All cell and organelle membranes have the same basic structure:
a lipid bilayer with associated proteins:
"protein icebergs floating in a lipid sea"
Fluid mosaic model of membrane structure:
- Lipid bilayer:
- mostly phospholipids, but may also contain cholesterol
- polar "heads" float on surfaces
- nonpolar hydrocarbon "tails" cluster together
- Proteins whose position depends on hydrophilic and hydrophobic interactions
- Integral proteins: embedded within the membrane; may snake through it
- Surface proteins float on the membrane surface
- Some proteins (glycoproteins) contain attached carbohydrates at the surface;
these aid in cell recognition
Organelles made primarily of membranes:
- Plasma membrane (cell membrane):
- surrounds and encloses cell;
- determines what can enter and leave
- Endoplasmic reticulum (ER):
- a series of folded membranes enclosing cisternae
- important for internal transport within the cell;
- may be rough (with ribosomes on surface) or smooth (no ribosomes)
- Golgi apparatus:
- a series of stacked vesicles, somewhat flattened
- important in packaging proteins into vacuoles
- Nuclear envelope, a two-layered envelope around nucleus
- penetrated by nuclear pores
- Vacuoles, spherical droplets surrounded by a membrane
- may contain fat, protein, ingested food, or secretion product
- often formed by budding off margins of Golgi apparatus
- Lysosomes, containing protein-digesting enzymes surrounded by a membrane
- Food vacuoles fuse with lysosomes as part of digestion.
- Rupture of lysosomes causes autolysis (cell death).
- Vesicles, spherical or teardrop-shaped sacs formed by pinocytosis or
phagocytosis, or during programmed cell death (apoptosis)
Transport processes:
- Passive transport (diffusion) along a concentration gradient
- Always goes from higher concentration to low
- Requires no energy
- Diffusion of water is called osmosis
- Active transport:
- Always opposes concentration gradient (from lower to higher concentration)
- Always requires energy (ATP)
- Bulk transport, always requiring energy:
- Exocytosis, secretion or release of material from cell
- Vacuole fuses with plasma membrane, releasing contents
- Endocytosis, the opposite process, in which vacuoles are formed:
- Phagocytosis, formation of large vacuoles by folds of the cell
membrane enclosing material from outside the cell
- Pinocytosis, formation of smaller vacuoles at the bottom of
funnel-shaped depressions or folds
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