Biological membranes

OCR A biology

What is a fluid mosaic model?

Theory of a cell membrane structure with proteins embedded in a sea of phospholipids

What is glycolipid?

lipid/phospholipid with a chain of carbohydrate molecules attached.

How do membranes act as a barrier?

The cell membrane is partially permeable and forms a barrier and separates the cell contents from the cell's exterior environment.
They also separate organelles from cytoplasm

What are the roles of the plasma membrane?

-Separates the cells components from its external environment
-Regulates transport of materials into and out of the cell
-May contain enzymes involved in specific metabolic pathways
-Has antigen so the immune system can recognise the cell as itself
-Contains receptors for chemical signals such as hormones
-Site of chemical reactions

How are membranes like within cells?

The membranes around organelles present in a eukaryotic cell , separate the organelle content from the cell cytoplasm

What is the fluid mosaic membrane structure and what does it contain?

-The fluid mosaic model proposed that the outline of a membrane consists of a phospholipid bilayer with proteins floating in it.
-The lipid molecules can change shape and some of the proteins move to give fluidity.
-The fabric of the membrane is made up of two layers of phospholipid molecules.
-The hydrophilic heads are outwards in contact with watery exterior
-The hydrophobic heads point inwards away from the water

What are spanning the membranes?

Various proteins

What is the function of the various proteins in membranes?

-Pores act as channels to allow ions , which have an electrical charge and surrounded by water molecules to pass through
-Some proteins are carriers by changing their shape they carry specific molecules across the membrane
-Proteins may be attached to carrier proteins and function as enzymes, antigens or receptor sites for complementary shape signalling such as hormones

What is the function of cholesterol?

Regulates the fluidity of the membrane, maintains mechanical stability and resist the effects of temperature change on the structure of the membrane

What are glycocalyx found outside the membranes?

They are formed from the carbohydrate chains attached to either lipids or protein in the membrane

Why don't all cell membranes have the same composition?

As cells become differentiated and specialised , their membranes may have a particular distribution of protein in order to enable them to carry out specific functions

What is diffusion?

Movement of molecules from an area of high concentration to an area of low concentration across a concentration gradient

What is in the fluid mosaic model?

-Phospholipids
-Cholesterol
-Glycoproteins and glycolipids
-Transport proteins

What is facilitated diffusion

movement of molecules from an area of high concentration of that molecules to an area of a low concentration; across a partially permeable membrane via protein channels or carrier - no ATP involved.

What is simple diffusion?

When molecules diffuse they move down their concentration gradient until reached equilibrium- no net diffusion
All molecules have kinetic energy and can move freely and randomly within gas or liquid media.

What is different about the movement of water molecules?

They are polar , insoluble in lipid , the phospholipid bilayer would seem to be an impenetrable barrier however water is present in high concentration so direct diffusion is present.

What are aquaporins and what happens in them?

In membranes where a high rate of water movement is required, specific water protein channels known as aquaporins allow water to cross membranes easily moving through a lipid layer.

What are the factors that affect the rate of simple diffusion?

-Temperature - temps increase - more kinetic energy , rate of diffusion increases
-Diffusion distance - thicker membrane - slower diffusion rate
-Surface area - larger SA - more diffusion takes place
-Size of diffusing molecules - smaller ions/molecules diffuse faster then rapid ones.
-Concentration gradient - steeper the concentration gradient - faster the diffusion to side with fewer molecules

What are passive processes?

they don't need to use cell's metabolic energy and only use kinetic energy of the molecules and don't use ATP

What are examples of Passive processes?

Oxygen, Co2 are exaples of passive processes

How is the gradient is maintained

Many molecules entering cells then pass into organelles and are used for metabolic reaction- this maintains the concentration gradient and keeps more molecules entering.

What is the width of the membrane?

0.8nm

How is glucose transported?

Glucose molecules are too large to diffuse through the water-filled protein channel so they bind to a transmembrane carrier protein, which then opens to allow glucose to pass out on the other side of the membrane.

What do cholesterol molecules do to small water-soluble membranes?

Cholesterol molecules within membranes reduce the permeability of the membranes to small water-soluble molecules.

What small molecules can't interact with the hydrophobic tails of lipid bilayer?

Small molecules that have polarity are insoluble in lipids

Why are small polar molecules insoluble in lipids and how is it overcome?

Small molecules that have polarity are insoluble in lipids because they cannot interact with the hydrophobic tails of lipid bilayer.
They diffuse through water filled channels (pores) embedded in the membrane

Why do cell types have differing proportions of transmembrane protein channels and transmembrane protein carriers

This allows cells to control types of molecules they pass in and out.

What does increasing and decreasing temperature have on kinetic energy?

Increasing the temperature gives all molecules more kinetic energy, and as a result these molecules move faster

Decreasing the temp lowers the kinetic energy of molecules so they move slower.

Many organisms don't generate heat to maintain their body temperature + so their temperatures........with the .......

Varies, environment

What happens when temperature drops to fatty acids?

-Saturated fatty acids become compressed
-unsaturated fatty acids in phospholipid bilayer , as they become compressed the kinks in their tails push adjacent phospholipid molecules away- this maintains their fluidity.

What does cholesterol do when temperature drops?

Cholesterol in the membrane buffers the effect of lowered temperature, to prevent reduction in the cell's fluidity. By, preventing the phospholipid molecules from packing together too closely as cholesterols are in-between groups of phospholipid molecules.

