Plasma membranes

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  • Created by: Elliew176
  • Created on: 09-06-17 13:29

Membrane structure

The cell surface membrane which separates the cell from its external environment is known as the plasma membrane.

Membranes are formed from a phospholipid bilayer. The hydrophilic phosphate heads of the phospholipids from both the inner and outer surface of a membrane, sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane. 

Cells normally exist in aqueous environments. The inside of cels and organelles are also usually aqueous environments. Phospholipid bilayers are perfectly suited as membranes because the outer surfaces of the hydrophillic phosphate heads can interact with water.

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Cell membrane theory

Images taken in the 1950s showed the membrane as two black parallel lines.

In 1972 American scientists Singer and Nicolson proposed a model in which proteins occupy various positons in the membrane. This mode is known as the fluid-mosaic model becasue the phospholipids are free to move within the layer relative to each other (they are fluid), giving the membrane flexibility, and because the proteins embedded in the bilayer vary in shape, size, and position. 

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Cell membrane components

Membrane proteins:  2 types of proteins in the cell-surface membrane - intrinsic and extrinsic proteins. Intrinsic proteins are embedded through both layers of a membrane. They have amino acids with hydrophobic R-groups on their external surfaces which interact with the hydrophobic core of the membrane, keeping them in place. Channel and carrier are both intrinsic proteins.

  • Channel proteins - provide a hydrophillic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes. They are held in position by interacttion between the hydrophobic core of the membrane and the membrane and the hydrophobic R-groups on the outside of the proteins.
  • Carrier proteins - role in bothpassive transport (down a concentration gradient) and active transport (against a concentration gradient) into cells. This often involves the shape of the protein changing.

Glycoproteins: they are intrinsic proteins. They are embedded in the cell-surface membranes with attached carbohydrate (sugar) chains of varying lengths and shapes.  They play a role in cell adhesion and as receptors for chemical signs. When the chemical binds to the receptor, it elicits a response from the cell. This may cause a direct response or set off a cascade of events inside the cell. This process is known as cell communications or cell signcalling.

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Cell membrane components

Glycolipids: similar to glycoproteins. They are lipids with attached carbohydrates chains. These molecules are called cell markers or antigens and can be recognised by the cells of the immune system as self or non-self.

Extrinsic proteins: present in one side of the bilayer. They normally have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phopholipids or with intrinsic proteins. They can be present in either layer and some move between layers.

Cholesterol: a lipid with a hydrophilic end and a hydrophobic end like a phospholipid. It regulates the fluidity of membranes. Cholesterol molecules are positioned between the phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails, pulling them together. In this way cholesterol adds stability to membranes without making them too rigid. The cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molceules from grouping too closely and crystallising.

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Cell membrane components

Ion pump transport protein: Ion channels are pore-forming membrane proteins and have a role of gating the flow of ions across the cell membrane, controlling the flow of ions and regulating cell volume. Ion channels are present in the membranes of all cells. 

Gated protein transport channel: These can open and close and they can control what comes in and out of the cell

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Factors affecting membrane structure

Temperature: Phospholipids in a cell membrane are constantly moving.  When temperature is increased the phospholipids will have more kinetic energy and will move more. This makes a membrane more fluid and it begins to lose its structure. If temperature continues to increase the cell will eventually break down completely. This loss of structure increases the permeability of the membrane, making it easier for particles to cross it. Carrier and channel proteins in the membrane will be denatured at higher temperatures. These proteins are invloved in transport across the membrane so as they denature, membrane permeability will be affected.

Solvents: water, a polar solvent, is essential in the formation of the phosphlipid bilayer. The non-polar tails of the phospholipids are orientated away from the water, forming  a bilayer with a hydrophobic core. The  charged phosphate heads interact with water, helping to keep the bilayer intact. Many organic colvents are less polar than water e.g alchols. Organic solvents will dissolve membranes, disrupting cells. 

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Passive and active transport

Passive transport:

Whenever diffusion occurs, molecules will always be moving in both directions, but if there is a concentration gradient, their net movement will be towards the region of low concentration e.g diffusion. Diffusion is the net, or overall, movement of particles from a region of higher concentration to a region of lower concentration.

Active transport:

Active transport is the movement of molecules or ions into or out of a cell from a region or lower concentration to a region of higher concentration. This process requires energy and carrier proteins. Energy is needed as the particles are being moved up a concentration gradient, in the opposite direction to diffusion. Metabolic energy is supplied by ATP.

Active Transport involves the movement of molecules across a Membrane using Proteins in the Bilayer similar to Carrier Proteins. These use energy in the form of ATP to 'pump' molecules in one direction across a membrane.

Active Transport means that a substance can move against the Concentration Gradient and at a much faster rate than Diffusion alone.

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Diffusion

The net movement of particles froma  region of higher concentration to a region of lower concentration. 

It is a passive process and will continue until there is a concentration equalibrium between the two areas. 

Diffusion happenes because the particles in the gas or liquid have kinetic energy. This movement is random and an unequal distribution of particles will eventually become an equal distribution. Equalibrium doesn't mean the particles stop moving, just that the movements are equal in both directions.

Particles move at high speeds and are constantly collliding which slows down their overall movement. This means that over short distances diffusion is fast but as diffusion distance increases, the rate of diffusion slows down because more collisions have taken place.

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Factors affecting rate of diffusion

Temperature - the higher the temperature the higher the rate of diffusion. This is because the oarticles have more kinetic energy and move at higher speed.

Concentration difference - the greater the difference in concentration between two regions, the faster the rate of diffusion because the overall movement from the higher concentration to lower concentration will be larger.

