Biology- cells, exchange and transport.

Notes and past exam questions with answers

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  • Created by: Shivani
  • Created on: 22-04-11 10:09

Membranes

Cell surface plasma membranes surround all the cells and control what enters and leaves the cell.

Membranes are made up of two layers of phospholipds known as the Phospholipid bilayer and proteins.

The polar heads of the phospholipids are hydrophillc and are attracted to the water, this is why they face up to the cytoplasm and the exterior of the cell.

Proteins are scattered around the membrane, and transmembrane proteins pass right through them

Carbohydrates are attached to the protein and face the outside of the cell. this structure is called the fluid mosaic

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Components of cell membrane

Phospholipids:

  • Forms a bilayer between the cytoplasm and the exterior of the cell
  • Are fluid, so components can move within the membrane
  • Are permeable to non-polar molecules such as oxygen and fatty acids
  • Are permeable to small polar molecules such as ethanol, water and carbon dioxide
  • Are impermeable to ions and large polar molecules such as sugars and amino acids

Cholesterol:

  • Stabalises the phospholipid bilayer by binding to polar head and non- polar tails
  • Controls fluidity b preventing phospholipids solidifying at low temperature and becoming too fluid at high temperatures
  • Reduces permeability to water, ions and polar molecules
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Proteins

  • Are transmembrane proteins acting as channel and carriers
  • Are receptors for chemicals made by other cells
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Glycolipids are Glycoproteins

  • Are carbohydrate chains found on the exterior surface of cell surface membranes
  • Act as recptors for signaling molecules
  • Act as cell surface markers that identify the cell to others
  • Are involved in sticking cells to one another (Adhesion)
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Exam Questions on membranes

Q1.  Describe the structure of a phospholipid bilayer.

Q2.  Describe one function of a Channel protein.

Q3. Describe the function of Cholesterol in a phospolipid bilayer.

Q4. Outline three functions of gycoproteins in membranes.

Q5. Explain why the phospholipid bilayer is arranged with fatty acid tails pointing inwards and phosphate heads outwards.

Q6. State on function of each of the following components of the cell surface membrane: Carbohydrates and Proteins

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Answers

Q1. Membranes are made up of two layers of phospholipds known as the Phospholipid bilayer and proteins. The polar heads of the phospholipids are hydrophillc and are attracted to the water, this is why they face up to the cytoplasm and the exterior of the cell.

Q2. Channel proteins allow the movement of some substances across the memebrane.

Q3.

  • Stabalises the phospholipid bilayer by binding to polar head and non- polar tails
  • Controls fluidity b preventing phospholipids solidifying at low temperature and becoming too fluid at high temperatures
  • Reduces permeability to water, ions and polar molecules
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Answers

Q4.

  • Are carbohydrate chains found on the exterior surface of cell surface membranes
  • Act as recptors for signaling molecules
  • Act as cell surface markers that identify the cell to others
  • Are involved in sticking cells to one another (Adhesion)

Q5.The Phosophate heads are attracted towards the cytoplasm as they are hydrophillic and the fatty acid tails project inwards as they are hydrophobic and move away from water

Q6. Carbohydates give structural support and protein create channel to transport substances across the membrane

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