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Higher Human Biology. Chapter 6: Cell Membrane and Transport of Materials. Unit 1: Cell function and inheritance. Lesson Aims. To study the structure and function of the plasma membrane

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Chapter 6: Cell Membrane and Transport of Materials

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Higher Human Biology

Chapter 6: Cell Membrane and Transport of Materials

Unit 1: Cell function and inheritance

Mrs Smith

Lesson Aims

  • To study the structure and function of the plasma membrane

  • To examine the transport of substances across the membrane by diffusion, active transport, endocytosis, exocytosis, pinocytosis, phagocytosis,

Mrs Smith

All living things are surrounded by a thin boundary called a cell membrane (plasma membrane).

  • The only exceptions to this rule are the nucleoli and ribosomes.

  • Cell organelles (e.g. Nucleus mitochondria, Golgi apparatus and lysosomes) are also bounded by a cell membrane.

Mrs Smith

Functions of Cell Membrane

  • All living things are surrounded by a thin boundary called a cell membrane (plasma membrane).

  • Controls entry/exit of materials including water and soluble molecules.

  • Communicates with other cells e.g. hormones binding with receptors.

  • Important for the connections between cells.

Mrs Smith

Investigating the chemical nature of the cell membrane

  • The cell sap present in the central vacuole of a beetroot cell contains a red pigment. Bleeding (the escape of this pigment) indicates that the cell’s plasma and vacuolar membranes have been damaged.

(Red cell sap)

Plasma Membrane

Mrs Smith

Beetroot experiment con’t

  • 4 identical cylinders of beetroot are prepared using a cork borer.

  • These are washed to remove red cell sap from the outer damaged cells.





Water bath at 70oC

Water bath at 25oC

Mrs Smith

Beetroot experiment CONCLUSION

  • Bleeding is found in tubes B,C and D showing that the membranes have been destroyed.

  • Molecules of protein are known to be denatured when exposed to acid or high temperatures. Molecules of protein are known to be soluble in alcohol.

  • It is therefore concluded the cell membrane contains protein (results b and d – denatured protein allowed pigment to leak out) and lipid (results c – lipid dissolved in alcohol causing the same.)





Water bath at 25oC

Water bath at 70oC

Mrs Smith

Danielli and Davson 1930's-40's

  • Studied triglyceride lipid bilayers over a water surface.

  • Lipids arranged themselves with the polar heads facing outward.

  • Formed droplets (oil in water) and the surface tension was much higher than that of cells

  • Added proteins, the surface tension was reduced and the membranes flattened out

Danielli and Davson’s early model of the cell membrane.

Mrs Smith

Structure of the plasma membrane

  • The PM consists of protein and phospholipid molecules.

  • The best model we have for the arrangement of these molecules is called the FLUID MOSAIC MODEL of cell membrane structure.

Mrs Smith

Fluid mosaic model

  • Here the plasma membrane consists of a fluid bilayer of constantly moving phospholipid molecules containing a patchy mosaic of protein molecules

  • Proteins act as:




  • Phospholipids form a bilayer that acts like a barrier between the cell and the environment.

Mrs Smith

Fluid Mosaic Model

Mrs Smith

Remember Phospholipids- have different properties at each end!

  • The phosphate (‘head’) end of the molecule is hydrophilic (water loving) and is therefore soluble in water.

  • The fatty acid (‘tail’) end of the molecule is hydrophobic (water-hating) and is therefore insoluble in water.

Mrs Smith

Phospholipid Bilayer Structure

In the company of other similar molecules, phospholipid molecules arrange themselves into a bilayer.

Where the Phospholipid:

  • Hydrophilic head points towards the outside environment and the cytoplasm, this makes up the two outside surfaces of the bilayer.

  • Hydrophobic tail repels water and points in.

Mrs Smith

Phospholipid bi-layer

Hydrophilic phosphate head

Hyrdophobic lipid tail

  • The arrangement of phospholipid molecules is fluid yet at the same time it forms a stable and effective boundary round the cell.

  • It allows tiny molecules such as water to pass through it rapidly.

  • Larger molecules such as glucose depend on the membranes protein molecules for entry and exit from the cell.

Mrs Smith

Freeze-fracture of Plasma Membrane

Mrs Smith

Freeze-fracture of Plasma Membrane

  • This electron micrograph shows the inside of a membrane and bumps, grooves, ridges.  These were later found to be proteins. 

Mrs Smith

The membranes possessed by the cell’s various organelles tend to vary from oneanother in function and arrangement.

  • The nucleus, has a double membrane perforated by pores to allow exit of mRNA for protein synthesis.

  • The inner membrane of mitochondrian is convoluted giving a large surface area for aerobic respiration to occur

Mrs Smith

The membranes possessed by the cell’s various organelles tend to vary from oneanother in function and arrangement.

  • The membranes of the Golgi apparatus and endoplasmic reticulum become pinched of releasing vesicles containing protein.

  • The enzymes around a lysosome isolates digestive enzymes until they are required.

Mrs Smith

Membrane composition

Despite this diversity of function, all of these membranes have the same basic structure as the plasma membrane and operate as selective barriers, each contributing in its own way to the integrated working of the cell.

Mrs Smith

Constant cell environment.

