2.5 Membranes Flashcards
What organelles have membranes?
Mitochondria
Golgi apparatus
Chloroplasts
Lysosomes
What is the role of membranes?
To regulate conditions within the cell and organelles by controlling material entry and exit
Why is the role of membranes important?
Each of organelles needs specific conditions in order for reactions to occur
What do plasma membranes surrounding cells form?
A boundary between the cells environment and the cytoplasm
What is the boundary between the cells environment and the cytoplasm needed for with regard to conditions?
- establishing different conditions
- keeping conditions separate
Give a property of the cell membrane:
It is partially permeable
Why is it necessary for the cell membrane to be partially permeable?
To precisely control exchange between the cell and its environment
Allow some molecules through but not others
Where are cell membranes found?
Surrounding cells
Surrounding organelles
How is the cell membrane/ boundary vital for cell signalling?
It is vital for receiving messages at receptors bound to the membrane
Allows ends and exocytosis
Allows formation of a membrane network throughout cells
What do membranes within cells do?
Divide the cell into different compartments
They act as a barrier between organelle and the cytoplasm
What can membranes within the cell form relating to transport?
Vesicles to transport substances between different areas of the cell
What do membranes within the cell control? Give an example:
Which substances enter and leave organelles (e.g. RNA leaves the nucleus via the nuclear membrane)
What do membranes within organelles act as? Give an example:
Barriers between the membrane contents and the rest of the organelle
E.g. Thylakoid membranes in chloroplasts
What can membranes within the cell be the site of? Give an example:
Chemical reactions
E.g. The inner membrane of the Mitochondrion contain enzymes needed for respiration
What is exocytosis?
How things get out of the cell
What is endocytosis?
How things get into the cell
What is the cell membrane made up of?
A phospholipid bilayer
What is the structure of a phospholipid?
A phosphate group attached to a glycerol backbone with two fatty acids joined
What area of the phospholipid is hydrophilic?
The phosphate heads
What area of the phospholipid is hydrophobic?
The fatty acids
What do phospholipids automatically form when added to water?
Bilayer or Micelle
Describe the arrangement of the bilayer:
Hydrophilic Phosphate heads face outwards toward the water whilst hydrophobic fatty acids stay in the centre away from the water
Because the centre of the membrane is hydrophobic what does it not allow through?
Water soluble materials such as ions
What is the term for organelles being divided by internal membranes?
Compartmentalisation
Why is compartmentalisation important?
Some reactions are incompatible and can not occur in the same environment as they need specific conditions
Give an example of a compartmentalised organelle:
Mitochondria - divided by an external and internal membrane
The smaller and less polar a molecule…
… The faster it will diffuse across a cell membrane
Give examples of small non polar molecules that rapidly diffuse across a cell membrane:
Oxygen
Carbon dioxide
Do small polar molecules diffuse across plasma membranes?
Yes but much slower than small non polar molecules
Give examples of small polar molecules that diffuse across the membrane:
Water
Urea
What molecules are unlikely to diffuse across the membrane?
Charged particles (ions)
Describe the fluid mosaic model?
“Cell surface membranes consist of a double layer if phospholipid molecules studded with proteins”
In the fluid mosaic model why is the bilayer described as fluid? What does this make the membrane?
- Because the phospholipids can move
- This makes the membrane a flexible structure that constantly changes shape
Why is the membrane described as mosaic?
It appears like tiles in a mosaic with proteins embedded in the bilayer
What is cholesterol?
A type of lipid
Where is cholesterol present?
In all cell membranes (except bacterial)
What is cholesterol important in?
Controlling membrane fluidity
How are cholesterol molecules arranged in the membrane?
They fit between phospholipids and bind to the hydrophobic fatty acids causing them to pack more closely together
The more cholesterol in the membrane…
The less fluid and permeable it is
At normal body temperature what is cholesterol important in?
Keeping membranes stable
Why is maintaining membrane fluidity important?
Otherwise cells would not be able to function
What processes is a fluid membrane necessary for?
- Diffusion of substances across membranes
- Membrane fusion (e.g a vesicl me fusing with the cell membrane during exocytosis)
- cells to move and change shape e.g macrophages during phagocytosis
