Module One: 3. Cell Function

Answer 1

Cellular Movement and Diffusion

Movement of Substances in and Out of Cells
* Cells interact with their environment and surrounding cells to function effectively.
* Substances needed by cells include gases, nutrients, water, and mineral salts.
* Substances that must leave cells are wastes like urea, uric acid, and excess carbon dioxide.

Cell Membrane Structure
* Cell membrane is selectively permeable, controlling what passes across it.
* Cell membrane controls the passage of water and other molecules into or out of living cells.
* Cell membrane structure is permeable, allowing water and most molecules to pass freely inwards or outwards.

Answer 2

Molecule Movement across Cell Membranes
* Permeability of a cell membrane to a molecule depends on size, electrical charge, and lipid solubility.
* Small molecules can move quickly across membranes, larger molecules have more difficulty.
* Water is highly permeable to membranes due to pores in the membrane, called aquaporins.
* Molecules with low permeability rely on carrier proteins to transport them across membranes.

Diffusion
* Movement of materials into and out of cells takes place either passively or actively.
* Diffusion is the net movement of any molecules from a region of high concentration to a region of low concentration of that substance, until equilibrium is reached.
* Equilibrium is reached when there is no net movement of molecules in either direction.
* Diffusion can be likened to the movement of rocks rolling down a hill, needing no energy input.

Diffusion across a Cell Membrane
* Small uncharged particles like carbon dioxide and oxygen move easily through the cell membrane by simple diffusion.
* The concentration gradient is usually maintained for the diffusion of oxygen molecules.
* Facilitated diffusion is required for relatively large molecules and charged particles.

Answer 3

Cell Membrane Diffusion Overview

• Carrier proteins bind to molecules on one side of the cell membrane, changing shape and releasing the substance on the other side.
• The direction of movement depends on the concentration gradient.
• Facilitated diffusion moves particles from high to low concentration until equilibrium is reached.
• At equilibrium, no net movement of particles occurs in either direction.
• Diffusion does not require energy input and occurs faster with higher temperatures or steeper concentration gradients.
• Small, uncharged molecules like oxygen and carbon dioxide can easily diffuse across the cell membrane.
• Larger molecules and small electrically charged ions can diffuse across the cell membrane aided by carrier or channel proteins.

Question 4

What are the type of molecules are able to move through the membrane?
Which molecule is more resistant?

Answer 4

1/ Specific non-lipid-soluble molecules or ions
Lipid-soluble molecules

2/ Non-lipid-soluble molecules

Question 5

What is Osmosis? What solvent determines the concentration of a solution? How do water moves through the plasma cell membrane?

Answer 5

Osmosis: A Type of Diffusion and Its Importance

• Osmosis is a diffusion process where solvent molecules move from high to low concentrations through a semipermeable membrane.
• Water is the most common solvent in a solution, with concentration determining the concentration.
• Water moves through aquaporins, tiny protein channels in cell membranes, through the cell membrane.
• When water is more concentrated outside the cell than inside, it moves into the cell, causing cell swelling.
• If water concentration is lower outside the cell than inside, it moves out, causing cell shrinkage.
• The pressure created by water moving across the membrane is called osmotic pressure.
• If fluids inside and outside a cell have equal solute concentration, the external solution is isotonic to the cell contents.
• If cells are surrounded by a solution with lower solute concentration than their cytoplasm, the external solution is hypotonic to the cell contents, allowing water molecules to move through the membrane into the cells.
• If cells are surrounded by a solution of higher solute concentration, the external solution is hypertonic to the cells, causing water molecules to move out of the cells.

Answer 6

●Active transport is the movement of molecules from a region of low concentration to a region of high concentration.
●Active transport moves against the concentration gradient and requires the input of energy.

Answer 7

Understanding Osmosis in Cellular Systems

• Osmosis is the process of moving water from a high concentration of water (dilute – low solute) to a low concentration of water (concentrated – high solute).
• Osmosis requires no energy input and the more water that moves across the membrane, the higher the osmotic pressure created.
• Osmosis in animal cells is a problem as it can cause the cell to swell and burst the cell membrane.
• Plant cells do not burst when soaked in fresh water (hypotonic solution), but water molecules move into the vacuole, causing the cell membrane to stretch against the cell wall.
• If placed into a hypertonic solution, the water in the cell would leave the cell by osmosis, the vacuole would shrink and cause the cell membrane to move away from the cell wall in a process called plasmolysis.
• Active transport is the movement of molecules from a region of low concentration to a region of high concentration, requiring the input of energy.
• Active transport requires a carrier protein that spans the membrane to actively move chemicals from a low to a high concentration, utilising cellular energy.

Answer 8

Endocytosis and Exocytosis: Transport of Large Molecules

Endocytosis:
* Process where a large particle is engulfed by the cell membrane.
* Phagocytosis: This process occurs when a solid particle is engulfed, forming a vesicle.
* Pinocytosis: This process occurs when the cell membrane engulfs a drop of extracellular fluid.

Exocytosis:
* Specialized animal and plant cells produce substances like antibodies, neurotransmitters, and enzymes.
* These substances are transported to the external environment of the cell through exocytosis.
* A membrane-bound vesicle moves to the cell membrane, fuses with it, and releases its contents to the cell’s exterior.

Answer 9

Factors Affecting Material Exchange Across Membranes

Chemical Factors
* Uncharged molecules like ethanol can easily penetrate the cell membrane due to their phospholipid bilayer.
* Hydrophilic, charged ions like sodium and potassium cannot cross the hydrophobic centre of the membrane.
* Channel proteins specific for each ion allow their movement through the cell membrane.
* Water, being non-lipid-soluble, cannot move through the hydrophobic ‘tails’ in the cell membrane.

Physical Factors
* Size and shape of the molecules affect the movement of substances across the cell membrane.
* Small molecules can easily diffuse between the phospholipids.
* Large molecules use carrier proteins to move through the cell membrane.
* Large molecules are moved through the process of endocytosis or exocytosis.

Concentration Gradient
* The relative concentration of the substance on either side of the membrane affects the rate of diffusion.
* A high concentration gradient results in rapid diffusion, while a decrease decreases the rate of diffusion.
* Plant cells maintain a steeper concentration gradient through cytoplasmic streaming.

Surface-area-to-Volume Ratio
* The surface area divided by the volume (SA:V) affects the movement of substances into and out of the cell.
* A smaller cell has more surface area in relation to its volume, allowing faster movement of substances.
* A larger cell has a smaller amount of surface area in relation to its volume, reducing the efficiency of obtaining nutrients and removing wastes.