  • For a cell to function efficiently, its internal environment must remain fairly constant with respect to water and soluble substances present in the cytoplasm.

  • E.g. If O2 was to be used up without being replaced, the cell would eventually be unable to respire aerobically.

  • It is the cell membrane which regulates the entry and exit of substances to maintain a constant environment.

Mrs Smith

TASK – Testing you knowledge!

  • Complete Torrance Pg 46 Questions 1-3

Mrs Smith

Essay Question

  • Describe the function of lipids and proteins within the plasma membrane.

Mrs Smith

Modes of transport – 3 main types.

  • The movement of small molecules or ions into or out of a cell normally occurs as a result of:

    • Diffusion

    • Osmosis

    • Active transport.

  • In addition:

    • Endocytosis: cells engulf and take in large molecules

    • Exocytosis: discharge secretory products.

Which depends on the nature of the substance involved.

Mrs Smith

Passive Transport: Diffusion & Osmosis

  • Molecules move down the concentration gradient.

  • No energy is required .

  • Diffusion - molecules moves from high concentration to low concentration e.g. minerals, oxygen and carbon dioxide

  • Osmosis - water molecules through a selectively-permeable cell membrane from HWC to LWC

Channel-forming proteins

Mrs Smith

Role of the plasma membrane in diffusion: IT IS SELECTIVLY PERMEABLE.

  • The PM possessed tiny pores and is freely permeable to small molecules e.g.O2.

  • It is not equally permeable to all substances.

  • Larger molecules pass by diffusion slowly

  • Even larger molecules are unable to pass by diffusion.

Mrs Smith

Role of the plasma membrane in OSMOSIS: IT IS SELECTIVLY PERMEABLE.

  • Whenever a cell is in contact with a solution of different water concentration, osmosis occurs.

  • This because the PM is selectively permeable.

  • Allowing rapid movement of H2O through it (but allows larger molecules to move slowly or not at all).

  • The direction of water molecules depends upon the water concentration of the liquid the cell is in compared with the cell contents.

Mrs Smith

Osmosis in Red Blood Cells

When RBC’s are placed in salt solution, there is a water there is a HWC inside and LWC outside. Water moves out of cells and cells shrink

Mrs Smith

Osmosis in Red Blood Cells

When RBC’s are placed in water there is a LWC inside and HWC outside. Water moves into cells and cells burst.

Mrs Smith

Osmosis in Red Blood Cells

When the water concentration of the RBC inside = water concentration outside. No net gain or loss of water

It is essential that the water and solute concentration of blood plasma and body cells is maintained at a steady state so that the body is not damaged by the cells bursting or shrinking

Mrs Smith

Ecstasy tablets: interfere with the water balance in cells

  • Immediate effects include an increased heart rate and raised blood pressure. Dry mouth and raised body temperature. Ecstasy interferes with the body's fluid control mechanisms and salt balance, and it's easy to drink too much fluid and overhydrate (water poisoning, causing the brain to swell dangerously.

Mrs Smith

Active Transport

  • Molecules move across the PM against the concentration gradient.

  • Energy (ATP from respiration) required.

  • Proteins act as carrier molecules

  • Proteins are specific

  • Rate affected by temperature, respiratory substrate, oxygen concentration

Mrs Smith

Sodium/potassium pump

  • Active transport carriers = pumps. Some carriers have a duel role, they exchange one type of ion for another.

  • Above shows Na ions being pumped out of the cell and K ions being pumped into the cell against the concentration gradient. The same carrier molecule is able to convert ATP to ADP + Pi to produce energy for this process. This balance is very important for nerve cells.

Mrs Smith

Conditions required for active transport

Factors such as

  • Temperature

  • Availability of oxygen.

  • Availability of respiratory substrate (e.g. Glucose)

  • All affect the cell’s respiration rate also affect the rate of active transport.

  • For example increase in temp causes an increase in ion uptake until at high temps the enzyme becomes denatured and the cell dies.

  • Mrs Smith


    • Cells absorb material e.g. Proteins, hormones or lipids from the outside by engulfing it with cell membrane.

    • This involves the PM folding inwards to form a pouch. When this becomes closed off and detached from the cell membrane, it is called an INTRACELLULAR VESICLE.

    • It is used by all cells of the body because most substances important to them are large polar molecules so cannot pass through the hydrophobic plasma membrane.

    Mrs Smith


    • The engulphing of large solid particles by the cell is called PHAGOCYTOSIS “Cell-eating”.

    • The contents of the vesicle are then digested.

    • e.g. white blood cell eating fungus Candida albicans

    Mrs Smith


    • The formation of small liquid-filled vesicles by the cell membrane is called PINOCYTOSIS “Cell-drinking”

    • e.g. Amoeba showing pinocytosis

    Mrs Smith


    • This is the reverse of endocytosis

    • Vesicles formed inside the cell fuse with the PM allowing their contents to be expelled from the cell membrane.

    • Vesicles contain:

      soluble proteins to be secreted to the extracellular environment

      membrane proteins and lipids that are sent to become components of the cell membrane

    Mrs Smith

    TASK – Testing you knowledge!

    • Complete Torrance Pg 49/50 Questions 1-3

    Mrs Smith

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