BIO Ch.4 movement of substances across the cell membrane

What is diffusion?

• Passive movement

• when there is a difference in concentration of particles between two regions (concentration gradient)

• Net movement of particles down the concentration gradient

How does the particles move when they are spread evenly throughout the space they occupy?

• Reach equilibrium

• No net movement

• Still move randomly in all direction

how do different particles (non-polar, polar, macromolecules) diffuse across the cell membrane?

• Non-polar: directly diffuse through the phospholipid bilayer (simple diffusion)

  • ✅kinetic energy ❌energy from respiration in cells

  • Example: oxygen, carbon dioxide

• Polar: diffuse through the channel proteins or carrier proteins, repelled by the phospholipid bilayer (facilitated diffusion)

  • ✅kinetic energy ❌energy from respiration in cells

  • Example: glucose, amino acids

What factors affect the rate of diffusion?

• Difference in concentration between two regions (how steep the concentration gradient is)

  • ⬆steeper the gradient ⬆ rate

• Distance over which diffusion takes place

  • ⬇distance ⬆rate

• Surface area of the membrane

  • ⬆surface area ⬆rate

• Temperature

  • ⬆temperature ⬆kinetic energy ⬆rate

• Size and nature of particles

  • ⬇size ⬆rate

  • Non-polar ⬆rate

Examples of diffusion.

• Gas exchange in the lungs

• Absorption of digested food in the intestines

• distribution of substances within cytoplasm

What is osmosis?

• Net movement of water molecules from a region of higher water potential to a region of lower water potential

• across a differentially permeable membrane

• water molecules move across the cell membrane through specific channel protein for water

What is water potential?

• tendency of water molecules to defuse from one place to another

• Water potential: <= 0 (highest: distilled water)

• ⬆concentrated the solution ⬇water potential(inversely proportional)

How is cells in isotonic solution?

• No net movement of water molecules into or out of the cells.

How is cells in hypotonic solution (high water potential)?

• water enter the cell by osmosis

• Animal cells: swell (膨脹) and eventually burst

  • Example: haemolysis: bursting of red blood cells leading to the release of haemoglobin

• Plant cells: turgid (硬漲) and will not burst

  • Strong and rigid cell wall prevent cells from bursting

  • No more water can enter this plant cell when the cell membrane is pressed firmly against the cell wall

How are cells in hypertonic solution (low water potential)?

• Water leaves the cells by osmosis

• Animal cells: shrink and become wrinkled

• Plant cells: plasmolysed and become flaccid (軟縮)

  • Cytoplasm and the vacuole shrink due to water loss

  • The cell membrane detached from the cell wall (plasmolysis 質壁分離）

What is the importance of osmosis?

• Animal (human): most of the water in food is absorbed by osmosis in the small intestine

• Plant: provide support in young seedings and non-woody plants

  • gain water: turgid and press against the other cells

  • Lose water significant: flaccid and wilts (萎蔫)

How does osmosis occur in the onion cells in the hypertonic solution (sucrose solution)?

1. The onion cells are plasmolysed and flaccid

2. This shows that the water potential of the sucrose solution is lower than that of the onion cells

3. When water leaves the onion cells, they become flaccid and plasmolysed

How does osmosis occur in the onion cells in the hypotonic solution (distilled water)?

1. The onion cells become turgid

2. This shows that the water potential of the distilled water is higher than that of the onion cells

3. When water enters the onion cells, the onion cells become turgid

How does osmosis occur in the red blood cells in the isotonic solution?

1. The red blood cells appear normal

2. This show said there is no difference in the water potential between the cells and their surroundings

3. There is no net movement of water into or out of the cells

Why do we have to put a cover slip over the epidermis?

• Prevent evaporation of water from the sucrose solution

  • change the water potential

  • affect the experimental result

• Flatten the specimen

• Prevent the object from touching the specimen

what are the applications of osmosis?

• Soaking food in a solution with high salt or sugar concentration (hypertonic solution)

  • Remove water from the food and microbes by osmosis

• Use slightly hypertonic solution to store red blood cells

  • Prevent cells from drawing in water and bursting

• Isotonic saline solution to dissolve drugs for intravenous injection (靜脈注射)

What is active transport?

• Substances are absorbed against the concentration gradient

• usually occur when plants absorb mineral ions from the surrounding

How does active transport occur?

• ✅energy from respiration ❌kinetic energy

  • ⬆rate of respiration ⬆rate of active transport

• In living cells only

• Transported through specific carrier proteins

Example of active transport.

• Active minerals from the soil into the roots of plants

• We absorption of glucose and amino acids in kidney tributes

• Absorption of small water soluble molecules (e.g. monosaccharides, minerals) in small intestine

What is phagocytosis?

• Large particles cannot enter cells by the diffusion or active transport (so use phagocytosis)

How does phagocytosis occur?

1. The cell membrane folds in to form a pit or it extends out to form pseudopodia

2. The particles is enclosed by vacuole and is taken into the cell

3. A lysosome containing enzymes move towards the vacuole

4. The digested products diffuse into the cytoplasm

5. The lysosome fuses with the vacuole. The particle is digested by the enzymes

Example of phagocytosis.

• White blood cells can engulf invading microorganisms by phagocytosis

• Some unicellular organisms use phagocytosis for feeding

What is endocytosis (胞吞) and exocytosis (胞吐)?

• Endocytosis: intake of minerals into the cells by the infolding of the cell membrane to form a vacuole (e.g. phagocytosis)

• Exocytosis: removing wastes or release secretory products by enclosing them in vesicles which then move to the cell surface for release