Describe the structure of cell membranes and relate this structure to the movement of
substances through it.
Membranes are found surrounding all organelles within an animal cell. The nucleus, however, is
encased by the nuclear envelope, a double membrane that separates the cell's genetic material
from the cytoplasm. While membranes are often described as selectively permeable, a more
accurate term is partially permeable. This means they allow the passage of small molecules,
such as water and gases (e.g., oxygen and carbon dioxide), but restrict the movement of larger
molecules. Larger substances like glucose and amino acids can still pass through, but their
diffusion is slower and may require specific transport proteins to facilitate their movement
across the membrane. This selective permeability ensures that the cell can maintain its internal
environment by regulating the substances that enter and exit.
Membranes are composed of a variety of molecules, including lipids, which play a crucial role in
their structure and function. One of the most common lipids found in biological membranes is
the phospholipid. A phospholipid consists of a polar head and two non-polar hydrocarbon tails,
which are derived from fatty acids. The polar head contains a phosphate group, giving it an
uneven distribution of charge. This uneven distribution, or polarity, causes the molecule to
interact differently with water compared to the non-polar tails. The hydrophilic (waterattracting) phosphate head is attracted to water, while the hydrophobic (water-repelling)
hydrocarbon tails avoid contact with water. This distinctive arrangement of polar and non-polar
regions makes the phospholipid molecule amphipathic, meaning it has both water-loving and
water-hating components. The polarity of the phospholipid causes it to be soluble in water, but
the hydrophobic tails are repelled by water, leading to the formation of unique structural
arrangements in water. This behavior is a fundamental characteristic of phospholipids and plays
a key role in the structure of cellular membranes. In the case of biological membranes,
phospholipids typically arrange themselves into a bilayer, where two layers of phospholipids
align with the hydrophilic heads facing outward toward the aqueous environment, and the
hydrophobic tails face inward, away from the water. This arrangement forms the lipid bilayer,
the basic structure of all biological membranes. The bilayer creates a semi-permeable barrier
that separates the internal environment of the cell from the external environment, allowing the
cell to maintain homeostasis and regulate the movement of substances.
The fluid mosaic model describes the structure of cell membranes, where proteins are
embedded in a fluid phospholipid bilayer. The bilayer consists of two layers of phospholipids
with hydrophilic heads facing outward and hydrophobic tails inward. The proteins are scattered
across the bilayer, forming a mosaic pattern. Because the bilayer is fluid, the proteins can move
laterally within it, creating a dynamic structure. This fluidity, akin to the consistency of olive oil,
is essential for membrane flexibility and functions like transport, signaling, and interaction with
the environment. Cell membranes contain different proteins with specific functions. Protein
channels act as gates with a fixed shape, allowing only specific ions (e.g., chloride) to pass.
Carrier proteins change shape to transport molecules, like a "ping pong" mechanism. Protein
pumps, similar to carrier proteins, also change shape but require ATP to move substances
against their concentration gradient.
Diffusion is the process by which molecules move from an area of higher concentration to an
area of lower concentration, driven by the random movement of particles. This movement
continues until equilibrium is reached, where the concentration of molecules is the same
throughout. For example, oxygen diffuses from the lungs into the blood while at the same time
carbon dioxide diffuses in the opposite direction. There are many factors which effect diffusion
and these factors affect the rate at which molecules cross cell membranes by diffusion are
summarised by Fick’s Law This states that the rate is proportional to:
Facilitated diffusion is a type of passive transport where specific molecules, such as glucose or
ions, are assisted in crossing the cell membrane by transport proteins. Transport proteins which
allow the passage of ions are called ion channels, they are ‘gated’ which means that they can
open or close. Gated ion channels are important in the conduction of nerve impulses. Unlike
simple diffusion, facilitated diffusion requires proteins such as; channel proteins and carrier
proteins because the molecules being transported are too large or charged to pass directly
through the lipid bilayer. Both diffusion and facilitated diffusion do not require energy, as the
movement of molecules is driven by concentration gradients.
Osmosis is the movement of water molecules across a selectively permeable membrane, driven
by differences in water potential. Water moves from an area of higher water potential (lower
solute concentration) to an area of lower water potential (higher solute concentration). The
solute potential is a key factor in determining water potential, as the presence of solutes
decreases the water potential. In hypertonic, isotonic, and hypotonic solutions, the movement
of water across red blood cells plays a crucial role in their shape and function. In a hypertonic
solution, the solute concentration outside the cell is higher than inside, causing water to move
out of the red blood cell to balance the concentrations. This results in the cell shrinking and the
cell becomes dehydrated and loses its normal shape. In an isotonic solution, the solute
concentration is the same inside and outside the cell, meaning water moves in and out of the
red blood cell at equal rates. This keeps the cell at a stable size and shape, which is ideal for
normal cell function. In a hypotonic solution, the solute concentration outside the cell is lower
than inside, causing water to flow into the red blood cell. As water enters, the cell swells, and if
too much water accumulates, the cell can burst releasing its contents. These processes
illustrate how osmosis regulates the volume and integrity of red blood cells, which is essential
for maintaining proper fluid balance and ensuring the cells can function effectively in the body.
Active transport is the process by which cells move substances across their membrane against
their concentration gradient, from areas of low concentration to areas of high concentration.
Active transport requires energy in the form of ATP to power the movement of molecules. This
energy is needed because the cell is working to move substances in the opposite direction of
natural diffusion, which would normally move them from high to low concentration. The
sodium-potassium pump is a type of membrane protein found in most animal cells, and it
actively moves sodium ions (Na⁺) out of the cell and potassium ions (K⁺) into the cell, both
against their concentration gradients. For each cycle, the sodium-potassium pump moves three
sodium ions out and two potassium ions into the cell. This movement is crucial for maintaining
the cell's resting potential, regulating cell volume, and supporting nerve function. The process
uses energy derived from ATP, which is broken down to provide the necessary power for the
pump's action. The sodium-potassium pump is essential for maintaining proper cellular function
and plays a key role in processes such as nerve signal transmission and muscle contraction.
Endocytosis occurs by the infoldings or extension of the cell surfaces membrane to form a
vesicle or vacuole. There are two types: phagocytosis and pinocytosis. In phagocytosis material
is taken up in solid form and cells are specializing in the process phagocytes, and are said to be
phagocytic. An example is the white blood cells which take up bacteria by phagocytosis and the
sac formed is called phagocytic vacuole. In pinocytosis the material is taken up in liquid form.
Vesicles are often formed and are extremely small, in which case the process is known as
micropinocytosis and the vesicles as micropinocytic vesicles. Pinocytosis is used by the human
egg cell to take up nutrients from the surrounding follicle cells. Exocytosis is the reverse. Waste
materials may be removed from the cells, such as solid, undigested remains from phagocytic
vacuoles, or useful materials may be secreted. Secretion of enzymes from pancreas is achieved
in this way. Plant cells use exocytosis to export materials needed to form the cells walls.
High temperatures denature proteins, causing them to lose their shape and function. This
disrupts essential cellular processes, and if enough proteins are affected, the cell can no longer
function properly. As a result, the cell may die due to the failure of critical processes.