Biological Membranes - Biology A-Level OCR A

What is the fluid mosaic model

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

What is a glycolipid?

Lipid/phospholipid with a chain of carbohydrate molecules attached

What is a glycoprotein?

Protein with a chain of carbohydrate molecules attached

What is the plasma membrane?

Cell surface membrane. A selectively-permeable phospholipid bilayer forming the boundary of the cells

What is permeability?

The ability to let substances pass through

What does partially permeable mean?

Allows only certain substances to pass through

What is the role of membranes at the surface of cells

- Separates 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 antigens

- May release chemicals

- Contains receptors

- May be the site of chemical reactions

Why do cell surface membranes have antigens?

So that the organisms immune system can recognise the cell as being 'self' and not attack it

Why do cell surface membranes contain receptors?

For chemical signals, it is a site for cell communication or signaling. Hormones and drugs may also bind to membrane-bound receptors

What is the role of membranes within cells

- Separate the organelle contents from the cell cytoplasm so that each organelle is a discrete entity

- In some organelles, metabolic processes occur on the membrane

Who proposed the fluid mosaic model?

Singer and Nicholson in 1972

What does the fluid mosaic model propose?

It proposes that the fabric of the membrane consisted of a phospholipid bilayer, with proteins floating in it, making up a mosaic pattern. The lipid molecules may change places with each other and some proteins may move, giving fluidity

Fluid mosaic model components

Glycoprotein, protein receptor site, carbohydrate molecules, cholesterol, glycolipid, channel proteins, carrier proteins, peripheral protein, phospholipids water-filled channel, phospholipid bilayer, cytoplasm

What is the glycocalyx?

Formed from the carbohydrate chains attached to either lipids of proteins in the membrane

Not all cell membranes have the same composition

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

They have a long myelin sheath formed by flattened cells wrapped around then several times, giving several layers of cell membrane. The membrane forming the myelin sheath is about 20% protein and 76% lipid

The plasma membranes of white blood cells contain special protein receptors that enable them to recognise the antigens on foreign cells, usually from invading pathogens but also from tissue or organ transplants

Root hair cells in plants have many carrier proteins to actively transport nitrate ions from the soil into the cells

The inner membranes of mitochondria are 76% protein and 24% lipid. This is because their inner membranes contain many electron carriers that are made of protein, and hydrogen ion channels associated with ATP synthase enzymes

What is diffusion?

Movement of molecules from an area of higher concentration to an area of lower concentration.

What is facilitated diffusion?

Movement of molecules from an area of high concentration of that molecule to an area of low concentration across a partially permeable membrane VIA protein channels or carriers; it does not involve metabolic energy (ATP)

What is simple diffusion?

Diffusion that doesn't involve a direct input of energy or assistance by carrier proteins.

What is net diffusion?

net movement from the region of higher to the region of lower concentration until the concentration difference no longer exists

How the concentration gradient is maintained

Many molecules entering cells then pass into organelles and are used for metabolic reactions; this maintains the concentration gradient and keeps more of the molecules entering the cell

Factors that affect the rate of simple diffusion

Temperature, diffusion distance, surface area, size of diffusing molecule, concentration gradient

How temperature effects rate of simple diffusion?

As temperature increases, molecules have more kinetic energy, so their rate of diffusion will increase. Conversely, as they lose heat their rate of diffusion will slow down

How diffusion distance effects rate of simple diffusion?

The thicker the membrane across which molecules have to diffuse, the slower the rate of diffusion

How surface area effects rate of simple diffusion?

More diffusion can take place over a larger surface area. Cells specialised for absorption have extensions to their cell surface area called microvilli to increase surface area

How the size of the diffusing molecule effects rate of simple diffusion?

Smaller ions or molecules diffuse faster than larger molecules

How concentration gradient effects rate of simple diffusion?

The steeper the gradient, the faster the diffusion to the side where there are fewer molecules down the gradient

What is osmosis?

Passage of water molecules down their water potential gradient across a partially permeable membrane.

What is water potential?

Measure of the tendency of water molecules to diffuse from one region to another

What is the water potential of pure water?

Has the lowest possible water potential 0 kPa

What happens when solute molecules are added to water?

