BIOLOGICAL MEMBRANES

what are cell surface membranes / plasma membranes?

the membrane at the surface of cells

what is the main role of cell surfaces membranes?

acts as a partially permeable membrane between the cell and its environment

what does "partially permeable" membrane mean?

the membrane will allow some substances to cross but not others

Other than being partially permeable, what else is a cell surface membrane involved in?

- cell signalling: communication between cells

- cell recognition: allows the immune system to tell whether the cell is "self" (don't attack it) or "foreign" (attack and destroy"

- site for chemical reactions: as enzymes may be attached to the cell surface membrane

What is a membrane-bound organelle?

Membranes within cells surround organelles

What are the roles of membranes surrounding organelles?

- form partially permeable barriers between organelles and cytoplasm

- allow compartmentalisation

- membranes within an organelle worked to increase surface area

Define compartmentalisation

Different chemical reactions are kept separate by being contained in different organelles

What is an example of how membranes within organelles worked to increase surface area?

The inner membrane of mitochondria (cristae) were the last stage of aerobic respiration takes place

Who developed the fluid mosaic model and when?

Singer and Nicholson in 1972

Outline the fluid mosaic model

- lipid bilayer with cholesterol

- specialised proteins are inserted into the lipid bilayer

- polysaccharides (sugar chains) are attached to proteins on the outside part of the membrane

What is the width of the membrane in the fluid mosaic model?

7nm to 10nm

What is the membrane in the fluid mosaic model made up of?

PPCP

Phospholipids

Proteins

Cholesterol

Polysaccharides (long sugar chains called glycocalyx)

Fill in the gaps:

Phospholipid heads have phosphate groups which are ___________ (point _________ into a watery environment).

Phospholipids tails are ____________ (point ________ away from water)

This creates a barrier between the internal and external tree environment of the cell. The tales effectively creates a barrier to the movement of _______/________ molecules.

Phospholipid heads have phosphate groups which are hydrophilic (point outwards into a watery environment).

Phospholipids tails are hydrophobic (point inwards away from water)

This creates a barrier between the internal and external tree environment of the cell. The tales effectively creates a barrier to the movement of charged/polar molecules.

Why is the membrane in the fluid mosaic model fluid?

- Due to the fatty acid tails (hydrocarbon chains)

- unsaturated fatty acids cause the tails to bend

- this causes the phospholipids to be further apart, making the membrane "fluid"

What are glycolipids?

Sugar chains attached to lipids

What are glycoproteins?

Sugar chains linked to protein molecules

Fill in the gaps.

In neurones (____ cells), the protein channels and ------- in the plasma membrane covering the long ---- allow ----- and ---- of ions to bring about the ---------- -- ------- ------- along their length.

In neurones (nerve cells), the protein channels and carriers in the plasma membrane covering the long axon allow entry and exit of ions to bring about the conduction of electrical impulses along their length.

Neurones have a myelin sheath. How is it formed?

the myelin sheath in neurones is formed by flattened cells wrapped around them several times, giving several layers of cell membrane

What is the membrane forming the myelin sheath in nerve cells made of?

20% protein, 76% lipid

what do the plasma membranes of white blood cells contain that enable them to recognise antigens on foreign cells, usually from invading pathogens but also from tissue or organ transplants?

special protein receptors

myelinated motor neurone synapsing with muscle tissue

notes (diagram)

What do the root hair cells in plants have that allow them to actively transport nitrate ions from the soil into the cells?

many carrier proteins

What is the inner membrane of mitochondria made up of?

76% protein, 24% lipid

Why are the inner membranes of mitochondria made up of 76% protein and 24% lipid?

Because they're inner membranes contain many electron carriers that are made of protein and hydrogen ion channels associated with ATP synthase enzymes

What is the role of cholesterol in membranes?

Membrane stability and fluidity

How does cholesterol work in membranes?

"plugs" any gaps in the bilayer making the barrier more complete

What is the role of channel proteins in membranes?

Membrane transport

How do channel proteins work in membranes?

Allows water soluble ions to move across the membrane

What is the role of carrier proteins in membranes?

Membrane transport

How do carrier proteins work in membranes?

Allows water soluble molecules to move across the membrane. Also involved in actively pumping ions across the membrane.

What is the role of glycoproteins and glycolipids in membranes?

Recognition and communication

How do glycoproteins and glycolipids work in membranes?

Act as docking sites (receptors) for some hormones and drugs. Act as cell markers so white blood cells know which cells not to destroy.

What is the role of enzymes and co-enzymes in membranes?

Metabolic processes (chemical reactions)

How do enzymes and co-enzymes work in membranes?

Enzymes are protein molecules. Many enzymes are found in or within membranes to speed up chemical reactions.

exocytosis

diagram in notes

endocytosis

diagram in notes

What are three examples of bulk transport?

- protein secretion

- phagocytosis (white blood cells)

- secretion of neurotransmitters

What is phagocytosis?

Movement of solids into a cell

How can the rate of active transport be affected?

- temperature

- oxygen concentration (a cell needs lots of oxygen in order to produce lots of ATP)

- presence of poisons (e.g. Cyanide) ( prevents ATP from being made)

What is pinocytosis?

liquid movement

What are the factors affecting membrane structure and therefore it's permeability?

