the purpose of cholesterol in the cell surface membrane?

To cause the phospholipids to pack more closely together, making the membrane less fluid and more rigid

What type of protein would a channel protein be?

Intrinsic

What is the function of a channel protein?

Allows the movement of smaller charged molecules through the membrane via diffusion (different ones for different particles)

What is the function of a carrier protein?

Moves larger molecules through the membrane via facilitated diffusion or active transport

How do carrier proteins work?

When a molecule enters them, they change shape (this can either be using the molecule's own energy in facilitated diffusion or the cell's energy in active transport), allowing the molecule through the phospholipid bilayer and into the cell

What is a glycoprotein?

A protein with a polysaccharide chain attached

What are 4 functions of glycoproteins/glycolipids?

Stabilise the membrane by forming hydrogen bonds with water molecules, act as antigens, receptors in cell signalling, site where hormones and drugs bind to the cell

How thick is the plasma membrane?

Around 7nm

What is a fluid mosaic structure?

One which is constantly moving and made up of many small pieces (i.e the cell surface membrane)

What are 3 examples of attachments that plasma membranes allow to occur?

Ribosomes to Rough ER, enzymes to cell surface membrane and to inner membrane of mitochondria

Where are proteins for the cell surface membrane made?

Ribosomes on the Rough ER

What is the difference between intrinsic and extrinsic membrane proteins?

Intrinsic- go all the way through the membrane
Extrinsic- on the surface of the membrane

What percentage of the plasma membrane proteins are extrinsic and what percentage are intrinsic?

~30% extrinsic, ~70% intrinsic

Are extrinsic proteins more hydrophobic or philic?

Hydrophilic

Are intrinsic proteins more hydrophobic or philic?

Hydrophobic

What are 3 examples of intrinsic membrane proteins?

Channel and carrier proteins, some enzymes

What are 3 examples of extrinsic membrane proteins?

Receptors, antigens, glycolipids/glycoproteins

How is membrane structure affected below 0 C?

Phospholipids have less energy, so pack more tightly and membrane is more rigid. Channel and carrier proteins denature, ice crystals may penetrate membrane, making it more permeable

How is membrane structure affected between 0 and 45 C?

As temperature increases, phospholipids get more energy and move more, so membrane permeability increases

How is membrane structure affected above 45C?

Phospholipid bilayer begins to break down and melt, so membrane more permeable. Expanding water in cell puts more pressure on membrane. Channel/carrier proteins denature.

Why are polar solvents such as water essential in the structure of plasma membranes?

To keep the heads and tails of the phospholipids arranged correctly

Why do organic solvents dissolve membranes?

Many are less polar than water (i.e alcohols) and some are completely non-polar (i.e benzene). This means that phospholipid bilayer is disrupted and cell membrane dissolves

Why is alcohol used in antispetics?

Becuase it is less polar than water, so dissolves the plasma membranes of bacteria

Why are alcoholic drinks harmful to cells?

While they do not dissolve plasma membranes like strong or pure alcohols, they still damage them because non-polar alcohol molecules get between the phospholipids, disrupting the membrane and making it more fluid and permeable.

Why does people's behaviour change if they drink alcohol?

Because it disrupts the membranes of neurones (nerve cells) in the brain, whose membrane is vital in the transmission of nerve impulses

Which direction do substances move during endocytosis?

Into the cell

Which direction do substances move during exocytosis?

Out of the cell

What type of substances are transported by phagocytosis?

Solids

What type of substances are transported by pinocytosis?

Liquids

What is an example of endocytosis?

Glycogen molecules entering liver cells to be stored

When do endo/exocytosis occur?

When large amounts of molecules need to be moved across the cell surface membrane quickly

Are exo/endocytosis active processes?

Yes, as they require energy to move the vesicles and cell surface membrane

When does endocytosis occur?

When a high concentration of useful substances are present outside the cell surface membrane

What happens during endocytosis?

The cell surface membrane folds inwards (a movement controlled by microfilaments) until it pinches off and forms a vesicle around the desired molecules; this vesicle can then be transported around the cell via the cytoskeleton

What triggers exoctosis?