They lower the water potential of the solution. The more solute molecules added, the lower the solution

What happens if two aqueous solutions are separated by a partially permeable membrane?

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

What happens when the water potential on both sides of the partially permeable membrane are equal?

There will be no net osmosis, although water molecules continue to move randomly

What is the unit for measuring water potential?

Kilopascals (kPa)

What happens as solute molecules are added?

The water potential of the solution is lowered so, in numerical value, it becomes more negative. The more negative the value, the lower the water potential

The water potential of cells

The water potential inside cells is lower than that of pure water as there are solutes in the solution, in the cytoplasm and inside the large vacuole of plant cells. When cells are placed in a solution of higher water potential, then water molecules move by osmosis down the water potential gradient, across the plasma membrane, into the cell

What happens in animals cells if too much water enters?

Cytolysis

What is cytolysis?

Cytolysis is the cell swells and bursts and the plasma membrane breaks

What happens in plant cells if too much water enters?

The cell becomes turgid and does not allow any more water to enter. The cell contents push against the cell wall. Turgidity helps to support plants

What is a turgid cell?

A cell that is swollen as from a fluid; bloated

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

Water leaves the cell by osmosis, across a partially permeable plasma membrane. Animal cells shrivel and are described as crenated

Crenated / Crenation of cells

Shrinkage of cells giving crinkled appearance

Plant cells in solution of lower water potential

The cytoplasm of the cell shrinks and the membrane pulls away from the cellulose cell wall. The cell is described as plasmolysed.

What is the term for plant tissue with plasmolysed cells?

Flaccid

What are plasmolysed cells?

Cells that suffer a degree of dehydration and their metabolism cannot proceed, as enzyme catalysed reactions need to be in solution

What is active transport?

movement of substances against concentration gradient (low to high concentrations) across a cell membrane, using ATP and protein carriers

What is endocytosis?

Bulk transport of molecules that are too large to pass through a cell membrane (even via channel or carrier proteins) into a cell

What is exocytosis?

Bulk transport of molecules that are too large to pass through a cell membrane (even via channel or carrier proteins) out of a cell

Why active transport needs ATP?

The molecules are moving against the concentration gradient and needs more energy than just the kinetic energy of the molecules. This energy is provided by the hydrolysis of ATP.

What are carrier proteins?

Reversibly bind to molecules and change shape to shuttle them across the membrane

examples of bulk transport

endocytosis and exocytosis

Examples of endocytosis

phagocytosis and pinocytosis

What is phagocytosis?

A type of endocytosis in which a cell engulfs large particles or whole cells

What is pinocytosis?

A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes.

Examples of exocytosis

Vesicles - A vesicle containing the molecule is moved towards and then fuses with the plasma membrane

Synapses - Chemicals in vesicles are moved, by motor proteins moving along the cytoskeleton threads, to the presynaptic membrane. Here, the vesicle membranes and plasma membranes fuse and the neurotransmitter chemicals are released into the synaptic cleft

Factors affecting membrane structure and permeability

Temperature, Kinetic energy

When temperature drops

- Saturated fatty acids become compressed

- Unsaturated fatty acids make up alot of the phospholipid bilayer, as they compress, the kinks in their tails push adjacent phospholipid molecules away, this maintains membrane fluidity

- Therefore, the proportions of unsaturated and saturated fatty acids within a cell membrane determine the membranes fluidity at cold temperatures

- Cholesterol also buffers the effect of lowered temperatures

When temperature increases

- Phospholipids acquire more kinetic energy and move around more, which increases the membranes fluidity

- Permeability increases

- It also affects the way membrane-embedded proteins are positioned and may function

- An increase in membrane fluidity may also change the ability of cells to signal to other cells by releasing chemicals, often by exocytosis

- The presence of cholesterol molecules buffers, to an extent, the effects of increasing heat as it reduces the increase in membrane fluidity

- An increase in membrane fluidity may affect the in folding of the plasma membrane during phagocytosis

Effect of solvents of phospholipids

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

Proteins and temperature

- High temperatures cause the atoms within their large molecules to vibrate and this breaks the hydrogen and ionic bonds that hold their structure together. They unfold

- Their tertiary structure changes and cannot change back again when they cool. They are denatured.