Biology, Chapter 7 – Cell Structure and Function
Section 3 – Cell Boundaries
I.
Cell Membrane
A. Function
1. Surrounds cell, separating it from its surrounding environment
2. Selectively permeable – meaning some substances can pass through and others can not.
3. Provides protection and support
B. Structure
1. Lipid bilayer – double layer sheet that forms the core of nearly all cell membranes
2. Contains two layers of lipids – they give the membrane a flexible structure
3. Contains proteins – form channels and pumps for moving materials
4. Contains carbohydrates – act as chemical identification cards, allow cells to be identified
II. Cell Wall
A. Found in plants, algae, fungi and bacteria
B. Function is to provide support and protection for the cell
C. Structure:
1. Composed of cellulose
2. Has a rigid shape
3. Contains small openings for materials to pass
III. Diffusion
A. Diffusion – when particles move from an area of high concentration to an area of low concentration.
1. Depends upon random particle movements, substances diffuse across membranes without requiring
the cell to use energy.
B. Equilibrium – when the concentration of a solute is the same throughout the system
IV. Osmosis
A. Osmosis – the diffusion of water through a selectively permeable membrane.
B. Flow is from an area of high water concentration to an area of low water concentration.
V. How Osmosis Works
A. Water tends to move across a membrane until equilibrium is reached.
B. The effects of osmosis on cells:
1. Isotonic – when the concentration of water and another substance are in equilibrium
a. Isotonic means “same strength”
b. Everything is equal
2. Hypertonic – when the concentration of a solution is high
a. Hypertonic means “above strength”
b. Water leaves the cell for the solution
i Animal cells shrink or shrivel
ii Plant cells central vacuole collapses and the cytoplasm shrinks
3. Hypotonic – when the concentration of a solution is low
a. Hypotonic means “below strength”
b. Water moves into the cell
i Cells swell and burst
ii In plants, water collects in the central vacuole.
VI. Facilitated Diffusion
A. Movement of specific molecules across cell membranes through protein channels
B. The cell membrane channels “facilitate” or “help” the diffusion across the membrane
1. Very fast and specific, does not require energy
2. Still diffusion, movement of molecules only occurs if there is a high/low concentration
VII. Active Transport
A. Active Transport - The movement of particles through proteins against the normal direction of diffusion.
B. Particles move from LOW concentration to HIGH concentration
C. The cell must use energy
D. Three types:
1. Molecular Transport
a. Small molecules are carried across the cell membrane by proteins that change shape
2. Endocytosis
a. Process of taking materials into the cell by creating “pockets” of the cell membrane.
b. Pockets break loose and form a vacuole within the cytoplasm.
i Phagocytosis – “cell eating”
Extensions of cytoplasm surround larger particles and create a food vacuole within the cell
ii Pinocytosis – “cell drinking”
Tiny pockets form along the cell membrane, fill with liquid and pinch off to form vacuoles
within the cell
3. Exocytosis
a. Releasing of large amounts of materials from the cell
b. The membrane of the vacuole fuses with the cell membrane forces its contents out of the cell
i Example – removal of water by a contractile vacuole