cell transport mechanisms

What is diffusion

The natural movement of particles from an area of high concentration to low concentration- it occurs because all the in liquids and gases are moving but there is no net movement

Facilitated diffusion

Certain substances such as large polar molecules or ions cannot diffuse through the phospholipid bilayer of cell membranes

Need proteins

Channel proteins

Water filled pores

Allow charged substances to diffuse

Gated channels- can open and close

Carrier proteins

-can switch between two shapes, channel proteins have fixed shape

-has specific shape

Open one side closed the other side

Rotates or flips

Osmosis

Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane

Water potential

• A dilute solution has a high water potential (the left-hand side of the diagram below)

• A concentrated solution has a low water potential (the right-hand side of the diagram below)

• The water potential of pure water (without any solutes) at atmospheric pressure is 0kPa;

  • Any solution that has solutes will have a water potential lower than 0kPa (it will be a negative value)

  • The more negative a water potential value, the lower the water potential is said to be

Plant cell osmosis

• Plant and animal cells are affected differently by osmosis, as plant cells have a cell wall

• In a hypertonic solution, water leaves the plant cell by osmosis, the protoplast shrinks and pulls away from the cell wall — this is called plasmolysis

  • Without enough water, cells lose turgor and the plant wilts

• In a hypotonic solution, water enters the plant cell, the vacuole expands, and the cell becomes turgid — the cell wall prevents bursting

  • Turgidity supports the plant, helping it stay upright and catch sunlight

• In an isotonic solution, water moves in and out equally, so there is no net change, and the cell is neither turgid nor plasmolysed

Incipient plasmolsis- 50% plasmolysed

Osmosis in animal cells

Lysis- water moves into cell and they burst

Crenated- moves put, shrivelled