A concentration difference is said to be a concentration gradient which goes from high to low concentration. Diffusion takes place down a concentration gradient. It takes a lot more energy to move substances up a cncentration gradient.

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Diffusion across membranes

Diffusion across membranes involves particles passing through the phospholipid bilayer. It can happen if the membrane is permeable to the particles. Non-polar molecules such as oxygen diffuse through freely down a concentration gradient (the process of particles moving through a solution or gas from an area with a higher number of particles to an area with a lower number of particles. The areas are typically separated by a membrane).

The hydrophobic interior of the membrane repels substances with a positive or negative charge (ions) so they cannot easily pass through. Polar molecules, such as water with partial positive and negative changes can diffuse through membranes, but only at a very slow rate.
Small polar molecules pass through more easily than larger ones. Membranes are therefore described as partially permeable.

The rate at which molecules or ions diffuse across membranes is affected by two things:
Surface area – the larger the area of an exchange surface, the higher the rate of diffusion.
Thickness of membrane – the thinner the exchange surface, the higher the rate of diffusion.

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Facilitated diffusion

Phospholipid bilayers of membranes are barriers to polar molecules and ions. However, membranes contain channel proteins through which polar molecules and ions can pass. Diffusion across a membrane through protein channels is called facilitated diffusion.


Membranes with protein channels are selectively permeable as most protein channels are specific to one molecule or ion.


Facilitated diffusion can also involve carrier proteins which change shape when a specific molecule binds. In facilitated diffusion the movement of the molecules is down a concentration gradient and does not require external energy.


The rate of facilitated diffusion is dependent on the temperature, concentration gradient, membrane surface area and thickness, but is also affected by the number of channel proteins present. The more protein channels, the higher the rates of diffusion overall.

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Active transport

Active transport is the movement of molecules or ions into or out of a cell form a region of lower concentration to a region of higher concentration.
The process requires energy and carrier proteins.
Energy is needed as the particles are being moved up a concentration gradient in the opposite direction to diffusion.
Metabolic energy is supplied by ATP.
Carrier proteins act as pumps.
Transport from outside to the inside cell
The molecule or ion to be transported binds to receptors in the channel of the carrier protein on the outside of the cell.
On the outside of the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate.
Binding of the phosphate molecule to the carrier protein causes the protein to change shape – opening up to the inside of the cell.
The molecule or ion is released to the inside of the cell.
The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP. The carrier protein returns to its original shape.

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Bult transport

This is another form of active transport. Larger molecules like enzymes and hormones are too big to move through channel or carrier proteins so they are moved into and out of cells by bulk transport.


Endocytosis is the bulk transport of material into cells. There are two types of endocytosis – phagocytosis for solids and pinocytosis for liquids. The cell surface membrane first bends inwards when it comes into contact with the material to be transported. The membrane enfolds the material until eventually the membrane fuses, forming a vesicle. The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell.


Exocytosis is the reverse of endocytosis. Vesicles usually formed by the Golgi apparatus move towards and fuse with the cell surface membrane. The contents of the vesicle are then released outside of the cell.

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Osmosis

Osmosis the diffusion of water through a partially permeable membrane.

A solute is a substance dissolved in a solvent forming a solution.

The amount of solute in a certian volume of aqueous solution is the concentration. Water potential is the pressure exerted by water molecules as they colllide with a membrane or container. Measured in units of pressure pascals (Pa) or kilopascals (kPa).

Pure water is defined as having a water potential of 0Kpa. This is the highest possible value for water potential, as the presence of a  solute in water lowers the water potential below zero. All solutions have negative water potentials. The more concentrated all the solutions the more negative the water potential.

When solutions of different concentraions, and thereofre different water potenitials, are separated by a partially permeable membrane, the water molecules can move between the solutions but the solutes usually cannot. There will be a net movement of water from the solutions with the higher water potential (less concentrated) to the solution with the lower water potential (more concentrated).  This will continue until the water potenial is equal on both sides of the membrane.

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Effects of osmosis on animal cells

The diffusion of water into a solution leads to an increase in volume of this solution. If the solution is in a closed system, such as a cell, this results in an increase in an increase in pressure. This pressure is called hydrostatic pressure and has the same units as water potential - kPa. At the cellular level this pressure is relativily large and potentially damaging.

If and animal cell is placed in a solution with a higher water potential than that of the cytoplasm, waterw illl move into the cell by osmosis, increasing the hydrostatic pressure inside the cell. All cells have thin cell-surface membranes (around 7nm) and no cell walls. The cell-surface membrane cannot stretch much and cannot withstand the increased pressure. It will breaks and the cell will burst - this is called cytolysis.

If an animal cell is placed in a solution that has a lower water potential than the cytoplasm it will lose water to the colution by osmosis down the water potential gradient. This will cause a reduction in the volume of the cell and the cellsurface membrane to pucker, referred to as crenation. 

To prevent either cytolysis or crenation, multicellular animals usually have control mechanisms to make sure their cells are continually surrounded by aqueous soltutions with an equal water potential. In the blood the aqueous solution is blood plasma.

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Effects if osmosis on plant cells

Plant cells have strong cellulose walls surrounding the cell-surface membrane. When water enters by osmosis the increased hydrostatic pressure pushes the membrane against the rigid cell walls. This pressure against the cell wall is called turgor. As the turor pressure increases it resists the entry of further water and the cell is said to be turgid.

When plant cells are placed in a solution with a lower water potential than their own, water is lost from the cells by osmosis. This leads to a reduction in the volume of the cytoplasm, which eventually pulls the cell-surface membrane away from the cell wall - the cell is said to be plasmolysed.

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