Topic 2: Cells

Topic 2: Cells

2.4 Membranes

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2.4.1 Draw and label a diagram to show

the structure of membranes (1).

2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of the cell membranes (3)

2.4.3 List the functions of

membrane proteins (1).

2.4.4 Define diffusion and osmosis (1).

2.4.5 Explain passive transport across

membranes by simple diffusion and facilitated diffusion (3).

2.4.6 Explain the role of protein pumps and

ATP in active transport across

membranes(3).

2.4.7 Explain how vesicles are used to

transport materials within a cell between

the rough endoplasmic reticulum , Golgi

apparatus and plasma membrane(3).

2.4.1 Draw and label a diagram to show the structure of membranes

en.labs.wikimedia.org Try this interactive

2.4.2 Explain how the hydrophobic and hydrophilic properties ofphospholipids help to maintain the structure of cell membranes

The plasma membrane is almost made up entirely of protein and lipid

The structure of the plasma membrane is called the fluid mosaic model because the membrane moves like a fluid and the proteins are randomly scattered about like a mosaic pattern

There are three points to remember:1. The phosphate heads are

hydrophilic 2. The tails are hydrophobic3. Imbedded cholesterol affects its

flexibility and permeability

2.4.2 Explain how the hydrophobic and hydrophilic properties ofphospholipids help to maintain the structure of cell membranes

1. Hydrophilic heads• A phospholipid is composed of glycerol

head with an attached ionized phosphate group (polar or charged).

• The phosphate group easily forms bond with water molecules

• This part of the phospholipid is hydrophilic or “waterloving”

2. Hydrophobic tails• The tail of the phospholipid is composed of

two long fatty acid chains(tails).

• The tails have hydrophobic properties or “water hating”

2.4.2 Explain how the hydrophobic and hydrophilic properties ofphospholipids help to maintain the structure of cell membranes

So…The phospholipids arrange

themselves in a bilayer with the hydrophobic tails pointing inward, away from the water and the hydrophilic heads facing out towards water of the cytoplasm or the outside of the cell

This is how the membrane maintains its structure

The plasma membrane forms a good barrier, preventing molecules from either leaving or entering the cell

2.4.2 Explain how the hydrophobic and hydrophilic properties ofphospholipids help to maintain the structure of cell membranes

3. The plasma membrane is also composed of cholesterol

The amount of cholesterol influences the permeability and flexibility of themembrane

More cholesterol makes it less permeable

2.4.3 List the functions of membrane proteins

There are two types of membrane proteins

a) peripheral proteins – found on the outside of the plasma membrane

b) integral proteins – pass completely through the plasma membrane

• these proteins have contact with both the cytoplasm and the external environment

en.wikibooks.org

2.4.3 List the functions of membrane proteins

1. Hormone binding sitesHormone will only bind to certain proteins. These proteins are called protein receptors

2. Pores (channels) for passive transportSome proteins can form hollow tubes to allow molecules to pass through the membrane

3. Cell adhesionallows cells to stick together

4. Cell-to-Cell communicationThe cells can communicate with each other by using hormones or neurotransmitters

5. Membrane-bound enzymesSome enzymes are attached to the membrane in a sequence to allow certain reactions to happen step-by-step

2.4.3 List the functions of membrane proteins

6. Pumps for active transport

Some proteins on the cell membrane can change form in the presence of ATP (energy). This will allow some molecules to be pumped across the membrane such as ions

See tutorial at:

http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP1101

2.4.4 Define diffusion and osmosis

Diffusion – the movement of gas or liquid particles from an area of high concentration to an area of low concentration

See animation at:http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

2.4.4 Define diffusion and osmosis

Osmosis – the diffusion of water molecules across a partially permeablemembrane, from an area of low solute concentration to an area of highsolute concentration

Solute – the substance dissolved in the liquid

Solvent – the liquid doing the dissolving

Example: Sugar and teaSugar is the soluteTea is the solvent

See animation at:http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

2.4.5 Explain passive transport across membranes in terms of simple diffusion and facilitated diffusion

Passive transport is the movement of molecules across a membrane without energy

The molecules generally move from an area of high concentration to an area of low concentration until equilibrium is reached

There are two types of passive transport:1. Simple Diffusion2. Facilitated Diffusion

1. Simple DiffusionNon-polar molecules (steroids, glycerol, fatty acids, O2 and CO2) can easily diffuse across a membrane without help

This is called “selectively permeable” – allowing only certain molecules to pass through

ckgbio.blogspot.com

2.4.5 Explain passive transport across membranes in terms of simple diffusion and facilitated diffusion

2. Facilitated Diffusion

Polar molecules are unable to pass through a membrane without help

They need the help of two types of globular proteins that can be found on the surface of the plasma membrane

a) Channel proteins

create pores which allow charged particles to pass

b) Transport proteins (carrier proteins)

have binding site for certain types of molecules.

Once the molecule binds to the membrane protein it is transported across the membrane.

Does not require energy

http://www.bio.miami.edu/~cmallery/150/memb/c8.7x15.facilitated.diffusion.jpg

2.4.6 Explain the role of protein pumps and ATP in active transport across the membrane

Active transport requires energy in the form of ATP (adenosine triphosphate)

Active transport is different from diffusion (simple and facilitated) in the following ways:

1. Occurs against the concentration gradient. It moves from low to high concentration

The advantage of this type of transport is that cells can keep a reserve of ions inside the cell

Even when the concentration is lower outside the cell, the cell can pump these ions across the membrane using active transport

2.4.6 Explain the role of protein pumps and ATP in active transport across the membrane

2. Active transport is highly selective They are specific for a particular substance

3. Active transport requires special molecules of the membrane called pump molecules

Pump molecules pick up particular molecules or ions and transport them to the other side of the membrane where they are released

They are integral globular proteins

4. They require a reaction with ATP to get the energy and drive the pump

biologie.uni-hamburg.de

2.4.7 Explain how vesicles are used to transport materials within a cell between the rER, the Golgi apparatus and the plasma membrane

Intracellular transport requires the use of vesicles

The plasma membrane of the cell is basically the same as the membrane of the Golgi apparatus, the rER, and the nuclear envelope

When the rER makes proteins and sends these proteins to the Golgi apparatus there is no direct connection between these organelles, because the proteins are packaged in membrane-bound vesicles

Cell animation from Harvard

Vesicle transport begins at 5:39 in the video

2.4.8 Describe how the fluidity of the membrane allows it to change shape and break and reform during endocytosis and exocytosis

Material can be taken into the cell (endocytosis) or removed from the cell (exocytosis)• The membrane of the golgi and ER is composed of the same structure as the plasma membrane• When the membranes touch, they become one• If solid material is taken in it is called phagocytosis• If liquid material is taken in it is called pinocytosis

Fact - macrophages (white blood cells that eat debris and pathogens) engulf bacteria and dead or dying cells using endocytosisIn 24 hours, the macrophages in your body breakdown 2 x 1011 red blood cells