Cell Membranes Osmosis and Diffusion

Functions of Membranes 1. Protect cell 2. Control incoming and outgoing substances 3. Maintain ion concentrations of various substances 4. Selectively permeable - allows some molecules in, others are kept out

Solutions Solutions are made of solute and a solvent Solvent - the liquid into which the solute is poured and dissolved. We will use water as our solvent today. Solute - substance that is dissolved or put into the solvent. Salt and sucrose are examples of solutes.

Methods of Transport Across Membranes 1. Diffusion 2. Osmosis 3. Active Transport 4. Facilitated Diffusion

Methods of Transport Across Membranes 1. Diffusion -passive transport - no energy expended 2. Osmosis - Passive transport of water across membrane 3. Active Transport- requires energy to transport molecules against a concentration gradient – energy is in the form of ATP 4. Facilitated Diffusion - Use of proteins to carry polar molecules or ions across

Diffusion Movement of molecules from an area of high concentration to an area of low concentration till equilibrium is attained. Diffusion can take place anywhere , in living and non living organisms It is a passive process Eg in plants: Movement of gases like oxygen and CO2 during respiration In animals: Diffusion of oxygen into the blood stream in the lungs.

Factors affecting diffusion: 1. Shorter the distance (or thinner the membrane) faster the diffusion 2. high concentration gradient is essential 3. The larger the surface area, faster the diffusion ( example: Small intestine folded, placenta villi folded )

Diffusion

Osmosis

Osmosis Movement of water molecules from a region of its higher concentration to a region of its lower concentration across a semi permeable membrane.  Can take place only in living organisms.  It is a passive process.

Tonicity is a relative term Hypotonic Solution - One solution has a lower concentration of solute than another. Hypertonic Solution - one solution has a higher concentration of solute than another. Isotonic Solution - both solutions have same concentrations of solute.

When an animal cell (RBC) is placed in: Hypertonic solution: Since salt concentration is more, water from the RBC moves out of the cell into the surrounding, exosmosis occurs RBC shrinks as more and more water leaves the cell Appearance of the cell changes completely. Edges become wavy or crinkled and the cell becomes Flaccid. Cell becomes crenated (star shaped with edges sticking to each other

When an animal cell (RBC) is placed in: Hypotonic solution: . When RBC is put into a hypotonic solution, since water potential is higher outside the cell, water moves into the RBC by endosmosis. The cell becomes turgid gaining water. Appearance: Cell becomes rounded, swollen and edges of the cell are smooth. RBC swells up and bursts ultimately as there is no cell wall .

When an animal cell (RBC) is placed in: Isotonic: Water potential inside and outside the cell are the same. There is no net movement of water molecules into or outside the cell. There is not much difference seen in the RBC as the concentration is the same inside and outside the cell.

When a Plant cell is placed in: Hypertonic solution: When a plant cell is put in a concentrated solution, exosmosis occurs and water leaves the cell. Water potential is lower in the cell so water moves out. The plant cell becomes flaccid after losing water. Appearance: cell shrinks and becomes smaller with wavy margins Further loss of water makes the cell contents shrink to the centre, cell membrane tears away from the cell wall. The cell is said to be plasmolysed and the process is called plasmolysis.

When a Plant cell is placed in: Hypotonic solution: Water potential is higher outside the cell, so endosmosis occurs. Water moves into the cell, cell becomes turgid. Gains water and swells up. Size increases and edges of the cell smoothen out The cell doesn’t burst because of the elastic cell wall present.

When a Plant cell is placed in: Isotonic solution: There is no net movement of water molecules into or outside the cell. Cell doesn’t gain or lose water, appearance remains the same. osmosis is reversible in plants cells but not in animal cells. in some cases where the particles are too large to cross a membrane by diffusion or active transport, phagocytosis is used, where the WBCs engulf the particles and help transport.

Plant and Animal Cells put into various solutions

Active transport: substances like minerals move from a region of lower concentration to a region of higher concentration against the concentration gradient using energy. It is an active process since it requires energy. ATP provides the energy Carrier proteins in the membranes play a very important role in transporting the substances across the membrane. Example in plants: uptake of minerals from soil In animals: In humans absorption of glucose at the villi of the small intestine.

Types of Transport A Bird’s View…