Some form of external factor, such as a certain hormone being present in sufficient amounts

What occurs during exocytosis?

Vesicles containing the required molecule are moved along the cytoskeleton to the cell surface membrane. The vesicle then fuses with the membrane, and the molecules are released from the cell

What is an example of exocytosis?

Insulin being released in response to low blood glucose concentrations

What is the general term for exo/endocytosis?

Bulk transport

What is cell signaling?

When cells communicate with each other, i.e using hormones or in the nervous system

What would cells designed to respond to a hormone likely have?

A cell receptor (usually a glycoprotein or glycolipid) with a complementary shape to that hormone

What is a primary messenger?

The hormone or other molecule which binds with the complementary cell receptor

What happens when a hormone binds to the cell receptor?

It causes an enzyme attached to the receptor to begin working

What happens when an adrenaline molecule binds to a glycoprotein?

It causes an enzyme to begin catalyzing the conversion of ATP into cAMP, which triggers the necessary reactions within a cell

What is a secondary messenger?

Something which causes the necessary reactions within a cell once the primary messenger has bound to a receptor

What is an example of a common secondary messenger?

cAMP

Why are secondary messengers necessary?

Because primary messengers cannot always pass through the cell surface membrane

Why don't some primary messengers, such as steroid hormones, need secondary messengers or cell receptors?

Because they can pass through the cell-surface membrane themselves (steroid hormones are lipid-based)

What are reactions caused by a secondary messenger called?

Enzyme controlled reactions

What is an example of an enzyme controlled reaction?

More carrier/channel proteins being placed in the cell surface membrane

Why is diffusion a passive process?

Because it uses particles' own kinetic energy

What is diffusion?

The net movement of particles across a partially permeable membrane and down a concentration gradient

Which molecules can diffuse into cells between the phospholipids in the cell surface membrane?

Lipid-based molecules and ones which are very small (such as CO2, O2 or H20)

Which molecules can diffuse into cells, but only via facilitated diffusion?

Larger or polar/charged molecules

What is simple diffusion?

Diffusion in which the molecules move between gaps in the phospholipids

What is facilitated diffusion?

Diffusion of larger or charged molecules across a cell membrane through channel (for smaller molecules) or carrier (for larger ones) proteins

Where does the energy that a carrier protein needs to change shape come from?

The particle's own kinetic energy when it collides with the carrier protein

What is osmosis?

The net movement of free water molecules across a partially permeable membrane.

How is the number of free water molecules measured?

As water potential

What is the highest possible value for water potential?

0, the water potential of distilled water

What is the effect of addition of solutes to a solution on its water potential?

It lowers its water potential

What units is water potential measured in?

kPa (i.e water with water potential of 0 puts 0kPa of pressure on a membrane)

What is the protoplast?

The plant cell except the cell wall

What is crenation?

Where a cell shrinks due to a lack of water

What is haemolysis?

Where a cell swells, bursts and releases its contents due to having too much water

What is an isotonic solution?

One with the same water potential as a cell, meaning that there is no net movement of water in or out of it

What type of solution should blood be?

Isotonic

What is a hypotonic solution?

One with a higher water potential than cells, meaning that there would be a net movement of water into the cells

What is a hypertonic solution?

One with a lower water potential than cells, so there would be a net movement of water out of the cells

What would happen to an animal cell placed in a hypertonic solution?

It would crenate

What would happen to an animal cell placed in a hyp otonic solution?

It would haemolyse

What should the protoplast of a plant cell in an isotonic solution be doing?

Slightly pulling away from the cellulose cell wall

What happens to a plant cell placed in a hypertonic solution?

Its permanent vacuole fills with water, causing the protoplast to swell and press against the cell wall, making the cell turgid

What happens to a plant cell placed in a hypotonic solution?

The cell surface membrane pulls away from the cell wall (also known as plasmolysis) and the hypertonic solution fills the space

What are 5 factors which maximize rate of diffusion?

Concentration gradient, diffusion distance, temperature, surface area, amount of substance

What are some adaptations of exchange surfaces to maximise rate of diffusion?

Thin membranes, good blood supply, large SA:V ratio, large amount of carrier/channel proteins, good ventilation (in the lungs)