What happens when temperature increases?

-The phospholipids has more kinetic energy + move around more, randomly- this increases the membranes fluidity
-Permeability increases
-Affects the way membrane embedded proteins are positions and may function eg enzymes
increase in membrane fluidity may change the ability of cells to signal to other cells by releasing chemicals, often by exocytosis

What does cholesterol do when temperature increases?

The cholesterol molecules buffers, to some extent the increasing heat as it reduces the increase in membrane fluidity.

How does temperature affect protein in the membrane?

-proteins aren't as stable as lipids.
-High temps cause the atoms within their large molecules to vibrate , and this breaks the hydrogen bonds and ionic bonds that hold their structure together- they unfold.
-Their tertiary structure (shape) changes so they are denatured.

What is an example of proteins in a membrane and what are the effects if they denatured?

Underneath the plasma membrane are cytoskeleton threads made of proteins. If both the membrane - embedded proteins and cytoskeleton thread denatured , then the plasma membrane will begin to fall apart. - membrane becomes more permeable as holes will appear in it.

What are the effects of protein on solvents?

Organic solvents such as acetone and ethanol will damage cell membranes as they dissolve lipids.

What is osmosis?

The net movement of water molecules across a partially permeable membrane from a region of high water potential to a region of lower water potential without the use of energy.

What is active transport?

The movement of substances against their concentration gradient ( low to high concentration of substance) across a cell membrane requiring ATP

How is the energy of active transport released?

Hydrolysis of ATP

What are carrier proteins?

These proteins have specific sites that combine reversibly with only solute molecules or ions. They have a region that binds to and allows the hydrolysis of a molecule of ATP, to release energy and they act as enzymes

What is an example of Active transport?

ATP made by chloroplast provides energy to actively transport potassium ions from surrounding cells into the guard cells

What does root hair cell use active transport for?

To absorb ions from soil.

What does bulk transport do?

Transportation of large molecules and particles that are too large to pass through the plasma membrane, requiring energy from ATP

What is endocytosis?

This brings in large particles into the cell by a segment of the plasma membrane that surrounds and encloses the particles and brings it into the cell enclosed in a vesicle.

What is pinocytosis?

When the cells ingest liquids by endocytosis

What is phagocytosis?

Type of endocytosis that intakes solid matter

What is needed for all for endocytosis ?

ATP is needed to provide energy to form vesicles and move them using motor proteins , along cytoskeleton threads into the cell's interior.

What is exocytosis?

large molecules exported out of the cells. They do not pass through plasma membranes but rather a vesicle containing them is moved towards and fuse with the plasma membrane .

What is an example of exocytosis?

At synapses where chemicals in vesicles are moved along cytoskeleton threads to the presynaptic membrane. Here, the vesicle membranes and plasma membranes fuse and the neurotransmitter chemicals are released into the synaptic cleft.

Outline how the vesicles are moved from one organelle to another

Cytoskeleton move along microfilaments
The microtubules are broken down
Uses ATP

Explain how cell surface membranes contribute to the processes of cell signalling

Releases signal molecule by exocytosis
The specific protein have receptors
The receptors have a specific shape
The receptors are complementary
Cells from surface membranes allow entry of signal molecules,

What is cell signalling?

The process by which cells communicate with each other

Describe how substances are released by exocytosis

A membrane-bound vesicle , containing the substance to be secreted is moved towards the cell surface/plasma membrane
The plasma membrane and the membrane of the vesicles fuse together
The fused site opens , releasing the contents of the vesicles

What is water potential?

The measure of the tendency of water molecules to diffuse from one region to another

What has the highest water potential?

pure water

What does adding solute molecules have on the water potential?

The more solute molecules in as solution , the lower the water potential

Describe the movement when two aqueous solutions are separated by partially permeable membrane or cell organelle membrane?

Water molecules move from the solution with higher water potential to the solution with the lower water potential

What happens when the water potential on both sides of the membrane becomes equal?

There will be no net osmosis , but water molecules will move randomly

What is the unit for water potential?

kPa

What is the water potential of pure water?

0 kPa

What happens to the water potential as solute molecules are added?

The water potential of the solution will be lowered , so becomes more negative

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

lower

What is the water potential inside cells?

The water potential inside the cell is lower than the water potential of pure water as there are more solutes in the solution, in cytoplasm and large vacuoles in plants.

What happens when cells are placed in a higher water potential?

Water molecules move by osmosis ,down the water potential gradient, across the plasma membrane , into the cell

What happens to animal cells if a lot of water enters the cells ?

The cell will swell and burst as the plasma membrane breaks. This is called cytolysis.

What happens to plant cells if a lot of water enters the cells ?

The cellulose cell wall will prevent bursting as the cell will swell up to a certain size when contents push against the cell wall resisting any further swelling. The plant cell is turgid.

What happens when cells are placed in a solution of lower water potential?

Water leaves the cell by osmosis , across the partially permeable plasma membrane

What happens to animal cells in a lower water potential?

They shrivel,crenate

What happens to plant cells in a lower water potential?

The cytoplasm of the cell shrinks ,and the membrane pulls away from the cellulose cell wall. The cells are plasmolysed

What happens to cells that are plasmolysed?

Plasmolysed cells suffer a degree of dehydration and their metabolism cannot proceed as enzymes catalysed reactions need to be in solution.