Transport Across the Cell Membrane

Transport Across the Cell Membrane

Cell Membrane Structure in Relation to Transport

  • The cell membrane is a phospholipid bilayer, with hydrophobic (water repelling) tails facing inwards and hydrophilic (water attracting) heads facing outwards.
  • Peripheral proteins positioned on the surface, and integral proteins embedded in the bilayer contribute to functionality, including transport.
  • The feature of the cell membrane as selectively permeable allows it to control the passage of substances across it.
  • Cholesterol molecules within the cell membrane maintain membrane fluidity and stability.

Types of Transport Across Cell Membranes

  • Passive transport involves the movement of molecules down their concentration gradient without the need for energy.
  • Three main types of passive transport include diffusion, facilitated diffusion, and osmosis.
  • Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.
  • In facilitated diffusion, carrier proteins or channel proteins aid diffusion of larger or charged molecules.
  • Active transport uses energy (ATP) to move substances against their concentration gradient, often using protein pumps.

Role of Carrier and Channel Proteins in Transport

  • Channel proteins form hydrophilic channels in the membrane for specific molecules (usually ions) to pass through by diffusion.
  • Carrier proteins combine with a specific molecule before changing shape to allow movement across the membrane, these proteins are used in both passive (facilitated diffusion) and active transport.
  • Protein pumps involved in active transport are carrier proteins that utilise energy to move molecules against their concentration gradient, including the sodium-potassium pump.

Importance of Membrane Transport in Cellular Function

  • The transport of substances across the cell membrane is vital for nutrient uptake, waste elimination, maintaining ion concentration balance and responding to changes in the cell environment.
  • Active transport is crucial in muscle contraction, nerve impulse transmission, and maintaining cell volume among other cellular activities.
  • Facilitated diffusion is important in glucose uptake in cells, as seen in the use of glucose transport proteins.
  • The inability of certain solutes to cross the cell membrane assists in creating and maintaining cell potential.

Endocytosis and Exocytosis in Cellular Transport

  • Endocytosis is a form of active transport where the cell membrane invaginates (folds in) to form a vesicle and engulf substances into the cell.
  • In phagocytosis, a form of endocytosis, the cell engulfs large particles or even whole cells.
  • Pinocytosis is another form of endocytosis where the cell engulfs extracellular fluid along with its dissolved substances.
  • Exocytosis is the opposite process, where substances are expelled from the cell via vesicles that merge with the cell membrane.
  • Both endocytosis and exocytosis are critical in immune response, cell growth, and neurotransmitter release.