-temperature

- solvents

-detergents

Explain how temperature affects membrane structure and therefore it's permeability

- this affects how much the phospholipids will move

- increasing temperature increases kinetic energy

- increased movements caused the membrane to become more fluid and it will start to lose its structure

- loss of structure means that it is easier for particles to move across the membrane therefore permeability increases

What is the effect of high and low temperature on membrane structure?

- High temperatures will cause the membrane structure to completely breakdown

- Extremely low temperatures and high temperatures will cause proteins in the membrane to denature

- This permanent change structure of the proteins will also affect the permeability of the membrane

What is a solvent?

a solvent is a substance that can dissolve other substances

Explain how solvents affect membrane structure and therefore it's permeability and give examples

- water is essential informing lipid by layers due to the hydrophilic and hydrophobic properties of phospholipids

- ethanol and acetone are organic solvents which will dissolve membrane, therefore majorly affecting the permeability of the membrane

Explain how detergents affects membrane structure

notes diagram

What is diffusion?

The overall net movement of molecules or ions from a region where they are more highly concentrated to one where the concentration is lower.

Is diffusion an active or passive process, why?

Diffusion is a passive process as input of energy is required

What are the factors affecting the rate of diffusion?

- concentration gradient

- diffusion distance

- the area across which diffusion occurs

- structure through which diffusion occurs

- size and type of diffusing molecule

Explain how concentration gradient affect diffusion rate

The greater the difference in concentration between two regions, the faster the rate

Explain how diffusion distance affects diffusion rate

The shorter the distance, the faster the rate

Explain how the area across which diffusion occurs affects diffusion rate

The larger the area, the faster the diffusion

Explain how the structure through which diffusion occurs affects diffusion rate

pores (holes) in a barrier may speed up the process

Explain how the size and type of diffusing molecule affect diffusion rate

Small molecules diffuse faster than large ones. Fat soluble molecules diffuse faster than water soluble ones.

What are some examples of molecules that use simple diffusion to move through a membrane?

- water

-oxygen

- ethanol

- carbon dioxide

- fat soluble vitamins (vitamin A, D, E, K)

- steroid hormones (e.g. oestrogen)

What is facilitated diffusion?

A special form of diffusion which speeds up the movement of diffusing particles across a membrane

What are the two main types of protein involved in facilitated diffusion?

- Channel proteins

- Carrier proteins

What is the role of channel proteins in facilitated diffusion?

- for water-filled channels

- allow water-soluble ions (e.g. Na⁺ and Ca⁺) to pass through the membrane

- channels are specific to one type of ion/molecule

- channels have "gates" which mean they can be opened/closed

What is the role of carrier proteins in facilitated diffusion?

- attach to specific molecules and carry them across the membrane

- Carry your proteins transport larger molecules (e.g. glucose and amino acids) across the membrane. Part of the carrier protein will have a complimentary shape to the specific molecule it is moving.

What is active transport?

Uptake of molecules/ions against the concentration gradient using energy (ATP) from respiration

How does active transport work?

1. The molecule/ion attaches to the carrier protein

2. ATP transfers a phosphate group to the carirer protein on the inside of the membrane.

3. The carrier protein then changes shape and carries the molecule/ion to the opposite side of the membrane

4. The molecule/ion is released and the carrier protein returns to its original shape.

What does a cell need a lot of if it is using active transport to produce lots of ATP?

Lots of mitochondria

What can the rate of active transport be affected by?

- temperature

- oxygen concentration (a cell needs lots of oxygen to produce lots of ATP)

- presence of poisons (e.g. cyanide.) (prevents ATP from being made.)

What are the uses of active transport?

- muscle contraction (transport of Ca²⁺ ions)

- absorption of amino acids from the small intestine

- absorption of mineral ions (Mg²⁺ / Na³⁻) by root hair cells

- loading of sugar from leaf into the phloem

- protein synthesis

What is bulk transport / cytosis?

Large quantities of molecules can be transported into or out of a cell in bulk

What is endocytosis?

bulk movement of materials into the cell

What is exocytosis?

Bulk movement of materials out of the cell

What is water concentration known as a A-level?

Water potential

What is osmosis?

The diffusion of water molecules from a region of their high water potential to a region of their low water potential across a partially permeable membrane

What type of water has the highest water potential and what is it?

Distilled water has the highest water potential where water potential equals zero

What can we add to water that lowers the water potential of a solution?

Solutes

fill in the gap.

The more ----- the water potential value, the lower the water potential.

The more negative the water potential value, the lower the water potential.

What is a hypotonic solution?

Dilute therefore a high water potential

What is the effect of a hypotonic solution on an animal cell?

Water enters the cell, the cell swells and may burst (lysis)

What is the effect of a hypotonic solution on a plant cell?

Water enters the cell, the cell swells a bit, and becomes turgid

What is an isotonic solution?

Same water potential as the cell - the water is equally in and out (osmosis is still occurring)

What is the effect of an isotonic solution on an animal cell?

No net movement of water, cell stays the normal size

What is the effect of an isotonic solution on a plant cell?

No net movement of water, cell stays the normal size

What is a hypertonic solution?

Concentrated, therefore a low water potential

What is the effect of a hypertonic solution on an animal cell?

Water leaves the cell, the cell shrinks and crenates

What is the effect of a hypertonic solution on a plant cell?

Water leaves the cell, the cytoplasm shrinks, and plasmotyses