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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Cell Membranes and Walls
Every living cell is surrounded by a cell membrane, which acts like a security guard for the cell. It controls what goes in and out while protecting the cell's contents. Plant cells have an additional protective layer called the cell wall. Together, these structures help maintain cell shape and function while allowing necessary interactions with the environment.
Key Definitions:
The cell membrane is made up of a double layer (bilayer) of phospholipid molecules with proteins embedded within it. This structure is called the fluid mosaic model because:
Phospholipids are fascinating molecules with two different ends:
This arrangement creates a stable barrier that prevents water-soluble substances from freely crossing the membrane.
Proteins embedded in the cell membrane perform various crucial functions:
Create tunnels through the membrane that allow specific molecules to pass through. Think of them as doorways that only let certain visitors enter.
Change shape to move specific substances across the membrane, like a revolving door that picks up molecules on one side and deposits them on the other.
Recognize and bind to specific molecules (like hormones), triggering changes inside the cell. They work like satellite dishes receiving signals.
Substances need to move in and out of cells for survival. There are several ways this happens:
Passive transport requires no energy from the cell and moves substances from areas of higher concentration to areas of lower concentration (down the concentration gradient).
The movement of particles from an area of higher concentration to an area of lower concentration. Small molecules like oxygen and carbon dioxide can diffuse directly through the phospholipid bilayer.
Example: Oxygen diffuses from the lungs into the bloodstream, where its concentration is lower.
The diffusion of water molecules across a partially permeable membrane. Water moves from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration).
Example: If a plant cell is placed in pure water, water moves into the cell by osmosis.
Active transport requires energy from the cell (in the form of ATP) and can move substances against their concentration gradient (from lower to higher concentration).
Carrier proteins use energy to change shape and move specific substances across the membrane against their concentration gradient.
Example: Root hair cells actively transport mineral ions from the soil (where concentration is low) into the plant (where concentration is higher).
Active transport is essential for:
Unlike animal cells, plant cells have an additional protective layer outside their cell membrane called the cell wall.
Plant cell walls are primarily made of cellulose, a tough carbohydrate that forms fibres. These cellulose fibres are arranged in a cross-linked network, giving the wall strength while still allowing water and dissolved substances to pass through.
When plant cells are placed in different solutions, they respond in fascinating ways:
Scientists use various techniques to study cell membranes and walls:
Understanding cell membranes and walls has important applications:
Cell membranes are essential boundaries that control what enters and leaves cells. They consist of a phospholipid bilayer with embedded proteins that serve various functions. Substances move across membranes through passive processes like diffusion and osmosis, or through active transport that requires energy.
Plant cells have an additional cell wall made of cellulose that provides structural support and protection. This wall allows plants to maintain their shape and stand upright, even when their cells absorb water and become turgid.
Understanding how cell membranes and walls function helps explain many biological processes, from how plants grow to how medicines work in our bodies.
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