Cell Membrane Function: Diffusion, Transport, and Homeostasis
Cell Membrane Function: Maintaining Cellular Equilibrium
7.3
– Cell membrane function: Keep the cell’s internal conditions constant.
– Diffusion: Process in which particles move from an area of high concentration to an area of lower concentration. The driving force behind the movement of many substances across the cell membrane.
– Once the concentration of a substance on both sides of the cell membrane is the same, equilibrium is reached.
– The movement of material across the cell membrane without using cellular energy is called passive transport.
– Proteins act as carriers, or channels, making it easy for certain molecules to cross.
– Only glucose can pass through these protein channels.
– Facilitated diffusion: Process in which molecules that cannot directly diffuse across the membrane pass through special protein channels.
– Inside the cell, the lipid bilayer is hydrophobic. Because of this, water molecules pass through water channel proteins named aquaporins.
– Osmosis: Diffusion of water through a selectively permeable membrane.
– When the concentrations on both sides are considered, they can be isotonic (“same strength”), hypertonic (“above strength”), or hypotonic (“below strength”).
– Osmotic pressure: Movement of water out of or into a cell produces this force. This can cause a cell in a hypertonic solution to shrink and one in a hypotonic solution to swell (in animal cells).
– What happens when cells come into contact with fresh water?
Water moves into them so slowly that osmotic pressure is not a problem.
– Bacteria and plant cells are surrounded by tough walls.
– The movement of materials against a concentration difference is known as active transport (small molecules).
– Cell membrane transport is generally carried out by transport proteins – protein pumps.
– Large molecules and clumps of material can also be actively transported across the cell membrane by processes known as endocytosis and exocytosis.
Molecular Transport (small molecules)
A considerable portion of the energy used by cells in their daily activities is spent providing the energy to keep this form of active transport working.
Bulk Transport (large molecules)
Transport by movement
– Endocytosis: Process of taking material into the cell by means of infolding, or pockets, of the cell membrane. Phagocytosis is a type of endocytosis in which extensions of cytoplasm surround a particle and package it within a food vacuole. Blood cells use this to “eat” damaged cells.
Tiny pockets form along the cell membrane, fill with liquid, and pinch off to form vacuoles within the cell: pinocytosis.
– Exocytosis: Cells release large amounts of material.
7.4
Homeostasis: Constant internal physical and chemical conditions.
To maintain it, unicellular organisms grow, respond to their environment, transform energy, and reproduce.
Unicellular organisms include prokaryotes and eukaryotes. Prokaryotes are found in bacteria.
The cells of human beings and other multicellular organisms do not function by themselves. They are interdependent and work together.
– The cells of multicellular organisms become specialized for particular tasks and communicate with one another to maintain homeostasis.
Cell specialization allows cells to move, react to the environment, and produce substances.
– Specialized cells of multicellular organisms form tissues, organs, and organ systems.
Tissue: A group of similar cells that performs a particular function.
Organ: Many groups of tissues working together.
Organ system: Groups of organs that work together to perform a specific function.
Cells in large organisms communicate by means of chemical signals that are passed from one cell to another.
Receptor: Responds to one of these chemical signals. Some receptors are in the cell membrane; receptors for other types of signals are inside the cytoplasm.